JPH0422130B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for controlling the injection pressure of an injection molding machine, and in particular to a method for smooth injection by eliminating the hydraulic shock when switching from injection filling pressure to injection holding pressure. The present invention relates to an injection pressure control method for an injection molding machine to obtain pressure.

(Prior art) Conventionally, in order to perform proper injection molding, a predetermined amount of uniformly plasticized molten resin is filled into the mold at an appropriate injection pressure and speed according to the mold structure, and the molded product is The filling pressure must be maintained accurately and cooling must be carried out at an appropriate cooling temperature for an appropriate amount of time until the product is cooled and solidified.

This appropriate injection pressure, that is, the pressure range for maintaining good products, has become significantly narrower as product designs become more complex, and sometimes contradictory defective phenomena such as burrs, short shots, weld lines, sink marks, shrinkage distortion, voids, warping, whitening, and cracks occur. There are many cases where both appear at the same time, and we have no choice but to accept one or both.

For this reason, it is necessary to be able to change the holding pressure at any time within the holding pressure time.

Therefore, the injection pressure is controlled in a closed loop based on the block diagram shown in FIG. That is, in order to make the injection cylinder oil pressure actual measurement value signal PV follow the injection oil pressure control signal SV so as to match it, PID calculation control is performed so that the steady deviation that occurs between the SV signal and the PV signal is set to zero.

As is well known, in PID operation, in the adjustment element,
An adjustment signal is obtained by adding the control operation signal, its differentiated signal, and its integrated signal at an appropriate ratio.Here, the above deviation is controlled by a control signal (HV) that turns ON when controlling the injection holding pressure. , PID calculations are performed, the obtained output is added to the injection hydraulic pressure control signal SV, amplified, and a control signal is output to the electromagnetic proportional valve, servo valve, etc.

Open control, as shown in FIG. 4, is performed by a circuit without PID calculation, and generally the linearity between the setting and the control value is poor and the control value is affected by disturbances such as hydraulic oil temperature.

FIG. 5 shows the injection pressure P with respect to the time from injection start A to injection end B with respect to the injection oil pressure control signal SV and the injection cylinder oil pressure actual measurement value signal PV under open loop control.

Here, point C indicates the point at which the injection filling pressure switches to the holding pressure, section X ₁ is the filling time, and section X ₂
is the pressure holding time.

In addition, in closed-loop control, the injection cylinder oil pressure is controlled in stages during the pressure holding process based on feedback signals from the injection cylinder oil pressure controlled by an electromagnetic proportional valve or servo valve, and the set injection oil pressure control signal SV is controlled. The injection cylinder oil pressure actual measurement value signal PV can be tracked.

Therefore, closed loop control is generally more preferred.

(Problems to be Solved by the Invention) However, in the open loop control described above, the set pressure of the electromagnetic relief valve rapidly drops at the pressure holding switching time point C, so that a hydraulic shock occurs. Then, there is a delay in the operating state of the injection cylinder with respect to the injection oil pressure control signal SV, and the injection pressure that is the object of control cannot follow the control target value, which poses a problem in control under severe molding conditions.

Also, in closed loop control, the sixth
As shown in the figure, since there is a sudden change in the set injection pressure during each step of operation, there was a problem in that the operating oil pressure of the injection cylinder was likely to cause a hunting phenomenon P _h .

When this hunting phenomenon occurs, a hydraulic shock usually occurs when switching from filling pressure (flowing pressure) to holding pressure, and if a peak (overshoot) occurs, the molded product is likely to have burrs and undershoot. When this occurs, the molded product suffers from defects such as sink marks, short shots, and weld lines, causing molding defects.

The present invention was made in view of the above circumstances, and uses pressure control by a microcomputer to ensure stable molding without causing hydraulic shock even during closed-loop pressure holding control when switching from the filling process to the pressure holding process. An object of the present invention is to provide an injection pressure control method for an injection molding machine for obtaining a product.

(Means for Solving the Problems) In order to achieve the above object, the injection pressure control method of the present invention involves filling a mold with a molten molding material using an injection pressure set to a predetermined pressure, and in this filling process. At the time of switching from the pressure holding process to the pressure holding process, the actual measured hydraulic pressure of the injection cylinder that advances the screw should match the injection pressure setting, and during the pressure holding process, the CPU
The injection hydraulic pressure control signal outputted from the control means at each sampling time is sequentially linearly changed along an inclined broken line trajectory toward the set value of the pressure setting device provided for each control section of the pressure holding time. The pressure is controlled and closed loop control is performed using the output signal PV of the pressure sensor that detects the actual measured value of the oil pressure as a feedback signal.

(Function) With this configuration, the filling oil pressure for filling the mold with molten resin gradually increases during the injection filling process, and matches the set injection pressure of the injection cylinder at the time of switching the holding pressure. Then, the process will be switched to the holding pressure process. Thereafter, the actual holding pressure detected by the pressure sensor is controlled in a closed loop toward the pressure setting value of the pressure setting device for each control period divided by the holding time setting timer.

By outputting the injection hydraulic pressure control signal at a very short cycle of the sampling time of the control means such as the CPU, it is possible to control the injection pressure by reducing the difference between the actual hydraulic pressure value of the holding pressure and the set pressure value. It becomes possible.

Therefore, a stable molded product can be obtained by eliminating hydraulic shock caused by a large pressure difference between the set pressure and the actual holding pressure.

(Example) Embodiments of the present invention will be described based on the drawings.

In FIG. 1, 1 is an injection device in which the molding material from a hopper 2 is kneaded by a screw 4 rotated by a hydraulic motor 3, and melted by a heating cylinder 5.
By providing hydraulic pressure into the injection cylinder 6, the screw 4 is advanced, and the molten molding material is injected and filled into a cavity (not shown) in the mold.

7 is an electromagnetic proportional valve, a servo valve, or the like, which controls the flow rate of the oil sent from the pump 8 to the injection cylinder 6. 9 is an electromagnetic relief valve that sets the oil pressure to the injection cylinder 6 to a predetermined pressure.

The injection pressure control method in this embodiment uses the CPU 10 for process control to control the injection pressure at the injection cylinder 6.
The injection pressure given to the injection pressure is controlled by the operation of the hydraulic mechanism 20, and in particular, the pressure is controlled more precisely during the pressure holding process after the completion of injection and filling, and the hydraulic shock is prevented from occurring when changing the injection pressure setting of the pressure setting device 11. It is made so that there is no such thing.

Therefore, multi-stage control of holding pressure is not performed by sequentially switching to the set injection pressure of each pressure setting device at each time of a timer that sets each holding pressure control period, as in the conventional method, but by changing the operating hydraulic pressure of the screw 4. The pressure sensor 12 detects the actual oil pressure of the injection cylinder 6 and inputs the signal to the CPU 10.
Based on the sampling time of , set injection hydraulic pressure control signal to follow the above input signal.
The pressure is controlled by output from the CPU and during the pressure holding process of time T shown in FIG. 2, using a control pattern based on a broken line in which there is no sudden drop in the injection pressure.

The specific method is to first use open loop control during the injection filling process, and at the start of injection (A), the electromagnetic relief valve 9 is set to an appropriate pressure necessary for injection, and this injection pressure a is maintained during the injection stroke. is held up to a certain position. As a result, the filling pressure is increased along the curve shown by the broken line PV in FIG.

Next, at a certain screw stroke during filling, the injection hydraulic pressure is switched to a slightly lower pressure b using the electromagnetic relief valve 9. This control signal is set to a value approximately 20 kg/cm ² higher than the oil pressure setting value for pressure holding switching.

Due to the pressure control using the injection pressures a and b, the actual measured oil pressure value PV of the injection cylinder (filling pressure during the filling process) increases, and when it reaches the oil pressure setting value for pressure holding switch, injection filling is completed and the holding pressure Switch to

This holding pressure switching (starting of holding pressure control) is determined when the timer 13 times up or by a signal indicating that the screw 4 has reached the holding pressure switching position due to the operation of the screw position detection limit switch _LS1 . b is made to match the actual measured oil pressure value PV of the injection cylinder at that time, and thereafter closed loop control is performed during the pressure holding process.

In the embodiment, the injection pressure is lowered from a to b by the time the holding pressure is changed, but the holding pressure may be changed without changing the injection pressure a. Furthermore, although open loop control was used during the filling process and closed loop control was used during the pressure holding process, any system may be used as long as pressure control can be performed in accordance with the present invention.

Next, the pressure holding step (X) is performed from c to c in FIG.
g, and each section is obtained by dividing the timer 13 with time setting T for holding pressure control into n equal parts.
or g.

The first section c is a set value obtained by drawing a straight line from the actual value of the injection cylinder oil pressure at the time of switching the holding pressure to the first holding pressure setting value D over T/n time,
It is obtained through arithmetic processing by the CPU 10.

Below, the holding pressure set value is obtained sequentially along a broken line connecting (n-1) set values after the holding pressure second set value E with a straight line.The calculation for creating this gradient pressure setting signal is Explain the method.

The time of the first control section T ₁ T ₂ shown in FIG. 3 is set by TIM ₁ of the timer 13. Since this time t has been set in advance on the operation panel, it is input to the CPU 10 as data. This time t
CPU10 sampling time (scan time)
The value t/T _S divided by T _S becomes the number N of arithmetic processing in the first control section. That is, N=t/T _S Also, the time point when switching to injection holding pressure (T ₁ )
Then, the injection cylinder oil pressure at that time is measured by pressure sensor 1.
2, and if the pressure value is P ₁ , then
The amount of change ΔP in the pressure setting value for each sampling from time T ₁ to time T ₂ is ΔP=P ₂ −P ₁ /TIM ₁ × _TS . Therefore, a _certain time t (however, t
≦TIM ₁ ) The pressure P _t after elapsed time is P _t = P ₁ + ΔP × N = P ₁ + P ₂ − P ₁ /TIM ₁ × T _S × t/T _S = P ₁ + P ₂ − P ₁ / TIM ₁ t ... It becomes. Therefore, according to the formula, it can be seen that the pressure P _t after an arbitrary time t has passed is on the line segment _{1 2} .

Therefore, the actual measured oil pressure signal PV of the injection cylinder during pressure holding changes moment by moment based on the sampling time of the CPU using polygonal pattern control so as to follow the injection oil pressure control signal SV set by each pressure setting device. This makes it possible to prevent the hunting phenomenon and eliminate the need for a hydraulic shock.

According to this embodiment, the injection hydraulic pressure control signal SV is
The setting method for outputting from the CPU is performed using a timer 13 that sets the pressure holding time T and n pressure setting devices 11 that set pressure holding in each pressure holding control section. There is no need for the operator to use a timer for each pressure-holding control period and operate the set value, and the pressure-holding control period can be set by simply dividing the pressure-holding time T into n equal parts. Condition settings can be simplified.

For this reason, for example, in Figure 2, n
= 5, the holding pressure set values are D, E, F, G, H, and from the point C when switching from filling pressure to holding pressure, it takes n/T time to move to D, and thereafter, E at a cycle of n/T time. , F, G, H, the injection hydraulic pressure control signal is
It is output from the CPU and can be controlled with a clove droop.

(Effects of the Invention) As is clear from the above explanation, the present invention, in injection pressure control, matches the injection pressure setting to the actual measured oil pressure value of the injection cylinder at the time of switching from filling pressure to holding pressure, and maintains pressure. CPU during time
The injection hydraulic pressure control signal output at each sampling time is controlled by the control means such as the following, following a broken line trajectory toward the set value of the pressure setting device set for each control section, so the hydraulic shock is eliminated and stability is achieved. The precision of the molded product can be maintained, and the holding pressure can be finely and reliably controlled.

[Brief explanation of drawings]

Fig. 1 is a schematic explanatory diagram showing the configuration of an embodiment according to the present invention, Fig. 2 is a pressure control diagram showing changes in injection pressure with respect to time of the injection process in this embodiment, and Fig. 3 is a pressure control diagram of the embodiment. FIG. 3 is a diagram for deriving an arithmetic expression for explaining a control method. Figure 4 is a block diagram showing a normal injection pressure control method, Figure 5 is a pressure change diagram showing conventional open loop control, and Figure 6 is a pressure change diagram showing conventional closed loop control. be. 4... Screw, 6... Injection cylinder, 10...
...CPU, 11...Pressure setting device, 12...Pressure sensor, SV...Injection hydraulic pressure control signal, PV...Injection cylinder hydraulic pressure actual value signal.

Claims (1)

[Claims] 1 Fill the mold with molten molding material using an injection pressure set to a predetermined pressure, and at the time of switching from the filling process to the pressure holding process, match the injection pressure setting to the actual oil pressure value of the injection cylinder that advances the screw. Furthermore, during the pressure holding process, the injection hydraulic pressure control signal output from the control means such as the CPU that controls the injection pressure at each sampling time is set to the set value of the pressure setting device provided for each control period of the pressure holding time. An injection pressure control method for an injection molding machine in which holding pressure is controlled by injection pressure by drawing a sequentially inclined broken line locus by changing the direction of the injection pressure.