Hi all.
I have been noticing that my acrylic parts have micro cracks in them, please see photo of what i am talking about. 3mm acrylic.
Have 100w co2, run at 36w max power and 6.9w min power, 18mm cut speed.
Any help would be appreciated.
Micro and large cracks in acrylic come from tensions in the material itself and are often released by the use of chemicals during cleaning, e.g. alcohol, thinner, acetone…
p.s. something completely different, can’t you drive faster to get the parallel stripes away?
I suspect this may be something specific to this crazy “spotted” acrylic.
Can’t tell from the pic- is it at a specific part of its thickness? Like only in the middle or only in the back?
@bernd.dk sounds like you’re asking about the edge striations. Striations in acrylic are not well understood, but they don’t change spacings with the speed of the beam.
…But it looks funny and is definitely fine for some products 
It is quite possible, but I work a lot with acrylics and do not have this pattern. I have made many stands for shielding in the covid era, in 6mm cast and extruded acrylic.
These micro cracks are even more noticeable after a week or so. They also appear on straight cuts as well as curves. Sometimes they dont appear at all, even when cut from the same sheet.
They are evident as soon as the item is pulled from the laser, so there has been no cleaner applied.
I thought the striations were as a result of the laser firing on an off? Would love to get rid of them…but how?
I’m sorry, I just checked to see if I remembered correctly - I don’t.
I go even slower like you do, depending on the thickness between 6 and 7mm/s.
And there are also the irritating streaks, but much less visible.
(4 and 6mm extruded acrylic)
It may very well be correct that the colored dots give off something that triggers the internal tensions.
These sound like stress cracks. If you use alcohol on acrylic it will do this also.
If you’re not using any kind of cleaner, then I follow up on @Dannym comment about where the material actually changes… dark spot wider uneven heating and increased stress.
I have my edges do this a number of time… so far it’s a belt adjustment for a fix.
Is that a tighten or loosen on the belt ?
Overall, they tend to get lose, but a maladjusted belt is a problem, either way.
Generally, I keep mine snug and move that axes as I adjust belt tension for the most ‘snug’ feel and the least drag. As the belt gets too tight, it will increase the drag.
It a touch like an artist, you have to develop it… once you figure it out, the same technique will work for other belted machines… screw driven machines have their own issues.
It’s something you have to figure out yourself, there isn’t anyway I know to tell how to tension a belt… it’s on the job training of yourself.
These are really timing belts from cars and are designed for one direction of motion. They have little stretch as they are usually in the timing cover of an engine and difficult to get to…
My own experience with acrylics is that they tend to have residual stresses remaining from the manufacturing processes. It has been most apparent in acrylic glasses that go through the dishwasher a couple of times… they end up with lots of microfractures that continue to propagate every time they go through a heat-cool cycle, no matter how slow you try to do it.
I don’t know, I’ve never tried it… but is it possible to “anneal” residual stresses out of acrylic sheet by very slowly heating, and then very slowly cooling it back down? Would this prevent those microfractures?
Probably, but impractical.
I have the same issue with the dishwasher… One of mine actually started leaking… Good fodder for the laser 