10.1002/spepro.000059 Morphology for microcellular injection molding Jingyi Xu Injection molding makes it possible to better control the structural alterations in microcellular foam, which can result in minimal mate- rial property changes for plastic parts. Microcellular foaming technology was originally conceptualized and invented at the Massachusetts Institute of Technology in 1984. 1, 2 The idea is simply to add tiny bubbles that are smaller than the preexisting flaws of the material into a polymer, which reduces the amount of ma- terial while maintaining toughness. 1 Suh defines microcellular plastics as foamed plastics with a cell size that is less than 30 microns. 1 In fact, this technology offers the potential to manufacture transparent supermi- crocellular foams with cell sizes <0.05 microns. Microcellular plastics were not successfully used in industrial applications until 1998, when we developed the first reciprocating injection-molding machine. 3, 4 To- day, cell sizes typically range from 5 to 100 microns. Microcellular technology has already had a significant impact on the worldwide plastics industry. It requires precisely metered quantities of atmospheric gases (nitrogen or carbon dioxide) in any of the three most common thermoplastic conversion processes (injection molding, extru- sion, and blow molding) to create millions of nearly invisible micro- cells. Fabrication on this scale brings a wide array of benefits, including reduced weight, smaller amounts of material needed, and lower cost, as well as compatibility with environmental friendly blowing agents. The microcellular injection-molding process is primarily used where foam- ing has not historically been deployed, producing less-expensive preci- sion parts with consistently high quality and exceptional dimensional stability. Our research has revealed that numerous factors influence the quality of the microcellular foam. For instance, the pressure drop rate must be high enough for the necessary nucleation. In addition, a minimum gas percentage is required, and the first stage of gas mixing is critical for uniform cell distribution in the molded part. (Specifically, CO2 gas can create close-packed cell structures more easily than N2 gas can.) 2–11 Since microcellular structures measure about half the diameter of a strand of human hair, they cannot be seen by the naked eye. As a result, the scanning electron microscope is the most popular way to define the morphological changes of different microcellular parts. Examining Figure 1. Morphology of polystyrene microcellular foam. Average cell size: 25 microns. Cell density: 8.1×10 7 cells/cm 3 . 5 White bar is 100 microns. WR: Weight reduction. (Image courtesy of John Wiley and Sons.) cross-sections requires the use of a carefully broken section, which is typically done cryogenically. For instance, liquid nitrogen is used to deep-freeze the sample, which is then broken along a predetermined direction to reveal a flat fracture section view. This in turn is magni- fied about 200 times or higher to see the microcellular structure. 12 The amorphous material polystyrene, shown in Figure 1, has an average cell size of 25 microns and a cell density of 8.1×10 7 cells/cm 3 . Other amorphous materials will have a similar structure. Injection-molded crystalline and semicrystalline materials have be- come increasingly popular in a variety of industries. Typical examples include polypropylene, polyethylene terephthalate, and polyamide. In these materials, crystallization during cooling may expel gas near the crystalloid, and the cell structure may not be as uniform as that of an amorphous material. Our work has shown that amorphous material has a better cell structure than crystalline material, whose morphology is also greatly influenced by mold temperature. Continued on next page