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Attempting to spin-cast parabola

  1. Feb 7, 2012 #1
    I am trying to make a mold of a deep parabola, by putting plaster of paris in a spinning flower pot. Ive got another thread going as well. So far Ive tried using and making all sorts of spinning bases, however inconsistencies in the rotation of the pot have to be almost invisible for the plaster to set correctly. This is surprisingly(!) hard to do, I'm hoping hanging the pot from a motor will help with this.

    My question is more about the substrates. I want a positive mold, so the plan is to cast the plaster, then form a fibreglass mold. But, the plaster of paris ineviatably contains bubbles, and these bubbles all migrate to the lowest point, right where the most accuracy needs to be. This is a big problem, and I'm trying to think of other materials to use, I was thinking liquid rubber or foam of some sort? I could perhaps use the plaster as filler, and do I final thin coat of epoxy resin, but I dont even know how the epoxy will perform.

    The ultimate test is water. Ive got close but it is very hard to get water to hold itself motionless as a (very deep) parabola. A good trick is to glad wrap the top, so the air inside spins up to speed as well. I'm working on the smooth rotation, should be just a few more days, but in the meantime can anyone else thinkk of a good setting liquid to use?
    cheers all, tom
     
  2. jcsd
  3. Feb 7, 2012 #2
    tiviny--

    You might try fiberglass resin and catalyst that comes as part of a kit for doing automotive repairs. It's sold in most auto stores in a quart can with fiberglass fabric stored in the lid. It's nearly water-thin, sets in 15 minutes, and is pretty inexpensive. I've used it to impregnate rotten wood before doing restoration work as it penetrates the old weathered wood very well.
     
  4. Feb 7, 2012 #3
    That sounds good. The epoxy I have is fairly thick. Ive decided thats the way to go, plaster as bulk then a thin layer of epoxy as a finish. Ill just have to be careful to avoid bubbles.
     
  5. Feb 8, 2012 #4

    Bandit127

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    Gold Member

    Having been well used to spin casting contact lenses (i.e. optics) in my career I would suggest you could improve your process as follows:

    1. Use a more circular former. Perhaps a paint pot would be more circular - even a plastic one?
    2. Be precise on centering your former on the axis of spin. Within 0.02 mm should reduce the probability of wobble.
    3. Try to avoid vibration at all costs - include the quality of bearings etc in your thinking.
    4. Consider degassing your casting liquid if bubbles are a problem.

    Contact lens tolerances may not be appropriate for your application, but thinking about precision will certainly help the quality of the shape you can produce.

    Good luck.
     
  6. Feb 8, 2012 #5
    thank you for your suggestions banndit127!
    I plan to get a container made by a potter, so it is circular.
    Hanging will hopefully solve the centering issue, it is very hard to centre something that spins.
    Im doing all sorts to eliminate vibration.
    Im trying to now make a vacuum degassing chamber, because I have a vacuum pump just lying around.
    Cheers for the good luck, am probably going to need it.
    -tom
     
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