Full Version: Reverse Engraving

From: Brian (BRIAN_R) [#1]
 15 Apr 2007
To: ALL

Hi folks,
So I decided to try reverse engraving a piece of clear cast acrylic. I sprayed with paint, engraved, and resprayed the second color. I was not too impressed with my results. I played a little more and the engraved area cracked. Wrong paint? Wrong settings? Just won't work?

25 watt Mini
100 % power
100 % speed
300 dpi & 600 dpi

Thanks,
Brian

From: David Wilhite (DAVID-ULS) [#2]
 16 Apr 2007
To: Brian (BRIAN_R) [#1] 16 Apr 2007

Brian,

There's a tutorial on how to reverse engrave acrylic on the Universal Laser Systems website. Go to: http://www.ulsinc.com/ and click "View All" on the "How To" section to access the information.

All the best,
Dave Wilhite

From: Brian (BRIAN_R) [#3]
 16 Apr 2007
To: ALL

Thanks,
Theirs turned out nice. I think I found the problem.
The type of paint I used seems to cause cracking in the acrylic.Do I need a water based acrylic paint?
Any suggestions on spray paints?

From: Harvey only (HARVEY-ONLY) [#4]
 16 Apr 2007
To: Brian (BRIAN_R) [#3] 16 Apr 2007

Some paint solvents, (most), contain chemicals which will at least 'craze' the area engraved. (Most of the crazing will disappear in a few days, but some not.)

If you use 'acrylic latex enamel' it will not affect acrylics at all. And it will clean up with water!

From: Rodney Gold (RODNEY_GOLD) [#5]
 16 Apr 2007
To: ALL

The laser stresses the Acrylic very badly, any process that puts heat into it does. (sawing , routing , bending)
Solvents will promote and cause stress cracking , thus dont use any paint with a sovent in it near or on lasered acrylic.
What makes it worse is that the clear acrylic best used for reverse engraving is extruded (engraves clear and not frosted) which is stressed more by lasering , its a short polymer chain acrylic and thus will stress crack way before cast will.
Dont think that time will stop the problem either , the stresses are there permanently and even worse is the fact that the stress cracks might not appear immediately when solvent is introduced , they can happen weeks later.
There is a fix tho , you can put the pex in an oven at 70-80c for 1 hr per mm thickness and let it air cool , this will stress relieve the pex and then you can use solvent based paints.

EDITED: 16 Apr 2007 by RODNEY_GOLD

From: Harvey only (HARVEY-ONLY) [#6]
 16 Apr 2007
To: Rodney Gold (RODNEY_GOLD) [#5] 16 Apr 2007

That is a long way to go to stress relieve it, but when it is the only way on a certain project, it sure is great to know.

For the rest of us that is 158* F to 176* F.

From: Brian (BRIAN_R) [#7]
 16 Apr 2007
To: ALL

I think I'd be better off finding the proper paint.
Is there a spray I can use?
Maybe I can dilute acrylic water based paint and shoot it through my paint gun?
How about color filling with a brush and then repainting with a spray can to get a consistent color on the back?

From: Harvey only (HARVEY-ONLY) [#8]
 16 Apr 2007
To: Brian (BRIAN_R) [#7] 16 Apr 2007

I either paint it on with a brush, or dilute with water and use an airbrush.

One trick to brighten colors is to overspray the finished painted area with white acrylic primer.

From: basehorawards [#9]
 16 Apr 2007
To: Mick Unread

Mick,

Have you ever tried to engrave acrylic that you have laser sublimated?

From: Boz (CHEDDARHEAD) [#10]
 16 Apr 2007
To: ALL

Brian, like Rodney wrote, sometimes acrylic micro crazes by itself after laser engraving. In a former life, when I worked for someone, we laser cut a whole bunch of parts. They were perfeect. We shipped half of the order to the customer and kept the rest for stock. A week or so later, the customer called and said, What the #$$%, these parts are a mess. So just to prove him wrong, we started ulling parts off the shelf. Everyone of them had micro cracking all around the laser cut edges. OOOPPPPSSSS!!!! That's when we learned about stress relieving acrylic after cutting. It is even worse if you throw on some solvent based cleaner or paint. They tend to craze quicker, and more thuroughly. Good luck.

From: ACJ (LADYCUTTER) [#11]
 16 Apr 2007
To: Boz (CHEDDARHEAD) [#10] 18 Apr 2007

I've only done a couple of test pieces, but I have had good results with an acrylic spray paint from Ace Hareware.

From: Brian (BRIAN_R) [#12]
 16 Apr 2007
To: ALL

Well, I'm still plugging away at it. Maybe I should try extruded clear instead. What about speed, power and dpi?
The sample last night had lines across the clear. I only used 200 dpi, 100 power 100 speed.

From: sprinter [#13]
 17 Apr 2007
To: Brian (BRIAN_R) [#12] 17 Apr 2007

Slow it down and use less power. It will give you better results.

From: Harvey only (HARVEY-ONLY) [#14]
 17 Apr 2007
To: Brian (BRIAN_R) [#12] 17 Apr 2007

I have done some extruded and hate it. There are always lines from the uneven mix of monomer and polymer in it. They will be less apparent if engraved lightly, but will still be there. I would never use it for unfilled lettering.

Just my opinion.

From: laserman (MIKEMAC) [#15]
 17 Apr 2007
To: Brian (BRIAN_R) [#12] 17 Apr 2007

Brian,

Try the texturizer switch in the driver under the advanced tab put a check mark into the Enhancements and then into the texturizer.

This will randomly scatter the dots and gives a unique background it works well on large engraved out areas of acrylic. The file will be larger so be aware.

From: Rodney Gold (RODNEY_GOLD) [#16]
 17 Apr 2007
To: ALL

Isn't 200 DPI rather low if you want to clear/hog out an area for filling?
With the spot size of a 2" lens it means you are barely at a smooth stepover - ie the distance between scan lines in the y axis is too high and you dont have a spot overlap leading to a ridged finish. Its like furrowing a Field with too high a spacing tween furrows.

From: Brian (BRIAN_R) [#17]
 17 Apr 2007
To: ALL

I've tried 200, 300 and 600 dpi.
I think it's several things that I'm doing wrong. One is that the area I'm lasering through is a very wide number. The small logo next to it didn't do so bad.
I just got back from Lowes and I just don't see any acrylic spray paint, laquer or enamal is it. Someone mentioned Krylon someplace, maybe I'll try that. I'll slow it down to 50% speed and 50% power also.
I'll try a few more things tonight!
Thanks for all the help.

From: dun_it (OREGON) [#18]
 17 Apr 2007
To: Brian (BRIAN_R) [#17] 17 Apr 2007

Try the local craft store for paint. Most of what they carry is a water based and it comes in both spray and bottled and a huge varity of colors. B-)

From: Peter [#19]
 17 Apr 2007
To: Brian (BRIAN_R) [#17] 17 Apr 2007

hit the yellow pages and ask your hardware stores and paint supply shops for acrylic latex or water based pressure pack paints.

Krylon make solvent based paints, they may have a water based version as well, pay to check.

any solvent based paint will give you trouble with crazing as previously mentioned.

regards
Peter

From: Harvey only (HARVEY-ONLY) [#20]
 17 Apr 2007
To: Brian (BRIAN_R) [#17] 17 Apr 2007

Krylon paints do craze badly, voice of experience. (Some may be worse than others.)

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