Cutting acrylic and have top/bottom burrs

[Brian Lamb]

I got back to working on some acrylic parts for some of my other product, basically a hair line pointer. I raised the acrylic sheet off the table using same angle aluminum and a ceiling light plastic grid and that solved a lot of the nasty edges and bounce back (I assume that's what caused it). Tried it with and without the plastic film on top, better without, haven't tried it without on the bottom yet.

I adjusted power so that the cut of the laser goes through the part, and then where it crosses the plastic grid underneath, I can see maybe .100" to .150" of further cutting. My big question is, I seem to get like a raised burr top and bottom on the part, it's not terrible and I can take a sharp box knife and scrape it along the edge and deburr it, but is there a better way to cut and get a nicer top and bottom edge?

Power settings, 80%, speed .8 and 1000 PPI, Universal Laser 50 watt. I'll try dropping the vector cut power to 75% tomorrow and see if I can get thru the parts still (.200" thick Acrylic from Lowes). How critical doe that become, or maybe I should say does the depth of cut vary? If I set it to just barely sever, will I run into situations where it might not cut through?

Here is a picture of the finished part, you can't really see the burr, the edges seem to be pretty smooth and clear looking.

IMG_1095.jpg

[Jerome Stanek]

I would say that the acrylic is extruded and not cast. I have had the same problem with extruded

[Brian Lamb]

Yes, it appears to be extruded, label doesn't say, but given it engraves and cuts clear and not frosty, I assumed it was extruded. So that is just the nature of the beast, so to speak? I don't need to make very many parts at this time, so I can deburr these, but would like to figure it out for future projects.

So, does the cast acrylic cut better? A frosty edge wouldn't hurt any, I used to machine the shape of this on my CNC mill, it's just a lot easier and faster to produce these on the laser.

As a relative newbie on laser cutting, given I started with the factory recommended 90% power and reduced power, would I have been better off to keep power the same, or at 100%, and increase the speed of movement from .8 to maybe 2? Would the cut edge improve or get worse? If this was cutting metal on the mill I could tell you exactly what would work better, but this laser work seems to be different.

[Kees Soeters]

Normally, when it is cast acryl it is mentioned on the protective sheet. If not, it is almost certainly extruded.. Cast Acrylic sheet is better in many ways: it works better and it is a bit stronger (IMHO)

Kees

[Glen Monaghan]

Cast acrylic cuts with a clear edge same as extruded; the frosted effect is only when engraving, not cutting. I think though that extruded has more tendency to flow when heated, with surface tension pulling sharp corners into rounded edges, so that may be why the edges feel raised. All the extruded acrylic I've had used plastic film for protection, while cast had paper. I would remove top and bottom plastic film before cutting because the film tends to get fuse into the cut, but I leave the paper in place.

[Rich Harman]

You can tell if it is cast or extruded by lighting a small piece on fire. If it drips flaming balls, it's extruded. If it burns and crackles, it's cast.

[Brian Lamb]

I played with my power, dropped it to 75% and it still cut through and left less of a burr. I will use this material for jigs and fixtures and find some cast to try next time I need the parts I was making. Thanks for all the answers, a lot to learn with this machine.

[Mike Null]

Brian
I'm quite sure that HD and Lowe's sell only extruded acrylic.

I would try cutting in two passes to see if that helps in achieving a smooth edge. One thing I keep handy for smoothing rough cuts is a mill file. It's mostly for laminates when the material isn't pefectly flat but it works well.

[Kev Williams]

Just to be a pill-- I've yet to achieve an acceptable (to me) cut edge on any acrylic thicker than 1/16" with a laser. In many cases, such as making my own 1/16" thick legend plates, I've long ago given up cutting them by laser in favor of an endmill. Endmills take about 50% longer to do the job, but there's no cleanup- After spending 30 minutes cutting plates in a laser, I spend at least that much time cleaning goo from the edges, and holes, and smooth-sanding a few serrated edges, and the backs... It's simply easier the machine them...

About a month ago I spend 2 hours trying to get make a single a 'go-nogo' gauge from 1/4" acrylic, spec was +/- .005", and it simply wasn't possible, due to burrs, cut angle, cut width etc... With any of my tool machines I can get that part to .0005" tolerance no problem.

Only suggestion I have is what Mike just said, only I'd take it to at least FIVE passes if a fairly high-precision result is necessary.

edit--- ALSO, are you using, or have you tried transfer tape if the acrylic you're cutting isn't paper-masked? Makes a big difference as to the surface edges...

[Brian Lamb]

Hi Kev,

I ran the previous batch of parts on the CNC mill, cut 1" wide strips 6" long, used a set of aluminum soft jaws and could get 3 pieces out of each 6" long strip. Used one tool an 1/8" 2 flute 90º point end mill so I could drill the holes, mill the profile, deburr the profile and scratch the hairline (with the spindle off and the tool oriented by hand). It was fast and of course very accurate, although as my mill warms up the Z depth of the scratch varied somewhat, leaving me with some of the hairlines not so "hairline".

Nothing is that close of tolerance, so I thought I'd try the laser this go around. Cutting back on the power makes the parts acceptable, not perfect, but form, fit and function is good. I did not try transfer tape, did remove the clear plastic film, and that seemed to leave 90% of the parts clean and usable. I have the occasional mess on the bottom of one when the laser flares up the plastic grid underneath. I will find/acquire a honeycomb cutting grid and hopefully that issue will be gone.

[Kev Williams]

I would try transfer tape on both sides, and go for at least 3 passes. The lighter the first pass, the better the cut edge. The transfer tape really helps keep the 'mushrooming' to a minimum. And until you get a honeycomb table or similar, I swear this works-- put just enough dish soap in a spray bottle of water to make the water 'wet' so it doesn't bead up, then mist the table before cutting...

[Brian Lamb]

I will give the transfer tape a shot. As a question, when you go from no tape to transfer tape, do you need to add power to get thru the transfer tape? The hair line portion of this part I had to gradually increase my power to get the depth of the line deep enough to hold red fill paint. Is there a rule of thumb that transfer tape is going to suck up 5% power or something like that?

Second portion... how would I get second and third passes over the same lines? For example, I don't want to recut the hairline three times, so how would I go about cutting the other colored lines multiple times?