Edge cracking when cutting acrylic

[Roger Lueck]

I cut some .125" and .25" acrylic and experienced short cracks (.o625" - .25" in length) on the edge of the acrylic that were cut. The occurance of the cracking appears to have been random as it did not consistently happen on every piece.

I know the cracking is the result thermal expansion/contraction of the material. However, what I don't know is the correct % power as well as the correct speed mm/sec that would eliminate the cracks. Also, is the setting of one (either power or speed) more effective in controlling the cracking than the other?

Chinese Laser 50W, Air assist 18 psi
Setting were as follows:

.25" acrylic Power 98% Speed 3mm/sec
.125" acrylic Power 60% Speed 10 mm/sec

Thank You,
Roger

[Matt Turner (physics)]

Can you post some pictures?

[Rodne Gold]

The cracking is stress cracking , its most likely Extruded acrylic you are cutting as lasering stresses extruded way more than cast due to short polymer chains extruded is made of.
Its normally the result of cleaning the acrylic after the lasering with some sort of solvent or exposure to solvent fumes.
If you Burn a piece of extruded it has a silent flame (unlike cast which is crackly flame) and it will drip flaming molten balls of acrylic whereas cast wont

[Joe Pelonio]

Early on after I got my laser I discovered that you can actually make cracks on the edges appear before your eyes by rubbing them right after cutting with denatured alcohol. It only took one piece wasted to learn not to use it for cleaning. I have never seen this happen without a solvent being used on it.

[Roger Lueck]

Thanks for the response. The material was some scrap I had in the scrap bin and I'm sure it was extruded. I did clean the material with a window cleaner which I'm sure is comprised mostly of alcohol.

thanks again everyone!
Roger

[Dan Hintz]

Thanks for the response. The material was some scrap I had in the scrap bin and I'm sure it was extruded. I did clean the material with a window cleaner which I'm sure is comprised mostly of alcohol.

thanks again everyone!
Roger

Ammonia, more likely... and just as deleterious.

[Joe Pelonio]

You can use a window cleaner that does not contain ammonia. I found the stuff from Costco (Sprayway) to be OK. I also find regular rubbing alcohol to be fine. It's the denaturing (added gasoline or equivalent) that caused the cracking.

[Roger Lueck]

You can use a window cleaner that does not contain ammonia. I found the stuff from Costco (Sprayway) to be OK. I also find regular rubbing alcohol to be fine. It's the denaturing (added gasoline or equivalent) that caused the cracking.

I was curious about the chemical makeup of the cleaner I had used. It did not contain any ammonia, however, it did contain alcohol.