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RTV Silicon Vulcanizing Rubber Molds by: Steve Rivas


  • 2-part RTV molding rubber $30
  • Lego base and blocks (for your mold "holder" encasement)
  • Talcum Powder
  • Stir stick
  • Razor
  • Glue (Elmers)
  • Superglue

This method of molding uses RTV silicon vulcanizing rubber. Sounds fancy and quite frankly I don't know what the heck RTV stands for or what vulcanizing means, but... This is the method great for making entire parts, such as arms, legs, torso halves, heads, probably even guns, weapons, lightsabers and more... As well as "the spirit of..." figures using clear casting resin with dyes. You can get the rubber in 24 hour cure or 4 hour cure varieties. (This is the time it takes for the rubber to "harden" or be ready for molding. I prefer 4 hour cure, I'm hasty....) This method requires Lego blocks. If you don't have legs AND are only molding a head (or other small objects) please go to One Part Head Mold.
[by Stan Seel: RTV means "room temperature vulcanizing". Which more or less means that it sets at room temperature.]

Step 1

The best method for making an encasement or base for housing the mold is Lego's. Use the thin long thin blocks and short thin blocks on a Lego base (ya know: the "floor" parts of Lego houses). You will be pouring rubber into the encasement. You will not be coating the master like with liquid latex, this will cover the master in one shot. So build a base housing wide enough and tall enough to contain whatever you are casting into the encasement.

Step 2

Build a "T" piece from a popsicle stick (as the top part of the "T" i.e.: the horizontal part of "T") and some kind of "rod" as the base of the "T" (i.e.: the vertical piece). I suggest cut-up Marriott pens (the neck pegs of head fit snuggly into those cheap hollow pen casings). Any thin rod will work. And even recently pencils, glue the master to the eraser (with Elmer's glue) and cut the pencil very short and attach the popsicle stick to form the "T".

Step 3

After the "T" is made attach the master to the vertical part of the "T". This will be hanging down into the encasement of Lego's and suspended in the rubber. Glue it (you can even use super glue, you should be able to make a clean break after the mold is cured and you take it out). Now place it into the housing of Lego's you made for your mold so to measure if your housing is tall enough. After you figured out the measurements (guesstimations, no real exact measurements required). Talc your piece to be molded.

Step 4

Mix parts A and B of the RTV in appropriate ratios according to package instructions. Try to mix enough to fill the Lego box. After you have stirred the parts thoroughly, pour into the Lego box.

Step 5

Place the master into the liquid RTV rubber hanging the horizontal part of the "T" on the top of the Lego encasement (I'm sure you know what I mean)

Step 6

Dip it in and out a few times making sure there are no air bubbles. Then let sit. After 4 hours or 24 hours (depending on what kind of RTV rubber you bought) it will be cured.

Step 7

Now take apart your Lego housing and with a razor carefully slice your mold just enough to get your piece out. Slice it from one side to the other (NOT top to bottom, slice it side to side) and carefully pull it apart not tearing it. And pull out your piece (You don't have to cut all the way past your molded piece, only part of the way just enough to get the master out).

Step 8

Dust the inside of the mold with talc. Then VIOLA!, you are ready to pour resin in and make casts. (You might want to lightly wrap a rubber band around the mold before you pour your resin into the mold to keep it from coming apart. Not too tight though, you don't want a deformed version of what your trying to mold).

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