PDA

View Full Version : 3/8" Cast Acrylic Sheet-How to vector cut?



Clyde Baumwell
01-23-2007, 8:31 PM
Anybody have any suggestions as to how I could vector cut 3/8" cast acrylic with my laser (30watt universal)? Is there a trick.

Gary Hair
01-23-2007, 8:42 PM
I don't know that your laser will effectively cut 3/8" thick acrylic. I have a 30 watt also and it is down to 1.4 speed at 100% power to cut 1/4". If you do try, use full power, start around .7 speed and use air-assist. I would draw a 1" square and try cutting it at various speeds until you make it through - keep moving it around, don't cut the same spot... If you don't have/use air you might not cut through it as easily.

Gary

Joe Pelonio
01-23-2007, 11:21 PM
I have 45 watts and 3/8" is the max I can cut, at speed 3. With 30, I'd try 2-3 passes, manually focus a little deeper each pass. It's not going to be the usual nice clean smooth edges.

Lee DeRaud
01-23-2007, 11:41 PM
I'm scratching my head...

The "normal" settings for acrylic for a 30W ULS combine very slow speed (<1%) with reduced (~50%) power, but that's for 3/16", not 3/8". I haven't tried anything thicker than that, but I'm under the impression that more power doesn't help much because the extra heat just rewelds the cut. (Uh, no, I don't have air-assist, thanks for asking. :p )

Time to try a double thickness, I guess...maybe tomorrow.

Mike Null
01-24-2007, 6:52 AM
Joe's on the mark.

Clyde Baumwell
01-24-2007, 11:05 AM
thanks all for your responses. i don't have air assist in place and the suggestions you made did not do the job. a lot of melting going on and not much cutting at all. I have to refigure the project-use thinner pieces and then fuse them. So the question is, what is the max thickness of cast acrylic should i order to vector cut without any difficulties such as melting or burning? thanks in advance

Joe Pelonio
01-24-2007, 12:00 PM
My first job this morning was replacing a damaged letter that was CNC cut from 1/2" acrylic. I cut two out of 1/4" and used acrylic cement to put them together, and since they are going to be painted will look fine.

In your case it's a matter of what you machine will cut smoothly, probably 1/4". You can do one layer 1/4" and the other 1/8".

Without air assist you will have problems, I'd suggest you get it or else stick to 1/8" acrylic. You might also try removing the protective paper (both sides) and replacing it with transfer tape. Then mist it with water just before cutting. That tends to reduce the edge melt.

Jerry Allen
01-24-2007, 1:01 PM
I have cut 3/8 extruded acrylic on my Merc 25 using .8%speed, 100% power, and 250 DPI, air assist. Using a low DPI will help with the melt. I probably focused down a click or two. The 2" lens focal cone is only good for about 1/4", but acrylic, as mentioned before by Rodney, acts somewhat as a waveguide. I wouldn't try it without air assist. Cut in the upper left hand corner of the table for sure. Don't expect a real straight edge. In my experience, cast acrylic cuts cleaner than extruded, but I haven't really tried cutting it at 3/8".
Don't walk off while cutting. There is a real potential for a flame here. Make sure your cutting table is free of combustible crud.

Rodne Gold
01-24-2007, 1:31 PM
I can cut 10mm on my 25 W machines (GCC)
anything thicker has more problems with beam divergence than power. Acrylic is actually a wave guide , theoretically you shouldnt even be able to cut thinner stuff without sloping edges and so forth , however the acrylic stops the beam diverging so much and you can indeed do thick.

Air assist is absolutely vital for acrylic cutting , you might kludge thin stuff without it , but it serves many functions.
Firstly it ejects the melt thru the kerf (the cut slot)
Secondly it cools the edges so you dont get a remelt and in cooling the edges , it also promotes a smoother edge (over cooling results in a frosty edge)
Thirdly It clears the melt from the cut , so the laser is always cutting "virgin" acrylic.
Fourthly , and pretty importantly , it stops fires.

Multipass cutting is a disaster , firstly the melt has nowhere to go and resolidifies and secondly you are cutting thru melt in the groove for the 2nd pass , you run the risk of registration problems and warpage of the material due to heat (which will almost guarantee that the 2nd or 3rd pass will be out of register)
Its not to say it cant be done , but its a real bad strategy.
One thing to look at when cutting acrylic or any thick stuff is the alighnment of the beam , if it hits the focus lens anywhere else but dead centre , the spot is going to be eliptical and will lose power density.
Jerry , dpi is not relevant when vector cutting , PPI is , is that what you meant?
In general Extruded "should" cut better than cast and in fact with your 25 w and a 4" lens , you can cut up to 12mm. The extruded is a weaker polymer and thus breaks down easier when cutting , its also normally much flatter than cast , cast thickness tolerances are +- 15-20% on a sheet!!!
Extruded is much more flammable than cast , it drips flaming molten balls, so as jerry says , never walk away when cutting acrylic, especially extruded.
You MUST be aware of one other issue , and that is cutting thick acrylic , extruded or cast , puts MAJOR heat stress into it , and cleaning it with any solvent after is almost guaranteed to lead to edge stress cracking.

Lee DeRaud
01-24-2007, 1:54 PM
Ok, here's a quick-and-dirty experiment, for whatever a single data point is worth:

ULS VL200, 25W, no air assist. Taped together two scraps of clear acrylic from Home Depot (probably extruded), transfer tape on bottom only, total thickness 0.41".

Started at 100% power, 1% speed, and dialed the speed down in stages: depth of cut seems to be pretty linear with speed (which is what you'd expect, but doesn't always happen).

Cuts nicely with 100% power, 0.3% speed, 1000PPI, clean edge, no flaming. I tried diddling the focus: setting it about 40% down into the slab (0.25") gave the most polished edge. The cut piece is tight in the hole, so I wouldn't try this with a delicate design, but I can't see why you'd be cutting something delicate out of 3/8" material anyway.

(A useful side effect: it welded the two pieces together as it cut.)

Extrapolating up to 30W and down to 0.375" thickness...looks like 100% power and 0.4% speed should work. I have absolutely no idea how much you would need to compensate for cast vs extruded and/or colored vs clear.

Jerry Allen
01-24-2007, 1:59 PM
Rodney,
When I set the DPI with PPI at Auto, I assume PPI is set the same.
My main point here was to get the power density down and reduce melt.

I am aware of your posts regarding DPI and PPI and I assume you know what you are talking about. I have received vague info from GCC telling me that both DPI and PPI affect cutting. Not much more than that. Also on GCC's site with some of their test examples, they specify DPI, perhaps for the same reason that Auto takes care of it.

I have cut a lot of gears for clocks using both extruded and cast. The cast seems to cut cleaner and straighter although the kerf seems a bit wider. It also seems to drop free better. But because of the fact that the tolerance is much worse across the faces of cast, there a lot of parts that I selectively cut using extruded to ensure that I get the fit I need.

Jerry Allen
01-24-2007, 2:12 PM
Lee,
I guess if you want a smoother edge higher PPI might be right, but I think the cut is more reliable and straight at a lower setting.
I never use any cover on the sheets because it either sticks or causes burn marks.
Two sheets might cause weird refraction or reflections at the interface.
The GE acrylic from Home Depot is definately extruded. It's about .220 for 1/4" nominal and I suspect that 3/8" cast is probably more like .320-.350.

Lee DeRaud
01-24-2007, 3:07 PM
Lee,
I guess if you want a smoother edge higher PPI might be right, but I think the cut is more reliable and straight at a lower setting.Possibly, but I've never had an issue with cut straightness using the 1000PPI setting. In any case, I suspect that for most things using 3/8" acrylic, edge quality is more important than edge accuracy. I suppose you could polish the edge afterwards, but that's not exactly a precise operation either.
I never use any cover on the sheets because it either sticks or causes burn marks.
Two sheets might cause weird refraction or reflections at the interface.Neither of those seemed to be a problem. I use transfer tape on the bottom to eliminate burn marks from the aluminum honeycomb ULS cutting table.
The GE acrylic from Home Depot is definately extruded. It's about .220 for 1/4" nominal and I suspect that 3/8" cast is probably more like .320-.350.It actually says .22" on the label, but it usually runs .205-.210, at least at my local HD.

Jerry Allen
01-25-2007, 10:24 AM
Lee,
.22, right. The point wass that acrylic is always smaller than nominal (why is that?). I always mic it and mark it when I get it. The stuff from HD is pretty consistent but I usually have to go through a bunch to get some without scratches. The Lucite from True Value is usually in nicer shape. But Delvies is the best for price and cast acrylic if you need it.
I don't use a honeycomb table. Mine is a roll pin on 1" centers and I rarely get any flashback. I used to leave the bottom protective cover on but got tired of picking off fused pieces of poly. On the few occasions I used paper, I got some char marks that stuck to the bottom of the acrylic.
The nice edge is due to melt. If that's the look you are going for it's worth it I guess. But the high heat density adds to the potential for sticking, burning, and sloped or rounded edges, especially at a very low speed. For a plaque that's probably fine. For gears and interlocking parts, not so. There is a little more faceting, but generally the edges are still smooth but more importantly, a lot straighter.

Rob Bosworth
01-25-2007, 11:06 AM
Hey C, you might be able to cut 3/8" acrylic with your 30 watt ULS system. If your system is set up perfectly, you will be able to cut the material. Make sure all of your optics are clean and in great shape. Make sure that your beam alignment runs down through the center of all of your optics. Set your focus 1/3 of the way down into the material you are trying to cut and make sure you are using a vector cutting table. Set your power at 100%, your speed at the minumum amount. I would set your PPI (pulses per inch of lineal travel) at 250 to 300. I would use O2 as an air asssist gas. Don't blow the lens out of your unit, but use enough O2 air flow to help blow the molten and vaporized acrylic out through the bottom of your cut. You should be able to cut through the 3/8" acrylic in one pass if all of the above is perfect.

Using O2 (oxygen) as an assist gas will enhance the "burn". IF you use compressed air, you arer using mostly nitrogen, which actually retards the "burning" process. Think of the use of the assist gas the same way you would use gasses in a cutting torch. When you hit the oxygen handle, it almost helps super heat the material you are cutting and blows it through the kerf.

If you get your first piece to cut through, look at the edges of the cut for smoothness. If you are getting too many striations (lines) on the edge of your cut part, try increasing your PPI to help smooth it out. The higher your PPI setting, the more consistent the laser power is through the life of the pulse. The lower your PPI setting, the higher the peak pulse power will be, but the power will drop off quicker towards the end of the pulse.

Rodne Gold
01-26-2007, 12:07 AM
Synrads site has a few good articles in terms of pex cutting
http://www.synrad.com/search_apps/application_briefs/43-1.htm
http://www.synrad.com/search_apps/application_briefs/3-1.htm

Also go here on their site for a lot more info on applications
http://www.synrad.com/newsletter/archives.htm