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Monday, April 2, 2012

My First RTV Silicone Mold

Since I want to eventually offer resin dolls through Tamara Henson Studios, I need to learn how to mold and cast Master doll samples. Enter RTV Silicone Moldmaking and Casting Resin. I had ordered some doll ball joints from Batchix's Shapeways account and got her permission to use 'em however. I decided to practice my moldmaking with the parts.  

I read up on the process, ordered supplies and made my first mold. Turns out, practice moldmaking was the best route for me. The rest of the Internet can explain the science and specifics of the process. I'm no expert, but I'll do my best to answer any questions you may have. I also welcome constructive criticism. Hope this helps anyone who's looking for basic 2-Part silicone mold-making procedures:


RTV Moldmaking Silicone (I bought mine here.)
Casting Resin, of some type... (Mine is polymer. Sold by the same guy as above, here.)
Disposable plastic cups
Disposable plastic spoons or wooden spatulas... or new chopsticks!
Oil-based, non-drying modeling clay
A small cardboard box that allows at least 1/2" on all sides, top and bottom of your part
Petroleum Jelly
A scale that accurately measures grams (kitchen scale, $20, Wal-Mart!)
A ruler
A simple part or non-porous object to replicate
A wooden bead to make register marks
Hot glue gun
Plastic (To protect surfaces)
A soft, small scrub brush
Elbow grease ;)

1. Add 1/2" clay to taped-closed bottom of box, pressing to make no cracks remain. Press parts into clay so that no silicone can seep under them. I also sealed the slit in the side to keep out the silicone. Dig a shallow registry trench around the perimeter using a chopstick (or an actual sculpting tool, if you're feeling ambitious). Press and twist wooden bead in spaces between parts, no more than halfway up the side of the bead. Using the paintbrush and your hand, smear petroleum jelly on exposed sides of box and anything that silicone will stick to. You can see the register running around the perimeter between the part and the cardboard wall, as well as a couple other registry depressions between the parts:
Swirly modeling clay. Note the chopstick on the right.

2. Mix silicone and catalyst by volume according to package directions. (Use your scale!) A chopstick is not the optimal stirring device for this task, since the silicone is REALLY thick, worse than cake batter, and the catalyst is basically colorful water. Also, stir slowly, being careful not to pull in air and make undesirable bubbles. This is true especially for the silicone touching the object you're molding. 
The catalyst is purple, the base off-white. And, no kidding, it smells like grapes!
3. Pour the silicone. I poured the first layer, making sure all the parts had a layer of silicone. But then again, I'm silly and mixed by the plastic cupful, so I did this several times. You want to cover up the object, giving no more or less than a half inch over the highest point of the object. (Use your ruler!) My last couple batches have bubbles. I don't consider that a problem, because those bubbles aren't affecting the casting surfaces. A tip from someone online: Pour in a thin, steady stream in the corner of the box. The silicone will spread out on its own. (Too late, buddy. Next time, though!)
Lubed up box, and such a pretty color! Also, oops-bubbles!
4. Let it cure, probably overnight. Then cut a piece of cardboard and hot-glue tack it over your smooth silicone to support it when you flip the box. Assuming you didn't fill the whole box. Your stirring arm must be tired, and your wasted silicone was expensive! (No more than 1/2"... No wasting!)
Strategically placed hot-glue blobs
5. Get ready for side 2. Flip the box and cut the tape. Scoop out the modeling clay and clean the silicone with a paper towel. Do not remove the parts yet! You can see that some modeling clay is still crammed down in the holes. Trusty chopstick dug that out for me! You can clearly see the bumps I created with the bead and the ridge running around the outside to help the two pieces match up. 
Getting ready for the Core Mold step!
6. Apply a thin layer of petroleum jelly to EVERY EXPOSED SILICONE SURFACE and the cardboard box! Aw, heck. Apply the stuff to every surface, like I did. A brush will leave ridges that the silicone may pick up, though, so be careful not to leave streaks on your object. (I lubed the parts as well, because I felt some rough edges I hadn't sanded, AND because all the doll joints I cast from these will be sanded anyway...) Then, mix more silicone and pour it in. The "pour it in the corner and let it seep into crevices" advice would've worked better here. But I was stuck poking at big bubbles with my chopstick. Let it cure overnight again. 

7. Rip off the box and gently separate the two parts of the mold, using an X-Acto knife, if needed. You should be able to gently pry apart the halves with your fingers. Remove the original part(s) at this time, using a combination of your fingers and some minor X-Acto work. (My mold had a couple places where the silicone covered the edge of the original part. I just carefully cut that thin stuff off. This is the mold before I washed it. You can see the shiny, overly liberal application of petroleum jelly AND the remnants of some modeling clay I neglected to remove before. (Since this mold is for casting doll joints, not finished products, this doesn't bother me.)
Left: Core Mold (looks like Spaceballs city), Right: Base Mold

8. Gently scrub the mold with warm water and dish soap, removing any gunk and as much jelly as possible. Let the mold dry thoroughly. At this time, I cut some vents in the core mold so that air could escape. I just slipped my X-Acto knife up through the thin ridge you see around the base of the dome shapes (where the resin will rise), a couple slits for each piece. Not really necessary for this mold, but I wanted to practice. Place on plastic for the horrid mess you'll make in a few minutes...

9. Mix resin base and catalyst together, following manufacturer's instructions. Most resins are exothermic, meaning they give off heat as they cure, and they have a very fast working and cure time. Even so, stir thoroughly and slowly so you don't get bubbles. Pour into mold wells, and set core mold on top, making sure it fits into register ridge. The excess resin will either spill over the sides or rise up through the vents. Yeah, I made a big mess... (No pic. I was rushing to not waste the stuff!) Since this is not a complicated mold, I didn't need to do any fancy resin injection or holding it together with rubber bands or bungees. That type of mold will come along soon enough...

10. De-mold. See the fruits of your labor, and the extent of the mess you made. I for instance overfilled to the point of having a thin cover of resin across the whole mold and drizzled all across my makeshift Wal-Mart bag table cover, which had a perfect resin mold-shaped rectangle. Since the mold is flexible, you can bend it and twist it to remove your pieces. My parts needed a lot of trimming, but it didn't take a lot of time. There was a thin film of resin in the slit up the side of the dome and all around the base of each part. Also, some parts were actually UNDER-filled. Don't know how that happened, considering...
The Mess! All that not-purple is resin, covering the whole surface of the mold!
11. Finished parts! Not bad for a first-time mold. You can see that some parts are a little rough around the edges, but the next casting should be better. 
Finished parts: On my kitchen scale, in the disposable plastic lid I used to contain spills! :)

Moldmaking tutorial, complete! :) I take questions and constructive criticism seriously. Lemme know what you think, and how I can improve.




  1. I think you did a great job! I wish I knew how to do that! so no criticism from me, you did it pretty well!


    1. Thanks so much! :) I've since learned so much about the process. Trial and error is an expensive process, but I think I've got the hang of it now.