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The problem I have with this thread is that the A&E editor summarized this question as "How do I avoid horizontal banding when laser engraving?" However, it does NOT appear that the original poster actually has a banding problem, even though the word "banded" was used in the problem description.
Many of the responses may be correct and quite relevant to the issue of banding, but I don't think this is what the OP was describing.
It is my impression that in printing and laser engraving, banding is generally interpreted to be interspersed contrasting lines (usually whiter) parallel to the raster direction, which may be spaced a few millimeters apart to centimeters apart. It appears as a discontinuity in the photo or bitmap image.
I believe that what the OP was referring to is not actually "banding" per this definition but the fact that solid areas show discrete "troughs" where the laser has passed. It may look like a regular woven fabric pattern, or parallel grooves. In any event, it is objectionable because it does not look like a "soft" fill which I think is the objective.
If this is the poster's problem, I think Mr. Korbyl's response is correct. Actually, I don't believe that you need to go as much as 80% black - a small deviation from true black will often correct the problem and soften the solid fill areas. (If you change too much from 100% black it will cause the edges of characters and line art to become jagged.) In the case of large text, I fill with 90%+ black with a 100% black outline, and convert to b/w using an algorithm such as Stuki. This creates a solid outline with a soft even fill.
There is a lot of good info posted here as well as some not so good. The banding is a problem many of us face at one time or another. Next time and issues like banding comes up, I think it would be prudent of A&E to ask the experts at the laser manufacturers like Universal or Epilog to respond first. then others stating their opinions and experiences on the subject can do so. I'm just saying...
Banding is caused by a number of reasons the most common reason is laser based and a misunderstanding by users. Unfortunately it can be hard to separate mechanical problems from image design problems to maintenance issues from outright laser tube failure. Here are the main issues we have found.
Reason 1:
A Laser on/off time is a significant factor and is based on the design and gas mixture used with in the laser tube. Some older laser tube designs have very long on/off times or what is know as rise and fall times. If the speed of the laser system combined with the DPI of the file exceeds the rate at which the laser can turn on and off then the laser can never achieve full power and the laser becomes unstable so the actual power will vary across the engraving hence banding.
To mitigate these effects you can actually calculate the maximum speed at a given DPI of file how many times per second the laser will have to fire and match this to the rise and fall rate of the laser. A good rule of thumb is to always run the laser at full power and to run the feed rate as slow as you can without over burning the material.
Reason 2:
Banding can occur when an engraving file includes areas 100% white which causes the laser to go unstable. This issue can be mitigated by reducing the white areas below 80%. A better way is to diffusion dither the file using a software like Photograve.
Reason 3:
The laser is failing OR the optic on the end of the laser has dust on it or your mirrors or focus optic is dirty. I have also seen users who use the wrong focus optic or an out of focus optic. When processing large areas of white space it is always best to use the shortest focus optic you can to achieve the highest energy density which can over come some of the issues described above.
Reason 4: (And most common problem)
The way gantry systems are designed they use guide systems
known as linear bearings many laser engraving systems use roller guides. Both vertical and horizontal lines within an engraving are most commonly caused by these bearings. What happens is as a the head travels across the material the play within the bearing is forced to one side when it reverses direction the head deflects slightly to the opposite direction. What this cases is what called line pairing where the sea wave effect is out of sync so you get areas where the the line from one pass touchs the proceeding line and then arcs out and leaves a wide gap between the the proceeding line so a wave effect.
Many laser users misunderstand the causes of banding these are the most common reasons banding occurs and from we have observed in building laser systems over the last 20 years.
Good Luck
We had to replace our laser tube which was not holding it's power, causing banding.
I am sorry but Richard is incorrect:
Banding is caused by the new breed of high speed laser gantry, (x-motion). The laser is asked to be fired faster than if can recover after eash firing. CO2 laser are know for going into oscillations or randam fireing before they are totally recharges. In working with a raster design the laser is best used at a maxium speed of ~50% speed, espically when engraving photos. If you look at a laser burn spot with a 10x lens, you will note that the spot is out of round at 100% speed, therefore caueing photo images to have poor contrast. Slow the laser down and the photos will look much sharper
I have the same issue on large photos and it is always in the lighter areas like the sky or maybe a lake, can't I have seen it in darker areas. It seems to help if I cut down the head speed by 20%. Also making sure the table is level seems to be important.
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