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Need help, mold rubber does not stay in place within shell

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Need help, mold rubber does not stay in place within shell

Postby dacaldera » Mon Apr 11, 2016 7:16 am

Hi everyone! I am new to this forum and am glad to find a place where we can discuss technical things about casting with resin and mold-making in general. I see that the majority of the casting being done here is relatively small scale parts, and so perhaps my question may difficult to answer...but I'll ask anyways in hopes of somebody being able to help me.

And I will post a photo down below.

I am in the process of making castings of a life size femal torso, originally taken from a living model. I have made the original plaster copy, on which I used brush-on 40 urethane rubber to create the mold. Over the rubber I applied polyester resin and fiberglass to make the supporting shell. The shell comes apart into two haves (the front of the body and the back of the body with the parting line down the sides), and then the rubber mold has a seam running up one side of the body, just enough to be able to pull the rest of the rubber off the model like a glove.

My question is about the registration keys and how they link to the fiberglass support shell. It seems as though the registration keys that I made with the urethane rubber are not adequate enough to keep the rubber in place on the fiberglass support shell. Essentially what is happening is that rubber and resin shell come together nicely, but the weight of the rubber pulls itself out of the registration keys which are supposed to hold it in place. What can I do to keep the rubber in place within the support shell? I am thinking that perhaps spraying an adhesive between the shell and the rubber might hold it in place long enough for the rotational casting which I plan to do. Do you think a spray adhesive will work? I'd like to know before I ruin the rubber with a sticky adhesive that doesn't work. And advice appreciated. I hope my explanation is clear, but if it is not then please say so.

Thanks!
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Re: Need help, mold rubber does not stay in place within she

Postby dacaldera » Mon Apr 11, 2016 7:21 am

The link to an image showing my mold. You can see that the rubber is floppy and heavy under its own weight, and when it is in the resin shell, it bows out of the registration keys.

Image
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Re: Need help, mold rubber does not stay in place within she

Postby mangozac » Thu Apr 14, 2016 1:57 pm

Sorry this is such a late reply but I think this is possibly the largest casting project anybody has posted on here! Very cool!

I've not done a supported mould before but it sounds like you were on the right track with the registration keys.Perhaps they're just not the right size, shape or quantity?

Something you could do to fix the existing mould would be to drill some holes, say 12mm in diameter, through the fibreglass shell in the trouble spots. Mount the silicone mould inside the shell and support it so that the trouble spots are correct (just do one half of the mould at a time). Pour some silicone into the holes so that it overflows a bit and let it cure. These new "plugs" should help the mould stay in place in the future.
Oh yeah I can make that....
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Re: Need help, mold rubber does not stay in place within she

Postby dacaldera » Thu Apr 14, 2016 2:34 pm

Hi mangozac,

Yeah I usually don't work this large either, but a friend referred me to a man who wants a series of bust casts, so I took it up. It has been an enlightening experience working on this scale and with all of the different materials, from alginate for the body mold, to plaster, then urethane rubber and fiberglass for the production mold. I've done alot of reading on the subject before "going for it" and it is hard to find good online info on the little details. Of most help (to anybody that is interested in larger scale like this) is a website I found with lots of good info called juxtamorph.com. I'm not affiliated in any way with it but thought I would share since it has been so helpful to me. The "techniques" section has tons of good info.

Thanks for the suggestion, the thought of cutting holes in the shell and filling them with urethane had not crossed my mind and sounds like a good solution. If the spray adhesive thing doesn't work then I'll try something along those lines.

Best!
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Re: Need help, mold rubber does not stay in place within she

Postby mangozac » Thu Apr 14, 2016 8:56 pm

Glad I could help - please let us know how it turns out!
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