Laser Cutting backing material instead of grid?

Question

My research group recently purchased a 40W CO2 laser cutter. We work in the biosensing field and some of the materials we wish to cut are very thin nano/micro fibers that will be cut into relatively small pieces.

When cutting the material we hope to keep the samples as clean and uncontaminated as possible. So the current grid is not the best option as it will most likely get contaminated by other materials we cut and also the small samples will most likely fall through the grid anyway.

Can anyone suggest an alternative solid material that can be used instead of the grid? Ideally the material should not be affected by laser cut at all or at least not give off any smoke, particles or other contamination. Also the material should not reflect the laser back towards the sample.

Thus far I have tried two options on an external laser cutter before we got our own.

1st) Ceramic tile as I was told that it would not reflect any of the laser underneath the samples and also not get cut by the laser. BUT after testing it was found that a whole lot of molten ceramic was deposited on the samples.

2nd) Stainless steel metal sheet that was orbital sanded to reduce reflection. This was a little better than the ceramic but I think there was some reflection of the laser under the laser that reduced the quality of the cut. I only assume this because there was a little burnt material left on the steel after cutting the material.

I'm thinking possibly glass above the grid but I have heard that even glass can be etched and so if the sample material retains heat this may cause the glass to etch or shatter? We are yet to setup and switch on our new laser cutter and some experimentation and optimization is needed.

Thanks in advance for any advice!!

Answer 1

Steel with a black passified surface (as mentioned by Jason in the comments) would probably work very well. This surface is Magnetite and has a melting point of 1590°C. The carbon steel has a high thermal mass and reasonable thermal conductivity to pull the heat away from the focal point.

Similarly, copper plate black oxide surface treated to have a layer of cupric oxide would also work well. Cupric oxide can withstand 1,326°C and the very high thermal conductivity of copper will pull the heat away from the focal point. Either of these solutions will likely require buying a metal plate and applying the surface treatment yourself, as they are not widely available.

Another thing to consider is the focal length of your laser. A shorter focal length will permit the kerf of the beam to be smaller. However for normal cutting a shorter focal length also reduces the depth of cut, so manufactures seek to strike a balance between these. Since your depth of cut is likely very small, you will want to source the shortest focal length you can and closely test the focus each time before proceeding with a cut.

It is also important to note that cheap lenses will suffer from spherical aberration because a sphere is cheaper to manufacture. An aspheric lens will cost more money, but will result in a smaller focal point. 1 inch focal length lens from ThorLabs

You may also want to experiment with different laser powers and different cutting gasses to reduce contamination. Selection of the material in the product you are cutting is also important.

This is an absorption graph for uncoated steel and copper at the 10.6um wavelength. I unfortunately did not find one for the coating materials. If the material you are using is fixed, and the other items above have not addressed the issue, you may want to look a different wavelength cutting laser.

Answer 2

If money is no object, an alumina plate could work well, maybe 1/4" thick. It is a fairly nonreactive material and we use it in crucibles up to 1500C.

Answer 3

I make Acrylic game pieces with laser printed artwork on a peal and stick full page poly label. Some of the pieces are double sided so you can flip them over for a half strength game piece.

The metal background grid was a real problem for these double sided pieces, the pattern of the grid would get scorched onto the back of the game piece.

The solution was quarter inch bar stock cut into blocks, one for each corner, and a small post in the middle. This raised the plastic a quarter inch off the grid and I no longer get scorch marks on the backs. I had to alter the cutting path so that the page size cutout came last. Now the only scorch marks are from impurities in the plastic, and those clean up with 100% alcohol.