Methods of manufacture
SMP devices have been successfully made from both thermoplasts and thermosets. Thermoplastic SMPs have the ability to be molded or extruded into complex shapes, but are highly dependent on the degree of crystallization and more susceptible to factors such as stress relaxation and creep [106]. Crystallinity is especially important in the shape-memory behavior of thermoplasts [107]. Thermoset SMPs can also be produced into complex shapes, and their functional attributes can be tailored via control of their polymer chemistry, but they are more susceptible to a lack of homogeneity due to difficulties with the polymerization process. The authors have extensive experience in controlling the thermomechanics of thermoset SMPs, as well as managing the degree of crystallinity in thermoplastic SMPs. Both structural homogeneity and crystallinity are discussed briefly below within the context of detail design and with respect to manufacturing methods.
Manufacturing with SMPs can take numerous forms. Additional methods of manufacture exist, and are discussed briefly at the end of this section, but here we will focus on the two most successful approaches, casting and molding. Casting is primarily for thermosets, has been utilized in a variety of commercial SMP products, and commonly involves the synthesis of the SMP in a final or near-net form. Molding is primarily for thermoplasts, and has also been employed with equal success. However, both manufacturing methods can overlap in final component production. Zero strain geometries can be produced directly from casting and molding, but can also be the result of near-casting and extrusion or bulk molding, followed by secondary processing such as machining. Both approaches have been utilized successfully, with bulk processing followed by secondary machining being the most common.
In addition, recent work has proven the viability of SMP foams [108—111]. In particular, polyurethane SMPs have been thoroughly studied. Traditionally, polymer foams are defined as polymeric substrates that, after introduction to porogens (gaseous agents, solvents, etc.) during curing or thermoplastic processing, can be made to have porous macrostructures [112]. However, this typically results in the creation of closed-cell structures, whereas alternate porogens (such as salts or other particulates) create open cell structures upon subsequent porogen removal [113]. This includes the creation of porous structures not commonly thought of as foams, but instead as three-dimensional scaffolds, and have held significant interest within the biomedical industry.
