Molding prosthetic ears with deep undercuts

This sculpt has some pretty deep undercuts. I'm planning to cast the final prosthetic in silicone, so if i'm using silicone for the mold, I very definitely need to be able to get mold release ALL the way inside.  I don't think a block mold is going to cut it - would it be better to pour the mold in multiple parts*? or do a glove mold?
* I know it will have at least two parts, the silicone and the plaster ear form - I'm trying to figure out if the silicone should be in multiple parts too, and if so, where to put the seam.

(I'm also happy to get any feedback on the sculpt - I'm not sure if I've chosen good locations for the  edges etc.)


  • If you plan to cast the final appliance in silicone you may want to use a rigid mold.  

    This course series covers mold-making for similar pieces:

  • I admit, dealing with the undercuts in a rigid mold kind of scares me. At least with silicone I don't need to seam it or worry too much about undercuts.  I'd also need to drill pour holes in the back of the ear lifecast, right?
    That said, I'm not concerned about ruining my sculpt (I've already ruined a few so I've gotten fast at sculpting them.) so it's worth a shot; plaster is cheap.

    Does this look like a reasonable case line to you?
    Case line continues down the near edge of the ear to the slanted line on the earlobe - couldn't really sketch it in with a toothpick like the other areas.

  • You can do a silicone mold and still cast silicone, you'll just need to be very thorough with your release agent.

    The seam line you laid out for the back of the ear in the images above looks good to me.

  • Quick update - I ended up going with silicone for the molds; somehing about how I was doing the stone mold just wasn't working for me (I had a really tough time with the parting wall - I was probably doing something wrong). Anyway, so far so good! Time to crank out some extras and experiment with pigments and paint.

    Learnings so far:
    • Ears are not positioned flat on the head. Judging the angle and shape by holding the ear lifecast straight on will result in the wrong angle once it's on your actual head.
    • If I intend to flip the silicone glove mold inside out, use an extra print coat and don't go straight to the thickened silicone on the second layer, because the print coat "skin" can pop  under the stress where there are bubbles in the thickened layer, leaving bumps in the final product.
    • Related: consider cutting a slit in the glove mold to make it less of an upper body strength exercise to flip it inside out for cleaning/applying release (the slit doesn't even have to go all the way up). A little bit of powermesh at the top of the area where the slit ends will do a fine job of keeping it from propagating. I also reinforced the heck out of the very tip of the ear, as well as adding a little strip of powermesh along each ear edge.
    • A few drops of isopropyl alcohol help a lot with de-molding silicone from silicone, since it helps break the suction between the ear and the mold.
    • It is way too difficult to apply the ear if I don't fill in some of the undercuts, particularly under the top curl of the ear. The undercuts don't cause any problems molding or demolding in silicone, but there's plenty of area for glue to hold everything in place without it.
    • Mineral spirits and a stipple sponge do a perfectly good job for ear skin texture!
    • Related: Rinsing in acetone and then spraying with crystal clear is really really helpful to keep the solvent texture from being super mushy and easily damaged. Putting it in the freezer is also not the worst thing, if you find yourself with more work to do after the texture.
    • It is extremely helpful to pre-dilute your silicone pigments (silicone oil works fine) so that you don't accidentally put in way too much. And if you make it thin enough to be runny, you can add it by the drop, which is a lot easier to measure. You can also mix flocking into silicone oil to make a paste so that you don't accidentally dump too much flocking out of the container.
    • Green pigment or flocking made the color look MUCH more natural on me, because otherwise everything was too saturated - even if I only added a tiny bit of color. I think I have gray undertones.
    • Guitar string and coping saw rakes MVP!!
    • Big chonky mold keys do a pretty good job of locking the mold into place in the support shell, not only just helping to align it. The capsule shaped key molds from BITY were solid.
    • Silicone... floats... make sure you've got your silicone mold stuck in place before pouring the support shell. Oops. Ended up using way too much plaster for that one.
    • If you have a tendency to break your lifecasts... just spend the money on lifecasting siilcone to make a reusable ear mold. Then you can just keep making plaster ears until you get something functional.
    • I actually didn't need an armature when using Chavant NSP hard, even when paring the clay super thin. Go figure. I also cast in Dragonskin 30, which is rigid enough to hold the long ear shape without a lot of internal structure.
    • Weirdly, RS-9115 silicone works just fine with an accelerator, and with a thixotropic additive, but if I use both at the same time it won't cure. Don't ask me why.
  • Glad you are getting good castings out of the mold!  Hope you can share your progress with pigments, paint, and application!

  • Haven't painted this ear yet, but here's one applied with skin-tite, worn with the character wig (a Super Maru from Coscraft in Midnight Green, with about half a wig's worth of extra fiber sewn in, and a whole lotta shading with alcohol markers to give the curls definition.) The wig's not quite at its best in these pics - I still need to style a couple bits around the ears, and I'll be gluing the bangs to my face when I wear it for real
    The ears stay on pretty solidly, even without glue; with glue they're even better. I'm not super happy about how much you can see my natural ear from some angles, but I'll see how far I can get by just using the wig to obscure my real ear.

  • Looking good!

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