Building a Custom USB Flash Drive -- Part 2: Resin Casting in a Silicone Mold

So, you've got a silicone mold all ready to go?...now you just need to make something in it. This post shows some of the steps to pouring urethane casting resin into a silicone mold. Other types of resin (like epoxy) could be cast into a silicone mold, but some resins cannot. For example, polyester casting resin won't set up well in a silicone mold because the silicone actually inhibits the chemical crosslinking (and hence, curing) of the polyester resin. If you want to cast polyester, you're better off making a urethane rubber mold rather than a silicone one (but you'll also have to use mold release as a surface treatment before pouring the material into the mold since urethane isn't self-releasing like silicone is). Part of my reasoning for using a paper cup as my mold form (aside from the simplicity and low cost of them) was that I could re-insert the mold into a cup of the same size to seal it off before pouring resin into it (as shown below). Without any seal formed around the mold's parting line, liquid resin poured into the mold will just run right out of it. For custom or irregular-shaped molds, it's good to either place the mold back into the original mold forms, or to otherwise seal off the mold with packing tape. If you're using the latter, wrap the whole mold so the tape can stick to itself since it won't stick to the silicone mold or otherwise make an effective seal.

Before going hog-wild with encapsulating my fancy-shmancy USB drive in the resin, I figured it was wise to do a test pour with the resin first--just to make sure there were no issues with the mold. I always prefer to do a test part before assuming that things will work our perfectly because there's always so much that could go wrong, and so little working capital to burn on faulty projects. So, to make my test piece, I measured out equal parts of casting resin (since the Alumilite requires a 1:1 mix ratio by volume), and poured them into a cup...

...and then I mixed them thoroughly. As with the silicone resin, I made sure to completely mix this thermosetting resin to avoid soft spots or areas of uncured resin in the final cast part.

Next, I poured the resin in a thin stream through the sprue into the mold. If the sprue isn't wide enough, the surface tension of the resin can actually bridge across the sprue and stop it up so resin won't pour in smoothly. For safety sake, it's good to make sure there's at least 3/8" diameter or more space down the length of the sprue.

Once the cavity of the mold was filled, I let the resin sit for about ten minutes (the recommended cure time for the resin) and double-checked that the residue spilled around the sprue was completely hardened. Thin sections of excess resin like these are usually a good indicator of the state of resin cure within the mold. I then peeled off the cup...

...pried open the mold, and extracted the cast test piece. Aside from the a little bit of "flash" (excess material that formed within the parting line) and a big plug o' material at the sprue that needed to be trimmed off, the piece came out rockin' sweet.

Next, I needed to prepare the USB drive for encapsulation in resin. This required a couple extra steps to keep the casting resin from seeping into the connector end of the drive and filling it up. Resin that gets into the connector end can actually make it impossible to plug the USB drive into a computer in the future--a very bad thing, indeed. If you inspect the back side (or circuit board side) of the connector, you'll notice that there are some very small openings around the wires that attach the connector to the circuit board. These holes can allow resin to leak into the connector, so I used a quick bead of super glue to seal them off quickly and effectively.

Just to make sure there weren't any other areas open for resin leakage, I used some packing tape to seal off the rest of the connector end of the drive. Urethane resin won't stick to the packing tape, so this tape helps promote good clean up of the final part as well.

After sealing up the connector end of the drive, I inserted it into the silicone mold, right where the dummy connector had formed a cavity in the bottom of the mold. This cavity created a good registry for the USB drive so it would fit properly in the mold.

I closed up the mold by putting it into yet another paper cup, mixed up some resin, and poured it into the mold through the sprue.

After waiting long enough for complete cure of the resin, I pulled the newly formed part out of the mold with the USB drive encapsulated in it.

A little bit of trimming with a utility knife was all that was needed to clean up the part. I cut off the sprue plug, cut out the knock-out center (for the finger hole), and removed the tape from around the connector. To get the part prepped for final painting, I sanded the surface of the urethane with 400-grit sand paper, just to ensure that there weren't any chemical residues on the part that could foul-up the paint job.

After a quick paint job, here's what the final part looks like (below)...super-groovy and ready to store up all that important data that I've got kickin' around.

While this drive-forming demonstration shows a somewhat simplistic part, the flexibility of a silicone mold and the robustness of the urethane casting resin can help you build a wide variety of nifty shapes and parts. And since these molds will work for over a hundred castings (if you play nice with them), you could even make bucket-loads of these little gems for whatever purposes you deem necessary. Aren't plastics fun?  I'm a big fan. Have fun, ya'll.