Novel microcellular foam injection-molding technology
Microcellular polymer foams are plastics fabricated to contain billions of tiny bubbles. They are defined as having cells that are <100μm in size and a cells per unit volume density of >108/cm3. The first commercial microcellular foam injection-molding technology (i.e., Trexel's MuCell injection-molding process) was developed in the 1990s, based on research conducted at the Massachussetts Institute of Technology.1, 2 In this original and most commonly used microcellular foam injection molding process, a supercritical fluid (SCF) pump unit is used to pressurize a physical blowing agent (PBA)—nitrogen (N2) or carbon dioxide (CO2)—to supercritical pressure levels. The PBA is then introduced, through an injector valve, into a barrel that contains the molten polymer. In this way, the gas is dissolved in the polymer prior to injection. The benefit of the foam-generation process is that it allows plastics to be recycled, i.e., so their carbon footprint and the cost of raw materials are reduced. Since the introduction of the MuCell approach, different microcellular foaming technologies have been successfully developed,1, 2 and the method has been used to great success in the automotive, computer, and home appliance fields. In these newer technologies, however, an SCF pump unit is still required (i.e., for pumping CO2 or N2 from the cylindrical pressure to the supercritical pressure), which gives rise to ever-increasing machine and operation costs.
In this field of research, there is still a strong-held, yet baseless, belief that supercritical CO2 or N2 are required for the preparation of the microcellular foam and that such SCFs provide special properties in the produced foams. In contrast, it has been experimentally proven that the solubility of CO2 and N2 in thermoplastic polymers follows Henry's Law (i.e., the gases can be dissolved in polymers even below the critical pressure).3, 4 It is therefore theoretically possible to conduct physical foaming even when the saturation pressure (which corresponds to the dissolved PBA concentration) is lower than the critical pressure. An injection-molding machine without a high-pressure pump system, however, has not yet been developed.
In our work,5 we have thus developed a new foam injection-molding approach (which does not include an SCF pump unit) for the production of microcellular foams. We simplify the injection-molding process by directly delivering the PBA (i.e., CO2 or N2) from their gas cylinders into the molten polymer. This delivery takes place through an injection valve, which we can control by using a specifically designed operation sequence and screw configurations.
Our foam injection-molding apparatus is illustrated in Figure 1. It includes a vent hole, with a venting vessel, in the middle of the machine. Through this venting hole, the excess PBA gas (i.e., the residual PBA that exists as a gas in the molten polymer) can be discharged from the molten polymer to the atmosphere. Alternatively, it can be used to introduce additional PBA gas to the molten polymer (when the PBA concentration in the polymer is below the saturation point). Specifically, we can adjust the concentration of the PBA dissolved in the molten polymer by tuning the pressure in the venting vessel. We manipulate the pressure of the venting vessel by using a back pressure regulator at the end of the discharge line.
Scanning electron microscope (SEM) images of core-back foam injection-molded products that we fabricated with our technology are shown in Figure 2. We prepared these foams at three different expansion ratios (2, 3, or 5). In addition, we used either CO2 or N2 as the PBA, an injector valve opening time of 0.2 seconds, a vent vessel pressure of 5MPa, and a secondary pressure of 8MPa in the gas cylinders. These SEM images show that the cell size of the foams is about the same, or less, than that of foams we obtained with a conventional foam injection-molding approach. We have thus demonstrated that our foam injection-molding machine and method can be used to successfully produce microcellular foams.
By delivering the PBA only through the vent hole, we can also produce microcellular foams with our system. An example of a polypropylene injection-molded product and its corresponding microcellular foam are shown in Figure 3. We prepared this microcellular foam by delivering air through the vent vessel and hole and into the molten polymer, with the injector valve completely closed. The air was provided from an air compressor, in which air is simply compressed from atmospheric pressure to 4MPa. Through our process, the size of the original disk-shaped plate—with a diameter of 100mm and a thickness of 1mm—was expanded to have a thickness of 2mm. In addition, the plate became white in color because of reflection from the microscale bubbles.
In summary, we have developed a new injection-molding system, without an SCF pump unit, for the fabrication of microcellular polymer foams. In our technique, we can produce stable microcellular foams with the use of non-supercritical N2, CO2, or compressed air as the physical blowing agent. We have thus demonstrated that the pressurization of N2 or CO2 to a supercritical state is not necessary for successful microcellular injection molding. In our upcoming work, we will be advancing this technology by optimizing the design of the screws and the performance of the vent-hole PBA delivery. This will allow us to easily convert conventional injection-molding machines to our system design, at low cost.
- J. Xu, Introduction, Microcellular Injection Molding, pp. 1-11, Wiley, 2010.
- M. Berry, Microcellular injection molding, Applied Plastic Engineering Handbook, pp. 215-226, Elsevier, 2011.
- Y. Sato, K. Fujiwara, T. Takikawa, Sumarno, S. Takishima and H. Masuoka, Solubilities and diffusion coefficients of carbon dioxide and nitrogen in polypropylene, high-density polyethylene, and polystyrene under high pressures and temperatures, Fluid Phase Equilibria 162, pp. 261-276, 1999.
- Y. Sato, T. Takikawa, S. Takishima and H. Masuoka, Solubilities and diffusion coefficients of carbon dioxide in poly(vinyl acetate) and polystyrene, J. Supercrit. Fluids 19, pp. 187-198, 2001.
- A. Yusa, S. Yamamoto, H. Goto, H. Uezono, F. Asaoka, L. Wang, M. Ando, S. Ishihara and M. Ohshima, A new microcellular foam injection-molding technology using non-supercritical fluid physical blowing agents, Polym. Eng. Sci., 2016.