Unhappy with my engraving results on cast acrylic! What can I do to improve?

[MjB (Martin B)]

So, I have been ‘playing’ around for a few months now, trying to sort this issue out. I am unhappy with my results when engraving cast clear acrylic from the reverse side.

I wish to make a door number to mount on the wall.

I have tried a lot, and I seem to get one issue or another every time.

Keeping the protective film on - the film itself seems to contaminate the engraving. When I remove the film, I get a fog/halo effect around the engraved text that I can’t remove. I have tried neat washing up liquid but to no effect. I’ve tried tape but ended up with the tape being burnt into the lettering.

I’m using a big Chinese 1390 100 watt laser. If I fire at less than 20%, the tube is not happy. The maximum output has been tested and reads 129 watts. I have a Ruida 6332G control system with a live focus head controlled by RUIDA LFS-ANM-T43-V2 (Though this is switched off)

Most of the engraving has been done with a speed of around 200mm/s and power at 20%; DPI is about 300.- Though over the months, I’ve tried most speeds and power settings.

I use a 63.5mm 20mm diameter lens and 25mm mirrors. The spot size is extremely small and very clean/circular. (See photo below. difficult to photograph)

I have had a perfect result, but that was with air off entirely and no film. But I still get the odd mark, and I need this to be consistent. I am worried about contaminating the lens?

I have two compressors; the smallest is a silent ‘nail bar’ compressor and the air delivered is just right to offer some positive pressure out of the nozzle. I’m at a loss; such a small amount of air can interfere with cast acrylic engraving quality.

Any suggestions as to what I am doing wrong?
VIDEO link at bottom of page - if you are interested to see the laser working.

This is the result I am after the lettering is solid and white - but I must keep air off and film removed. Is this correct?

With the film and air assist on the quality of engraving is very poor.

The Dot size is small - the black dot you see above is the hole made through 4mm ply when the beam is pulsed at 20%.

Video of laser at work

The above link shows the laser at work on clear acrylic.

[jkwilborn (Jack Wilborn)]

@MjB

100 watt laser … output has been tested and reads 129 watts

This is questionable. Generally these Chinese tubes don’t even make the ‘advertised’ power. Mine is a 50 watt and measures 44. Really curious how this is happening. A 130 watt tube is physically bigger than a 100 watt tube.

My tube is happy down to just below 10%.

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And the video link chokes with some kind of error…

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You minimum output is 20% of 130 watts or 26 watts, pretty hot for acrylic.

I think you are vaporizing so much acrylic it’s condensing on the surface.

I would say what you are seeing is the vaporized acrylic condensing on the surface. Of course it will ‘bind’, leaving the type of marks you are seeing. Same kind of condensation you get on the metal near the engraving/cutting area on all materials. It just condenses out.

With acrylic, I will run about 200mm/s@20% (8.8 watts). About 1/3 or the minimum power you can run.

Don’t know how fast your lps is but you could push the speed and see if that would give you a better results. You’d probably outrun the lps, but that’s your only real power control.

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@MjB

spot size is extremely small

I would expect it to be less than the width of the lines on your ruler. Somewhere around the 0.1mm range. You should to be able to put about 10 of them in 1mm gap. Looks like you can get 2 with that dot size. Making it about 0.5mm at best.

You definition of very small is not equal to mine, I guess. However I doubt that’s the problem.

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I run a very small amount of air on acrylic, even when I cut it. One of the materials you don’t want higher pressure for a cut.

I haven’t eliminated it but you have to control it as best you can. I’ve tried leaving the protective layer, tape … doesn’t seem to work the best. Of them all, the blue painters tape was OK. If I had to suggest something like a coating that would be it.

One of the drawbacks of a high powered dc excited laser is that the ‘low end power’ control isn’t there for images or graphics.

Good luck

[MjB (Martin B)]

Hello Jack,

Sorry for the delay in responding - I’ve been unwell.

I don’t know if my hole size is the lens I am using. 0.1mm would be fantastic! I think you are correct in saying my laser is producing a larger hole at 0.5mm

I have now fixed the issue (sort of). It all comes down to the nozzle. Perhaps my nozzle was designed for cutting. It’s less than 3mm to the material surface. So I removed it and replaced it with a piece of gaffa tape. I pinged a pulse hole through the tape, and now I get air through and a perfect engraving.

I will need to find a replacement metal nozzle 20mm in diameter with 0.8 pitch - but I am finding that a challenge.