JPS62127218A - Improved vent type injection molder - Google Patents

Abstract

PURPOSE:To prevent molten material in an injection molder from overflowing by a method wherein pressurized gas is forced in an air vent hole simultaneously with the stopping of the rotation of the screw in the vent type injection molder having the air vent hole. CONSTITUTION:Under the state that a screw is being rotated for the plasticization and metering of material after the finish of an injection stroke, a three-way selector valve 6 is changed over in the direction of a piping 7 communicating to a vacuum pump. Accordingly, volatile matter is sucked through a vent hole 3 and a piping 5 into the vacuum pump so as to obtain high deaeration effect. When the metering comes to an end, the screw stops and the three-way selector valve 6 is changed over in the direction of a pressurized gas piping 8 in interlocking with the stopping of the screw. After being cooled down, a molded part is released from molds through the opening of the molds. When the molds are closed, the injection stroke is started and the screw advances. When a cooling stroke starts after the finish of the injection stroke, the rotation of the screw starts and the three-way selector valve 6 is changed over in the direction of the vacuum pump side piping 7 in interlocking with the starting of the rotation of the screw so as to start the deaeration again.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application Field The present invention relates to an injection molding machine for synthetic resin, and more particularly to an improvement of a vent type injection molding machine having a vent hole (deaeration hole) in the cylinder. It is something.

(b) Conventional technology Conventionally, many plastic materials have water absorption properties, so
It is necessary to perform preliminary drying before injection molding.

In other words, when molded in a mold containing moisture, silver streaks may appear on the surface of the molded product, or air bubbles may be generated inside the molded product, resulting in a defective molded product. For materials,
The hydrolysis reaction causes cleavage of the main chain, resulting in a significant decrease in physical properties.

Furthermore, in recent years, plastic materials containing reinforcing materials and fillers have become widespread, but some of these reinforcing materials and fillers also absorb moisture on their surfaces or have some degree of water absorption, and are generally not reinforced. Since materials are often molded at high temperatures, even a small amount of residual moisture has a significant effect on the appearance and physical properties of molded products.

Further, even when such water-absorbing fillers and the like are blended with non-water-absorbing materials such as polyethylene and polypropylene, the above-mentioned adverse effects due to moisture occur.

For this purpose, injection molding is generally performed after preliminary drying using a hot air dryer, dehumidifying hot air dryer, vacuum dryer, etc., but these dryers are relatively expensive equipment; In addition to consuming a large amount of electricity, it is difficult to dry the material sufficiently depending on the weather environment, and the degree of drying is not reproducible, so there is a risk of unexpected product defects. Ta.

In addition, when plastic materials are heated in the cylinder of a molding machine, they partially decompose and generate gas, which, like moisture, can cause problems such as poor surface condition of molded products and the generation of bubbles. This may cause product defects. Materials in which this phenomenon is particularly noticeable include methacrylic resin and the like.

In order to solve the above-mentioned drawbacks of the method of pre-drying the molding material, vent holes (a-pores) are provided in the cylinder of the injection molding machine to release the generated water vapor, decomposition gas, etc. into the atmosphere. Alternatively, a method of continuously degassing during the molding process by vacuum suction, the so-called vent injection molding method, has been developed, but it has not yet become widespread due to various problems as described below. .

(c) Problems to be Solved by the Invention The vent-type molding method has been adopted and practiced for a long time in extrusion molding, and has been proven to be highly effective.

However, although a similar method has been adopted for screw in-line injection molding machines, it has not yet been generally used in injection molding machines, mainly due to problems with work stability.

That is, this problem is that during the molding process, the molten resin in the cylinder often overflows from the vent hole and blocks the vent hole, so that a stable deaeration effect cannot be expected.

In the case of an extruder, the screw continuously rotates at a constant speed during operation, and the molten resin is continuously transported forward, so the screw flight passes through the lower part of the vent hole, causing moisture and volatilization. Materials are continuously degassed through vent holes.

In other words, since the extruder always operates in a steady state,
This is because the molten resin does not overflow and accumulate from the vent hole.

On the other hand, in a screw in-line injection molding machine, the screw plays the same role as an extruder in that it plays the role of material supply and plasticization, but a certain amount of molten material is plasticized and the retracted screw After the material is measured and accumulated in front of the screw, the screw is forced forward by hydraulic pressure, etc., and the screw tip plays the role of injecting the measured material into the mold at high pressure and high speed, and then the next material supply and plasticization. This is done while the material injected into the mold is cooled and solidified, but this time is set to be somewhat shorter than the required cooling time due to the need to keep the molding cycle constant.

As a result, the screw temporarily stops rotating when the metering process ends, and starts rotating again when the injection process ends.

In this case, the ratio of the time the screw is rotating to the time it is stopped is often between 1:4 and 1:1.

While the screw is stopped, the material at the bottom of the vent hole foams and expands due to the volatilization of water, etc., and tends to swell into the vent hole, and this swelled material is outside the screw mountain.
Moreover, since it is not in contact with the inner wall of the cylinder, it may get caught up in the screw during the next rotation of the screw and not be transported forward. When this phenomenon is repeated, the molten resin accumulates in the vent hole and eventually completely blocks the vent hole.

Under this condition, degassing is no longer performed.

This tendency becomes more noticeable especially when attempting to remove volatile substances by strong suction under a vacuum or reduced pressure condition through the vent hole.

That is, under vacuum or reduced pressure, the molten material at the bottom of the vent hole significantly increases in volume as a result of releasing a larger amount of volatile matter, and easily blocks the vent hole. Therefore, in the case of injection molding, the only way to operate a so-called vent-type injection molding machine, which has a vent hole in the cylinder, is to open the vent hole to the atmosphere and allow the volatile matter to naturally dissipate into the atmosphere. Even in this case, the molding operation may be interrupted due to the vent hole being blocked, or the molding operation may be continued without realizing that the vent hole is blocked, resulting in defective products due to insufficient deaeration effect in the molded product. There was a risk of an outbreak.

Furthermore, due to the natural dissipation of volatiles into the atmosphere, sufficient degassing effects cannot be obtained in the case of materials that undergo hydrolysis reactions that cause scission of the polymer main chain due to the presence of trace amounts of moisture. .

Therefore, it is unavoidable to take measures such as using a drying device in combination, and the actual situation is that advantages cannot be achieved in terms of equipment or energy saving.

The present inventor has worked diligently to solve the above-mentioned problems of conventional vented screw in-line injection molding machines, and to provide an improved vented injection molding machine that can perform stable molding operations. As a result of research, the present invention was completed.

(d) Measures to solve the problem and their effects:
The structure of the present invention will be explained based on the drawings showing examples.

FIG. 1 is a partial sectional view of an embodiment of the vented injection molding machine according to the present invention, with the injection process completed and the screw rotating to plasticize and meter the material.

In this step, the three-way switching valve (6) is switched in the direction of the line (5) to the vacuum pump (not shown). Therefore, the volatile matter is sucked into the vacuum pump through the vent hole (3) and the piping (5), and a high degree of degassing effect can be obtained. Furthermore, in this step, the molten material is continuously conveyed forward by the screw crest and does not bulge into the vent hole.

FIG. 2 is a cross-sectional view of the same state after measurement has been completed.

When the metering is completed, the screw stops, and in conjunction with this, the three-way switching valve (6) is switched to the direction of the pressurized gas pipe (8).

The pressurized gas pipe (8) is connected to a pressurized gas storage container (not shown), and the pressurized gas is supplied to the pipe (5) and the vent hole (3).
) to pressurize the molten material in the vent section. As a result, the volatilization of volatile matter from the molten material at the bottom of the vent hole is suppressed,
Since the material does not foam or expand, it will not bulge in the vent hole and block the vent hole.

When the cooling process is completed, the mold is opened and the molded product is released, and the mold is closed, the injection process begins and the screw advances.

When the injection process is completed and the cooling process begins, the screw starts rotating, and in conjunction with this, the three-way switching valve (6) is switched to the direction of the vacuum pump side piping (7), resulting in the state shown in Figure 1. to return to.

That is, with the above structure, pressurized gas is injected into the vent part each time the screw rotation stops, and pressurized gas is injected into the vent part every time the screw rotation stops, which is often experienced in conventional vent-type injection molding machines. This can be avoided by suppressing evaporation of volatile components and suppressing foaming and expansion of the molten material.

The pressurized gas to be injected at this time may be of any type as long as it does not cause undesirable reactions with the molten material or deterioration of the polymer, but nitrogen gas is generally preferred from the viewpoints of cost, safety, etc.

The pressure during pressurization varies depending on the concentration of volatile components, but is generally in the range of gauge pressure 2-/- to 20 kg/-.

By repeating the above operations, the molten material is degassed in vacuum as it passes through the vent section, then accumulates in the metering section (9) and is transferred to the injection process, resulting in generation of moisture and polymer thermal decomposition. The volatile matter is injected into the mold in a sufficiently degassed state.

(e) Effects of the Invention As described above, the improved vent-type injection molding machine with the structure according to the present invention uses a pre-drying device etc. by stably molding a sufficiently deaerated resin material. It achieved improvements in the surface condition of the molded product and prevention of air bubbles inside the molded product without causing any damage, and more importantly, significant improvements in strength and elongation were observed in polymers that easily undergo hydrolysis. It shows the effectiveness.

Of course, the present invention can operate in a stable degassing state even under natural dissipation operating conditions in which vacuum or reduced pressure is not drawn from the vent hole, since the vent hole is not clogged with molten material.

Furthermore, the structure according to the present invention can be easily attached to a conventional vent-type injection molding machine using simple parts and work, and the operation can be easily automated by connecting it to the limit switch of the molding machine. Its practical industrial effects are enormous.

[Brief explanation of drawings]

The drawing is a partial cross-sectional view of an embodiment of an improved vented injection molding machine according to the present invention. Figure 1 shows a state in which the injection process has been completed, the screw is rotating, and the material is being plasticized and measured (during degassing).
The figure shows the state after measurement has been completed (gas is being pressurized). In the figure 1-----Injection molding machine cylinder 2-----
・Injection molding machine screw 3------Vent hole provided in the cylinder 4------1------Vent part chamber 5---------1----Vent part piping 6--- −・−
1----Three-way switching valve 7-----11111 Piping to the vacuum pump or piping opened to the atmosphere

Claims (1)

[Claims] In a vent-type injection molding machine with a vent hole for degassing in the cylinder, deaeration is performed while the screw is rotating, and when the screw rotation is stopped, the vent hole is pressurized with pressurized gas to remove the molten material. A vent-type injection molding machine characterized by preventing overflow of.