Bumpy final layer?

[joatrash 1]

I'm getting a lot of prints where there are raised bumps in the top layer. These bumps occur exactly and evenly where there is no infill (i.e. hollow underneath). It almost looks like expanding air inside the print is causing the plastic to bulge upwards. They also only seem to occur on perfectly flat (horizontal) layers. Sometimes the bumps are so bad that there are small pinsized holes in the middle.

It seems like it MAY be more common on low prints (prints that seal up closer to the print bed), which leads me to suspect a print bed heat issue (PLA not cooling fast enough) but it is not consistent enough to confirm.

Changing the infill amount doesn't seem to matter much (usually 10-30%) besides making the bulges bigger or smaller. It also doesn't appear to make any difference how many models are printing (i.e. how long layers have to cool until the next pass).

Using a 65 print bed and 220 nozzle temperature.

Should I try turning down the bed temp?

 

[ian 32]

the problem is.. when you use a normal infill like 20 percent... and a thin top bottom layer... the layer lieing above the opening between the infill will pop up a little. if the layer is set to thin.. the pop will actually break the top of the surface and then you have bumps and openings... not good.

Im not 100% sure why the bubble happens.. but maybe because of trapped heat being trapped and pushing up the new thin layers or... when the material is printing, it cools and bends upwards... and because there is no material under it to hold it down.. it bobs up like a baloon... probably that !

best solution is to increase your top and bottom layer height in Cura !

Something like 1.2... or 1.5 should be good... If you increase infill that wont really help so much.

Hope that helps.

Ian :-)

 

[jonnybischof 60]

I wonder why the default setting is 0.6 mm top/bottom thickness... I never ever got good results with that..

My preference is using 23% infill and 1mm or even more top/bottom thickness. That gives you a pretty strong part, but it's not the best choice for saving material..

 

[joatrash 1]

Yes, but it's still curious as to why it happens on some prints and nother others that are similar in shape, and printed with identical settings. If it were only down to thickness of the layer, it would probably be more consistent across all models.

I've gotten perfect results with some prints and horrible results with others and the only difference I've been able to come up with is the model height (top layers being printed too close to the print bed causing them to stay soft, thus reacting to expanding air inside the print). Thickening the top layer that much doesn't sound like it would help that much if that's the case. There may also be some correlation to surface area (wide spaces without outer walls that give extra support) but that does not seem to be consistent either.

 

[ian 32]

trapped air heated by the hot bed expands.. and so the larger the opneing between the infill. the more air is there to expand and so cause the thin top surface to bulg outwards....

Ian :-)

 

[ian 32]

could be sometimes an advantage to be able to release air from the hotbed... like tiny small air release wholes that can be opened and closed ?

 

[joatrash 1]

trapped air heated by the hot bed expands.. and so the larger the opneing between the infill. the more air is there to expand and so cause the thin top surface to bulg outwards....

Ian :smile:

That goes directly against what I seem to be observing. HIGH models (with more air) are less affected by it.

 

[IRobertI 521]

This is what we call "pillowing" and it happens even if you don't have a heated bed. More cooling helps greatly, crank the fans up to max if they aren't already. Since you say that it happens more on lower parts it could be that the fans aren't up to 100% speed by the time the print finishes. By default cura ramps up the fan speed gradually over the first Xmm (I can't remember the default I'm afraid) so you could try lowering this threshold.

A thicker top also helps. Thicker layers help.

 

[joatrash 1]

I think the wost example was with a wide, flat model that was only about 10-15mm high. Is the fan ramping a setting in Curas expert panel?

 

[peggyb 171]

slowing down the print really helps.

 

[gr5 2,268]

This is called "pillowing". There are many tests and discussion of this and the solution is more fan. The problem is not heated air although it looks like it. The problem is "breaking threads". So you can also improve by increasing infill or layer thickness (thicker threads) or just thicker "top" surface. But the simplest solution is "more fan":

post #10 is a summary by illuminarti with photos but there is a whole thread somewhere:

http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/?p=17300