Cutting Acrylic letters - Help?

[Ted DeRuyter]

I having been cutting 1/8", 1/4", & 3/8" acrylic letters on my Epilog 75 watt laser for a customer. They are asking if I can get a more "polished" edge. The edge looks "jewel" like (faceted), but has distinct lines in it from what looks like the laser pulsing action. Since I am fairly new to this, I am not sure if this is normal or If I should be getting a much smoother finish on the edge. I am using 20 speed, 100 power and 5000 ppi on the 1/8" material. on the 1/4", I am using 16 S, 100P & 5000ppi but with 2 passes. The 3/8" is 10 S, 100P & 5000ppi with 2 passes. I also use a conical air assist for all vector cutting.

The other issue is that the acrylic sheet has a 3M adhesive layer on the bottom. When the letters are cut it appears that the adhesive is creeping up the sides of the letters causing the edges to be sticky.:(

Does anyone have any insights on how to improve the quality of the laser cut edges? Any insights on eliminating the adhesive from creeping up the edges?

Also any insights on how to speed up the total cuttings process would be much appreciated. Thanks in advance for all you help! :)

[Joe Pelonio]

I do a lot of these on my 45 watt, and it's sounds like you are going too slow.

My edges have what might be considered almost microscopic vertical lines on the edges but only on hard curves where it goes slower. Straight lines and gentle curves are more like flame polished.

Lets' compare 1/4". I use settings 100 power, 8 speed and 5000 frequency.
My manual says 6-100-5000 but faster is cleaner.

That's one pass. With 75 watts you should be able to go faster and do it in one pass, unless something is wrong.

I also will use 3m laminating film such as 468MP, which most always cuts cleanly through. Once in a while a letter will have that liner perforated here and there so I turn it over and hit it with an xacto or utility knife.

[Scott Shepherd]

I don't know if this helps or not, but I've cut stuff that looks flame polished and a month later, not so much. Nothing has changed with the laser, but I have messed with the air pressure. In my mind, I think more air=better cut on acrylic, but I believe that's not true. I think it needs a little air, but not a lot.

That's just a theory I haven't had time to play around with, but the logic is there for it to be true. Maybe someone else can give some insight into whether or not that's true.

Seems like I'm running 40PSI and it probably needs to be about 10-15 (just enough to keep it from flaming up).

Just a thought. Might be totally wrong.

[Peck Sidara]

Ted,

A few things come to mind:

You're using the cone lens, this means you have the EXT-75W?

If so, I'd start by checking your laser beam alignment, especially true if you're using the cone lens. There's not much of a hole at the cone lens exit point so we want to make sure the beam is coming thru completely and accurately. Tech support can assist with the alignment.

Swap from the cone lens to the standard lens using the same power/speed/frequency settings. Does it make a difference?

Try adjusting the air-pressure from low/medium/high (if applicable) for both lens assemblies.

As for the adhesive, try raising the acrylic away from the vector table, use 4-5 peices of something sturdy of the same height, support in all four corners and in the middle if possible.

HTH,

[Ted DeRuyter]

Hello Peck,

Thanks for the ideas. I only have the cone lens assembly. I will have to check with my distributor on getting a regular lens assembly. I will test changing the air pressures as this might help.

I am concerned about raising the acrylic sheet and having the letters fall through and get nicked or cut by the laser beam as it is cutting other letters. Any concern on that?

Thanks.

[Dan Hintz]

In my mind, I think more air=better cut on acrylic, but I believe that's not true. I think it needs a little air, but not a lot.

That's just a theory I haven't had time to play around with, but the logic is there for it to be true. Maybe someone else can give some insight into whether or not that's true.

Scott, your thinking is correct. Too much pressure and the acrylic cools off before the laser has a chance to melt and punch cleanly through. Too little and the meltoff is not blown away and recrystallizes. You'll have to find a good balance for your specific machine.

[Joe Pelonio]

Hello Peck,

Thanks for the ideas. I only have the cone lens assembly. I will have to check with my distributor on getting a regular lens assembly. I will test changing the air pressures as this might help.

I am concerned about raising the acrylic sheet and having the letters fall through and get nicked or cut by the laser beam as it is cutting other letters. Any concern on that?

Thanks.
Raise it no more than the thickness of the material you are cutting, and hopefully they will fall straight down and be safe. I have cut up card stock letters that blew or were sucked into the wrong place and ruined, and had to remake some, but without a vacuum table there not much that can be done.

[Keith Outten]

I have experienced the same issues cutting large letters on my CNC router but it's magnified due to the larger letter sizes. It was suggested that I take a very close look at each letter in the font to see if the edges are irregular. Sure enough lots of fonts appear to be smooth on a very small scale but the edges actually are very rough when you scale them up or zoom in for a close look.

I have also noticed that the 60 watt machine I use at work will produce a much smoother edge than the 35 watt machine I own.

.

[Ted DeRuyter]

Thanks again for all the great ideas. I have run a trial on my laser with 1/4" acrylic adjusting the air pressure on the conical air assist. I honestly don't notice much difference. If I had to be super critical, I think the sample at 10psi looks best. I ran the trial at 10psi, 25psi, & 40psi. Using 8 Speed, 100 Power, & 5000 ppi and focus on the top surface with 1 pass. I did notice some improvement with 1 pass versus 2 pass runs. With 2 passes you get a line horizontally across the edge where I assume the first pass penetrated.
I cut several different fonts but I am getting similar results.

I have attached photos of my current cuts. They are better. Perhaps this is what is expected and I am asking for too much.:confused:

Thanks,

[Frank Corker]

I personally think you should reduce your frequency and your speed. I have cut similar thicknesses (but mine is only a 45watt) two or three stops lower than the recommended speeds, reduced the frequency from 5000 right the way down to 500 and had extremely good results.

Don't be afraid to go even lower than that because I have had very conflicting results from my laser and the recommended settings from the instruction manual. With cutting you can also make changes to the focal point using a manual focus, again I would say try dropping it down a couple of stops from the top of the work piece.

[Peck Sidara]

Hi Ted,

The standard 2" lens is pre-installed from the factory. All optional lenses are packaged separately in a small box within the machine. You should definitely have this lens and I think it's worth a shot.

In regards to raising the material above the vector grid, Joe's suggestion is a good one. Color mapping your vectors so it'll cut inside out would take care of any concerns on cut letters interfering with uncut letters.

I recommend trying the 2" standard lens, low air-pressure, focus a little closer into the material and have the material raised. (raising the material will help with the adhesive creeping back up and will prevent any flashback from the vector grid).

There are a lot of variables to adjust but with some patience, you'll improve on the cut quality.

[Rodne Gold]

There is no way to stop micro striations when laser cutting acrylic , its not the fault of the machine , its not doing "steps" , its the way the material cuts melts and gets ejected.
The only 2 remedies are to slow the cut down and very finely adjust air assist .
Problem is , no1 method places incredible stresses into the material due to the heat affected zone from slow cutting. If you use extruded acrylic , this WILL result in stress cracking of the edges at a later stage or when a solvent is applied or nears the stressed edge. Still will happen with cast , but to a lesser extent.
2 pass engraving into acrylic is a very bad strategy for various reasons. 75 watts is more than enough to cut 10mm pex in a single pass.
With 2 passes you get resolidificiation at the bottom of the 1st pass , huge amounts of heat stress and can get slight misregistration due to movement and expansion and contraction and you tend to get "melt" all over , your air assist cannot do its job.

I dont think the fact you have the 3m adhesive sheet is helping either but have no solution as to what to do to stop its stickyness apart from applying it after you cut the letter.

Co incident air assist is also not ideal , the ideal is to have the air assist directed just after the cut and blowing backwards , but this assumes you are cutting with a single pass.

I think you can try to do what you can to minimise the less than ideal edge quality but it will never be perfect.
Your customer would be paying a huge amount more if they used a process to give a flame polished edge , if indeed the letters could be cut that way.
Speeding up cutting and better edges in this case are mutually exclusive. A single pass operation will do so , some elementary nesting might also help and will save some material.

[Ted DeRuyter]

Hi Ted,

The standard 2" lens is pre-installed from the factory. All optional lenses are packaged separately in a small box within the machine. You should definitely have this lens and I think it's worth a shot.

I recommend trying the 2" standard lens, low air-pressure, focus a little closer into the material and have the material raised. (raising the material will help with the adhesive creeping back up and will prevent any flashback from the vector grid).



Hello Peck,
Thanks for the info. Our epilog distributor changed out the standard air assist with the cone air assist upon installation due to our need to cut a large volume of printed foam core sheets. I am asking them for a quote on a standard lens today to try this with the acylic.
Thanks,
Ted

[Michael Simpson Virgina]

The Synrad site has the best writeup for cutting acrylic that I have seen.

http://www.synrad.com/e-newsletters/08_24_06.htm#app2