Best material to lay under thin stock to absorb the beam?

[Bert McMahan]

I need to cut very thin plastic on a semi-regular basis, and it needs to be clamped down to a flat surface. It comes in rolls and holds the curve quite a bit; my usual "use some weights" trick doesn't work.

I used MDF to cut some last night and it worked fine for cutting, but the MDF "goo" deposited onto the plastic and required a lot of cleaning to get it out of the nooks and crannies. I need something that'll absorb the beam with little or no outgassing.

My best guesses are glass or anodized aluminum. Any thoughts on those? Glass will absorb it but I worry about thermal cracking if I use it a bunch. Theoretically not a ton of beam is making it through the material but there will always be a small amount.

[Gary Hair]

anodized aluminum will work really well, glass probably not as well - cracking, as you mentioned, would be a concern of mine.

edit: if the design isn't too terribly intricate you could use transfer tape to reduce, or eliminate, the goo.

[Kev Williams]

How are you holding it down if not with weights?
How thin is thin?
How large a piece are you cutting?

Depending on the answers, what will work great is water...
I cut this thin mylar while it was floating on water in a bake pan...
381464381465

You don't necessarily need to float it, if you use an aluminum plate as a 'backstop', try spraying the plate down with water as a buffer -- add a titch of dishsoap to the water to break its surface tension, makes it more wet... A soaked paper towel might work, but the paper burn may leave deposits, but maybe not, just depends!

[Bert McMahan]

Right now I'm holding it down with a piece of MDF with windows around the area to cut. The MDF is screwed to another backer piece of MDF. The laser cuts through the window.

Kev I love the water idea! Unfortunately this film is about 0.020" thick and super curly, so it needs a positive "lock" to hold it down to something. A wet paper towel behind it might work wonders though, I hadn't thought of that.

The parts themselves are very small- think "fingernail" sized.

I'm going to try screwing my "frame" down over an aluminum panel next. I found a scrap one sitting around that should work pretty well.

[Matt McCoy]

There's a sticky self-healing mat that some use:

https://www.johnsonplastics.com/tapes-fastening-systems/table-mat/rowmark-fat-mat-12-x-12-engraving-table-hold-down-mat

[Dave Sheldrake]

sandblasted aluminium sheet, works flawlessly, just blast with a 40 to 60 grit

[John Lifer]

I'm assuming you have a solid table? I've a honeycomb and with the exception of acrylic, I've not had anything flash back and cause issue. I cut out a bunch of plywood hold down fingers for curly stuff. Magnets where I could use them also.... Water is intriguing.

[Ian Stewart-Koster]

That rubber mat that is naturally sticky, and made for lasering on, is great.
The piece I have is 24" x 12", and was worth about $200 some years ago.
But you can put a piece of paper on it, and laser away.

Another way it to use aerosol spray glue, on shiny cardboard. That'll hold a floppy flimsy substrate, but might burn itself.

[Bert McMahan]

Thanks for the help all, the anodized aluminum worked great. I couldn't just use the honeycomb or a solid table as I needed to drive screws into it to hold down the plastic with my "stencil clamp". Here's a pic I took with the MDF backer:

https://i.imgur.com/KYmWra3.jpg

(It's not actually that bowed, I think I jiggled my camera or something when I took the pic!)

The stencil trick worked fine but the MDF was flashing back, so adding the aluminum sheet in there soaked the beam after the plastic just fine. You can see in the pic above the black spots along the cut lines; that was MDF soot/goo flashing back onto the plastic. Adding an aluminum layer totally fixed the problem but I didn't think to take a pic of that one.

[Kev Williams]

typical lens fisheye effect ;)

Those screws you're using are why I had one of my customers make me up a bunch of these steel bars for weight and shim use- I have a dozen of each, the big ones are 3/4" thick x 1-3/4" wide x 12" long, the small ones are 1/2" x 1" x 12" long, they were cut from (I guess) "premium" bar stock, because they're measurements are dead on- 4 of the large ones add up to exactly 7" wide, very accurate for spacer use... They're heavy enough to eliminate the need for screws in most cases... Right now I'm doing Hydroflasks and SIC cups-tapered- in the GCC. To account for the cup angle I raise the rotary tailstock 2", and I put 2ea of the large bars at both ends to keep it in place- Done hundreds of cups, and in normal use the rotary doesn't budge...

Of course, in use the lens needs to be able to pass over them, but you're not using a cone and I'd bet a pair of my big bars would hold that fixture down just fine with about 1/4" lens clearance...
:)
381577

[Bert McMahan]

I've used similar "heavy things sitting around" in the past- I wish I had some bars like that!

I actually had to use a special purpose jig on this one, as holding around the edges alone didn't work. Initial attempts showed that the plastic expanded around the cut area slightly, and bulged out the middle of the plastic, getting me all kinds of trouble.

I do have a replacement head and gantry that's going on the laser "one of these days" but I need to find some free time first ;)