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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 content of HDPE. When the content of HDPE increased from 0% to 5%, a complex cellular structure (CCS) appeared in the PLA/HDPE blending foams. With a content of HDPE more than 10%, the CCS disappeared gradually.
The interface between PLA and HDPE acted as heterogeneous nucleation points for both the crystallization of PLA and the cell nucleation of PLA/HDPE blends. Finally, the influence of foaming temperature on the foaming behaviors of pure PLA and PLA/HDPE blends was also investigated and the foaming mechanism on pure PLA foam and PLA/HDPE blending foams with various blending ratios was presented (47).
Nanofoams. The use of physical foaming agents to form nanofoams in semicrystalline polymers is still a tremendous challenge. The change from microcellular to nanocellular bubbles (the micro/nano transition) in PLA foam was investigated by adjusting the type of crystals induced in PLA during cold crystallization (48).
The results showed that at saturation temperatures below the micro/nano transition temperature, α′ crystals formed. Their coarser surfaces and higher crystallinity resulted in a higher cell nucleation efficiency, and thus a microcellular foam was converted to a high-cell-density nanofoam. After chain extension, the micro/nano transition temperature decreased, and the properties of the foam were improved at temperatures above the micro/nano transition temperature. A nanocellular foam with a cell density of 1015 cells cm–3 and a cell size of approximately 30 nm was obtained (48).
1.9.2 Water-Assisted Injection Molding
In the early 1970s, the idea of injecting fluid into polymer melts in the mold was proposed to produce hollow parts, and later the water-assisted injection molding (WAIM) technology was developed (49, 50).
Due to the development and wide applications of GAIM (42–44, 51), WAIM had been set aside in the last century.
Recently, this promising polymer processing technology has attracted attention not only for academic reasons but also for its industrial applications (52). The WAIM technology provides a new way to fabricate hollow or other complicated products due to its faster cycling time and light weight.
An overview has been given of the basic principles and applications of WAIM as well as the current research status (52). Here, first, the origin and the development of WAIM technology are described and then their advantages and applications are given. The review also focuses on the experimental trends of WAIM such as computer simulation, the effect of processing parameters on the WAIM samples, and the morphology as well as related WAIM-molded composites and polymer blends’ work (52).
1.10 Molding Machine for Granules
During a conventional injection molding process, plastic granules are melted by heating and then injected into a mold to form a plastic product (53). However, the plastic granules may be contaminated so as to cause the molded product to have defects, which lowers the product yield. Therefore, there is a need to provide a system that can perform defect analysis of a molded plastic product at an early stage.
Such a molding machine, which is operated to mold plastic granules into a plastic piece, includes a machine body, a platform, a heater, and a pressing unit (53).
The platform is installed at the machine body and defines a molding cavity adapted to receive the plastic granules. The heater is installed at the machine body adjacent to the platform and is adapted for heating the platform.
The pressing unit is installed at the machine body and disposed above the platform. The pressing unit includes a pressure cylinder, a telescopic rod connected to and driven by the pressure cylinder to extend downward and retract upward, and a pressing plate coupled to the telescopic rod and disposed to correspond in position