JP4085103B2 - Holding pressure switching control method - Google Patents

Description

  The present invention relates to a holding pressure switching control method in a holding pressure process after filling a mold cavity with a resin in an injection molding machine.

  Injection molding in an injection molding machine generally includes a mold closing process for closing a mold, a mold clamping process for clamping a mold, a nozzle touch process for pressing a nozzle against a mold sprue, and a screw in a cylinder is advanced. An injection process for injecting molten material collected in front of the screw into the mold cavity, a pressure holding process for applying a pressure for a while to suppress the generation of bubbles and sink marks, and a mold cavity filling. For the next cycle in the time between the molten material being cooled and solidified, the screw is rotated to plasticize the resin and melt the resin in front of the cylinder, and the solidified molded product is removed from the mold. In order to take out, it consists of a mold opening process and a molded product projecting process in which the mold is opened and the molded product is pushed out by a projecting pin provided on the mold.

  Conventionally, in the control of injection and pressure holding processes in an injection molding machine, resin is injected at a set speed, and after reaching a set condition, for example, a screw position enough to fill a mold cavity or an injection limit pressure In addition, there have been those proposed in Patent Documents 1 to 3 and the like for performing control in a closed loop of pressure.

  FIG. 4 is a system diagram of a main part of a control circuit that performs the holding pressure control disclosed in Patent Document 1.

  The mold 10 is provided with a pressure sensor 18 for detecting the pressure in the cavity 10a, and the injection cylinder 16 is provided with a pressure sensor for detecting the control hydraulic pressure supplied from the variable displacement hydraulic pump 20. 22 is provided. Furthermore, a position sensor 24 for detecting the screw position of the injection screw 14 and switching from the filling process to the pressure holding process of the injection process is provided. Outputs from the sensors 18, 22, 24 are input to the control device 26, and the control device 26 supplies a variable control hydraulic pump for supplying a required control hydraulic pressure to the injection cylinder 16 based on these input values. 20 is controlled. The control device 26 includes two control systems, a multi-stage hydraulic pressure control unit 30 having a pressure holding setting unit 28 and a constant pressure control unit 32 in the cavity, as means for performing pressure holding control. The switch 34 is selectively switched and a required control signal is output from the D / A conversion output unit 36, whereby the variable displacement hydraulic pump 20 is controlled.

  Patent Document 2 discloses that an injection molding machine including a cylinder 2 and a screw 7 provided in the cylinder 2 so as to be driven in a rotational direction and an injection direction, at the initial stage of injection, When the detected resin pressure reaches the set resin pressure, the detected resin pressure 26 directly measured in the molds 22 and 23 is controlled by injection speed feedback control in which the speed 15 in the injection direction of the screw is the main feedback amount. Is controlled by feedback control of the resin pressure in the mold using the main feedback amount, and when the screw position reaches the holding pressure switching position, the injection pressure using the detected injection pressure 61 for driving the screw 7 in the injection direction as the main feedback amount. A control method for controlling by the feedback control is disclosed. That is, the control method disclosed in Patent Document 2 injects resin at a set speed, and after reaching a set condition, for example, a screw position sufficiently filled into the cavity 10a or an injection limit pressure, It is controlled by a closed loop of pressure.

Further, in Patent Document 3, in the holding pressure switching control method for an injection molding machine, the holding pressure process is divided into a plurality of stages so that the holding pressure of each stage can be set individually, and the adjustment at each stage is adjusted. A holding pressure switching control method is disclosed in which the rate of change in pressure response can be set individually.
JP-A-10-264223 Japanese Patent Application Laid-Open No. 11-077770 JP 2001-293762 A

  Usually, since the resin pressure is transmitted more slowly at the end portion of the mold cavity than at the gate side, the resin tip entering the cavity is cooled and fixed, and the size is reduced at the end portion of the molded product. If an appropriate pressure is set in the gate portion, the end portion is caused by insufficient or unfilled resin pressure. On the other hand, if an appropriate pressure is set in the end portion, the gate portion is overfilled with resin. In addition, even when the resin can be properly filled at the gate and the end, in the case of a mold in which a thin part and a thick part are mixed in the molded product, the resin cooling time at each part is different, and sink marks etc. Will occur. In addition, when switching from speed control to pressure control and controlling with the pressure setting pressure, it takes time to reach the setting pressure (on the order of several hundreds msec), and the resin enters the mold cavity too much, and the thin part in the cavity Swarfs and overfilled molded products, and in the worst case, burrs.

  As a means for solving this problem, the invention disclosed in Patent Document 1 can be applied. However, a special mold, a sensor, and a pressure switching control device for that purpose are necessary, which is expensive. In a molded product with a mixture of thin and thick parts, the resin is filled up to a certain filling pressure by filling the resin up to a certain filling pressure, and it is not possible to fill the resin more than the appropriate filling amount of the thin part. It is necessary to perform a pressure reducing process and then apply pressure to prevent sinking of the thick part.

  Further, in the case of the control of Patent Document 3, even if the rate of change of pressure fluctuation in the pressure holding process can be changed, it is complicated to set the setting conditions, and the molder is forced to perform trial and error.

  Accordingly, an object of the present invention is to provide a pressure holding switching control method capable of applying a uniform pressure to a mold cavity, a terminal portion or a thin portion, and further to a gate portion or a thick portion.

  In order to solve the above problems, the holding pressure switching control method of the present invention advances the screw in the cylinder by advancing the piston in the injection cylinder and injects the molten material into the mold cavity of the mold. In a holding pressure switching control method of an injection molding machine by a control device that performs pressure control when molding a molded product, a primary pressure set value that is an injection limiting pressure and a primary pressure are held in advance before injection molding is started. The time T2 and the switching pressure set value that is lower than the primary pressure set value are input to the control device, and after the injection starts, the time after the pressure value in the injection cylinder reaches the primary pressure set value is After T2, the control device sets the pressure value to 0% output and lowers the pressure value in the injection cylinder to the switching pressure set value.

  As described above, the holding pressure switching control method of the present invention prevents overfilling of the molten material by rapidly releasing the pressure from the primary pressure set value after the mold cavity is filled with the molten material. In addition, if the pressure is suddenly removed, sinks or the like occur when the molded product cools. Therefore, the pressure during pressure reduction is monitored, and when the pressure drops to the switching pressure set value, control is performed to compensate by pressure control. By performing such control, the molten material in the mold cavity has a uniform thickness between the thin portion and the thick portion, the dimensional difference between the end portion and the thin portion, and further the gate portion and the thick portion. The dimensional difference can be minimized. In addition, the warpage of the molded product can be improved.

  The holding pressure switching control method of the present invention is a servo valve that controls the supply of hydraulic pressure to the injection cylinder until the pressure value in the injection cylinder reaches the switching pressure set value by setting the pressure value by the control device to 0% output. It may be one that controls.

  Further, the control device in the holding pressure switching control method of the present invention performs control to gradually decrease the pressure value in the injection cylinder after the pressure value in the injection cylinder reaches the switching pressure set value. Also good.

  According to the present invention, since a uniform pressure can be applied to the mold cavity, the end portion, the thin portion, the gate portion, and the thick portion, a molded product having no dimensional difference can be obtained. Further, it is possible to improve warpage, burrs, “shine” appearing on the surface of the molded product caused by uneven pressure, and the like.

  FIG. 1 shows a schematic diagram of a system configuration for performing pressure holding control of an injection molding machine according to the present invention.

[Injection molding machine]
The injection molding machine of this embodiment includes a cylinder 62, a screw 63 inserted into the cylinder 62 so as to be able to rotate and move forward and backward, an injection cylinder 64 including a piston 73 that moves the screw 63 forward and backward, and an injection cylinder 64. And a control device 69 that performs injection control and pressure holding control.

  The position of the screw 63 is detected by a position sensor 68. The injection cylinder 64 is provided with pressure sensors 65 and 66 for detecting control hydraulic pressure supplied from a hydraulic pump (not shown) and an accumulator. The pressure sensor 65 detects the hydraulic pressure on the hydraulic pulling side, and the pressure sensor 66 detects the hydraulic pressure on the hydraulic pressure side. Detection values detected by these sensors 65, 66, and 68 are input to the control device 69.

  The control device 69 includes a pressure holding setting unit 70 for setting the pressure holding in the pressure holding process, a timer 71 for setting various times in molding, control signals from the pressure holding setting unit 70 and the timer 71, and each sensor. And a hydraulic pressure control unit 72 that controls opening and closing of the servo valve 67 based on the detected values of 65, 66, and 68. The control device 69 performs speed control in the injection process and pressure control in the pressure holding process.

  Various setting values are input to the control device 69 in advance. The holding pressure setting unit 70 of the control device 69 includes a primary pressure set value P1 that is an injection limit pressure, and a switching pressure setting that becomes a monitoring value when the pressure is lowered after the primary pressure set value P1 is held for a predetermined time. Value P2, pressure control first stage set value P3, pressure control second stage set value P4, pressure control third stage set value P5 and pressure control fourth stage as pressure value when pressure is gradually reduced The set value P6 is input in advance as a pressure value. The pressure value is the highest at the primary pressure set value P1, which is the injection restriction pressure, and then the switching pressure set value P2 is the highest pressure. The primary pressure set value P1 is set as a pressure at which 80 to 95% of the molded product shape can be formed. The switching pressure set value P2 may be the same pressure value as the pressure control first stage set value P3. P3 to P6 are values for controlling the holding pressure in stages.

  In the pressure holding process in the present embodiment, the pressure holding after the primary pressure set value P1 is an example of performing control to reduce the pressure in the injection cylinder 64 in four stages within the pressure holding time T7. Although shown, it is not limited to this, The holding pressure control may be performed in more stages, or may be three stages or less.

  Further, in the timer 71 of the control device 69, the control time T3 to T6 as the time for holding the pressures of the set values of the primary pressure holding time T2 and P3 to P6, the pressure in the injection cylinder 64 is the switching pressure set value. The holding pressure maintaining time T7, which is the time from reaching P2 to ending the holding pressure process, is input in advance as the control time. The primary pressure holding time T2 is a time for holding the primary pressure set value P1.

[Injection molding process and pressure holding process]
The injection molding process by the injection molding machine of this embodiment will be described with reference to FIGS. FIG. 2 is a graph showing temporal changes in the pressure in the injection cylinder 64 and the moving speed of the cylinder 62 in the injection molding. FIG. 3 is a flowchart for explaining the injection molding process and the pressure holding process.

  Injection of resin into the mold cavity 61 starts from an injection start time T1 (step S1). The forward movement of the screw 63 at the time of resin injection is controlled by speed control (step S2).

  The control device 69 determines whether or not the pressure feedback signal PW1 that is the pressure of the injection cylinder 64 detected by the pressure sensors 65 and 66 has reached the primary pressure set value P1 (step S3). If the pressure feedback signal PW1 does not reach the primary pressure set value P1, the screw 63 is continuously controlled by speed control. If the pressure feedback signal PW1 reaches the primary pressure set value P1, the screw 63 is controlled. Is switched to pressure control and the pressure is held at the primary pressure set value P1 for the primary pressure holding time T2 (step S4).

  After the primary pressure holding time T2 has elapsed, the hydraulic control unit 72 of the control device 69 outputs 0% as a pressure command value (step S5) and controls the servo valve 67. As a result, the pressure feedback signal PW1 rapidly decreases. That is, the pressure in the mold cavity 61 decreases rapidly.

  The control device 69 determines whether or not the pressure in the injection cylinder 64 has decreased from the primary pressure set value P1 and has reached the switching pressure set value P2 (step S6). The control device 69 continues the 0% output if the pressure value does not reach the switching pressure set value P2, stops the 0% output if the pressure value reaches the switching pressure set value P2, and sets the pressure control first stage set value P3. Output to hold. Note that pressure control is performed so that the pressure goes to 0 MPa with 0% output, but this transition time is on the order of several tens of msec. The control device 69 performs control to maintain the pressure control first stage set value P3 for the control time T3 (step S7), and then determines whether or not the control time has expired the holding pressure maintaining time T7 (step S7). S8). When the control device 69 determines that the control time has not expired the holding pressure maintaining time T7, the control device 69 controls the holding pressure value from the pressure control first stage setting value P3 to the pressure control second stage setting value P4. Then, after performing the control to maintain the pressure control second stage set value P4 during the control time T4 (step S9), it is determined whether or not the control time has expired the pressure keeping time T7 (step S10). Similarly, if the control device 69 determines that the control time has not expired the holding pressure maintaining time T7, the holding pressure value is controlled from the pressure control second stage setting value P4 to the pressure control third stage setting value P5. (Step S11). The above steps are sequentially repeated, and when the pressure holding time T7 has expired, the pressure holding step is terminated (step S12).

  As described above, according to the present embodiment, after injection by speed control, the pressure in the injection cylinder 64 is held at the primary pressure set value P1, and after the primary pressure holding time T2 has elapsed, the pressure is suddenly released, and the resin is discharged. Overfilling can be prevented. Further, if the molded product is still cooled, sink marks and the like are generated. Therefore, the pressure during pressure reduction is monitored, and when the pressure drops to the switching pressure set value P2, control is performed so that the pressure does not drop any further. Switch to pressure control after the eye setpoint P3. As a result, the resin pressure in the mold cavity 61 is uniform in the end portion, thin portion and gate portion, or in the thick portion of the molded product required for refilling the resin, and the end portion, thin portion and gate portion or wall portion. In addition to minimizing the dimensional difference of the thick part, it is possible to improve warpage and burrs of the molded product, “shine” appearing on the surface of the molded product caused by uneven pressure, and the like.

It is the schematic of the system configuration which performs pressure holding control of the injection molding machine of this invention. It is a graph which shows the time change of the pressure in a metal mold | die cavity in the control method of this invention. It is a flowchart for demonstrating the injection molding process and the pressure-holding process by the control method of this invention. It is the schematic of an example of the system configuration which performs pressure holding control of the conventional injection molding machine.

Explanation of symbols

60 Mold 61 Mold cavity 62 Heating barrel 63 Injection screw 64 Injection cylinder 65 Pressure sensor (hydraulic pulling side)
66 Pressure sensor (hydraulic push side)
67 Servo valve 68 Position sensor 69 Control device 70 Holding pressure setting unit 71 Timer setting unit 72 Hydraulic control unit 73 Piston VW1 Speed feedback signal PW1 Pressure feedback signal P1 Primary pressure setting value P2 Switching pressure setting value P3 Pressure control first stage setting Value P4 Pressure control 2nd stage setting value P5 Pressure control 3rd stage setting value P6 Pressure control 4th stage setting value T1 Injection start time T2 Primary pressure holding time T3 Pressure control 1st stage control time T4 Pressure control 2nd stage control Time T5 Pressure control 3rd stage control time T6 Pressure control 1st stage control time T7 Holding pressure maintenance time

Claims (3)

The piston (73) in the injection cylinder (64) is advanced to advance the screw (63) in the cylinder (62) to inject the molten material into the mold cavity (61) of the mold (60). In the holding pressure switching control method of the injection molding machine by the control device (69) for controlling the pressure when molding the molded product,
Prior to the start of injection molding, the primary pressure set value (P1) that is the injection limiting pressure, the primary pressure holding time (T2), and the switching pressure that is lower than the primary pressure set value (P1). The set value (P2) is input to the control device (69),
After the start of injection, after the time (T2) has elapsed after the pressure value in the injection cylinder (64) reaches the primary pressure set value (P1), the control device (69) sets the pressure value to 0% output, A holding pressure switching control method, wherein the pressure value in the injection cylinder (64) is lowered to the switching pressure set value (P2).   By setting the pressure value by the control device (69) to 0% output, the hydraulic pressure to the injection cylinder (64) until the pressure value in the injection cylinder (64) reaches the switching pressure set value (P2). The holding pressure switching control method according to claim 1, wherein the servo valve (67) for controlling the supply is controlled.   The control device (69) performs control to gradually decrease the pressure value in the injection cylinder (64) after the pressure value in the injection cylinder (64) reaches the switching pressure set value (P2). The holding pressure switching control method according to claim 1 or 2, wherein the holding pressure switching control method is performed.

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