JPH08281688A - Hollow injection molding of thermoplastic resin - Google Patents

Abstract

PURPOSE: To facilitate the hollow injection molding of a thermoplastic resin excellent in mold transfer properties, dimensional accuracy, surface appearance and productivity by forming a hollow molding in a cavity while extruding a molten thermoplastic resin into the cavity by a pressurized fluid. CONSTITUTION: The region from the sprue part of a mold to the runner part or gate part thereof is filled with the molten resin 7 injected into the mold by a molding machine and, after the filling with the resin 7, the injection of a pressurized fluid into the resin is started and a hollow part 8 is formed while the resin of the sprue part and the runner part is extruded into a cavity 2 to form a hollow molding in the cavity. Next, the pressure of the pressurized fluid injected into the resin 7 in the cavity 2 is kept for a definite time. Thereafter, the pressure of the pressurized fluid is released to take out the molding. By this constitution, the shrinkage of the resin 7 is suppressed and surface appearance is enhanced and mold transfer properties can be enhanced. Therefore, a product constituted of a plurality of parts is integrated to be enhanced in dimensional accuracy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for hollow injection molding of thermoplastic resin, which is particularly excellent in dimensional accuracy and surface appearance,
The present invention relates to a hollow injection molding method for a small molded product having no sink mark or warpage.

[0002]

2. Description of the Related Art The conventional hollow injection molding methods have been focused on relatively large molded products such as automobile bumpers and TV exterior parts for the purpose of reducing weight, improving rigidity, and reducing the number of parts. It has been developed and established. However, in recent years, in order to improve the dimensional accuracy of relatively small molded products such as gears and rollers, or to integrate the components, there is an increasing demand for hollow molding of these components.

As conventional hollow injection molding methods, the following methods have been generally performed. A hollow injection molding method in which a molten resin is injected into a mold cavity without leaving an unfilled portion, and then a pressurized fluid for forming a hollow portion in a molded product is injected into the resin. While the molten resin is being injected into the mold cavity, the injection of the pressurized fluid for forming the hollow portion inside the molded product is started into the resin, so that the resin and the pressurized fluid are simultaneously filled into the cavity. Hollow injection molding method. After injecting the molten resin so that the unfilled part remains in the mold cavity, the pressurized fluid for forming the hollow part in the molded product is injected into the resin, and this pressurized fluid is used to partially remove the resin. A hollow injection molding method in which a resin reaches a non-filled portion of a mold cavity while forming a hollow portion by extrusion.

However, in the above-mentioned method called "full shot method", since the hollow portion is obtained by the amount of the pressurized fluid that compensates for the molding shrinkage of the molten resin, the hollowness is almost the same as the molding shrinkage of the molten resin. It is a value, and the effect of pressurized fluid is small. Therefore, for the purpose of improving the effect of the pressurized fluid, a method of providing an auxiliary cavity (also referred to as an auxiliary chamber or a waste cavity) in the mold has been proposed. This auxiliary cavity is a cavity for allowing a part of the resin in the cavity to escape when the pressurized fluid is injected, and by adjusting the volume of this auxiliary cavity, it is possible to adjust the hollowness of the product (Japanese Patent Laid-Open No. Hei 10-1999) No. 3-121820).

In this method, the resin is injected so that an unfilled portion does not remain in the cavity, and then a part of the resin is extruded into the auxiliary cavity by the pressurized fluid to form a hollow portion. A molded product having an excellent appearance can be obtained. By this method, a product having an excellent surface appearance can be obtained if the shaft has a simple shape. However, in this method, the pressurized fluid selectively passes through the low pressure portion of the resin filled in the cavity to form a hollow portion. As a result, a hollow portion centered on the shortest distance is formed between the injection port and the auxiliary cavity. Therefore, it becomes difficult to obtain a hollow molded product having a uniform wall thickness as the target shape becomes complicated. For example, when a hollow molded product is obtained by this method from a component in which a gear and a shaft are integrated, a hollow portion is formed in the shaft portion, and the dimensional accuracy of the shaft portion is improved. However, it is difficult to form an effective hollow portion in the gear part in order to improve the dimensional accuracy, and the gear accuracy may be equal to or lower than that of a general injection molded product.

Further, by promoting the integration of parts, the product shape as a single part becomes complicated. Therefore,
Intricate mold structures are inevitable. As a result, it is often not possible to provide an auxiliary cavity in the mold. The above method is called a "simultaneous injection method", and is a method in which a molten resin and a pressurized fluid for forming a hollow portion inside a molded product are simultaneously injected into the cavity. However, the pressure of the molten resin injected into the cavity, it is common in the nozzle portion is 600~1000kg / cm ^2, pressure of the injected pressurized fluid body is 50~300kg / cm ² Is common, and it is said that it is preferably 50 to 200 kg / cm ² . After the resin is injected into the mold,
The pressure of the resin in the runner portion is lower than the pressure in the nozzle portion, but is often higher than the pressure of the pressurized fluid injected. Therefore, the pressurized fluid cannot form a hollow portion in the resin until the pressure of the resin in the cavity becomes lower than the pressure of the pressurized fluid. Therefore, when the injection pressure is equal to or higher than the pressurized fluid pressure, there is no great difference from the above method.

In practice, it is possible to keep the injection pressure of the resin low by taking measures such as slowing down the injection speed. Hollow injection molding by the simultaneous injection method is possible if the injection pressure is equal to or lower than the pressure of the pressurized fluid injected to form the hollow portion inside the molded product. However, when the resin passes through the sprue part and the gate part, at the injection speed set so that the injection pressure of the resin becomes equal to or lower than the pressure of the pressurized fluid, the inflow speed of the resin into the mold becomes slow and the sprue It becomes difficult for the resin, whose fluidity is lowered, to pass through the gate part due to the progress of cooling and solidification in the part and runner part. Further, it is necessary to delay the injection timing of the pressurized fluid by reducing the injection speed. This means that the pressurized fluid is injected into the resin while the resin injected into the mold is cooled and solidified. As a result, it can be said that it is difficult to form a hollow portion inside the molded product. Therefore, the molding conditions are not suitable for hollow injection molding of a thermoplastic resin having a high crystallization rate.

In the hollow injection molding method by the simultaneous injection method, it is conceivable that the pressurized fluid is provided with an injection port at the nozzle portion or the runner portion of the cylinder. The pressurized fluid injected into the resin from the pouring port passes through the gate and reaches the inside of the molded product. By this method, the occurrence of surface appearance defects called hesitation marks tends to be suppressed. However, it is difficult to completely suppress the generation of hesitation marks in hollow injection molding using a resin having a high crystallization rate. It is speculated that this is because when the resin passes through the gate portion, the resin flow path is narrowed and the resin pressure increases. Therefore, the pressurized fluid injected into the resin cannot pass through the gate portion, and there is a difference in the time for the resin and the pressurized fluid to reach the cavity. The pressure of the resin that has flowed into the cavity decreases, and part of the resin is cooled and solidified while the pressure on the cavity surface is insufficient, so the mold transfer property and surface appearance tend to be poor.

As a method of suppressing the generation of hesitation marks in hollow injection molding by the simultaneous injection method, it is conceivable to provide an injection port for pressurized fluid in the cavity. However, the product after the completion of the molding process has a defect that a trace of the pressurized fluid is left on the surface of the product in addition to the gate portion. The above-mentioned “short shot method” is a method in which a resin is injected so that an unfilled portion remains in a cavity, and then a pressurized fluid for forming a hollow portion in a molded product is injected into the resin and the addition is performed. It is characterized in that a hollow molded article is obtained by filling a resin in an unfilled portion while forming a hollow portion by a pressurized fluid.

In this method, the resin is cooled and solidified from the portion in contact with the mold cavity surface until the pressure of the resin injected into the cavity is reduced to the pressure of the pressurized fluid. From the time when the pressure becomes equal to or lower than the pressure of the pressurized fluid, the pressurized fluid forms the hollow portion in the resin and spreads the resin throughout the cavity. Therefore, a hesitation mark is generated at the boundary between the portion formed by the resin injection process and the portion formed by the pressurized fluid.
Moreover, the portion formed by the injection process of the resin tends to have poor mold transferability and surface appearance because cooling and solidification proceed with insufficient pressure bonding to the cavity surface.

[0011]

SUMMARY OF THE INVENTION The present invention provides a hollow injection molding method for a thermoplastic resin which is excellent in mold transferability, dimensional accuracy, surface appearance and productivity.

[0012]

The present invention is as follows. 1. In the hollow injection molding method of a thermoplastic resin, after filling the space from the nozzle to the runner section or the gate section with the thermoplastic resin, it was injected from any of the nozzle section of the molding machine, the sprue section of the mold, and the runner section. A method for hollow injection molding of a thermoplastic resin, characterized in that the molten thermoplastic resin is extruded into the cavity and a hollow molded article is formed in the cavity by a pressurized fluid for forming a hollow portion inside the molded article. . 2. Pressurized fluid injected to form a hollow part inside the molded product by connecting the runner part and cavity with the process of blocking the runner part and cavity and filling the thermoplastic resin into the sprue part and runner part. And a step of forming a hollow molded product in the cavity while extruding the thermoplastic resin in the sprue part and the runner part into the cavity, the hollow injection molding method of the thermoplastic resin according to the above item 1.

The present invention will be described in detail below. In the present invention, the "hollow injection molding method" refers to an injection molding method in which a thermoplastic resin is injected into a mold cavity in injection molding or a pressurized fluid is injected into the resin after completion of injection to obtain a hollow molded article. Is. In the present invention, the “cavity” refers to a portion of the depression provided on the parting surface of the mold, which is filled with resin to be a product after the completion of the molding process. In the present invention, the “hollow part” means a nest (void).
It does not mean a hollow portion formed by a foaming agent. In the hollow injection molding method, by supplying a pressurized gas into the resin, it is possible to bring the resin into close contact with the mold while compensating for the shrinkage of the resin, so that a molded product with good dimensional accuracy can be obtained. Guessed. Even in normal injection molding, it is possible to compensate the shrinkage of the resin by applying the injection secondary pressure (holding pressure), but since the resin is solidified at the gate after gate sealing, the effect of holding pressure is increased. Can not be expected.

In the hollow injection molding method, since pressure is applied from the inside of the molded product by the pressurized gas even after the gate sealing, it is presumed that the mold transfer property is improved. However, it is difficult to improve the dimensional accuracy of the entire molded product by simply injecting a pressurized fluid into the product. In the hollow injection molding method of the present invention, the resin injected into the mold fills the space between the sprue part and the runner part or the gate part, and the nozzle part of the molding machine, the sprue part of the mold, and the runner part. The hollow molded product is formed in the cavity while the resin is extruded into the cavity by the pressurized fluid for forming the hollow portion inside the molded product injected from any of the above.

In addition, the process of shutting off between the runner portion and the cavity and filling the resin into the sprue and the runner portion,
The process of forming a hollow molded product in the cavity while pushing the resin of the runner part into the cavity by the pressurized fluid injected to form the hollow part inside the molded product by communicating between the runner part and the cavity. It can also be implemented by having it. This is preferable because it is possible to prevent the resin from flowing into the cavity at the time of injecting the resin into the molding die by the molding machine by providing the step of blocking between the runner portion and the cavity.

A valve provided on the runner portion or the gate portion of the mold and having a function of arbitrarily switching between disconnection and communication between the runner portion and the cavity is called a "shut-off valve". This shut-off valve uses hydraulic pressure or the like as a drive source, and receives a signal from the molding machine to switch between shutoff and communication between the runner portion and the cavity. In the present invention, while the resin filling is filled from the gate portion to the flow end in the cavity, the resin filling proceeds continuously and at a uniform speed.
It is presumed that the generation of hesitation marks, which has been regarded as a problem in the prior art, is suppressed. The hesitation mark refers to an appearance defect that occurs at the boundary between the portion formed by the resin injection process and the portion formed by the pressurized fluid.

Further, in the process of filling the resin into the cavity, the resin is uniformly pressed onto the mold cavity surface by the pressurized fluid. Further, since the pressure of the pressurized fluid injected is lower than the secondary injection pressure (holding pressure) in general injection molding, it is possible to obtain a molded product with excellent dimensional accuracy without applying excessive stress to the molded product. It is possible to obtain a molded product with a small molding strain. In the hollow injection molding method of the present invention, the space from the sprue portion to the runner portion or the gate portion is filled by the injection process of the resin by the molding machine. Therefore, in the mold used in the hollow injection molding of the present invention, the total volume from the sprue portion to the runner portion or the gate portion is preferably a structure larger than the total volume of the cavity, and the sprue portion or the runner portion is preferable. It is preferable to provide a part where the cross-sectional area of the resin channel is enlarged and a part where the total volume of the sprue portion and the runner portion is enlarged, because the hollowness of the molded product can be adjusted.

In ordinary hollow injection molding, a nozzle for a molding machine, a sprue portion of a mold, a runner portion, a product portion and the like are provided with an inlet for pressurized fluid, and the pressurized fluid injected from the inlet is used. It is possible to form a hollow portion in the molded product. However, in the hollow injection molding method of the present invention, the process of filling the space between the sprue portion and the runner portion or the gate portion with the resin injected by the molding machine,
There is a step of pushing into the cavity by the pressurized fluid. At this time, it is preferable that the amount of resin pushed out into the cavity by the pressurized fluid is large, so that the inlet for the pressurized fluid is preferably provided at a position as far as possible from the gate portion.

FIG. 1 shows an example of a mold structure used when carrying out hollow injection molding according to the present invention. The mold shown in FIG. 1 has a structure in which a runner portion 4 is provided with an injection port 6 for pressurized fluid. FIG. 2 shows an explanatory view of an example of the hollow injection molding method when carrying out the present invention. The first step of the method is shown in FIG. By the molten resin injected into the mold by the molding machine, the mold sprue part 5 to the runner part 4 or the gate part 3
It is the process of filling up to.

The second step is shown in FIG. 2 (b). After the filling of the resin in the first step is completed, a pressurized fluid is started to be injected into the resin, and the hollow portion 8 is formed while pushing the resin of the sprue portion and the runner portion into the cavity, and the hollow molded product is formed in the cavity 2. It is a journey. The injection time of the pressurized fluid depends on the shape and size of the target hollow molded product, but is 0.1
It is preferably -60 seconds, more preferably 1-3.
0 seconds. The injection time of the pressurized fluid is the minimum time required to form the hollow portion in the resin of the sprue portion and the runner portion and push out so that no unfilled portion remains in the cavity.

The third step is shown in FIG. 2 (c). In the second step, the pressure of the pressurized fluid injected into the resin in the cavity 2 is held for a certain period of time. In the present invention, a hollow molded article is obtained by injecting a pressurized fluid into the resin in the cavity 2. However, until the resin in the cavity is sufficiently cooled, the pressurized gas injected into the resin Need to hold pressure. The pressure holding time of the pressurized fluid required to effectively form the hollow portion 8 is preferably 5 to 150 seconds, more preferably 10 to 60 seconds.

Further, the size of the molded product can be easily adjusted by adjusting the pressure of the injected pressurized gas and the pressure holding time of the pressurized fluid. After completion of the third step, the pressure of the pressurized fluid is released and the molded product is taken out. In the example of the mold structure shown in FIG. 3, a shut-off valve 9 that can arbitrarily shut off and switch communication between the runner portion 4 and the cavity 2 is provided.
On the runner 4. The shut-off valve 9 may be installed in any location as long as it is a section from the sprue section to the gate section, but it is preferably located near the gate section 3. This is because the hollow injection molding method according to the present invention has a process of pushing the resin filled between the sprue portion 5 to the runner portion 4 or the gate portion 3 into the cavity 2 by the pressurized fluid, This is because it is preferable to increase the resin filling amount on the nozzle side as much as possible.

Since the die shown in FIG. 3 adopts a method of injecting a pressurized fluid from the nozzle portion of the molding machine, the die body is not provided with an inlet for the pressurized fluid. FIG. 4 is an illustration of an example of hollow injection molding using the mold of FIG. The first stroke is shown in FIG. With the shut-off valve 9 blocking the runner portion 4 and the cavity 2, the sprue portion 5 up to the shut-off valve 9 and the runner portion 4 are filled with resin 7. The second step is shown in FIG. After the completion of the filling of the resin 7 in the first step, a pressurized fluid is started to be injected into the resin from the nozzle portion of the molding machine, and after the injection of the pressurized fluid is started, the shutoff valve 9 is operated so that the runner portion 4 and the cavity are By releasing the interruption between the two, the pressurized fluid pushes out the resin 7 of the sprue portion 5 and the runner portion 4 to form a hollow portion 8, and the hollow portion 8 is pushed into the cavity 2 to hollow-form the cavity 2. This is the process of forming a product.

The third step is shown in FIG. 4 (c). In the second step, the pressure of the pressurized fluid injected into the resin in the cavity is maintained for a certain period of time. After completion of the third step, the pressure of the pressurized fluid is released and the molded product is taken out. As the pressurized fluid used to obtain a hollow shape inside the molded article in the present invention,
A gas or liquid is used at room temperature. Further, those which do not react or be compatible with the molten resin used for molding under the temperature and pressure of injection molding are preferable. For example, nitrogen, carbon dioxide, air, helium, glycerin, liquid paraffin and the like can be mentioned. Usually pressurized gas is used,
In particular, it is preferable to use an inert gas such as nitrogen, helium, neon or argon. Further, although the use of these gas bodies usually contains impurities, if too much impurities are contained, the resin may be decomposed or burned during molding, which is not preferable. Considering economic efficiency, nitrogen gas is industrially preferable.

A general example of the hollow injection molding method using a pressurized gas will be further described below. Hollow injection molding using a pressurized gas is performed by a combination of an ordinary injection molding machine and a gas injection device. The gas injection device is a device that injects a gas into the resin through a pipe after starting the injection of the resin and holds the gas for a set time. For this, the gas to be injected is compressed to a high pressure in advance and stored in an accumulator, and a high pressure gas is introduced through a pipe when injecting the gas, or a certain amount of gas is measured and continuously fed by a pump for pressurization. However, any method is possible as long as a gas body is sent into the resin during injection and after injection. At this time, the gas inlet may be provided in the nozzle of the cylinder, the sprue of the mold, the runner, or the product section. Any method can be used as long as a high-pressure gas can be injected into the resin.

The thermoplastic resin used in the present invention includes polyethylene, polypropylene, polystyrene and ABS.
Resin, polyvinyl chloride, polyamide, polyacetal,
Polycarbonate, modified polyphenylene ether, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, polyimide, polyamideimide, polyetherimide, polyarylate, polysulfone, polyethersulfone, polyetherketone, liquid crystal polymer, polytetrafluoroethylene, thermoplastic elastomer However, any thermoplastic resin can be used as long as ordinary injection molding is possible.

In the present invention, the thermoplastic resin may contain a filler if necessary. Suitable fillers include glass fibers, carbon fibers, metal fibers, aramid fibers, calcium titanate, asbestos, silicon carbide, ceramics, silicon nitride, barium sulfate, calcium sulfate, kaolin, clay, pyrophyllite, bentonite, and seri. Site, zeolite, mica, mica, nepheline sinite, talc, atalpulgite, wollastonite, PMF, ferrite, calcium silicate, calcium carbonate, magnesium carbonate, dolomite, zinc oxide, titanium oxide, magnesium oxide, iron oxide, Reinforcing fillers such as molybdenum disulfide, graphite, gypsum, glass beads, glass powder, glass balloons, quartz and quartz glass can be mentioned, which may be hollow.
Two or more of these reinforcing fillers can be used in combination, and if necessary, they can be pretreated with a coupling agent such as a silane-based or titanium-based coupling agent before use.

The molded product obtained according to the present invention has a hollow portion, but the preferred hollow ratio is 10 to 90%, more preferably 20 to 80%. This has a hollow ratio of 90%
Hollow molded products that exceed the limit will be extremely thin.
It is not preferable because it causes a decrease in strength of the product. Extreme thinning of the product is preferable because the pressurized fluid injected to form the hollow part in the product may flow out of the product from the thin part, and a hollow molded product of the desired shape may not be obtained. Absent.
Further, in a hollow molded article having a hollowness rate of less than 10%, the effect of the pressurized gas is weakened, and improvement in dimensional accuracy, which is the object of the present invention, cannot be expected. The hollow ratio is defined by the following equation. Hollow rate (%) = {(V × ρ−M) / (V × ρ)} × 10
However, in the above formula, V is the volume when the hollow part is filled with the same resin, ρ is the specific gravity of the resin used, and M is the mass of the hollow molded product.

The hollow injection molding method according to the present invention includes mechanical parts such as gears, rollers, shafts, pulleys, rollers, cams, clutches, drums, trays, sliders, reels, and integrated parts combining a plurality of these parts. In addition, select lever, knob, buckle, door handle, shift knob, assist grip, chassis, tank body, tube, stopper, cap, pedal, joint, shower handle, chain, screw, connector, handle, switch, pen tip, etc. It can be applied to hollow injection molded products. The hollow injection molding method according to the present invention is effective for hollow molding using a crystalline resin. In addition, an additive for enhancing the function of the resin itself is particularly effective for hollow molding of a resin having an extremely high crystallization rate. Hereinafter, the present invention will be described in more detail with reference to examples.

[0030]

EXAMPLES When carrying out each example and comparative example, a polyacetal resin (manufactured by Asahi Kasei Corporation Tenac-C 4
Hollow injection molding was carried out using 520, Lynex-T TFC67N).

[0031]

Embodiments 1 and 2 Gear portion 10 and shaft portion 11 shown in FIG.
A mold capable of molding an integrally molded product including a, 11b, and a roller portion 12, and having a structure in which a pressurized gas injection port 6 is provided in the runner portion 4 as in the mold shown in FIG. I prepared. The cylinder temperature of the molding machine was set to 200 ° C and the mold temperature was set to 100 ° C. The gas injected to form the hollow portion in the product is nitrogen gas, and the pressure of the nitrogen gas is 100
It was set to kg / cm ² . 2 (a), (b), and (c) show the molding process when Examples 1 and 2 are carried out. The first step is Figure 2
As shown in (a), the molten resin injected into the mold by the molding machine causes the mold sprue part 5 to the runner part 4 to flow.
It is the process of filling up to. The second step is Figure 2 (b)
As shown in, the completion of resin filling in the first step 0.5
After a few seconds, a pressurized gas is started to be injected into the resin from the pressurized gas inlet 6 provided in the runner portion 4, and the resin in the sprue portion 5 and the runner portion 4 is pushed into the cavity 2 and the hollow portion 8
And forming a hollow molded product in the cavity 2. The pressure of the pressurized gas at this time is 10 as described above.
The pressure was 0 kg / cm ² and the injection time of the pressurized gas was 5 seconds. As shown in FIG. 2C, the third step is a step of maintaining the pressure of the pressurized fluid injected into the resin 7 in the cavity 2 for 40 seconds, as shown in FIG. 2C. After completion of the third step, the pressure of the pressurized fluid was released and the molded product was taken out.

In Example 1, polyacetal resin, Tenac-C 4520 manufactured by Asahi Kasei Co., Ltd.,
In Example 2, the same resin, the same company's Linex-
Hollow injection molding was carried out using T TFC67N. FIG. 6 shows a cross-sectional view of the obtained integrally molded product. The evaluation of the surface appearance of the obtained integrally molded product was based on the result of measuring the surface roughness of the roller portion 12 shown in FIG. The roller portion is a portion where a hesitation mark is generated during ordinary hollow molding. The measurement is carried out by measuring the roughness curve and cross-section curve specified in JIS (JIS B
0601). The measurement items are the center line average roughness (Ra) and the maximum height (Rmax). The length of the roller portion 12 was 30 mm, and the measurement range was 28 mm in the central portion. As a measuring instrument, a surface roughness profile measuring instrument Surfcom 575A manufactured by Tokyo Seimitsu Co., Ltd. was used. The measurement results are shown in Table 1.

[0033]

[Embodiments 3 and 4] Similar to Embodiments 1 and 2, a mold capable of molding a resin integrally molded product including the gear portion 10, the shaft portions 11a and 11b, and the roller portion 12 shown in FIG. Figure 3
A mold having a shut-off valve 9 that can arbitrarily shut off and switch communication between the runner portion 4 and the cavity 2 was prepared similarly to the mold shown in FIG. Since the pressurized gas was injected from the nozzle of the cylinder, the mold was not provided with a pressurized gas inlet. The temperature conditions and the type of pressurized gas were the same as in Example 1.

Example 3 is shown in FIGS. 4 (a), 4 (b) and 4 (c).
The explanatory view of the forming process at the time of carrying out 4 is shown. As shown in FIG. 4A, the first step is a step of filling the sprue to the shutoff portion and the runner portion with resin in a state where the runner portion and the cavity are shut off by the shutoff valve.
In the second step, as shown in FIG. 4 (b), 0.5 seconds after the completion of the resin filling in the first step, injection of pressurized gas into the resin from the nozzle of the molding machine is started. 0.5 seconds after the start of injection, the shutoff valve is driven to release the cutoff between the runner portion and the cavity, so that the pressurized fluid forms a hollow portion in the resin of the sprue portion and the runner portion,
This is a process of forming a hollow molded product in the cavity by extruding resin into the cavity. The pressure of the pressurized gas to be injected was 100 kg / cm ^2, and the injection time of the pressurized gas was 5 seconds. As shown in FIG. 4C, the third step is a step of holding the pressure of the pressurized fluid injected into the resin in the cavity by the second step for 40 seconds. After completion of the third step, the pressure of the pressurized fluid is released and the molded product is taken out. In Example 3, a polyacetal resin, Tenac-C 4520 manufactured by Asahi Kasei Kogyo Co., Ltd., and in Example 4, the same resin, Reinex-T TFC67N manufactured by the same company were used to perform hollow injection molding. The surface roughness of the obtained integrally molded product was measured, and the results are shown in Table 1.

[0035]

[Comparative Examples 1 and 2] In Comparative Examples 1 and 2, hollow molded articles were obtained by hollow injection molding by a normal short shot method, in which a nozzle of a molding machine was provided with an inlet for pressurized gas.
FIG. 7A shows the parting surface of the molding die used when carrying out Comparative Examples 1 and 2. 7 (b), (c),
The molding process for carrying out Comparative Examples 1 and 2 is shown in (d). The first step is as shown in FIG.
This is the process of injecting the molten resin so that the unfilled portion remains inside. As shown in FIG. 7C, the second step is a step of injecting a pressurized gas from the nozzle portion of the molding machine 0.5 seconds after the completion of the resin injection in the first step. The pressurized gas was injected for 5 seconds. As shown in FIG. 7D, the third step is a step of maintaining the pressure of the pressurized gas in the resin for 40 seconds after the injection of the pressurized gas in the second step is completed. After the completion of the third step, the gas pressure in the resin was released and a product was obtained. Molding conditions such as temperature conditions, types of pressurized gas, pressure and molding cycle were the same as in Example 1. In Comparative Example 1, a polyacetal resin, Tenac-C manufactured by Asahi Kasei Corporation
In Comparative Example 2, 4520 was subjected to hollow injection molding using the same resin, Reinex-T TFC67N manufactured by the same company. The surface roughness of the obtained integrally molded product was measured, and the results are shown in Table 1.

[0036]

[Comparative Examples 3 and 4] In Comparative Examples 3 and 4, hollow molds were obtained by performing hollow injection molding by a normal simultaneous injection method by providing the mold runner portion 4 with the injection port 6 for pressurized gas. FIG.
The parting surface of the molding die used when carrying out Comparative Examples 3 and 4 is shown in (a). 8 (b), (c), (d)
The molding process for carrying out Comparative Examples 3 and 4 is shown in FIG. The first step is the step of injecting the molten resin into the cavity 2 as shown in FIG. As shown in FIG. 8C, the second step is a step of injecting a pressurized gas from the injection port 6 provided in the die runner portion 4 during the resin injection in the first step. The pressurized gas was injected for 5 seconds. Figure 3 for the third step
As shown in (d), it is a step of maintaining the pressure of the pressurized gas in the resin for 40 seconds after the completion of the injection of the pressurized gas in the second step. After the completion of the third step, the gas pressure in the resin was released and a product was obtained. Molding conditions such as temperature conditions, types of pressurized gas, pressure and molding cycle were the same as in Example 1. Example 3
In Example 4, hollow injection molding was performed using a polyacetal resin, Tenac-C 4520 manufactured by Asahi Kasei Kogyo Co., Ltd., and in Example 4, the same resin, Linex-T TFC67N manufactured by the same company. The surface roughness of the obtained integrally molded product was measured, and the results are shown in Table 1.

[0037]

[Table 1]

[0038]

The hollow injection molding method according to the present invention is a hollow injection molding method capable of suppressing resin shrinkage, improving surface appearance, and improving mold transferability. Therefore, it is a hollow injection molding method that can improve the dimensional accuracy of the entire hollow molded product having a complicated shape by integrating the product that is composed of a plurality of parts, and industrially,
It can be said that this is a very useful hollow injection molding method.

[Brief description of drawings]

FIG. 1 shows an example of a parting surface of a molding die used in the present invention.

2 (a) to 2 (c) are explanatory views of a molding process when carrying out the present invention.

FIG. 3 shows an example of a parting surface of a molding die used in the present invention.

4 (a) to 4 (c) are explanatory views of a molding process when carrying out the present invention.

FIG. 5 is an external view of the integrally molded products obtained in Examples 1 to 4 and Comparative Examples 1 to 4.

FIG. 6 shows an example of a cross-sectional view including axes of the integrally molded products obtained in Examples 1 to 4 and Comparative Examples 1 to 4.

FIG. 7A shows a parting surface of a molding die when performing Comparative Examples 1 and 2. (B)-(d) shows the molding process when performing Comparative Examples 1 and 2.

FIG. 8A shows a parting surface of a molding die when performing Comparative Examples 3 and 4. (B)-(d) shows the molding process when performing Comparative Examples 3 and 4.

[Explanation of symbols]

 1 Parting surface 2 Cavity 3 Gate 4 Runner 5 Sprue 6 Pressurized gas injection port 7 Resin 8 Hollow part 9 Shut-off valve 10 Gear part 11a Shaft part 11b Shaft part 12 Roller part

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display area // B29L 22:00

Claims (2)

[Claims] 1. A hollow injection molding method for a thermoplastic resin, wherein after filling the space from the nozzle to the runner section or the gate section with the thermoplastic resin, any of the nozzle section of the molding machine, the sprue section of the mold, and the runner section. A thermoplastic resin characterized by forming a hollow molded product in the cavity while extruding the molten thermoplastic resin into the cavity by a pressurized fluid injected from the inside to form a hollow part inside the molded product. Hollow injection molding method. 2. A process of cutting off between the runner portion and the cavity and filling the thermoplastic resin into the sprue portion and the runner portion, and connecting between the runner portion and the cavity so as to form a hollow portion inside the molded product. And a step of forming a hollow molded product in the cavity while extruding the thermoplastic resin in the sprue portion and the runner portion into the cavity by the pressurized fluid. Molding method.