Expelling warm air from voids - what is left afterwards?

[EddieP]

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?

 

[Lnewqban]

The heated air that can escape should be replaced by melting plastic, I believe.

I see additional problems due to the possible difference of densities between metal and plastic, inducing stratification, as well as air oxidizing the metal at high temperatures.

Those negative effects could be reduced by applying vacuum to the mix previously to the beginning of the heating process.
Is that feasible?

 

[EddieP]

The heated air that can escape should be replaced by melting plastic, I believe.

I see additional problems due to the possible difference of densities between metal and plastic, inducing stratification, as well as air oxidizing the metal at high temperatures.

Those negative effects could be reduced by applying vacuum to the mix previously to the beginning of the heating process.
Is that feasible?

Thanks Lnewqban,
I appreciate your answer. The vacuum would be ideal, but it is an extra step I am trying to avoid if I can.
With regards to the oxidation, I should have said the material in question is actually a graphite powder. Sorry, I overlooked mentioning that. It seems that graphite starts oxidizing at 400C - if I keep the temperature under this, is it safe to assume that oxidation will not be an issue, or does oxidation occur in minute amounts at lower temperatures?

 

[Lnewqban]

Thanks Lnewqban,
I appreciate your answer. The vacuum would be ideal, but it is an extra step I am trying to avoid if I can.
With regards to the oxidation, I should have said the material in question is actually a graphite powder. Sorry, I overlooked mentioning that. It seems that graphite starts oxidizing at 400C - if I keep the temperature under this, is it safe to assume that oxidation will not be an issue, or does oxidation occur in minute amounts at lower temperatures?

You are welcome.
In that case, the melted mix may allow some of the reduced density bubbles to slowly reach the surface, but not without fighting the viscosity and possible convection movement of the hotter mix.
I don't know enough about it to give you any opinion on oxidation of graphite, sorry.

 

[Chestermiller]

It is also possible that the air bubbles will dissolve in the molten plastic if the plastic is not initially saturated with air at the higher temperature.