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).
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).
In the CIM part, the degree of lamellae orientation was also roughly centro-symmetric, which increased and then decreased from the skin layer to the core layer. The degree of lamellae orientation reached a maximum at the shear layer and a minimum at the core layer. For the RHCM part, the degree of lamellae orientation of the different layers varied slightly, and the degree of lamellae orientation of all layers was less than those in the CIM part. The maximum degree of lamellae orientation was at the skin layer next to the moving
