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 with the molding cavity.
The pressure cylinder drives the pressing plate via the telescopic rod to move the pressing plate relative to the platform between an initial position, where the pressing plate is away from the molding cavity of the platform, and a molding position, where the pressing plate is close to the molding cavity of the platform so as to cooperate with the platform to form a mold for molding the plastic granules into the plastic piece.
Another system for defect analysis of the plastic piece includes the molding machine, an optical sensing unit, and a computing unit. The optical sensing unit is configured to detect the plastic piece formed using the molding machine. The computing unit is coupled to the optical sensing unit and is operable for defect analysis of the plastic piece.
Here, the method for forming a plastic piece includes the following steps (53):
1 Heating a raw plastic material,
2 Pressing and molding the heated raw plastic material to form a light-transmissive molded plastic piece,
3 Cooling the molded plastic piece, and
4 Based on defects of the molded plastic piece, determining whether or not the molded plastic piece conforms with predetermined criteria.
1.11 Foam Curing of Footwear
An article of footwear may be constructed with a foam sole portion (54). The formation of the foam portion may involve heating the foam portion utilizing one or more heating elements. However, a traditional heating element, such as an oven, used to heat and cure a foam item is continuously energized to maintain sufficient thermal energy and may be inefficient at heating the foam item. Furthermore, the ability to control the heating and timing of the foam item may be limited with a traditional heating element.
An induction heating assembly has been used to cure an expanded foam structure (54). Modular induction heating assemblies can efficiently and controllably heat a mold that contains a foam item. In order to achieve efficiencies and controllability, inactive zones and cooling mechanisms were used in connection with induction heating assemblies to regulate the temperature of the mold during the curing process. Figure 1.4 shows an induction heating assembly that is configured to house induction coils and a mold.
Figure 1.4 Induction heating assembly (54).
The induction heating assembly may house coils having a combination of coil patterns. Chamber rollers may allow a belt, such as a conveyor belt, to pass over the chamber rollers and through the cavity of the inner chamber. Also, the chamber rollers may allow a foam item to pass through the cavity so that it may be cured utilizing the induction coils (54).
Various induction heating assemblies and soaking zones may be used. For instance, four induction heating assemblies and four soaking zones may be used where each soaking zone is between each induction heating assembly. Similarly, it is contemplated that a dwell time within a soaking zone or as influenced by an induction heating assembly may be adjusted to achieve a desired intermediate temperature.
For example, on a moving conveyor having a constant speed, the dwell time may be changed in the soaking zone by changing a length between induction heating assemblies. The rate of the conveyor belt movement may also be manipulated to change a dwell time in the soaking zone — and exposure to induction heating effects within the induction heating assemblies.
1.12
