10.2417/spepro.005005 Foam injection molding using nitrogen and carbon dioxide as co-blowing agents Xiaofei Sun and Lih-Sheng Turng A novel microcellular injection molding approach using nitrogen and carbon dioxide significantly improves the morphology and mechanical properties of melts. Microcellular injection molding (MIM) is a very special injection molding process. It injects a gas such as nitrogen (N 2 ) or car- bon dioxide (CO 2 ) in the so-called supercritical state—i.e., simul- taneously liquid and solid—into the polymer melt as a blowing agent to produce lightweight, foamed plastic parts. MIM continues to attract attention because it saves on material costs and energy while improving dimensional stability and production efficiency com- pared with conventional solid injection molding. 1 With its unique properties, MIM has encouraged a range of innovative applications, such as packaging materials, insulation, filtration membranes, sports equipment, automotive components, and aircraft parts. 2–5 Recently, Lee et al. 6 proposed a method of producing microcellular injection- molded parts known as supercritical fluid-laden pellet injection molding foaming technology (SIFT). This method generates gas-laden pellets from an extruder equipped with a gas-injection device. Whereas conventional MIM requires modification and additional equipment for every injection-molding machine used to make microcellular parts, only one extruder needs modification with an add-on gas pump when using SIFT technology. The gas-laden pellets produced can be used by several conventional injection-molding machines without having to modify them. 6 These two technologies work seamlessly with each other, enabling the introduction of two different types of gases independently into the injection-molding foaming process. One type of gas (gas A) can be embedded into gas-laden pellets using SIFT extrusion. These pellets can then be used in MIM, during which gas B will be injected (see Figure 1). Different gases have distinct physical properties and different benefits in polymer foaming. When used together as co-blowing agents, their strengths can be combined and fully exploited, to superior Figure 1. Schematic of the combined supercritical fluid (SCF)-laden pellet injection-molding foaming technology/microcellular injection- molding (SIFT/MIM) process. effect. Carbon dioxide and N 2 are the two most commonly used physical blowing agents because of their low cost and environmen- tally friendly nature. Carbon dioxide typically shows high solubility in polymers and, therefore, can be readily dissolved into the polymer melt at a high concentration. Moreover, CO 2 shows a strong plasticization effect that serves to reduce the surface energy of the polymer melt and enhance bubble nucleation. Nitrogen, on the other hand, shows a much lower solubility, which causes a high degree of supersaturation once the polymer-gas mixture is injected into the mold cavity. This supersat- uration is a major driving force for bubble nucleation. Consequently, N 2 usually leads to a finer bubble structure and higher bubble density. When both CO 2 and N 2 are introduced into the same foaming process at appropriate concentrations and ratios, N 2 triggers a high bubble nucleation rate, while CO 2 —which can be uniformly dispersed and dissolved at high concentration—ensures uniform bubble growth af- ter nucleation. As a result, the final bubble structure achieved using N 2 C CO 2 as co-blowing agents is much finer and more uniform than using either type of gas alone. Continued on next page