JPH0725111B2 - Injection molding method - Google Patents

Description

The present invention relates to an injection molding method suitable for obtaining a precision molded product such as an optical disc, and more particularly to a pressure holding step.

[Conventional technology]

Conventionally, in order to obtain a precision molded product, for example, (1) near the end of the measurement (charge) process, accurately stop at the set value, and after the measurement is completed, the molten resin pressure stored at the tip of the screw is In order to lower it, the screw speed and back pressure are calculated and controlled.

(2) At the start of filling, a multi-step control in which the molten resin is switched to a low speed when passing through the cavity gate and then a high speed, and then to a low speed again before switching the holding pressure.

Further, in the case of a complex cavity having a thick portion and a thin portion, the filling speed is finely changed in a plurality of stages so that the resin passes through and fills each portion at a uniform speed.

(3) As a method of switching from the filling process to the pressure-holding process, the reaction force exerted on the screw by the molten resin filled in the cavity is detected, and the pressure is switched to the holding pressure when a certain set value is reached.

(4) In the pressure-holding step, the pressure-holding pressure is lowered in multiple stages according to the state in which the molten resin is cooled and loses its fluidity. Finish with no deformation due to residual stress.

Was adopted.

[Problems to be solved by the invention]

By the way, in the conventional pressure-holding process, even when the pressure is changed in a plurality of stages, the filled resin in the cavity is almost in a static state at the same pressure. For this reason, there is a problem that uniform holding pressure is not applied to the end of the cavity, a "sink" occurs in the thick portion of the molded product, and internal distortion occurs in the molded product.

Conventionally, an injection molding method as described in JP-A-63-95920 has been proposed. In this method, a vibrator is attached to a movable mold, and the vibrator is applied to the movable mold during the pressure-holding step to eliminate residual strain in the molding resin.

However, the mold is clamped during molding, and the movable mold is extremely firmly fixed to the fixed mold, so even if the vibrator is driven as described above, the movable mold is Does not vibrate, and therefore the effect cannot be exhibited with a vibrating machine attached to the mold.

In order to apply vibration to the movable mold by the vibration exciter, it is necessary to make the mold clamping force extremely low so that the movable mold can vibrate. Then, the dimensional variation of the molded product is large. In addition, there are drawbacks such that molding defects such as burrs occur and the molding quality deteriorates.

Further, conventionally, an injection molding method as described in JP-A-61-116520 has been proposed. This method continuously changes the injection hydraulic pressure in the pressure-holding step into a pulse shape to ensure the transferability of the molten resin to the cavity.

The stress (shear stress) σ generated in the molten resin is obtained from the following equation.

σ = η · η; viscosity; shear strain rate The viscosity η is calculated from the following equation.

η = k · ⁿ⁻¹ k; constant n; structural viscosity index (n> 0) Therefore, the stress (shear stress) σ becomes σ = k · ⁿ , and eventually the stress (shear stress) σ has a high shear strain rate. Will increase.

As described above, applying a pulsed injection hydraulic pressure to the resin in the cavity means that the shear strain rate (= d
/ dt) is instantly extremely increased, and a great amount of stress is generated in the resin in the cavity. This causes unnecessary heat deterioration of the resin, and the stress is not completely relaxed and remains in the molded product as residual strain.

Further, when a pulsed injection hydraulic pressure is applied, the pressure is abruptly applied so that the propagation of the pressure is delayed and concentrated on the gate, and the stress of the resin in the vicinity of the gate becomes large.

Further, if the value of the pulsed injection hydraulic pressure is as large as possible, the mold may momentarily open, which causes various problems such as adversely affecting the molding quality.

The object of the present invention is to eliminate such drawbacks of the prior art,
An object of the present invention is to provide an injection molding method capable of higher precision molding.

[Means for solving problems]

In order to achieve this object, according to the present invention, a molten resin is filled in a cavity formed by a fixed mold and a movable mold, and then the fixed mold and the movable mold are clamped in a stationary state. , Injection servo motor, for example, 10 to 10
It is characterized by applying a sine wave signal having a low frequency of about 0 Hz to hold the pressure while slightly vibrating the filling resin through the injection screw.

〔Example〕

 Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram of a main part of an injection molding apparatus according to an embodiment, and FIG.
FIG. 3 is a characteristic diagram of injection speed and pressure in the filling process and pressure holding process of the injection molding apparatus, and FIG.

In FIG. 1, 1 is a fixed mold, 2 is a movable mold, 3 is an injection molded precision molded product such as an optical disk, 4 is a heating cylinder, 5 is an injection screw, and 6 is a resin pressure at the tip of the screw. Measuring load cell, 7 is a ball screw, 8 is a servomotor for injection, 9 is a pulse encoder, 10 is an injection speed signal detected from the number of rotations of the pulse encoder 9, and 11 is charge completion depending on the rotation position of the pulse encoder 9. Position signal for detecting holding pressure switching etc., 12 is a microprocessor unit (MPU), 13 is a servo control CPU,
Reference numeral 14 is a pressure command signal, 15 is a speed command signal, and both the pressure command signal 14 and the speed command signal 15 are sent to the MPU 12 by preset values set by key-in.

Reference numeral 16 is a load cell amplifier, 17 is a resin pressure signal obtained by measuring the resin pressure at the tip of the screw, 18 is a servo amplifier, 19 is a motor control signal, 20 is a hopper, and 21 is a nozzle.

The I connexion molten resin Sukuriyu 5 within the cavity in the mold clamping completion state, for example, a first filling speed V ₁ as shown in FIG. 2
Then, the second filling speed V ₂ and the third filling speed V ₃ are changed at three different speeds. The injection speed command at each stage is output from the servo control CPU 13 to the servo amplifier 18 as a speed command signal 15, the injection speed signal 10 from the pulse encoder 9 is fed back, and the servo amplifier 18 outputs the servo control signal 19 as an injection servo. Output to the motor 8.

As shown in FIG. 2, in the pressure-holding process following the filling process, the fixed mold 1 and the movable mold 2 are normally clamped, that is, both molds 1 and 2 are stationary. The holding pressure P is applied in three stages of a _first holding pressure P ₁ , a second holding pressure P ₂ and a third holding pressure P ₃ . The holding pressure P at each stage is output from the servo control CPU 13 to the servo amplifier 18 as a pressure command signal 14, the injection resin pressure signal 17 obtained from the load cell 6 is input as a feed back signal, and the servo amplifier 18 outputs a motor control signal 19 to the motor control signal 19. Is output to the servo motor 8.

Further, in each pressure holding stage, since the resin filled in the cavity is slightly vibrated, a sine wave signal having a frequency of, for example, 10 to 100 Hz / sec is superposed on the pressure command signal 14 to the servo motor 8. . By doing so, reciprocating motion (fine vibration) is applied at a predetermined minute angle about the point where the rotor of the servo motor 8 rotates at an angle corresponding to the pressure command signal 14.

FIG. 3 is an enlarged view showing the fluctuation of the holding pressure at one holding pressure stage. As shown in the same drawing, while keeping the predetermined holding pressures P _{1 to} P ₃ almost, the microscopic holding pressure is maintained. Repeatedly increases and decreases. For example, in the figure, the center value A of the holding pressure is 100 kg / cm
^{When set to 2} , the upper limit is about 150 kg / cm ² and the lower limit is about 50 kg / cm ² . These upper and lower limits vary depending on various molding conditions, and the lower limit can be set to 0 kg / cm ² . Further, the fluctuation range of the pressure can be changed in each pressure holding stage.

By repeatedly changing the holding pressure in this way and making a slight fluctuation, the filling resin in the cavity is effectively vibrated,
The molten resin is kept in a state where it is evenly dispersed in the cavity, and a holding pressure is applied to the molten resin to cool it.

〔The invention's effect〕

As described above, the present invention is a method of giving a slight vibration to the filling resin from the nozzle side through the injection screw, and therefore, compared with the method described in JP-A-63-95920, the filling resin is On the other hand, a slight vibration can be effectively given.

Further, since it is a slight vibration caused by a sine wave signal, it is disclosed in Japanese Patent Laid-Open No. 61-11.
Compared to the method of applying a pulsed injection hydraulic pressure described in Japanese Patent No. 6520, the change in the shear strain rate is slower, and therefore stress does not occur to the resin in the cavity, causing unnecessary heat deterioration of the resin. However, the stress does not remain in the molded product as residual strain.

Further, compared to the method of giving a pulsed injection hydraulic pressure, there is no stress near the gate and the mold does not open instantaneously.

Furthermore, in the method in which the injection servomotor gives a slight vibration to the filling resin through the screw, the servomotor and the screw are mechanically connected without a hydraulic system or the like, so that the slight vibration caused by the servomotor is generated. It is surely transmitted to the screen, and vibration energy can be applied to the filling resin with good reproducibility.

As a result, the internal strain of the molded product can be reduced and the concentration of the internal strain can be suppressed, and higher precision molding can be performed. Particularly in the case of thick-walled parts, since slight vibration is applied to the filled molten resin, the gate seal time will be slightly extended, but during that time the resin in the cavity will be sufficiently held in a uniform state, and Can be effectively prevented.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a main part of an injection molding apparatus according to an embodiment of the present invention, FIG. 2 is a diagram of injection speed and pressure characteristic of filling and pressure holding steps of the injection molding apparatus, and FIG. It is a characteristic view which expands and shows. 1 ... Fixed mold, 2 ... Movable mold, 5 ... Injection screw, 6 ... Load cell, 8 ... Injection servomotor, 11
...... MPU, 12 ...... CPU, 14 ...... Pressure command signal, 16 …… Load cell amplifier, 17 …… Injection resin pressure signal, 18 …… Servo amplifier, 19 …… Motor control signal, P ₁ …… First Holding pressure, P ₂
…… Second holding pressure, P ₃ …… Third holding pressure.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seiichi Tsuchiya No. 523 Nishinoyama, Futari-cho, Futami-cho, Akashi-shi, Hyogo Toyo Kikai Metal Co., Ltd. (72) Hiroshi Kamei Fukusato, Futami-cho, Akashi-shi, Hyogo Nishinoyama No. 523 No. 1 in Toyo Kikai Kinzoku Co., Ltd. (56) Reference JP-A-61-116520 (JP, A) JP-A-63-95920 (JP, A)

Claims (2)

[Claims] 1. A cavities formed by a fixed mold and a movable mold are filled with molten resin, and then a sine wave is applied to an injection servo motor with the fixed mold and the movable mold being stationary. An injection molding method, characterized in that a pressure is applied to the filling resin while giving a slight vibration to the filling resin through an injection screw by applying a signal. 2. The injection molding method according to claim 1, wherein the holding pressure is divided into a plurality of stages, and microvibration is applied in at least one holding stage.