Thickness of Plate to withstand 100psi

  1. Hi all. I was wondering if anyone knows of an equation to use to calculate the thickness of a plate to withstand a pressure of 100psi. I am using an injection molding process where the walls of my mold will be subjected to 100psi, so I need to calculate a plate thickness that can withstand this pressure.
     
  2. jcsd
  3. boneh3ad

    boneh3ad 1,687
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    You first need to determine what you mean by "withstand", because otherwise you might design something that won't break but deforms and either leaks like a sieve or else just gives you misshapen parts.

    Second, I assume the exterior of said part is at atmospheric pressure? Pressure inside of a mold will only exert a net force on the mold beyond whatever is balanced by the pressure outside the mold.
     
  4. Baluncore

    Baluncore 2,728
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    The thickness needed will be dependent on the size of the plate, it's method of attachment and the shape, is it a flat or a cylindrical surface.
     
  5. Sorry for the vague info. The plate will be made of aluminum, so it will be pretty ductile. I don't want this plate deforming at all. Yes, the outside is at atmospheric pressure. The plate is flat and rectangular shaped

    The mold will pretty much be a rectangular box (with all the components of interest on the interior), so there are 6 sides of aluminum plates. A rough dimension of this box is 10"x10"x4".

    What do you mean by the statement "Pressure inside of a mold will only exert a net force on the mold beyond whatever is balanced by the pressure outside the mold?"
     
    Last edited: Feb 5, 2014
  6. boneh3ad

    boneh3ad 1,687
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    There is no way to keep it from deforming "at all" unless you've found some infinitely stiff material, which would break physics. You need to have some kind of tolerance on allowable deformation.
     
  7. Hmmm, I want the deformation to be as small as possible, but our tolerance for our manufactured parts about +/-0.005".
     
  8. SteamKing

    SteamKing 9,178
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    I don't know about molds being made from plate. Most of the molds in my experience have either been cast around a part or have been machined from a solid billet.
     
  9. Baluncore

    Baluncore 2,728
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    You will have to allow for shrinkage of the injected material as it cools. You may as well allow for the flexure of the mould plate at the same time. What type of plastic are you injecting?

    If the mould is held together in a 10” x 10” hydraulic clamp then the 100 psi will be opposed by the clamp jaws, not just the plate. The plate itself could then be lighter.
     
  10. boneh3ad

    boneh3ad 1,687
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    Again, physics doesn't work that way. You could make the plate 100 miles thick and it would still deform a little. You need to to figure out what is allowable. That's part of being an engineer.
     
  11. Bandit127

    Bandit127 206
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    100 bar Is very low for injection moulding. Especially for tolerances of 0.005" (I will guess over 1"). The processes I work with typically have specific pressures in the cavity between 200 and 1,000 bar. Typically 500 bar.

    Aluminium will give you a mould suitable for short runs. The softness means the vents on the spilt line will last a few hundred thousand shots at most.

    The deformation under pressure will be predictable for a given cavity pressure. You will probably capture that in your shrinkage measurements and final tool sizing. I.e. leave the cavity metal safe and perform final machining after you have characterised the shrinkage.

    As for a safe wall thinkness. Without a detailed part design and a software package to work it out I can't say.

    All I can say is that my process makes parts about 1" in diameter and nothing load bearing is less than 1" thick. And that is in hardened steel...
     
  12. If you think 100 bar is low, then imagine what you think of 100 psi (6.9 bar)...
     
  13. Bandit127

    Bandit127 206
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    Ha :smile: Thanks for that.

    Makes my point about 14.503 times more pertinent then.
     
    Last edited: Feb 6, 2014
  14. I am injecting resin. The final piece will be made of carbon fiber.
    Is there an equation that is used to calculate these sort of things or is it more based one experience?
     
  15. Q_Goest

    Q_Goest 2,974
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    Roark's is a very common handbook for stress analysis. They have a section for flat plates.
     
  16. boneh3ad

    boneh3ad 1,687
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    The odd's are you would be better off using FEA software, as molds are generally going to be complex shapes that don't have analytical solutions for their stress and strain fields.
     
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