Molding questions

Discussion in 'Replica Props' started by hydin, Jun 3, 2015.

  1. hydin

    hydin Sr Member RPF PREMIUM MEMBER

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    I was curious about something. I'm working for the first time on a complex mold (i.e. a 2 piece mold that's in a weird shape), and I had the idea of using either foam board or acrylic to build a mold box. Would this be something that would work or would it be doomed to fail utterly?

    In my head, it'd probably work with acrylic because I can hot glue the "walls" together and cover the gaps on the bottom with glue. In reality though, I don't want to watch silicone leak out of the mold box and destroy my work table.

    I thought maybe foamboard but I wasn't sure if it would handle the stress of a mold. It's a kinda big piece (life sized mask) so there'd be some weight to it.

    I figured a flat piece of acrylic for the bottom, and 4 pieces to use as the sides, and just move em/glue em down as needed for making the walls.

    Does that make sense to anyone else or am I alone in this?

    Chris
     
  2. franz bolo

    franz bolo Sr Member

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    Foam board shouldn't be a problem.
     
  3. hydin

    hydin Sr Member RPF PREMIUM MEMBER

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    I was thinking the arts and craft foam board. I just did a google search and found some people talking about some kind of home insulation foam board which I don't think I have at the moment (unless it's in my walls).

    It's about 1/4" thick or so. Saw like a 15x20 inch slab at walmart for about 2$ a sheet. Thought I might grab a few for this.

    Chris
     
  4. nomuse

    nomuse Sr Member RPF PREMIUM MEMBER

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    Yeah...just watch out for the size. Think of the molding material as being at least as dense as water. For a box 4" on a side with 2" walls, foam core is strong enough to hold up under the weight of the liquid. If you are making a box that's two feet across with walls 8" tall, it's going to bulge, distort, collapse.

    And make sure to caulk up the seams, with an inner bead of hot glue or better yet, pack some modeling clay in there.
     
  5. ultraman

    ultraman Sr Member RPF PREMIUM MEMBER

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    go with the foam board!

    and if you get worried about it you could always cut out more pieces of foam board to reinforce the joined pieces!:ninja
     
  6. franz bolo

    franz bolo Sr Member

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    You can also reinforce the outside walls with some wood taped around it. After the mold dries, the wood pieces can come off and be reused.
     
    minifig likes this.
  7. Contec

    Contec Master Member

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    Eva foam works good also.
     
  8. hydin

    hydin Sr Member RPF PREMIUM MEMBER

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    Cool, appreciate the help guys.

    It's a kind of big piece (normally the stuff I work with is smallish), so I was worried it'd end up falling apart.

    I'll swing out and hit the store and grab the foam board, and hopefully get my mold finished up over the weekend :)

    Thanks again!
    Chris
     
  9. DrewSmith007

    DrewSmith007 Well-Known Member RPF PREMIUM MEMBER

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    I like to use the blue house insulation board. The kind I use is about 1/2" thick, cuts super easily with an exacto, and hot glues to itself really well. And it's cheap, I think a 4'x8' piece is like $11.

    If you're worried about wall distortion, you can use gussets to support it.
     
  10. Kevin Gossett

    Kevin Gossett Master Member RPF PREMIUM MEMBER

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    Use the glossy side of the foam board. I normally would use the white foam board (with the glossy surface) with no issues, but once used the matte black foam board. The silicone slightly warped the foam core and stuck to the mold a bit.
     
  11. Pro Mod

    Pro Mod Well-Known Member RPF PREMIUM MEMBER

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    I've used acrylic for mould boxes for more than twenty years and never had a problem with it.
     
  12. minifig

    minifig Well-Known Member RPF PREMIUM MEMBER

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    I use foam core all the time, but always cover the inside with 50mm sellotape, just to make sure that it doesn't stick to the silicone, it also makes the outside of the mould really smooth which means you can tape it up easier. If it is big, I would recommend supporting the outside with ply or mdf just to stop it bowing under the weight. I also only ever tape the sides of the mould box together and have never had any trouble, it also helps to make sure its watertight (just make sure you run tape along all the open seams).

    M
     
  13. robstyle

    robstyle Sr Member

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    foam core works great. As prior mentioned coating the inside where silicone will be will help in removal. Vaseline will also work.

    In reality most anything will work for a mold box as long as its up to the task. Some people use clay, legos, cardboard boxes, wood, paper cups... If it fits the role, it will work.
     

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