Silicone prototyping

by Devin on August 2, 2011

My silicone prototyping setup. Back row (L-R): RTV (room temperature vulcanization) silicone, hardener, and pigments; Nalgene vacuum bell and mixing container; Harbor Freight vacuum pump. Middle: hot glue gun, modeling clay, silicone mold release agent. Front: Finished silicone prototype; machined aluminum positive; silicone mold; acrylic mold framing.

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.

[ 9 comments… read them below or add one ]

Robert August 3, 2011 at 12:37 am

Thanks. I have wanted to do this and felt it was a bit unapproachable.

Rob August 3, 2011 at 6:27 am

Have you looked at Sugru for your silicone rapid prototyping? Works out of the sachets without need for heat or vacuum….
http://sugru.com
(no business interest, happy customer, etc)

Devin August 3, 2011 at 7:55 am

Robert – Glad to hear – I had felt the same way until I spent an afternoon perusing those instructional videos. I was really impressed with how, well, instructive they were. :)

Rob – I give Sugru a big thumbs up. With all the different colors it comes in, it’s probably the most attractive putty silicone. For the kind of finish quality I needed on this part, the liquid setup was better suited, but for casual use, Sugru is definitely the way to go.

Robert August 12, 2011 at 2:43 am

I have considered Sugru. Most of the examples I see are crudely formed and a bit aesthetically disappointing.
Using Sugru with a mold as a pressmold rather than a pourmold might work great and allow for a quality appearance.
I understand there is a non-RTV silicone process also.

Jacob D August 16, 2011 at 1:53 pm

Hey Devin, thanks for sharing your process. I used to work in the tool and die industry. It’s fascinating to see this done in small scale.

Devin August 16, 2011 at 2:04 pm

Hey Jacob! You’re a many-talented man. The small scale is where I live, so it’s fascinating for me to see this stuff in the big time. I’m really looking forward to seeing 1000 of these little guys, all exactly the same. :)

Jacob D August 16, 2011 at 3:07 pm

Jack of all trades, master of none… Thats me :) I think the tool design-production-assembly is tops, but no doubt watching that press spit out your parts will be a fulfilling moment. Keep up the awesome work D!

Mary W May 23, 2013 at 4:39 pm

Nice pictures and instructions. Can you provide more details/info on the exact model of vacuum pump and nalgene chamber? I’m having a hard time finding reasonably priced items. Thanks.

Devin May 30, 2013 at 8:21 am

Sure! The pump is from Harbor Freight: http://www.harborfreight.com/25-cfm-vacuum-pump-98076.html, and here’s the cheapest Nalgene bell I can find: http://www.opticsplanet.com/nalge-nunc-vacuum-chambers-nalgene-5305-0609.html. Not super cheap, but I’m sure you could find the bell for less on Ebay if you can wait for the right auction, and HF is always having sales. Curious – what are you making?

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