How to avoid cracks on large objects?

Hello,

after printing many objects without problems, I got serious problems with the largest part I have printed up to now.

It is a box with 110x190 mm ground surface and 100mm height. The walls are 3mm thick. The box is printed from transparent Bendlay ABS filament at 230°C over a PET tape covered heat bed set to constant 120°C temperature. My M90 was running in a heated chamber with 44°C inside temperature.

During printing developed several cracks between the layers, obviously caused by thermal shrinkage of the ABS material. The first crack is about 10mm above the surface, but also far above from the glass plate developed cracks.

What are your experiences about such large objects?
Is there a way to avoid this cracks ( reduced bed temperature after first layer, better inter-layer connection by higher extruder temperature, special filament types, other box design ...???)?

Finally, when traying to remove the box from the glass plate, I broke the glass plate :-(
Can I replace it by some standard, common glass plate or have I to look for some special glass?


Cheers, Gerd

I haven't any experience with Bendlay but your setup is very close to mine for regular ABS which I've had no problems with on tall prints.

As for replacing the glass, apparently picture frame glass is 2mm and people here have had success in getting their local picture framer to cut 200x200 sheets for them.
I use 3mm thick SORLI mirror tiles from IKEA, which are already 200x200 (albeit with rounded corners) and are £6 for four. It takes a little longer to get up to temperature given the extra thickness, but I have been printing on it with no problems. Having lots of tiles means I can clean off and prepare spare ones with PET tape and just swap them over when the tape eventually degrades without interrupting my printing too much.

230C seems low for ABS but I have never seen transparent ABS before. I extrude natural ABS at 250C and coloured at 240C.

Something that size and shape would be challenging in ABS. Would natural PLA be suitable for your application?

---

[www.hydraraptor.blogspot.com]

Thanks for the infos.

The datasheet of the transparent ABS quoted 225 to 240°C printing temperatures. So I tried 230°C, which worked fine for small objects.

But the box must not be transparent or from ABS (btw, if the infill is not solid, it looks not very transparent, more like crushed ice).
I still have a spool of green PLA. Or has natural PLA even better characteristics?

I have printed 200 x 190 x 60 similar boxes in quite a few colors of PLA. I have had problem with cracks using grey but it seems to have quite a lot of additives. I print on blue tape.

Edited 1 time(s). Last edit at 03/02/2014 10:46PM by Ralph.Hilton.

As nophead proposed, I printed the large box in natural PLA and the result is perfect: no cracks, no warping, correct dimensions :-)