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Liquid nitrogen

  1. Apr 30, 2016 #1
    I have a plastic injection mold, the part I am running has hot spots in the part when it is being shot in the mold.
    Would like to try to incorporate a closed loop plumbing system to carry liquid nitrogen past the back side of the cavity in hopes to pull the trapped heat from the hot spot area. Injected under pressure for 20 seconds. Then deplete the liquid nitrogen from the line and return it to it's holding tank. Do not want to contaminate water cooling lines, plastic material or the air quality. That is my reason for the closed system.
    Now the question I have is: What do I need to maintain the −320 °F of liquid nitrogen in the holding tank?
    I'm pretty sure the ambient heat when it pass through the mold will sooner or later cause the liquid nitrogen in the holding talk to boil when it reaches room temperature.
    I would appreciate any input that could steer me in the right direction of maintaining -320°F
  2. jcsd
  3. Apr 30, 2016 #2


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    The LN2 is going to boil long before it reaches room temperature. It's going to be boiling once it reaches -319° F. In order to cool any hot spots on your mold, the LN2 must absorb some heat, and that's going to raise its temperature immediately. LN2 can remain liquid only between -346° F and -320° F, a rather narrow temperature range.

  4. May 1, 2016 #3
    The pressure build up will explode in a closed system. I know it's expensive, but your best bet is to retool the mold with cooling passages. Are you using chilled water in your cooling system? You can always insert a chiller in line.
  5. May 1, 2016 #4
    So instead of the LN2 being in the line behind the mold for 20 sec. after the material is packed. Do you think it would serve a better service
    to give a quick shot before the plastic is packed. Chilling the cavity before the hot plastic reaches the area would have the same effect.
    The tool I made is with cooling lines and the water going into the mold is coming from a chiller. What I have when I say closed loop is a hose used only for LN2 it would be on demand usage the pressure pump maintains the low line pressure. On/off valve going to intake port (copper tube) on mold. The ports are directed at and close to hot spots. Without knowing what cycle time for LN2 would be needed I would want a return line to the tank. Once it is dialed in and working It may not need a return line. It may only need an exhaust tube. LN2 has been used in molds for years but delivered a different way.
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