Tuesday, October 4, 2011

Arisu's Head Silicone Rubber Mold: Part 2

 Now it is time to make the second part of the mold.

The volume of the space to be filled by silicone rubber is 2.5"x2.5"x1.25", which is 7.8 cu.in.

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.

The neck area is approximately 0.5" long with a 0.25" radius.  The volume of the neck is .1 cu.in.

The volume of silicone rubber needed = volume of box + volume of neck - volume half head = 7.8 cu.in. + 0.1 cu.in. - 0.89 cu.in = 7.01 cu.in.

This part will need 0.34 pounds or 5.44 oz of silicone rubber.

So I would need 2.365 oz of A and 3.075 oz of B.

I will also be making the silicone mold for a 10mm eye at the same time.  The eye mold will need .91 oz of silicone rubber.  That is a total of 6.35 oz.

So I will actually need 2.76 oz of A and 3.59 oz of B.  

For the first mold I poured 7.36 oz of A+B = 2/3 cups.  So if you do the math 1/3 cup = 3.68 oz.  That makes 1 cup = 11.04 oz.  Then 1/2 cup = 5.52 oz.  These of course are rough estimates but should give me a good ball park number.

For the first mold I estimated needing 6.56 oz of silicone rubber but ended up pouring 7.36 oz, which is a 12% increase.  Some of the silicone remains in the cup, on the stirring tool, missing the mark, etc.  So it is good to take into account needing a little more, but you also want to minimize waste.  If I did a 10% increase of 6.35 oz, I would need to pour 7 oz.  That would be 3.04 oz of A and 3.96 oz of B.  Or I could do by volume which would be 5/8 cup (A + B).

One thing I did notice is that the area around the ears will not cast the way I want them to.  Instead of pouring the facial features first I should have poured the back of the head.  This would have allowed me to separate the face plate better where the ears are.  Sometimes you don't see an issue until you try.  At least I know how to fix it for next time.

The two halves came apart rather easily, which means I used enough release agent.

There is a tiny air bubble on the bottom of one of the ears but the rest looks good.  Not bad so far.  You can even see where the nostril holes are.  (I am actually very pleased with the molds so far, but I won't know how good the molds are until I get a chance to cast with urethane resin.)

The next part is going to be the most difficult since I need to add clay inside the head where I want resin to be.  I would be also adding the pour holes and vents.  To get a better idea of what I am doing, check out my post about my BJD Head Mold.

When mixing A and B for the silicone rubber, you can reuse your cup and stir rod if they are non-porous.  The cured silicone rubber comes off easily enough with the thicker parts pealing like a sheet.

For this mold I weighed the waste silicone rubber and came out to 0.5 oz.  The waste from the first mold came closer to 2 oz.  I have almost used 1 pound of silicone rubber thus far and the kit I am using is 2.9 pounds of OOMOO 30.

Part 1
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.

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