JPH0999452A - Control of injection molding machine for gas injection molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the bonding strength of a weld line part and to make a weld line inconspicuous. SOLUTION: In an injection molding machine for gas injection molding wherein a molten resin is injected into the cavity of a mold and high pressure gas is injected into the resin within the cavity, in at least a part of the period of a dwelling process period applying dwelling pressure to the resin in the cavity from the interior of the resin by the high pressure gas injected into the resin, the pressure of the high pressure gas injected into the resin is vibrated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to injection for gas injection molding, which is one type of hollow injection molding, in which a molten resin is injected into a cavity of a mold and a high-pressure gas is injected into the resin in the cavity. In particular, the present invention relates to a method for controlling a pressure-holding process in a molding machine.

[0002]

2. Description of the Related Art A molten resin is injected into a cavity of a mold, and a high-pressure inert gas (usually nitrogen gas is used) is injected into the molten resin before the molten resin is solidified. In the gas injection molding, a holding pressure is applied to the resin from the inside of the resin and the surface side of the resin is pressed against the inner wall surface of the cavity, so that a good product without sink marks can be molded and the weight of the molded product can be reduced.

[0003]

By the way, even in the gas injection molding method having the above-mentioned advantages, inevitable molded product defects may occur, and a typical one is a weld line.

As is well known, the weld line corresponds to a fusion mark (joint trace) generated at a position where the molten resin branches and flows in the flow path in the cavity and merges again. That is, since the tips of the molten resin that have branched and flowed are cooled until they join again, they cannot be completely fused at the joining portion, and a streak-like weld line is generated. Therefore, the weld line cannot be completely eliminated except to change the design of the molded product so that the molten resin does not branch into the cavity and merge again.

However, in many cases, such as when holes are required or inserts are required, the molten resin branches and flows in the channel in the cavity, and there is no choice but to merge and fuse again. In such a case, a weld line occurs. This weld line may cause a problem in appearance, but rather, the bonding strength in that part often becomes a problem, and there is a problem in the bonding strength at the stress concentration portion when the molded product is used (weak bonding strength is weak). ) When the weld line is located, the reliability of the molded product with respect to mechanical strength is significantly reduced.

Therefore, the technical problem to be solved by the present invention is to solve the above-mentioned problems of the prior art. The purpose is to increase the bonding strength of the weld line portion and to improve the weld line. To make it inconspicuous.

[0007]

In order to achieve the above-mentioned object, the present invention is an injection for gas injection molding in which a molten resin is injected into a cavity of a mold and a high pressure gas is injected into the resin in the cavity. In the control method of the molding machine, the high-pressure gas press-fitted into the resin is pressed into the resin during at least a part of the holding-pressure stroke period in which the holding pressure is applied to the resin in the cavity from the inside of the resin. Make the gas pressure of the high-pressure gas oscillate.

After injecting the molten resin into the cavity, a high-pressure gas is injected (injected) into the molten resin, and the gas pressure of the high-pressure gas injected into the resin is oscillated at a frequency of, for example, about 5 to 100 Hz. The entire resin including the confluent portion of the molten resin (weld line portion) receives the force from the inside from the inside, and especially at the confluent portion of the molten resin, the molecular orientation of the resin is improved well, and the bonding strength of the weld line portion is greatly increased. In addition, the weld line becomes inconspicuous. Furthermore, the transferability of the molded product is further improved.

[0009]

Embodiments of the present invention will be described below. FIG. 1 is a configuration diagram of a gas supply device in an injection molding machine for gas injection molding according to an embodiment of the present invention (hereinafter referred to as this embodiment). In FIG. 1, 1 is an air source,
Reference numeral 2 is a nitrogen gas generator, 3 and 4 are check valves, 5 is a gas pressure increasing mechanism, 6 is a pressure gauge, 7 and 8 are electromagnetic switching control valves, and 9 is a fixed side mold and a movable side mold. A molding die, 10 is a cavity forming a molding space, 11 is an electric servomotor, 12 is a servo amplifier, and 13 is a system controller.

The air from the air source 1 is supplied to the nitrogen gas generator 2 after dust is removed by a filter (not shown) and water and oil are removed by a mist separator. In this example, a known membrane separation type (filter type) is used as the nitrogen gas generator 2, and nitrogen gas is separated and generated from the supplied air (air) and continuously output. That is, the membrane separation type nitrogen gas generator 2 is
It is equipped with a hollow fiber membrane module consisting of multiple bundles of hollow fiber membranes, each of which has a unique permeation rate for each component in the air. There are two streams:
It is configured to separate nitrogen and other streams (oxygen, carbon dioxide, other trace components), and thereby separate and generate nitrogen gas.

The nitrogen gas output from the nitrogen gas generator 2 is supplied via a check valve 3 to a gas booster mechanism 5 whose details will be described later, and the gas booster is driven and controlled by an electric servomotor 11. The mechanism 5 raises the pressure to a set pressure that is pressed into the resin in the cavity 10. The high-pressure nitrogen gas output from the gas booster mechanism 5 is supplied to the electromagnetic switching control valve 8
By opening the electromagnetic switching control valve 7 with the
It is pressed (injected) into the resin (not shown) in the cavity 10. Even after the gas is injected into the resin, the electric servomotor 11 is continuously driven by the servo amplifier 12 controlled by a command from the system controller 13 during the pressure-holding stroke period, whereby the pressure-holding condition set in advance is set. The gas pressure value is controlled so as to be commensurate with, and the gas pressure is vibration-controlled at a predetermined cycle as described later. Then, when the timing of the exhaust gas after the molded product is solidified is reached, the electromagnetic switching control valve 7 is closed, and then the electromagnetic switching control valve 8 is opened, whereby the nitrogen gas is released into the atmosphere or the like.

In this example, the high-pressure nitrogen gas is press-fitted into the resin from the molding die 9, but the high-pressure nitrogen gas may be press-fitted at the nozzle of the injection mechanism or the like.

Next, the gas pressure increasing mechanism 5 will be described.
In this example, the gas pressurizing mechanism 5 includes two first gas compression cylinders 21 and one second gas compression cylinder 22, and the nitrogen gas from the nitrogen gas generator 2 is: It is adapted to be introduced into the compression chamber 21a of the paired first gas compression cylinder 21 via the check valve 3. Further, the nitrogen gas pressurized (compressed) in the first gas compression cylinder 21 is introduced into the compression chamber 22a of the second gas compression cylinder 22 via the check valve 4, and the second gas compression cylinder The pressure is further increased (compressed) by 22.

As shown in FIG. 1, the electric servomotor 11
A pulley 23 is fixed to the output shaft of the electric servo motor 11 and a timing belt 25 is stretched between the pulley 23 and the nut body 24 with the pulley.
The rotation of the pulley-equipped nut body 24 is rotationally driven. The pulley-attached nut body 24 is held so as to be rotatable but not displaceable in the axial direction. A ball screw 26 is screwed onto the pulley-attached nut body 24 so that the pulley-attached nut body 24 is rotated. 26 is adapted to move in the axial direction. That is,
A known ball screw mechanism constitutes a rotation → linear motion conversion mechanism for converting the rotation of the electric servomotor 11 into a linear motion.

A piston body 22b of the second gas compression cylinder 22 is connected to the ball screw 26 through a connecting mechanism as needed, and connecting members 27 and 2 are also provided.
Piston body 2 of the first gas compression cylinder 21 via
1b is connected. Therefore, the electric servomotor 11 rotates in the first direction, and the ball screw 26 is shown in the figure A.
When driven in the direction, the nitrogen gas in the compression chamber 21a of the first gas compression cylinder 21 is compressed by the piston body 21b and the pressure is increased, and the nitrogen gas in the compression chamber 22a of the second gas compression cylinder 22 is reversed. It is introduced via the stop valve 4.

When the electric servomotor 11 rotates in the second direction and the ball screw 26 is driven in the direction B in the figure, the compression chambers of the first gas compression cylinder 21 to the second gas compression cylinder 22 are compressed. The nitrogen gas introduced into the inside 22a is compressed by the piston body 22b and further pressurized.

Here, the accurate value of the gas pressure in the compression chamber 22a can be detected by the pressure gauge 6, and the compression stroke can be detected by an encoder (not shown) attached to the electric servomotor 11. Therefore, the system controller 13 that has acquired these detection information sets the piston body 2 of the second gas compression cylinder 22 based on the gas pressure condition value of the pressure holding stroke by the gas pressure set by the operator.
Servo amplifier 12, electric servomotor 1
Control via 1. In addition, the system controller 13
Is a predetermined cycle of the electric servomotor 11 via the servo amplifier 12 so that the gas pressure is oscillated during the entire holding pressure period or a predetermined partial period based on the gas pressure oscillation condition value set by the operator. To drive the piston body 22b of the second gas compression cylinder 22 back and forth in a predetermined cycle.

FIG. 2 (a) is a diagram showing an example of the state of gas pressure control in the pressure-holding process according to this example, and FIG. 2 (b).
FIG. 6 is a diagram showing an example of a state of gas pressure control in a pressure holding process according to a conventional technique. 2A and 2B, the horizontal axis represents time and the vertical axis represents the gas pressure value of the nitrogen gas press-fitted into the resin. In this illustrated example, the holding pressure is divided into two stages. However, it is needless to say that the pressure can be controlled in one step or in three or more steps.

In FIG. 2 (a), the gas pressure is oscillated during the entire pressure-holding stroke period, whereas in FIG. 2 (b), the gas pressure is simply changed to two stages during the pressure-holding stroke period. Only the control is performed, and the vibration control of the gas pressure is not performed. It should be noted that the vibration in FIG. 2A is schematically shown for convenience of illustration, and in actuality, a vibration having a shorter cycle is given.
The gas pressure is oscillated at a frequency of about 0 Hz. Even if the gas pressure is oscillated and controlled in such a cycle, the gas compression response is good, so highly accurate gas pressure oscillating control can be performed, and in this example, the drive source for the oscillating control is electrically driven. Since the servo motor 11 is used, the control accuracy on the drive side becomes extremely good.

FIG. 3A is a view showing the state of the resin near the weld line when the gas pressure control of FIG. 2A is performed, and FIG. 3B is the view of FIG. FIG. 4B is a diagram showing a state of the resin near the weld line when the gas pressure control of FIG. 3B is performed, and the right side of FIGS. 3A and 3B is a cross-sectional view taken along the line AA of the resin. Is shown for reference.

In FIG. 3, 31 is a resin, 32 is a weld line (a position where molten resin branches and flows in the flow path in the cavity, and merges and fuses again), 33 is resin 3
Hollow part formed by nitrogen gas pressed into 1
4 is the molecular orientation of the resin.

As shown in FIG. 3B, when the gas pressed into the resin is not vibrated, the molecular orientations 34 are arranged on both sides of the weld line 32 in directions in which they are not entangled with each other. On the other hand, as shown in (a) of FIG. 3, when the gas press-fitted into the resin is vibrated, the molecular orientations 34 on both sides of the weld line 32 are oriented so that they are displaced in a direction in which they are entwined with each other. As a result, the joint strength of the weld line 32 portion is greatly increased, and
The weld line becomes inconspicuous. Furthermore, it is possible to further improve the transferability of the molded product.

In the example shown in FIG. 2 (a) described above, the gas pressure is oscillated during the entire pressure-holding stroke period, but the gas pressure is oscillated only during an arbitrary part of the pressure-holding stroke period. The pressure may be oscillated. Further, although the amplitude of the gas pressure vibration is arbitrary, it is necessary to have a certain amplitude or more in order to repeatedly exert a stress that spreads from the inside of the resin to the outside and exert it on the resin. In any case, the period for vibrating the gas pressure, the vibration cycle, and the amplitude can be set to arbitrary optimum conditions according to the resin material, the shape of the molded product, and the like.

Although the present invention has been described above with reference to the illustrated embodiments, it goes without saying that various modifications can be made by those skilled in the art without departing from the spirit of the present invention. The gas may be an inert gas other than nitrogen gas, or may be air in some cases. The high-pressure gas supply device for injecting gas into the resin also has a configuration in which high-pressure gas is injected into the resin from a high-pressure tank or the like via a high-speed response pressure control valve or a servo-type pressure control valve. In this case, the gas pressure may be controlled to oscillate by a high-speed response pressure control valve or a servo-type pressure control valve.

[0025]

As described above, according to the present invention, the joint strength of the weld line portion can be increased and the weld line can be made inconspicuous, and its value is great in the field of the injection molding machine. Is.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a gas supply device in an injection molding machine for gas injection molding according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of pressure holding control showing a comparison between a case where gas pressure oscillation control is performed during a pressure holding process and a case where gas pressure oscillation control is not performed.

FIG. 3 is an explanatory view showing a state of the resin in the vicinity of the weld line in comparison between the case where the gas pressure oscillation control is performed during the pressure holding process and the case where it is not performed.

[Explanation of symbols]

 1 Air Source 2 Nitrogen Gas Generator 3, 4 Check Valve 5 Gas Boosting Mechanism 6 Pressure Gauge 7, 8 Electromagnetic Switching Control Valve 9 Mold for Mold 10 Cavity 11 Electric Servo Motor 12 Servo Amplifier 13 System Controller 31 Resin 32 Weld Line 33 Hollow part 34 Molecular orientation of resin

Claims (2)

[Claims] 1. A method for controlling an injection molding machine for gas injection molding, which comprises injecting a molten resin into a cavity of a mold and injecting a high-pressure gas into the resin in the cavity, the method comprising: The gas pressure of the high-pressure gas injected into the resin is oscillated during at least a part of the pressure-holding stroke period in which the high-pressure gas applies the holding pressure to the resin in the cavity from the inside of the resin. A method for controlling an injection molding machine for gas injection molding, which is characterized. 2. The gas boosting mechanism according to claim 1, further comprising a gas compression cylinder having a gas compression cylinder for boosting the pressure of the gas to be injected into the resin by the driving force of the electric servomotor. A method for controlling an injection molding machine for gas injection molding, wherein the gas pressure is oscillated by reverse rotation.