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I just added a fan to my printer because very small layers seem to come out very badly. For example, the 5mm PLA cube that's the top level of the test shape shown below. Watching closely, I can see that the newly-extruded fiber is pushing the previous layer(s?) around pretty freely. And when the object is finished, the little top cube is bulging, rounded, and still soft to the touch.

The 2nd-to-top level of the object is also quite small and quick, but often comes out nicely (if anything, it was better before I added the fan).

The fan is a squirrel-cage with about a 2.5cm square outlet, pointing at the nozzle from about 5cm away, running full speed. The extruder is a Mk9 from http://www.makergeeks.com/duexretopr.html.

I also tried telling pronterface to wait if a level was too brief, but that setting seems not to do anything.

What else can I try? It seems like this is a not-enough-cooling problem, but perhaps something else too?

A sample step-cube

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looks like pillowing : support.3dverkstan.se/article/… – Lars Pötter Feb 9 at 20:50
    
Is this the first of our questions that made it into the "Hot Network Questions"? Even if not, nice to see some question from 3D printing show up there. +1 – the third dimension Feb 9 at 23:43
    
@LarsPötter -- Great link, thanks! – TextGeek Feb 10 at 15:39

Layer Times

See my answer to this question and pay particular attention to my suggestion about a minimum layer print time. I'm not sure if all slicing engines provide this option, but I know MakerWare/MakerBot Desktop and (possibly) Slic3r allow this setting.

Basically, when you're extruding smaller features like this, the previous layer(s) are still very hot and possibly very pliable. So, as your nozzle moves around above the previous layer, the nozzle may (and probably will) push some of this molten plastic around. Chances are you can see it to a certain degree while it's printing. You can definitely see this in a most drastic state if you print a tall and small diameter cylinder. You'll notice that the part will become almost exponentially unstable the higher it goes.

By increasing the time your printer takes to print a single layer, you are allowing the previous layer(s) to cool closer to the ambient temperature of the build space, and hopefully not as molten.

Please refer to this calculator or a similar one for material cooling times. For a standard shell setting of about 2-3 (0.4mm nozzle) will yield about 130sec to cool down to room temperature. I would recommend (for ABS/PLA at least) about a 15second minimum for each layer, possibly longer depending on the size and spread out of the features.

Also note that this can be cheated by simply printing multiple items in the same build plate with the same heights (ie. multiples of the same part). Naturally, it will take longer for the machine to print the rest of the parts and therefore allow each layer to cool slightly before being printed over.

Active Cooling

Again, some slicing engines have an Active Cooling setting. I don't personally have this option setup on my machine, but I believe it regulates the flow of air directed at your nozzle (usually by use of a mounted fan). This can help cool the layers a bit faster. With ABS, this might result in some pretty bad warping mid-print.

Feedrates

Try bringing down your feedrates to provide the printed portions of the current layer more time to cool if the above options aren't available. Note that you might also bring down your hotend temp to shorten the time it takes to cool the plastic.

All else fails

My only other suggestion is stated above, try printing duplicates on the same plate. My diagnosis is that the previous layers aren't cooling down enough before the next layer begins.

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+1 for minimum layer time – Leo Ervin Feb 9 at 20:18
    
I was going mad printing something with lots of bits that had little 'posts' as connectors - most of them printed perfectly but one of them the post always seemed to go all melty and squidgy - it was taller than the rest and was the last thing that was printed and this is exactly the problem! Thank you! – MalphasWats Feb 10 at 9:24
    
Nice post! I can confirm that Slic3r provides good support for controlling printing speed and fans with regard to minimum layering time. For the reference, I will add a separate answer on that soon. :) – Tormod Haugene Feb 11 at 8:26
    
I found that the REASON my top 5mm cube was mushy was mostly because I had set a minimum layer time of 15 seconds. Slic3r handles this by slowing down if the layer will otherwise be printed too quickly. This causes the very hot print nozzle to be held in the very-tiny print area for too long and it keeps the plastic warm and mushy. When I increased the time to 30s it got worse. I turned it off completely and now it's much better. Still need a fan on PLA, though. – phord May 3 at 18:44
    
@phord how large of an object were you trying to print? If the feature was small on top, then when the printer is idly waiting to continue the next layer, it is most likely waiting directly above the small feature and slowing the cooling process. For smaller parts/features it might be best to "cheat" the process by adding a small object in the corner of the print bed that the printer can idle over. – tbm0115 May 3 at 19:29

I believe you might be having an issue with insufficient cooling. Remember the suggestion to have wait times per layer to let the previous layer cool? Well, I think with your setup and print not only do individual layers overheat, but individual parts of each layer. You either need to try more direct or dual fan cooling or maybe use a nozzle with thinner outer diameter which will have less surface to heat the layed down plastic surrounding it.

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