Cracks?
I am trying to find the minimum layer height on my Prusa i3. The prints were bound to fail at some point but cracks have started to show in the corners of this cube at 0.1mm. Is this a speed/temperature issue? Something else?
Natural ABS. 205c hot end (higher temperatures start to sag, maybe the thermistor is off). 120c bed. 0.2 honeycomb infill.
Are those cracks crossing layer lines? I’ve never seen anything like that…
@Whosa_whatsis , yes, they do cross several layers so I assume they are stress fractures of some sort and not poor layer adhesion. It may be related to the corner curling and platform adhesion issues that i have been working through. I wish I had recorded the print so I could determine when the cracks formed.
What’s the ambient temperature around your print, and have you checked for drafts across it?
@Whosa_whatsis , ambient temperature is generally stable between 21 and 22 (read from the thermistors). After I built a vent hood around the back half of the machine (front is open), the bed got 10c hotter, suggesting that there was a draft. I have not tried anything more scientific. Any thoughts on how the reduced layer height could cause it? Prior to this, all prints were 0.4m.
Well, the reduced per-layer tensile strength due to decreased cross-sectional area of extrusion, combined with the increased inter-layer adhesion due to the reduced diameter of the rounded ends of the cross-sectional profile of the extrusion, should make breaks across layer lines a bit more likely, but I haven’t seen any evidence that it would be enough to result in some thing like this, especially with the layer height only reduced to 100 microns.
I have been able to do .025 layer heights on my Prusa. That was with PLA though.
I think it may be due to shrinkage. As the plastic cools it shrinks putting the cube under stress… more plastic = more stress. The larger the dimensions are, the more likely stress cracks from shrinkage are to form.
Also… there are many different formulations of ABS plastic, and some are more likely to have large amounts of shrinkage. Using plastic from a different company may help.
Could also be too hot, plastic is more likely to crack if it crystallizes which only happens at higher temperatures, but only if it is under stress… like with shrinkage due to normal cooling.
