Laser cutting through mirror acrylic

[James Cogbill]

Does anybody have any advice or tips for cutting through Mirror Acrylic? I typically leave the plastic masking on the face and cut with the mirror face up. I'm concerned that I might be bouncing the laser beam back into the machine. Has anybody got any advice? I certainly would appreciate some. Thank you.

[Tony Lenkic]

James,

If you cut mirror side up I would suggest to mask it with application tape to prevent beam reflection.

[James Cogbill]

What about flipping it, and cutting it from the back?

[Tony Lenkic]

That is how most shops would do it.....cut from coated side.

[James Cogbill]

Very good to know, thank you for the tip

[Chris DeGerolamo]

I cut it from the back, almost all the way through, then run a second pass at a faster speed. I find that it reduces the amount of damage to the underside (i.e. front).

[James Cogbill]

That's a good suggestion, I'll try it out. Thanks!

[Mike Null]

I just ran a fairly good sized job using reverse engraved .125" acrylic. I attempted to engrave and cut from the back which is my normal procedure. I left the plastic on the front (bottom) and proceeded to ruin most of the signs due to marks left on the front side. After several experiments I followed a suggestion by Steve Shepherd to cut the signs from the front then flip and engrave. That worked flawlessly with and without the plastic.

Granted, I wasn't dealing with reflective surfaces but cutting from the front was the answer for me.

[Ross Moshinsky]

Regular acrylic = Face Down
Reserve Acrylic/Mirror Acrylic/Acrylic with Adhesive on back = Face Up

The kerf when entering is "much" wider than exiting.

[Kev Williams]

I've never much bought into the reflected-back-beam-will-kill-my-laser theory. At least not on the low power machines we use. I do know that reflections happen, my laser singed my mustache one day when engraving a stainless cylinder as the beam came down front side of the tube and hit me in the face. Got a blast of heat and the smell of burning hair. Lesson learned the hard way ;)

But having the having the beam bounce straight back into the tube from something being cut or engraved? Just the act of lasering something is altering the reflective properties of what's being lasered to the point any reflected light would be so out of focus as to be insignificant. So, what about lasering something whose reflective properties WON'T be affected? Such as a bare piece of mirror polished stainless steel-- Now it seems to me that firing the laser onto some mirrored SS would be the most likely way to create the best (or worst) relections. And the reflected light should transfer significant heat onto whatever it hits on the way back, like it did on my face from 2' away. So I just did a little experiment. I took a 2 x 3" or so piece of paper, cut some holes so it would fit over lens tube and focus rod, so the bottom of my lens was surrounded by paper. My thought being, any significant reflections may start the paper on fire. And I know my beam isn't dead straight, it's angled somewhat, so some of the reflected light should definitely come back at an angle and hit the paper, or at least the lens tube. I drew a black rectangle and ran the machine at full power over the bare SS for about a minute. Nothing happened. The paper, the lens tube, the lens itself, none of them even got warm, all were cool to the touch. I test cut some 1/8" thick Romark afterward to make sure I did no damage, it cut it fine...

***disclaimer*** All of the above is purely my opinion, and as isn't unusual I'm likely totally wrong. All I know is what works and doesn't work for me, and I've never much worried about lasering shiny stuff. ;)

[Mark Ross]

This is what I recommend. It is actually 3 steps and we have had less material come back from our customers. Acrylic is fragile, if it is extruded it can be a testy thing. Believe it or not, it can absorb enough moisture that you can send out what looks like a perfectly good piece and it comes back with cracks.

We bake it (step 1) to dry it out. We cut through it through the backing, then we cut it out. Haven't had a piece come back since then.

[Michael Reilly]

By the time the beam travels through the acrylic, if it reflects, it has to travel back through the acrylic again, there won't be anything of significance left. As others have said, it can be a concern with high power lasers, but nothing any of us use.

Acrylic can only absorb 0.3% by weight of water. Cracking at the edges is called crazing. It is caused by the rapid heating/cooling of the material along the edge which creates a stress imbalance that we can't see. If the material is exposed to anything with solvent properties, such as acetone, acrylic glue, etc. it allows the stress to be released in the form of micro-cracks. The only way to prevent it is to avoid solvents or anneal the material after cutting. Unfortunately, annealing is impractical for most of us. It involves slowly heating the material up to 180 degrees F over a 2 hour period, hold at that temperature for 30min per 1/4" of thickness, and then cooling down at a rate of 50 degrees per hour. Not only is this time consuming, but requires an oven large enough for whatever pieces you've cut. As a result, I've never done it. We work primarily with cast acrylic which is less likely than extruded to craze during gluing. Crazing is also more likely to happen with the quick cure glue than the slow set glue. I've recently discovered that superglue actually works pretty well for certain gluing applications and it's not a solvent I don't think. Also, Naptha is an acrylic-safe adhesive solvent.

FYI, mirror is always on extruded acrylic.

[Mary Geitz]

Michael: Can you tell me a little more about Naptha a an adhesive? I am looking for a good adhesive for acrylic. I know about Well Bond, but I think it can be difficult to work with.
Thank you!

[Michael Reilly]

Naptha is a solvent for adhesives like labels and the paper backing. So if you have old acrylic where the paper doesn't want to come off easily, dab a naptha soaked rag on it and it will peel right off. But unlike solvents like Acetone that would do the same, Naptha won't attack the acrylic itself. We use it as a cleaner for adhesive, oils, etc. After that, we use an ammonia free windex or anti-static glass/plastic cleaner.

Adhesive wise, we've always used solvent adhesive for acrylic. It works by dissolving part of the acrylic and welding it together. There are typically two types... Weldon 3 and 4 are one, Acrylite Pure and Quickset are another brand. The one is a pure acrylic solvent that is non flammable, non carcinogenic, won't leave a white mark if dripped on the material, but slow setting. The other is basically the opposite. These are water thin, so they're typically dispensed via fine needle along the edge of two pieces touching and capillary action sucks it in underneath. Unfortunately, it rarely sucks it in evenly and the fast setting stuff in particular will close off the outer edge before you get the middle filled. Acrylite sells a product made by an employee that lowers the surface tension and causes it to go in more evenly. Because it's so thin, it's not gap filling. So even the slight striations from laser cutting can mean it doesn't make a smooth glue joint. They make a thickened version of the slow set called Weldon 16 or Pure-V, but in my experience it doesn't work very well. I'm willing to accept that I'm doing it wrong, but it never seems to setup hard. Acrylite has a UV reactive glue that is similarly thick and cures rock hard. Unfortunately it seems more likely to cause crazing, but it is otherwise great to use. They also offer 2-part epoxies that I haven't tried but are used in large aquarium fabrication.

Recently, I've been turned on to using superglue (cyanoacrylate) for acrylic. It doesn't seem highly likely to cause crazing. It comes in gap- and ultra gap-filling varieties, and they sell a related accelerator spray. It's great for making pieces that stand up on a base of acrylic. I spray the base with the accelerator and apply the thick glue to the bottom edge of the piece, set it in place and within seconds it's set. The only real downside is it's difficult to estimate glue coverage, so some joints may not fill out fully, while others have lots of squeeze-out. The solvent for superglue is acetone, so squeeze-out is generally not something you want to try to clean up. The other issue is if you have fingerprints or moisture on the sides of the acrylic when you glue it, the glue fumes will turn them white. That will clean off with a quick wipe of acetone but you have to be careful not to get it on the laser cut edge or it'll cause crazing.

It is possible to use a polyester tape around glue joints to mask the acrylic from the glue. In theory you can let the thick glue squeeze out and harden over the tape, then break it off before removing the tape. While it looks easy in the videos, it's a little harder in real life.

One of my biggest complaints in acrylic fabrication is that there aren't any classes. Companies that manufacture devices and accessories for them teach classes in sand blasting, screen printing, etc. But the acrylic manufacturers don't seem to do more than publish white papers on fabrication techniques. And there is a long way between reading, or even seeing it demonstrated in a quick youtube video, and actually refining the technique to one that is repeatable. I would totally invest in a week-long training class on the subject if I could. But instead, every company is left to find it on their own and as a result, they don't share their knowledge with others.