DEVICE FOR AND METHOD OF MOLDING FOR INJECTION MOLDING MACHINE

Abstract: A device for and method of molding for an injection molding machine, by which fluctuation in weight of an injection-molded resin product is reduced to stabilize its quality. For satisfying the purpose, a screw (61) of an injection unit (30) is moved forward to start injection of molten resin when the upper metallic mold (15) reaches a specified position, decelerated to a specified speed when advanced to a fixed position, and stopped when it reaches a predetermined stop position; and molten resin is reduced in injection flow quantity and pressurized until the gate (71) is closed.

Description: Technical Field The present invention relates to a device for and a method of molding for an injection molding machine, and more particulary to a device for and a method of molding for an injection molding machine by which fluctuation in weight of an injection-molded resin product is reduced. Background Art The following methods have been implemented to manufacture resin molded products by a molding device for an injection molding machine, an injection compression molding machine, and an extrusion molding machine. (1) A method by which resin is molded by moving forward a screw along a specified distance to inject resin and controlling a screw drive hydraulic pressure. (2) A method by which a resin passage is closed and molds are pressurized by a press mechanism or the like after moving forward a screw along a specified distance and injecting resin. Document EP-A-0 365 684, for example, discloses a molding device and method of an injection molding machine having a mold clamping unit, an injection unit, a gate opening/closing mechanism to give a precompression to molten resin. However, in the case of molding a product with a large projection area (that is, a large compression stroke), molded products have been insufficient in strength due to insufficient density or have been unstable in quality due to a number of flashes because molded products substantially vary in weight despite that the products are satisfactory in dimensional accuracy. In other words, there has been a problem including the following points. (1) A hydraulic actuator is used to move forward the screw and its position is controlled in accordance with information fed back from a position sensor. However, it is difficult to accurately stop the screw at a specified position because of a time lag. (2) It takes a certain lot of time to close a resin passage and, moreover, the amount of resin fed into a mold varies with a time necessary for closing the resin passage and fluctuation of the screw position at which the resin passage begins to be closed, and therefore, it is difficult to maintain a stable weight of molded products. An object of the present invention made in view of the above problem is to provide a device and a method of molding for an injection molding machine capable of reducing fluctuation in weight of a resin molded product and providing stable quality. Disclosure of the Invention A molding device in accordance with the present invention for an injection molding machine is recited in claim 1. The molding method of the present invention is recited in claim 2. The screw is decelerated to the specified speed at a position 1 to 7 mm in front of the predetermined stop position. After deceleration is completed, the screw further moves forward at a speed of 10 mm/sec or less. According to the above described configuration of the present invention, the advancing speed of the screw is reduced and the molten resin feed rate is also reduced before the injection of a specified amount of molten resin into the mold is completed, and, therefore, the molten resin feed rate can be reduced before the gate closes, so that feed rate control is facilitated, and the molten resin feed rate can be accurately controlled because molten resin continues to be pressurized until the gate closes. These advantages result in smaller variations in weight of molded products and stable product quality. Molded products free from surface concavities and with satisfactory dimensional accuracy and structure can be obtained. Brief Description of the Drawings FIG. 1 shows a general view of an injection molding machine according to an embodiment of the present invention; FIG. 2 is an electrical circuit diagram including a hydraulic circuit diagram of the embodiment shown in FIG. 1; and FIG. 3 is an enlarged cross-sectional view of the resin passage varying mechanism. FIGS. 4(a), 4 (b), and 4(c) are respectively graphs showing the relationship of the resin pressure, resin feed rate, and gate opening area to the stroke. Best Mode for Carrying out the Invention By reference to the attached drawings, preferred embodiments of a device for and a method of molding for an injection molding machine of the present invention are described below in detail. FIG. 1 shows a general view of an embodiment of the present invention; FIG. 2 is an electrical circuit diagram including a hydraulic circuit diagram of the embodiment; FIG. 3 is an enlarged cross-sectional view of the resin passage varying mechanism. In FIG. 1, a molding device (hereafter referred to as "molding device 1") for an injection molding machine having a clamping unit 10 such as a press, which opens or closes the mold in a vertical direction and an injection unit 30 which is located below the die plate of the clamping unit 10 and feeds resin into the opened mold, comprises a base 45 which horizontally slides below the clamping unit 10, a tractor unit, not shown, which actuates the base 45 to slide horizontally, a rocking block 48 which is supported by the base 45 at its one end and is rocked by an actuator 47 at its other end, an injection unit 30 which is fixed to the rocking block 48, and a hydraulic unit 49. The clamping unit 10 comprises four lift guide rods 11, a lift frame 12 which is supported by the lift guide rods 11 and moves in a vertical direction, an upper die plate 14 supported by four compression cylinders 13 incorporated in the lift frame 12, an upper mold 15 attached to the upper die plate 14, a lower mold 16 provided faced to the upper mold 15, a lower die plate 17 which holds the lower mold 16 and supports the lift guide rods 11, and a frame 18 which fixes the lower die plate 17, and vertically opens and closes the mold by the actuator 20. A mold position sensor 50, which detects the positions of the lower mold 16 and upper mold 15, is fixed to the lower die plate 17. The injection unit 30 comprises an injection mechanism 60, a gate opening/closing mechanism 70, a resin passage varying mechanism 80, and a controller 90. As shown in FIG. 2, the actuator 20 and compression cylinders 13 are actuated by a hydraulic pressure supplied from the pump 21 through solenoid valves 22a and 22b. A flow control valve 23 is installed in the hydraulic circuit for the compression cylinders 13 to control the flow rate of hydraulic oil to the compression cylinders 13 in accordance with a command signal from the controller 90 which is responsive to a detection signal generated by the mold position sensor 50, to move up and down the upper mold 15 which is kept in parallel with the lower mold 16. This mold parallel drive mechanism comprises a mold position sensor 50, a flow control valve 23, a controller 90, and compression cylinders 13. The flow rate of oil in the hydraulic circuit for compression cylinders 13 can be controlled by a pump instead of the flow control valve 23. The injection mechanism 60 comprises a screw 61, a screw drive motor 62, an injection cylinder 63, a flow control valve 64, a solenoid valve 65, and a pressure regulating valve 66. The screw drive motor 62 rotates the screw 61 to knead and meter molten resin and the flow control valve 64 adjusts the oil flow rate to control sliding movement of the injection cylinder 63 to control the injection rate of molten resin. The quantity of resin to be injected is measured by detecting the position of the injection cylinder 63 with the injection cylinder position sensor 67. The gate opening/closing mechanism 70 and the resin passage varying mechanism 80 are installed on the injection nozzle 71 located between the lower mold 16 and the injection mechanism 60. The servo motor 91 is driven with a command signal from the controller 90 to open or close the valve 72 of the resin passage varying mechanism 80 through the gate opening/closing mechanism 70. The valve position sensor 92 detects the position of the valve 72. The gate opening area is determined by varying the relative position of the passage 81 provided inside the valve 72 and the injection nozzle 71. A molding method by means of a molding device for the injection molding machine of the present invention is described below, referring to FIG. 1 and the graphs 4(a) - 4(c). When the lift frame 12 is lowered to the lowest point (i.e., the stroke of the actuator 20 is maximized) and the compression cylinders 13 are actuated to make the upper mold 15 and the lower mold 16 come in contact with each other while the stroke is maximized, the heights of four lift guide rods 11 are individually adjusted by height adjusters 19 to ensure close contact of upper and lower molds 15 and 16. With the two molds kept in close contact, hydraulic pressure is transmitted to the port 20a of the actuator 20 to eliminate backlash, and adjustment is thus completed. The upper mold 15 is first lifted from the position at which it is in contact with the lower mold 16 and then lowered to a specified position (where the upper mold 15 and the lower mold 16 are slightly opened). When the mold position sensor 50 detects that the upper mold 15 has reached the specified position, it transmits a detection signal to the controller 90 to stop the upper mold 15. Simultaneously, the solenoid valve 65 is actuated with the command signal of the controller 90 to supply hydraulic oil from the pump 21 to the injection cylinder 63 to advance the screw 61 at the specified speed. When the stroke position sensor 67 detects the screw 61 which has reached the specified position, the controller 90, which has received this detection signal, transmits a command signal to the flow control valve 64 to reduce the advancing speed of the screw 61. As shown in FIG. 4(b), the deceleration of advancing of the screw 61 can be started at a position 1 to 7 mm in front of the predetermined stop position. A preferable advancing speed of the screw after deceleration can be 10 mm/sec or less. When the screw 61 has moved forward at a speed of 10 mm/sec or less until it reaches the predetermined stop position, the stroke position sensor 67 detects the screw 61 and transmits a detection signal to the controller 90 to change over the solenoid valve 65 and stop the screw 61. At the same time, the servo motor 91 is driven with this command signal from the controller 90 to start closing of the valve 72 of the gate opening/closing mechanism 70 as shown in FIG. 4(c). The valve 72 is closed while the screw 61 is continuously moved forward and molten resin remains pressurized. At the same time, the valve position sensor 92 detects the start of closing of the valve 72 and the controller 90, which has received this signal, transmits a command signal to the pressure regulating valve 66 to start the reduction of pressure to molten resin as shown in FIG. 4(a). After the pressure is appropriately reduced, the compression cylinders 13 are actuated to apply a pressure to the upper mold 15 to feed molten resin into the full range of the mold, thus reducing variations in the density of resin. Molding is completed. The test conditions and test results in the above embodiment are shown below. (1) Test conditions Material: PMMA (polymethyl methacrylate) Screw speed: 85 mm/sec Advancing speed after deceleration: 3.5 mm/sec Number of sample moldings tested: 50 pcs (by continuous molding) (2) Test results The table below shows the comparison data with respect to the quality of moldings according to the method of the present invention and the conventional method. As known from Table 1, the deviation σ n-1 and inter-group fluctuation R of data obtained by the method according to the present invention are less than half those according to the conventional method, showing remarkable reduction of the variations.

Claims: A molding device (1) for an injection molding machine having a mold clamping unit (10), which feeds molten resin into an opened mold and clamps said mold for compression molding, an injection unit (30) composed of a screw (61) which melts, kneads and injects resin, a gate opening/closing mechanism (70) which stops injection of resin, mold position detecting means for detecting a position of said opened mold; stroke position detecting means (50) for detecting a stroke position of said injection unit, pressure control means for controlling an injection pressure of resin, resin passage varying mechanism (80) for closing a gate after a specified amount of resin is fed through said gate, and gate position detecting means (72) for detecting an opening position of said gate, comprising: speed control means for controlling an advancing speed of said injection unit; and a controller (90) for transmitting command signals to said speed control means, said resin passage varying mechanism, and said pressure control means in response to signals from said respective detecting means. A molding method for an injection molding machine, which feeds molten resin into an opened mold, compresses, and cools, detecting a position of said mold by a mold position detecting means when said mold is moved to a specified position, starting injection of molten resin by advancing a screw of an injection unit according to a signal thereof, detecting by a stroke position detecting means when said screw advances to a specified position, reducing the feed rate of molten resin while a gate is closing by said gate opening/closing means, and pressurizing molten resin by said pressure control means until the closing of said gate is completed, comprising: reducing the advancing speed of said screw to a specified value by a speed control means in accordance with a signal from said stroke position detecting means; and stopping said screw by said speed control means when said screw reaches a predetermined stop position by similar means. A molding method for an injection molding machine according to Claim 2, wherein the speed of the screw of said injection unit is reduced to said specified value at a position 1 to 7 mm in front of said predetermined stop position. A molding method for an injection molding machine according to Claim 2, wherein the advancing speed of the screw of said injection unit after the screw speed has been reduced is 10 mm/sec or less.

Inventors: FUJIMOTO SATOSHI, NOGAWA MAKOTO, FUJIMOTO, SATOSHI, NOGAWA, MAKOTO
Applicants: KOMATSU MFG CO LTD, KABUSHIKI KAISHA KOMATSU SEISAKUSHO
Application year: 1991
Main classification: B29C45
IPC codes: B29C45