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Making highly polished divots


Apr 13, 2007
I have an application that calls for 1/16" +/- 1/64" dia dents with a 1/4" +/- 1/16" radius of curvature in something that I mold silicon into. The dents should have an RMS roughness of less than 0.5 microns. The dents should ideally be close together -- a hexagonal grid would be ideal, but a random arrangement is fine, too. I've had a few ideas that I'll list here -- maybe others will have some ideas. The domes don't really have to be perfect domes. The radius of curvature around the top half is important, and the high polish is important.

  1. Coin it into copper. Take a sheet of copper and polish it to better than 0.5 microns roughness, take a ball bearing and whack it or press it in with a vise. I've had good luck with this but I don't have much experience getting high polishes. I'm pretty inexperienced with finishing, and am pretty strapped for time. I tried polishing copper but wound up with a bunch of tiny pits. Maybe I loaded up my sandpaper. Might try again with much more careful unloading of my sandpaper, and using a full series of lapping film. What would be nice is to find something commercially available that just comes with a high polish. Perhaps discs from a hard drive?
  2. Place a silicone membrane over a box with some holes in it, and pull a very slight vacuum. Silicone will self level and make an extremely smooth surface that the box will pull into the desired shape. I did some quick calculations and found that for reasonable sizes, the volume of air I would have to remove is so small that this feels too touchy.
  3. Hot press. Take a bunch of ball bearings and epoxy them into a grid on a flat surface. Take a sheet of acrylic with a high polish (commercially available!), heat it, invert the grid of ball bearings, and press them down into the acrylic. Repeat this for various positions of the ball bearings to uniformly cover the plastic with dents. This seems like my best bet. Bit worried about it sticking -- maybe some mold release would help?
  4. Diamond turn it. I don't have the $5-10k this would take!
  5. Greyscale lithography. I'm at a university and am trained on a lot of cleanroom equipment. You can expose photoresist at a variety of doses and make arbitrary profiles. Could work, though not that controllable for the thicknesses I want to use. Also very expensive, since greyscale lithography requires switching doses for each level and rewriting.
  6. Resist reflow. I could expose a grid of dots, and heat the resist so it flows and makes a dome and cast an inverse to use as a mold. Also, not that controllable for large diameter bumps.
  7. Wet etch silicon -- Again hard to control the radius of curvature
Does anyone else have any ideas for how to do this?
Polish a peening hammer and tap with authority your pre-polished metal substrate. Copper and silver are joys to planishing, pure iron is alright.