Plastics Process Analysis, Instrumentation, and Control. Группа авторов

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Plastics Process Analysis, Instrumentation, and Control - Группа авторов


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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.

Schematic illustration of weldless-type injection mold apparatus.

      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.

      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).

       1.6.2.2 Lamellae Orientation of Isotactic Poly(propylene)

      Earlier reported studies on RHCM focused mainly on controlling the mold temperature distribution and temperature history (22, 25) as well as the relationship between process parameters and macroscopic properties of parts (9, 22).

      Although it is well known that the morphology determines the macroscopic properties of parts, there are only a few studies on morphologies in the RHCM part.

      Isotactic poly(propylene) (iPP) is a typical crystalline polymer with polymorphism that is commonly used to study the relationship between the molding process, morphology, and part quality in injection molding (26). A multilayered structure was prepared that was divided into the skin layer, shear layer, and core layer depending on the morphology. In particular, the crystal was highly oriented in the skin layer, there were a large number of fibrous shish-kebab crystal structures in the shear layer, and there were spherulite crystal structures in the core layer.

      The thickness difference of the multilayered structures, crystallization and lamellae orientation at different sampling sites or molding and between RHCM and conventional injection molding (CIM) processes also reflected the variations of the temperature and shear gradient during the filling process.

      The main difference between these two processes was the different temperature histories in the mold, which introduced various thermal and shear histories to the polymer. Accordingly, the multilayered structures differed from each other, which could be observed from SEM images (26).

       1.6.3 Injection Molding: Heating

       1.6.3.1 Modeling a Fast Mold Temperature

      The modulation of the mold temperature during injection molding is a strategic issue since it allows modulating and calibrating interesting properties of the moldings (27).

      Thin heating devices (28) were layered on the cavity surface allowing its fast temperature evolution between injection and cooling channels temperatures.

      The heating device was made up of four layers (27): an 80 µm thick electrically conductive layer of carbon black loaded poly(amide imide), which induces the cavity surface heating by Joule effect, two Kapton electrically insulating layers, one on each side of the electrically conductive layer. A steel layer, 100 µm thick, protects the heating device from the incoming melt. The heating power was chosen such that, when the polymer reaches the cavity, the cavity surface temperature was intermediate between the mold temperature, as held by the cooling channel, and the injection temperature.

      The heating devices were made by a conductive layer between two insulating layers with thicknesses selected in order to realize a heating/cooling cycle as fast as possible.

      Several tests were performed, injecting iPP, using different heating powers and heating times to analyze the effect of the fast cavity surface temperature evolution on the molding morphology and properties. The heat transfer through the mold was modeled, accounting for the Joule effect in the conductive layer of the heating devices (27).


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