Thermoforming and the Coefficient of Thermal Expansion

By Thom Murray, Sr. Materials Engineer

By definition, the Coefficient of Thermal Expansion (commonly referred to as CTE), is “the amount of expansion (or contraction) per unit length of a material resulting from a one degree change in temperature”. In simplest and most practical terms for the thermoformer, it can be thought of as how much a material will grow when its temperature increases.  In English units it is typically expressed in length/length/per degree Fahrenheit. For example, inch/inch/°F. In metric, it is m/m/°C.

When we think about CTE in relation to plug assists, the problem is a bit more complex.  Solid polymers, by their very nature, have a relatively high CTE.  They like to grow when heated.  For example, Delrin® has a CTE of 59 x 10-6 in/in/°F. Nylon can range from 40 – 60 x 10-6 in/in/°F.  This means they grow and change greatly during the plug assist thermoforming process as they absorb heat from the sheet during contact. With materials that are specifically designed and engineered for thermoforming, like HYTAC® syntactic foam, thermoformers can greatly reduce variation in their process.

Syntactic foams are filled with hollow glass spheres, so even though they are polymeric in nature, the stable fillers mean that the CTE is relatively low, around 17 – 20 x 10-6 in/in/°F.  When you couple this with the fact that syntactics are excellent insulators and therefore take less heat from the sheet and run at much lower operating temperatures, you have a material that maintains a much higher dimensional stability that other plug material types.  With more stable materials, the thermoforming process itself becomes much more stable.

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