Silicone Monolithic Laser Cover (aka "The Laser Cozy")

To further isolate our new monolithic lasers from accoustic vibrations and thermal fluctuations, we make a floppy cover using a casting compound. We tried two different compounds:

• Two-part urethane, Shore hardness 20A (from McMaster-Carr). Came from "Forsch Polymer Corp."

  This material did not meet the requirements for the laser cover. While soft and most likely a good material for damping vibrations and temperature fluctuations, it was brittle and very difficult to demold. In fact, even with a simple test piece, we were never able to remove the casting from the mold without damaging it in some way. Also, the material was easy to tear by both pulling and twisting (and this happened without much force). Finally, the urethane rubber had a strong odor that never really went away.



• Two-part silicone compound (from Smooth-On, Inc.)

  This material worked so well, we didn't even need to try a third. The silicone casting compound flows very nicely; is able to contour and fill small, detailed shapes; and releases superbly from its mold--even without the aid of a release compound. Furthermore, the cured material is quite resilient and does not readily rip or tear, even under violent pulling and twisting. It does, however, cut very nicely with a razor blade (and can even be pieced back together!). Like the urethane compound, the silicone casting has an odor, but it dissipates after a few days. Lastly, the silicone rubber has the added bonus that we can color it using food coloring or a specialized pigment.

  

  We tried two variants of a translucent silicone mold, one of Shore hardness 15A and the other 27A. While both showed the excellent qualities described in the previous paragraph, the softer version provided more of the flexibility and "squishiness" we wanted in a cover. Smooth-On also offers a 10A variant, but that is probably too soft. These are the products we use for the "Laser Cozy":

  • Mold Max 15T. Shore hardness 15A, translucent two-part silicone compound. There are two sizes: trial size (2.2 lbs NT WT) and gallon unit (9.9 lbs NT WT), with the latter producing about 4 laser covers.
  • Silc Pig 9-Pack Color Sampler: Basically a lifetime suppply of pigment for our purposes.
  • Ease Release 200: Labeled as "Mann Release", this is what we use as our silicone release agent.

Machine Drawings

To make the cover, we created a mold, which, due to the complicated nature of the shape we are casting, needs to be assembled from several pieces. The mold can be used for any of the beta-version monolithic lasers; simply swap out the appropriate pieces ("Isolator Mold" and "Front Wall") depending on the size of isolator used at the particular laser's wavelength.

The mold consists of Delrin (acetal) internal pieces, which approximate the shape of the laser, and polycarbonate external pieces, which form the outside wall of the mold and allow us to see the compound when we pour it. After the casting has set, the exterior pieces are easily removed without damaging the cover. Here are the drawings for the mold:

• Assembly (.vwx) (.pdf)
• Compilation of drawings (.pdf)
• 3D files for CNC machining (.igs)
• All short laser cover technical drawings (.zip)

• Assembly (.vwx) (.pdf)
• Compilation of drawings (.pdf)
• 3D files for CNC machining (.igs)
• All short laser cover technical drawings (.zip)

Note that instead of having material left in the front pieces to mimic the cable and fiber (as in the drawings), we use posts which slide through to the interior. This eased both the machining of the mold and disassembly after casting.

Due to a typo in the drawings when they were submitted to the shop, the mold ended up being 1/16" shorter than intended. At first we used a 1/16" piece of laser-cut acrylic to make up this difference, but it turns out that the material expands as it cures and we ended up cutting off about 1/16" of material anyway. So the corrective acrylic is no longer applied and we get a laser cover with nearly appropriate height and minimal aesthitic treatment. This "feature" has been accounted for in the long laser cover drawings, but the short model has yet to be changed (It's a quick fix, and I'll do it when I have some down time...07/2012, PJM)

Casting procedure

• Apply release agent to all surfaces the compound will come into contact with. Spray briefly with the Mann Release at a distance of ~8" and then use a small paint brush to lightly brush the entire surface, ensuring that there is a relatively even coat of release agent. Let dry for ~10 minutes, then spray lightly once more (I haven't been brushing it after this second round). Let it dry again, then assemble the mold. Remember to include in the laser-cut acrylic text if you want to label your laser.
• Mix the Mold Max 15T compound in a disposable container (needs to be 800 mL!) as instructed in the data sheet (10:1 mixing ratio, stir for 3 minutes). Add desired pigment...it doesn't take much! Pour slowly so that no large bubbles form. Tilting the mold a bit will help somewhat, but we're pretty much stuck with some bubblage unless we degas the compound (during which it will expand to roughly 3x the volume). We haven't bothered with this yet, but it's something to keep in mind.
• Use a flat-edged object to level the compound by scraping away the excess. Allow the compound to solidify for 24 hours, then carefully cut away overflow material (this comes from the expansion of the material as it sets).
• Remove exterior pieces of the mold, starting with the front. Carefully peel the cover off the interior pieces and then clean up corners and text regions.
• Use a razor blade to slit the material from the base to through-holes for the fiber and connector cable. This allows for the cover to be opened and wrapped around these obstacles as it is put into place. Lastly, let the cover dry and outgas for a few days (possibly bake it at a relatively low temperature).