Protocol of Photo Engraving by CO2 Laser

What are the most important parameter in engraving photo by laser: power, speed, focus lens, scan gap, or others?

Introduction: Handling (Tips and Tricks):

The shorter the focal length of the lens, the better resolution of the dots can be generated.

For transparent materials, invert the image.

Sketch is the key element on the image processing.

If the material is more dense in structure (like glass, acrylic, stone), use the lower scan gap size and high dpi (over 330).

If you are using cardboards or thin materials, use small magnets to keep it fully flat during the engraving.

Procedure:

1. Use a laser head lens with the shortest focal length

, such as 1 ½” or 38.1mm. The shorter the focal length, the smaller the spot size and shorter depth of focus. 

2. Determine the best DPI / PPI(dot per inch or pixel per inch)

100 PPCM (pixel per cm), means 100pixel/1000mm or 1pixel=0.1mm;

In theory with a 1 ½ “ or 38.1mm focus lens, on cardboard materials, we can expect to get 0.075mm resolution of a single pixel (hit of a laser light) and with a 2” / 50.8mm focus lens, it will be 0.1mm resolution. In practice, with a 1 ½” or 38.1mm focus lens, we can reach around 0.15mm diameter of one pixel on some cardboard. 

3. Find the best focal point by scanning

Using the 1 ½” or 38.1mm focus lens, start at 6mm distance from the laser head tube and scan it up to 4mm, every 0.5mm, i.e. for 6.0, 5.5mm, 5.0mm, 4.5mm and 4.0mm distances. Then observe the printed pattern images under microscope. Look for separation of dots and thickness of the line. The thinner the line, the better resolution. If dots start joining together, that is not a good choice of focus distance. 

As you will see, with 10x or 20x magnification, both 6.0 and 5.5mm distance normally shows better resolution of dots. Between these two, the 6.0mm distance shows some dots start joining up and it is better to go to 5.5mm distance. In addition the 5.5mm distance shows the thinnest line. 

4. Next step is using 5.5mm focus distance, and find the actual dot size:

We can use dots at size of 0.1mm and the gap of 0.1mm, as well as 0.2mm dots to figure out the pixel size (laser hit size) in mm. In other word, we need to figure out what resolution of image we can produce.  

The measurement shows the size of dots is 0.18mm on cardboard. This number can be as low as 0.05 on PMME.

5.Calculate the resolution of picture we can produce:

Resolution of a picture is known by PPI or Pixel per inch. We have a 1” or 25.4mm per pixel. How many of the laser dots do we have in one inch?

Lasered output image resolution (dot per inch) = 1 inch / (size of a laser dot) = 25.4mm / 0.18mm = 141 DPI. This is the best resolution of an image we can produce with the lens of 38.1mm at focal distance of 5.5mm, so far. 

Note: Changing the output image resolution from 141 dpi to 282 will not cause getting the dots at half size (0.09mm), but it will rather cause to have double dense laser hits.  

The conclusion is the white region becomes terribly lost when we use an excessive amount of laser hits by choosing the double dpi. If we zoom in the 282 dpi output image, you will see how we loose the resolution (below pic). At below picture black dots are 0.18mm size with 141dpi and red circles represent the 282 dpi. As you see the red circles shows the double burn by overlapping the regions, which results in loosing the white space. Using this dense dpi causes extra amount of laser to hit, and more likely a 3D picture to come out (with depth due to more hits).

6. Software (RDWorks) Works:

6.1.  Convert the image to grey scale.

6.2.  Try to increase the brightness, more than a photography pic is supposed to be.

6.3.  We need to resize it to what needs to be lasered to, for instance 6” or 150mm wide. 

6.4.  The actual image resolution is 600 dpi. Go to Handle > Bitmap handle, drop down the image resolution from 600 to 141 dpi. Thus we set the output image resolution to this number, apply the source then OK.

7. Set the Laser Parameters:

Calculate the proper speed,

Scan Mode

No Below Air

Power: start at 12 or 13% which needs to be optimized  from 9% to 15%

: 0.18mm

Calculate the speed:

The amount of time that takes for a single laser bit to move is ~3ms. You may try 1ms, 2ms, 4ms or 5ms numbers as well, depedning on your machien mechanics. 

Speed (in mm/sec)= travel distance / time = 0.18mm / 1ms x (1000ms/1s)=180mm/s 

Experience of the printing speed vs DPI: https://www.youtube.com/watch?v=bSsaYZTmAGo

 

8. Optimize the proper laser power, around %9-15%

You can run multiple tests of the following pattern at 9% till 15% with 1% increase to find the lowest possible power, which burns. Look at the relative darkness of the lines and dots, to see what lower power can generate that matching quality of lines and dots.  

In comparison, the 13% power gives better resolution of dots than 12% and thinner line than 14%. Meanwhile the dots start making joining up at 14%

9. Make sure you keep the lid is closed and have uniform airflow for exhaust.

10.Enjoy the generated picture.