draft as in a mold pull

Discussion in 'Fiberglass and Composite Boat Building' started by brokensheer, Feb 1, 2011.

  1. brokensheer
    Joined: Jun 2009
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    brokensheer Senior Member

    I was having a conversation with someone and we got on the subject of draft as it relates to a part coming from a mold is it positive of negative that will pull easy??
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Draft is usually the distance from the waterline to the deepest part of the boat. Depending on the shape a mold may have to be made in several parts that separate to take the part out. I am not sure what you mean by positive or negative.
     
  3. brokensheer
    Joined: Jun 2009
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    brokensheer Senior Member

    Thanks Gonzo, say you are building a five sided cube mold, when you build the four sides the parts that are connected to the fith side are slightly smaller then the open end so you can remove the part!. my question is is this , positive or negative draft?
     
  4. jehardiman
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    jehardiman Senior Member

    Draft, as refering to a permanent mold, is all ways positive when added to allow the part to be removed from the mold without damage to the mold. Draft is the slight taper from root to tip given to straight sections to allow easy removal. Some materials, like certian metals, do not need draft because of shrinkage in the mold. Patterns for sand casting are often made with draft so that the mold is not damaged when the pattern is removed.

    Distortion allowance, sometimes refered to as "spring", "springback", or "pre-set", can either be positive or negative depending on the shape, material, molding method. Many exothermic molding processes need to have the mold slightly smaller than the finished product because the piece will be hot when setting, making it pre-stressed so it will "spring" when removed from the mold. Wether this is a concern depends on the required dimensional accuracy of the part. In this case the required distortion allowance is designed into the mold so that the finished part meets the correct dimensional tolerances. Stamped metal parts are a good example of designing a die/mold so that the finished part ends up with desired shape, which is not the same as the shape of the die/mold.
     
  5. gonzo
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    gonzo Senior Member

    I just learned new terminology.
     

  6. brokensheer
    Joined: Jun 2009
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    brokensheer Senior Member

    Jehardiman,, Thanks that is what I was looking for!
     
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