Vector Cutting Acrylic, revisited

[Barb Macdonald]

Hi all
I've got a epilog legend ext 60 watt, and I've been mostly happy with it's performance. I wish I'd found this forum before I bought the darn thing, though. I've been vector-cutting 0.25" cast clear and black acrylic rectangles.
I remove the premask, and use my own high-tack sign premask, because I've found it reduces soot/blackening/flaming. I am getting a nice shiny edge, but I still get chipping on the back side of the pieces. I've been focusing to 0.125" (due to a post I read on this forum, it made sense to me), and then using vector settings between the .125 & .25" settings supplied by epilog. 12 speed, 100 power, 5000 frequency, with air assist on, on a vector grid. Is there anything else I could do? I haven't used spacers, between the plex and the vector grid, would that help? I'll be experimenting again, tomorrow, I really thought you couldn't get around the lack of perfection in the edges.
I have a customer with 100+ pieces I've done, and she aint' happy:mad: as they need to back-paint them, and the paint is showing off the chip marks like crazy.
Can I achieve a "perfect" like flame-polished edge with this laser?
Thanks to everyone, and could the moderator please PM me, as to how I can donate?? I'd much rather do it over t'phone, I don't like giving credit info on the web. I know, I know, I just don't wanna!

Thanks again
Barb

[Nancy Laird]

Barb. I'm not a moderator, but if you'll go to this thread: http://www.sawmillcreek.org/donate.php, you'll get the address to send a check for a donation to the Creek.

As for your acrylic cutting problem, I just finished cutting pieces for about 20 acrylic clocks with no chipping or flaming whatsoever. We are cutting on a 40W ULS, focusing to 1.5" with our 1.5" lens, using air assist, and a lower speed than you are using, on the vector grid. PERFECT shiny edges, no chipping, just beautiful cutting (I'm at my day job and don't recall exactly what powers and speeds). Your problem may be that you are removing the premask, as the acrylic people recommend that any cutting (saw or laser) be done with the masking on. Try it without removing the premask and see how it turns out. Also, slow your speed down - I think we are cutting at about 1.5-2% and getting wonderful results.

Nancy (130 days)

[Kim Vellore]

Just yesterday I was trying out Delrin and one thing I did for a similar problem was I went to low frequency 800 and lowest speed then slowly increased the frequency till I was able to cut easily and clean with one pass. The higher frequency increases the temperature, causes re-sticking and soot marks, It might work for Acrylic too, experiment with a spare. You might want to start with 1% speed and increase the frequency.

Kim

[Lee DeRaud]

I remove the premask, and use my own high-tack sign premask, because I've found it reduces soot/blackening/flaming. I am getting a nice shiny edge, but I still get chipping on the back side of the pieces. I've been focusing to 0.125" (due to a post I read on this forum, it made sense to me), and then using vector settings between the .125 & .25" settings supplied by epilog. 12 speed, 100 power, 5000 frequency, with air assist on, on a vector grid. Is there anything else I could do? I haven't used spacers, between the plex and the vector grid, would that help?Check the spacing on the "chips": if they match up with the vector grid, they're due to beam bounce-back from the grid and the spacers will definitely help. (I'm not familiar with the Epilog grid...on the ULS grid the honeycomb is oriented diagonally, so any bounce-back marks look somewhat random even though they really not.)

The subsurface focusing doesn't hurt but it really isn't necessary on material that thin.

[Joe Pelonio]

I too get clean cuts with 45 watts, set at speed 8 power 100% freq 5000. i hope your tube is not going!

Mine did that when my tube last went. I gradually had to keep slowing down the speed until I could no longer cut 1/4" and got a new tube. I suppose it could also be a loss of power due to other things like power supply, mirror or lense problems.

[Eric Allen]

I'm with Lee. I've had all kinds of little things like that on my acrylic initially, especially my scrap that I got free with no mask. It's not chipping so much as the vector grid bouncing the laser back into the plastic. I noticed it happening during my cutting, little flare ups every time it crossed a piece of grid material, so I space all that material now. It really does solve the issue, just have to make sure your spacers aren't anywhere they will be hit and cause the same problem.

[Mike Null]

As I believe Nancy has suggested, I'd go back to focusing to the surface.

If that doesn't work to your satisfaction try two passses.

[Joe Pelonio]

If I can cut 1/4" in one pass on my 45 watt, and Nancy can on her 40 watt, the certainly something's wrong if Barb can't do it on a 60. I have a stack of acrylic 3' high in the storeroom for recycling, most all of it 1/4 with bits and pieces cut out of them. The only flare I get from cutting directly on the honeycomb is the X pattern in burn marks here and there on the backing paper. the cuts are clean through and the edges glossy, no chipping.

A closeup of the "chipping would help, so we can see if it's really chipping from not cutting all the way but instead breaking out on the bottom, or a flareup issue. Maybe it's something something like a leak in the air assist, or dirty lenses/mirrors, even a bad autofocus mechanism if not doing manual.

[Scott Shepherd]

I noticed when trying to cut 1/2" extruded, it worked okay, but 1/2 cast chipped out a lot. It's like it didn't burn all the way through, but just barely. If I slowed it down 1% more, then it toasted the backside. I determined after much trial and error that I couldn't burn 1/2" cast with 45 watts. Then I did some 3/8" cast last week with a speed around 6% and it chipped out as well. It clearly wasn't going deep enough.

My suggestion is, as others have said, slow it down some more. Also, one trick that's been working well for me is to soak the masking (paper backed) with water. I just mist it and let it sit for 5 minutes or so and rub it in my with hand until the masking is soaked (backside masking only), then I burn it and it seems to greatly reduce flare ups as well as reducing the backside getting heat damage.

Oh, one more thing- all my acrylic is cut spaced off the table. I have 1/2 dozen decks of playing cards still in the wrappers and I just drop them on the table and it's spaced up almost 5/8" off the table.

[Scott Challoner]

If they truly are chips, then you should find evidence of shards under your vector table (or hear them getting sucked into your exhaust). I've been doing a lot of research lately about forming acrylic and it turns out it tends to absorb moisture. We have the same problem in my day job (electronics manufacturing) where PCB material (FR4 (basically fiberglass)) absorbs moisture and then outgasses when heated to soldering temperature. I would look at your process. You say you are applying a mask. Is it liquid? The acrylic may be absorbing it and then outgassing. If the acrylic sits around for awhile after you peel the paper off, it could absorb moisture too. I would try another piece (maybe even from a different source) and leave the paper mask on and see what results you get. Also, you can bake the acrylic to dry it.

[Scott Challoner]

Just to be clear... I think it may be possible that when the laser heats the acrylic, it gets hot enough for any moisture absorbed to vaporize and outgass, causing chipping. We call this popcorning.

Just my $.02

[Rodne Gold]

If the paint they use has solvents , it will stress crack the pex (any heat operation on perspex stresses it incredibly) and worsen any chipping.
Water absorption isnt really a problem with perspex , we have never experienced it and we laser cut and form/bend and glue pex all day.
Key to cutting pex with a good edge is actually air assist. It has to be the right pressure to eject the melt thru the back of the cut and not too high as to cause rapid cooling of the edge thus leading to a dull finish. We never cut pex on a vector grid due to flashbacks and we never focus on the middle of it rather the surface cos pex acts as a wave guide - ie it doesnt let the beam diverge when cutting.
If you cant get rid of the "chipping" , then use a deburring tool on the back edge of the rectangles , this will remove "stress crack originating" edges etc.

[Hilton Lister]

I never use my vector grid any more for Acrylic. I have made up spacers from scrap acrylic. 2x 6mm laminated together. If the beam strikes it while cutting, any residue is easily polished off with a light abrasive. My air assist is pathetic as I have not been able to, so far, source a decent mini compressor locally, but I cut acrylic quite regularly up to 10mm with my 25watt GCS Mercury, (albeit quite slowly). I must give credit to Rodney for the original idea.

[Frank Corker]

I think the 'chipping' is probably the bounceback you get from the vector grids. Sometimes when I have that problem I put golf tees in the vector grid dotted across the sheet, it raises the workpiece high enough to stop the bounce and makes a cleaner cut. Another thing when working with acrylic sheet I have sometimes found it just as neat a cut when I have reduced the frequency from 5000 to as low as 1000 and had a good result. The cast acrylic does sometimes have different consistency when it is made, you can cut one sheet perfect and the next is a devil of a job. All cast acrylic is clear when it is made, to get colour they have to add ingredients also the reason why black acrylic is more expensive to make, it may be that the added ingredient is more dense.

[Dave Schneider]

To eliminate bounceback I add a .125 stroke to the laser cut file after I cut the first sheet. Then place your consecutive sheets to be cut on top of the oversized cutouts. For ease of removal you may also want to put some folded pieces of lexan taped to the grid inside the oversized cutouts to act as springs to lift the cut pieces. I cut a lot of Pinball machine plactics and this method works well. We keep the original masking on the front of the acrylic, but after printing, mask the printed side with polyethelene premask.

[Barb Macdonald]

I've used the golf tee method this morning, on top of the grid, and so far, so good. When the piece drops I get a little bit of a mark, but nothin' to write home about:) I also decreased the frequency to 1500, it does make a better cut.
Great ideas, thanks to all of you. We've had extremely hot humid weather the past few weeks here in Toronto, I wonder??
Time to A/C the shop??
$$$$$$$$$, for a whole month of the year needed?
Thanks again, back to work:)
Barb
Corelx3, epilog legend ext 60watt (maybe), xenetech rotaries, cas-mate/mimaki plotters etc.etc.
"Hope is the thing with feathers, that perches in the soul, and sings the tune, without the words, and never stops at all"
Here's hoping!

[Stephen Beckham]

Barb,

Not part of this subject, but in case you missed my problems with my Epilog.... First off - no - I'm not bashing Epilog... But I noticed your comment about adding A/C.

Just want to remark that I had moisture buildup during the high humidity times that caused my air assist tube to sweat (cold air moving fast in a little tube - lots of moisture - yada - yada). At any rate, it started binding on itself instead of properly retracting towards the right.

Long story short - if the lense head starts disengaging while in the middle of a job (not left to right traversing, but the arm continues down through the job and you have to reset it by power switch). Check your air assist tube for binding.

Otherwise - yes - it's time to put an A/C in the shop. Since I've moved mine to the store - it hasn't happened again. If you can't get it controlled - my other option was to limit my job size to keep from reaching that magic binding point (it was about 18" on my Mini 24 machine).

[Barb Macdonald]

Barb,

Not part of this subject, but in case you missed my problems with my Epilog.... First off - no - I'm not bashing Epilog... But I noticed your comment about adding A/C.

Just want to remark that I had moisture buildup during the high humidity times that caused my air assist tube to sweat (cold air moving fast in a little tube - lots of moisture - yada - yada). At any rate, it started binding on itself instead of properly retracting towards the right.

Long story short - if the lense head starts disengaging while in the middle of a job (not left to right traversing, but the arm continues down through the job and you have to reset it by power switch). Check your air assist tube for binding.

Otherwise - yes - it's time to put an A/C in the shop. Since I've moved mine to the store - it hasn't happened again. If you can't get it controlled - my other option was to limit my job size to keep from reaching that magic binding point (it was about 18" on my Mini 24 machine).

Hi Steve

Thanks for the input, I had a terrible time with the air-hose binding, needed to do some fancy (i.e. butt-head simple) adjustments in order to stop the hose from catching on the cover assembly. My white computer cable also flies around like crazy, under the cover, discovered this myself too, the cable was catching on screws nuts on the inside, needed again simple fixes to solve a REALLY bad design problem, in my opinion:) (Masking tape to the rescue!)
Have a great day
Barb