Deep engraving in Arylic

[Paul Phillips]

Ok, I have my first engraved acrylic job to do and I am running into a problem. I am trying to reverse engrave some ½” tall letters on to the backside of some ½” thick clear acrylic and paint fill them black. The problem I’m having is in trying to achieve a 1/16” depth cleanly. I keep getting a white “haze” around the inside edge of the letters. I have looked through the archives for a solution and the only useful thing I could find is using someone’s suggestion to defocus the lens slightly and also try several passes at a lower power which helps reduce it slightly but does not eliminate it enough. When I paint fill the letters you can see a white halo effect around the edges when you look at it from the front side and it gives them a “gray look”.
The other issue is the horizontal lines that are visible on the letters, I have the raster image density set to 6 (high quality) and have tried both the halftone and diffusing settings, can’t tell much difference.
At 150watts I realize that I have a little more power than most laser engravers, maybe I’m putting too much power down per pass? I started by using the default factory recommended settings in the Universal driver settings which are 75% pwr, 100% spd, 500 ppi, and then began adjusting them down in increments of 10 until I reached about 10% pwr, 100% spd, 500 dpi. At the lower power I can achieve a nearly perfect looking letter but it also takes about 7-8 passes to get there which is just too long. When I defocused, I tried it about 1/8” up with my 2” lens which definitely helps but, it seems to me if I go much more than that the edges start to get fuzzy.
Ideally, it would be nice to be able to do one pass with high power to achieve the desired depth and then one or two additional passes to eliminate the haze, any more than this seems too time consuming, I can achieve a decent looking result on the CNC machine using a 30 deg engraving bit with a 3d tool path but was hoping for a quicker cleaner result with the laser. Anybody have any good suggestions?
Thanks,
Paul

[Rodne Gold]

You have the right tool for the job, the laser isn't it

[Mike Null]

It is normal to get a whitish finish when engraving cast acrylic which is likely what you're engraving.

The paint fill will not be clear and sharp. Your option is to engrave it from the front for color filling. Then you will get the true paint color without any diffusion.

[Martin Boekers]

Haven't used much of it so I can't recommend it, but you may check on extuded acrylic instead of cast. Not sure if a bit of flame polishing will take away the etch.

Maybe a fellow engraver with experience in the extuded and flame polishing can help out.

Marty

[Mark Winlund]

You have the right tool for the job, the laser isn't it

I would suggest a rotary for this. I assume you're kidding, Rodne.

Mark

[Scott Shepherd]

Also, for the finish pass, try out of focus by about 1/16" and then use about 10% power, or maybe less on your system. No more than 10%, I'd say.

[Chris DeGerolamo]

you could always mask the back and spray paint instead...

[Angus Hines]

I always leave the paper on and engrave through it then I load it up with rustoleum spray paint which has a tendency to melt its way into the acrylic if applied in a heavy first coat.

Then just peel away the paper.

[Paul Phillips]

Rodney, I assume that you are talking about using the CNC machine to do this. I can do a nice job with the CNC however, my machine is a little "loose" from wear, I need to replace the Z axis screw and tighten up the other axis', so as a result I get a bit of chatter in the letters which is more evident in that small size of letter.
I like the sharpness I can achieve with the laser, I just wanted to know if there was some setting I'm overlooking or technique I can try to achieve the 1/16" depth with laser. Scott, I will try your suggestion, thanks.
The design is for reverse engraved letters. These are for donor plaques for a new hospital wing so they have to be perfect.
Correct me if I'm wrong but I thought all acrylic over 1/4" only comes as cast.
Chris, yes I am using a pre-mask so I can paint fill with spray paint.
I think we are going to be able to change the design to use white paint instead of black and use a darker background so I think we dodged the bullet on this job but I gather there is no simple solution to avoid the "white haze"?
Thanks for the responses,
Paul

[Rodne Gold]

You will have unresolved/able issues with using the laser to do this , the haze is vapour which has nowhere to go and the striations are due to remelt,expansion etc and are impossible to remove.
Multiple passes will worsen this.
I would use my Tekcel for this job and get far better results. Why dont you use one of the multicams?

[Mike Mackenzie]

Hey Paul,

A lot of the suggestions are good and correct. When trying to engrave deep into cast acrylic it will be very difficult to get it clear.

A couple of suggestions don't use image density 6 this will heat up the material more causing the lines.

Do not use the air assist when engraving the letters. The way our air assist works is it blows a steady stream of air directly at the cut, So once it is raster engraving you are blowing the liquid acrylic and splattering it into the engraving sort of like sand blasting.

What I would try is this.

do it in multiple passes maybe 3

Duplicate the image on top of each other and change the color to three different colors. (black top red middle, & blue bottom) Make sure the duplicates are directly on top of each other.

First pass (black) 100 p 60 speed 500 ppi and image density 5 focus to surface
Second pass (red) 70 p 60 speed 500 ppi and image density 5 Focus up 0.020 (move table up)
Third pass (blue) 20 p 80 speed 500 ppi and image density 5 Focus up 0.050

Set the Z height to adjust by the color and run the file I do not know if this will give you the necessary depth you will have to try it.

I don't know if you will get desired looks but it should get better than what you are getting.

[Martin Boekers]

Correct me if I'm wrong but I thought all acrylic over 1/4" only comes as cast.
Paul

I haven't used these guys but it appears they fabricate extruded up to .5"


http://acrylicparts.com/acrylicprice.html


Marty

[Mark Ross]

Not sure if your laser driver has a similiar setting but on our Epilog we have a frequency selection which can go to 5KHz. We always get nicer edges with the frequency cranked all the way up.

[Paul Phillips]

Mike, I tried your suggested settings, the letters look clean and deep but they still have the "white haze" around the inside edge so turning the air assist off didn't seem to make a difference with that.
Martin, I stand corrected, you're right I talked to my local acrylic supplier and they do indeed make 1/2" extruded acrylic. He did say that he doesn't stock it because it's not much cheaper than cast which is a purer form of acrylic that tends to cut, polish and glue much nicer than extruded.
Mark, I think the equivalent setting in the Universal driver would be DPI? Mike, maybe you can answer that one best.
So for anyone else who might do a search for deep engraving in acrylic, it can be done but if you are wanting to reverse engrave and paint fill a dark color, the white haze seems to be impossible to eliminate to an acceptable level of quality.
Hope this helps someone down the road.
Thanks for all the suggestions as I continue to learn the limitations and capabilities of my machine.
Paul

[Dan Hintz]

Paul,

A suggestion, but be forewarned, I have not tried this myself...

What about giving a light coat of clear before filling with black? The clear should increase the clarity of the edges by filling in the microchasms that cause the cloudy look, and it won't affect the look of the black fill.