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243 Posts
Discussion Starter #1
This is a journal/notes on trying to make a mold based on my right arm for zombies.

Step 1: Acquire all the things needed.
  • Alja-safe (3# ... but this was not enough, should have bought more)
  • Plaster of Paris (25#, used slightly less than half)
  • Tube or similar for holding the cast (made one from 24" wide shelf liner, duck tape, and a linoleum floor tile)
  • Rebound-25 silicone
  • Plasti-paste for outer shell
  • Mold Release
  • Urethane Foam (Foam-It4)
  • Flesh color tint for foam and assortment of other colors
  • Ure-coat (to paint arms and seal "wounds" I carve in foam flesh)
  • Plenty of buckets in different sizes
  • Stirring sticks and drill powered stir thing (blue disc with a shaft sticking up to put in a drill)
This is the container I made for the mold. I wanted something that would cover fingertips to mid-bicep, and allow enough room to bend my elbow a little.


Step 2: Follow instructions and make a small mold to get an idea of the feel of materials. I used a prescription bottle and copied my thumb.

Step 3: Assess learning so far:
  • If you don't have a timer nearby, you are forced to sing to keep track of time (most songs are 2-3 minutes, sing 5 songs while alginate sets). Get a timer or a clock to watch.
  • Wear comfortable shoes since you can't move. Standing on concrete in socks is not fun.
  • Be careful to leave a gap between your body part and the container.
Step 4: Create alginate form for the plaster casting. Very important to pre-plan how you will hold perfectly still for 10 min and not be uncomfortable. I did poorly at this. Also, if you do this alone be sure you can reach into the mold to help free your trapped arm!
  • Mix it with the blue stir stick on a drill.
  • It's a 1:1 mix by volume, so I measured the alja-safe first and added the same amount of water to a 5 gallon bucket. There was not a lot of Alja-Safe ... 3 pounds is roughly 3 gallons in volume. 3 gallons is not a lot of liquid :(
  • Pour mixed Alja-safe into the vessel (cue sad face when you see how little it is) and use target hand to scoop extra into vessel. That helps ensure the alja-safe has good contact with your skin and captures all the details.
  • Put hand/arm into container and wait required time. Carefully extract hand, this requires a lot of gentle wiggling to break the seal between the gel and your arm.
Step 5: Cast Plaster into Alginate mold (quickly)
  • Mix 2:1 by volume (clean off drill mixer thing, use a clean bucket). Should feel like thin pancake batter. I used 2 gallons plaster and 1 gallon water.
  • Pour some plaster in, tilt the mold and tap to release air bubbles.
  • I poured the extra in to act as a base because it seemed like a good idea.
IMG_20200215_174723.jpg IMG_20200215_162604.jpg IMG_20200215_175312.jpg

Step 6: Wait a few hours and unmold the sculpture
  • Use a Xacto blade or similar to score the alginate. Pull off chunks CAREFULLY. I was not careful enough.
  • Gleefully notice how much detail you captured. Mourn any fingers you snapped off.
  • After it was all cleaned off, I noticed odd bumpy spots on my plaster arm. They popped off easily with a fingernail.
  • I used TiteBond to glue my plaster fingers back on (thin the glue with water). Holding the finger for 30 sec seemed long enough.
IMG_20200215_181340.jpg IMG_20200215_181358.jpg IMG_20200215_183857.jpg

That's it for today. Tomorrow is time to fill the little holes in the plaster cast and make any touch-ups. Then let it dry awhile and start with the silicone mold and plasti-paste shell.

243 Posts
Discussion Starter #3
Tomorrow is time to fill the little holes in the plaster cast and make any touch-ups. Then let it dry awhile and start with the silicone mold and plasti-paste shell.
I learned more things today :)

First, I need to let the plaster dry for a few days. Maybe I will be lucky and the combination of dry winter air and heat in the house will speed that process.

Second, even if it was dry ... I need to SEAL the plaster before using it to create a mold. Guess who didn't buy sealer? Moi.

So what I did do was touch up the plaster around the broken fingers, fill in any odd divots or pinholes, and clean up the base. I noticed the plaster had formed in a way that hid 1/4 - 1/2" of my arm. So I carved that down to clean up the base and make a better flange.

To patch little areas I wet down the area with clean water and then mixed a tiny amount of fresh plaster with some water. Bamboo skewer or wooden toothpick is great for this. Dab the damaged area with water again, then apply the plaster with the skewer/pick. Use the brush to smooth it out and then add in any detail lines with the pointy part of the skewer. I should have used a chip brush for this, but I found a foam brush first so I went with that.

For the base, I used a utility knife and paint scraper to CAREFULLY shave the plaster down. Super glad I did this, it looks much better and I will be able to get a clean, flat flange for the multipart mold. Until it dries I am kind of stuck for now, though.

IMG_20200216_111439.jpg IMG_20200216_111336.jpg

243 Posts
Discussion Starter #4
Next step - making a silicone mold of the arm. I am going with a brush on mold backed by plastic shell. This is supposed to help keep the weight down.

The first step here is to figure out if you can make a glove mold that is a single sleeve of silicone you peel off the arm (like a long glove) or if you have to go with a 2 part mold. Based on how I positioned my fingers, I felt a glove mold would have to stretch WAY too much to be usable over the long haul. I am probably being overly cautious. But I decided to go with a 2 pc mold.

Now that this is known ... determine the seam line. I used clay to establish the line that will separate top and bottom half of the mold. The clay has to be sulphur free if you are using silicone. I drew a line in pencil and then used the clay to follow the line. I am showing way more pics than needed here because a lot of thought goes into this step.
IMG_20200222_182126.jpg IMG_20200222_182826.jpg IMG_20200222_191145.jpg

Then I mixed up the silicone and painted on the first layer (unthickened) and used a parchment paper sheet to make a mold for some keys. Do NOT do this. Silicone sticks to silicone and parchment paper is impregnated with silicone. Duh.

After 1 hour it was tacky but stable (did not come away with my finger) so I mixed up a second, thickened layer of the Rebound 25 (thivex is the thickener). And slathered that on.


That cured overnight and in the morning I removed the clay. I also realized that while I had sealed my plaster hand with Krylon Clear, I had not used a mold release. Hmmm. I wonder if I can take this thing off? I verified that I could but realized a little plaster had stuck (in the wrist area, its whitish). The second half is the palm side with more crevices, so I used Paste Wax to act as mold release. Fingers crossed that works. For a mold release between the silicone layers I am using 2 parts rubbing alcohol to one part dish soap.
IMG_20200223_070541.jpg IMG_20200223_070933.jpg

The downside of breaking the seal on the top side is that I needed to get creative about how to hold the mold firmly to the plaster so I could make the second half. Had I left the first half firmly attached this would not have been an issue. I used clay to build little supports all over the place around the edges. Then did the "apply thin coat, wait an hour, thickened coat" sequence again. Painting it onto the thumb was difficult since that was upside down.

20200223_115205-COLLAGE.jpg 20200223_115105-COLLAGE.jpg 20200223_115137-COLLAGE.jpg

That last pic shows where I called it a day. It needs to cure before the plastic cast gets made. That might happen this week or more likely next weekend. I think doing a bunch at once would make more sense. It takes 10 min to apply the silicone, if I had 5-6 hands ready I could apply silicone to each one and be ready for second coat by the time I was done. Gotta get more hand positions cast in plaster!


243 Posts
Discussion Starter #5
Quick update ... this project lives :)

I created a plaster outer mold for the silicone mold and cast my first hand. I learned many things:
  1. Do not use rigid foam for something like this. I used a 4# rigid foam that turned out to be BRITTLE as hell. Hard pass on that. Carefully peeling the mold off the hand resulted in a broken thumb.
  2. When making the mold, do not remove or "check" any of it until you are done. Make BOTH silicone sides, and BOTH plaster sides, let the final product cure as long as needed before separating anything. My mold had some odd lines where rogue silicone from side 2 was able to seep under the edges of side 1.
  3. Use lots of whatever you are using. Go for thick, durable pieces. Even if someone else swears that 3 layers of plaster bandages is plenty ... use 5.
I decided to remake the silicone mold and create some new hands (severed hands for holding candles and such). I also bought a different foam that is FLEXIBLE and highly tear resistant. The rigid stuff was the consistency of a rice cake. I will use it to cast something, maybe an axe or other weapons that are blocky (sledgehammer?).

Pics below are of new hands (when cast in flexible foam they should be able to grip candles) and the in process silicone mold for the second try. I changed the parting line placement and the thumb will now be a glove mold. I also made small keys to align the silicone mold halves (the small round bits on the edge). Making a mold takes a LONG time, huge amount of effort. Hopefully this next try is production worthy and not just another learning experience :)



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