We previously developed a resilient and innovative cellular foam injection molding (RIC-FIM) technology to illustrate that pressurization of the physical blowing agent (PBA), such as carbon dioxide (CO2), to the supercritical state is unnecessary for preparing microcellular foams. Herein, our developed FIM machine is further simplified by using the previously auxiliary venting unit as the only PBA delivery unit. Subsequently, high void fraction polypropylene (PP) foams were successfully obtained using this novel FIM machine with 4–6 MPa of nitrogen (N2). The results showed that an increase in the vessel pressure reduced the PP foam cell size and enhanced the cell density as well as the void fraction, signifying that the concentration of dissolved gas was controlled by the pressure vessel. Furthermore, compared with the N2 foaming process, CO2 foaming revealed a finer cellular structure for PP. This simplified FIM technology holds great potential for the industrial manufacturing of lightweight injection-molded products with better economic benefits.

A new foam injection-molding technology was developed to produce microcellular foams without using supercritical fluid (SCF) pump units. In this technology, physical blowing agents (PBA), such as nitrogen (N 2 ) and carbon dioxide (CO 2 ), do not need to be brought to their SCF state. PBAs are delivered directly from their gas cylinders into the molten polymer through an injector valve, which can be controlled by a specially designed screw configuration and operation sequence. The excess PBA is discharged from the molten polymer through a venting vessel. Alternatively, additional PBA is introduced through the venting vessel when the polymer is not saturated with PBA. The amount of gas delivered into the molten polymer is controlled by the gas dosing time of the injector valve, the secondary reducing pressure of the gas cylinder and the outlet (back) pressure of the venting vessel. Microcellular polypropylene foams were prepared using the developed foam injection-molding technology with 2-6 MPa CO 2 or 2-8 MPa N 2 . High expansion foams with an average cell size of less than 25 lm were prepared. The developed technology dispels arguments for the necessity to pressurize N 2 or CO 2 to the SCF to prepare microcellular foams. POLYM. ENG. SCI., 57:105-113, 2017. 109 FIG. 11. SEM micrographs of iPP/MD (0.5%) foams with N 2 at different expansion ratios. The foaming temperature is indicated in the upper left corner of each micrograph.

We report a novel approach to prepare injection-molded foams using air as the foaming agent, which is realized through our newly developed foam injection molding technology. Microcellular polypropylene (PP) foams exhibiting cell size of ∼7 μm and cell density of 1.63 × 109 cells/cm3 were successfully obtained employing air foaming. Fourier transform infrared spectroscopy and rheological results illustrated that the molecular structures and melt properties of PP were greatly altered in air foaming, resembling a reactive foaming process. Furthermore, compared with nitrogen and carbon dioxide foaming, air foaming gave us a more flexible PP foam with finer cellular structure.

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