Material Madness

Due to my curiosity and love of searching out and testing new materials to work with on my laser, my laser center has shelves full of untested or tested-but-not-perfected items waiting for my attention. Occasionally, I need to focus on just one material and make a decision of whether it should be kept and promoted in future products or should it go to my bin of dashed dreams (big gray trash barrel).
Sometimes the bin fills up about as quickly as my shelves of hopefuls, but that is OK. Like I said, I love finding and testing new materials. For me, "new" materials fall into two major categories
  1. Materials that others use, but I have not.
  2. Materials that possibly nobody has used, but look interesting to me.
Last night, I decided to spend a little time working with a product from category 1. This time, it was Laser Tile.

I have used Laser Tile for a few small projects in the past; mostly single-tile awards, paper weights, coffee cup coasters, trivets, etc. But I always wanted to use it to do photos and corporate logos and some other products that potentially have large contiguous areas of solid black. Designs like that have always given me real headaches on Laser Tile; the major one being an inconsistent shade of black. When I get close to what I think is right, I often notice that holding the finished tile at a different angle shows a different reflection from the blacks in one area than in another; even to the point of making a solid black area look striped. Albeit faint, I know it is there and refuse to allow that into a customer's hands. Midwest Laser Arts will never knowingly give a customer a flawed product. So, the goal last night was to come to a final determination as to whether I can keep Laser Tile as a viable material for my work or to forget it and move on to something else.

Of course I have made past attempts to get it right. I would intuitively vary my speed, vary my power, vary the scan interval (DPI). Like I said, I would get close, but never close enough to be satisfied. And the bin of dashed dreams has already seen way too many dollars worth of failed attempts with Laser Tile. This time, I vowed to make it different. This time I would steer away from my intuition and stick with the more scientific approach that has worked well with other materials in the past. Thinking others might be interested in how to make relatively short work of determining your settings for a new material, I thought I would detail here what I did and let you see the results.

While I want to limit my intuitive influence, you have to start somewhere; and there is no better place to start than with your own experiences of the past. Yes, you can try to correlate manufacturer's recommendations to your system, but that can be pretty tough. (What does 40% speed really mean? My old laser maxed out at 20mm/sec. This one tops out around 750mm/sec. So what speed is 40%?) True, I look at manufacturer's recommendations, but only to get a feeling for whether the material likes "fast" or "slow", "hot" or "cool", "fine" or "course" on the resolution. My database has lots of notes about all kinds of materials, including Laser Tile. But many are from my old laser, so I decided to start from scratch and throw in some numbers that worked well for some other materials with this laser - 200mm/sec, 40% power, and .05 scan interval (~500 DPI).

When working intuitively, I would just throw those settings in and fire away. The better approach is to use a trick from photography; bracketing. Bracketing is where you use what you think is right, but you also take some shots with settings that result is more exposure and additional shots with settings that results in less exposure. Then you can see a range of results, side by side, and use the ones that turned out best.

I combined that idea with an concept from my 35+ years of software development. When trying to debug a problem you change one variable at a time and see how the system runs differently. Changing more than one variable just adds confusion. For my tests, I decided to vary the speed and hold the power and scan interval constant. I generally like operating in the 40% to 60% range as it lets me keep the speed up and the time to complete a job down. The .05 scan interval (~500 DPI) was chosen because I know that Laser Tile has a very smooth surface and would probably benefit from the higher resolution. On wood, I am usually nearer to .10 or .09 (254 DPI, or 300 DPI). The grain in wood, and it's charring, don't usually stand up to higher resolutions.

A quick note here about the test image is probably in order. The goal is to build a test image that is representative of the kind of work you want to do. I mentioned doing photos. I could test using a photo, but photos are so different from one another, that I might get it right for the test photo but every other photo would need something different. Instead, I decided to use an image that is representative of the other kinds of work I want to do with Laser Tile, but has some of the characteristics that have caused my previous grief - solid blacks. A solid square would work - I have tried that in the past, but I also wanted areas that force the laser to be turned on and off somewhat rapidly during a scan line. Why not use MY logo?

It has some nice solids and some lettering with narrow spacing, especially when I make it small as I did for this test - only about 1.3" by .75".

Back to the test method now. The intuitive method would have me run a test, then try to determine if faster or slower or hotter or colder would improve it. The better method is to run multiple tests at once - using the bracketing method mentioned earlier. I set up a job with 4 copies of the logo, each a different color. This would allow me to set 4 different speeds - 400 mm/s, 300 mm/s, 200 mm/s and 100 mm/s. This brackets my 200 mm/s intuitive start point and gives me wide enough brackets that I should see some real differences between them.

In each of my tests, I set them up so that speed increased right to left and power increased left to right and scan interval decreased from left to right. With this theme in mind, the output should always range from weakest "exposure" at the left and progress to higher "exposure" as we move right.

Time to test. Send it to the laser and hit Start.

400 mm/s    300 mm/s    200 mm/s    100 mm/s

(All tests run on a piece of 6" cream colored Laser Tile.)

The 200 I was thinking might be good was sort of OK (looks better in person than on camera), but the 100 certainly was better. Time to move my brackets and try again; this time 125 mm/s, 100 mm/s, 75 mm/s and 50mm/s.

125 mm/s    100 mm/s    75 mm/s    50 mm/s

This is starting to look pretty good. But choosing between 100 mm/s and 75 mm/sec is really tough. Time for a third test; 95 mm/s, 90mm/s, 85mm/s and 80mm/s. I still have the 100 and 75 from the previous test run if I need them for comparison.

 95 mm/s        90 mm/s        85 mm/s        80 mm/s

At this point, I cannot really tell any difference between the 100 mm/s and the 95 mm/s. Looking at it with a magnifying glass makes me think that, if I were to made a choice right now, I would probably go with the 95. In fact, if I had to run a job right now for a customer, I would probably be able to get a good product out the door and be happy with it. But, since I am playing, I may not be ready to choose yet. What would happen if I changed the power? Would it make any noticeable difference? So let's make the speed constant at 95 mm/s and start bracketing the power with 30%, 35%, 40% and 45%.

30%            35%            40%            45%

As it turns out, it is a tough call between the 40% and the 45%. For now, at least, I think I will leave it at 40%.

OK, good enough, right? Well, maybe not. What if we now hold the speed (95 mm/s) AND power (40%) constant and vary the scan interval? Would it make a difference? After all, higher resolution (lower scan intervals), should give finer detail. But higher scan intervals (lower DPI) will run a job faster than the lower scan intervals (higher DPI). Let's try this -  .08 (~ 325 DPI), .06 (~ 425 DPI), .04 ( ~635 DPI),  and .02 (1000+ DPI).

 .08              .06              .04              .02

The .02 is definitely out of the question. That many close scan lines actually ate the glaze off the tile, which we do NOT want. Laser Tile has special coating UNDER the glaze that turns various shades of gray or black when heated by the laser. The outer coating glaze protects it and makes it weather proof and very scratch resistant. Removing the outer glaze defeats the purpose of the glaze.

At the other end, .08, I can begin to see slight shadows of the individual lines. Don't think I want that either.

The other two, .04 and .06 really don't appear to be much different than the .05 we had been running at. The .04 may be starting the flake off the glaze, so I'll eliminate it. That leaves me to choose between the original .05 and the .06. Visually, I doubt I could tell one from the other if I did not know which one was which. Since the .06 runs at 2.5 minutes and the .05 was taking about 3.25 minutes, I'll take the significant speed increase by going with the .06. After all, this is a small test, only 1.3" by .75". A real job on a 6" square tile will hugely magnify the time difference. Besides, if I ever notice a problem, I can always drop it back to .05.

Just as a note, while the .06 is running at about 2.5 minutes, the same test at .02 took 7.75 minutes, 3 times the processing time of the .06. Stands to reason as we are doing only 1/3 the scan lines, but wanted you to know how important it is to optimize your scan interval to get the shortest time while maintaining the quality you want.

Finally, I have what appears to be a quality output of my logo with power conservation and optimized scan interval. I have something I can work with.

95 mm/s, 40% power, and .06 scan interval

The question is: Will it scale? Will these settings hold up to larger areas of black? As it turns out, they do quite well. Here is the same logo blown up to about 4.5" by 2.5". That's nearly 340% of the original size.

Is this the BEST I can do? Maybe not. Maybe I could up the power and increase the speed. That would make the job run faster, but it is not likely going to improve the product, in this case. Still, it would be nice to run faster. I'll save those tests for another time, knowing that I at least have something NOW that I CAN work with for production jobs with filled vectors.

For me, Laser Tile is a definite keeper!

Eventually, I will need to test these settings with photos. That will likely mean additional bracketing, but now I have a great starting point.

A final question? How do these settings compare to my previous intuitive setting results? I looked back at my database. The "best" results I ever had were at 50 mm/s, 45% power, and .10 scan interval. The power was about right, but I sure was a long way off on the rest of it. And the proof is in the stunning visual results.

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