JPH09254221A - Back pressure control system for motor-driven injection molding machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent forward speed of a screw from increasing suddenly and to uniformize density of a resin even when the pressure of the molten resin during metering is low by inserting and connecting a restricting means for restricting forward speed of a servo motor into a control system. SOLUTION: A detecting signal of a pressure detector 21 provided in an injection device by a servo motor 1 and detecting back pressure of a molten resin is added in a subtractor 22. The subtractor 22 outputs deviation between a back pressure set value and the detected value of the pressure detector 21 to a compensator 23 and the deviation is converted into a speed command value of a servo motor and it is outputted. Then, a speed restrictor 24 outputs a restricted speed command value when the speed command value from the compensator 23 exceeds a specified restricted value. In addition, a servo amplifier 25 drives the servo motor 1 in accordance with the speed command value. As the result, in metering, when the back pressure is low, the speed command value becomes large but this is restricted by means of a speed restrictor 24 to prevent the forward speed of a screw from increasing excessively and to unify filling density of the molten resin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a back pressure control system for an electric injection molding machine equipped with an injection device driven by a servo motor.

[0002]

2. Description of the Related Art Recently, instead of hydraulic actuators,
A so-called electric injection molding machine using a servo motor has been widely used. The operation of the injection device driven by the servo motor will be briefly described below.

(1) A screw is rotated, and a fixed amount is fed to the tip of the heating cylinder while melting the resin that has fallen from the hopper to the rear of the screw (measurement step). This time,
The screw retreats while receiving the pressure (back pressure) of the molten resin accumulated at the tip of the heating cylinder.

A drive shaft is directly connected to the rear end of the screw, and the drive shaft is rotatably supported by a pressure plate via a bearing. The drive shaft is driven via a timing belt by a motor supported on the same pressure plate. The pressure plate moves forward and backward along the guide bar by the injection servomotor via the ball screw. The back pressure of the molten resin described above is detected by a load cell and controlled in a closed loop, as described later.

(2) Next, the pressure plate is moved forward by driving the servomotor, and the molten resin is fed into the mold by using the tip of the screw as a piston (filling step).

(3) At the end of the filling process, the molten resin fills the cavity of the mold, at which time the forward movement of the screw switches from speed control to pressure control. This is called V (speed) -P (pressure) switching and affects the quality of the molded product.

(4) After the V-P switching, the resin in the cavity of the mold is cooled under the set pressure (pressure holding step). The resin pressure is controlled in a closed loop as in the back pressure control described above.

In the injection device, after the step (4),
Returning to the step (1), the next cycle is started. On the other hand, in the mold clamping device, in parallel with (1), after opening the mold and taking out the product cooled and solidified by the ejector mechanism, the mold is closed and the process of (2) is started.

Next, referring to FIG. 2, an injection molding machine will be described in which the rotary motion of the servomotor is converted into a linear motion by a ball screw and a nut to fill the molten resin. In FIG. 2, the rotation of the injection servomotor 1 is transmitted to the ball screw 2. A nut 3 that moves forward and backward by the rotation of the ball screw 2 is fixed to a pressure plate 4, and the pressure plate 4 is movably mounted on guide bars 5 and 6 fixed to a frame (not shown). The forward / backward movement of the pressure plate 4 is transmitted to the screw 10 via the bearing 7 and the load cell 8.

As the screw 10 advances in the injection cylinder 11, the molten resin stored in the measuring step is filled in the mold and pressed to perform molding. At this time, the force pushing the resin is detected as a reaction force by the load cell 8, amplified by the load cell amplifier 12, and input to the controller 15. A position detector 9 for detecting the amount of movement of the screw 10 is attached to the pressure plate 4, and this detection signal is amplified by an amplifier 13 and input to the controller 15. The controller 15 outputs a current (torque) command according to each process to the servo amplifier 14 according to the setting of the operator, and the servo amplifier 14 controls the drive current of the servo motor 1 to output the output torque of the servo motor 1. It is designed to be controlled.

[0011]

By the way, in a hydraulic injection molding machine provided with a hydraulic actuator by an injection cylinder, back pressure control at the time of measurement is performed as follows. That is, when the molten resin is measured by the rotation of the screw, the operation in which the screw moves backward is controlled by squeezing the hydraulic oil in the injection cylinder with a relief valve or the like. Since the hydraulic injection molding machine has no means to move the screw forward, even if the resin pressure in front of the screw is low at the start of weighing, the screw will move forward rapidly to raise the back pressure. do not do.

On the other hand, in an electric injection molding machine using a ball screw and a servomotor as shown in FIG. 2, the servomotor 1 can be moved forward with respect to the screw 10. For this reason, if the resin pressure in front of the screw, that is, the back pressure, is low at the start of weighing, a sudden forward motion is applied to the screw, which makes the weighing unstable and makes the resin density in the mold uneven. There is.

Therefore, an object of the present invention is to provide a back pressure control system for an electric injection molding machine which can improve the back pressure control at the time of measuring the molten resin to improve the quality of the molded product.

[0014]

The present invention is provided with an injection device driven by a servo motor, detects a back pressure of molten resin measured in front of a screw with a pressure detector, and compares the detected value with a set value. In the electric injection molding machine having a control system for controlling the speed of the servo motor based on the deviation, a limiting means for limiting the forward speed of the servo motor is inserted and connected to the control system.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing a back pressure control system of the electric injection molding machine, and is applied to the electric injection device as shown in FIG. The back pressure control system applies a detection signal of a pressure detector 21 provided in an injection device using the servo motor 1 to detect the back pressure of the molten resin to a subtractor 22. The subtractor 22 outputs the deviation between the back pressure setting value and the detection value of the pressure detector 21 to the compensator 23. The compensator 23 converts the deviation into a speed command value for the servo motor 1 and outputs it.

The speed limiter 24 is given a limit value in advance, and when the speed command value from the compensator 23 exceeds the limit value, the speed limiter 24 suppresses the speed command value to within the limit value and outputs the restricted speed command value. Of course, if the speed command value is less than the limit value, it is output as it is. The servo amplifier 25 drives the servo motor 1 according to the speed command value from the speed limiter 24.

According to such a back pressure control system, the molten resin pressure in front of the screw 10 shown in FIG.
That is, when the back pressure is low, the deviation of the subtractor 22 becomes large and the speed command value becomes large.
Limited by 4. From this, it is possible to prevent the forward speed of the screw from becoming excessively high, and it is possible to make the packing density of the molten resin in the mold uniform.

[0018]

According to the present invention, by providing the speed limiting means in the back pressure control system by the injection servo motor,
Even when the molten resin pressure at the time of measurement is low, it is possible to prevent a rapid increase in the screw advancing speed, to make the resin density in the mold uniform, and to improve the quality of the molded product.

[Brief description of drawings]

FIG. 1 is a block diagram of a back pressure control system by an injection servo motor in an injection device of an injection molding machine according to the present invention.

FIG. 2 is a diagram showing an example of an electric injection device to which the present invention is applied.

[Explanation of symbols]

 2 ball screw 3 nut 4 pressure plate 5, 6 guide bar 7 bearing 8 load cell 10 screw 11 injection cylinder 21 pressure detector 22 subtractor 23 compensator 24 speed limiter

Claims (1)

[Claims] 1. An injection device driven by a servo motor, wherein a back pressure of molten resin measured in front of a screw is detected by a pressure detector, and the servo motor's back pressure is detected based on a deviation between the detected value and a set value. A back pressure control system for an electric injection molding machine, comprising: an electric injection molding machine having a control system for controlling a speed, wherein a limiting means for limiting a forward speed of the servo motor is inserted and connected to the control system.