Divot pulled from glass bed

[JimG]

Perhaps others have seen this: my most recent ABS print pulled a small divot (6mm x 6mm x 0.4mm thick) of glass off of my build plate! The divot remained stuck very tightly to the underside of the print after cooling.

The divot was shaped like a a wedge of pie with a 90 degree angle in the "corner." It did not otherwise damage the glass, and the print came out fine. The print was completely free of the bed when I removed it, having cooled overnight. So the divot was pulled out of the plate while it was cooling, I suppose. I was not there to see or hear it happen.

I use inexpensive window glass, 3mm thick, from a big box store. The bed was heated to 110C, which is typical for my ABS prints.

The object was a terrain model of a mountainous area. It is thick (>20mm) at 3 of the corners, and I had been having problems with those corners lifting slightly off of the bed. But the divot occurred under the thinnest corner (< 3mm) at a location where I had never seen any lifting on previous prints.

The change I made in the model right before this print was to increase the bottom layers from 3 to 5. I did this because the thick corners had been pulling up enough to actually break the bottom layer on previous prints. Making the base thicker solved that problem, and the print was perfectly flat across the bottom.

I had just reapplied a very thin layer of All Purpose Glue Stick to the bed before this print. My practice is to apply a few swipes of the stick, then even it out using a damp paper towel. I was actually concerned that I had not applied enough since this was a thinner application than normal for me. And the print was completely loose after cooling. So I am not blaming this on too much glue?

Comments and suggestions to avoid a similar occurrence in the future are welcome!

Jim

[TopJimmyCooks]

The hardware store window pane is not especially high grade material and may have defects, inclusions, stresses that would never be an issue when used as designed in a picture frame. We are applying stresses that it wasn't designed for. I would treat it as disposable/replaceable substrate and and not worry about it. I pry prints off the bed with razor blades and tweezers sometimes, and haven't damaged it beyond some small scratches. I wouldn't be surprised if I broke one eventually. I keep two on hand but usually just clean and coat one in place and reuse it 5-6 times before switching it out for more complete cleaning.

[JimG]

I have an expensive piece of borosilicate glass that I am keeping in reserve, until I "know what I am doing." I wonder if the borosilicate also will be prone to this kind of damage?

Jim

[TopJimmyCooks]

Pyrex is a lot stronger and more heat resistant. Very different than window pane. Not likely to chip as easily. you can scratch it however.

[JimG]

I have the nominal 1/8" borosilicate/pyrex plate installed and have done a few test prints. First observation is that the boro is a full 1 mm thicker than the window glass (3.2 mm vs 2.2 mm). It looks very beefy on my heated bed!

It also seems to distribute the heat more evenly across the bed, as theory would suggest. This is a very unscientific observation, and might just be wishful thinking. But the extreme edges of the boro glass seem to be hotter than the plate glass was.

Jim

[dacb]

Wow! I use cheap hardware store glass and print ABS onto an ABS slurry and I've never seen this! Can you post a photo? I'm kind of impressed.

[JimG]

Photo of the divot with pencil for scale.
photo1.jpg

This is the object that pulled the divot off of the glass.
photo2.jpg

Another perspective on the divot.
photo3.jpg

I think the primary cause of the pulloff was the glass trying to restrain the shrinkage in the plane of the base of the object. There was probably also some out-of-plane force (tension) being applied at the same time. But the shape of the wedge suggests strongly that the shearing force in the plane of the base of the object caused the pulloff/divot.

Jim

[curious aardvark]

that is pretty impressive.

And yet another reason to use pla :-)

[AbuMaia]

I had a divot get pulled out once. I think I was using only hairspray with ABS that time.

However, in a later attempt to combat warping, I used blue tape, with hairspray on it, with ABS juice on that. The part still tried to warp, but the hold was so strong it broke the glass. I never did get the part off of the glass, tossed the whole mess as one piece.

[dacb]

Wow! Wild shrinkage. Are you printing in a cold space? I wonder if the rapid cooling to ambient caused this?

Maybe just an unlucky piece of glass.

Interesting piece you've printed. What is it?