Pint Glass Engraving Issue

[Jeff Heinrichs]

Hi Gang - I'm at my wits end with my latest job. I have a few hundred pint glasses to do and my laser isn't cooperating with me, or I'm not cooperating with my laser (the latter is more likely).

On this job the trouble I'm having isn't 100% of the time, but it is occurring on at least 8 out of every 10 glasses I'm running.

The trouble is the engraving isn't consistent across the entire surface of the graphic. You can see in the example images that the leading edge of the art has a different surface finish than the rest of the engraved area.

The one consistent theme with the trouble I'm having is that it's always on the leading edge of the engraved area. That said it doesn't always occur at the same point on the art and it happens to differing degrees as seen in the images.

IMG_0442.jpg IMG_0443.jpg IMG_0445.jpg IMG_0447.jpg IMG_0446.jpg

Now for my settings and technique. I've tried many different settings and techniques in an effort to improve or at least make the trouble consistent so I can begin tweaking settings, but up until now the settings and techniques have not shown much in the way of a difference to the outcome.

I am using a Trotec 400 Flexx (120W CO2) with the rotary accessory and using the hot dog roller attachment (not the clamp).

I've tried settings from 90% power at 100 speed to 75% power at 65 speed - No real difference
I've tried it as vector art and raster art (using CorelDraw) with Ordered Dithered from 333dpi to 1200dpi - No real difference
I've tried liquid soap on the surface, wet newspaper and dry - No real difference
I've cleaned the mirror and lens (2.85" flexx lens) at the laser head - No real difference

At this point I am not getting consistent results to where I can start chasing down what's really happening here.

I would point out that when I started running the pint glasses, I really only saw this occurring subtly on the first paw print, but since then it has gotten worse.

[Kev Williams]

funny how the thing is engraving the E and the heart ok within the same scanlines that are messing up in the paw pads...

Why aren't you using the other rotary? At least you can repeat with it-

Suggestions: not many, but-- can you engrave from the bottom up? If so, try that-
Also, with 120 watts on tap, I'd seriously consider about 30-40% power and whatever speed % works out to around 35" per second. Even at the slowest speed and power settings you listed, you're WAY above my glass settings! I run glass with my 40w LS900 at full power and 50% speed, which is about 35ips with wonderful results. I'm kinda wondering if the problem you're having is with the glass itself? It seems logical/reasonable to me to believe that running that hard and fast, could be the laser's just bouncing off the glass. If there's one truth about lasers I've found over the years, is that in many cases, Less Is More...

[Mike Null]

First, I am by no means a glass expert. I agree with Kev, use the cone; that will allow repeats. Second, I scale my power and speed back a lot. I'm doing a large glass job right now. I'm getting a perfect mark. (it's a small logo and I run three passes) My objective into get a uniform frosted mark--not the fractured glass look. I've been doing this particular job for the past three years with perfect results. Low power, low speed, 333 dpi and 70% black image I'm fortunate that 3 passes takes 35 seconds.

Oh yes, the reason for the three passes is that I get random open spots if I don't. That seems to be common with glass.

[Jeff Heinrichs]

Thanks for the inputs. I have scaled back the power and learned a little more about the issue and a painful workaround that appears to be my course of action right now.

If I run the job at 333dpi, the issue appears the worst, with the most area that's not the preferred engraving. I've changed the dpi to 1000dpi and it makes the smallest "un-preferred" engraving area.

I'm running the job at 38% power at a speed of 100 and I make two passes and then I finish up with steel wool to clean up the fractured areas. It looks good, but what should be a job that takes 4 minutes is now taking 12 minutes... Uggh

I'm just happy that I'm able to complete the job, but I sure would like to figure out what's going on with the leading edge "low power" start to some of the engravings.

[John Lifer]

By no means an expert on glass, but ran a dozen different liquor bottles yesterday with customers logo. Had been getting chipping, I started using masking paper and it totally eliminated chipping. Only equivalent of 300 DPI, 40% power of my 80 watt and 275mm/s which would be slow for a trotec.

[Kev Williams]

Glass is weird. Last week I was asked to test etch a bottle with a flat side. Piece o' cake

Nope.

First thing I noticed was it Would Not Fracture. It was instead melting slightly, emphasis on slightly. Way too subtle.

So I went over it again. And again. Different speeds and settings. The etching never changed for better OR worse.

But WAIT! ( ) -- after a couple of minutes, oh, I guess it WILL fracture!
glass1.jpgglass2.jpg
- just not exactly the fracturing I was going for - I've never seen glass crack like this?

Customer then explained these bottles are high-content borosilicate glass. Which I guess explains it, but, Pyrex is borosilicate glass, and I've etched dozens of those hot/cold Pyrex 'travellers' with no issues. "High content" may be the key? About a year ago I had similar results when etching some wedding goblets- at least they left here without cracks!

Is it possible those tumblers are leaded? LC is a low temperature glass, and depending on the lead content you could be riding a fine line between the laser fracturing the glass and melting it...

[Mike Null]

I forgot to mention that the glass I am engraving is tempered. They are cookware lids.