JPH0474628A - Back pressure control device of injection molding machine - Google Patents

Abstract

PURPOSE:To correct a deviation in condition transition in a plasticizing process, by a method wherein an established value of back pressure is amended based on the established value according to a variation of a resin temperature. CONSTITUTION:A resin temperature theta and resin pressure P within a reservoir at the time of molding of a good product are measured by a temperature sensor and resin pressure sensor. Then they are taken into a control system 3 as time sequence data at every sample time T through an A/D conversion part. Injection molding is started and a resin temperature theta' and resin pressure P' within the reservoir are measured by a temperature sensor and resin pressure sensor and taken into the control system 3 similarly. The time sequence data such as the resin temperature theta, resin pressure P and resin temperature theta' are substituted for a formula I and time sequence data P'(nT) such as the pressure P' is calculated. The P'(nT) is converted into driving pressure of a hydraulic cylinder by multiplying the P'(nT) by a sectional area ratio As/Ac and stored into a time sequence memory 62. The stored data is read out by the next molding cycle in a back pressure feed back control device 5 and made into an established value of back pressure in a plasticizing process and back pressure control of the next plasticizing process is performed. With this construction, change in condition transition of molten resin with time can be covered.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a control device for an injection molding machine, and more particularly to an improvement in back pressure control.

(Prior Art) Generally, injection molding is performed through the steps of plasticizing resin, injection, holding pressure, and cooling. In order to make the weight, dimension, photoelasticity, etc. of the molded product obtained through such a process uniform, the state transition of the resin (specific volume of the resin) is necessary.

It is necessary to improve the reproducibility of temperature and pressure changes. Note that the specific volume is the reciprocal of the density of the molded product.

Holding pressure is the total resin pressure within the mold that is controlled in the holding pressure process.

Incidentally, it is known that the relationship between specific volume, temperature, and pressure of a resin in a molten state is expressed by Spencer's equation of state below.

(P+π)(V-ω)-R,, θ (0) However,
P is resin pressure, ■ is specific volume, θ is resin temperature, π, ω, R
, are constants determined depending on the material.

If this relationship is expressed in terms of three dimensions of p, v, and θ, it will be as shown in Figure 3, and in one molding cycle of plasticization, injection, holding pressure, and cooling, as shown by the solid line in Figure 3. Draw a trajectory. It is desirable that this trajectory remains the same even if the injection molding operation is repeated.

(Problem to be Solved by the Invention) However, in reality, the resin temperature fluctuates from the start to the end of the molding cycle, so after one hour it deviates from the trajectory shown by the solid line in FIG. I started to draw a trajectory like the one shown.

As a result, the quality of molded products varies. This is because when the resin temperature changes, the specific volume in the reservoir in the heating cylinder immediately before injection starts changes, and the initial conditions (initial state) of the resin when filling the mold changes.

The problem of the present invention is that even if fluctuations in resin temperature occur.

A back pressure control device that adjusts the set value of back pressure for each molding cycle accordingly makes the state transition of the molten resin uniform throughout the molding cycle, thereby reducing variations in quality of molded products. It is about providing.

(Means for Solving the Problems) A back pressure control device for an injection molding machine according to the present invention includes a feedback control means for controlling back pressure based on a back pressure setting value and a back pressure detection value, and a heating cylinder reservoir. back pressure correction means including correction amount calculation means for calculating a correction amount of the back pressure set value based on the output of the temperature sensor and the back pressure detection value or the oil pressure sensor output of the hydraulic cylinder; The present invention is characterized in that the back pressure set value can be corrected for each molding cycle using the output of the calculation means.

(Function) In the present invention, the state transition (pressure) of the resin changes due to changes in the resin temperature from the start to the end of molding.

We are focusing on the fact that the specific volume (specific volume, temperature) also fluctuates, resulting in differences in the specific volume when solidified, resulting in variations in the quality of molded products.

From this, the correction amount calculation means according to the present invention is as follows.

By correcting the back pressure set value in response to fluctuations in resin temperature based on a value determined in advance through tests, it acts to correct deviations in state transition in the plasticizing process.

(Example) First, an injection molding machine to which the present invention is applied will be described with reference to FIG.

This injection molding machine 100 includes an injection device 1 for injecting molten resin, a mold 2 for solidifying the molten resin to obtain a desired molded product, and a control system 3 for the injection device 1.

In the injection device 1, the screw 11 is rotationally driven to send the molten resin to the reservoir 13 provided at the tip of the heating cylinder 12. This process is called the plasticization process.

Thereafter, in the injection process, the control system 3 determines the amount of operation to be performed on the servo valve 4, and operates the servo valve 4.

This operation controls the hydraulic pressure to the hydraulic cylinder 14, thereby driving the screw 11, and the molten resin in the reservoir 13 is filled into the mold 2 through the gate.

Next, in the pressure holding step, the control system 3 controls the resin pressure sensor 41 in the reservoir 13 under control to be described later. Alternatively, based on each detection signal from the oil pressure sensor 42 in the hydraulic cylinder 14 and the temperature sensor 43 in the reservoir 13, the amount of correction of the back pressure setting value is calculated to determine the operation amount to the servo valve 4, and the servo valve 4 is Manipulate. By this operation, the molten resin filled in the mold 2 is pressed.

The amount of movement of the screw 11 and the oil pressure for driving the screw 11 are measured by the amount of movement detector 44. It is converted into an electrical signal by the oil pressure sensor 42 and fed back to the control system 3 through amplifiers 46 and 47.

Next, the control system of the present invention will be explained with reference also to FIG.

The control system 3 includes a CPU 31. In addition to the memory 32.

An input/output interface 34 that exchanges data with the input console 33. Resin pressure sensor 41. Temperature sensor 43
a first A/D converter 35 for converting detection signals from the detectors 44 to digital signals; A second A/D converter 36 for converting each detection signal from the oil pressure sensor 42 into a digital signal. It has a D/A converter 37 for converting a digital command value for driving the servo valve 4 into an analog signal. 38 is an amplifier.

The functions of the CPU 31 are represented by the functional block diagram shown in FIG. 2, and are comprised of back pressure feedback control means 5 and back pressure correction means 6.

The back pressure feedback control means 5 includes an adder 51. which adds the back pressure set value P7 and the back pressure correction value. It has a subtracter 52 for subtracting the back pressure detection value P obtained from the detection signal of the resin pressure sensor 41 from the output of the adder 51, and a back pressure feedback control compensator 53.

On the other hand, the back pressure correction means 6 includes a calculation unit 61 that calculates a correction amount of the back pressure set value based on the detected value θ of the temperature sensor 43;
It includes a time series memory 62 for temporarily storing the calculated correction amount.

Note that the calculation in the calculation unit 61 may be performed on the detected value of the oil pressure sensor 42.

The back pressure correction means 6 will be explained in more detail.

Simply put, the present invention attempts to make the time trajectory of the specific volume V of the resin in the reservoir uniform in each molding cycle in the plasticizing process by correcting the back pressure set value when the resin temperature fluctuates. can be.

It is based on the following principles.

■Once the molding conditions for a good product are determined by determining the molding conditions, the resin temperature θ in the reservoir 13 during the plasticization process at that time is determined.
and resin pressure P (or cross-sectional area ratio A to the driving pressure in the hydraulic cylinder 14 detected by the hydraulic sensor 42./A
The value p converted to resin pressure by multiplying by s) (However, AC
: cross-sectional area of the hydraulic cylinder 42, AS : cross-sectional area of the screw 11), and using these two values as the following equation (1), calculate the time-series change in the specific volume V of the resin in the reservoir 13. calculate.

V-(R,, (θ+273.5)/(P+π)) 1ω■ When injection molding is actually started, the resin temperature θ' and resin pressure P' (or 1”).

The specific volume V' during injection molding is calculated using equation (1).

V' - (R, (θ'1273.5)/(P' + π Month 10ω (1)' Here, the difference between the specific volume V when molding a good product and the specific volume v' when actually molding is ΔV If so.

Δ v−v−v′ −(R, (θ+273.5)/(P +π))IR
, a (θ' +273.5)/ (P' 1π) 1
In order to make this Δ■ zero, it is sufficient that the resin pressure P' is low.

■ Convert the resin pressure P' obtained by formula (2) to the driving pressure of the hydraulic cylinder 14 (multiply by the cross-sectional area ratio A s / A c ) so that it becomes the set value of the back pressure for the 11th molding cycle. Make it.

The control operation will be explained with reference to FIG.

■When the molding conditions for a non-defective product are determined, the resin temperature θ and resin pressure P in the reservoir 13 during molding of a non-defective product are determined in the plasticizing process.
are measured by the temperature sensor 43 and the resin pressure sensor 41, respectively, and input into the control system 3 through the A/D converter 35 as time series data at each sample time T. In addition,
When the resin pressure in the reservoir 13 cannot be measured, the cross-sectional area ratio A is applied to the drive pressure detected by the oil pressure sensor 42. As described above, the value p may be multiplied by /As and used as the set value p of the resin pressure P in the reservoir 13.

- Injection molding is started, and the resin temperature θ' and resin pressure P' in the reservoir 13 are measured by the temperature sensor 43 and the resin pressure sensor 41, respectively, and sent to the control system 3 through the A/D converter 35 for a sample time ゛r. Import as time series data.

(2) Substitute the time series data of resin temperature θ, resin pressure P, and resin temperature θ' into equation (2), and calculate the time series data of pressure P'. Namely.

P′(・T). The cross-sectional area ratio As/A is calculated as P' (nT).
The driving pressure of the hydraulic cylinder 14 is converted by multiplying by C and stored in the time series memory 62. The back pressure feedback control means 5 reads out this stored data in the next molding cycle so that it becomes the set value for the back pressure in the plasticizing process, thereby controlling the back pressure in the second plasticizing process.

■By repeating the following operations from ■ to ■ for each molding cycle, the back pressure set value is corrected according to fluctuations in resin temperature, and as a result, control that covers changes in the state transition of the molten resin over time can be performed. It will be done.

(Effects of the Invention) As described above, according to the present invention, even if the resin temperature fluctuates during the molding cycle, the back pressure set value is corrected in accordance with the fluctuation.

In the plasticizing process, the state transition of the molten resin is controlled so that it follows the same trajectory regardless of the number of molding cycles. Therefore, according to the injection molding machine of the present invention, molded products with small variations in quality can be provided.

FIG. 3 is a diagram showing the trajectory of state transition of molten resin during the molding cycle of an injection molding machine.

In the figure, 1 is an injection device, 2 is a mold, and 3 is a control system.

4 is a -bo valve, and 5 is a pressure holding feedback control means.

6 is a back pressure correction means.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an injection molding machine according to the present invention, and FIG. 2 is a functional block diagram of the CPU shown in FIG.

Claims (1)

[Claims] (1) Feedback control means for controlling back pressure based on a back pressure set value and a detected back pressure value; and a back pressure correction means including a correction amount calculation means for calculating a correction amount of the back pressure set value based on the back pressure setting value, and the back pressure set value can be corrected for each molding cycle with the output of the correction amount calculation means. A back pressure control device for an injection molding machine, which is characterized by: