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A cylinder contains a powder made up two substances - spherical grains of metal and spherical grains of plastic. In between the grains are voids, so that the cylinder is made up of 75% powder grains and 25% air. The cylinder is oriented vertically. It is sealed at the bottom and open at the top.

The cylinder is heated, and the plastic grains start to melt. The air in the voids gets less dense as the temperature increases, and rises. Some of the hot air leaves the cylinder, but some of the air gets trapped in the melting plastic, and forms small pockets of air encased in the now melted plastic.

Would heating the cylinder slowly allow for more of the air to leave the voids before the plastic is melted? Or, would this rising air just be replaced by falling cooler air that fills the voids?

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  • $\begingroup$ Vacuum chamber + shaker when heating. Control temp and keep it shaking in the heated vacuum chamber. $\endgroup$
    – Abel
    Sep 10, 2022 at 1:43
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    $\begingroup$ Yep vacuum. Chamber. I'm not sure if also vibrating it would improve things or make it worse but in a vacuum I am pretty sure it would speed things up. $\endgroup$
    – DKNguyen
    Sep 10, 2022 at 2:23
  • $\begingroup$ Shaker helps for settling epoxy and thereby removing air bubbles so should work for molten plastic. Heck works for a bottle of soda (slightly different reasons but...) $\endgroup$
    – Abel
    Sep 10, 2022 at 4:15

2 Answers 2

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To answer your question as stated:

Not so much heating slowly as holding it above the plastic's melting point for a while. But this depends heavily on the surface properties of the metal and the plastic -- if the plastic doesn't want to wet out on the metal, or (I think!) it has a high surface tension and wants to "hold on" to the bubbles of air, then they won't preferentially rise.

The suggestion to use vacuum is a good one, because in the absence of surface interactions between the metal and plastic, when you re-apply atmospheric pressure the plastic would be pressed into any voids. If you could conduct heat into the mixture in a vacuum, then you'd probably want to pull a vacuum first, to avoid pulling a bunch of metal-filled plastic foam into your vacuum line. Whether you melt first or vacuum first, you'd want to make sure things are thoroughly melted and under vacuum and then apply atmospheric pressure.

Depending on the metal's grain size, the plastic's surface tension, and its propensity to wet out the metal when it's molten, you may still end up with some voids.

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There are so many causes of bubbles, it is necessary to identify the cause before any solution. The characteristics to be determined include:

  1. locations of bubbles ?
  2. recurring locations ?
  3. void or gas ? ( use to slow reheat test to determine if bubble expands or collapses)
  4. bubbles from eddy currents?
  5. race-track effect?

A few of the possible causes;

  1. lack of vents? vacuum vents? inadequate pressure, use of vacuum, venturi effect? preheat volatiles? misaligned nozzle? Too much decompression? heat back end less or more? more back pressure? vacuum pull before injection?
  2. Insufficient plastic is the main reason for sinks or voids
  3. gate is freezing
  4. too hot, reduce melt temp.
  5. Thin down the nominal wall. Core out the thick section,

The info was extracted from here. https://www.ptonline.com/articles/injection-molding-how-to-get-rid-of-bubbles

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