Tuesday, October 4, 2011

Arisu's Head Silicone Rubber Mold: Part 1

Arisu's head will have a 4-part mold.  This is the first mold being made.  To be precise, it is the first silicone mold I have ever made.

Clay is built up around the sculpt and mold keys are made.

I created a mold box using foam boards which were sealed using a low temperature glue gun.

Now that the mold box is ready, I have to figure out how much silicone rubber I need to pour for the first part.

The volume of the space to be filled by silicone rubber is 2.5"x2.5"x1.5", which is 9.4 cu.in.
Volume of a rectangle = l w h

The head is approximately 1.25" in diameter.  The volume of a circle with 0.75" radius (which is half a diameter) is 1.767 cu.in.  Half the head is in the space so I would need to remove 0.89 cu.in.
Volume of a circle =  (4/3) π r 3

The neck area is approximately 0.5" long with a 0.25" radius.  The volume of the neck is .1 cu.in.
Volume of a cylinder = π r 2 h

The volume of silicone rubber needed = volume of box - (volume half head + volume neck) = 9.4 cu.in - (0.89 cu.in +.1 cu.in.) = 8.41 cu.in.

According to the Technical Bulletin for OOMOO 30 the Specific Volume is 20.6 cu.in per pound.  So 8.41 cu.in / 20.6 cu.in will tell me I need .41 pounds of silicone rubber.
Weight of silicone rubber needed = Volume of silicone rubber needed for the mold / Specific Volume of the silicone rubber

The Technical Bulletin also states that the ratio by weight is 100A:130B.  So I would need 0.1783 pounds of A and 0.2317 pounds of B.
A needed = (Weight of silicone rubber needed for mold / (A+B)) * A
A = (0.41 pounds / (100+130))*100 = (0.41 pounds / 230)*100 = 0.1783 pounds
B needed = (Weight of silicone rubber needed for mold / (A+B)) * B
B = (0.41 pounds / (100+130))*130 = (0.41 pounds / 230)*130 = 0.2317 pounds

Converted to ounces it is 2.85 oz of A and 3.71 oz of B.
Convert pounds to ounces:  ounces = pounds*16

So using my ounce scale I will be measuring out 2.85A:3.71B.  When they are poured, they should be 1A:1B by volume.

After applying sealing agent and releasing agent, it is finally time to measure and mix the silicone rubber.

I ended up pouring 3.2 oz of A and 4.16 oz of B, which is 1/3 cup of each.  (I tried to stop at 2.85A but it flowed a little to quickly and I figured I was close to the 1/3 cup mark anyways, so I continued to pour until the 1/3 cup line.)  This created just the right about of silicone rubber to fill the mold box. This minimizes waste but also ensured I had enough silicone rubber for the mold.

After 6 hours of curing, my mold is finally cured and it is time to remove the mold box.  The clay build up is removed but the head is left in the silicone mold.

I am rather happy with the results.  There were some tiny bubbles in the tiny key dots, but this shouldn't be a problem.

Part 2
Part 1 & 2 REDO
Part 3 & 4 

NOTES:  I am using the Mold Making & Casting Pourable Starter Kit that I had purchased from DickBlick.com.  This kit is made by Smooth-On.  The silicone rubber that comes with the kit is OOMOO 30.  The kit comes with an instructional booklet and a DVD with video on how-to make molds and to cast.


  1. Thank you for making this silicone rubber mold making tutorial. I am so happy you decided to mold the head first. Congratulations on a successful first mold half. It looks really good. I love all the details that you have posted. Very helpful. Don't forget to apply a parting agent !!! (^_^)

  2. It was hard to decide how much detail to include and what would be redundant. Since there are great resources already available that go more in-depth with the step-by-step process, I decided to include information pertaining to my particular project.

    And yes, the parting agent is VERY important. I actually applied it 3 times. First spray, brush, and dry. Second spray and dry. Third spray and dry. I was worried about not getting enough parting agent in the neck area dip. We will see soon enough if I used enough parting agent.