Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 20

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 mold 20 or 30 to prevent the upper or lower mold 20 or 30 from being overheated due to repeated injection molding processes, and includes a heat-blocking unit for preventing heat from being transferred to the outside of the upper or lower mold 20 or 30. The second cooling unit 60 is installed between the heating unit 40 and the first cooling unit 50 and cools an area surrounding the cavity 12 and an injection molded product.

The first circulating conduit 43 may include a first control valve 46 installed to control steam to be supplied to the first fluid flows 41. The second fluid flows 51 are spaced a predetermined distance apart from the bottom surface of the lower mold 30 toward the cavity 12. The second fluid flows 51 are connected to each other by communication holes 52. In addition, the second fluid flows 51 are sealed by a blocking plate 53 engaged with the lower mold 30. The blocking plate 53 may have partitioning plates 54 inserted into the second fluid flows 51 to elongate a fluid flow track of the second fluid flows 51. Here, each of the partitioning plates 54 may be shorter than each of the second fluid flows 51.

Figure 1.1 Weldless-type injection mold apparatus (24).

The first cooling unit 50 includes a first refrigerant supply unit 55 for continuously supplying coolant to the second fluid flows 51. The first refrigerant supply unit 55 includes a first refrigerant tank 57 in which refrigerant 56 such as coolant or cooling oil is stored, a first pump 58 connecting the refrigerant tank 57 and the first fluid flow 51, and a third circulating conduit 59. The first refrigerant tank 57 is connected to a makeup water tank 57a for refilling the refrigerant 56. In addition, a refrigerant cooling system for cooling the refrigerant may be installed in the first refrigerant tank 57.

The third fluid flows 61 and the branch conduit 71 may be connected to each other by the first circulating conduit 43 of the boiler 42 and a purge conduit 73, so that the refrigerant of third fluid flows 61 may be exhausted when heating is carried out by the heating unit 40. The second control valve 72 may be a three-way valve installed at a connection part of the purge conduit 73 and the branch conduit 71 to supply steam or coolant.

Images of the upper mold and the lower mold during injection molding were obtained using a forward-looking infrared camera. This can illustrate the heated states during injection molding, as shown in Figure 1.2.

As evident from the photographs in Figure 1.2, heat accumulated around the cavity, while heat did not accumulate in the upper and lower molds. That is to say, since heat is not transferred to a lower portion of the cavity, the heat capacity for the overall injection molding process is not so high.

Since heat accumulation is prevented in such a manner, a cooling and heating time for injection molding, specifically the cooling time, can be reduced, thereby shortening the overall cycle time required for injection molding of a product, ultimately enhancing the manufacturing efficiency (24).

Figure 1.2 Heated states during injection molding (24).