Plastics Process Analysis, Instrumentation, and Control. Группа авторов
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figure shows an example in which the mold heaters 7, 7 are respectively embedded in the fixed mold and the movable mold of the mold 2.
The mold heating-cooling system has a control panel 21 including a control portion 22, which controls the mold heater 7, the cooling medium supply pump 11, and the valves 12a, 14a, 18a, 19a. The control panel 21 has the control portion 22 such as a CPU, a display operation portion 24, and a memory portion 23, which are respectively connected by signal lines. The display operation portion 24 constitutes a display portion and an operation portion for setting up, inputting or displaying. The memory portion 23 is constituted of various memories and stores information about conditions and values inputted and set up by an operation of the display operation portion 24, various programs such as a control program for executing the respective operations mentioned below, various predetermined operation conditions, various data tables or the like (39).
In an example of the operation, after a first cooling step constituted by the main cooling step, a second cooling step in which air is supplied to the medium flow path 4 of the mold 2 is executed. In other words, when the cooling medium valve 12a is closed, and the air valve 19a is opened, air is supplied to the medium flow path 4. Upon closing the cooling medium valve 12a, the cooling medium supply pump 11 can be stopped. An aspect can be such that a bypass path which connects the supply side path 12 with the discharge side path 15 is provided, a bypass valve provided for the bypass path is conversely opened and closed relative to the opening and the closing of the cooling medium valve 12a, thereby generally activating the cooling medium supply pump 11 constantly.
The first cooling step and the second cooling step can be respectively executed until the predetermined time elapses in such a manner that cooling medium in the medium flow path 4 is generally discharged (purged) by executing the second cooling step and without excessive cooling or the like. In other words, the temperature does not greatly fall below the target cooling temperature upon finishing the cooling step. Based on the detection temperature of the temperature sensor 6, the first cooling step can be switched into the second cooling step and the second cooling step can be finished. So, the air valve 19a can be closed.
In the second cooling step, in place of or in addition to air, a small amount of cooling medium can be intermittently supplied or steam can be supplied as in, for example, an operation in the second embodiment mentioned below. In such a case in which steam is supplied, a small amount of cooling medium can be intermittently supplied in addition to steam. When steam or a small amount of cooling medium is supplied as mentioned above, the supply can be appropriately controlled in such a manner that the total amount in the medium flow path 4 gasifies and almost all of the medium in the medium flow path 4 does not remain at the end of the second cooling step. For instance, heat quantity released from an inner wall face of the medium flow path 4 is calculated based on capacity of the medium flow path 4, mold temperatures before and after filling of molten material such as resin; it can be experimentally or empirically determined based on the heat quantity, gasification heat quantity of the medium, the target cooling temperature, or the like. Since gasification occurs in a mold opening or a demolding step, a relatively small amount of remaining medium can be allowed.
After the cooling step, in the mold 2, the mold opening and the demolding of the molded article are appropriately executed, and the heating step and the cooling step are repeatedly executed. The heating step in which the mold 2 is heated and the cooling step in which the mold 2 is cooled and executed as mentioned above, improve the transfer property (transfer rate) of the cavity face to the molded article and shorten the molding cycle. The mold-cooling system 1 (the mold heating-cooling system) in the embodiment and the mold-cooling method (the mold heating-cooling method), which is executed by the mold-cooling system 1 constituted as mentioned above, enhance the cooling efficiency while suppressing the discharge of gasified cooling medium.
In other words, the discharge side path 15 connected to the outlet 5 side of the medium flow path 4 of the mold 2 is communicated with the heat exchanger 20 condensing gasified cooling medium discharged from the medium flow path 4. Accordingly, gasified cooling medium is condensed in the heat exchanger 20 and discharge of gasified cooling medium is suppressed. By condensing in the heat exchanger 20, pressure raised by gasification of cooling medium drops. Consequently, cooling medium is easily fed into the medium flow path 4, cooling time is shortened, and cooling efficiency is enhanced (39).
1.9 Microcellular Foam Processing System
A microcellular foam processing system for a reciprocating screw injection molding machine has been successfully developed (40). The necessary conditions for creating and maintaining a single-phase solution in the overall system of the plasticizing unit are a supercritical fluid delivery unit and a hydraulic unit.
An overall systems approach is the key to successfully implementing a microcellular foam process. The necessary modifications and the component designs that are required for a microcellular foam molding machine have been described. The important components are the plasticizing unit, injection unit, hydraulic unit, clamp unit and supercritical fluid unit (40).
1.9.1 Gas-Assisted Injection Molding
A novel gas-assisted microcellular injection molding method was created by combining gas-assisted injection molding (GAIM) with microcellular injection molding (41). Here, with the assistance of the high-pressure gas from GAIM, the amount of the polymer melt required for fully filling the mold cavity is reduced in the gas-assisted microcellular injection molding in comparison to that in the microcellular injection molding, thus leading to a high weight reduction of the foamed part.
In addition, the high-pressure assisted gas from the GAIM can dissolve all the cells generated in the melt filling stage back into the polymer melt, thus improving the molded part’s surface appearance by eliminating the surface sliver marks. Also, the secondary foaming process in a steady state triggered by releasing the high-pressure-assisted-gas results in a foamed part with a fine cellular structure and a compact solid skin layer, which can help to enhance the part’s mechanical properties.
In order to verify the effectiveness of the gas-assisted microcellular injection molding, experiments that can compare the microcellular injection molding and the gas-assisted microcellular injection molding were conducted. The results demonstrated that the gas-assisted microcellular injection molding can not only significantly increase the weight reduction, but also greatly improve the surface appearance and the mechanical properties of the foamed part (41).
1.9.1.1 Flow Visualization
The basic filling phenomena in a gas-assisted injection molding process with flow visualization were assessed (42). Here, a high-speed video camera was used to record the mold-filling phenomena of rectangular cavities with various arrangements of gas channels. The mold-filling phenomena were compared and guidelines for layout of gas-channel ribs were drawn.
The following conclusions can be drawn from this investigation (42):
1 Gas-assisted filling, with a near-exponential velocity rise, is much faster than conventional filling, with nearly constant speed. Ribs with similar area but different geometry significantly affect the speed of gas tip and melt front advancement during the gas-assisted filling.
2 Without any rib in a gas-assisted molded plate, severe rigidity degradation will occur along the gas penetration path, especially near the thickness transition edges. Similar rigidity degradation will occur even in a plate with improper gas-channel rib layout. If there is no proper gas-channel rib along the melt front advancement in the far end, gas will penetrate into non-rib regions along the direction of melt front advancement to fill the free space.
3 With a symmetrical rib layout, the gas penetration in symmetrical ribs can never remain symmetrical.
1.9.1.2 Computer-Aided Engineering
The development of computer-aided engineering (CAE) technology for GAIM has been reported (43). To achieve this goal, efforts have been made in developing a numerical