text:
E3D V6 nozzle vs MK8 nozzle, first layer adhesion
Does anyone notice that when they upgrade from an MK8Makerbot(?) to a E3D V6 hotend that when using the same settings and bed-leveling/z-distance-setting procedures, that the filament is much more likely to be pulled up and bunch up around the nozzle while printing the first layer?
It occurs mostly when printing small details, such as 3 mm bolt holes, and not so much when laying down long lines.
Is this due to the shape of the nozzle, which on the V6 is much more flat when compared to the MK8 which is more sharp, or is it due to something else that can be easily fixed?

what kind or Mk8? Mk8 is a way to show that this is itteration 8 of some hotend. Who's manufacturer? which design? On which printer?

It seems there are unspoken rules that you are supposed to follow when switching to an e3d v6 or other all metal hotend. People say to increase the temperature 10-30 degrees and the bed temp as well. I dont see how this should matter if both thermistors are accurate. All I know is after installing the V6 and keeping everything else the same, the filament is much more likely to get pulled up and stick to the nozzle!

@Trish good points, I thought the same thing; but I don't think that's how it works. Everyone simply calls them "MK8". Search Amazon for "MK8 nozzle" and you will get this specific shape of nozzle and a specific thread type

So many things wrong with that last statement. First off, lets get the first layer adhesion out of the way... ANY time you work on your hotend, you need to adjust your Z offset because things rarely go back to the exact same. Even just a different torque spec (your hand is not a torque wrench nor is it as consistent as one) changes it. 10 degrees more turn on the thread from harder cranking and now youre higher off the bed. Believe it or not metal does actually flex when you tighten, especially aluminium and soft copper alloys like brass and bronze.
Secondly, the E3D V6 head is shorter than the mark 8. Thus you will need to adjust offset closer, by default, when switching to a V6.
That will fix bed adhesion in most cases. Follow procedure and recalibrate when you make changes and youll be fine. Now, as for temps... raising bed that high is a bandaid setting. We dont use bandaids. Fix the root, stated above, not a symptom of it. This prevents other problems. In this case your bed temp (assuming you're on PLA) is above glass transition. Thats a no-no. Even with proper offset, that causes other issues like the "other" type of elephant's foot. From the bottom layers being fluid and having a taller and more diffuse heat gradient. Basically the bed layer stays put and then layers above it shrink. Till about 5 or 10 layers up. Giving it a bow at the base of the print. You want it hot. But not melted. In regular PLA, thats 55c. 60c is glass transition. You can go to 59c with disgression, but generally you wanna keep 5c away from GT. Be careful with forums and FB posts, people say "do 75c" etc... these are beginners. Be careful taking advice from someone in their first few years of printint. FDM is tricky. It will fool you and teach you bad habits, and you wont realize its hurting you till much later on when you start moving out of the box.
Now all metal temps... who told you +30c? Thats not a thing. If you are having to heat up another 30c to print with all metal, you have a throat clog. Fix your throat cooling system. This can also be from incorrectly installing or assembling it. Same with steel nozzles needing to be hotter. This is a myth. They will equalize just as any other nozzle. The difference is steel has a lower rate of heat absorbtion, and thus takes longer to heat up. It also drops faster than brass as you get into speed printing. Think pouring water on red hot steel. The filament is the water. So * when * printing at very high rates of flow, which most people wont reach anyway, then yes, you run it hotter. But this is true for all hotends and nozzles during speed printing, not just because its steel. Speed printing often has nozzle temps above thermal breakdown of a material because it never has a chance to reach that due to rate of flow and time to heat, but still needs that abnormally high temp so the rapidly flowing filament can grab enough energy to melt in time. Larger bore nozzles also need a small bump because the core of the extrusion is farther from the walls of the nozzle.
But no, V6 and/or all metal hotends absolutely do not need extra temps under normal use (like 0.2 layers at 60mm/s). The run at the same temps. And so does the bed. If it still doesn't stick and you know for sure your z offset is good, and you preheated and all the stuff youre supposed to do when setting that offset, then your bed is dirty, wipe with alcohol. Or the coating is worn out. Or, on bare glass and large base prints, you need an adhesive like 3D Lac or gluestick (not regular white, or gel, you need Xtreme or extra strength, or purple). If you have to heat up the hotend to extrude on an all metal or nozzle change, figure out why youre having throat clogs, because thats not a normal thing for a well-functioning hotend and extruder. That indicates a problem somewhere. Either user error or poor throat cooling. It can even happen with really cheap and poorly made nozzles. V6 does not mean its genuine E3D. If you see V6 and its not directly from E3D or a company in a contract with them, then its a Chineese clone. Yes, it matters... Their geometry inside the hotend needs to be machined properly, not rushed out cheaply. Thats why genuine ones cost more, and its how they got their reputation for being amazing hotends. They take care with machining, and they test and refine stuff, in house, daily.
-The guy that started on a Cupcake back in 2008-2009 and now teaches it.

Turns out this can be fixed by increasing the bed temp to 65-70 °C and of course increasing the extruder temp by 30 °C or so, which is standard for the all metal hotends (no idea why)

Have you measured the temperature of the two hot ends with an actual thermometer or are you just going off the print settings? The thermistors used to measure temperature can have wide tolerances and your V6 might actually be running colder than your original hot end with the same settings. Plus, is your V6 a real E3D or a clone? All-metal hot-ends have much stricter machining/material requirements for the internal throat and heat-break than the ones using a ptfe liner, and some of the cheaper V6 clones have inferior thermal characteristics compared to the E3D originals.

authors: Андрей Шлегель


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How to format an SD card for Monoprice Select Mini V2?
I bought a 32 GB SDHC (Sandisk) I'd like to use for my Monoprice 3D printer.
I have downloaded Partition Wizard and partitioned a 2GB primary partition formatted as FAT and it still does not show any files.
What am I doing wrong?

Welcome to 3dPrintnig.SE!

For an SD card to work with the printer firmware Monoprice suggests to format the SD card to FAT32.
However, the Monoprice Select Mini V2 is not able to read SDHC memory cards, it is advised to use an SD card (smaller than 4 GB) instead.
According to the Monoprice support website: "Why is my printer not reading my SD card?":

If the SD card is not recognized on the printer or if the files are
  not reading, it could be an issue with the actual card itself. The
  first thing we recommend checking is that none of the print files on
  the SD card contain a space in their name. This shows in the printer
  as an unidentified character and can cause issues. If none of your
  prints contain spaces, we recommend reformatting your SD card.
Note:  If you choose to purchase an SD card, please make sure that it
  is not labeled HC (High Capacity) as it may not be compatible with the
  printer. This means that the card must be smaller than 4GB in size.

The last part of the support page is probably applicable to your card.
Some further information can be found in What is the largest microSD card that a Monoprice Select Mini can read?, specifically this answer. From this latter answer, I quote:

Cards between 2 GB and 32 GB might work, depending on the specifics of
  the card

Basically there are no guarantees when using large cards.

This is kind of scary... HC is the standard way to make and sell SD cards now, and "regular" non-HC cards are getting harder to come by.

@JoelCoehoorn Yes, bad choice of the printer developer to choose for the old format. I'm just the messenger, it is an official statement release from the printer OEM.

I was having problems formatting micro SD cards for my Monoprice Mini v2. I have some 256 MB cards tried formatting them FAT FAT32 etc, no luck.
Then I downloaded a program called SD Memory Card Formatter did the format on the card with this and the card is now readable in the printer.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


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Marlin 1.1.x on Ender 3 changing PREHEAT_1_FAN_SPEED has no effect?
I dont want the part cooling fan on during preheat, especially when I'm only heating the bed, it is just unnecessarily loud and serves no purpose at that time.
However when I set PREHEAT_1_FAN_SPEED to 0 it has no effect, the fan still spins at full speed as soon as I preheat either the hotend or the bed.
So why is this setting not working and how do I fix this?
I am using the latest version of Marlin.
The only one that is defined is the one that I am editing and it is the one that appears on line ~1260 of the stock configuration.h
#define PREHEAT_1_FAN_SPEED 0 // Value from 0 to 255

And this is the grep result of searching the entire firmware folder:

This question is a little terse and could use some extra information, please update the question by [edit] to include where you set this constant (as it could be that it is overridden later!). The default Marlin configuration.h file already contains a PREHEAT_1_FAN_SPEED of zero, part cooling fans should therefore never spin unless the configuration is altered. In that effect, which sources do you use? Original Marlin sources or a repository of someone else?

Apparently, I forgot a critical step:
While I have completely reconfigured the LCD menus, setup custom 25-point mesh leveling, changed a bunch of other numerical values, and flashed the firmware dozens of times, certain values will never be updated unless you remember to initialize the EEPROM after the flash!
Honestly I kinda wish they would make it all or nothing but I guess there is limited space so particular things take priority.

This answer is now obsolete now that the OP has updated the question with additional information. It is left here as a possible solution for those who have tinkered with there firmware.

Part cooling fan speed during preheat (from menu) is controlled by the setting
#define PREHEAT_1_FAN_SPEED     0 // Value from 0 to 255

in Configuration.h where a value of 0 implies no rotation, or 0 % and 255 implies 100 %.
If changing this value in your configuration does not result in a reduced fan speed, you could have this constant be defined somewhere else overriding this value.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Important build plate characteristics/qualities
I managed to tear my build plate trying to get some very stubborn plastic off it.

I'll need to buy a new one, what characteristics are important for me to ensure it's compatable with my printer?
I can think of:

Dimensions
Power requirements
Connector types
That my printer moves the plate in the z axis (?)

NB  I'm so new at this I didn't realise that this was just a sticker on the plate, so I just need to replace this sticker.  However, as a question, I'm still curious as there's a good chance I will break it at some point.

What kind of printer is that? it looks like you have a PEI coating that is chipped - that can be easily refurbished.

Do you intend to replace the (heated?) bed, or mount a print surface on top of it?

@Davo  no, I was mistaken thinking that I had to.  Just need to replace that too layer.

@Trish it a Monoprice one.  https://www.monoprice.com/product?p_id=15710

If your printer's heated bed still works, but the sticker has been ripped in some places:

You can try and remove the rest of the sticker, clean the metal plate under the sticker (perhaps with isopropyl alcohol) and then apply a new sticker once there is no adhesive remaining on the heated bed.
You can remove the sticker with the method above, but instead of replacing it with a new sticker, you can buy a PEI sheet and cut it to size if required (read more about PEI here) and then stick it to the metal plate.

Personally, I would recommend the latter option, since PEI is low maintenance and the prints automatically pop off the sheet once it cools down, however just make sure not to print PETG on it, since it sticks too much.
On the other hand, if your entire heated bed is broken, the best-case scenario is to look for a heated bed that was designed specifically for your printer. Especially since your printer's heated bed has 3 mounting screws instead of 4.
If you somehow have a solution for mounting a heated bed with 4 mounting screws on a printer that uses 3, the things you need to watch out for are the heated bed's operating voltage (it should be 24v) and the dimensions must be the same. If the wires are not provided with the heated bed, make sure to get wires that have high enough gauge so that it is able to handle the high currents that the heated bed will need.

You could add a link to How to clean up my build plate for a new build surface?!

I would say put a glass bed on it and you won't have to worry about tearing it ever again. Many places which sell picture frames can cut out a piece of 3 mm glass for very little (like 1-2 dollars). I've had a normal 3 mm glass bed (no expensive "heat glass" or anything) that's worked fine for 2 years of printing. You can use simple metal clips to hold the glass in place.
As an added bonus the prints pop off by themselves when the glass cools down. I'm usually impatient and remove the glass from my printer and cool it down under water. I wouldn't recommend doing that tho, since you risk cracking the glass due to the heat shock.
Only down side is that it might be a tiny bit tricker to get the print to stick, but I don't really have anything to compare to so I don't know for sure. I only have problems getting prints to stick when my initial layer height is wrong or when printing very small things.


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Can I 3D-print a PET bottle?
I want to print a transparent PET bottle for my homemade lemonades and thought about 3D-printing them.
I would like the printed bottle's quality to be as fine as a Coca-Cola PET bottle for coke and the printing substrate or material be something that's cheap or readily available. I thought I might recycle some old PET bottles to print the custom one for my lemonades as I don't want something that would require me buying new materials or re-exporting from the manufacturer.
Is it possible to recycle PET and print it into food-certified containers?

Technically recommendation questions are forbidden, but in this case there is a doable core: "Can I print recycled PET fully transparent and food rated?"

Glass bottles and homebrew-style caps would probably be your best packaging suggestion.  Or reusing cleaned PET bottles, if permitted.   You could personalise the packaging with labelling, or print some kind of nubbin to stick to the top of the cap/lid.

No, due to 3 reasons
PET is not (easily) printable.
There is a lot of confusion on what Filaments you can buy: most times filament branded PET is actually PETG, sometimes PETT.
PET is not an easily printable material at all. With expert knowledge and the right machine settings it can be printed, but even then, it is not as easily recyclable into a useable 3D-printer-filament as you might think. You need full reprocessing capabilities, which means the need for machinery to allow thorough cleaning, grinding to dust, melting it up, pelletizing and finally extrusion as a fresh filament.
The closest related material that is easily printable is PETG, a modified PET that also contains glycol. You can't convert PET into PETG with home or hobbyist applications at all - they are totally different in their chemical behavior, even as just one material was added in production. PETG is not brittle like PET, it does not haze on heating, but it ages in UV light, scratches easily and can't be autoclaved like PET. But the chemical modification has to be done during the initial manufacturing of the material, and it is a huge mess to try to recycle the two together, which can and will happen if you try to work with material you source from recycling.

“When they’re processed together, PETG melts and becomes sticky while PET remains solid. PETG sticks to PET chips and forms large clumps that pose processing problems.” Resource Recycling (magazine/blog)

3D printed objects are very unlikely to become food certified.
You can't easily manufacture (certified) food-rated printed products, like food containers due to the requirements that a machine that manufactures food-certified products needs to comply to. I advise looking at this answer regarding food rating for more elaboration.
It is hard to print really transparent with FDM.
Due to the method how FDM works - extruding lines next to each other - it is often impossible to print fully transparent objects right of the bat - there is almost always air inside a printed object, and there are so many boundaries between the extrusion paths that refract and change the photon paths that the best one can achieve somewhat easily is translucent (=semi-transparent). Read this answer for further information.
But if you manage to get the object really solid, you might get some near-transparent, icy results from some orientations while looking in others still will look matte.
To get them fully transparent you then will have to post-process them to become fully transparent by grinding the surface up to 4000 grit, but that is very labor intensive and most likely not possible for the inside of a bottle. To be clear, you spend hours polishing one surface.
Could it be economic in the slightest?
On a side tangent, the viability of printing a bottle via buying new ones will need to be expored. Shapped PET Bottles with caps start at \$0.01 per piece and top out at \$1 per piece - you get the better prices if you order in larger quantities. You will have to compete with getting under \$1 per bottle, or rather with what the price of a typical bottle you want is.alibaba.com
A typical PET bottle ordered from China weighs 30 g for a 300 ml bottle, and the particular example I looked at comes \$0.22 to \$0.28, depending on the bottle cap, with a minimum order of one parcel with something around 300 items. That seems to be in the average range.
A roll of 1 kg of PET(G?) filament starts at ~\$30 at the moment. That is the weight of 33 shaped bottles per roll. Your print will most likely be heavier than the blown up bottle to get it watertight, but let's just assume you might manage the same weight. Then it's about \$0.90 in the material alone - so we are at more than 300% of a bought product with cap already!
Atop that comes the running cost of the printer, which depends on your print time, printer and electricity price. I know my hobbyist machine comes, maintenance and electricity combined, down to 0.21€/h, so roughly \$0.25. Printing a bottle will take several hours.
PET preforms that can be blown up to almost any bottle shape, type and size and ship much cheaper come to prices due to better density. Which means you compete against \$0.015 to \0.15 per bottle in material costs.
Conclusion
It is not economically viable to even attempt to print bottles beyond a prototyping stage.

I think there's a fourth issue: 3D printing is designed to replicate the shape of an object. It's not designed to replicate the structure of an object.  One shouldn't expect a 3D printed replica of an object to have the same physical properties, even if the same materials are used. It's a little bit like expecting to be able to replicate a forged sword using cast iron.  PS. "I advice looking at this answer" ->  "I advise looking at this answer"

PET bottles are made by heating and expanding a thick-walled blank inside a mold.  Would a PETG blank balloon out if heated and inflate4d in the same way?  Or would it jus tear?

@Criggie that is a different question, one for engineering.SE under Material Science. In general: PETG does melt while PET at the same temperature becomes moldable. With the right temperature and timing... maybe?

Forget about transparency and food certification.  The first problem you'll get with FDM is that your bottle won't be watertight, due to small gaps caused by the layering process and minor variations in the extrusion rate during the print.  Especially if using consumer-level hardware.

@aroth I have managed to print watertight objects with china machines. You just need to print more walls.

@Trish: I've printed watertight with single-wall/vase-mode. You mainly just need to make sure nothing is badly out of tune. Localized underextrusion due to material loss during travel and lagged response of extrusion with respect to extruder motor motion are probably the biggest threats to messign it up, and largely fixed by using retraction and a printer with linear advance feature in firmware.

@R.. which makes it extremely hard, but yes, technically it's possible.


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How can I decrease the thickness of a wall in an STL file?
There seem to be a lot of search results answering the question of how to increase wall thickness, but how can I decrease wall thickness? I have an STL model of a mechanical keyboard key where the outside dimensions are correct, but the walls are slightly too thick so the key gets stuck in the down position. I'd like to shave some of the thickness off of the inside of the walls to better match the original key I'm replacing, while leaving the rest of the model dimensions as-is. How can I go about doing this? I consider myself technical in general, though I'm a novice at 3d modeling software.

Welcome to 3dPrinting.SE!

Be careful with nomenclature: When you open your slicing tools (e.g. slic3r or Cura), the word "Wall thickness" refers to how thick of a solid layer is created before the slicing software switches to designated infill for the object.  In your case, it sounds like what you want to do is change the dimensions of the part you're designing, i.e. total part thickness.

@CarlWitthoft It's basically a 5-walled cube, where the bottom is open and the inside is hollow. It's the 4 side walls that I want to decrease the thickness of, leaving the overall outer dimensions in place. So yes, I do mean the thickness of the solid walls, though not overall. Does that make sense?

The process is rather simple:

import your model into either a modeling software (e.g. Blender et al.) or a CAD-program (e.g. fusion360, Design Spark Mechanical et al.) that can import and export STL.

if needed, convert the STL into a useable model with your chosen program's functions or switch to edit mode.

select inner walls and extrude them the desired amount.
export as STL again.


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What causes this "stringing" and can my print recover from this?
Here's what it looks like

This is the model
thingiverse linky
It looks like it couldn't print the edge, but this happened many many hours after printing the brim.
This did not happen with my 1st attempt at this print. The last print lost adhesion and I had to scrap it. This time, adhesion looks good so not sure why this happened.
Printing with Monoprice Select V2 with ABS, sliced with Cura. 100C bed / 250C extruder. 15mm/s initial layer speed. 60 mm/s print speed.
Update
It looks like the printer is starting to smooth it out like so. Still not sure if this will lead to an ok print or will fail because of this layer.
And it seems the stringing area does not have a brim underneath it. Did Cura just not calculate the brim size correctly?

Update2
Here's a few screenshots from Cura to show that the model is lying completely flat.

I let the print go on overnight and here's where I stopped it

It almost seems like the print shifted completely after printing the initial layer. Have you ever seen anything like this or is there anything in my Cura model that would make it do this?

The strange thing is that all the models in the Thingiverse link have a perfect flat bottom, there should absolutely be no need for supports on that end.

good that you had the print run on for a couple more hours, resulting in identifying layer shift.

No, your problem is not related to slicing, this is a hardware problem. Your complete print has shifted, this is called layer shift.
This could happen when the nozzle hits an obstruction while printing while the Y stepper continues. This could lead to skipping teeth on the belts, slipping of the pulley or missing steps. This results in printing over air as the print progresses. This manifests itself as stringing, but in fact is unsupported printing (in the air). In this case it is unrecoverable as the printer has lost the reference frame, it just continues to print with the new reference frame caused by the layer shift.
A Prusa MK3, or any printerboard using trinamic stepper drivers would be able to recover (if the belt and pulley are correctly attached, and steps are missed) as the skipping of steps is detected, in case of a Prusa  MK3 the machine will re-home when it detects skipped steps and continue printing. See also this answer for more details.
Possible solutions are increasing the belt tension, increase the stepper torque by increasing the current through the stepper drivers or re-tighten the pulley on the stepper of the Y belt.

This seems like a reasonable explanation. To be honest, I don't understand everything you said, but I'm going to do some research on it

In my Case a missing Set screw on the bed stepper would cause it to shift when it did a high speed transition from the back to the front of the piece. the small set screws were not particularly visible and its loss went unnoticed.

No, the print is lost.
First thoughts after Update 1
Your print will not perfectly recover from this stringing issue, which to me looks to be unsupported bridging. I postulate that your slicing seems to be made with the model not aligned flat on the bottom correctly.
The biggest indicator for this is, that the brim is passing right underneath the model, so in that area, the slicer did not detect the model touching the surface, which means it is considered floating.
Re-slice the model and check in layer view that you absolutely have the bottom touching to the sheet everywhere. Use the "lay flat" function (which should suffice) or enable at least 10% support to the build plate (which in this case should not be needed). Even though the model doesn't need this because the base is flat as to inspection, the last resort could be to move the item 0.1 mm into the sheet, cutting off any odd protrusion your slicer identifies that can result in parts of the object "floating" and demanding to bridge long ways into the unsupported nirvana.
New Thoughts/Revision after Update 2
After the new photos, it became apparent that it was not a slicing issue of a slightly angled print but a layer shift.
You will have to check the tension of your bed belt and if anything might have come into your bed movement. On my Ender 3 I had tried a cable chain, which did hinder Y-Axis movement - make sure that it has not even a movement of short snagging.
Make sure that your nozzle does not snatch on the print as it moves. Maybe activate Z-hopping.

All models are perfectly flat, there should be no need for support for the bottom (other than the coin dispenser).

@Trish this is a reasonable theory and something I'll look out for in the future. But please take a look at my updated photos. The model is laying flat and I even have supports enabled though I believe Oscar is right that it's not needed here. It almost looks like the print shifted y-axis after printing the initial layer

It was reasonable at the time we didn't had the new picture... layer shift can't be distinguished from bad slicing in the infancy at times.

@kane updated with a couple ideas what might be the cause of layer shift. bad cable chains or improperly installed ones can be a huge problem..

@Trish I believe I identified the cause of the layer shift. I've posted it as a follow-up answer to this question

Both @Oscar and @Trish have identified the problem in their answers. Upon further investigation, I believe I found the root of the problem.
The Monoprice Select V2 has what I think is an inherent design flaw with how its wires are routed. The wire can sometimes (1) get caught on the frame (when the y-axis is moving towards its maximum position), (2) get caught between the frame and the print bed preventing the y-axis from reaching the minimum position, and (3) it even hits the y-limit-switch sometimes.
Here's a backview of the printer where you can see problem (1) where the wire can get caught by the frame. You can also see the y-limit-switch and I saw first-hand when the wire bundle caught it, forcing another layer shift in my print.

For problem (1) above, I found this solution seems to help
thingiverse link
To resolve all of the wiring issues, you have to unwrap the wires and reroute them. People online say this voids the warranty.
The wire jamming doesn't always happen. But when you printer moves back and forth along the y-axis hundreds of times for a large print, there's a high probability it snags at some point. This is very frustrating after many hours of printing. And I'm thinking of getting another printer for this reason.
I should note that it looks like Monoprice's next version printer, the Monoprice Select Plus, doesn't seem like it would have this problem since the control unit is integrated and the wires don't seem like they would be in the way.

Snagging cables are a PITA... but nailed it (well, I was looking at a chain, but...)

hm, judging from the design... you don't need to remove the cable binders, if you are comfortable to unscrew all the screws...

@Trish What are you proposing?

unscrew the backplate &  rail on one side, then slip in the cable for the bed, then re-screw it.

Are you suggesting routing the wires to the heat bed under the backframe?

yes, that would solve the snagging on that corner.

Let me look into this a bit later. I think I've already resolved issue (1) but your proposal could also resolve issue (3). I think it would exacerbate issue (2) though so I'll have to check. Also, I may have to unwrap cables so it would fit under the frame. I think it is too thick as it

There are many problems caused with this. It could be a faulty motors, an unlevel bed, a dirty nozzle, or even something as simple as using the wrong filament settings. It would be best to go through a checklist of things that might be wrong with your printer. It could also just be caused by the wear and tear of a  really old printer.

please be more specific.


text:
How to build a CT scan into a printable 3D model?
I have a set of 2D pictures from a CT scan.
How can I convert them into a 3D model for 3D printing? An example looks like this:

What is your question? how to convert 2D slices int 3D model?

You can use MITK

Using the terms "convert CT scan to 3D model," I found a number of links of tutorials. One of them is described as free, with registration and appears to be web based. The link, Embodi3D, appears to have a relatively comprehensive set of instructions to accomplish your goal.
Instructables also has a similar tutorial. Should neither of these prove suitable, the search terms above may be of value.
Image below via Instructables:

Fred, this is a bit of a link only answer. If the links die, then the answer won't be terribly useful, apart from the search terms... :-) Any chance of including the main steps?

I attempt tp avoid link only answers, as I've seen reference regarding dead links. In both linked suggestions, the quantity of steps are substantial and detailed. My primary purpose was to suggest useful search terms.

Knowing the layer thickness, you can manually re-engineer/design the object:

take your CAD or 3D modeling program of choice

import one picture into the XY axis
trace the outline on that layer
add a construction layer one layer thickness above

Rinse 1-3 and repeat for all layers.
Combine the outlines, method depending on your program

this might be quite easy in blender if you add the new layer by extruding the previous one up by one layer thickness and just move the vertices to match the new outline

finally, export the model as .stl depending on your program, slice and print.


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How specific can I get setting layer heights?
When choosing a layer height, I know that often you go as fine as your printer will do for better precision, but sometimes you go a little thicker, for speed, for example.
I also see 0.1 mm and 0.2 mm as common thicknesses.
What are my options here? When I'm working on a part where I want to print a draft piece, and the quality matters less, can I set it to 0.15 mm? 0.11 mm? The Ultimaker Cura slicer I normally use will let me put in almost anything, but what can it really do? If I can use values in between simple 0.1 mm increments, are there reasons I might want to do so?
For reference, I have a Monoprice Maker Select Plus with a 0.4 mm nozzle and, again, Ultimaker Cura as the slicer. But more general answers for other printer types and slicers are also encouraged. I want to know about this generally, and not just for one printer.

Layer hight can be theoretically anything as long as it fits into these ranges:

it needs to be at least one step for the Z-Axis motor to be physically possible
it should be at max 3/4th of your nozzle diameter to create an adhesion surface

In praxis, the lowest setting for layer height due to physical limitations of the extrusion systems is around 0.05 mm.
Also, not any number is possible, it is dependant on the Z-axis system: since one or one-half step is the smallest rotation that the stepper can achieve, the raise that belongs to this partial rotation is how much one and the next layer height can be together. The step limitation usually is of no concern though, unless one has a very steep lead screw.
As a rule of thumb: doubling layer thickness results in almost half the print time. 0.1, 0.2 and 0.3 mm are common because they allow easy checking the printed accuracy in Z-axis with calipers.

@JashJacob No, stepper motors only allow full or half steps, where a step depends on the number and arrangement of coils - and the range is given in degrees. 5 microns are a length, not a rotation. 5 microns is what is the result of the turn via the leadscrew.

You decide which layer height you want based on the quality you desire, but never go over about 75 % of your nozzle diameter, so with your 0.4 mm nozzle never choose layer heights larger than 0.3 mm. The rationale of this rule of thumb is that the filament leaves the nozzle as a tube and needs to be flattened to make it adhere to the previous layer.

The thicker the layer, the smaller the number of interlayer contact zones (this is a weak point), the higher the strength of the part.
For good interlayer adhesion, the thickness of the layer should not exceed half the diameter of the nozzle (this is pure geometry). The recommended value is 0.8 * 0.5 * D.
There is an opinion that the thickness of the layer should be a multiple of the height of the Z climb to one full engine step (in order to avoid accumulation of error).

Thus, for an engine with a step of 1.8 degrees and a screw with a step of 8 mm per revolution (most common configuration of Z-axis), the layer thickness must be a multiple of 0.04 mm. For a nozzle with a diameter of 0.4 mm, the recommended layer thickness is 0.16 mm, maximum valid is 0.2 mm.

No, sorry, I meant exactly count of interlayer contact areas. The smaller the number of such areas, the higher the strength.

Ahhh, yea, I got that twist... but count and area are proportional. you might mean "number of extrusions/contact area"? that is antiproportional to the strength, as you gain in-layer-inter-extrusion boundries.

Translation difficulties... I meant "the number of interlayer contact zones". Finally fixed.

I would like to debate the first point, many of my failed prints have failed through layers, not only alongside the layers. Could you explain where the formula comes from?

Sorry, @0scar, I did not understand. The first point does not contain formulas, only general considerations. The strength of the interlayer adhesion zone is less than the layer itself, this is obvious. The strength of the interlayer adhesion zone varies from layer to layer and is distributed between the minimum and maximum values. The more layers, the greater the likelihood of a zone with a strength close to the minimum. The ideal case is a molded part (one layer, no interlayer adhesion zones, maximum strength).

@DmitryKasatsky The second point does contain a formula, comments cannot contain an enter (new line), therefore it looks at though I meant it to be for the first point while I meant the second, please explain. Nice explanation about the strength of layers, but I seriously doubt that considering the failures I've seen. It all depends on the loading of the part.

@0scar, the effect of the number of layers on strength is small, probably it can be neglected.
I'll look for the source of the formula from the second point.

@0scar, sorry, but the source in Russian: https://3dtoday.ru/blogs/filamentarno/the-intricacies-of-3d-printing-part-2-theory-and-practice/
This is an article by a famous russian filament manufacturer. Try to use Google translator if you want.

If I understand this right: 1.8 degrees is 200 steps to get a full revolution (360 degrees ÷ 1.8 = 200). Therefore the 8 mm per-revolution for the threads is also 8 mm per 200 steps, such that 8 / 200 results in the .04 mm per step. Thus, rather than .1 mm layers (the advertised machine minimum), I might do much better at .12 mm, which is an exact multiple of .04 mm we move at each step. Does that sound right?

Doing more research, and the rods used in a given printer can vary widely, so the 8 mm revolution distance is just one example. The main thing is the firmware in the printer knows what it needs to do. Also, stepper motors can take partial steps, so we can get a lot more granular.

Let's start with the stepper motors used to move the z-axis. Most printers use a Nema-17 motor, and most Nema-17 motors turn 1.8° per step. For the motors that don't use 1.8°, almost all of them are .9°, such that assuming 1.8° is pretty safe for most us (if you have a .9° motor and wrongly assume 1.8°, the motor will just take two steps instead of one).
But these motors don't move the print head or bed directly. Rather, they will turn a threaded rod to lift or lower the necessary part. We need to figure out the linear distance traveled along the rod per step.
There are two ways to determine this. First, we can look at the threads on the rod, and think about the linear distance traveled if the rod turns one complete revolution. Use a sharpie marker to trace a thread, and then measure the distance.
Alternatively, if we check the specs for the rod in terms of threads and pitch. Most 3D printers use 8 mm rods, but looking at my own printer these aren't standard metric threads. Indeed, the pitch can vary quite a bit from printer to printer. Both 2 mm and 3 mm pitch is common, with the number of threads on the rod varying from 1 to 4. To find the linear distance of a revolution, you multiply the threads by the pitch. A 3 mm pitch with 1 thread has a linear distance of 3 mm. A 2 mm pitch with 4 threads has a linear distance of 8 mm.
With either method, we know the linear distance for one complete revolution (360°). But a single step only moves a small portion of that: 1.8°. Thankfully, this works out to an even number: 360/1.8 = 200. There are 200 steps per revolution.
Now we know enough to figure out the step distance. The math looks like this:
StepDistance = (RodPitch*RodThreads) / 200

As an example 2 mm pitch and 4 threads produces this expression:
(2 * 4) / 200

and the final result is:
.04 mm per step

Those values are common, but not universal, so you need to know what numbers to plug in for your printer.
You should also be aware that many stepper motors can take half steps or even divide a single step up to 256 times. If you can watch your printer smoothly move from very low to very high, or vice versa, without jerking for each step, this is what it's doing. That means your printer may be able to make adjustments as fine as .00015625mm. At this point, we're down in nano-meter territory. I think it's probably best to assume your printer wants to take at least a half step when moving from layer to layer.
Now lets look at applying this .04 mm per step value to a real printer. Please remember: this is just an example, where the .04 mm number is based off a specific rod type. You need to know the values for your specific z-axis rods.
Let's say you have a printer that advertises a .1 mm minimum layer height and a .4 mm nozzle. Given the .04 mm steps, you might do much better actually using .12 mm layers, which is an exact multiple of .04 mm. But then, if we consider half steps, .1 mm could be just fine. On the other end of the range, we don't really want to go above 75% of our nozzle width. That's .3 mm. Given a .04 mm step height, a better max is really .28 mm... but, again, considering half steps, we could in theory choose to do the exact .3 mm height.
The main thing I want to take away here is for when you're looking for an in-between layer height: .1 mm is too slow, but .2 mm is too course. It's probably best to bump up the layer in exact half-steps. For this example, that's .02 mm increments. So after .1 mm we try .12 mm, then .14 mm, and so on. Finer adjustments may be possible, but there's a power of two law at play here, so keep dividing things in half, rather than by 10ths. If you don't get good results this way, try assuming full steps, and start at .12 mm for the layer height, then go to .16 mm, and so on.

Hi Joel, see also Full steps mode for Z axis stepper controller


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I can't level my bed any more
I have a Monoprice Select v2 that I just bought less than 2 months ago. I've manually leveled it on several prints prior.
I noticed a couple of holes in my print bed after my last print, like the extruder pushed into the sheet really hard. They're both on the same side.
I then noticed that the extruder's horizontal bar wasn't leveled so I leveled it. I did this by unplugging the wire to one motor and modified the z-position so that the other connected side would go up. (Is this the proper way of adjusting the horizontal bar?)
And then I moved the extruder back into its Home position at z-position 0.
That's when I noticed that it seems my extruder barely reaches my print bed even though I have the screws almost loosened to the point of detaching from the thread.
The extruder z-limit-switch (I think that's what it's called) seems to be pressed so the extruder is as far down as it can go. The controls also show that the z-position is at 0.
So I'm guessing my print bed somehow lowered. But I don't see how that could possibly be.
Any ideas?
Here are some photos but I'm not even sure what to show really
you can see the z-limit-switch in the right of this pic next to the letter A's

you can see my extruder still has some distance to go before reaching my print bed but you can also see that my screws are loosened as much as possible

showing the horizontal extruder bar is now leveled

Kane, your portal is not level ut shows clearly to be tilted downwards.

@Trish what do you mean by "portal"?

The last picture show me, that you have a tilted X-axis portal - the bubble should be in the middle of both lines, it is to the left side. You might want to brush up your spirit-level knowledge... http://www.johnsonlevel.com/News/SpiritLevelInformation

You can't adjust your z-limit switch downward any? I'm not familiar with the monoprice, but it seems the easiest way to fix this is to move it downward so it will travel further down in the z-axis.

@Pᴀᴜʟsᴛᴇʀ2 No, the z-limit switch can be moved but it's at the lowest possible position

Is there adjustment to the nozzle which you could do? If there's a jam nut between the heater block and the extruder motor, you might be able to loosen it and run the nozzle down a couple of threads, putting it closer to the bed. I'm not sure how much of the nozzle threads into the extruder motor, but it might be worth a look-see.

The bar where the level is attached is the support cross-bar for the X-axis travel.
It's important this bar be level, but it's even more important the bar be true to the rest of the machine. We call this "leveling" the printer, but really the more accurate term is "tramming". If the bar is level, but the machine sits on an unlevel surface, things are out of sync. This may be the case here.

I did this by unplugging the wire to one motor and modified the z-position so that the other connected side would go up. (Is this the proper way of adjusting the horizontal bar?)

No. The first image in your question shows two threaded vertical rods: one on the left that is exposed, and one on the right that is more hidden (only a small portion is visible between the two "A" labels). These rods control travel in the vertical "z" direction. Each rod has an inline coupler connecting it to the Z-stepper motors. The coupler for the right side is visible just below the square label "A". You adjust the level of the X cross-bar by turning the couplers while the printer is turned off or the motors are otherwise disengaged. This allows you to make finer adjustments for the bar, often without needing to re-home the Z axis.
You can also use this method to correct your immediate problem.
I own a Select Plus, which uses similar construction to your Select V2. When I first got the printer one of my couplers was not adequately secured to the rod. This made for all kinds of difficulty leveling the bed at first, including at one point an issue similar to what you are experiencing.
Re-leveling the X cross-bar is easiest if you are first certain the printer itself is on a level surface. If this is the case, you should be able to use the level in the images to check the cross-bar in the same way you did previously, making adjustments via couplers. But check that surface first. If the surface is not level, you need to adjust the X cross-bar to be parallel to something in the machine other than the bed. While we want the bed to match the rest of the machine, the very fact you can adjust this means it's not guaranteed at any point in time, and so you should use something more structural to compare against.
In this case, you can place the same level across the two steel Y-axis travel bars below the bed, and check where the bubble sits. Then make sure the bubble ends up in exactly the same position for the X cross-bar, regardless of whether or not that's level to the ground. The greater precision you can get here, the better things will be.

I'm gonna add some notes about leveling the beds on these specific printer models.
There are three basic types of leveling:

Manual. You do everything yourself.
Assisted. You still do most things yourself, but the machine will help by moving the head quickly to certain positions for you to check. This is what many machines refer to as "automatic", even though it really isn't.
Automatic. This is where the machine does everything, using lasers to map the bed or some other contact sensor to detect when the head approaches the bed and make it's own adjustments. Often this only uses software to compensate for the bed, and so you still want to get the bed as level as possible first without this option.

The Select V2 and Select Plus only do #2: assisted.
When I was starting out, my research indicated you should generally level your print beds "hot", because heating a print bed will expand it and potentially change the results. This makes sense to me.
However, the instructions included with my Select Plus said nothing about heating the bed, and if followed precisely meant I leveled a cold bed. Additionally, the so-called "automatic" (really: "assisted") leveling tool in the firmware also does not keep the bed heated, meaning if you want to level it hot you must do it entirely manually.
This caused me great difficulty when first starting out getting a good level, because I was trying to do it manually on a hot bed. I finally gave up on that, and started using the assisted tool on a cold bed. To my surprise and relief — and counter to what I've read elsewhere — I've had much better results when doing it cold this way. But this may be some peculiar to these printers, perhaps involving the special mat it uses on the surface of the bed.
I'm also considering this upgrade (Update: I recently installed it, but it's too soon to know if it's helping), to help improve things even further, since there is some curvature in my plate:

https://www.amazon.com/dp/B07B251KBS/

Finally, there's an interesting seesaw effect when working on a square print bed with four adjustment knobs. I have a post about it here:

Leveling a square 3d printer bed with four mounting posts

I hadn't even considered that the surface the printer rests on could be un-leveled. I just measured it, and to my surprise, you're right. It's way off! Let me try what you suggested. Thanks!

Y is according to the right hand rule starting front going back, X is starting left, going right, Z starting down, going up.

@Trish That is correct. That's what I get for posting late at night. I've corrected the post.

Thanks for your helpful insight on my unlevel surface and on how to adjust the rod correctly. I think I'm also experiencing the "seesaw" effect you posted about. Very frustrating!

@kane I just added another post you may find helpful.

For leveling your bed you do not require the use of a bubble level! This is not necessary, if you do, you need to place the printer on a bubble level underground. It is far easier to level the bed relative to the printer frame rather than to the outside world. Basically the bed and the X-axis (the axis that moves your nozzle) need to be parallel and each side of the X-axis needs to have the same distance to a reference point on the frame.
In order to level your X-axis you need to position both of the sides at a certain, same distance from a reference point on your frame, e.g. you could measure this with respect to the top of the Z stepper attachments. If you have 2 steppers, it is easy as you can individually move them by turning the lead screw when the stepper motors are not powered. Once you have each side at the correct distance you can commence a homing sequence and start leveling your bed.
You mention that you are running out of threads for proper leveling of the bed while the Z minimum limit switch is pressed/activated. What you need to do is lower the switch a little so that you have some extra height available. If the switch is at the lowest, you could consider drilling extra holes or filing it down, or print one of the Z-adjustment mods that can be found on the web. This should be enough to level your bed properly. Leveling can be done cold or hot, the difference is that the heat expands the plate a little or causes some changes in the shape of the bed due to the expansion or uneven heating. This difference is usually very small, but sometimes causes your prints not to stick to a hot plate when leveled cold.

You're right (as always), @0scar and I upvoted you for this answer. But I accepted Joel's cuz he was the 1st to guess my unlevel surface, gave me specific instructions on how to adjust my printer's horizontal rod, and identified the seesaw effect I'm experiencing

@kane no problem, this one is a little more generic answer, and we need more answers per question anyways, good luck with printing!


text:
Printing rectangular grid
I am trying to print an item with a rectangular grid (using PLA on Ultimaker 2+):

Holes are 4x4 mm, distance between holes (wall thickness): 1mm. I am printing with 0.4 mm nozzle.
Unfortunately, Ultimaker Cura generates G-code which prints each rectangle on its own and in a quite a bad way:

The printer head tries to draw a rectangle, then moves in the direction opposite to a last laid line, which with not perfect adhesion of single line messes up the print pretty badly: it often picks up last rectangle side and drags it.
Any way to change the way Ultimaker Cura approaches to lay out the grid? I would imagine that long lines that are connected to other lines would adhere to bed much better than individual squares, but I don't see any options that would allow to alter it.

I agree, the slicing is not optimal. Maybe you can make an Issue at github.

Another thought why this might be bad: the printed piece will will have very little tensile strength as there are no strands running along the whole thing keeping it together.

It's difficult to slice for 1mm walls with a .4mm nozzle. You'll do much better with walls of .8mm or 1.2mm. Once you go beyond 1.2mm the slicer can start to use diagonal lines to fill in for any value you want, but less than that you really want to stick with multiples of your nozzle size.

In Cura 5.10, I saw an interesting tactic for this, where it first printed tiny squares in the corner of each grid square, and then connected them with larger squares.

Slicers never create a grid by crossing the already printed lines - they print perimeters and thicken them into a shell. So if the picture that is to be printed is a #, it does not lay down first the || and then the = crossing over it, it lays down an inner square and then builds the rest to get to the shape. This operation rule is held for shells, but not for infill patterns.
As you describe the printer picking up the printed, you have adhesion problems and your first layer might be not level or too thick.
You might also want to enable "print thin walls" and use 2 or 3 perimeters.

I ended up re-leveling the bed and made the walls slightly thicker (1.2 mm). It, of course, didn't change the printing pattern, but it did manage to print properly. Still, I think printing internal holes this way is wrong, especially with an adhesion plate present: it would hold much better if as much lines as possible ran across bottom layer ending on both ends at the adhesion plate...

This sounds like you have an adhesion problem if it catches laid down filament, you might want to address that first. E.g. use a PVA based glue or spray to get better adhesion. This will result in not dragging laid down filament.
To my knowledge, Ultimaker Cura has no option to choose how you print the squares (direction and start point). However, you could use Z-hop so that it will lift your nozzle (or lower your build plate in your case) prior to moving to the next rectangle.
It looks as though you are using an older version of Ultimaker Cura as it only prints one line of each of the small rectangular holes (or are you actually using a single wall/shell), in later versions of Ultimaker Cura this is fixed (e.g. the image below is created with Ultimaker Cura 3.4.1), it will print all walls/shells before commencing to the next small rectangular hole. This way you have more lines deposited which have a possible better adhesion to the increase of laid down material:

Further investigation of your image shows that you are using a very fine grating (< 1 mm?) resulting in very limited amount of walls. In your case the version is not that important, but the latter information is just left as a possibility for people that use an older version of Ultimaker Cura.
Also note that there is an option to put the brim on the inside of your models (option called Brim Only on Outside), when disabled, this would also increase the surface area for better adhesion.

There are also option available to start with the outer or inner wall (option Outer Before Inner Walls), but in this case that would not help you as there is only 1 wall at each side of the rectangular hole.

Tried PVA, it's better than without it, but it still drags the filament. Yes, walls are 1mm, probably need to make them thicker.

Use the latest stable Cura 3.6.0
Ensure that the setting "Wall Line Count" > 1, recommended value is 3-4
Uncheck the setting flag "Outer Before Inner Walls"

Try slicing with 0 perimeters (so it's all infill).

That would imply that there are no perimeters everywhere, this might not be a good idea as the rest of the object doesn't have perimeters as well. What you might suggest is to alter the infill/perimeter amount for the grating alone, this means that you would want to have different slicing option in certain parts of the model. This question has some answers to accomplish different settings within the model (the method is not limited to solely infill).

@0scar I have no experience with that to contribute. Please feel free to edit.

Tried that, it still prints each rectangle on its own.

@n0rd Did you try that with a rectilinear infill pattern at 100%, and 0 solid bottom layers?

Perhaps a slicer is not the ideal tool for this application.  It should be possible to use the slicer to generate a 'framework' file, doing coordinate conversions, setup and such, and then write raw G-code or a script to do so.
Whether you find that a desirable approach is another question.


text:
Bad layers in otherwise good prints
I can print a reasonably high quality print, but then there will be a millimetre or two of rubbish.

This can happen at the start of a print, in the middle or at the end.
It doesn't seem to be bad G-code as I can reprint with good results.
There seems to be a correlation between the bad layers and walls.  They seem to occurs when there is a transition to printing a horizontal wall.
The latter if there two images if the tail end of a thirteen hour print.

What's causing this?
Is it something I can fix or is it that the (new) printer is failing?

that looks like 3 or 4 bad layers before recovery and then again... can you do a 2 cm diameter, 10 cm high pillar test print and check if that error always happens at the same height?

@Trish I did some narrow cylindrical prints, 0.5 cm by 10 cm with a curved base for support.  The first failed the first millimetre then reduced, the second no failures (both had wobbly profiles but good adhesion).

@Trish I'll print your recommendation, any specific settings that are good for a test print?  The prints above were 0.1mm layers.

Thanks for the test. a larger diameter would allow to rule out thermal troubles with printing narrow objects (at some point they just blob). Just the diameter and some arbitrary hight that is larger than where you get errors is needed - if it always appears in the same hight on prints one after another it hints to the movement while random placement hints to extrusion. I suggest no infill and 2 or 3 walls - and no top layers

This can be caused by various problems, of which an aforementioned filament diameter is only one of multiple possible causes.
The observed phenomenon is called under-extrusion. Temporary under-extrusion can also be caused by filament friction on the spool, too low or too high filament extruder gear friction (pressure on the filament by the roller), or friction in a Bowden tube (dust particles).
Please ensure your spool unrolls freely (not entangled; entangled filament is audible like "ker-chunk" when the filament is rearranged on the spool by force of the extruder pulling) and check the extruder for friction or slip. If the extruder gear slips, this is also accompanied by an audible "click" that is called clicking. This generally implies that there is too much friction in the hot end, or an obstruction/clog.
Considering that your print recovers from the problem, and that it occurs randomly, the best guess is that your filament in entangled now and then.

Thanks Oscar.  How do I check the extruder for friction and slip?  Is there a calibration process or similar for it?

I just realised, I occasionally hear a loud "ker-chunk" which I'd attributed to the spool turning ungracefully around the holder.  I'm wondering not if that's when the bad layers appear.

@BanksySan If you hear that, it means your filament is entangled on the spool, this causes extra friction, it is most probably your actual problem. Checking the filament that comes out if the extruder will reveal if you have too much or too less tension on the roller, you see that by the indentation the gear leaves in the filament. There are many pictures to be found on that topic. If you have a direct drive extruder you need to un-mount the hotend.

@BanksySan Ker-chunk can also be a skipping extruder. It hints to a clogged nozzle often

I've had similar problems related to friction in my bowden tube. The extruder will make frequent clicking sounds as it skips. Certain filaments seem more prone to this as well. I've found that lubricating the tube and/or filament fixes it. There are several sponge holders on Thingiverse; add a little cooking oil to the sponge and install it where the filament enters your extruder.

@mbmcavoy Shortening the bowden tube a little also can help

One of the reasons for such a defect may be a local decrease in the diameter of the filament. Carefully check your filament.

Thanks Dmitry, what do you mean by check the filament?  It looks OK.  It's gray and long and thin.

@BanksySan, measure the diameter of the filament with the micrometer or caliper in several points (>=10) on length about 5-10 meter

@BanksySan, can you disassemble the extruder? The extruder gear can be loosely fixed to the motor shaft.


text:
Designing lenses
Recently, I did discover that it might be possible to print fully solid, air free and thus optical homogenous, even with PLA.
How can I design a proper focusing lens (or rather: single sided lens with a flat bottom) with this, ignoring the post-processing needed to get the surface smooth?

I guess I'll ask, if not about post processing or print orientation...how does this apply to 3D printing?

To calculate the focal length of an optical element, the two main factors are the refraction index and the shape of the lens.
For a cylindrical lens with one optic active side (that is, one domed or bowled side), we can ignore the whole bottom cylinder and just take into account the top dome. The shape of the dome is determined by the radius of the circle that created it.
Thin, single sided lenses
For a thin, single-sided lens the rather complicated Gullstrand’s formula to calculate the focal length of lenses becomes rather simple:
$f = \frac {r}{(n-1)}$ for the bend facing the object
$f = \frac {r}{(1-n)}= \frac {-r}{(n-1)}$ for mounting it in reverse.
A Polymer database did give a refraction index of PLA as $n=1.465$.
Thick, single sided lenses
For a thick lens with a total thickness of $d$ and one active side, we solve first for the one active side, and then insert:
$$f_1=\frac{r}{n}  \land \frac {-r}{(1-n)} ; f_2=\infty ; P_i = \frac 1 {f_i}$$
$$P=P_1+P_2 -P_1P_2\frac d n ; P_2\to0$$
$$f=\frac{1}{P_1}=f_1$$
As long as one side of our lense stays flat, the thickness of the lense is mathematically not relevant (save for increasing dispersion).

That last line is not strictly true.  The thickness doesn't affect the paraxial trace a lot, but does affect higher-order distortion terms.  In addition, since your material is almost certainly dispersive ( index varies with wavelength), the thickness will lead to spectral distortion.  BTW,  your "mounting in reverse" formula is identical to the first formula: just multiply by $\frac{-1}{-1}$

@CarlWitthoft duh! missed a minus sign...

aha!   Tho' I will say that us "Optickers" usually leave the sign of the radius of curvature as positive for a convex surface with the understanding that the focal length is always positive for a real image and negative for a virtual image.  But so long as your notation is self-consistent, no big deal.


text:
Setting up a brim/raft for only a portion of the overall print
I'm trying to build an assembly with one larger part (about 50 mm x 50 mm), two small parts (about 10 mm x 5 mm) each, and one part that starts with two 2 mm x 2 mm squares that eventually bridge into a sort of flap. The larger part has decent adhesion for a couple of layers, but the small parts are slipping off the table right away and the nozzle drags them onto the large part since it's the last part being printed in the group, resulting in some stringing.
My bed is adequately leveled and can print larger models well so my suspicion is that the small parts are too small for good bed adhesion and need a brim or a raft to increase the surface area. Is this correct? If so, is there a way in Cura to add a brim or raft to the small parts and not the large part? Ideally I would print all pieces at once instead of the large part separate since I will be doing batches of these assemblies in the future.
I'm using Ultimaker Cura 3.6, printing with PLA filament set to 200 °C at nozzle and 60 °C on the bed.

For information, Super Slicer has per-object brim. https://github.com/supermerill/SuperSlicer

This is a "No" and "Yes" answer, it depends on the version of Cura you are using!

No, there are no options available to set the brim to some of the parts in Ultimaker Cura (at the time of writing this answer, pre Cura 4.5.0). You could however change the smaller model to include the brim in your design, or add so-called "Mouse Ears".
At the time of writing this answer, the image below shows the Ultimaker Cura interface for "per model" settings, basically different settings with respect to the other models. As can be seen, you cannot expand the Build Plate Adhesion (it already shows the arrow downwards, so expanded):

But, yes as of Cura 4.5.0 (see this answer) the "per model" settings have changed and the Build Plate Adhesion is added with the option to add a Brim Distance to trick Cura into de-attaching the brim (it will print a brim, but it is not attached to the part):

When applied to several models this could result in models with a brim attached and others without the brim attached:

By "change the smaller model to include the brim" you mean that I'd open the model in some kind of modelling software (Fusion360, Blendr, etc)?

@LuxClaridge Yes exactly, there are already a few questions on that topic here.

In the newest version of Cura (4.5.0), in the per model settings, the only thing you can change for options set Build Plate Adhesion is Brim Distance. What this allows you to do however is create a brim for everything with the distance of a millimeter or two, and then for the part that needs the brim, you just set the distance to zero in the individual part settings. It's a little janky, but you essentially create a skirt for the parts that don't need a brim.

Good answer! Would be great if you added an image! :-) Welcome to 3DPrinting.SE!

The problem still persists if you need - for instance - to add a brim to small parts that are... support structures. While the normal model has got a sufficient adhesion part that does not need any brim.

This post is quite old, but I describe how I solved this for anyone has the same problem I had: areas moved by the nozzle as the base of some appendices are too thin and tall.
In Cura 4.8 (I don't know since which version this feature is available) it is possible to add some custom shapes (cubes or cylinders) to your model. You can find this tool after move, resize, etc,, in the prepare page (left side icons).
So you can add some custom supports to protect from falling/moving some areas of a single model that need to be kept in place. These added parts can be moved and resized as you would do with the any other imported model.
I found this very useful to avoid that the nozzle moves parts with a small base area that are connected to the rest of the model on a higher level and the lower part is not stable enough. Especially when printing TPU or other flexible filaments.


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Mystery filament cleaning fluid
Whilst looking to fix a different problem I had I came across this video.
It's a filament reel holder, but at 02:40 the person uses a mysterious fluid.  I haven't seen anything like this before (though had wondered).
Mystery cleaning fluid.
What practical problems would this cleaning fluid mitigate?

.... and what new problems would it introduce?   Gotta be careful there.

That's a filament cleaner/oiler combo. The black bottle is a variety of machine oil.
The effect of oilers on prints is heavily disputed in the community. The suggestion of plant oils is a very bad one as they can create residue that stays in the hotend and create clogs.

For peoples information, that is where the debate starts. Others claim that the residue is actually what coats the hotend and thereby making it more slick, synthetic oil does not create such a coating.

For really long Bowden tubes, some people use Teflon lubricant in a cleaner bead.
Never used it myself, since I use a small tube my printer with the stepper on top of one of the Z axis steel rod mounting plates (Anet A8).
Off topic, but by cooking and being a vaper, I vouch that organic compounds leave gunky residues on hot surfaces/parts.


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X and Y homing issues
I get an error: printer halted. (kill() called! Any time I home my machine's X or Y axis. I have just installed a 3D touch auto bed level sensor, ironically it works fine. I was using Sprinter firmware but it doesn't support auto bed level, so thus I'm using Marlin. I am using the latest version of Marlin and it gave me nothing but issues, printer wouldn't move or home at all. I used the latest Marlin 1.1.x bug fix edition (1.1.9) and now everything is working correctly except X and Y homing, the machine throws the error: printer halted. (kill() called! on Repetier-Host.
When I use the M119 command all endstops (optical, not mechanical) show as open unless triggered, the endstops are working correctly. When I home the X and or Y it will do as it should and stop when it hits the endstop but that is when the errors pop up and the printer freezes/no longer to gives commands without disconnecting and reconnecting to the printer again.
If I home my Z axis it will deploy the probe and come down and touch the bed 2x and no error code and I can still manually move the printer around with the program movements after homing Z axis. I'm not sure what would cause this to happen only on X and Y but any help would be appreciated.
I use Repetier-Host but I downloaded Printrun just to see what would happen and it shows the exact same effect/errors. I also tried the Marlin bugfix 2.0 and it gave the same errors too.

qould you mind telling the verion number of your marlin (the one you downloaded)? "latest version" becomes a wrong information within short weeks often.

Welcome to 3dPrinting.SE, btw!

@Trish The latest version of the 1.1.x branch will always be 1.1.9 as of sometime last year. X) They abandoned that branch, new developments take place in branch 2.

all of the versions i used were downlaoded with in the last 3 days, I've tried all the verisons they have listed.

Also, the board I'm using is a ramps 1.4 all of the electronics I use are the most basic 3D printing stuff you can find online. I also tried removing the auto bed level and its settings to test and see if it was the Z probe causing the issue and it did not change any thing. I'm assuming its some sort of homing setting or maybe possibly a endstop setting but the end stops seem to work correctly.

I also did forget to mention I am using optical endstops not mechanical.

I made a small amount of progress. When I turn on pull ups for X and Y min I can now home the printer with out error codes, but If i try to print some thing I get the error code again.
theres a catch though, If I home the Y axis first it will give me the error code, if I home X thenY no error code unless I try to print.

@user3335963 I've added a bounty to attract more people to help you out. In the mean time, please update the question by [edit]; comments are not really meant for prolonged information addition and storage or discussion. The better. more complete the question, the more chance you get an appropriate answer.

@user3335963 Please add images of the optical endstops and the wiring of the endstops (also wiring to the board). This is e.g. interesting to see whether you have wired it incorrectly and induce a short circuit.

also include information if your optical sensors are Normally Open or Normally Closed.

Please help us help you! Please add the requested data into your question by [edit] and ping Trish or 0scar by leaving a comment where you put the @ in front of the user name to give him a notification! This does not sound as a configuration problem, but a hardware problem. You may also add a link to your configuration.h file if you want us to check it for you.

at least give us a wiring diagram, labeling which input/output is wired to which sensor input/output and where your optical sensors get their power from. Also, the specs of the optical sensors (working voltage) would be a necessary information

Without the images of how you connected the endstops, the best guess for your problem is that the endstops cause a short circuit, once pressed, the microprocessor trips and shuts down. If you provide more information, other people may even add better answers based on your added information.
E.g. how is everything connected at both sides of the cable (board and endstop), does the message occur when you press an endstop, maybe it is even wise to add a link to your configuration files.

I apologize I should have got back with you guys sooner. I downloaded a fresh copy of the Marlin firmware again and pulled up the Sprinter config.H folder. Since the firmwares are very similar I was able to just glance at my Sprinter firmware and noticed certain endstops for my optical endstops required "pull ups" to correctly work. I thought I had tried this before but either I did some thing wrong the first time or I didn't save the changes, who knows. Thanks to every one trying to help.


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What is this material?
In the context of a personal project I would like to reproduce the appearance of a commercial product of which I send you a cropped image.
I would also like to point out that I do not have the object in question, but it would seem that it is made from a polymer.
The product is a case with an embedded electronical card, so heat dissipation is important.
I'm interested by what kind of plastic is really used here. I plan to have the part manufactured by a company, so I think the method used will be SLS
I therefore rely on your expertise in the field of 3D printing to try to identify the material used.

Thanks

Plastic has markings in a triangle. Tell us what your markings are, they tell what your plastic is.

As I may have said initially, I don't own this product....

Does it matter what kind of plastic it really is, or are you just interested in a similar finish? There are a couple of filaments that can reproduce/mimic the looks. Maybe you should make that clear in your answer by [edit]. Furthermore it would be great if you say something about 3D printing, i.e. how are you trying to make it: FDM, SLS. As is, the question has not got a real 3D Printing value/link.

@joshuac without knowing the product or the markings we can't know. The https://polymerdatabase.com does know 86 different basic types of plastics, at times lumping very different ones together based on their chemistry. One example: Polyamids is a group of  16 different plastics, among them 11 different Nylons, Nomex and Kevlar - while chemically similar to some degree, their physical characteristics differ vastly!

Are you solely concerned with visual appearance, or must the part meet any functional needs such as strength, resistance to fluids, etc.?

Looks like Shapeways sandstone.

@0scar I'm interested by what kind of plastic is really used here. I plan to have the part manufactured by a company, so I think the method used will be SLS.

@Trish, I know what you mean. The product is a case with an embedded electronical card : https://passel.io/flter. I want obtain the same result for mine.

@CarlWitthoft, as I just said, the material have to be adapted for electronical usage : heat dissipation is an important property.

The model you showed (the AD look) differs from the shipped look quite a bit - the later is sleek, shiny, the former a rough surface. It seems like the design was changed to allow easier production. There are no data on the material used online. Call the maker? We can't identify the material without some information you don't provide - if you seek a print material and give us temperature ranges you expect, we can give you options what you might use.

@Trish, You're right. It seems that the shipped version differs from the initial one... Probably easier production. I have already contacted the company, but I haven't received an answer...

cura has a fuzzy skin setting that's somewhat like what you show in the photo.

@dandavis, applied on which material ?

Surface finish does not really map to the substrate material, Visually, what you have shown could be glass, ceramic, plastic, epoxy or metal.
The surface finish is a combination of the shaping process, any post processing, and any surface finishing. Most significantly, there are a wide variety of custom paints which are designed to mimic specific surface finishes. This means you could carve an object out of clay, then spray it to give the appearance of being sand-blasted steel (to give a specific example).
The underlying material is mostly irrelevant to the appearance. It will be driven by mechanical/thermal considerations (is this a mock-up, or does it need to have functional wall-thickness), and production volume/cost considerations (is it a one off, or are you making hundreds/millions)?

Is there anything else about this object, but its picture? Softening temperature, biodegradability, is it stiff of flexible, hard or soft, anything could help identifying its material.
Also, post-processing (sanding down or chemicals like acetone bath) greatly enhances the range of filaments that can be used.
Just from the picture, my first guess would be: it looks very much like what ceramic powder added filaments can yield with when you sand them down afterwards. Take a look at pictures of prints with LayBrick, CERAMO, etc.
On second thought, the answer is: Polyamide/Nylon. It must be either a polyamide case or polyamide coating of some other material. For home projects, MJF polyamide prints can easily be ordered online. Ordering polyamide coating of plastic parts can be trickier depending on your location, although definitely doable.
Going with Nylon at home prints may require a non-basic 3D printer and some expertise.

In fact, it is a box containing an electronic card. I can give you the brand of this product : FLTER.

What you base the PA/Nylon on? There are 16 different types of Polyamid(relative)s.

They all look similar while their mechanical properties are different. Not sure if all copolymers are available for hobby printing, most brands don't even mark what PA they are, you can try to guess by properties. Pure Nylon 12 is hard, resistant though brittle. Nylon 11 is more flexible, and so on. Reinforced nylons have wide variety of properties. Choose the one that is right for your printer and your application.

If you want to 3D print this, you could try post processing with some acrylic or fiber glass filler, and then use some stone texture spray paint like this one.
Edit: SLS printing will never be heat efficient, if that product (in your example image) is an electronic case, it was either machined or molded, and the finish is some sort of paint with a ruff or stone like finish.
However, you might be able to pull it of with air flow redesign, and as long as the temperature does not exceed the glass transition temperature of the thermoplastic or resin used, you should not face any deformations on the case.


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Font suitable for 3D printing
I am looking for a font, which is connected and has a good line width (so that it can be 3D printed). Something like in the image below:

Something like in the image above would be great, but I don't know the name. Do you have a suggestion for a font?

What is wrong with the font you posted? It looks like you already found it!

I don't know the name of it. The pic came from a web generator

See https://meta.stackexchange.com/questions/260057/site-for-typography-question - which suggest that https://graphicdesign.stackexchange.com/ is the correct place to ask.

For better or worse, there are websites with access to thousands of fonts. It can take a while to filter down to ones you like.

That particular font is Sarina Regular and is part of the Google Fonts collection. Just Google "connected handwritten fonts" to find more. Kimberly Geswein does a good selection of handwritten fonts. A lot of them are constant width, but most are unconnected.


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Bed too close to nozzle?
I've been having some bed adhesion problems that I have been trying to solve by leveling the bed. I think that it's pretty level now but when I start a print the lines seem pretty flat. Is this true? I used a feeler gauge to have a 0.2 mm gap between the bed and nozzle, but the center feels as though there's more space, despite the tape (ie I didn't feel any resistance when leveling the center) so I'm not sure if the plastic is too squished or not.

I am printing PLA with 210 °C at Nozzle and 60 °C on bed. I also used a 130% extrusion factor for the first layer on a Creality CR-10S. It did come without a black print surface but with a glass sheet and a roll of painters tape instead, so I opted for Blue-tape.
Also, I see some stringing, which seems to happen with skirts and brims (this is my first brim) and a bit of under extrusion in the center. But those are probably concerns fit for a separate question.

I'm not sure why it would matter as this should be a pretty general thing to troubleshoot, but added just the same.

It matters a lot! Is it a bowden or a direct drive are the main differences, but also if the bed is heated or not. Most of these info comes from the printer type. CR-10S is a bowden with a heated bed... and should come with a Build-Tak, yet you added blue tape... is that a mod (removed buildTak-clone)?

It did not come with Build-Tak. In the package there was a sheet of masking tape, but I have seen/read somewhere that the included tape is garbage and just use painter's tape.

ok, that is good to know - oddly enough there are like 10 slightly different itterations of CR10 out there, about 3 or 4 of them under the label 10S. So you have bare aluminium. Do you have chosen a bed temperature? all information I have is, that all 10S should have a jeated bed.

Yeah, I just learned last night that there is a 10S Pro, which is different from the S4, S5. Creality is getting out of hand...  I have bare aluminum and a glass sheet on top. I set the bed temp to 60 (and edited the question to reflect that).

imho, too close is better than too far; if it's not grinding it's not too close.

No, the bed does not look too close to the nozzle, it could well be that it is too far from the nozzle. However, the brim looks okay, but the method you follow is questionable.
Using a feeler gauge of 0.2 mm is larger than the recommended paper method which is in the order of half that value (0.1 mm). You compensate this larger leveling gap with an over-extrusion of 130 %. The preferred method is using a thinner feeler gauge or a sheet of paper without over-extrusion. The reasoning is that if you have a 0.2 mm to start with, adding the first layer thickness (e.g. 0.2 mm) would imply that you are already starting at a 0.4 mm gap on your first layer. You now try to push out more filament to fill that gap (resulting in not really pressing the filament to the plate).
Furthermore, your tape does not look like it is laid down next to each other, it looks as though it is laid over each other. This creates an uneven bed surface.
Finalizing, you should do whatever you do to get the filament to stick to the build plate, if it works for you, use it. If it doesn't produce viable prints, or you are not satisfied with the surface finish, change it.

I tried 0.1 mm but something seemed off so I changed it back. I don't remember what the problem was because I've had a myriad of problems since that adjustment. I've been able to print a couple of things despite my apparent large gap, but having to tolerate poor beginning layers. Guess I'm back to leveling some more. :)

Thanks for the comment on the tape. I wondered if that would be a concern. I could have sworn that was how I saw it done on YouTube, but to be honest I wasn't paying that much attention.

After clarifying the printer type and specs, I have to go d'accord with 0scar's answer about the bed leveling thickness: try to go down to 0.1 mm.
About the bed adhesion I have a couple of pieces of advice though:

Go with what works for you (Rule #1: Never change a running system)
Try out the bare glass bed coated in...

Gluestick left to dry a little (that is PVA!)
a super thin layer of Wood Glue (also PVA)
Hairspray (also PVA)
3D-Lac (PVA again)

When sticking with tape (pun!), lay it down side by side with no overlap. Wider tape is better.

try different brands of tape, I prefer the rougher surface tapes

don't fear to use your heatbed! 60°C is the common PLA print bed temperature.

if you can, take a thermosensor/thermometer and check what the temperature on top of the glass actually is: you might need to adjust the heater setting up to get a bed surface of 60°C.

Thanks for the answer. I'm marking 0scar's as best as it answers the question better (dealing with the gap between nozzle and bed), but yours is very informative on the actual adhesion so +1. :)

@LuxClaridge I saw you added that you have a glass sheet, ditch the tape and invest in a can of 3DLAC, never failed me. And you get a super shiny bottom surface. :) Levelling glass is also easier, glass is perfectly flat caused by the production process it uses.

@0scar I'll check out 3DLAC. I got some dollar store aquanet over the weekend but haven't been too successful yet, though I haven't tried much yet.

@0scar regarding a level glass sheet, that doesn't seem to always be the case. I've seen quite a few videos talk about how Creality tends to send uneven glass (though they seem to be getting better). I haven't had a chance to get to Ikea to buy mirror sheets.

I cannot stress enough how much purple glue stick helps with this exact problem.
I had a cheap XYZ printer that I used nearly half a spool of filament on trying to align and get prints to stick. After ten hours straight of hair pulling and desperation, I smeared some purple glue stick across the build plate -- worked like a charm. Works great even just on bare glass, although I usually use blue tape underneath.
Also had one of those cheap Monoprice MP Mini and a 3D Solutions Cube which both benefited greatly from the glue stick.
I've had a few cheap printers that I carried with me for educational purposes in a backpack and I'm sure the alignment wasn't ideal, but the glue always made it work perfectly.
Elmer's makes one, I picked up a 3 pack of cheap ones on Amazon with great success.


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Heatercartridge bolt stuck
After a brownout a print failed and it got the whole hotend covered in PLA. I am now in the process of replacing some parts on the hotend (one of the thermistor legs broke of) and also wanted to take the heater cartridge out. The problem is that the bolt that locks the heater cartridge is stuck and I am now afraid to strip the head.
Is there a trick to remove the bolt with the smallest probability of stripping the head? Is it better to apply heat or not? Should I soak it in Acetone or some other solvent? etc.

Don't Panic!
First of all, a printhead caked in PLA is usually not a death sentence, it is often a temporary setback. Let's start in steps!
Step 1: remove the extruder
We want to work on the hotend, so we remove the extruder feed first. For direct drive, we unload it, then remove it depending on your printer, so the cooling of the hotend remains. For a Bowden, unload and, if possible, remove the Bowden tube from the cooling block. If not disconnect it from the extruder.
Step 2: remove from the carriage
Now that we have the whole hotend assembly bared, we remove it from the carriage. Usually, it is 2 to 3 bolts, more if the cooling solution is mounted separately and has to be removed to access it.
Step 3: remove the cool-end
If you can, remove the cooling fins - we want to have the heatbreak to hold on to.
Step 4: Clamp it up
Take a fire-proof surface (ceramic tile!) and put down the hotend. Rig it up that it can't jump away, best with a small vise. Don't bend the cables!
Step 5: Free the thermosensor
Let's start to clean! If your cartridge still works, get 12V onto the heater and let the stuff melt a little. Use pliers and a scraping tool to clean the thermistor cartridge.
As you have no heat control only heat in short bursts to prevent fire and destroying the cartridge too.
As an alternative and if you can't get the cartridge to work, use a hot air gun or a hot-air soldering station. A soldering iron with a broad tip also works well to scrape off the plastic with controlled heat an as a scraper to remove large chunks.
If you use an external heat source, free the heater cartridge first and remove it, hoping that it is not also dead.
As soon as you can, get the thermosensor out. If you run on the heater cartridge,  install a fresh Thermosensor, even into the goopy heater block, and wire it to the board to regain temperature control and use the board to provide the power.
Step 6: Finish cleaning
I usually clean up the final stretch (and as far as I can: all steps) under board controlled heat:
Make sure that heater cartridge and thermosensor are installed well and working. Set the hotend to 170 to 180°C and do the last cleaning under the use of regulated temperatre.
Step 7: Do steps 1 to 4 in reverse order.
Reassemble, following the steps backwards.
Step 8: Hot Tighten!
Heat the assembled hotend to 240°C, then tighten the nozzle against the heatbreak to ensure tightness. Let cool, done.
Alternative
Replacement/Upgrade!
One could go down to step 3 and replace the hotend assembly (Heaterblock, Thermosensor, heatbreak, nozzle) with fresh parts and reassemble. This is much more expensive than reusing but cleaner and faster - if you need to print now, you might want to keep one hotend assembly on hand as spare and clean the one replaced while the machine runs.
External heat sources
As mentioned, a hot air gun or soldering iron can provide heat to remove the plastic caking.
The soldering iron has the benefit of doubling as a scraping tool and providing localized heat, allowing to possibly free the thermosensor without unsoldering anything in it, and it won't melt the heater block.
A heat gun provides gentle, overall heating of the beater block, but needs extra care where the hot air is going - it can easily char wood and might remove the solder from the heater cartridge.
A gas torch might be used to burn off any residue on a totally stripped heater block, but it also would be able to melt and deform an aluminium heater block! Do not use it on a still assembled heater block, or it will melt any solder in the heater cartridge and destroy it.
In any case, working on a fireproof surface is mandatory!
Chemically cleaning (for non-PLA)
PLA can be removed chemically, but the solvents are rather nasty and some take quite some time to work. Very toxic dichloromethane was used to make a solution of PLA to create thin layers used in this paper, and most other solvents the study mentions are at least equally nasty. The available working options - ethyl acetate and propylene carbonate - have high price tags associated with them. So chemically cleaning PLA from cheap nozzles is not an economically viable option. However, it is a viable option to use acetone if you have your hotend caked in ABS.

This is often a great time to upgrade to an extruder with an all-metal hot end, if you haven't already.

@JoelCoehoorn An all-metal hotend is not an upgrade, this is a widely spread misconception! It is an alternative, not an upgrade. It would be an upgrade if you want to print materials over 250 ℃ for a prolonged time, mostly used filaments have lower print temperatures. Many problems are associated to all-metal hotends, several questions can be found on this.

Note, that it would be economically viable to save an Olsson Ruby nozzle that costs about 100 €.

Seconding what @0scar said. All-metal hotend will significantly interfere with your ability to get good prints with typical materials, e.g. introducing problems with retraction that might require reducing or disabling it, which in turn results in catastrophic lost/misdeposited material problems. Cooking food in PTFE pans doesn't significantly harm you and printing at 250°C or even slightly above with a PTFE tube near the heat won't either.

Acetone will not dissolve PLA, there are other nasty solvents that do that, but it is not recommended to go that way.
You could apply 12 V (if the cartridge is 12 V that is) directly. As the thermistor does not work, you cannot use the printer board. You should take caution not to heat it too far or too hot, but from your experience with your own machine you should know approximately how long it takes to heat up the hotend to working temperature. E.g. start with a minute and try to pry off the lump of plastic with pliers. If you can reach the bolt/worm screw, try heating until you are able to insert the hex key or screw driver.
This video from Josef Prusa shows exactly what you need to do what is described above. The only difference is that you need to connect the heater element directly to the power supply as your thermistor is broken. So don't leave it on too long or you will bake the filament to carbonization, which is much harder to remove. Try to cycle it on/off.

Actually I soaked the block in acetone and de PLA did not dissolve completely but it disintegrated into an easy to clean compound. So soaking it in acetone actually helps.

@EDoe Only for PLA that contains other plastics like ABS. Glad it worked out for you!

I have just gone through this with an E3D v6 Aero hot end.
Working on the hot end while hot is the best way, but using the internal heater is filled with several problems:

The thermister may be broken
The heater may be broken
The electronics of the heating loop (sense, decide, control heater) may be broken
The wires may be too short to make it easy to work on.

It is best to remove the hot end and work on it on your bench -- not inside the printer.
Someone recommended a torch.  This is possible, but it is hard to control the heat with a torch.  The flame may be hot enough to melt the metals.
Instead, I use a hot air gun.  It gets hot enough, but the maximum temperature and the rate of heating is lower.  The heat source is more diffuse.
I had an immoveable thermister set screw, which responded well to heat.  When reassembling, the heater clamp screw was filled with some plastic which prevented me from clamping the heater while cold.  After heating, it screwed in easily.
Don't be afraid to strip the hot end down to the basic parts.  Probably you'll be doing it several times over the printer's life, and knowing how to fits together makes everything easier.

If you use a brass heating block (which is a bit heavier than aluminum), you can first use a heat gun to soften the residues to remove the cartridge/sensor.
After removing all electronics, you can use a gas torch and burn the residues off. However, the far best option would be to use a heat block made of stainless steel. I guess heat conductivity is anyway not very important on the hotend as long as the cartridge is powerful enough.
Using a torch makes cleaning very fast. In five minutes you are typically done.

I got a blob of around 5 x 8 cm out of PETG! Setting extruder temp to PETG or even a little higher. I did it all with an older soldering iron.
Simply (but carefully!) go with the point into the blob and take it slowly off. With a copper 'toothbrush', I cleaned the extruder and nozzle more times.
After mostly all was down I started with the first layer test (Prusa MK3S+)
That was surprisingly almost perfect. Then started to print not very high subjects like rings etc. With that several burned drops came down during printing. Use the 'toothbrush' as many times you can between the first prints!
Done without any damage to the printer as a whole!