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Pressure on medium in vacuum mold - formula needed

  1. Mar 14, 2015 #1
    I'm a little confused as to how to figure out the pressure exerted on a material pressed between two solid "press plates" Let's say we have 2 .5" thick x 12" x 24" hardened steel plates. I have a 1" thick layer of cloth (say 50+ layers) of 8" x 18" (1ft^2).

    The fabric is placed between the plates in the center, placed in a vacuum bag, and air is extracted to say 24" Hg (12psi). Now, since the area of the plates is 288in^2 but the fabric is 144 in^2, would there pressure exerted on the fabric be:
    2,880 lbs (only the area of the fabric gets the pressure)?
    or would it be
    5,760 lbs (total psi over the entire press plate)

    I'm assumming that only one side would be calculated for this application, is that correct as well?

    What I'm getting confused about is if I use the same plates and use 4 C-clamps placed on the corners, say at 400lbs per clamp, wouldn't the pressure on the fabric be 1600 lbs?

    Thanks for any help, I'm stuck "arguing" with someone over this, lol :)
  2. jcsd
  3. Mar 14, 2015 #2


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    2017 Award

    Staff: Mentor

    You can use a simple force balancing: the force from the atmosphere on a plate is given by the area of the plate. There has to be an equal force from the other side - subtract the pressure from your partial vacuum, and the remaining pressure has to come from the cloth. Include gravity if necessary.

    All those weird units...
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