How to make biomed clear part uniformly opaque like frosted glass

Anyone know how to make biomed clear resin parts look uniformly opaque, like frosted glass?
Nearly all posts about clear resin are seeking ways to make it transparent and polished, but I have the opposite problem. I’m using biomed clear to meet FDA biocompatibility/cytotoxicity requirements for a part that contacts skin, but I don’t like the semitransparency, which shows the circuitry inside the finished device and makes any scratch very apparent. A contractor hired to make these parts (using a different printer and biocomp resin) sanded then painted the finished parts, but the paint wears off over time. I tried just making the parts with Formlabs standard black resin, and they look great, but they failed biocomp/cyctoxicity testing miserably. Requiring cytotox test success for a part that has only occasional, incidental skin contact is dumb, IMHO, but I don’t see a way around it from an FDA and CE Mark regulatory point of view. Formlabs biomed clear parts should pass cytotox testing, but I just dislike the appearance and want to avoid painting. Sanding by hand is a poor option for these parts. Tumble sanding might work, but my attempts so far in a steel shot jewelry tumble sander just made the parts look dirty. I wonder if anyone has a chemical vapor means to achieve a frosted glass like surface on biomed clear.

Your best best is probably bead blasting.

That being said, changing the surface roughness of the material might cause FDA and biocompatibility issues though, so keep that in mind.

Thanks leonhart88. The cytotox test protocol actually requires grinding the part into pieces before the test, so surface mechanical features probably don’t matter in this context. I agree that it could affect outcomes with an implant.

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Go to your local Harbor Freight and buy a Rock Tumbler and some crushed Walnut shell abrasive. Put parts and Walnut grit in tumbler, tumble until desired surface texture is achieved. I would guess 5 or 10 minutes of tumbling might be all it’d take to frost the surface.

I tried steel beads in water, steel irregular pieces in water, silicone carbide grit in water, and dry walnut shell grit in a tumble sander and none gave a good result. I suggest others not bother with those approaches if the goal is to make cured biomed clear look uniformly frosted like frosted glass. I suspect I’ll need a chemical approach. Or painting.

I’d really rather tjuts just opaque standard resin, but FDA regs preclude doing do, because they mandate passing cytotoxicity testing, which standard black resin failed completely.

Tumbling is usually used for polishing and smoothing out surfaces so that will do the exact opposite of what you want.

Bead blasting or sand blasting is the way to go. I’ve done this before for turning clear acrylic frosted. You can get pretty small cheap sand blasters.

Also available at Harbor Freight. :slight_smile: