Profiling clear acrylic with clean edges

[Drew Montell]

Hello all! I'm new here, and recently purchased a new Trotec 80W C02 Speedy 400. So far, the machine is great! However, I am trying to determine a repeatable way to setup laser cutting a bunch of small parts of mirror backed clear acrylic with clean edges. I have been successful with some small runs (4"x4" sheets), but I noticed some warping of the <=.125" thick material the more pieces I cut out of a single blank. I would like to increase the initial blank size to about 12"x12" and I have been told that lifting the part off of the tooling plate helps to keep the edges "clear" or clean." However, I think that if I used something like a multimat to hold the entire piece down when cutting, I could get the best yield with minimal warping. The only problem is, the multimat would be directly behind the acrylic and I am afraid of not getting clean edges.

So, before I purchase a multimat, or make a jig to hold the material as flat as possible to "float" above the table, I was wondering if any of you have some advice. I will be cutting these sheets all of the time, with thousands of parts coming off of the machine.

Thank you in advance, and I'm glad to finally post something!

[Mike Null]

You should have a cutting grid. If you cut with your material on the table the back will be a mess.

[Keith Upton]

I have a honeycomb "vector table" and when I cut .125 acrylic on it, the back is still a mess. Leaving the paper masking on helps. I've had to raise mine up quite a bit to keep from getting flash back. The problem with that was unless I had several pins under each part, they would fall down and get under the next part to be cut... messing them up as the laser burned into them as it cut the next part. I recently bought two 12x12 tiles at Lowes and have been cutting directly on top of those. So far the cuts have been pretty good. I was afraid that the trapped heat would still mar the back of the acrylic, so far so good. I'm going to try drilling a bunch of holes into the tiles to help dissipate the heat and allow the vacuum from the exhaust pull the acrylic sheet down some.

[Drew Montell]

I recently bought two 12x12 tiles at Lowes and have been cutting directly on top of those. So far the cuts have been pretty good.

Thanks for the feedback! Keith, the tiles sound like an interesting idea. Can you specificy the actual color/material of the tile? Also, how are you keeping the 12x12 sheet flat as you cut the parts out (the heat as more parts get cutout tends to warp my workpiece)? I am trying to avoid taping the back side, if possible.

I too have had issues with things falling through the honeycomb. My finished parts are about .25" x .25"

One method I have tried before is making an open topped "box" and clamping the the outside edges of the workpiece to the top of the box. This way, the workpiece "floats" to avoid flashback, and the parts (if they fall) fall into the box. I still have issues with the laser hitting fallen parts when this happens. I also still hvae SOME issue with warping of the workpiece due to the large center peice not having any support.

The bed of nails approach seems reasonable too, however, i am not sure that the part will actually remain flat on the bed.

[Keith Upton]

My parts are not near as small as yours, so a dedicated pin table is what I will eventually move to, but until I have time to build that, the tiles should work. The tiles I got look something like these, but I don't know if they are the same ones. I just looked for some that had a flat finish to avoid flashback. I also thought they might look cool engraved if they did not work out for cutting acrylic on. lol

Your box idea might actually have potintual depending on how deep your Z axis is and what you make it out of. What if you made a deep frame that went around the outside edge of your material (you might need a small brace in the center where you do not cut for support) and in the center, you had a "lid" that slopped down to an open slot. Here's a crude top view drawing if it helps...


box.png
That would allow the small parts to drop down and slide under the "lid" and out of sight of the laser beam. You might need an angled peice just below the slot to get the parts to slide back behind the lid covers a well. Something like that should not be hard to make and might solve all your problems. Make it out of something that will stand up to laser for a while, and then just replace it when needed (or make it out of something your laser can't cut at all if you have the skills/tools).

Rereading this before posting, it might be easy to just have the slot on the bottom and have the parts slide down to it and then down another slope going back behind the top slope.

[Drew Montell]

GREAT IDEA about the parts sliding out of the way! Thank you!

[Keith Upton]

Your welcome, it might even work for me! Not sure I can get enough angle to have my larger parts to slide out of the way. Might have to have a go a making this using a cut up Teflon coated cookie sheet or something.

Now, does anyone know if the laser will etch and mess up the Teflon over time?

[Glen Monaghan]

maybe you are elevating too much. My first pin table provided a 1" elevation and pieces that fell tended to bounce around and get hit by the beam cutting nearby pieces, ruining them. Sometimes, a part would hang up on a pin and land on an angle that left one side sticking up high enough to snag the air assist tube and knock things around. Now when I cut acrylic (which is never warped so no need to flatten it like often needed with ply) by placing a few 1/8" thick ply or acrylic spacer strips around the perimeter and across the middle of the honeycomb (in places where I won't be cutting), and setting the sheet on top of those spacers. The honeycomb provides good air flow and exhaust underneath the sheet, and the 1/8" spacers keep the back side of the parts clean (no flashback or heating effect from the honeycomb), and dropping parts fall straight down onto the honeycomb without bouncing into the beam's path or getting hung up on pins.

[Keith Upton]

Glen, I tried doing this with only about an inch of elevation and I still got quite a bit of flash back from my honeycomb. I wonder how much the material it is made of effects this... or how stained it is from use?

[David Somers]

Keith,

Teflon heated above 500 gives off compounds that causes a flu like set of symptoms. Not long lasting or permanently damaging. But not fun either. Check the MSDS. Needs very good ventilation if it is heated to 500 F and above.

Dave

[Keith Upton]

Thanks Dave, I was going to pull the MSDS later when I had time and was getting closer to trying this out.

[Glen Monaghan]

Good question, I'm not having problems with flashback with 1/8" spacers.

[Scott Shepherd]

Glen, I tried doing this with only about an inch of elevation and I still got quite a bit of flash back from my honeycomb. I wonder how much the material it is made of effects this... or how stained it is from use?

Excessive flashback on the edges is normally caused by way too much power. The power should barely be getting through the material. Too much power gives a lot of energy to flashback. Try dialing your power right to the point where it's barely getting through.

[Mike Null]

Steve is absolutely right. I cut thousands of nametags and I often don't even have to clean the backs (probably 50%) using the correct power and speed settings and a plain old aluminum grid raised off the table with strips of plywood. very sophisticated

[Drew Montell]

Here is what I came up with. I am cutting it all out of acrylic today, and testing it out! The blue is the work piece (6"x6" gets me 250 parts!), clamped down by magnets on the base plate and top frame, the teal is the slide...I'll probably use polished aluminum or the tile idea. I'll keep you all posted, and thanks again for the help!

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