overall systems approach is the key to successfully implementing a microcellular foam process. The necessary modifications and the component designs that are required for a microcellular foam molding machine have been described. The important components are the plasticizing unit, injection unit, hydraulic unit, clamp unit and supercritical fluid unit (40).
1.9.1 Gas-Assisted Injection Molding
A novel gas-assisted microcellular injection molding method was created by combining gas-assisted injection molding (GAIM) with microcellular injection molding (41). Here, with the assistance of the high-pressure gas from GAIM, the amount of the polymer melt required for fully filling the mold cavity is reduced in the gas-assisted microcellular injection molding in comparison to that in the microcellular injection molding, thus leading to a high weight reduction of the foamed part.
In addition, the high-pressure assisted gas from the GAIM can dissolve all the cells generated in the melt filling stage back into the polymer melt, thus improving the molded part’s surface appearance by eliminating the surface sliver marks. Also, the secondary foaming process in a steady state triggered by releasing the high-pressure-assisted-gas results in a foamed part with a fine cellular structure and a compact solid skin layer, which can help to enhance the part’s mechanical properties.
In order to verify the effectiveness of the gas-assisted microcellular injection molding, experiments that can compare the microcellular injection molding and the gas-assisted microcellular injection molding were conducted. The results demonstrated that the gas-assisted microcellular injection molding can not only significantly increase the weight reduction, but also greatly improve the surface appearance and the mechanical properties of the foamed part (41).
1.9.1.1 Flow Visualization
The basic filling phenomena in a gas-assisted injection molding process with flow visualization were assessed (42). Here, a high-speed video camera was used to record the mold-filling phenomena of rectangular cavities with various arrangements of gas channels. The mold-filling phenomena were compared and guidelines for layout of gas-channel ribs were drawn.
The following conclusions can be drawn from this investigation (42):
1 Gas-assisted filling, with a near-exponential velocity rise, is much faster than conventional filling, with nearly constant speed. Ribs with similar area but different geometry significantly affect the speed of gas tip and melt front advancement during the gas-assisted filling.
2 Without any rib in a gas-assisted molded plate, severe rigidity degradation will occur along the gas penetration path, especially near the thickness transition edges. Similar rigidity degradation will occur even in a plate with improper gas-channel rib layout. If there is no proper gas-channel rib along the melt front advancement in the far end, gas will penetrate into non-rib regions along the direction of melt front advancement to fill the free space.
3 With a symmetrical rib layout, the gas penetration in symmetrical ribs can never remain symmetrical.
1.9.1.2 Computer-Aided Engineering
The development of computer-aided engineering (CAE) technology for GAIM has been reported (43). To achieve this goal, efforts have been made in developing a numerical analysis for predicting the filling and post-filling behavior and verifying the predictions through collaboration with the industry and universities.
The physics of this process has been described, which leads to the various advantages and the inherent difficulties associated with the design and processing. Secondly, the methodology of numerically modeling and simulating gas-assisted injection molding filling dynamics was discussed, together with experimental comparison.
Finally, the various advantages of applying the CAE technology for the GAIM process were detailed, such as evaluating the various gas-assisted injection molding processes and establishing preliminary design guidelines (43).
1.9.1.3 Rubber Processing
It could be proven in initial experiments that the manufacturing of rubber parts made of liquid silicone rubber and an ethylene propylene diene monomer with functional hollows is possible (44). Further studies involved the determination of the basic requirements for the mold design, the refinement of the process control, different GAIM variants and border restrictions of the process as well as the necessary material characteristics.
1.9.1.4 Poly(lactic acid) Foams
Poly(lactic acid) (PLA) is a kind of bio-based and biodegradable polymers. PLA foams with high expansion ratio have great application potential in heat insulation, adsorption and other areas. However, due to its low melt strength and slow crystallization rate, linear PLA has poor foaming ability, so it is hard to prepare PLA foams with high expansion ratio (45).
Although chain branching, blending, and other methods have been utilized to improve PLA’s melt strength and foaming ability, they easily destroy the biodegradability of PLA, cause chemical pollution, and raise production costs.
A new supercritical fluid foaming process, based on pre-isothermal cold crystallization, was proposed to prepare PLA foams with a high expansion ratio (45). To improve PLA’s melt strength and foaming ability, a pre-isothermal treatment was applied to induce sufficient cold crystallization.
SEM shows that a higher pre-isothermal temperature (Tc) leads to a larger spherulite size and higher crystal stability before foaming (46). The foaming experimental results demonstrate that, as the Tc increases, the expansion ratio and cell size first increase and then decrease. This is because proper crystallization helps to improve melt strength and promote foaming, but excessive crystallization restricts cell growth. Finally, as Tc increases, the high melting temperature crystals of the foam gradually increase, while the crystallinity of the foam first increases and then decreases, which is attributed to strain-induced crystallization.
The DSC and wide-angle X-ray diffractometer results confirm that the pre-isothermal treatment remarkably promotes the PLA’s cold crystallization, and endows the PLA sample higher crystallinity and more perfect crystalline structure (45).
Moreover, the high-pressure rheological testing results indicate that the pre-isothermal treatment improves the PLA’s melt viscoelasticity significantly. Finally, the foaming results show that the pre-isothermal treatment significantly enhances the PLA’s foaming ability. With the pre-isothermal treatment, the PLA’s maximum expansion ratio increases from 6.40-fold to 17.7-fold, and the uniformity of cellular structure is also improved obviously. The new process provides a green, flexible, and low-cost way to prepare fully biodegradable PLA foams with high expansion ratio (45).
A simple and effective methodology to improve the crystallization behaviors, rheological property, and supercritical CO2 foaming of PLA using high-density poly(ethylene) (HDPE) as modifier was proposed (47). Here, PLA was blended with various contents of HDPE using a melt blending method.
The results demonstrated that with the blending of PLA and HDPE, the crystallization behaviors of PLA and HDPE were improved simultaneously and the rheological property of PLA gradually improved. The morphology of HDPE dispersion phase in the PLA/HDPE blends was irregular and its average size gradually became larger with the content of HDPE increasing. Then resultant PLA/HDPE blends were foamed using supercritical CO2 as physical blowing agent in an autoclave. The cellular morphology evolution of PLA/HDPE blending foams had a relationship with the
