Problem with cutting cast acrylic

[joewino]

I'm in the process of cutting some letters out of 3/8" black cast acrylic. Since I don't have a vacuum hold down, I'm using double stick carpet tape. Also, the plastic still has the protective film on both sides.

When I profile a letter the cut is good but the adhesive from the mask and/or carpet tape is sticking to the sides of the letter and turning the edge a different color (gray). I can't get it off with acetone or any such solvent and even trying to sand it is useless.

I've tried several different bits and some are better than others but none will give me a good, clean edge on the letter.

Any ideas?

[Brady Watson]

I can't get it off with acetone or any such solvent

Any ideas?

Acetone is a no-no on acrylics. You can use denatured or rubbing alky with relative safety.

My advice would be to remove the protective layers & take care when handling it. Typically, the quality acrylics have paper protective layers - not plastic cling-type.

Black plastics have a lot of rubber in them & cut differently than the clears.

What are you using for a cutter? Diameter? - How many flutes? - O-flute, straight etc???


-B

[joe]

Greetings Raymond,

Have you tried a Belin combined with bridges?

J.

[Mayo]

I would try some of the new orange citrus based products to get the adhesive off.

When I have used carpet tape, I always made sure to use the kind with the fiberglass mesh in it. This allows for easier removal. The kind without the mesh in it is a real pain to remove because it breaks into small pieces.

Another option for hold down would be 3M Super 77 spray if you haven't already tried it. It won't be as thick as the carpet tape and depending on your application of it - one surface only (acrylic) or both surfaces (acrylic and spoilboard) it should stick tight enough to hold the letters but this can depend on the size of the letters and your cutting speed. The spray glue removes with mineral spirits or citrus based adhesive removers.

As always, your mileage may vary - test first!

[bleeth]

Ray:
Toluene will remove any tape adhesive residue without harming Acrylic.
My favorite bit for Acrylic now is a single "O" straight flute bit 1/4"D. Just got a couple from our friends at Centurian and they cut better than anything else I have used. I went all the way through 1/2" except for a skin in first pass and then cut the skin. The parts (many were fairly small) didn't budge at the end and the sides were smooth.

Dave

[sailfl]

Raymond,

I cut 1/8" and 3/16" Acrylic all the time. I don't have a vaccum hold down system either. I use a 1/8" bit to cut most of the Acrylic. I don't use my dust collection and I don't use any thing to hold the material down except 3 claps at the edge of the table. I use an upcut sprial O bit. I cut at 100 IPM. I have to watch that the material does not raise up and I use a stick to push it down.

I am currently cutting 34 sheets for one of my clients.

Hope that helps.

[jerry_stanek]

I have used OOPs or Goof off to clean the carpet tape residue off before.

[joewino]

Thanks for the advice.

Joe - I was using a Belin bit.

I discovered what I was doing wrong. My feed rate was too slow and it was heating up the bit and allowing the chips to mix with the adhesive and weld back to the edge of the letter.

When I increased my feed rate it cut even smoother and cooler but there was still some adhesive along the edge. It was easily removed with Rapid Remover, which is a citrus based product that us sign guys use to remove the adhesive left over when removing vinyl lettering.

These letters were about 4" tall, so trying to cut them without tape was not very practical. Of course, I could have used tabs but then trying to remove the tabs from hundreds of little letters would have been very time consuming.

So, I learned a little. It costs me a bit of money in time and materials, but then any education has a cost associated with it.

[ron_churches]

Hi,

I have a potential job for a local University that requires v-carving text in some green cast acrylic. I've seen a few posts that address profile cutting of acrylic but not v-carving. They want to use these as divider panels (framed in oak) in an auditorium. First question I have is; is it possible to do this on my PRT and achieve a good finish in the v-carve areas? The characters are 5.5" tall and I intend to use .25 or .375 thick acrylic. The 3/8" x 4' x8' variety goes for $320 a sheet and I am hoping to make this my first "mistake free" project. How deeply can I carve in this acrylic before I ruin the sheet and watch the profit disappear? I recently ordered a 2" 120deg V-bit from Beckwith Decor, will it work well on acrylic? .If so, what are safe feeds/speeds? Partworks shows the full v depth to be .215 leaving .16 before the bit goes through. Any advice would be greatly appreciated.

Ron

[bleeth]

Gary (Beckwith) will be happy to chime in or respond to this question sent directly to him. I believe he has a bit of experience with Acrylic. (LOL)
IMHO I would run the program twice with the second run being a clean up. Make one program a 1/16 less deep than you want and the second the correct depth. Probably keep feed and speed around 1 ips and 8-10k. Do a sample in mdf to check out your toolpath and depth settings before you cut the real thing (Very important here!!). I would zero to top so I know what I do to the acrylic depthwise is the same as the sample. Seems to me I would want to leave a good 1/8" of material. Your PRT will do the job but you didn't mention whether you have a spindle or router.

[Brady Watson]

You'll need a 'tight & tuned' PRT to pull it off, but it will not be as clean as a PRT or PRS Alpha with 7.2:1 motors. The carving shown here was carved on my 1st PRT with original 3G control box. It is 'Blue Edge' cast acrylic - same as green edge, but 'special order' blue.

http://www.ibild.com/images/Monza_Acrylic2.jpg

You are not going to get away from white powdery looking marks in the crotch of the Vee when vcarving. Slow & smooth is the name of the game here - like 1,0.5 IPS - depending on how deep & intricate you are going. Air cooling with a simple football inflator* and 25 psi of compressed air will help, but you should think about wet sanding, buffing or polishing to get a clear finish at the center of the v-groove.

You will not likely see much of a difference between a bargain and high-end V-tool. The tips are identical on either & they both drag the material at the point since there is no flute there to carry the chips away. This is why you get the white marks...

You must eliminate as many sources of slop & vibration as possible from the machine. It would not be a bad idea to replace your pinion gears and re-grease them. Adjust all eccentric v-roller bearings as needed. Any vibration or slop will get directly applied to the acrylic and 'telegraph' any dirt/chips on the rails, cogging from worn pinions or oscillation from a worn t-rail etc.

You should really get a hunk of scrap from your plastic supplier and experiment with v-carving it. Most shops have off cuts that you can buy for peanuts to practice with.

-B


*(correct spelling...spellcheck prefers 'inflater') :confused:

[ron_churches]

Dave, I have a 4 hp HSD spindle. BUT, unfortunately, it's still in the box waiting until I can afford an electrician to come hook up my power (3 phase). I am currently running the 3.25 hp P.C. router, are the speeds you mentioned for use with a spindle?

[joe]

Boy O Boy, this is critical work. I'd be inclined to send this job to my competition.

Be careful.

Joe Crumley

[bleeth]

Yes they are for a spindle. I would expect that it would be similar for your router but I don't know for sure as the router loses torque as you slow it down. You probably should pick up a piece of clear cast for sample attempts. It's much less than the colors. Brady's air suggestion is good and although I never tried using oil lube on acrylic with the cnc I used to use it all the time for wetsanding edges and it helped immensely better than water there.

[jerry_stanek]

You can use dish detergent as a lube on acrylic. How big are the panels going to be. If they are not full sheets you may be able to use the drops to test different bits and depths. Have you tried a ball nose endmill and just create a tool path