Making Silicone Molds – Big Ones!

If you’ve got one of something and you want more, duplicating it with a silicone mold can be a great way to go. This is applicable to 3D printing something you need many copies of, and a whole variety of other usecases. [Eric Strebel] prides himself on his abilities in this area, and has put out a guide to producing very large silicone molds in a simple and reliable manner.

The overarching process is simple, but followed properly, it produces great results. [Eric] starts by building a mold box out of wood, coated in shellac to ensure it doesn’t stick to the silicone. The master part is then stuck to the base, surrounded by a lasercut cardboard strip which acts as a seal and key. Once properly degassed silicone is poured in and cured, the second half can be made. The mold is flipped in the mold box, the seal key removed,  and release agent applied to the silicone surfaces. With another pour and cure, the mold is ready for casting new parts.

While simple, if the correct equipment isn’t used or steps skipped, you’ll end up with a useless mold full of air bubbles or surface irregularities. It’s useful to see just what it takes to get a mold of such scale (13″ x 19″!) completed without flaws. We’ve featured [Eric]’s work before, such as his fine detail improvements on the Apple Pencil. Video after the break.

11 thoughts on “Making Silicone Molds – Big Ones!

  1. Excellent video. Eric is a master. The difference between kinda knowing how something works to actually getting tip top results is huge, particularly on a large scale.

    I’m a bit confused about the economics of this though. That much silicone plus Eric’s time would be very expensive, especially for just 100 parts. What exactly are those sports car things he is making?

    1. And how expensive would it be to get 100+ parts any other way?
      Perhaps some money could be saved making this a fibreglass outer shell with thin inner layer of silicone, but those sort of molds are a pain (IMO a much much bigger pain) to make, harder to store, less convenient to fill up properly (except for expanding foams and slush casting – which makes hollow parts and is largely what they are used for). Still not cheap either, just perhaps cheaper than such a volume of casting silicone.

      If a thinner end result would suit then vacuum forming is the nobrainer cost wise. But I don’t think anybody can really expect to vacuum form more than 1 maybe 2mm thick material in a multi discipline home shop type environment.

      If the surface finish and turn around time doesn’t matter so much building the large format 3d printer to print it in one pass might suit, but that is going to cost heaps in running costs even though the raw materials used are probably cheaper (no certainty there either, many casting resins are downright affordable for the volume, its the super detail hugging and specialist mechanical/optical properties type ones that are fairly pricey).

      The economics of any ‘mass’ production, particularly this very much smaller scale stuff is in my view less about the absolute best cost in materials used or man hours per part, and almost entirely about what your team can actually produce with the tools and skills on hand easily and reliably – no point skimping on this mold and risk having it fail in short order, or throw more failed parts, those end up costing more overall.

      1. ” But I don’t think anybody can really expect to vacuum form more than 1 maybe 2mm thick material in a multi discipline home shop type environment.”

        Gru: Liiightbulb !

        The thoughts about evenly heating polystyrene sheet started running through the gears, and I thunk… “What if you drew, or mask sprayed, or otherwise caused to form, a graphite or other conductive ink, paint or material, grid or array on the back side of the plastic, that you could apply RF heating techniques to???”

        1. Hmm that might work – but I think the biggest issue with thicker work wouldn’t be getting it warm enough – just leave it in an oven long enough to saturate, and then work fast enough it doesn’t cool too far.

          Its the strong and even enough vacuum to pull that thicker softened sheet properly over the form I would think is the real challenge. Though if you could keep it evenly heated throughout with the RF for the rear and a grill element above perhaps that would give you the working time to really work the thicker materials.

          I’m not really an expert though, all I know for sure is I’ve never seen anybody vacuum form particularly thick material without some really impressive kit and never seen anything that looked to have been formed that way from properly solid thickness stock.

          1. I would say my vacuum forming setup was really impressive… in it’s shoddiness! But it worked.

            This is admittedly far from production quality, but it got the job done. I needed to make a floor pan for a CNC mill enclosure so I could use flood coolant. Everything I read said that you can not vacuum form thick material without heating both sides. Having bought the 5mm sheet of ABS and built the form already, I had nothing to loose. If it didn’t work, I was going to lay up a fiberglass pan in the form. I suspended the form and sheet over a BBQ grill and worked a vacuum pump on and off as I moved the grill around to heat as evenly as possible. Tested it on a cheap piece of acrylic before using the ABS.

            Again, very far from production quality, but if this worked this well on a piece with these dimensions, it’s certainly possible to put together a viable setup for something smaller when your goal is to make more than one.

            Here’s my vac forming setup:


          2. That looks like a very nice job, thicker than I’d have thought you could form with such a setup too.
            Seems RW may be on to something with the RF heating as well then – you have just proved what I suspected, that with long enough working time you can form thicker stuff well enough. Not sure if that is thick enough to do job for Eric above though, looks to be nearer 10mm than under 5mm thick his part wanted (though that might be because the resins tend to be brittle so thicker for toughness)

            Still this is why I keep coming back to this site, thanks so much Chris for sharing!

        2. I wonder how that would compare to pointing a radiant heater at the part to keep some of the problem areas warmer. The dish style ones are extremely light and would be pretty easy to handle.
          Or a heat gun, for that matter.

  2. You can get a huge variety of silicone to use for molds. Some are definitely cheaper than others. Platinum cures give you best detail and least shrinkage and you can find a few like Smooth-On Dragon Skin NV that are made to not require any degassing. Then there are a few simple pouring techniques that also can eliminate bubbles on most silicone. A silicone or urethane mold is probably the best way to make a few hundred parts.

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