Need help with laser cutting acrylic

[mondrota]

Hi all and thank you for taking the time to read and hopefully respond :)

So, I've got a Chinese 100w co2 laser (although I'm now suspecting it to be an 80W marked as 100W). I'm using it to cut parts out of 10mm cast acrylic. I know extruded cuts better but I can't find it in black in 10mm thickness. Anyway, I've had to align mirrors, etc to get a decent cut, however, it's inconsistent. It seems like the edges melt instead of cutting cleanly in some places, the other areas cut ok. I've noticed it does this mostly at the top of the cutting bed so I began placing the acrylic sheet lower down and the cut was better, however, I then made a trial cut at the top of the bed again and the cut was good. I've double-checked the mirror alignment, the beam enters the head in the same spot, regardless of the head's position. The bed is also level in relation to the head. Please see the attached pic. (I was having trouble adding more photos). I've also just ordered an ammeter so I can monitor the power as it cuts. Any ideas or suggestions would be very much appreciated!

Thank you!

[Nickhofen]

Give us some more info.
Have you tried to put some adhesive paper tape on the bed and fire your laser on different spots to see how the beam comes out of the laser head?
Also try to have a ramp test, to find the sweet point of the beam?
Do you use air assist?
It is known that the 50wchinese lasers are 40w and the 100 to be 80w,.

[mondrota]

Hi, I've done the ramp test and set the z height accordingly. I use the air assist, I've tried it on full air flow and restricted but it didn't seem to make much difference. I'm using the compressor supplied with the laser.

I'm not sure about the paper tape on the bed, I think I'd only get a dot as it's focused at that point?

Thanks

[Nickhofen]

It seems that at the points that the material is melted, that the beam is focused at a different height than at the rest of the material.
What I mean is that the beam is focused higher or lower.
Did you try to cut something else? That melt happened at the same distance from XY 0 on the new material?

[mondrota]

I was suspecting this myself, but now I make sure the sheet surface is at the same distance from the head at every corner before cutting, but I still get the burning in some areas.

So far I've mostly cut acrylic.

And yes, the melting seems to happen in the same XY place, although when I tried to cut a small piece as a trial in that place, that turned out perfect. I noticed that there is a bed rail running across, below the honeycomb bed so I was wondering if that could be restricting air flow so the vapours can't escape from underneath the sheet? Could this affect the finish? But then, these rails run across the middle and the bottom of the bed too and that doesn't seem to affect the finish.

[Nickhofen]

Do you use air assist and you have also a fun duct to move the smoke out of the machine's working area ?

[mondrota]

Yes to both of these

[Nickhofen]

The melt is on the top of the material or at the bottom?

[mondrota]

At the bottom.

[Nickhofen]

Ok that's the problem, you have to raise the parts you cut away from bed, because the beam reflect on the bed and melt or burn the material you cut.
You have to buy or make something like that !
https://images.app.goo.gl/Rj8cF7ugAghHtAB68

Make a search on the web , you can buy the materials and make one on your own. Usually people use spikes like these.
https://www.amazon.com/Surepromise-Bullet-Leather-Clothes-Silver/dp/B00LAYPNIY?ref_=ws_cp_eb079558b77d06760d3f_p_2_i_p

[mondrota]

Thank you for that, I've been looking for bed ideas! Ive ordered some spikes. However I'm not sure this was the problem, because as I said, there is no melting in the middle of the bed where there is another rail across underneath the honeycomb mesh.

Anyway, I'll try this and will report any findings.
Thanks

[Nickhofen]

As I understand from the photo,the melt has happened on a curve, that means that the laser speed is slowed down at that point,in the middle of the bed is it a straight cut or a parabolic?
Anyway it is just a theory, but the spike bed will be a good upgrade.
Let us know what happens after you replace your laser bed.

[mondrota]

30424
30423

OK, I've made a spikey bed and after trying it out it seems to be about the same or maybe even worse. I'm currently placing the sheets on an old scrap sheet of acrylic that's had parts cut out of it. I've attached a couple of photos of the trial cut. What's weird is that one edge is not too bad but then the other edge adjacent to it is a lot worse.

Any more ideas / suggestions?
Thanks

[mondrota]

Thanks to all for your help. It turned out to be a combination of different issues. I've since upgraded the tube to 120w so I don't have to push it so hard, also stiffened up the Y axis (got rid of the plastic wheel that was on one side and replaced with a linear bearing), realigned and cleaned all the mirrors, realigned the z bed and now the cut quality is much improved.
Oh, and I also focus further down into the material too.

Many thanks 👍