Injection Molding Silicone Parts For Under $50

You’ve likely seen many tutorials on making silicone parts using 3D printed molds online. The vast majority of these methods use a simple pour method to fill the mold. This relies on careful degassing and gentle pouring to reduce the presence of bubbles in the final result. [Jan Mrázek] has been working on an alternative method however, that allows for injection molding at low cost in the home shop.

The process relies on the use of printed resin molds. [Jan] notes that this generally necessitates the use of condensation-cure silicones, as additive types don’t cure well in resin molds. The condensation silicone is mixed up, degassed, and poured into a standard cartridge. From there, it’s installed in a silicone delivery air gun, which uses compressed air to force the silicone out of the nozzle and into the waiting mold.

It’s basically using a bunch of home DIY gear to create a cheap injection molding solution for silicone parts. [Jan] notes that there are a few mods needed to mold design to suit the process, and that 400-800 kPa is a good pressure to inject the silicone at.

Having the silicone injected under pressure is great for complex mold designs, as it forces the material into all the little difficult nooks and crannies. Of course, we’ve seen other methods for making silicone parts before, too. Be sure to sound off in the comments with your own favored techniques for producing quality silicone parts. Video after the break.

14 thoughts on “Injection Molding Silicone Parts For Under $50

  1. Thanks for sharing. Really interesting stuff.
    Does anyone know about the stiffness and hardness of the parts? I did see the inserts for rigidity but I still recon it would be pretty rubbery, but I guess I’ve got a limited understanding of silicone.

  2. Some info on silicone types.

    Someone should test many types of 3D printing resin for cure inhibition against both types of RTV silicone. Why, because condensation or tin cure silicone shrinks slightly and after a while begins to break down. If using it to make parts like bumpers, tool grips, and other soft features it’s suitable for prototype work but not for production.

    Addition or platinum cure silicone has virtually zero shrinkage and lasts pretty much forever. I have some molds over 20 years old still making good castings.

    1. Between baking the resin print, light lacquer coating, and a light rub with regular paste wax ( no citrus additive, it wouldn’t work for me), I haven’t had cure inhibition issues for a good range of resins. I suppose i could have tracked it, but after getting siraya blu working i just kept doing the whole for other resins i used since.

  3. Try pulling in the silicone using a vacuum instead of using positive pressure. That’s the best way to make sure there are not gas bubbles trapped as the silicone enters. That’s what we did when making items for mems. There we had very small channels we were trying to fill with photo resist and the slightest bubble would block off a channel. So we’d pull a decent vacuum and then introduce the liquid. It didn’t matter how complex the channels were, since the air was removed by the vacuum, the liquid filled every cavity.

      1. In the jewelry world you rely on the porosity of the mold. You expose the whole outside of the mold aside from the top to vacuum and pour the metal in; it can’t go through the holes the air can. I don’t know if this would work as well with silicone.

  4. I’ve recently used a similar approach with 2-part dental silicone (shore A hardness approx. 35) and a syringe.

    The mold was printed in PLA with a rasied “gasket” on one half where the two parts of the mold meet. This gasket was lightly sanded before assembly to provide a cleaner fit. Some leakage is desirable however to ensure that the silicone fills the mold completely (no risers needed like in metal casting). The mold halves were aligned using standard 4 mm ground pins, and M3 screws with captive nuts placed in strategic locations.

    Overall, the success rate is very high with this method provided you are careful about mixing more than the final volume needed. Mixing does introduce some air bubbles, but for my application it was not an issue.

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