# Thermal expansion of air bubbles in plastic

• EddieP
In summary, the potential for air bubbles in a plastic structure to expand with heat and put pressure on the plastic surrounding them depends on the size of the air pockets and the thermal expansion of the air. If the air pockets are formed at a temperature between the melting temperature and room temperature, keeping the structure below the temperature at which the air pockets were formed can minimize the pressure on the walls of the air bubbles. However, the presence of humidity in the trapped air should also be considered, as steam has a much higher expansion capability than air.
EddieP
TL;DR Summary
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?

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.

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

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

Last edited:
EddieP

## 1. What is thermal expansion?

Thermal expansion is the tendency of matter to increase in volume when its temperature increases. This is due to the increase in kinetic energy of the particles, causing them to move and spread out, resulting in an increase in volume.

## 2. How does thermal expansion affect air bubbles in plastic?

Air bubbles trapped in plastic will expand when exposed to higher temperatures, causing them to increase in size and potentially change the shape of the plastic. This can lead to warping or distortion of the plastic object.

## 3. What factors can influence the thermal expansion of air bubbles in plastic?

The thermal expansion of air bubbles in plastic can be influenced by the type of plastic, the amount of air trapped, and the temperature difference between the initial and final state.

## 4. Is thermal expansion of air bubbles in plastic reversible?

Yes, thermal expansion of air bubbles in plastic is reversible. When the temperature decreases, the air bubbles will contract and return to their original size and shape.

## 5. How can the thermal expansion of air bubbles in plastic be controlled?

The thermal expansion of air bubbles in plastic can be controlled by using materials with lower thermal expansion coefficients, reducing the amount of air trapped in the plastic, or by limiting the temperature difference the plastic is exposed to.

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