Problems post-cutting/painting PMMA

[Steve Crawford]

I'm making small signs from 3mm thick clear PMMA acrylic sheet. I cut a shape out (such as an oval) with my Speedy 300, I spray one side with acrylic primer, then etch a reversed design into it and back paint it with acrylic colours. I then bond another sheet to the back using clear PVA glue to create a double thickness laminate; this glue is not spread right to the edges. However often within a few hours I see small cracks appearing at the edge of the uppermost piece. Over 24 hours these get bigger.

Does anyone have any idea what might be causing these cracks to appear? Its going to be a real problem for me if I cant solve it. :confused:

[Mike Null]

Steve

Only a guess but I suspect the acrylic is extruded and when cut with the laser is stressed in such a way as to cause the cracking that you see. This is often accelerated with the use of chemicals such as bonding materials, paints and solvents. Sometimes you can see it before your eyes if you take a scrap piece and apply alcohol or some other solvent to it.

I have used cast acrylic without problem. You might also try tape instead of cement.

[Dan Hintz]

If you must use that acrylic, you'll need to anneal it after you're done cutting with the laser and before you use any form of solvent... paint, cement, etc.

[Steve Crawford]

...you'll need to anneal it...

Please elaborate as to the technique you recommend Dan... I'm not familiar with how to do it.

[Bert Kemp]

To relieve the stress in the acrylic sheets or to anneal you have to heat the sheet for a few hours then cool it slowly. You really need an over that can circulate the air.

[Scott Shepherd]

You're probably not going to anneal anything like that successfully, in your home, on a regular basis for production.

Any reason you have to use the liquid to glue it? If it's back painted, I'd use a glue sheet between the two. There are all sorts of glue sheets, or adhesives in roll form. Some are colored, some are clear, some are optically clear. I'd head in that direction if possible.

[Martin Boekers]

Is the "crazing" on both sheets or just the painted one? If it's just the painted piece, I'd guess it has to do with the paint. What brand and type of paint are you using?

[Steve Morris]

is the paint water based or solvent based?

have you tried cutting a piece and not doing anything to it to see if problem is with the material or your post cutting treatment?

[Clark Pace]

is the paint water based or solvent based?

have you tried cutting a piece and not doing anything to it to see if problem is with the material or your post cutting treatment?

Alot of spray paint based can will do this. I've experimented with several. Use cast instead. It will still craze, but it's more resistant.

What I have found that works well. If spray the paint, and put it right in front of a fan right away. This help the of gassing off ammonia quicker.

Also spray spray paint, then engrave / cut.

[Kev Williams]

Cast acrylic isn't immune to this problem... This is a piece of 1/8" cast mirror, and no chemicals helped it along. It started to craze in just a few minutes after I cut it. Nothing but air has ever touched it--

as a note, this piece is (was) probably close to 20 years old when I cut it. So age may be a factor...

http://www.engraver1.com/erase2/craze.jpg

[Scott Shepherd]

Kev, that's caused by a dull beam. You need to sharpen your beam.

You can get a beam sharpener at the hardware store. It's between the aluminum magnets and the slack for rope ;)

[Gary Hair]

It's across from the aisle where you find muffler bearings and metric crescent wrenches...


Kev, that's caused by a dull beam. You need to sharpen your beam.

You can get a beam sharpener at the hardware store. It's between the aluminum magnets and the slack for rope ;)

[Glen Monaghan]

It's across from the aisle where you find [...] metric crescent wrenches...

Laugh all you want, but I probably wouldn't have been able to pull the busted differential off my first car, an Opel Kadet wagon, had I not had a metric crescent wrench to supplement my metric bicycle tools :^)

[Michael Hunter]

I have had cracking problems too, but ONLY on very old acrylic stock.

Acrylic absorbs water from the air - extruded does this much faster than cast - and I believe that steam is the cause of micro-cracking on old stock (when solvents are not involved).

One of the "How to use" things (I think from Perspex) suggests an putting acrylic in an oven at 80 degrees C for 24 hours per 3mm thickness to drive out the moisture before lasering.

[Steve Crawford]

I think Martin is possibly correct... I am using a solvent based car spray, and may change to a water based one (yet to experiment).
Here is my experiments and findings so far...

Method:
I cut six identical discs with three different settings (two of each). I spray painted one side of one and left the other plain with no paint.
I changed the PPI/Htz of the cut to see if that had any effect... not too sure if it did yet, but my reasoning was if the beam temperature was different on each of the three cuttings, perhaps that would in some way anneal the edge more.

Findings:
So this is what I did and the results

Disc 1 & 2, 5k PPI - painted = cracks at edge, non-painted = still perfect after 3 days
Disc 3 & 4, 10k PPI - painted = cracks at edge, non-painted = still perfect after 3 days
Disc 5 & 6, 20k PPI - painted = still perfect after 3 days, non-painted = still perfect after 3 days

Conclusions:
So my current conclusions, right or wrong (yet to be confirmed with more experimentation):

1. It does appear to be the paint that is affecting the material, due to all unpainted ones still being pristine.
2. It is possible that a higher (and therefore hotter), ppi is more effective in reducing cracking when painted.

Comments appreciated, this is very interesting... Thanks for participating in the discussion so far. :)

[Mike Null]

My view is that the paint is the primary culprit. Depending on the area I have to cover I use acrylic craft paints or for larger areas spray paints for plastics. Here we have a brand called Krylon for plastics and I've found it works very well and dries almost instantly.

[Dan Hintz]

not too sure if it did yet, but my reasoning was if the beam temperature was different on each of the three cuttings, perhaps that would in some way anneal the edge more.

Annealing is a slow, controlled process... you cannot recreate it with a fast-moving laser.

[Steve Crawford]

Annealing is a slow, controlled process... you cannot recreate it with a fast-moving laser.

Thanks for clearing that up Dan... appreciated.

Well in the end all of the pieces where the solvent based paint was used, no matter how lightly applied, all cracked in the end, some only literally a couple of millimetres, but a crack non the less. Ive now tried two pieces with water based acrylics and all seems fine so far two days on. So from now I'm going to start spraying water based paints for my signs.

Thanks for all your help and advice people. If there are any further developments on this story I'll report back. But it seems the simple solution is not to use solvent based paint with PMMA/Acrylic Sheet.