Black streaks on anodized tags - Help!

[Joe Thorn 911]

While engraving text and graphics on anodized aluminum dog tags, I get some portions that have black streaks or areas where the lazer engraved. It's not in all of the engraved areas, just parts.

I have a 50 watt chinese laser that cuts and engraves fine on all other materials. Last week I made several tags with no problems. This week I have that problem, with no changes to the machines settings. I think the tags I'm using this week may be from a different batch. I have checked the alignment on the mirrors and cleaned them. I have tried rubbing the blackness off of the tags, but it doesn't come off. If the total engraved area was black, I could understand maybe a wrong power setting, but some parts are fine and others have the streaks or areas of black, almost like the black is on/in the aluminum, under the anodizing.

I have tried settings anywhere from speed 200/power 30 to speed 50/power 50. I can't get my pics to upload here, but if you write me at firejoe911 @ hotmail.com I'll send you some pics of the tags.

Anyone have any guesses?

Thanks, Firejoe

[Anthony Scira]

My guess would be bad batch of tags or too much power. Dial the power back a bit, it really does not take that much.

[Joe Thorn 911]

Thanks for the reply. I have used everything from 200 speed/30 power to 50 speed/50 power. Much below 200 speed/40 power doesn't produce much of a recognizable image. And, it comes and goes within the same line of text, or if I reproduce the same line of text on the same piece of aluminum, one line will be fine and another line will be black in parts. My thought was also the aluminum tag. Interestingly, the red tags seem to be less of a problem than the blue or green tags.

[Brian Jacobs]

Did you try increasing power? My guess is that you got hold of a batch of unusually 'thick' (and possibly uneven coating) anodized tags. (I have purchased different batches from the same manufacturer and have needed to change my settings.)

[AL Ursich]

Any chance the lens is in at a slight angle and not flat? Causing a beam splitting of sorts....

AL

[Dan Hintz]

Banding like what you describe is common when there's too little power (but right on the edge) for the thickness of coating. If the banding does not disappear by slowly increasing the power by 20-25%, it's most likely something else.

[Joe Thorn 911]

I found the problem! The focal length was way, way off from the factory setting. The little plastic depth gauge is 2 1/4" and this worked well for most materials. I started at 2 1/8" and tested at 1/16" increments and found the optimal focal length at 2 9/16"! After re-aligning the laser dot pointer to coincide with this, everything is fine!
Thanks very much for your input.
Firejoe