JPH04220324A - Control method of viscosity of resin of injection molding machine - Google Patents

Abstract

PURPOSE:To obtain a control method, which constantly maintains the viscosity of a molten resin at the time of injection and has highresponse, regarding the control method of a plasticizing device for an injection molding machine. CONSTITUTION:When the fluctuation of injection pressure is detected on filling, the number of revolution and/or back pressure of a plasticizing screw in the plasticizing process of the next injection cycle is controlled, thus constantly maintaining the viscosity of a resin. The control is used preferably together with the temperature control of a conventional band heater and a medium circulating in a mold.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of controlling a plasticizing device of an injection molding machine, and more particularly, to a method of controlling a plasticizing device for an injection molding machine, and more particularly, to a method of controlling a plasticizing device for maintaining a constant viscosity of a resin filled into a mold cavity.

0002

[Prior Art] The injection process of an injection molding machine consists of a filling process in which molten resin stored at the tip of the injection cylinder flows into a mold cavity, and a pressure holding process in which pressure is applied to the resin filling the cavity for a certain period of time. It is made. The filling process is
This is done by the forward movement of a screw (in the case of an in-line screw type) or an injection plunger (in the case of a pre-plastic type), and the forward speed of the screw or plunger at this time is usually set as a function of the moving position of the screw or plunger. It is. In general, the differential pressure in a pipe caused by the flow of fluid in the pipe is proportional to the product of viscosity and flow velocity, so if the injection speed corresponding to each stroke of the screw or plunger is the same every time, the injection pressure will be is approximately proportional to the viscosity. Therefore, if the viscosity of the molten resin stored at the tip of the injection cylinder increases for some reason, the flow resistance of the resin flowing through the spool, runner, and gate into the mold cavity increases, and the injection pressure decreases. The overall price will be higher. On the other hand, when the viscosity of the molten resin becomes low, the injection pressure becomes low due to the decrease in flow resistance of the resin. In devices where a hydraulic cylinder is used to advance the screw or plunger, this change in injection pressure is reflected in the hydraulic pressure supplied to the hydraulic cylinder.

The main molding conditions in the filling process include injection speed, resin temperature, and injection pressure.As mentioned above, the injection speed is controlled by a program in relation to the movement stroke of the screw and plunger. has been
If the resin viscosity is constant, the injection pressure will remain the same for each injection. Resin temperature has a large effect on resin viscosity, so by controlling the temperature of the band heater installed around the outer circumference of the injection cylinder and nozzle and the heat medium circulating inside the mold, the resin temperature can be maintained constant. The resin viscosity is kept constant. The success of such control can be monitored by detecting the injection pressure during filling.

[0004] When the resin viscosity decreases, it causes defects such as jetting, air marks, gas burns, overpacking, and burrs.On the other hand, when the resin viscosity increases, flow marks, weld lines, and mold surface defects occur. This causes defects such as transfer defects, short shots, and insufficient strength.

[0005]

[Problems to be Solved by the Invention] Resin viscosity of resins used as materials for molded articles often varies slightly even at the same resin temperature due to variations in molecular weight and differences in degree of dryness. In such cases, conventional equipment detects changes in resin viscosity as changes in injection pressure, and based on this detection controls the band heater of the injection cylinder and the temperature of the heating medium flowing inside the mold, thereby increasing the resin viscosity. The temperature is raised or lowered slightly to maintain constant resin viscosity. However, since resin temperature control, including its detection, relies on heat transfer, responsiveness is poor and rapid control is impossible. Therefore, even if a fluctuation in the injection pressure due to a change in resin viscosity is detected during operation of the injection molding machine, the set temperature of the band heater etc. will be changed accordingly, and the temperature change will not be transmitted to the resin in the injection cylinder. This process requires a considerable amount of time, during which the injection cycle is repeated many times, leading to the risk of producing a large number of defective products.

[0006] This invention was made to solve the above-mentioned problems, and its object is to obtain a control method with higher responsiveness to maintain a constant resin viscosity during molding operations. .

[0007]

[Means for Solving the Problems] In the present invention, the resin viscosity is detected by detecting the injection pressure during filling, and when there is a fluctuation in the resin viscosity, the injection cylinder 1,
In addition to conventional control in which the temperature of the band heater 14 wrapped around the nozzle 8 and the nozzle 13 and the temperature of the heat medium circulating in the mold are changed to slightly raise or lower the resin temperature, the above-mentioned injection pressure was varied. In this case, a method is adopted in which the resin viscosity is maintained constant by controlling the rotation speed of the plasticizing screws 2 and 7 during the plasticizing process of the next injection cycle or the back pressure applied to the plasticizing screws 2 and 7 at this time. are doing. That is, when an increase in the injection pressure is detected, the plasticizing screw 2,
7 or increase the back pressure of the plasticizing screws 2 and 7 during plasticization. Conversely, when a decrease in the injection pressure is detected, the rotational speed of the plasticizing screws 2 and 7 is reduced, or the back pressure of the plasticizing screws 2 and 7 during plasticization is reduced. Control of the rotational speed of the plasticizing screws 2 and 7 and control of the back pressure may be performed by either one or both at the same time.

In order to avoid errors caused by local variations during the process, the injection pressure for detecting resin viscosity is detected at multiple points and the average value or sum thereof is used, or the integral value is used. It's good. The injection pressure may be detected at a timing such that, for example, sampling is started when the screw 2 or plunger 9 moves a certain distance L1 after the filling process is started, and sampling is then performed every time the screw or plunger moves a distance ΔL. It will be held in The moving distance L1 of the screw or plunger up to the first sampling at this time is:
Preferably, the distance is such that the first sampling is performed after the tip resin (the tip of the resin being filled) passes through the gate of the mold. This is to avoid sampling at that part, since the injection pressure often rises rapidly when passing through the gate and shows a small but instantaneous surge pressure.

[0009]

[Operation] According to the method of the present invention, when the viscosity of the resin stored at the tips of the injection cylinders 1 and 8 changes for some reason during the injection molding operation, this causes a change in the injection pressure during filling of the injection cycle. This is detected and fed back in the form of controlling the rotational speed or back pressure of the plasticizing screws 2 and 7 during the next cycle of plasticizing process that is started thereafter. By increasing or decreasing the rotational speed of the plasticizing screws 2 and 7, the shear rate applied to the plasticizing resin can be increased or decreased, and by increasing or decreasing the back pressure of the plasticizing screws 2 and 7, the shear rate applied to the plasticizing resin can be increased or decreased. As the applied shearing pressure increases or decreases and the resulting shear heat generation increases or decreases, the internal calorific value of the resin increases or decreases, thereby controlling the resin temperature and maintaining the resin viscosity constant.

Furthermore, the detected fluctuation in resin viscosity is fed back to the set temperature of the band heater 14 that heats the injection cylinders 1 and 8 and the nozzle 13, and as the temperature of the band heater 14 rises, the resin temperature increases. Then, the rotational speed and back pressure of the plasticizing screws 2 and 7 also return to their original set values.

As described above, according to the control method of the present invention, the time delay in resin temperature control due to temperature control of the band heater 14 etc. is compensated for by controlling the rotational speed and back pressure of the plasticizing screws 2 and 7. This makes it possible to control the resin temperature with quick response.

In addition, by sampling the injection pressure at multiple points and finding the average value or total sum, or by detecting the integral value of the injection pressure, it is possible to eliminate the influence of local variations in the injection pressure and achieve accurate control. It becomes possible to do so.

[0013]

[Embodiment] Next, an embodiment shown in the drawings will be explained. The example shown in the figure shows the control of the rotational speed of the plasticizing screw. Figure 1 is a control block diagram for the in-line screw type, and Figure 2 is a control block diagram for the screw prep type. be.

In FIG. 1, 1 is an injection cylinder, 2 is a screw, 3 is a position detector for detecting the axial position of the screw 2 (for example, a rotary encoder rotated via a rack and pinion), 4 is an injection hydraulic cylinder, 5 is a hydraulic motor for rotating the screw. Similarly, in FIG. 2, 6 is a plasticizing cylinder, 7 is a plasticizing screw, 8 is an injection cylinder, 9 is a plunger, 10 is a position detector that detects the axial position of the plunger 9, 11 is an injection hydraulic cylinder, 1
2 is a hydraulic motor for rotating the plasticizing screw.

Furthermore, in FIGS. 1 and 2, 13 is a nozzle;
14 is a band heater wrapped around the injection cylinders 1 and 8 and the nozzle 13; 15 is a hopper for material input; 16 is a rotary encoder that detects the rotation speed of the screw 2 or 7; and 17 and 18 are injection hydraulic cylinders 4. , 11, 19 and 20 are hydraulic pipes to the hydraulic motor for rotating the screw, 21 is a pressure detector for detecting the injection pressure, and 22 is an electromagnetic proportional unit for controlling the rotational speed of the screws 2 and 7. It is a flow control valve.

23 is a microcomputer for control;
24 is a keyboard, 25 is a display, 26 is an input interface circuit, 27 is an output interface circuit, 28 is a temperature control device, and 29 is a driver circuit. A temperature detector 30 is embedded in the injection cylinders 1 and 8 and the nozzle 13, and its detection signal is input to the microcomputer 23 via an input interface circuit 26, and is also input to the temperature control device 28. . Further, the rotational speed of the screws 2 and 7 during the plasticizing process is detected by a rotary encoder 16 and inputted to the microcomputer 23 via an input interface circuit 26. In addition, the injection hydraulic cylinders 4 and 11 during the filling process
The pressure, that is, the injection pressure, is detected by the pressure detector 21, and this detected pressure is detected by the position detector 3 or 1.
Each time the position of the screw 2 or plunger 9 detected at 0 reaches a predetermined sampling position L1 to Ln (see FIG. 4), it is read by the microcomputer 23 via the input interface circuit 26. The signal read in this way is transmitted to the microcomputer 2.
Temporarily stored in memory within 3.

FIG. 3 shows the control procedure of the microcomputer 23. When one cycle of molding operation is completed (step 34), it is determined whether the scheduled number of molding cycles has been completed (step 3).
5) If the set number of cycles has been completed, the injection operation is completed. If the next cycle exists, the sampled pressure Pi of the injection hydraulic cylinders 4 and 11 (
i=1 to n) are added up, and it is determined whether it is within a predetermined setting range (step 36). If it is within the allowable range, the next molding cycle is started without changing the molding conditions.

On the other hand, when the total value ΣPi of the injection pressure is outside the allowable range, the temperatures of the cylinders 1 and 8 and the nozzle 13, and the speed of the screw 2 and plunger 9 during filling detected by the position detectors 3 and 10. , screw 2 during the plasticization process,
It is determined whether or not the number of rotations of the engine 7 is normal (step 37), and if there is an abnormality in any of these,
Outputs an abnormal signal. Furthermore, if there is no abnormality in each of the above control conditions, it is determined whether the sum of the injection pressures ΣPi is higher or lower than the set range (step 38), and if it is higher than the set range, the flow rate regulating valve 22 is opened and the plasticizing screw is used in the next molding cycle. (Step 39), and if it is lower than the set value, throttle the flow regulating valve 22 to lower the rotation speed of the plasticizing screws 2 and 7 in the next molding cycle (Step 40). Start the next molding cycle.

In the above embodiment, in the control step 39 or 40, the rotational speed of the screws 2 and 7 is controlled by controlling the opening degree of the flow rate regulating valve 22. However, when back pressure control is performed, the injection hydraulic pressure is By controlling the back pressure of the cylinders 4 and 11 with an electromagnetic proportional pressure regulating valve (not shown) provided on the hydraulic piping 17 side, the screws 2 and 7 during plasticization are
Controls the resin pressure at the tip. This pressure control valve is also used to control the holding pressure in the pressure holding process. In particular, in the screw pre-plastic type, the screw 7 of the plasticizing device does not retreat, so stable plasticization can be performed like in an extruder, and the resin viscosity can be controlled more accurately by the screw rotation speed control and back pressure control described above. .

Although not shown in the flowchart of FIG. 3, in parallel with the control shown in FIG.
A control output is given to the temperature control device 28 based on Σ
Control is performed in parallel to increase the set temperature of the band heater 14 when Pi becomes high, and to lower the set temperature when Pi becomes low.

FIG. 4 schematically shows changes in the resin pressure inside the injection cylinders 1 and 8 during resin filling into the mold in relation to the position of the screw 2 or the plunger 9.
The horizontal axis is the position of the screw 2 or plunger 9 (if the injection speed is accurately controlled, it can be replaced by the elapsed time after the start of the filling process), and the vertical axis is the resin pressure or the moving speed of the screw 2 or plunger 9 In other words, it is the filling speed and indicates the rate up to the completion of filling. An imaginary line a shows the set filling speed (three-speed control in this figure), and an actual resin filling speed curve corresponding to this is a curve as shown by a dotted line b.

Curves c, d and e show the changes in injection pressure during the filling process; c is when the resin viscosity is normal, d is when the resin viscosity is high, and e is when the resin viscosity is low. are shown exaggerated. L1 to Ln are sampling points of resin pressure, and the first sampling point L
1 is selected at the timing after the tip of the filled resin passes through the gate of the mold, as described above. Furthermore, at the time point Lp in the figure when the filling process is completed, peak pressure is likely to occur due to the kinetic inertia of the resin and the plunger, so the last sampling point Ln needs to be set at a timing that is not affected by such effects.

As mentioned above, a plurality of sampling points L
The reason why the pressure is detected at 1 to Ln and the sum (or average value) is used for control is to prevent errors due to local variations in pressure.
The influence of errors due to such variations can also be eliminated by using the integral value of the pressure from Ln to Ln. Of course, the positions and number of sampling points L1 to Ln must be constant.

Although the present invention has been described above using a hydraulic injection molding machine as an example, the present invention can also be applied to a case where plasticization and injection are performed using an electric servo motor. In this case, the injection pressure can be detected by interposing a pressure sensor between the axial components of a mechanism that converts the rotational motion of a motor into linear motion of a screw or plunger using a ball screw or the like. The rotation speed of the plasticizing motor can be controlled by detecting the rotation speed of the motor with a pulse generator and controlling the servo driver while giving feedback. Further, the back pressure control is performed while feeding back the detection signal of the pressure sensor.

[0025]

As explained above, according to the present invention, when a change in resin viscosity occurs during a molding operation, the resin temperature is quickly controlled by utilizing the internal heat generation of the resin during the plasticizing process. Since the viscosity of the resin filled into the mold can be kept constant, control with much better responsiveness than with conventional methods is possible, and the occurrence of defective products due to changes in resin viscosity during molding operations can be avoided. It becomes possible to prevent this.

[Brief explanation of the drawing]

FIG. 1 is a block diagram when using an in-line screw type injection unit.

FIG. 2 is a control block diagram when using a screw pre-plastic injection unit.

FIG. 3 is a flowchart showing a control procedure.

FIG. 4 is a graph schematically showing changes in injection pressure during filling.

[Explanation of symbols]

1 Injection cylinder 2 Screw 7 Plasticizing screw 8 Injection cylinder 9 Plunger 13 Nozzle 14 Band heater

Claims (2)

[Claims] Claim 1: In a resin viscosity control method for an injection molding machine that detects the injection pressure during filling and controls molding conditions based on the detection signal, when a fluctuation in the injection pressure is detected, the next injection A method for controlling resin viscosity in an injection molding machine, which comprises controlling the rotational speed and/or back pressure of a plasticizing screw (2, 7) during the plasticizing process of a cycle to maintain a constant resin viscosity. [Claim 2] The injection pressure during filling is detected at multiple points after the tip of the resin to be filled passes through the gate of the mold, and the fluctuations in the injection pressure are determined by the average value or integral value. A method for controlling resin viscosity in an injection molding machine according to claim 1, wherein the resin viscosity is detected.