Foam injection molding using nitrogen and carbon dioxide as co-blowing agents
Microcellular injection molding (MIM) is a very special injection molding process. It injects a gas such as nitrogen (N2) or carbon dioxide (CO2) in the so-called supercritical state—i.e., simultaneously 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 compared 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 effect. Carbon dioxide and N2 are the two most commonly used physical blowing agents because of their low cost and environmentally 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, CO2 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 supersaturation is a major driving force for bubble nucleation. Consequently, N2 usually leads to a finer bubble structure and higher bubble density. When both CO2 and N2 are introduced into the same foaming process at appropriate concentrations and ratios, N2 triggers a high bubble nucleation rate, while CO2—which can be uniformly dispersed and dissolved at high concentration—ensures uniform bubble growth after nucleation. As a result, the final bubble structure achieved using N2+CO2 as co-blowing agents is much finer and more uniform than using either type of gas alone.
We carried out an initial experiment using low-density polyethylene (LDPE). We dosed first with N2 in the SIFT process to create N2-laden pellets at three different levels of N2 content (0, 0.19, and 0.24% by weight). We then dosed with 2.5% by weight CO2 during the MIM process. Figure 2 shows the foam morphology under these three conditions. The addition of N2 in the gas-laden pellets improved theTable 1.
|Properties||1.0%N2 MIM||2.5%CO2 MIM||0.19% N2SIFT+2.5%CO2 MIM||0.24% N2SIFT+2.5%CO2 MIM|
|Young's modulus (bar)||503.7±54.38||583.3±33.27||623.1±23.78||685.6±29.8|
|Strain at break (mm/mm)||1.07±0.10||1.31±0.09||1.30±0.09||1.42±0.08|
morphology dramatically: bubble size was reduced from approximately 120μm on average to approximately 30μm.
We performed tensile tests on the molded parts using an Instron 5967 test machine following the ASTM D638 standard (see Table 1). Besides the three samples shown in Figure 2, parts molded using only MIM with 1% by weight N2 are also listed in the table for comparison. The combined SIFT/MIM approach significantly improved the Young's modulus and toughness thanks to the finer bubble structure.
To verify whether this method is suited to materials other than LDPE, we applied the same process conditions to high-impact polystyrene (HIPS) and polypropylene (PP), with three different levels of N2content (0, 0.19, and 0.24% by weight) in the gas-laden pellets, and 2.5% by weight CO2 injected during MIM. HIPS is a widely used amorphous material with good foamability, and PP is a semicrystalline material that is typically difficult to foam because of its crystallinity and low melt strength, which results from its linear molecular structure.7 Figure 3 shows scanning electron micrographs of the morphology of these samples. Both HIPS and PP showed a significant reduction in bubble size and an increase in bubble density once both N2 and CO2 were introduced, compared with CO2 MIM alone.
In summary, we successfully implemented a combined SIFT/MIM process using both N2 and CO2 as co-blowing agents. N2 SIFT+CO2 MIM produced remarkable improvements in foam morphology and mechanical properties compared with either blowing agent alone. We applied this approach with equally successful results to three polymers: LDPE, PP, and HIPS. We are currently working on using combinations of different types of SIFT gas-laden pellets to produce microcellular injection-molded parts, such that the benefit of co-blowing agents can be realized without the need for gas dosing during the injection-molding process.
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