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How to obtain high resolution prints in a shorter period of time?
When I've printed an object I've had to choose between high resolution and quick prints.  What techniques or technologies can I use or deploy to speed up my high resolution prints?

Did I just place the first upvote?  Congrats on getting this site off the ground!

What are you looking for in an answer? You are basically asking how to make your car drive faster while using less fuel.

@TomvanderZanden I'd heard of some experiments in the past that have two nozzles, one larger and one finer.  The fine detail would take two passes (two layers) to build up the needed high resolution exterior, while the larger nozzle would perform the infill with only one layer for every two high resolution layers, and could do so very quickly due to the nozzle size and the amount of plastic it could pump.  I haven't heard anything more, and I expect there are other methods that could speed this up by now (this was years ago).

which technologies are you currently printing in and which are you considering, this will affect the answer.

@plaintoothpaste I'm printing ABS using a filament based printer (Taz Mini, in this case)

If you can't decrease resolution, you have to increase print speed, period. All other methods (lower infill, partial resolution decrease, multiple nozzles, etc) highly depend on the model type thus can't provide reliable result and in many cases only decrease final delivery time (you have to spend time on fiddling with these techniques)

You could experiment with slicing. For example, you might not need high resolution all over the object, but you can speed up some straight parts by using greater layer high there. See a part of Slic3r manual about such thing.
It is also possible to print thicker infill every Nth layer, see Infill optimization in Slic3r.
Other slicers might have those features as well.

For FDM technologies in general with a single extruder, slicing modifications is your only options. However there will be a trade off between quality and speed.
For ABS, changing to a machine with a enclosed build (such as a zortrax) chamber may help and a heated build chamber (Stratasys machine) will help the quality and reliability but not the print speed directly. As ABS has a tendency to warp vase mode is not the best idea either.
If you only need high resolution and not strength then reducing the infill percentage or even using vase mode will speed up the print. Also changing to a material that you can print at higher speeds like PLA will magnify any of the previous settings.
If you have two extruders then changing to a wider nozzle and using that for infill may speed up the print, heating and cooling time during extruder changeover may actually make it slower.
For other technologies there are lots of options digital light projection (DLP) and stereolithography (SLA) both provide significantly higher resolution then FDM, with DLP being the faster of the two, comparison. Take the review with a grain of salt though as the technologies are far from standardised, for example out DLP at work offers far higher resolution and speed then our SLA which is quite old.
Sintering or melting technologies can scan the outline every layer then perform a infill of multiple layers at once to speed up the process.

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Is 3D printing safe for your health?
I would like to buy a 3D printer, but I'm concerned about the health risks that are associated with its operation. Some groups of scientists say it can be harmful for humans.
What do I need to consider before buying a 3D printer if I care about my health? Are there any safe printers?

You should probably narrow this question down to a specific type of printer. A cheeze wizz printer is less likely to be dangerous than one that laser-sinters enriched uranium powder.

There is very little information about safety available, as home 3D printers are relatively new. However, plastics such as ABS have a long history in making plastic products, and a study found that at traditional manufacturing methods (such as injection molding and hot wire cutting) do not release dangerous levels of carcinogens and/or respiratory sensitizers in to the air.
Of course, 3D printers are not among the processes covered in the study. In home 3D printing circles, this study that looks at ultrafine particle (UFP) emissions, is often cited. It finds that printing ABS releases relatively high levels of UFP's and PLA releases significantly fewer (but still quite a large amount). However, it is unclear whether/how dangerous these UFP's are in the amounts emitted.
It is often suggested that PLA, partly because of the reduced UFP emissions is safer to print than ABS, partly because of its "natural" origins as it can be derived from materials such as cornstarch. I would caution against this line of reasoning since "natural" materials can still be poisonous (snake venom is natural, after all) and the cornstarch is heavily processed so it hardly resembles its original form. The lower UFP emissions may suggest it is safer, but the study is only quantitative, not qualitative.
That said, PLA does probably pose less of a risk (despite my earlier argumentation against "natural" materials, PLA does play quite nicely with the human body), but I contend the risk with ABS is not too large anyways, given that it has been safely used in factories for decades.
Another study is often miscited, supposedly saying that 3D printing ABS releases hydrogen cyanide. The study only looks at the thermal decomposition of ABS, which happens at significantly higher temperatures than are reached during printing (but a significantly malfunctioning printer might cause toxic gasses to be released, but I contend that at that point you should worry about your printer being on fire, rather than temporary exposure to some toxins).
There are no printers out there that are fundamentally safer than others. However, some printers have an enclosure (containing the fumes) and some even have a carbon filter and a fan for fume extraction. If you would like to err on the side of caution, this might be a good choice (but again, it is not clear if a carbon filter is totally effective).
Finally, as printers are generally quite noisy it tends to be preferrable to keep your printer in a separate room from where you usually work. In this case, fume exposure (during the few minutes that you go to check on your print) is minimal, and the potential advantages of a "safer" printers or using "safer" materials diminish.
Incidental exposure as a hobbyist is probably not a big deal; workers in factories are exposed to the fumes of melted plastic their entire lives and they don't seem to be dropping dead. On the other hand, if you are going to be printing structurally then it is probably preferable to move your printer to a separate room, if not because of health and safety because of the noise.

I think I'm going to start turning on the vent fan in my workshop while printing, just in case... :)

Great answer. In the molecular level there is no such thing as "natural" or "innatural" type of the same ions and same isotopes or molecules. Just because some atoms or molecules appear in nature (the chemical reactions causing them happen without humans invloved) doesn't make them safe. It would seem to me this is commons sense but a lot of people seem to be predisposed to natural being inherently good or in this instance harmless.  Also great point on the UFP studies, we know so little. It's paranoid way of thinking that something is dangerous until it's proven 100% to be safe.

Almost all 3D printers have issues that could cause health problems.
FDM/FFF printers heat plastic to a temperature that may cause it to off-gas, and these byproducts may not be healthy.
SLA printers often use epoxies that may off-gas, or may be somewhat toxic prior to being cured.
Powder based printers can also off-gas, in addition to the powder itself presenting a possible hazard.
Many hobbyist and small companies dance around the problem, and suggest that the machines always be used in well ventillated areas.  Professional machines often have filters and ventillation systems built in.
Rather than trying to find a "perfectly safe" 3D printer, spend some time deciding what you want to use one for, find printers suitable for your use, and expect that you'll need to provide reasonable ventilation for almost any printer.  Plan your installation for that, and you should be able to make any printer safe for your required use.
If, however, you plan on setting up a printer farm with many printers, and plan to have yourself or others spend significant time operating them, I suggest you work with a health and safety professional and have them identify possible hazards and plan mitigation.

I'm not sure this applies worldwide: but in Canada, most products/machines that pose a risk generally need to have an icon that demonstrates the type of danger and severity of it (WHMIS/HHDS?)

"May not be healthy" can't be stressed out enough here.  As for the UFP studies, we know so little about UFP effects. It's paranoid way of thinking that something is dangerous until it's proven 100% to be safe. But if it doesn't take much to reduce or eliminate the potential hazard, it should be done too.

I am going to address the air issue as it is currently unresolved. the third dimension offers a great answer for common safety issues.
The short answer is that based on our limited knowledge at this point, there may be imperceptible health hazards related to FDM / FFF printers and therefore additional safety precautions are, in my opinion, necessary and not optional or secondary as suggested by some in the community.
In other words, if you can isolate your printer in a well-vented area where people rarely go, then of course it's not a health risk, but if people will be exposed to the air of the printer for any significant periods of time, you need to do something about it. This is my situation - where I live dedicated workshops and extra rooms are luxuries that most people do not have.

Realistic Chance of Being Dangerous --> Treat It As Dangerous
The key information at this point in time is the UFP (Ultra-Fine Particle) study that is linked in Tom's answer.
Leaving out the scary / detailed parts:

Therefore, results herein suggest that caution should be used when
operating these 3D printing instruments inside unvented or unfiltered
indoor environments due to their large emissions of UFPs.
One important limitation to this study is that we have no information
about the chemical constituents of the UFPs emitted from either type
of 3D printer [...]
[...] there may also be
differences in toxicity because of differences in chemical
composition.

This means that although many processes release UFPs (the authors of the paper compare to cooking), all UFPs are not created equal. Since the UFPs from 3D printing are still an unknown, the only real answer from a safety perspective is to treat them as dangerous.

This is not legal, safety, or professional advice!
I am not qualified to give an opinion on what should be done but I will share what I would do:

Venting - Active airflow pushing the envelope of air around the print into a large, unpopulated body of air.
Enclosure + Venting - By fully enclosing your printer, it will probably keep the UFPs mostly within the enclosure. You could combine that with either continuous venting or as some have suggested purge venting before opening the enclosure.
Enclosure + Filtering - A filter can be applied both to the vent to reduce the output of UFPs (e.g. if you have no access to a safe body of air) and as a recirculating system that removes the UFPs from the body of air within the enclosure.

A note on positive vs negative pressure related to venting and filtering: if you produce positive pressure within the enclosure, you are going to be blowing all the UFPs out into your environment anyway. Negative pressure vented to a safe body of air or neutral pressure with good seals and recirculated filtering may avoid that.
A note on filters: Activated carbon filters will not remove UFPs. HEPA filters may remove 3D printing UFPs.

Which Printer?
As long as the uncertainty exists, I predict that as the market matures, filtering and enclosures will become more standard. At this point in time, the only enclosed AND HEPA filtered consumer-grade FDM printers I am aware of are the Up! Box and the Zortrax Inventure. There are a number of enclosed printers without filtering.
As an alternative, at least one company has appeared with products targeted at those who are concerned about various safety aspects of 3d printing.

Apart from the inherent process itself and direct health hazards from that, many 3D printers also require some complementary technology to work.
fdm printers have a printing head that needs to move around in 3D space. Moving machinery parts can be a hazard. In a home/hobbyist environment with children for example, I would recommend to buy a printer with a housing.
"open" designs often feature bare electronics mounted directly to the printer structure. This rises the possibility of short circuits and electric shock.
The printers that heat material often do so at very high temperatures. Hot parts of the printer should not be touched.

Are these really health issues? Is "you might cut yourself" a health issue of using knives?

@TomvanderZanden I dropped my 3D printer on my hand and sustained a small bruise. I also received a paper cut whilst handling the documentation. And after a few repeated print fails (caused by poor bed levelling) I banged my head on the wall... :-)

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How important is the minimum layer height on a 3d printer?
I know the minimum layer height will effect how detailed of an item you can print and the amount of time it takes to print something, but is it necessary to have an extremely low minimum layer height if you plan to print only larger objects?

I think it would be a good idea to specify what type of 3D printer you are talking about. Layer sizes can very a lot between different technologies and printing materials. On top of that, the application plays an important role. There's no general answer to the question how small a layer should be. I suggest changing this question to ask how to know what layer thickness to use and what influences that decision. That would be a canonical question an much more useful.

There's a "in order to accomplish " missing from your question.  In my case, I print miniatures which are 20-80mm long.   A print resolution in the microns is what I need to produce detail.  So the question would be better asked if you included what the intended use of the print is.

Question edited, hopefully that fixes it.

Please define the type of resolution you are talking about.  There are positioning resolutions for the x, y, and z axis on the printer,  there is also the nozzle diameter, and the minimum layer height.  Each would be slightly different in your question.

After editing, you changed the question entirely. Instead of the importance of the "first layer height", you now ask for the importance of "layer height for larger objects". Was this intented? Do you mean "first layer" when you say "minimum layer"?

I believe the Slic3r Manual summarizes this quite nicely:

A thicker layer height will provide more flow, and consequently more heat, making the extrusion adhere to the bed more. It also gives the benefit of giving more tolerance for the levelness of the bed. It is recommended to raise the first layer height to match the diameter of the nozzle, e.g. a first layer height of 0.35mm for a 0.35mm nozzle.

In other words, larger layer heights for the first layer typically gives better adhesion, and makes inaccurate bed leveling (and auto leveling) still work.

As with any manufacturing process, you'll need to learn to "use the right tool for the job". It depends on the requirements of the part. To answer your question, I would suggest using a larger layer height for the sheer fact of reducing print time on larger objects.
However, it depends on the part and how small the details are on the part. If your part has sharp edges that are required for the proper functionality of the part, then you'll want to use a smaller layer height. Or if your part fits into another part, you'll probably want to use a smaller layer height.
Another variable might be whether or not post-processing is necessary. Is this part going to be purposefully printed larger/rougher with the idea to use a Dremel later to smooth everything out? If yes, then use a larger layer height.

In short: no.
Smaller layers will make your prints look nicer. That said, in my experience, most printers can handle roughly the same layer heights, no matter what the manufacturer says it can do. With a little tinkering, you can get any printer to do what you tell it to do.
That said, for larger prints, you really won't want to lower the layer height too much. Typically, the quality difference you see in a print that's, say, .2 mm layer height (typical) vs .1mm layer height (typically high quality) is not that much different.
Note that lowering your layer height increases the number of layers in your object, so it'll take longer to print and have more opportunities to fail along the way. For anything of significant size (150 mm + or so) it's really not worth the extra risk, I'd say.


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Are there any metals that exhibit a large glass state?
Plastic is used in 3D FDM/FFF printing partly because it had a wide temperature range for its glass state - where it can be flowed with some force, but won't flow due only to gravity.
Most metals have a very narrow, or non-existant, glass state.  They transition from solid to liquid with almost no flowable-but-not-liquid state.
Are there any metals or alloys that display a glass transition state?

A few things are required for effective extrusion-style 3d printing materials:

It must stay where placed by the nozzle long enough to harden (or, alternately for pastes and such, have a shear-thinning or thixotropic viscous profile so it will not flow under its own weight).
If using a filament extruder, it must have a wide range of viscosity that varies gradually over a considerable temperature range. This is necessary to develop the proper "cap zone" semi-melt shearing behavior that allows the incoming filament to act like a piston and generate pressure upstream of the nozzle. Pellet extruders have a similar requirement but related to screw/wall shearing rather than filament/wall shearing. If using neither filament nor pellets, such as clay printers, the material must be pumpable by a positive-displacement pump. (It is possible to pump molten metal, but the cost is quite high.)
It must form some kind of bond with previously-deposited solid material, without needing to be in a state that will rapidly flow and lose shape.
It must have some combination of low shrinkage, the ability to creep at the printer's ambient temp, and/or low stiffness that allows consecutive layers to be stacked without an unacceptable amount of warping.

Liquid metals tend to have a conflict between "Staying where you put it" and "bonding with the previous layer." In order for deposited metal to fully bond, the interface material needs to reach the melting point so a true fusion weld occurs. And in order to supply enough heat to remelt the interface without an additional heat source like an arc, the deposited molten metal needs to be very hot. So it will tend to run while it cools. High density and high heat capacity makes it run fast and cool slowly.
Pretty much every DIY metal 3d print (such as made by wire-feed MIG welders) ends up looking something like this:

https://3dprint.com/29944/diy-metal-printing-garage/
In comparison, polymers have long molecular chains that allow them to "diffusion weld" and adhere WITHOUT fully remelting the interface. Molten liquid plastic will stick to solid plastic quite effectively. The interface only needs to get hot enough for appreciable diffusion to intertwine the molecular chains. This will occur between the glass point and melting point, without true fusion occurring. So you can print molten plastic at a temperature where it will stay in place long enough to harden, and still get good bonding.
Metals also tend to be very stiff, which encourages warping. It is difficult to build a heated environment of sufficient temperature to properly stress-relieve the thermal contraction stress as the print progresses, whereas with plastic a heated build plate and warm enclosure can permit warping stresses to start relaxing as the print progresses.
It is possible to "FDM-style" 3d print filament/wire made of metal alloys that have a wide range between solidus and liquidus. It has been done using solder and similar alloys. However, between the warping stresses, poor layer bonding from inadequate interface re-melting, and use of soft low-melting alloys, the resulting printed parts will usually end up being weaker than if they had simply been printed in a strong plastic. For example, PEEK is nearly as strong as aluminum, and carbon fiber or fiberglass composite plastics can exceed metals on various performance metrics. So what's the point of printing with weak, brittle metal alloys?
Over the years, lots of people have tried FDM-style metal printing, but no one has found it worthwhile to pursue in the long run. More typical DIY metal printing approaches like 3D MIG welding following by cleanup machining will produce better results.

I"m no expert on this, but the article at https://en.wikipedia.org/wiki/Amorphous_metal may be relevant for you.
There are some special alloys, such as gold/silicon and various titanium-based ones, that become "bulk metal glasses" if cooled extremely quickly (for example, by sputtering onto a spinning cold surface). The speed of cooling prevents crystal formation. Early BMGs were quite strong but brittle; improvements have reduced brittleness and required cooling speed.

Link-only answers are frowned upon, because of link rot. Please elaborate on the information in the link.

Added brief summary.

I think the closest you're going to get is with a composite material. Over the last 2 years or so, there have been more and more composite filaments emerging on the market for consumer 3D printers. I good example of composite filaments can be seen on Proto-Pasta. Since the filament must mostly be comprised of the polymer "binder", the material will obviously not exhibit all the properties of both.
Without getting into too much Material Science, consider the composite of both ABS and Iron (or some other ferrous-based metal). You cannot expect to print a solid circuit out of the filament because the iron may not be represented throughout all directions of the filament, therefore resulting in resistance or flat out non-conductivity.
So, to answer your question: I'm not aware of a significantly larger glass state in a type of metal/alloy. Your best bet is a composite, but it depends on the requirements for your part if a composite will work. Then, you'll have another battle of finding the right type of composite and worst of all, a good supplier lol

I spent some time looking at making an FDM machine that would print bronze filament.  An alloy commonly made into wire had a difference between the solidus and liquidus temperature of only 50 degrees C.  I determined that one could make a conventional hot end, electrically heated, made of either molybdenum or tungsten.
I did not determine how the bronze would behave in the 50 degree solid-to-liquid zone.  I was more concerned about the solubility of the nozzle material in copper, for which I could find very little published data.
From my experience welding, and from printing plastics with FDM, there could be a problem with layer adhesion.  To really bond, the cooled material needs to be melted by the material being deposited in the next layer.  This is complicated by the temperature of the cooled material, the thermal conductivity of the material, and the propensity of the material to form oxides. These could be mitigated by heating the object in an inert atmosphere.
So, to answer the question, I would suggest looking at bronze alloys because they melt at moderate temperatures, and are less prone to oxidation than aluminum alloys.


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How is PLA different from ABS material?
What are the main differences when using ABS over PLA and vice versa?

I think this is too broad for a good exhaustive answer unless restricted to just ABS and PLA...

I'm voting to close this as too broad.  Asking generically about filament types is opening the answer to too many possible filaments, and any answer would become incomplete over time as more filament types are created.

@1337joe: I disagree.  While it is true that many filament types exist, it would be good to have a canonical post which addresses general characteristics and differences of ABS & PLA for general purpose printing.

@Paul Yes, if it's restricted to ABS and PLA FDM printing. If it's open to "different filaments" then I'd suggest closing it.

@CJK I'm editing it to limit the answers to just ABS and PLA, the two most popular FDM filaments.  I would otherwise vote to close this question as too broad, as this information would take a book if you included a comparison between all filament types. If you disagree, go ahead and roll it back.

Paraphrasing this site. Feel free to add suggestions in the form of comments and I will try to incorporate them.
Summary

ABS: Stronger, machinable, more flexible, and more temperature
resistant than PLA. Typically printed on a heated bed. Warping is a common problem when printing ABS.
PLA: Wider range of filaments available, easier and in some cases faster to print. Not as strong as ABS and the fact that its biodegradable could be seen as both a benefit and a drawback.

Material Properties:

ABS: Strong plastic with mild flexibility. Naturally beige in color. Can be filled and sanded. Higher temperature. Easy to recycle.
PLA: Not as strong as ABS but more rigid. Naturally transparent. More difficult to fill and sand. Can sag in hot temperatures. Sourced from organic matter so it can be broken down in commercial compost facilities.

Part Accuracy:

ABS: Part warping is a significant issue. Sharp corners will often be rounded.
PLA: Less heat required contributes to less warping. Becomes more liquid at common extruder temperatures so finer details can be printed.

Safety and Handling:

ABS: Strong burning/melting plastic smell is present when printing ABS. Health concerns have been raised regarding airborne ultrafine particles generated while printing with ABS (ref). ABS will absorb moisture causing popping when the moisture enters the hot end. This leads to discontinuities in the print job.
PLA: Doesn't smell as strongly when printing due to its organic nature. Moisture can also be absorbed into PLA and can irreversibly damage it.

Is a heated bed required for ABS? Our printer supports ABS but doesn't have a heated bed (but I've only used PLA)

Yes. I don't think it's possible without.

Here's the printer we have: https://printm3d.com It claims to print both ABS and PLA and doesn't have a heated bed.

So after researching I have found some reports of people printing ABS without heated bed. I think common practice is still to use a heated bed but I will revise my answer.

PLA becomes soft at around 50C, ABS at around 100. So if you print something that might sit under the sun in a hot sunny day PLA won't do. PLA is also very sensitive to UV rays.

Can you expand on "discontinuities" please? My ABS is popping. It was only opened from its sealed bag about a week ago and I like to think my house is warm and dry. Mine is "popping" and some of my prints have brown streaks in which makes me think that the moisture is making the ABS burn rather than melt. Is this likely?

@DA. it is possible to print tiny parts with ABS without a heated bed and get good results, but only for tiny parts.

I would disagree with PLA being weaker. I have done tests with ABS and PLA and using the same file I found PLA to be ~20% stronger than ABS. But the big thing is PLA is brittle were ABS can take stress before breaking.

The problem is that it's almost impossible to answer the PLA/ABS question just by looking at the material characteristics as it is so dependent on the application and even the specific object you're printing.
The decision guide in this infographic covers the following points that usually should be involved when deciding between ABS and PLA:
Ventilation - I wouldn't recommend putting your nose into the printer with any material, but being exposed to ABS fumes is likely a lot worse.
Heated Bed - This is an easy one: Without one, PLA should be your choice. If you do have one though, it might improve adhesion of PLA as well (set to ~60C instead of 90C-110C for ABS)
Exposure to Heat or Mechanical Stress - ABS would be the preferred choice as PLA melts at lower temperatures (it might start to deform when left in your car in the summer) and ABS can endure more bend before breaking.
Of course even ABS has upper limits and you might want to consider other materials like Polycarbonate for extreme applications.
On the other hand there are newer materials that share many PLA features (biodegradable, no heat bed required) but are more similar to ABS in their mechanical properties (e.g. BioFila Linen) and temperature stability (e.g. Advanced PLA, ExcelFil EVO).
Printer Enclosure - Not a must-have for using ABS, but especially objects with large footprints are susceptible to warping when they cool too fast. A fully enclosed printer ensures a slow and even cooling process.
Post Processing - Often an afterthought, but ABS has a slight advantage here as it is a little easier to sand and you have the option of working with acetone (which I personally wouldn't mess with unless you REALLY need to).
Print Temperature - Roughly 190C-220C for PLA and 220C-240C for ABS, so be sure that your printer can go high enough to extrude ABS.
Biodegradability - PLA is biodegradable (under specific circumstances) which is a consideration when printing test objects, prototypes and other objects that will be discarded.

Please consider updating your answer with more details to avoid issues with link rot

PLA/ABS general and thermal properties

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PLA (polylactic acid) melts at a lower temp and does not warp AS BADLY when cooled. It is non-toxic (in USA it comes from cornstarch, beets in some countries, or tapioca root)
It is less flexible than ABS, could rip or crumble.
ABS (Acrylonitrile Butadiene Styrene) is a harder and more sturdy plastic. (What Legos are made of). A heated bed is used to keep it from cooling too fast, as warping can be a problem is you cool it too fast (being close to an exterior door or an air vent) An enclosed printer helps regulate temp and avoids SOME of this problem. Some people get headaches from the smell of molten ABS over a prolonged time. It flexes better than PLA. Can be sanded or cut easily and maintain integrity.
Beware of CHEAP ABS as it may contain a higher amount of HCN (Hydrogen Cyanide)
that can be released when used (but usually around 1 part per million, about one fourth the dangerous limit.)
"GOOD" ABS CAN contain HCN as well, but it is not released by "normal" printing temperatures. (avoid burning ABS or Nylon)
so, all in all:
PLA is safe/non-toxic, biodegradable, uses less energy to print, less flexible.
ABS is harder, sturdier, will last nearly forever (if not bent or stressed or frozen). Somewhat flexible and will snap back.

Basic facts and tips for using and choosing ABS and PLA materials:
Made from:
ABS: Petroleum based

PLA: Corn or other plants

Smell:
ABS: smell of hot plastic,

PLA: gives off a smell similar to a semi-sweet cooking oil. Less intense smell than ABS.

Part Accuracy:
Both PLA and ABS are capable of building dimensionally accurate parts. However, there are characteristics that we need to memorized.
ABS: will be curling upwards of the surface in direct contact with the 3D printer’s print bed.  for fine and delicate features on parts involving sharp corners, such as gears, there will often be a slight rounding of the corner.
PLA: much less part warping. but it undergoes more of a phase-change when heated and becomes much more liquid. The increased flow can also lead to stronger binding between layer.
Recycling:

ABS: Recycling number 7, most local recycling programs do not accept ABS. It generally being recycled into plastic lumber.
PLA: it biodegrades, however the process takes a long time when outside of an industrial composting facility
Why we provide a vacuum sealed package for ABS and PLA products?
long term exposure to a humid environment without sealed package may result in detrimental effects, both to the printing process and to the quality of finished parts.
ABS: will tend to bubble and spurt from the tip of the nozzle when printing, part accuracy, strength are reduced. Raises the risk of clogging the nozzle. ABS can be easily dried by using dry or hot air.
PLA: bubbles or sporting at the nozzle, also discoloration and a reduction in 3D printed part are found when in long term moisture environment.
Improving the printing outcomes （Blue tape and Kapton tape pictures)
PLA: printing temperature approximately: 190°C - 240°C
ABS: printing temperature approximately: 225°C - 250°C (recommend to use a heated print bed)
A good first layer adhesion is of the utmost importance in obtaining the best results for the prints.
Using Blue Tape or Kapton Tape.
Hairspray the print bed.
To attain optimal result for the prints, you need to consider variables such as nozzel diameter, printing speed, and layer height.
Summary:
ABS:
A preferred filament for the prints with engineering and professional purposes with its strength, machinability, flexibility, and higher temperature resistance.
The bad plastic smell is due to the petroleum based origin.
Required a heated print bed to attain ABS printing reliability.
PLA:
Wide range of available colors, provide translucencies and glossy feel of the prints.
Plant based and semi-sweet corn like smelling are appreciated over ABS.
When properly cooled, PLA seems to perform higher maximum printing speeds, and sharper printed corners. Combining this with low warping on parts make it a popular plastic for household uses, toys, hobbyists, and educational purposes.
to know more please visit  abs vs pla


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Multi-color printing with desktop 3D printer?
My MakerBot printer supports only two filaments at the same time.
What are techniques to print objects with more than two colors for one object?

The most obvious solution is to pause the print and swap filament for another color.
Another option is to splice pieces of filament together, though this does not allow very precise control of when the switch happens. There is also a device that can automatically slice filament this way.
Finally, another option that uses very little external equipment is to use (permanent) markers to colorize light-colored filament.
Other options include upgrading to a printer with more hotends, or installing a hotend with multiple filament inputs and one outputs, but these options would involve significantly changing your printer setup.

I've seen where certain slicers and/or firmware installations will allow you to set pauses mid-print so you can insert a different filament and resume. Such firmware that I'm aware of is Sailfish. I haven't personally used this, but I've heard many great things for people who enjoy tinkering with their machine(s). Alternatively, there are 3D-printable attachments for your extruder that allow you setup markers to color your filament (mentioned by Tom in another answer).

Another approach is Mosaic Manufacturing's Palette – it appears to a single extruder 3D printer as a filament reel, but it is creating a custom filament on-the-fly by

pulling information from a multi-extruder .gcode file to determine the length, and the order, each color segment needs to be.

The device was a successful Kickstarter campaign and as far as I can tell it isn't shipping yet (as of January 2016), but they are taking orders and promising an April 2016 ship date.

Answering this question fully depends on the type of printer you have. Some printers have a pause capability, while others do not. Some have multiple extruders, while most do not. Some have a tube leading to the extruder and others do not. You specifically are asking about dealing with more than two colors when you have a dual-extruder, but the question generalizes to how to get more colors than you have extruders.
Markers probably offer the easiest solution. You could have different markers on the feeds for each extruder.
For printers with no pause ability, you might have to snip one filament and hand feed the second color after the first one until it can be caught by the feeder gear.
There are rigs that are available, or that you can make, that will let you connect two strands together lengthwise. You can then make up a single piece of filament that has multiple colors. Thus, one extruder head will be generating multiple colors.
The Kickstarter mentioned elsewhere essentially does this automatically for several strands whose lengths are calculated precisely by special software. The result is a single strand of filament that goes into one of your extruders - the other extruder would be unused with this solution.

The question specifies they have a dual-extruder makerbot. Perhaps you could specialize your answer for that situation?

Disagree somewhat - the original question is about providing more than the two colors that their printer already supports. This generalizes into a question about dealing with more desired colors than their printer supports, for any number of extruders. I'll modify my answer to be more explicit in that fashion.

The standard option is to change out filaments at certain times during the print. Software to add these pauses to your print automatically are around, with one (ColorPrint). This method works if you only want to change at a certain Z height, and not intermix colors on the same levels.
The other newer development that works for many printers (this works for single nozzle printers as well) are getting Y type adapters that allow more than 1 filament to be fed into a single extruder, and use a rapid retraction to pull the filament back out past the Y connector to allow the next color to be fed into the nozzle. Several companies are putting products like this out on the market at this time. An example of this can be seen at this page.
As mentioned in other answers, Mosaic Pallette is an option to drive more complicated multi-color prints, but is a rather expensive option at $999.
The E3d Kraken might be a possible hotend upgrade to a Makerbot, although it would involve a fair amount of work.


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3D printing for outdoor use: "What types of filament are most weather resistant?"
I'd like to print modifications for my bird feeder, both to patch over the hail damage from last summer and to try to deter the neighborhood squirrels.  I have an FDM printer (and experience with nylon, ABS, and PLA, though don't restrict answers to those if there's something else that's better), what kind of filament would stand up best to daily exposure to sun, rain, snow, etc?

PLA should work: https://web.archive.org/web/20170713072530/http://www.protoparadigm.com:80/news-updates/weathering-of-3d-printed-pla-objects

have a look at https://www.youtube.com/watch?v=qqNfa_zExRU

PET(G) is a strong contender. It is very strong and water-resistant, and as such is often used to make pop bottles.
PLA has a reputation for being "biodegradable" and therefore it is often discouraged to use PLA outside and/or in contact with water. However, PLA only biodegrades under very specific conditions which it won't generally be exposed to so it can be used (though, as a harder and less flexible material it is more likely to be damaged by hail).
ABS and Nylon are good choices as well. Basically, any plastic you have on hand will last for years, even in an outside application.

Are you aware of any resources comparing PLA/PET/ABS resistance to UV? I'd be interested to know which material would hold up better in direct sun for long periods of time.

That's obviously missing from my answer, but I don't know of any such resources.

Some research indicates that ABS is very susceptible to UV damage, and while PLA is less so, it softens considerably in high temperatures (think black structure in the desert Summer). In any situation a good coat of paint is likely to substantially extend the lifespan of your print.

Nylon is hygroscopic, it may not perform well under the rain.

@FarO Could you explain how the hygroscopic properties affect its performance?

@TomvanderZanden humidity worsens the mechanical properties and accelerates aging. Humidity also affects size of printed parts. https://en.wikipedia.org/wiki/Nylon#Bulk_properties https://www.infona.pl/resource/bwmeta1.element.elsevier-91cc2fe7-6c12-3286-a9ab-346b5efb7cbf

I have several items printed in PLA that have been outdoors for about a year now. Apart from some discoloration I don't see any structural damage (yet?). Some of them are at the south side of a building so they get maximum sunlight.
Of course Belgian summers can not be compared to Arizona summers!

I would think that ABS is a good choice if the application isn't in direct sunlight. ABS is what sprinkler lines are made from and those obviously do well under ground, though I have seen ABS that is left in the sun get super brittle over time and snap easily.
PLA also will slowly melt in direct sunlight. I have seen this one firsthand, having left a print on my windowsill and watching it slowly morph with the weight of objects on top of it.
PETG is good in terms of easier printing with a good strength, but I cannot say that it's particularly good for outdoor use, depending on your use-case. It can be food-safe though!
It would seem to me like Nylon is your best bet, since this is super strong stuff, and is used as gears for things that exist outside, like boats and motorcycle parts, etc. I don't have specific proof that this is the best, but depending on what exactly you're trying to accomplish, it's likely the best choice.
You can always spray paint some kind of coating on the resulting print that will increase the UV protection. Something like Krylon Preserve It Aerosol Spray perhaps?

I think ABS would be your best bet. It's not biodegradable and realtivley easy to print.
But you can use more or less any material if you use some form of coating on it. I would always go for coated PLA instead of other materials just because PLA is the easiest to print and it's nontoxic.

ABS would be toxic, I believe (contains BPA I think) so that wouldn't be a good choice. I think PETG is safe plus UV resistant so that would be a good one. I'm not sure about PLA or other materials.

Most plastic resins decompose under UV light from the sun.  The chains breakdown, and the plastic becomes a powder.
From examining plastic parts for outdoor use, they typically have a stable pigment built into the plastic.  The pigment prevents the UV from penetrating into the object because the pigment stops or reflects the radiation.  A good choice for the pigment is titanium oxide. TiO is stable under UV light and heat, so it will not bleach and lost effectiveness.  As the plastic degrades in the outermost layer, the TiO remain effective until it loses all support from the plastic and is removed from the object.
TiO is a bright white pigment, so the temperature of the object is lower than it would be if it were compounded with carbon black.
Because TiO is inexpensive, I would expect it to be the pigment in most bright white filaments.  Not "natural" white, but true, bright white.
So to answer your question, I would pick the whitest version of whatever filament you would otherwise favor for the outdoors application.

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How do I give 3D-printed parts in PLA a shiny smooth finish?
The surfaces of my printed parts using PLA plastic look rough and uneven.
Would changing filament to a better one make any difference?
If not, what kind of methods can I use to achieve a smoother finish for my for 3D-printed objects?

PLA parts can be finished with a coat of epoxy like XTC-3D from Smooth-On. This will smooth out the part and give it a pretty nice shine.
I've also had a fair amount of success sanding prints, giving them a coat of automotive filler primer, and using glossy spray paint.
You can also get great results with an acetone vapor finish if you're willing to switch to ABS. Though that will require a heated bed and can be a bit more finicky to work with than PLA.

XTC-3D is more of a resin coat.  It works as intended, and looks really good  if primed, painted, and sanded, but do know your model will be a little thicker as a result.

+1 for mentioning a filler primer. Just make sure to get an actual filler primer. I have found that Rustoleum's typical Paint/Primer does not have the same properties as a filler. The filler dries much faster and fills in print lines way better.

Acetone vaporing is a great way to smooth ABS prints. For PLA, however, acetone smoothing does not work. An article about smoothing PLA says:

This is a pity, since PLA is much easier to work with than ABS. We found some solutions for smoothing PLA, but most involve rather dangerous-sounding chemicals such as Tetrahydrofuran and Dichloromethane. The one exception we found is Ethyl Acetate which seems to give good results and is (relatively) safe.

Other article mentioned MEK Substitute, which is Ethyl Acetate as well. You could also try some kind of polisher manufactured for 3D print results, such as XTC-3D. Here is a real photo of XTC-3D on PLA:

Ultimaker also suggests using chloroform for smoothing PLA prints.
If your parts are really uneven, it might be a symptom of something bad going on with your print(er), you might want to read Taxonomy of Z axis artifacts in extrusion-based 3d printing.

The most recognizeable cause of this problem is Z-wobble. It is caused by the misalignment of layers in a repeating pattern with a period equal to the Z thread pitch (technically the lead, but this is the same as the pitch unless you are using a multi-start thread), and was a famous problem of the original Makebot, the CupcakeCNC.

Ultimaker suggests usage of chloroform for PLA post-processing: https://ultimaker.com/en/tips-tricks/post-processing

Tetrahydrofuran is not that evil, but it evaporates really quickly so it's not very practical to work with.

It takes quite an effort to make PLA shiny, and it's not as simple as ABS and acetone.
You have to sand down the print with sandpaper with different grit sizes (start with grit P100, then P240, P400, P600, P1500 and P2000).
To make the shining result you have to polish the print with plastic finish compound. Alternatively you can apply XTC-3D Print Coating.
See these posts:

Finishing 3D Printed Parts in 1 Simple Step
Ultimate Guide to Finishing 3D Printed Parts

Your two easiest options are dipping your print in acetone or giving it an acetone vapor bath. Note this process generally only works with ABS not PLA, with the exception of some brands. There are many articles online where you can learn more about the process.
Aside from finishing, you will generally get a smoother looking end result by lowering the layer thickness, and removing any hysteresis/wobble in your print head making sure it's well calibrated.

For a question that asks about smoothing PLA an answer that only addresses smoothing ABS doesn't seem very useful. Calibration is a nice starting point, but won't achieve "shiny smooth".

@Tom van der Zanden, It's pretty clearly addressing PLA specifying that the acetone method will in fact work for some brands. Provided links are for PLA. All other answers in this thread echo similar points.

Acetone does not work for any brand of PLA. The article you linked to doesn't use acetone, they use another solvent.

Not trying to be argumentative, just factual. The third link in the post (https://ultimaker.com/en/community/10412-acetone-finishing-on-pla) which you might have missed due to the way I formatted it, contains information on how one person was able to use acetone finishing with Colorfabb PLA. The reason is because they include a small percentage dissolvable plastics in their formulation, which is what allows this process to work, although marginally.

Make sure your printer is properly calibrated and that there are no jams as this can cause blobs or other defects on the object. Printing at a higher resolution would also make the object smoother, as there would also be less difference between each layer.
The filament that you are using could be a cause for the rough look as well. Make sure you play around with the temperature to find the right range for a specific brand of filament. I have noticed different brands tend to print better at different temperatures, and if it too low could cause flow issues.
Also leaving PLA out in the air tends to have it absorb moisture which is also no good. If you are not going to use it for a while put the filament back in a resealable bag with a desiccant to help keep moisture out.
If you don't want to see the printed layers you could also try melting the surface again with a heat gun to smooth out the object. Be careful not to overheat the object, because could you could discoloring or cause drooping.

Thank you for mentioning proper printer setup. Surprisingly, it seems that you're the first person to bring this up. So +1 for providing an insightful answer!

It is called Acetone Finishing
Basically the 3D printed part stays in acetone vapor and the outer shell turns to smooth surface. I have heard that it works better with ABS.
This article shows how with videos:

(Acetone Finishing on PLA - dead link).
New link: Acetone Finishing on PLA

It is worth noting here that acetone finishing works very well with ABS and (at very least) not as well with PLA.

Note that this particular method only works for certain brands of PLA. In most cases there is a good chance that the acetone will do little if anything at all to the PLA.

I have heard that Acetone isn't effective with PLA, but it certainly is for ABS. One place to get acetone is in nail polish removers. Some stores and salons sell 100% acetone variants, while others can be 70% or even as low as 20%, the stronger they are the quicker they will act.

This answer is incorrect. The question asks specifically about PLA, and acetone does absolutely nothing to PLA. You need something really nasty like MEK to affect PLA.

Because this answer is based on the wrong filament, it has extremely low quality and should be removed.

Whilst this is [almost] a link only answer, the link definitely deals with Acetone and PLA, and is a link to a massive thread (20+ pages), which could certainly be useful, but it is way to big to summarise. Hence, I think it should stay, IMHO, but it should be the only exception to the rule.

So we know what doesn't work but what does work (but not well) is alcohol! Only isopropyl alcohol 90% (and up) will work! Also at Walmart you can get stuff called Goo Gone or even better is Oops!
What you're doing is melting the plastic chemically. Since PLA is made from corn (more or less) the same chemicals won't work on this kind of bio plastic that PLA is. ABS is an oil based product so that's why acetone works.
I use those two mentioned products because they are made to remove glues and waxes and don't immediately evaporate so it has time to do its job and dissolve the plastic. Alcohol will evaporate, especially with less water content, and I absolutely positively don't want anyone doing this in a bedroom or kitchen especially do not do it without ventilation! The vapors are flammable and will build up fast at room temp and takes little to set off and it's just bad!
With alcohol I dip and soak the parts till I see them looking kind of soft like having a skin or depending on the PLA used silky. I print hotter than recommended with hot bed little to no fan and more feed because what happens is it gets denser past a certain temp and will look like its messing up and not print right like its clogged or not being fed fast enough and it's true but if you increase the feed rate to say 108 or 115 it compensates for the shrinkage and you get the same fill.
Now that being said your going to get smoother prints because it becomes like watery but with faster speed and or little fan snaps it in place but the hot bed will keep it from getting too hard for the next pass of the hot end and new layer so the next layer wont go on a cold surface and can easily get squished and fused to the last layer and now two  0.15 mm layers act like three but denser and smoother.
Then with the Oops! chemical or alcohol I just take what's left of the imperfections off. I take a dustless cloth and apply the solution onto it and wiping in the same direction with very little pressure till it feels like the cloth is getting stuck or its biting then I leave it alone for 15 seconds and wash it off by dunking it in a bucket of cold soapy water then rinse with fresh water and repeat if necessary. It's best if you use string to hold or suspend the parts because it's easy to leave prints behind.
Also even better (and I don't offer this because any liquid when atomized can be flammable - besides water) is to use spray bottles instead and lightly spray the same process on and dip-and-dunk to take it off - instead of wiping and risking leaving prints with marks on the finished parts.
There is sanding. Lots of sanding is another way to smooth parts.
I also tried conformal coating my parts which works but gets messy when trying to apply over a pre applied coat.
Then there's body filler or if you use paint I recommend either automotive paint with filler primer or a paint in the automotive section specifically for plastic and vinyl or heavy thick paints but not latex but lacquer paint that is heavy and fills in imperfections but lays flat when dried! There are Lexan paints for RC and model plastics but need primer.
But your best bet is make sure that your printer is running 100 % print in as fine detail with no bigger than 0.4 mm nozzle and make sure your slicer profile is setup to work perfectly with how your machine works like your stepper numbers and everything because all that matters first and foremost your first layer determines the rest of the print! Level bed and clean level surface! New nozzle every few dozen prints makes a difference unless you get a high end tip!
Keeping your Bowden tube fresh also makes a big difference! Get yourself feeler gauges and use them to level your bed instead of paper and it will be exact every time! And print the same speed and make as few adjustments as possible through the whole print and it will come out uniform. Taller prints if you need hot bed add heat as the print gets taller or nozzle temp because the print will lose heat as it gets higher up and won't print the same as the bottom layers.
I hope this helps or was the answers you were looking for! I was just passing through and hate useless answers that done have anything to do with the question asked! People like to just talk I guess.

Isopropylic alcohol does not solve PLA - I have had parts exposed to it for weeks and not soften at all. Further it has nothing to do with the origin in what a plastic is soluble but the chemical composition only.

This post is very hard to read as it is one big wall of text. Please consider improving the formatting.

For a cheap method to smooth out your prints, you can use clear nail polish. It will act as a resin-like coat.

Hi Logan, and welcome to SE 3D Printing! Whilst your answer may be technically correct, it is rather terse, and, as such, it has been recommended for deletion, unfortunately. If you could expand it then you may get a more positive response. I would recommend that in addition to reading some highly voted answers to gauge the standard expected, that you take a look at the help section relating to answering questions, in particular [answer], and take the [tour] for more information on how stack exchange works. Thanks :-)

I suggest using XTC-3D since your prints are small also you can add some UVO pigments in. It's difficult to use in large prints because has small working time. Anyway here is a print I did for a friend:

This now dead link shows more images and say how it is done.

This answer does not add anything over other answers (XTC 3D is already mentioned by a fair few of them). While there might be useful information hidden behind your link, we discourage answers where the main content is a link, as they may become invalidated if the linked content changes or becomes unavailable. Moreover, since the linked site is your own this is a bit of a grey area concerning self-promotion. We're trying to build a repository of questions and answers here, we're not looking to build a repository of questions and people saying "look, you can find the answer on my website".

I think Tom gives a nice analysis of your answers quality - we generally don't like link-only answers. I would recommend you to take the Stack Exchange Tour, in order to get more familiar with our site. :-) Also, welcome!

Your link has died and as you included no information from the link in your post, your post is effectively useless.

because of the explanation of @TomvanderZanden I vote to delete this as superfluous and contentless.

The only additional information that I can see in this answer is the use of UVO pigments.

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Resolution Drawbacks
With an ABS or PLA extrusion 3D printer, are there any potentially negative quality differences that could occur if I try to print at a higher resolution?
I am not concerned about print time as the equipment is not under high demand.  I am, however, worried the device may be more prone to fracture, likely to have defects, or have other issues I cannot currently imagine.

The question isnt actually about pla or abs at all; the wording was only used to indicate a plastic filament one as opposed to resin or metal.

I see, what about [tag:plastic-filament]?

@thethirddimension, thats better.  I think a "plastic-filament-fdm" tag would actually be useful.

@kaine Or just plain "fdm".

@TomvanderZanden "fdm" was already tagged. While far less common, there are other types of fdm that aren't based on plastic filament.

The biggest effect I've see on resolution is due to plastic stress due to thermal gradients.
The higher resolution prints build up more layers of material, and each layer has a cumulative effect on thermal stress.  The upper layers pulling up more as they cool, and the lower layers curling up more strongly as the layer count is increased.
To counteract this, a heated (or even just a draft free) enclosure makes a big difference.  Having a heated print bed helps significantly, as long as the bed itself resists deformation (a sheet metal or PCB bed will bend more than glass under the same tension, for instance).
The actual plastic strength, however, appears increased.  Laying down thinner layers of material appears to increase the bond strength between layers.

Finer layer heights tend to decrease warping, not increase it. The thermal contraction stresses are applied more evenly and gradually as layers build up, which allows the print to form a stiff and stable "foundation" that resists curling as additional layers are added higher in the print.

Regarding the sturdiness of the final print, I believe it depends on the inter-layer adhesion of the filament itself - which varies greatly. Also, normally, thicker layers would increase the strength of the print up to a certain point.
An informal study of strength/layer height ratio can be found here: this study suggests that the strength of the print increases up to a layer height of 0.25 mm, and then stabilizes.
On the other hand, printing at high resolution often will hide defects that occur from bad quality filament, in particular filament that has degraded by absorbing too much moisture. Due to the less amount of plastic extruded per layer at high resolutions, some general printing defects also tend to be less pronounced and easier handle afterwards.

It's also worth noting that the ratio of nozzle diameter to layer height affects strength. The layer height is typically set slightly smaller than the nozzle diameter, so the nozzle "squeezes" the new plastic onto the previous layer. This is especially important for the first layer, because it affects how well the object sticks to the bed; but it also affects inter-layer strength.

In my experience building with smaller layers also makes bridging and overhangs more pronounced and less likely to fail.
The smaller layers allow gradual changes for overhangs that are more abrupt with thicker layer.


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How to print metal-like parts?
I would like to print parts (e.g. jewellery) for use which I don't want to look or feel like a plastic, but metal-like, so briefly people won't see much difference.
Are there any specific type of home-printers that can achieve that? Or it's rather kind of filament that you should use?

couldn't a good metallic paint do this on any material?  The weight won't be the same, but it'll have the glossy look and cold temperature I suspect you're going for.

Adding to my answer, the best option is still probably lost PLA or ABS casting and polishing with a jewelers drill with the proper tips. Same thing what real jewelers use, except they use $10K+ Solidscape printers which print in wax and use lost wax casting instead.

If you'd like to print on RepRap like FDM printers, you cannot print from metal, but you can use some filament that tries to look like metal. I have good experience with Bronzefill, but there are plenty of others, just Google for metal filament 3d printing. Note that sometimes the parts need to be post-processed with a rock tumbler. There are several open source DIY tumblers you can build and use.
If you actually want to print from metal, you would need SLS (Selective laser sintering) printer, which is much more expensive.

If you do choose to use an FDM printer and a blended filament, I'd also make sure you purchase a stronger nozzle (such as a hardened steel nozzle). This reduces wear & tear on the nozzle, as blended filaments (such as metal fills, wood PLA, glow in the dark) contain particulates that can be abrasive against brass nozzles.

The Colorfabb metal filaments are the most metal-like filaments I've used (copper, brass, and bronze) and probably the ones that would make the most sense for jewelry.  While not as dense as solid metal, it's about 3 times as dense as regular plastic and when polished the metal shows through.  Print at 100% infill to make it heavier. Bronzefill is about 3.9g/cm³, about 80% metal by weight, maybe 30-40% by volume.  I wouldn't say the polished result feel like solid metal, but it feels even less like plastic.
There's also Filamet, which promises to be almost entirely made of metal and can be sintered into solid metal, but it hasn't been released yet.

Filamet is described at http://www.kickstarter.com/projects/879356311/filamet-the-metallic-printer-filament-for-artists

Especially for smaller parts, I would suggest looking into electroplating.  You can get a really nice, copper, nickel, or even gold finish using it.
The biggest issue is that to electroplate something, it must be conductive, but there are many conductive paints on the market which you can use to apply a very light coating to make the plastic conductive. I heavily suggest priming well (2-3 coats) before applying the conductive paint, as it will help to minimize the amount of conductive paint needed.
Here is an instructables link on a cheap, diy, electroplating solution.  It's assuming you are doing it on a metal, but once you apply the conductive paint, it's effectively the same thing.
Also here is an amazon link to a fairly well priced conductive spray paint which would do the job just fine.

parts ... I don't want to ... feel like a plastic

This is harder than looking like metal.  Plastic doesn't have nearly the density of metal, nor the thermal conductivity of metal.  So by touch people will be able to tell the difference between almost any metal item, and a plastic item that looks similar.
For jewelry, as long as the wearer doesn't mind that it's not metal (they will be able to tell) you can fool most viewers with proper finishing.  Sanding/smoothing, and then painting the printed part will work for most things.
For things which dangle significantly, the swinging and action of the item may give away its density, but some objects people expect to be hollow metal can be printed in solid or high density plastic and give the same weight, though they are lower density.

Are there any specific type of home-printers that can achieve that?

For things which feel metallic, no, except for very small items where the weight and thermal conductivity won't be significant.  A laser sintering printer could make real metal parts if this is needed, but these don't fall into the category of home machines.
If you don't mind a multi step process, you can make molds of the printed object, then cast real metal, or very high density epoxies, to accomplish your goal.
Beyond that, a high resolution machine with a lot of hand-finishing work is going to get you as close as you'll get to looking like metal.

You have several options:

Printing with filaments made up of plastic and metal powder mix. Bronzefill is one example.
While most may argue it doesn't look much like bronze or copper and rather clay, it can made to by some automatic polishing methods:

How to polish copperFill & bronzeFill filament
Polishing 3D Printed Bronze w/ Rock Tumbler & Steel Pins

Some don't like the idea of having a plastic and metal powder mixture and they say if it's not pure bronze/some other alloy it might as well be painted. Which is the second option:
Spray painting an ABS or PLA print with metal color spray paint. You might need to sand or acetone bath the print first for the paint to be applied evenly.
Metal plating can be done for more realstic texture.

Gold Plating a 3D Printed Eagle

The 3d print should again be sanded/polished first.
The first method of getting actual metal object from a plastic object: Printing with a metal clay, then firing it in a kiln. You'll need an appropriate extruder which can extrude paste instead of filament:

Thingiverse: metal clay extruder

There are 3D printers that can do that officially.

MiniMetalMaker

You'll need a kiln...
Second method of getting actual metal object from a plastic object, via metal casting ("lost polymer casting"). From all the above options this is the most dangerous if you're not experienced, skilled and careful.

Make Metal Parts with a 3D Printer (lost polymer casting tutorial)
Metal Casting with 3d Printed Molds

In addition to the aforementioned enriched filaments which give an excellent result, ABS can be plastered, painted and even chrome plated.

Why don't you embed some images yourself?

There's also an interesting discussion of printing with specially-designed solder alloys, at RepRap: Blog - A new approach to printing metals.
The author settled on 57.5%Sn, 41.3%Bi, 1.2% In, which begins to melt at 130 °C and finishes by about 170 °C. This has much better viscosity after melting, so it doesn't just drip away or bead up, and whose melting point is low enough that it can be applied directly on top of PLA or ABS.
Brass nozzles corrode quickly, however, so another material is needed (anodized Al was an improvement). It sounds difficult but feasible.

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Are there any ways to make a 3D print transparent?
I am aware of several "clear" filaments for a ABS or PLA printer.  They, however, have a cloudy or frosted glass appearance. I do not believe this is possible to eliminate but I believe it can be reduced.
Are there effective ways to make a print have a more transparent appearance?

It depends on a lot of factors, type of plastic, whether the parts need to be strong, can you use a vase print, etc.  Here's a few thoughts.
PLA - The brand of PLA makes a big difference, some can be printed very clear, some can't.  Most of the transparent PLAs I've used print much more clear at around 240°C.
ABS - I've seen some pretty impressive clear parts printed as a single layer shell in ABS and then vapor smoothed. I tend to find ABS more translucent and less transparent though.
PETG - Again the specific PETG you use matters, but I haven't seen nearly as much variation as with PLA.  I'm not sure how much temperature matters, but if it's too hot you get bubbles which will decrease clarity.
Thin Wall Prints - I don't have much experience here, but the Smooth On XTC-3D or vapor smoothing seem to be effective.
Solid Prints
If I want transparency, I usually print it at 100% infill (should be a real 100%, too much overextrusion or underextrusion will decrease transparency).  Printing slower and with less cooling sometimes helps.  It's easier to get the infill solid with a direct drive extruder, I couldn't get decent results with a long bowden tube (a short bowden tube works fine).
Here's a page where I tested 10 transparent filaments, the printed samples are 2mm thick, 100% infill - http://thrinter.com/10-transparent-filaments.  Those samples are all overextruded a bit, you can get better results if you dial in the extrusion precisely, but that's hard to get right, and the optimal settings may change slightly depending on the part geometry.  Larger nozzles and thicker layers may help to, but I haven't experimented much with that as there are significant downsides to that approach.

It would be nice to include the excellent image, in the link, in this answer, in case the link dies.

Use Taulman t-glase and after a light sanding with really fine paper (optional really, but go for it if you can), spray it with polyurethane varnish or something similar. Check out the article here.

This seems to work for a thin material as in their examples.  Will there still be visible striations in a thicker product?  I expect it wouldn't be too high if the temp is sufficient but you might know better.

You can make a mold from the print and then get a cast from that mold with a clear casting material.

I feel like this, while helpful information is contradictory to the spirit of the site.  This is Q&A.  You're choosing not to answer the question here but instead imply that the OP is asking the wrong question.

@deltree if you feel like this is not answering the question, please flag it. There's no such thing as a "spirit of the site". 3D printing has its limitations. Knowledge about technology includes knowledge about when said technology is not well suited for a job or if there's an alternative to it. I did answer the question. If I thought the question was wrong, I would have downvoted it. But I didn't because I think it's perfectly fine.

This method will work successfully if you can polish all the areas of the model that will be visible. So, with ABS it would be relatively easy to vapour polish something printed with really fine layers, make a mould and cast it.
Bear in mind that you will be introducing further complexity when opting for this method, such as shrinkage, mould cut lines etc.

@deltree I am OP and highly disagree. This acheives the goal I want by merely approaching it in a different way.

the question was: how do I make a 3d print transparent.  This answer says "make something else instead".  I didn't downvote I'm just pointing out that it explicitly doesn't answer the question.


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How to easily get rid of rafts and support structures?
My printed parts consist rafts, supports and other extraneous filament when printing with ABS or PLA.
What are efficient general techniques of removing them?

Can you describe the support structure style, the plastic being used, and whether you're planning on doing further finishing?

You should identify a specific issue in your question. There are many (well-known) techniques for doing this, why do the techniques that you already know of not work for you?

The best way to get rid of them is to change the design of the printed object to make them unnecessary.
Instead of printing the one part with support material, the piece can be split into two or more parts which can be printed without support material and assembled after the printing.

Given that this is not always fully possible, a convenient way to get rid of additional structures is to use a different fillament for them that can be removed easily. This list of printing materials includes Polyvinyl Acetate (PVA), which is water soluble. You can wash the support material away given that your actual printign material is not water soluble. Here's a quote from the website (emphasize mine):

PVA (Polyvinyl Acetate) filament prints translucent with a slightly yellow tint and is primarily used as a 3D printing support material because it is water-soluble, meaning that it will dissolve when exposed to water (and so MUST be kept dry prior to use). PVA is most often used with 3D printers capable of dual extrusion: one extruder printing a primary material (such as ABS or PLA) and the other printing this dissolvable filament to provide support for overhanging features. PVA 3D printer filament is available in 1.75mm and 3mm.

Increasing the distance between the support/raft and the print should allow for easier removal.  Some slicers (such as Slic3r) have settings for 0.0 mm spacing for water dissoluble supports, and 0.2 mm for regular plastic supports.
Changing the upper surface of the support also can help.  Some slicers will leave the support as lines all the way to contact with the print.  Others will put a full layer on the top of the support before the print.  The full layer allows for better adhesion and the print not falling between the lines, but it makes it harder to remove.
When the raft/support is done with lines, putting pressure on the lines in the opposite direction should cause them to break off.  As you break some of the outer ones, the support can be pulled down which will continuously rip the support off.  There may be a few pieces that remain.  These can be removed with wood chisels. Sliding the pointed end between the support and the print will cut into the plastic which removes the support.  Note: always push the chisel away from you as it may slip.  You do not want a sharp blade moving uncontrollably towards your body./
A raft with a solid top:

A raft with only lines for the top:

You might also note the pro's and con's for each one with regard to surface finish, removability, and other machine variables.

I really like this answer, but at the same time, I think this only partially answers the question.  The question was about techniques to remove rafts/supports.  This answer is about how to make them easier to remove.  If you add a section on hardware and techniques for removal, you will have my +1.

I use diagonal cutters and a pair of mini pliers.
I print with Simplify3d and the support structures are remarkably easy to remove, I find giving a quick pull pops most off. The diagonal cutters get rid of any small beads leftover.

For the last few pieces that don't want to come off some coarse grit sand paper can help smooth the surface as well.

That works well but it should be noted this frequently causes bleaching on areas sanded which may not be desired. Also removing supports may cause small white dimples or pimples where the supports were melted to the part.

I usually use a chisel or a flat-head screwdriver to easily remove the bottom plate that the printer auto-generates. I would also suggest using something like wire cutters or some mini pliers to pull them off.


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What are main differences between rafts, skirts and brims?
I would like to understand the differences between rafts, skirts and brims. They appear in the software which I'm using to edit my 3D objects.
Can anybody elaborate what are these and what are the main differences between them?

Rafts:
Rafts are a few layers of plastic a placed on the printing surface before the object is printed.  If non water dissoluble filament is used, a bit of an air gap will be placed between the raft and the print itself so it can be removed easily. A raft can help a print stick to the printing surface as it normally has a larger surface area than the bottom of the print.  It also allows the bottom layers of the print to contact another layer of plastic so there is less spreading compared to printing on the printing bed itself.

Skirts:
Skirts are loops of plastic that are extruded around the object that is being printed.  On the first layer this allows the filament to get pushed through the extruder so that a steady flow can be established.  A multiple layer skirt can be used to create a blockage around the print so there is less air movement on the print and the print can cool slower.

Brims:
A brim is a layer of plastic that is placed on the first layer of the print go allow for better adhesion to the printing surface.  It does not extend under the print as a raft does, but only goes from the edge of the print to a set distance away.

Credits: Images from Slic3r

All three of these features are used to improve the quality and success rate of prints, especially those failing due to issues on the first few layers, or due to the small size of the first layer.
Raft
A raft is a horizontal feature made as the first few layers of a print, and is used to help with bed adhesion issues, primarily used with ABS. The first few layers printed are the brim (typically prismatic), with the part itself on top of it (with a small separation distance to aid in separation, to allow the part to be removed from the raft). This separation distance needs to be adjusted to allow the first layer of the actual part to adhere, but also for the raft to be removed easily.
Skirt
A skirt is a single-layer feature designed to help extruder priming and to establish a stable filament flow for an optimal first layer. They are generally a few passes around the first layer "footprint" in the rough shape of the first layer, but they do not touch the part itself or help adhesion directly (although having a primed and ready extruder helps extrusion on its own).
Brim
A brim can be considered a skirt touching the first layer shape. It is used to help adhesion, and increases the first layer surface area (thus having more area to adhere to the bed). Brims are best used for parts with small first layers that fail to adhere properly. They are generally done as perimeters (as opposed to the crosshatching of infill) to be easily removable without damaging the part.

authors: ʇolɐǝz ǝɥʇ qoq


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Is the weather a problem for MDF frames?
I've acquired all the parts to build a Reprap Prusa i3 rework, the only missing part is the frame.
I'm in doubt between a MDF cut (cheaper) or acrylic (more expensive), of course a cheaper one is my preferred option until I see any disadvantage on making it of wood.
I thought about variables like heat and humidity and the possibility of expansion/contraction of the frame, is this a valid concern? Will I have more precision buying the acrylic one or is it irrelevant?

Acrylic is a poor choice for anything structural. It cracks and snaps under any sort of pressure. Tightening screws  in it is a game of chicken. MDF is much better on almost every front.

Generally speaking, MDF will weather OK. In areas of high humidity you might experience warpage, but you can mitigate that by sealing the surface with paint or varnish.  However you will probably find that of the two materials, acrylic will be more stable over a few years.

I want to add the following to the already direct and very good answer of @Dani Epstein. It does not answer your question, but hopefully may help many people who are reading the question when choosing between the two materials.
Acrylic is less stable and will probably wear off faster than a well-built MDF frame for a 3D printer! I will address the points separately.

An MDF frame has much(!) more mass than acrylic, which will reduce vibrations immediately. Almost more important, a box frame, as typical for the Prusa i3 for example, stabilizes itself intrinsically more by its 'redundant wall' design (don't beat me up over the wording here, I didn't find a better way to describe the property that results from the towers.)
Compare these two images taken from reprap wiki Prusa i3:

You will probably not find an acrylic frame with this design. Shear stress as produced by the moving carriages, as well as z-wobble from the rods should theoretically be much less.
The acrylic might not wear by 'natural causes' since it is not a biologic material as wood is, but it will wear much faster due to handling the material, maintenance and human errors. The material is very brittle. You can find lots of reports on the web where the frame cracked or broke during setup. Here is my favorite example from someone with a lot of experience (1:20-2:30): https://www.youtube.com/watch?v=wkkVk8c8XoU
Acrylic is a bad choice in terms of its mechanical properties: It is soft (I found low tensile strength as a number to compare in the given database), has lots of thermal expansion (5 times higher than stainless steel) and melts at 160 C (so beware of anything touching your hotend). Feel free to compare it to other materials here (no MDF or plywood in that database, though ;-) ): http://www.goodfellow.com/E/Polymethylmethacrylate.html

This altogether makes acrylic a very bad choice for a 3d printer frame material in my opinion. Choose MDF or plywood, you can make it look cool by painting it, too.

Yeah, I have so many cracked acrylic parts from previous builds. I have no idea why anyone thought it would be a good thing for 3d printers other than it is dead easy to Lasercut with relatively inexpensive cutters.


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How to 3D print a bike?
I've seen article about World's First 3D Printed Bike.
What kind of printer is required to do that, briefly how long it takes and how much does it cost? Is this even achievable at home? Doesn't need to be that specific one.

Renishaw has been using/manufacturing 3D printers since something like the '80's.... http://www.renishaw.com/en/additive-manufacturing-systems--15239

You will need a laser sintering or lasercusing printer, which will not be something you can buy for home use. They are horribly expensive.
You could always print this in PLA or ABS and cast it in aluminium. Then you have to find a safe method to test the result, because casting is not quite as simple as it looks and the bike could be seriously dangerous.


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How many times printed printer can print it-self?
There is a 3D desktop printer RepRap which can print most of its own components.
Assuming each printed printer will print the next one and so on. Are there any limitation how many times this can be achieved?
For example somebody printed for me printer and I do the same for my friends and they do the same for theirs. Can this go forever (since 3D model stays the same), or there are any serious side-effects/disadvantages of doing that continuously?

Other than it will destroy the 3D personal printer manufacturing industry?

They can still sell filaments, even more as more people will have printers.

Is there any particular reason why you've hyphenated "itself"? i.e. a stylistic reason, a technical reason, to highlight the reflexive printing aspect,  or to make a grammatical point?

The files used to print these objects are digital, and do not degrade in any way after each printing. There are no side effects or degradation that occurs over time due simply to printing them multiple times.
This is the RepRap philosophy, and the machines are actually designed with enough tolerance for printing and building mistakes that even if the print isn't perfect, it will not only work fine, but it can print a printer better than it was printed, with some care and attention to calibration.
The process still takes a lot of human intervention, in the way of building the new printer and properly calibrating it.  If there are errors in the printer or the prints it produces, they can almost always be attributed to the builder/calibrator/user, and not to the design or the fact it's the Nth generation of printer.

This answer does not actually address what was in the question, where the Nth printer prints the N+1st printer rather than the first printer printing them all. Unless you engineer it out, there is generational degredation from any inaccuracies of the previous generation getting reflected in the parts of the next generation in ways that affect its accuracy.

The reprap printers have often been compared to plants, providing fruits to you and the possibility to reproduce themselves.
This analogy holds in both good and bad ways. Any life form can reproduce itself only so often without artefacts (mutations) being introduced.
It takes a bit of skill to build, configure and run a reprap printer. While the parts can be passed on, that doesn't necessarily hold for gained experience.
Chances are that the parts your printer produces are not as good as those that you have received to build the printer. At least not until you caught up on the learning curve.
A reprap has a lot other parts that are not printed and can vary in quality independently from the printed parts. It makes a difference what steel rods are used, what driver circuit for the motors, etc. If you give printed parts away that are as good as those that you received yourself, the added parts are not necessarily as good as your.

My recommendation would be that you and your friends get printer parts from that somebody and you build your printers together. While giving parts to others is a great thing, building 3D printers together with friends is greater.

I don't see how your final paragraph relates to the passing on of skill. You could equally say, build a printer, build a better one, and pass on the first for someone else to learn with.

@Sean Houlihane that's covered in the paragraph about passing on of skill. If you build the printer with somebody else, you gain the skill together, which is something you cannot necessarily pass on as easily as the hardware itself.

As long as you maintain each printer and keep a proper calibration, go for it, this is what they were designed to do, I've even made replacement parts for myself.
Unfortunately the RepRap project just shut down on January 15th 2016 due to their lack of sales.
I have a reprap that came from a reprap, and has made another reprap.
Just make sure that when printing out the pieces for the next you are properly calibrated, otherwise the next machine might be built crooked;
Your only limitations will be the electronics pieces and the small amount of hardware that you will need to buy.

-1: The original RepRap project ( http://reprap.org/ ) continues to operate. The "RepRap Professional Ltd." ( https://reprappro.com/ ) has (temporarily?) closed, but they have not shut down their website and all of their firmware and design files are still available as open-source.

The link says RepRap did not shut down. A different project using the name RepRap shut down. You think a project to make a printer that didn't need to be sold would shut down due to lack of sales?


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Printing Plastic Replacement Parts
On a number of occasions I've broken small plastic parts that are nearly impossible to replace but could easily be 3-D printed.  The latest such mishap is the volume knob on the factory-installed radio on my car.
I have little experience in 3D printing, and would like to be able to replace these parts with something very close to the original.  Spending hours measuring and designing a replacement part that should be $5 isn't really an option.  I need something to scan the broken pieces in 3D and somehow just seal up the seam where it's broken.
Is there a scanning/printing/software system to do this that doesn't require a lot of 3D design experience?

Hello @James, I noticed your question has been up for a while now. Have any of the answers below been able to solve your question? If so, would you mind accepting the appropriate answer. If not, what is missing so that we may help you further? Also, if you have figured it out on your own, you can always answer and accept your own solution. Thank you.

The easiest way is as you currently do: model the pieces by hand, using (digital) calipers to measure them.
Scanning technology isn't very good, and the models are not of printable quality. Usually, fixing a scan is more work than modeling an item from scratch.

The better the measuring equipment, the easier it becomes. I've used a CMM at work to "scan" a part with the touch probe. This gets me the curved surfaces that are hard to measure with calipers. Then again not everyone has $100k+ measuring equipment at their disposal lol Try looking for a local machine shop!

I'd say this is probably the best answer out of the bunch. For something as simple as a volume knob, even for new CAD users, it isn't a long process. I used this exact approach in my CAD class for students to practice modelling.  For a volume knob, I can't imagine it should take more than 45 minutes even for a relatively new users

@Diesel 45 minutes for a $5 piece is still far from ideal.  Even if it's the best solution at the moment, it's still too high a barrier for some.

@Erhannis, I'm not quite sure what you're getting at. 45 minutes is an estimate of course and if the part is 5$  is too much then cost of  printing it is going to be a barrier. My comment was pointing out that modeling is a very easy skill to learn (not to be great at, but learn) and if it is a skill that is learned on free software the OP will then both not have to pay for either an expensive scanner and they'll be able to create whatever they need. If 45 minutes is too much time to learn something, how badly do you really need the thing?

@Diesel Sorry, I was basically complaining.  If I had to pick between spending $5 and spending 45 min designing something mundane, I'd pick the $5.  Now, as a learning exercise, it may be very worthwhile.  Similarly, if the 45 minutes decreased significantly with practice (say, down to 5 or 10 minutes), then I would be ok with settling for the design route.  (In general, I'm just dissatisfied with the state of 3d scanning technology, and wish it were magically perfect, haha.)

There is scanning technology: either hardware or software (such software typically works from multiple 2D photographs). It has limitations, but is an active research area, and getting better all the time. "Autodesk 123D Catch" and "3-Sweep" are a couple examples.
One tradeoff between automatic and manual is the complexity of the shape you need to duplicate. Sometimes you can make a much simpler shape that still works, and/or thicken the part so it doesn't break the next time. That can make manual design much more attractive.


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Conversion of 3 mm ABS filament to 1.75 mm
I have a few kg of 3 mm filament when I only have use for 1.75 mm.
How can I make 1.75 mm from 3 mm filament?

Note: I believe this will not be worth the cost/effort but am very curious to see what useful suggestions are provided.

In theory, making filament is easy.  You take a 3 mm hotend with a 1.75 mm hole, and extrude the 3 mm (sometimes actually 2.85 mm) filament, let it cool, and then reel it up.
In reality there are a lot of pitfalls to making filament - if the pressure isn't even, the hole not perfect, the temperature uneven, you can end up with oval filament, filament with bubbles, or worse.  If you are over temperature you may damage the filament and it could look good, but not melt correctly when used.  If you reel it too fast you may thin it out more than the intended diameter, or too slow and you may thicken it. A lot of hotends use steppers, which may result in ripples in the filament, so you may end up building a nearly custom filament machine.
Resolving all these problems is probably not worth simply selling or giving away the filament to someone that can use it, and buying the right size for your machine.
If you are still interested, though, you might as well go all the way and build a full filament extruder that accepts raw plastic feedstock (usually pellets) as well as your filament, and convert it that way, then continue using it to create your own filament.

Your "theory" is wrong, the size of the hole through which you push the plastic only has a small influence on its size. Generally it will swell a little to become larger than the hole through which it was extruded, but by pulling on the plastic you can stretch it out.

@TomvanderZanden Which is why I tried to make the distinction between "theory" and "reality" starkly apparent. The die size influences the filament diameter, but there are many factors, as stated, and you'll have to make adjustments to make good filament at a given diameter.

The best option is to find somebody in need of 3 mm filament and trade them for it (either in exchange for 1.75 mm filament or in exchange for legal tender with which to buy said filament).
The next best option would be to cut it into small pieces, and feed those into a filament extrusion system such as the filastruder.

Exactly. Sell it, exchange it, or I have even contacted my US based provider and simply exchanged it. If you buy if from a physical location or have more direct contact with the provider, they may be willing to exchange it for little or no cost.

You could build a machine that has a nozzle with 3 mm input hole and 1.75 mm output hole, based on some designs for filament making machines. Or you could just cut the filament into little peaces and use them instead of the granulate in an original filament making machines.
There are some open designs for such machines you can build, or you could buy one, such as Filabot.
However, as mentioned by kaine, this is very unlikely to be worth the cost/effort. Best option for you is to try to sell the 3 mm filament to someone who has a use for it, take the money and buy some 1.75 mm filament instead.


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Alternatives to RepRap project?
I would like to print 3D desktop printer, but currently I know only RepRap project. Ideally I'm looking for some better quality or larger alternatives (e.g. for printing larger parts than RepRap allows).
Are there any?

Did you look on github or any of the standard 3D model sharing sites? There are tons of options (not necessarily affiliated with the reprap project) for printers large and small.

The answer to this question is "Yes." A complete answer would fill a book.  If we are to allow printer recommendations on this site, we must insist you provide a set of requirements that narrows the field down to, at most, a few machines.

I agree with @AdamDavis, but I also want to know what alternatives there are, so I still care to see an answer

@deltree Consider contributing an answer to the question here: http://meta.3dprinting.stackexchange.com/questions/5/how-do-we-handle-recommendations

@AdamDavis - I actually took for granted that the OP meant other projects of self-replicating printers ("I would like to print 3D desktop printer", they say), not "other printers" in general.  It seemed like a fairly interesting question to me, in that case.