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Thread: Large Cast Part, Rigid Mould Advice

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    Default Large Cast Part, Rigid Mould Advice

    Hello all,

    I need some advice for a small run of parts I need to cast.

    The part is essentially a hollow hemisphere with a flange and o-ring groove and a filling port.
    For scale the sphere diameter is 170mm (~7") and the wall thickness is 5mm (~1/4").
    I need to make around 10 of these parts and a key factor is that they are as dimensionally accurate as possible. They need to be rigid and transparent, so I'm thinking of using a resin.
    Large Cast Part, Rigid Mould Advice-concept_1_outside-jpgLarge Cast Part, Rigid Mould Advice-concept_1_inside_section-jpg



    My concern is how best to produce the moulds for casting. I think it needs to be a two part design, but if I use silicone I worry that the large mass and cavities of the moulds would not remain dimensionally accurate.
    (This is just a concept section view to illustrate the point, I haven't designed the mould yet)
    Large Cast Part, Rigid Mould Advice-concpet_1_two_part_mould-jpg


    Does the mould need to be flexible? Could I just get the two mould halves CNC machined / 3D printed from a rigid material, or would removing a rigid cast from a rigid mould be too difficult?

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  2. #2
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    Default Re: Large Cast Part, Rigid Mould Advice

    Assuming that all the holes and edges in the model are drafted sufficiently, then a rigid mold should work. A fiberglass-reinforced polyester or epoxy layup would be lighter, cheaper, and more dimensionally stable than the silicone block you've drawn. A 3D printed mold would not be as strong, since they tend to come apart at the lamination lines. Use several layers of a good mold release and you should be fine.

    [FONT=Verdana]Andrew Werby[/FONT]
    [URL="http://www.computersculpture.com/"]Website[/URL]


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    Default Re: Large Cast Part, Rigid Mould Advice

    OK, I'm new to this. When you say polyester or epoxy layup, would that involve creating a master pattern and then forming the moulds over that by hand, including features like keys for alignment?

    Cost isn't a major issue, but I need to ensure very high quality parts and as this is my first time casting I'd like to simplify the process somewhat.

    I would need to get the master part machined anyway, could I instead design the two moulds and get them machined? They are around 100mm deep so not sure what material choices I would have. Would something like Renshape offer a quality surface finish for transparent resin, and hold up to 10+ castings? Overkill I know but how about solid aluminium moulds?



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    Default Re: Large Cast Part, Rigid Mould Advice

    Quote Originally Posted by bradlar90 View Post
    OK, I'm new to this. When you say polyester or epoxy layup, would that involve creating a master pattern and then forming the moulds over that by hand, including features like keys for alignment?

    [Right.]

    Cost isn't a major issue, but I need to ensure very high quality parts and as this is my first time casting I'd like to simplify the process somewhat.

    I would need to get the master part machined anyway, could I instead design the two moulds and get them machined?

    [The master part could either be machined or printed. I suppose you could machine the molds from big blocks of plastic instead of laying them up, but the amount of material and the cost would be a lot higher, while the strength would be lower.]

    They are around 100mm deep so not sure what material choices I would have. Would something like Renshape offer a quality surface finish for transparent resin, and hold up to 10+ castings?

    [Renshape would be acceptable as a material for the model, but not for the molds.]

    Overkill I know but how about solid aluminium moulds?
    [They would probably work, but be sitting down when you read the quotes for that...]

    [FONT=Verdana]Andrew Werby[/FONT]
    [URL="http://www.computersculpture.com/"]Website[/URL]


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    Default Re: Large Cast Part, Rigid Mould Advice

    I agree with Andrew. A CNC machined Renhape or Aluminum mold would work fine but cost may be an issue.
    If you take the time and expense to machine a master pattern you might as well machine a mold.

    Doug Pryor
    David Wolfe Design, Inc.
    Akron, OH
    www.davidwolfedesign.com
    depryor@davidwolfedesign.com


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