Tiny dots around small engraved areas

[Toasty]

Hey all! I have been using my converted K40 for over three years now and I've had great luck with it.

Since it was new I've had one little issue that's a bit annoying. When I engrave small areas, like text for example, I get faint tiny dots around some of the letters from time to time. The only material I really notice it on is black engravable plastic. I assume the black absorbs more laser light and makes the problem more obvious. It seems to happen more on thin vertical lines more than something like a fat line. I had originally thought it was just hot material landing back on the plastic and pitting it up a bit but in all my years of engraving I've never seen that before. I'm assuming it's some sort of beam scatter or interference (bad ground?) causing it. It appears to happen more when the beam has to fire and shut off quickly for a thin vertical line.

I did put an extra ground on the case recently and that seemed to help some. Hard to say because the problem seems to come and go anyway.

Would the PWM frequency setting have anything to do with an issue like this? Mine is currently set to 20000 in LaserCad.

Thanks for any help.

[Toasty]

No one has ever had this problem? Hmmm. Well, maybe it is just the material splattering back down on to the plastic and causing the dots. I'll try to get a picture of it the next time it happens.

[Tech_Marco]

May be you should post a picture or video to show the dots... I couldn't get a picture in my mind of how dots did you get from the engraving. I think seeing is better.

Marco

[Toasty]

Hey Marco!

I should have uploaded a picture in the first place. Here's one.

Keep in mind that these dots are very small, the picture makes them look big since it's zoomed in so far.

extra dots.jpg (274.31 KiB) Viewed 3700 times

[Tech_Marco]

Check all wheels or belt see if any small partical fell into it.
I think it is mechanical issue

Marco

[Techgraphix]

I can't see if the dots are ON the surface or IN.. As they have the same color as the surface i suspect they are ON it.. Maybe it is possible to scratch them of with your nail?
If this material is provided with a protective sheet (thin plastic foil) , you could leave that on and remove that after the lasering.. Maybe it helps.

Kees

[Toasty]

Check all wheels or belt see if any small partical fell into it.
I think it is mechanical issue

Marco

Thanks Marco! I'm not sure I follow how a mechanical issue could cause small dots like this. If I engrave a larger/fatter font it goes away, it only seems to do it on thin line small fonts and I only see it on black plastic. I don't seem to have the issue on any other material which makes me think it might be the material splattering back on the surface of the black plastic instead of vaporizing. Maybe the beam hits the material and a little hot piece of the plastic falls back down and melts the plastic a little causing a dot.

Like I said, I've been fighting this since I got the laser. I orignally thought it was the tube firing when it shouldn't but that doesn't make sense considering I don't see it on other materials. On the other hand black will absorb the laser more so maybe that's why I only see it on black plastic.

It's an odd problem for sure. Sometimes there are a LOT of dots when I engrave small letters on this type of plastic.

I guess the next thing I'll try is a little air assist. I really only engrave so I've never been worried about using it before. If it's a spattering problem maybe that will help. Any other ideas are much appreciated.

@ Techgraphix: They are in the surface for sure. Like a little tiny melted dot if I look at them under magnification. This is typical engravable plastic. It has a plastic protective sheet but you can't/don't want to engrave through it. I could put a masking film over it before I engrave but that's really a pain. I've never seen any other laser do this so I prefer to solve the actual issue.

[Techgraphix]

It looks like hot melted plastic from the lasering was landed on the surface again, rather than spurious laserfiring, seen from the form of the dots..
If it was laserfiring i suspect the dots to be white..
Maybe you can increase the air-assist-airflow? or add a little blower on the side to blow particles away?

Hot gasses with particles landing on the surface again is more or less common with engraving plastics but not like those dots..
I even tried to put off airassist during engraving.. That helped a bit but my lens poluted within minutes so i skipped that..
What helped best was to submerge the object in water, so that there was only a very thin layer of water on top of the surface and airassist off.. Results where not too sharp however..
Covering it with a wet tissue might help but you can have some coloring from the tissue..

Kees

[Toasty]

I tried air assist today and it did help with the dots. Problem is, I hate using air assist while engraving, it's fine for cutting but when engraving plastic it just makes every little bit of dust stick back on the plastic. I don't want to make that big of a mess so I trashed the air assist idea.

I then tried to lower the power a bit but got a LOT more dots. So... I increased the power and they went away. My current theory is that the higher power helps to vaporize the material more so there are no larger hot pieces of splatter to land back on the plastic. I dunno, I might be wrong but that's the theory anyway.

By the way, knock on wood, but I'm still using my original tube on my 3 year old machine that I use almost every day. I generally don't cut with mine and run it at pretty low power most of the time but treated well these tubes can last quite a while.

[Tech_Marco]

You, right it was not mechanical issue. I was thinking something else. I thought that you were complaining the Non-smooth cut line (zoom in issue)but didn't pay attention on those little dot randomly spread. In this case, I think you may want to clean the tube or air pump. You better turn o air assist even thought on engraving. You can set it very low but you still need it to clean the lens

Marco

[Toasty]

You, right it was not mechanical issue. I was thinking something else. I thought that you were complaining the Non-smooth cut line (zoom in issue)but didn't pay attention on those little dot randomly spread. In this case, I think you may want to clean the tube or air pump. You better turn o air assist even thought on engraving. You can set it very low but you still need it to clean the lens

Marco

I know you're a busy guy but I mentioned the dots in the title of the message and several times after . No biggie. I haven't been able to find anyone discussing a similar issue anywhere on the 'net so I think it's a combination of the material I'm using and my settings.

Really though, I won't be using air assist engraving plastic. Like I mentioned, it made a huge mess on the plastic and on the lens for that matter. Most people I know only use air assist for cutting. It appears that bumping the power up stopped the issue for now anyway.