JP3875921B2 - Control method of injection molding machine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for controlling an injection molding machine, and more particularly to a method for controlling an injection speed when a molten resin is discharged (injected) from a nozzle.
[0002]
[Prior art]
A conventional molding operation by the injection molding machine will be briefly described. In the following description, it is assumed that the injection device and the mold clamping device are controlled by a common controller.
[0003]
First, the injection device rotates a screw arranged in the heating cylinder, and feeds the resin supplied from the hopper to the rear portion of the screw to the tip portion while melting the resin. The molten resin sent to the tip side by the rotation of the screw is accumulated in the tip portion (reservoir) of the heating cylinder. The molten resin collected in the reservoir moves the screw backward by the pressure (back pressure). When the screw is retracted to a preset position, the injection device stops the rotation of the screw, assuming that a predetermined amount of molten resin has accumulated in the reservoir (plasticization / metering step). Then, the injection molding machine retracts the screw so that the pressure of the molten resin in the reservoir becomes a predetermined pressure (suck back).
[0004]
Next, the injection molding machine advances the screw, and sends the molten resin in the reservoir into the mold cavity attached to the mold clamping device (filling step). At this time, the operation of the screw is controlled based on the speed.
[0005]
When the molten resin fills the cavity of the mold, the screw operation control is switched from speed control to pressure control. This is to keep the pressure of the resin in the cavity of the mold at a predetermined value. Thereby, the resin in the cavity of the mold is cooled while being kept at a constant pressure (pressure holding step).
[0006]
Thereafter, the injection device returns to the plasticizing / metering step described above in preparation for the next cycle. On the other hand, the mold clamping device performs mold opening, product removal, and mold closing operation (mold releasing process) (after further cooling time if necessary) to prepare for the next injection process.
[0007]
As described above, conventionally, the molding operation by the injection molding machine is repeated.
[0008]
[Problems to be solved by the invention]
In injection molding, it is necessary to adjust various parameters in order to mold a good product. And in the filling process which sends molten resin in the cavity of a metal mold | die, the advancing speed of a screw must be set appropriately. This is because the forward speed of the screw determines the speed (injection speed) of the molten resin discharged from the tip of the nozzle.
[0009]
However, the change in the injection speed of the molten resin discharged from the tip of the nozzle is often not proportional to the change in the screw advance speed. This is because when the molten resin pushed forward by the screw passes through the flow path in the nozzle, pressure loss or the like occurs, so that the influence of the increase in the screw forward speed does not directly lead to the increase in the molten resin discharge speed. is there.
[0010]
An object of the present invention is to provide a method for controlling an injection molding machine capable of improving the controllability of the speed of a molten resin discharged from a nozzle.
[0011]
Japanese Patent Publication No. 7-115386 discloses a technique for controlling the injection speed by controlling the opening area of the molten resin passage. However, the technique described in this publication is for controlling the injection speed in accordance with the position of the screw, and does not disclose the object and effect of the present invention. That is, this publication does not disclose or suggest changing the cross-sectional area of the nozzle according to the viscosity characteristics of the molten resin.
[0012]
[Means for Solving the Problems]
According to the present invention, in a control method for an injection molding machine that injects molten resin from the tip of a nozzle, the nozzle is repeatedly injected in advance at a constant injection speed while stepwise changing the flow passage cross-sectional area of the nozzle. Detecting the injection pressure corresponding to each of the flow path cross-sectional area, the relationship between the flow path cross-sectional area of the nozzle and the detected injection pressure, the injection pressure decreases as the flow path cross-sectional area increases, and calculated viscosity characteristics convex of the molten resin toward the origin direction in which the injection pressure as the flow path cross-sectional area is reduced is increased, so that determine the optimum flow path cross-sectional area of the nozzle on the basis of the viscosity characteristics An injection molding machine control method characterized by the above can be obtained.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0015]
FIG. 1 shows a schematic diagram of a nozzle-related portion of an injection apparatus to which a control method according to an embodiment of the present invention is applied.
[0016]
The injection apparatus of FIG. 1 has a heating cylinder 11 provided with a screw (not shown) inside, and a nozzle 12 fixed to the tip thereof. Further, a needle valve 13 whose tip reaches the inner tip of the nozzle 12 is provided in the tip of the heating cylinder 11 so as to be able to move forward and backward in the nozzle hole.
[0017]
The injection device of FIG. 1 also has a needle valve driving lever 14, a needle valve driving rod 15, a needle valve driving cylinder 16, a servo valve 17, a hydraulic pressure source 18, and a tank 19 for moving the needle valve 13 back and forth. The needle valve position detector 20 and a control device (control device for an injection molding machine) (not shown) are included.
[0018]
In this injection device, the nozzle 12 is driven by driving the needle valve driving cylinder 16 under the control of the control device and moving the needle valve 13 back and forth via the needle valve driving rod 15 and the needle valve driving lever 14. It is possible to change the flow path cross-sectional area. At that time, the position of the needle valve 13 can be controlled with high accuracy by performing control based on the position of the needle valve driving rod 15 detected by the needle valve position detector 20.
[0019]
An injection molding machine provided with such a needle valve 13 is described in, for example, Japanese Patent Application Laid-Open No. 9-295326.
[0020]
Next, the principle of the present invention will be described.
[0021]
The molten resin exhibits characteristics as a non-Newtonian fluid and has a feature that the viscosity decreases as the shear rate increases. FIG. 2 shows the relationship between the shear rate γ and the viscosity η of various polymers (Oyanagi: Introduction to Polymer Processing Rheology, P. 143, Agne Jofusha (1996)). Moreover, the shear rate γ (s ⁻¹ ) of the molten resin flowing in the circular pipe is represented by γ = 4Q / πR ³ . Note that Q: flow rate (cm ³ / s) and R: flow path radius (cm).
[0022]
From these facts, it can be understood that if the flow path cross-sectional area (flow path radius) at the tip of the nozzle is reduced, the shear rate of the molten resin flowing therethrough increases, thereby reducing the viscosity. And if a viscosity falls, the fillability (fluidity) in injection molding can be improved. FIG. 3 shows the difference in the flowability (flow length) of the molten resin due to the difference in the flow path cross-sectional area at the nozzle tip.
[0023]
On the other hand, the pressure loss ΔP (MPa) experienced by the flowing molten resin (in this case treated as a Newtonian fluid) in the circular pipe is expressed by ΔP = 8LQη / πR4. Note that L: channel length (cm), η: viscosity (MPa · s), Q: flow rate (cm ³ / s), and R: channel radius (cm).
[0024]
As is clear from the above formula, the pressure loss increases as the channel cross-sectional area (channel radius) decreases. In the case of an injection molding machine, there is a limit to the power of the screw drive device, so when the pressure loss increases, the screw speed decreases and the molten resin discharge speed cannot be increased.
[0025]
As described above, when the nozzle cross-sectional area is reduced, two contradictory phenomena occur. And the moldability in injection molding can be maximized in that these two phenomena are balanced. Therefore, in the present embodiment, the optimum channel cross-sectional area is determined as follows.
[0026]
First, the control device repeatedly performs injection molding while changing the flow path cross-sectional area of the nozzle 12 stepwise by changing the position of the needle valve 13, and detection from an injection pressure detector (not shown) at each step. Receive a signal. The injection molding here is performed at a constant injection speed.
[0027]
The control device stores the relationship (viscosity characteristic) between the position of the needle 13 valve and the injection pressure indicated by the detection signal from the injection pressure detector. Here, the relationship between the cross-sectional area of the obtained nozzle 12 and the injection pressure is as shown in FIG.
[0028]
As shown in FIG. 4, the graph showing the relationship between the obtained channel cross-sectional area and the injection pressure can be approximated by two straight lines. The control device calculates these two approximate lines and performs an operation for determining the intersection. And a control apparatus makes the flow-path cross-sectional area of the nozzle 12 corresponding to the calculated | required intersection point the optimal flow-path cross-sectional area, and makes it the flow-path cross-sectional area of the nozzle 12 at the time of performing subsequent injection molding.
[0029]
As described above, in the present embodiment, an optimum nozzle cross-sectional area having excellent moldability can be obtained. Then, by setting the optimum channel cross-sectional area obtained based on the viscosity characteristics of the molten resin as the channel cross-sectional area of the nozzle 12, the injection speed of the molten resin can be controlled with good controllability in the subsequent injection molding operation. Can do.
[0030]
In the above description, the operation for obtaining the optimum channel cross-sectional area is described as if it were an independent operation, but this operation can be performed as part of a series of operations performed at the time of purging. It is.
[0031]
Further, the present invention is not limited to a hydraulic type and can be applied to an electric injection molding machine.
[0032]
Furthermore, the present invention is applicable not only to needle valves but also to injection molding machines equipped with other types of valves.
[0033]
【The invention's effect】
According to the present invention, controllability of the injection speed of the molten resin can be improved by changing the flow path cross-sectional area of the nozzle in accordance with the viscosity characteristic of the molten resin.
[Brief description of the drawings]
FIG. 1 is a schematic view showing nozzle-related portions of an injection apparatus to which the present invention is applied.
FIG. 2 is a graph showing the relationship between the shear rate γ and the viscosity η of various polymers.
FIG. 3 is a graph showing a relationship between an injection speed and a flow length for clarifying a difference in fluidity (flow length) of a molten resin due to a difference in flow path cross-sectional area at the nozzle tip.
FIG. 4 is a graph for explaining a method for obtaining an optimum flow path cross-sectional area according to a control method according to an embodiment of the present invention.
[Explanation of symbols]
11 Heating cylinder 12 Nozzle 13 Needle valve 14 Needle valve drive lever 15 Needle valve drive rod 16 Needle valve drive cylinder 17 Servo valve 18 Hydraulic source 19 Tank 20 Needle valve position detector

Claims (3)

In the control method of the injection molding machine that injects molten resin from the tip of the nozzle,
The injection pressure corresponding to each of the nozzle channel cross-sectional areas is detected in advance by repeating injection molding at a constant injection speed while changing the nozzle cross-sectional area in stages, From the relationship with the detected injection pressure, the injection pressure decreases as the flow path cross-sectional area increases, and increases toward the origin where the injection pressure increases as the flow path cross-sectional area decreases. the method of controlling an injection molding machine, characterized in that the calculated viscosity characteristics of the molten resin, was so that determine the optimum flow path cross-sectional area of the nozzle on the basis of the viscosity characteristics. The relation between the injection pressure and the flow path cross-sectional area of the nozzle is formed by two straight lines approximating the convex molten resin characteristics, and the optimum flow path cross-sectional area is determined from the intersection of the two straight lines. The method for controlling an injection molding machine according to claim 1, wherein:  The injection according to claim 1 or 2, wherein a nozzle provided with a needle valve is used as the nozzle, and the flow path cross-sectional area of the nozzle is changed by changing the position of the needle valve. Control method of molding machine.

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