How do you assure that you print layers that bond correctly? I'm fairly new to 3D printing but I'm getting the steam up and I chug out pieces without much hassle. One thing though is that I have the impression that sometimes, parts are easily broken, and I suspect bad adhesion between layers.

I'm printing eSun PLA at 208&nbsp;°C (212&nbsp;°C first layer), bed at 50&nbsp;°C first layer then 30&nbsp;°C.

When I print the Benchy boat, it feels extremely sturdy (even the small chimney is unbreakable by hand), but if I make, say, a 50&nbsp;mm diameter cylinder with 4&nbsp;mm wall thickness, It breaks fairly easy along some layer. I don't see any specific error, it "looks" okay.

So my question is:

How do you assure that you print layers that bond enough?

[Edit] I already know that might actually cause bad bonding (low heat, underextrusion, ...) but I'm looking for a way to see when it happens.

Here is an image from when I stopped mid print (you can see the little ooze string just at the start at the crack) for checking dimensions. The next layer didn't bond well because, I guess, the already printed part had cooled down when I un-paused the print (say 1 minute later) or maybe the z-axis went off a bit when I touched the build plate.



Is the only way of knowing layers bonded correctly, to try to break the part apart?
I think the reason why the large cylinder is breaking much easier than the smokestack would is because of leverage. If you print the same cylinder at a much smaller scale it might be more difficult to break it. Think of taking a wooden stick, if you try to snap it by holding both hands near the center, it would be difficult but if you push on the very ends, the leverage will make it easier to snap. I have the tug boat on my desk right now and I was unable to snap it, instead I broke the cabin section just right above the steering wheel.

Now to answer your question. One of the faults in larger prints having weak layer adhesion could be a number of things. I think a common fault is under extrusion. If the nozzle has a high enough gap from the previous layer and it extrudes just enough filament to touch the layer but not necessarily push it snug, it could be a weak bond. This is like tape, usually when you want to make sure it will stay stuck, you press hard and rub it in. So I imagine filament shouldn't loosely flow onto layers but instead be pressed hard against the previous layers. Visually this is hard to judge but I noticed getting my first layer very close to the print bed makes every other layer stick very stronger on the previous one. You get a small skirt on the first layer but it does tend to give me a stronger bond between layers. You can easily slice off the skirt with a blade. Increasing the extrusion multiplier could give the same affect without needing to adjust bed height/leveling, but this can end up giving you some nasty walls. 

Another thing I recommend is to check your Z-axis. This is difficult to measure and best to find out by print quality on tall structures. On my first printer I had the issue where my threaded Z-rods would bind and cause one of the two rods to get a slight bit ahead. This caused a layer split focused on the side opposite of the faulty rod. This made many prints very prone to breaking at that point. I had to rebuild my Z-axis assembly to correct this.

Print temperature can help, try raising it up by 5 to see if this improves the bonding.

Lastly, try another filament. Filament tends to go bad due to moisture exposure, that is why usually you receive it airtight with a small bag of silica to absorb moisture. If I use filament which has been sitting out for months, just squeezing my part separates the layers in almost a slinky kind of way.