Stress in Thermoplastics

When you melt and “work” a plastic resin at or above its crystalline melt temperature, like in extrusion, the plastic resin likes to remember the physical torture it was put through as it is then cooled quickly. Depending on the type of plastic resin that memory can be large amounts of stress.

Stress is locked in during that first cooling cycle after being melted, and will be relieved only at the next heating cycle. The stress in any plastic is “heat history sensitive” and will be removed only once during its first excursion back to the temperature at which it was locked in. Note that these stresses may be locked into the plastic across a range of temperature, and to remove all of it requires that it be heated back to those temperatures. Generally, it’s sufficient to revisit a temperature only somewhat higher than will be experienced in an application to stabilize the part.

Thereafter, the only expansion and contraction that will occur at that (relieved) temperature with be the typical amount for that plastic’s normal physical properties, i.e., coefficient of thermal expansion. Axial and bi-axially oriented films have huge amounts of stress built as a benefit, as with film to “shrink” package your items (like frozen Pizza) or shrink package your boat for the winter to keep the weather out.

 

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