Molding

Much like casting of thermosets, thermoplasts can also be inherently nonhomogeneous from primary processing. During injection molding or extrusion of thermoplasts, varying degrees of crystallinity may be present, which can have a negative impact on mechanical properties. Molding is the method by which thermoplasts are heated above their melt temperature and injected into a final form, often under high temperature and pressure. Fast cooling during forming of thermoplastic components can result in amorphous regions. If slow cooling occurs, such as the forming of components in heated dies, final parts should be semicrystalline. A semicrystalline polymer can have oriented crystallites resulting in anisotropy in the mechanical behavior [116].

During molding, cooling of the polymer upon contact with the die surface can result in very superficial amorphous superficial “zones.” This amorphous “jacket” can impact the shape-memory performance; especially in very thin-walled structures. It is important to convert the finished part back to a semicrystalline state by providing a heat treatment above the Tg. This can also be mitigated by heating the receiving die close to the Tg of the material. Once the finished device is molded, the designer should characterize the amount of crystallinity in the part. This can be measured by either differential scanning calorimetry (DSC) or using a (modified) Avrami relationship [117]. In DSC, heat flows of the semicrystalline samples can be compared against polymer’s heat of fusion. The Avrami relationship can calculate crystallinity based on isothermal hold heat treatments. However, if mold temperature is not maintained at or near the Tg, then the Ozawa method for nonisothermal conditions is better suited [118].

Once the crystallinity of the samples is easily controlled and quantified, stress—strain samples should be produced for further characterization. This includes varying of processing parameters to determine their impact on crystallinity (including postprocessing, such as heat treatments to reduce amorphous surface “zones”). The samples of varying crystallinity should be elongated to measure modulus, yield strength, and strain to failure. These parameters are extremely useful in aiding in the design of shape-changing devices to identify the material’s limits. Strain-to-failure tests should be performed at multiple temperatures including intended storage temperature, maximum potential environmental conditioning (such as during shipping), and in the vicinity of the Tg.

 
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