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IndustryArena Forum > MetalWorking > Moldmaking > Making a positive mold from a negative mold - how?
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  1. #1
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    Making a positive mold from a negative mold - how?

    I CNC machine little metal plugs that fit into recesses in bodywork on boats.

    Well, I have a customer who asked me to do a custom size - but it's not feasible for him to send the boat to me, obviously, so instead he took bondo and made an impression of the recess. So I have in my hand a piece of hard bondo that looks like a mushroom - the underside of the mushroom top would be the bodywork contour and the stem of the mushroom is the plug I need to make for the bodywork.

    How would I "re create" the bodywork from this piece? I want to put some sort of mold release on it and use some material to make a negative of the part (which is itself a negative of the bodywork) - so I'll be left with an approximation of the original.

    Whats the best material to use and how do I do it? I was thinking fiberglass but the only issue is it may not come out smooth when I am done. The Bondo is glass-smooth now and I want me negative-image of it to be smooth too. Also, what do I use to ensure it doesn't stick and I turn it into a solid mess

    When I get the negative of this part, I will mount it in the CNC machine with a probe head to measure it, then I'll machine the plug and use the part I cast from his plug to ensure a correct fit.

    Tolerances are huge because this fits into molded plastic pieces, which have sloppy tolerances, so I don't need to be within thousandths or anything.

    Thanks for any tips!

  2. #2
    Moderator HuFlungDung's Avatar
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    I've heard of a compound called "ReproRubber", but have never used it.

    http://www.newmantools.com/flexbar/repl.htm
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  3. #3
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    I would digitize the negative image and generate the inverse with software.

    Ken
    Kenneth Lerman
    55 Main Street
    Newtown, CT 06470

  4. #4
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    BCC products in Franklin, Indiana has several products depending on your speed. I would pressure cast it at 150 psi. If your model has holes or small air bubbles it may colapse. Call me I have done many reverse engineering projects and cast rapid prototype parts. Can you send me a group of photos. gsilberberg@progressive-plastics.net I can probably either talk you through the process over the phone or do it myself. I have a small pressure pot at home..when I am not brewing beer in it (just kidding). Call me on my cell 317-408-2684. Indiana time now till 8:30 pm.

  5. #5
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    Quote Originally Posted by lerman View Post
    I would digitize the negative image and generate the inverse with software.

    Ken
    I know I could do that but without having the "other side" (positive image) its not easy to determine the flow of the surface (bear in mind part of the surface I will be milling is hidden by the interface between the stem and canopy of the mushroom shape. If I have the positive side, I can just measure off either one, then machine, fit and see how it looks.

  6. #6
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    Quote Originally Posted by HuFlungDung View Post
    I've heard of a compound called "ReproRubber", but have never used it.

    http://www.newmantools.com/flexbar/repl.htm
    That putty looks very promising Hu! Thanks, you earned your paycheck this week You know the parts we make for bikes - I have some interest from some boat guys but no way am I going to ask them to ship me body panels so I can measure, and I ain't into taking a Faro and laptop down to the marina and balancing them on a dingy while I measure

  7. #7
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    Quote Originally Posted by jetski View Post
    BCC products in Franklin, Indiana has several products depending on your speed. I would pressure cast it at 150 psi. If your model has holes or small air bubbles it may colapse. Call me I have done many reverse engineering projects and cast rapid prototype parts. Can you send me a group of photos. gsilberberg@progressive-plastics.net I can probably either talk you through the process over the phone or do it myself. I have a small pressure pot at home..when I am not brewing beer in it (just kidding). Call me on my cell 317-408-2684. Indiana time now till 8:30 pm.
    Thanks very much for your offer jetski! Unfortunately the model does have some small holes and cavities because it was sort of a quickly put together mold. I will try the putty that Hu linked to and it I don't have any luck I'll give you a shout. Thanks again for your very generous offer!

  8. #8
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    http://www.jmoritausa.com/perfectim_systems.asp

    it creates very accurate negatives of the parent shape.
    sets up to the consistancy of soft silicone rubber, quickly i might add.
    1 to 2 minutes.
    it is a dental molding compound. works awesome you will like it.
    also if you need to check a feature that you can not put into a comparator
    take a mold and slice a thin section of the material, and "walla" check it
    under your optics.
    probably real similar to the stuff hu mentioned.

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