The additive process creates parts differently than either machining or injection molding. Additive processes tend to be low- or no-pressure process compared, for example, to the high pressures of injection molding.
Additive manufacturing often relies on the application of thermal energy to create inter-layer adhesion and layer-to-layer consolidation. Also, heat management systems in additive manufacturing vary, as well as the form in which material is supplied (filaments, liquids, powders and pellets) and delivered.
The heat profiles in printers and the thermal properties of polymers can influence rheological behavior and affect the way material layers adhere, resulting in different properties compared to those of injection-molded parts, for example. Even post-processing techniques for additively manufactured parts, such as support removal, vapor smoothing, sanding and sand blasting, and thermal curing, are different from techniques for injection-molded parts, which can include de-gating, de-flashing, cleaning, and so on.
An example of a special requirement for additive manufacturing materials designed for fused deposition modeling is compatibility of the build polymer with support structures.
The support polymer should provide a balance between adhering to and supporting the structure being built, and also be readily removable after the print job is completed.
Other key issues are differences in mechanical properties as compared to injection molded resin properties, and anisotropy variations across print directions that result from the way material is deposited in an additive manufacturing process. Not only do mechanical properties differ for the X, Y and Z axes, with the greatest challenge typically seen in the Z (vertical) axis, they are also process dependent. Fused deposition modeling is more susceptible to anisotropy than selective laser sintering (SLS) and stereolithography (SL). Efforts are being made to address this issue with innovations in materials development and printing techniques.