Thermal expansion of air bubbles in plastic

[EddieP]

TL;DR

Estimating the potential for problems caused by thermal expansion of air bubbles.

I am interested in the potential for air bubbles in a plastic structure to expand with heat, and put pressure on the plastic surrounding the air bubble.

In this case the plastic structure is formed by melting a thermoplastic powder. In between the grains of powder are voids with air in them. As the plastic melts and sets, pockets of air are trapped.

I am assuming that the size of the air pockets is influenced by the thermal expansion of the air. As the plastic melts, the expanded air pushes against the softened plastic. The plastic sets at below the melting temperature, but above room temperature. If the air pockets are always kept at below the temperature the air pockets were formed, I think there would be no potential for the air to expand to such a degree that would put pressure on the structure - is this correct?

If the melting temperature were 150C, I am assuming the air pockets would be formed at somewhere between 150C and 100C. If this is the case, would keeping the structure below 50C be enough to minimize the pressure put on the walls of the air bubbles by the thermal expansion of the air inside?

 

[Lnewqban]

If the volume of the rigid bubble is established at those close to melting temperatures, at room temperature, the internal absolute pressure should be lower than the achieved pressure of equilibrium (injection molding pressure, if any, plus atmospheric and hydrostatic pressures).

As normally, that trapped air contains humidity, the low temperature pressure should much lower, considering the huge expansion capability of steam versus air.

Please, see:
https://www.engineeringtoolbox.com/air-density-specific-weight-d_600.html

https://www.engineeringtoolbox.com/saturated-steam-properties-d_101.html