What are the important thermal properties of Plastics?
COEFFICIENT OF EXPANSION
With a change in temperature, plastic materials tend to change size considerably more than other materials, such as steel, ceramics and aluminium. A designer must consider these differences and even the shipping environment which may expose the part to a much greater temperature variation than the part will ever see in use. The measure of how much a part changes size as temperature varies is called the “thermal coefficient of expansion”. The units are usually given in m/m/⁰C.
DEFLECTION TEMPERATURE UNDER LOAD
In addition to changing size, the strength and modulus of elasticity of plastic materials tend to decrease as the ambient temperature increases. The standard test for determining the deflection temperature under load (DTUL) at 0.45MPa and 1.80MPa provides information on the ability of a material to carry a load at higher temperatures. The temperature of the loaded beam is raised until a certain amount of deflection is observed. The temperature when that deflection is reached is called the DTUL.
Many plastics are good thermal insulators; that is, heat does not travel through them easily. The conductivity of plastics is 300 to 2,500 times less than most metals. This is why we can pick up a hot pan by its plastic handle and shows why it takes a long time for a casting or other moulded part to cool down in the middle. Internal stress can be set up in a material because of the differences in the cooling rates between the outside of a part and the core.