I have been cutting acrylic, raised on pins ( approx 10mm ) above the honeycomb bed, to stop laser bounce back. This works, however, when the pieces drop through the sheet, they often dont drop straight down and move to one side. This means that the next piece cut often marks the first fallen piece. Any tips…other than leave more space between pieces ( which wastes material) Thx
If I have to process cuts very close to each other I will place a support under the centre of each piece to be cut so they don’t drop at all. If you use absolute coordinates, you can use the laser to guide you where to put the supports by clicking in the right place and asking the laser head to go there.
Hope that helps,
David
Excellent tip, David.
Another approach is to reduce the height of the supports. I’ve had similar problems with other materials and the “internal support method” proposed by DavidF is a good one, but when the parts are too small, all that’s left is to lower the work.
I’ve not tried it,but having the work a few mm above a sacrificial material such as MDF will have your cuts drop straight down and the MDF should absorb the excess laser power. Consider to spread (and let dry) dish detergent and water over the underside of the acrylic. It washes off easily and takes the smoke with it. I use the same magic potion on the top as needed.
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Thanks David…Agreed, great tip.
Hi Fred,
Some great tips there…thanks On a separate note…do you happen to know if you can buy sacrificial honeycomb? If so, have you tried it? Thanks
Sacrificial honeycomb made of what substance? I’ve purchased thin, fairly fragile aluminum honeycomb from eBay, but not to use in the manner being discussed. It arrived in a “compressed” form, requiring the edges to be pulled apart very carefully to create the shape. I suspect it would create back-flash in the same manner as the bed honeycomb, though.
It’s interesting to ponder cutting MDF in such a pattern, leaving only the barest minimum walls. One would not be restricted to a diamond shape. It could be squares, circles, Escher fishies or birdies!
I was thinking there may have been a material that I could sit on top of my existing honeycomb, that stopped bounce back, & didnt burn too much. That said, I think your idea in using MDF might be worth a try. I’ll experiment. Thx
Water?
What happens if you support the acrylic above a shallow tray of water and let the parts drop in?
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Interesting…would require an experiment…but not such a crazy idea. Thx
Before I had my own laser, I remember they guy I hired would put down a sacrificial layer of Coroplast (plastic-like material used for yard signs). The corrugated material prevented bounce-back. With that said, I don’t know if that is safe to laser but might be worth your time to investigate its MSDS.
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I have two ways to deal with that:
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Use fewer supports. Normally, I only supportmy acrylic sheet using 4 or 5 knife rails on my 28" bed. So, the recently cut pieces fall free into a wooden basket I installed 5" below the cutting bed. The piece won’t get damaged, and the laser won’t mark the pieces so down below.
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If you can not let them fall below, cut the pieces in batches. I don’t know the proper word in English, but if you have to cut 8 pieces in a row, cut number 1 3 5 7, collect your pieces and then send pieces 2 4 6 8.
I use both ways with excelent results.
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I’ve laser cut Coroplast™ with reasonably good results. It’s polypropylene and mostly melts, but if one is doing non-precision work, it cuts. Lower power settings also provide for quite good score lines. I’ll have to keep this in mind as a sacrificial undersurface.
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I had never thought of using Coroplast as a base material. After a bit of a rabbit hole, I came across a Laser Processing Guide for Plastics (PDF of PowerPoint presentation).
The entire guide is excellent, but the best part is a printable guide (table) that lists detailed characteristics of how each type plastic will cut/engrave along with any safety concerns (i.e. PVC, etc.). These printable guides/tables start on page 8. 
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Ive found that when the honeycomb is dirty from soot or just other deposited vaporised materials, the flashback is much greater as there is more gas to ignited under it. One thing to try is to direct your incoming air that pulls out your fumes across the material. Most of these chinese machines have horrible venting, no thought or testing went into them, other than the one they copied had vents too.
We cut out all of the grating on our thunder machine so there was no longer a restriction, and then on the front panel, Ive used sheet metal tools to bend and open the airflow up a bit front the front of the machine. Its not perfect, but it helps. Next step is to just cut those entirely out and bolt on my own front panel that we can test airflow and make sure that we can get the same volume of air in as the fan can pull out. (and we keep the blower near the exit of the system, so it keeps the pressure in the big hose negative, instead of spraying small leaks into the shop)
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Take a look at 304 stainless woven sheets and the like.
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Thanks Tim, I’ll check it out
Depending on the size of your pieces, you can also use tabs. I use somewhere around 0.08", but its worth experimenting with your application. Pick something small enough so the pieces punch out but large enough that they support themselves. I have one thing I make that uses the entire sheet of material, and without tabs, the pieces would completely collapse under their own weight partway through cutting.
I’ve been watching Trotec’s videos and one suggestion is to use 3-4 layers of wet paper towels. Place the acrylic directly on that so falling parts are no longer an issue. I’ve only tested it once and plan on doing it again. Youtube video https://youtu.be/avYzp4YEpyI?t=2701 " Trotec Third Thursday: Laser Processing Acrylic". Paper towel tip starts at 45min in.
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Might watch this video about honeycomb beds. I got one with my machine as a stock part.
If you want to stop bounce back, put something under there that will absorb it, such as another layer of acrylic and gap for air flow under the work piece. No bounce back. Sacrifice some acrylic over time, but eliminates reflections.
I put small 3mm spacers when cutting 4mm stock can’t fall too far out of line 