Hi. A really strange problem with ABS has come to me. Usually ABS brittless horizontaly but on me it happened vertically, so not along layers.

Cracks appeared when I was polishing object with acetone.

So what should I do? Printing speed was 50mm/s. My printer is on balcony, balcony has glasses, printer has acryl cover, bed temp 115 and print head 250. I read moisture can be one causing problems to me.

A picture included.

It looks like you print with a 100% infill.
That can cause a lot of stress as the material can't go anywhere.
If possible try with less infill, let's say 85% or with a slightly lower print temp.
Try to find the balance between proper layer adhesion, infill level and print temp.

Quote
Downunder35m
It looks like you print with a 100% infill.
That can cause a lot of stress as the material can't go anywhere.
If possible try with less infill, let's say 85% or with a slightly lower print temp.
Try to find the balance between proper layer adhesion, infill level and print temp.

Hi. That is an intresting idea. Over extrusion? However that part is only 2mm thick, and width is 0.4mm.

Ahh, now it almost makes sense.
With parts that thin you have the additional problem of not enough layers for stability.
I was fighting this issues with some thin parts that either de-laminated or broke like in your case.
So far the best way to prevent this while still getting a good surface finnish is to use a thin ABS juice - at least for me it is...
I print thin parts with 0.1 or 0.05mm layer heights to start with.
When the part is finninshed and before even trying to get it off the plate I give it a quick coat of the juice and let it dry.
Doing this all around the edges will also help to fight the delamination if the print temp was a bit low (helps to prevent warping though).
If you allow a bit of under extrusion for the top layers it will leave tiny gaps between the lines.
The juice flows into those gaps and seals them while at the same time releasing the stress on the plastic from the cooling down.
Allow to properly dry for at least an hour before applying a finnishing coat with thicker ABS juice, this way no sanding or polishing should be required, just get the consistency of the juice right.

So issue is thermal expansion right? Are there any ways to reduce ABS thermal expansion and stress?

Some plastics claim to have problem with that but I have not tried any of those expensive ones yet.
Try to print this thin thing again but increase the speed to 60mm/s and leave the heated bed off.
Reduce the nozzle temp so you can just push the filament through by hand or simply do some test prints and check how low you can go before the layer seperate or the extruder works too hard.
If you can print with 0.1mm layer height and turn the object so the "thick" side is up.
I have little to no problem with thin ABS parts so I hope it is just a matter of speed and nozzle temp for you.