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 (N2) and carbon dioxide (CO2), 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 CO2 or 2–8 MPa N2. High expansion foams with an average cell size of less than 25 μm were prepared. The developed technology dispels arguments for the necessity to pressurize N2 or CO2 to the SCF to prepare microcellular foams. POLYM. ENG. SCI., 57:105–113, 2017. © 2016 Society of Plastics Engineers

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