Silicone prototyping

Right now, a shop in eastern Pennsylvania is working on 1000 custom injection-molded silicone stoppers for the Backcountry Boiler. But the tooling and custom color formulation are costing a pretty penny. And they certainly weren’t available to me when I wanted to make a prototype for testing. So, as is my MO, I made one at home.

Silicone rubber is an amazing material. Soft and pliable, but with fantastic heat, chemical, and UV resistance. But unlike many other materials, it doesn’t lend itself to direct machining or rapid-prototyping production methods. However, if you can get a positive made to the dimensions of the silicone part you want, you can cast both the final part and its mold out of silicone.

And that’s what I did. Here are the steps:

Choose a silicone: There are several different kinds of moldable RTV silicone (those that don’t require heat or UV light to cure) from various suppliers, but not all are created equal. They come in all kinds of viscosities, cured hardnesses, even colors. “Platinum cure” silicones are the ones to get for contact with food and should indicate if they are FDA compliant.

Make a positive (the shape you want the final part to take): My positive was machined out of aluminum by friend Steve Evans of Suluk46 based on drawings I sent him, but could have just as easily been made through the increasingly popular 3-D printing techniques. Shapeways is probably the best known provider of the service, but there’s also one based out of Pittsburgh – CloudFab.

Make a form for the mold and prep the positive: I used acrylic sheet held together by hot glue for the form, and adhered the positive in place with modeling clay. See this video and this video for the fundamentals of two-part mold making.

Mix, degas, and pour mold silicone: Back to YouTube U, here’s a walkthrough of these steps. Getting precise proportions of silicone and hardener by weight should be no problem for the dedicated ultralight hiker with an analytic scale. There are ways to make your own vacuum chamber that can run off of an air compressor, but I sprung for a dedicated Nalgene bell and vacuum pump. Together they cost about $200, should have reasonable resale value, and saved me implosion worries. They can also make marshmallows get really big.

De-mold and pour second half of mold: Silicone sticks to very few things, but it sticks to itself veraciously. Make sure all exposed silicone gets a liberal blast of release agent. Otherwise, the two-part mold will be a one part mold. :)

Cast the final part: This is where you can add pigment to the silicone to achieve the desired color of the final part. Release agent is again your best friend, and I ruined the first mold I made by not applying it well enough. Once cast, let it set for the full recommended time, and voila: that custom silicone stopper or other component you’ve been yearning for.

Ahhhh, it feels good to write a post about making something with your own hands. If anyone out there is looking to do their own silicone molding, hit me up. I’d be glad to share more details of the limited wisdom I gained along the way.