Plastics Process Analysis, Instrumentation, and Control. Группа авторов
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and environmental effects for the aviation industry
In the weld line extension algorithm, 135° is set as a welding angle threshold for forming weld lines. Finally, the effects of cavity thickness, process parameters, and mesh densities have been investigated. Moreover, moldflow simulation results and real parts in production have been conducted to verify the proposed determination method, which demonstrate that the proposed method is correct and effective in actual production (9).
1.6 Extrusion Blow Molding
1.6.1 Rapid Thermal Cycling Molding
Blow molded parts made from engineering resins usually possess a poor surface quality, and thus cannot satisfy the requirement of high-gloss appearance in some applications. For this reason, a rapid thermal cycling extrusion blow molding (RTCEBM) technology was developed. The process principle was presented and its process procedure optimization was also analyzed (19).
With the aim of obtaining a uniform temperature distribution on both the mold cavity and the core surfaces, a two-step optimization method based on a sequential quadratic programming algorithm was proposed for designing the heating system in a RTCEBM mold. Its effectiveness was demonstrated by optimizing the electric-heating system for the RTCEBM mold of an automotive spoiler.
After optimization, the maximum core surface temperature difference is reduced by 77% from the initial value of 22.06°C to the optimal value of 5.05°C. The molding process coordination could also be ensured. So, an effective method has been assessed to optimize the heating system for these molds with cavity and core sides to be heated simultaneously (19).
1.6.2 Rapid Heat Cycle Molding
Rapid heat cycle molding (RHCM) is an advanced injection molding technology for producing spraying-free plastic products with excellent appearance (20). Rapid mold heating and cooling is the key technique of RHCM.
Despite being widely used in practice, the regular rapid mold heating and cooling methods still have some obvious defects. Thus, a new rapid mold heating and cooling method has been developed, characterized by electric heating and annular cooling.
Here, the temperature and pressure responses in the heating and cooling periods were experimentally investigated (20). The results of the study showed that the tool surface temperature increases almost linearly with the heating time after a short response time. The larger the heating power or the smaller the distance from heater to tool surface, the faster the heating rate.
The introduction of air bubbles into the working fluid can remarkably reduce the pressure growth of working fluid without affecting the heating rate. In the investigated range of flow rate, the cooling rate first increases significantly with the flow rate, and then reaches a plateau, while the running pressure of working fluid increases linearly with the flow rate in the whole range.
The optimum flow rate is around 6.0 l min–1, corresponding to a Reynolds number of 6700 (20). The Reynolds number helps to predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers flows tend to be turbulent (21).
The heat transfer coefficient in the cooling period increases sharply at the initial stage, and then reduces gradually, and finally reaches a plateau. The larger the Reynolds number, the higher the heat transfer coefficient. In particular, the heat transfer coefficient and the Reynolds number show a linear relationship on the double logarithmic scale. Finally, a mathematical model was developed for predicting and controlling the temperature fluctuation range of tool surface (20).
1.6.2.1 Reduction of Weld Lines
Rapid Heat Cycle Molding. The RHCM technique can greatly improve weld lines without prolonging the molding cycle. The effects of cavity surface temperature in RHCM on the mechanical strength of the specimen with and without weld line were investigated (22, 23).
Six kinds of typical plastics, including poly(styrene) (PS), poly(propylene) (PP), acrylonitrile-butadiene-styrene (ABS), ABS/poly(methyl methacrylate) (PMMA), ABS/PMMA/nano-CaCO3 and glass fiber-reinforced PP, are used in experiments. The specimens with and without a weld line are produced with the different Tcs on the developed electric-heating RHCM system. Tensile tests and notched Izod impact tests are conducted to characterize the mechanical strength of the specimens molded with different cavity surface temperatures. Simulations, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and optical microscope are implemented to explain the impact mechanism of Tcs on the mechanical properties.
Thermal cycling experiments are implemented to investigate cavity surface temperature responses with different heating and cooling times. According to the experimental results, a mathematical model was developed by regression analysis to predict the highest temperature and the lowest temperature of the cavity surface during thermal cycling of the electric heating RHCM mold (23).
The simulated cavity surface temperature response showed a good agreement with the experimental results. Based on simulations, the influence of the power density of the cartridge heaters and the temperature of the cooling water on the thermal response of the cavity surface could be obtained. A high cavity surface temperature during the filling stage in RHCM can significantly improve the surface appearance by greatly improving the surface gloss and completely eliminating the weld line and jetting mark (23).
Weldless-Type Injection Mold Apparatus. A general forming process of a polymer resin has problems such as an aesthetically poor appearance due to a weld line formed by the molten resin in the mold and a low degree of surface gloss.
To solve these problems, a mold heating method can be used, in which the mold temperature is set to be higher than the melting point of a formed polymer resin.
However, if the polymer resin is formed by setting the temperature of a mold to be higher than the melting point of polymer resin, a weld line is not formed while enhancing aesthetic appearance, such as gloss. But a high temperature of the mold extends the cooling time, and the overall forming cycle may be prolonged, thereby lowering the manufacturing efficiency.
In particular, since the polymer resin is not separated from the mold after being cooled to lower than the melting point thereof, deformation due to shrinkage may become more severe than in a conventional molding.
To overcome these problems, a weldless-type injection mold apparatus has been developed (24). This apparatus includes an upper mold, a lower mold engaged to the upper mold to form a cavity for injection molding of products, a heating unit formed on one side of the cavity of at least one of the lower and upper molds to heat a resin injected into the cavity, a first cooling unit formed in at least one of the lower and upper molds to prevent the injection mold from being overheated, and a second cooling unit installed between the heating unit to cool an area surrounding the cavity and an injection molded product.
A schematic diagram of a weldless-type injection mold apparatus is shown in Figure 1.1.
The lower mold 30 includes a heating unit 40, a first cooling unit 50, and a second cooling unit 60.
The first cooling unit may include a plurality of vertical cooling flows formed to extend from a bottom surface of the mold to the cavity, the vertical cooling flows may be connected to each other through connection flows, and an inlet and outlet may be formed on a lateral surface of the lower mold to supply and eject coolant.
The heating unit 40 is installed at a side adjacent to the cavity 12 and heats an area surrounding the cavity 12 and a resin injected into the cavity 12. The first cooling unit 50 is installed at the upper or lower