JP2022030088A - Injection-molded article and production method therefor, and hinged cap and tamper-evident cap - Google Patents

Abstract

To provide a production method for an injection-molded article that is reduced in weight by foaming and has a thin wall having sufficient strength.SOLUTION: The production method for an injection molded article according to the present disclosure, comprises: (A) a step for formulating a molten resin composition containing a resin material and a supercritical fluid; (B) a step for injecting the molten resin composition into the cavity of a mold; (C) a step for causing the molten resin composition to foam in the cavity by reducing the pressure; and (D) a step for recovering, from the mold, a molded article having a plurality of voids derived from foaming. The molded article has a thick wall having a thickness of 1.8-10 mm and a thin wall having a thickness of 0.3-1.7 mm.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to an injection molded product, a method for manufacturing the same, and a hinge cap and a tamper evidence cap.

In recent years, plastic molded products have been used in all everyday goods and industrial products around us. Demand for plastic molded products is increasing due to the progress of generalization by improving the quality of plastic molded products and reducing costs. On the other hand, as seen in the marine plastic waste problem, it has been noticed that microplastics affect environmental pollution, and there is a growing tendency to refrain from the deplasticization movement and the use of plastic products.

Regarding disposable plastic containers for food and daily necessities, there is a growing demand from users to reduce the amount of petroleum-derived plastic used as much as possible. In response to such demands, efforts are being made to use plant-derived resins as part of the raw materials, to utilize recycled plastic materials, and to reduce the amount of plastic used by devising dimensions and shapes.

Effervescent molding is known as a means for reducing the weight of plastic molded products. Foam molding can be roughly divided into chemical foam molding and physical foam molding. A foaming agent is used in chemical foam molding. On the other hand, in physical foam molding, a fluid in a supercritical state is used, and this method is called supercritical fluid molding. Chemical foam molding has problems such as concern about the adverse effect of the foaming agent on the environment and contamination of the mold. Supercritical fluid molding has traditionally been applied to relatively large industrial products such as automobile parts molding and office equipment. In recent years, with the improvement of supercritical fluid generation technology and kneading technology for resin compositions, it has been studied to apply supercritical fluid molding to high-cycle injection molding. Patent Documents 1 to 3 disclose food containers manufactured by supercritical fluid molding.

Japanese Patent No. 6085729 Japanese Patent No. 6430684 Japanese Unexamined Patent Publication No. 2020-040690

By the way, in addition to the food containers as described in Patent Documents 1 to 3, injection molding is also applied to the production of plastic molded products having a specific function. For example, a container for accommodating seasonings and the like is provided with a hinge cap that can be opened and closed. The hinge cap includes a cap main body, an upper lid, and a hinge portion connecting the cap main body and the upper lid (see FIG. 2). This hinge portion is thinner than the cap body and the upper lid. For example, when a hinge cap is manufactured by supercritical fluid molding, a gap is formed not only in the cap main body and the upper lid but also in the hinge portion, and there is a possibility that sufficient strength of the hinge portion cannot be ensured.

The present disclosure provides an injection-molded article having a thin wall portion which is reduced in weight by foaming and has sufficient strength, and a method for producing the same.

One aspect of the present disclosure relates to a method for manufacturing an injection molded product. This manufacturing method includes (A) a step of preparing a molten resin composition containing a resin material and a supercritical fluid, (B) a step of injecting the molten resin composition into a cavity of a mold, and (C). In the cavity, a step of foaming the molten resin composition by reducing the pressure and (D) a step of recovering the molded body having a plurality of voids derived from the foaming from the mold are included, and the molded body is 1.8. It includes a thick portion having a thickness of about 10 mm and containing a plurality of voids, and a thin portion having a thickness of 0.3 to 1.7 mm.

According to the study by the present inventors, when the injection-molded article to be manufactured has a thick part and a thin part, foaming due to the supercritical fluid mainly occurs in the thick part, while foaming is suppressed in the thin part. Will be done. Therefore, the weight of the injection-molded article as a whole can be reduced by foaming, and sufficient strength can be maintained for the thin-walled portion. When the molten resin composition is introduced into the cavity through the gate, it foams due to the decrease in pressure. In normal injection molding, a step of applying pressure to the inside of the cavity (holding pressure) is performed after filling the resin, whereas in the above manufacturing method, for example, by not performing this step of applying pressure, foaming occurs in a thick portion. Occurs. It is presumed that the main reason why foaming is suppressed in the thin-walled portion is that the resin is cooled and solidified before foaming occurs in the thin-walled portion.

In the above manufacturing method, it is preferable to select the type of supercritical fluid according to the resin material to be used and to set the injection speed in the step (B). For example, when the fat material is a polypropylene resin, the supercritical fluid is preferably carbon dioxide, and the injection speed in the step (B) is preferably 5 to 20 mm / sec.

According to the studies by the present inventors, from the viewpoint of suppressing foaming in the thin-walled portion, the distance from the gate of the mold to the portion corresponding to the thin-walled portion is preferably 40 mm or more in the mold. From the same viewpoint as this, in the mold, it is preferable that the portion different from the portion corresponding to the thin-walled portion is the flow end. That is, it is preferable that the flow end of the mold is not a portion corresponding to the thin wall portion.

One aspect of the present disclosure relates to an injection molded article. This injection-molded article includes a thick portion having a thickness of 1.8 to 10 mm and including a plurality of voids, and a thin portion having a thickness of 0.3 to 1.7 mm. This injection-molded article can be produced by the above-mentioned method for producing an injection-molded article. The voids contained in the thick portion are derived from the foaming of the supercritical fluid.

As described above, a hinge cap is mentioned as an example of an injection-molded article in which a thin-walled portion performs a specific function. The hinge cap includes a cap main body, an upper lid, and a hinge portion connecting the cap main body and the upper lid. The cap body and the upper lid correspond to the thick portion, and the hinge portion corresponds to the thin portion. Another example is the tamper evidence cap. The tamper evidence cap includes a cap, a tamper, and a connecting portion between the cap and the tamper. The cap and the tamper correspond to the thick portion, and the connecting portion corresponds to the thin portion.

According to the present disclosure, there is provided an injection-molded article having a thin wall portion which is reduced in weight by foaming and has sufficient strength, and a method for producing the same.

FIG. 1 is a cross-sectional view schematically showing a state in which foaming occurs in a region corresponding to a thick portion of a cavity, whereas foaming does not occur in a region corresponding to a thin wall portion. FIG. 2 is a cross-sectional view schematically showing an example of a hinge cap. FIG. 3A is a side view schematically showing an example (before opening) of the tamper evidence cap, and FIG. 3B schematically shows a state after opening the tamper evidence cap shown in FIG. 3A. It is a side view shown in.

Hereinafter, embodiments of the present disclosure will be described in detail. The present invention is not limited to the following embodiments.

<Manufacturing method of injection molded product>
The method for manufacturing an injection-molded article according to the present embodiment includes the following steps.
(A) A step of preparing a molten resin composition containing a resin material and a supercritical fluid.
(B) A step of injecting the molten resin composition into the cavity of the mold.
(C) A step of foaming the molten resin composition in the cavity by reducing the pressure.
(D) A step of recovering a molded product having a plurality of voids derived from foaming of a molten resin composition from a mold.
The molded product produced by this method includes a thick portion having a thickness of 1.8 to 10 mm and containing a plurality of voids, and a thin portion having a thickness of 0.3 to 1.7 mm. The thick portion may have a thickness of 1.8 to 3.0 mm or 3.0 to 7.0 mm. The thin portion may have a thickness of 0.3 to 0.6 mm or 0.7 to 1.0 mm. The thin-walled portion may contain voids derived from foaming of the molten resin composition as long as the strength does not affect the portion. The series of steps from (A) to (D) can be carried out using, for example, a MuCell injection molding machine (“MuCell” is a registered trademark of Trexel. Co. Ltd) (see Patent Documents 1 and 2).

[Step (A)]
First, a molten resin composition containing a resin material and a supercritical fluid is prepared. Examples of the resin material include polypropylene resin and polyethylene resin. Examples of supercritical fluids include carbon dioxide, nitrogen, argon and helium. According to the study by the present inventors, when the supercritical fluid is carbon dioxide, it is preferably 1.5 to 4.5 parts by mass, more preferably 3 to 4 parts by mass with respect to 100 parts by mass of the resin material. A molten resin composition is prepared by adding carbon dioxide in a critical state. When the carbon dioxide content of the molten resin composition is 1.5 parts by mass or more, it is easy to form a sufficiently uniform foam layer in the thick portion, and it tends to be possible to realize weight reduction by foaming. On the other hand, when the carbon dioxide content of the molten resin composition is 4.5 parts by mass or less, excessive foaming tends to be suppressed.

When the supercritical fluid is nitrogen, preferably 0.1 to 2.0 parts by mass, more preferably 0.5 to 1.0 part by mass of supercritical nitrogen is added to 100 parts by mass of the resin material. To prepare a molten resin composition. When the nitrogen content of the molten resin composition is 0.5 parts by mass or more, it is easy to form a sufficiently uniform foam layer in the thick portion, and it tends to be possible to realize weight reduction by foaming. On the other hand, when the nitrogen content of the molten resin composition is 2.0 parts by mass or less, excessive foaming tends to be suppressed.

When the resin material used is polypropylene resin, the temperature of the molten resin composition (screw cylinder temperature) is preferably about 210 to 230 ° C. When the resin material used is polyethylene resin, this temperature is preferably about 220 to 240 ° C. When this temperature is at least the lower limit value, the resin tends to flow easily in the cavity, while when it is at least the upper limit value, for example, the scorching of the resin tends to be suppressed.

The molten resin composition may contain components other than the resin material and the supercritical fluid. That is, the molten resin composition may further contain, for example, a filler, a colorant, a slip agent, an antistatic agent, and the like, if necessary.

[Step (B) and Step (C)]
(A) The molten resin composition prepared in the step is injected into the cavity through the gate of the mold. When the molten resin composition is introduced into the cavity, the bubble cells grow due to the decrease in pressure and independent bubbles are generated in the resin composition. The injection speed is preferably 5 to 100 mm / sec, more preferably 10 to 30 mm / sec. When the injection speed is 5 mm / sec or more, it is easy for the resin to reach the end of the flow, and the occurrence of short shots tends to be suppressed. On the other hand, when the injection speed is 100 mm / sec or less, it tends to be possible to suppress the formation of sparse air bubbles in the molded body and the formation of irregularities on the surface of the molded body. According to the studies by the present inventors, when the resin material is polypropylene resin and the supercritical fluid is carbon dioxide, the injection speed in the step (B) is preferably 5 to 30 mm / sec, preferably 10 to 20 mm / sec. More preferably, it is seconds.

FIG. 1 is a cross-sectional view schematically showing a state in which foaming occurs in the region R1 corresponding to the thick portion of the cavity C, whereas foaming does not occur in the region R2 corresponding to the thin wall portion. The cavity C is formed by the upper die D1 and the lower die D2. The average diameter of the voids in the thick portion is, for example, 100 to 600 μm. The size of the voids can be controlled by adjusting the injection amount, injection rate, resin temperature or pressure in the cavity of the molten resin composition. The average diameter of the void can be obtained from the image of the microscope.

In the present embodiment, from the viewpoint of sufficiently promoting foaming in the region corresponding to the thick portion of the cavity, a step (holding pressure) of applying pressure to the cavity after filling the cavity with the molten resin composition is carried out. You don't have to. On the other hand, from the viewpoint of suppressing foaming in the region corresponding to the thin portion of the cavity, it is preferable not to carry out a step called "core back" for reducing the pressure in the cavity. The core back is a step of moving the movable part of the mold to expand the volume of the cabidi before the molten resin filled in the cabidi is solidified (see Patent Document 1). In the present embodiment, as described above, the pressure drop accompanying the introduction of the molten resin composition into the cavity can cause foaming in the region corresponding to the thick portion in the cavity, while the thin portion can be foamed. Foaming in the corresponding region can be suppressed.

From the viewpoint of further suppressing foaming in the thin-walled portion, the distance from the gate of the mold to the portion corresponding to the thin-walled portion is preferably 40 mm or more, more preferably 40 to 60 mm in the mold. From the same viewpoint as this, in the mold, it is preferable that the portion different from the portion corresponding to the thin-walled portion is the flow end. That is, it is preferable that the flow end of the mold is not a portion corresponding to the thin wall portion.

[Step (D)]
When the temperature of the molded product drops to about 30 to 60 ° C., the molded product is recovered from the mold. By forming voids derived from foaming in the thick portion, weight reduction is achieved and the amount of plastic material used is reduced. Since the voids are formed, it is preferable that the weight is reduced by 5% by mass or more as compared with the molded article (ordinary injection molded article) in which the voids are not formed. Further, it is preferable that no appearance defects such as minute irregularities (dimples) and foam marks (swirl marks) are observed on the surface of the molded product.

<Molded body>
Examples of the molded product produced by the above method include a hinge cap and a tamper evidence cap. FIG. 2 is a cross-sectional view schematically showing an example of a hinge cap. The hinge cap 5 shown in this figure includes a cap main body 1, an upper lid 2, and a hinge portion 3 connecting the cap main body 1 and the upper lid 2. The cap body 1 and the upper lid 2 correspond to the thick portion, and the hinge portion 3 corresponds to the thin portion.

FIG. 3A is a side view schematically showing an example (before opening) of the tamper evidence cap, and FIG. 3B schematically shows a state after opening the tamper evidence cap shown in FIG. 3A. It is a side view shown in. The tamper evidence cap 10 shown in these figures includes a cap 6, a tamper 7, and a connecting portion 8 of the cap 6 and the tamper 7. The cap 6 and the tamper 7 correspond to the thick portion, and the connecting portion 8 corresponds to the thin portion. For example, when the cap 6 is rotated to open the PET bottle, the connecting portion 8 is cut and the cap 6 is separated from the tamper 7 (see FIG. 3 (b)).

Hereinafter, the present disclosure will be described with reference to Examples and Comparative Examples. The present invention is not limited to the following examples.

(Comparative example)
A hinge cap was manufactured by ordinary injection molding using the following resin materials.
[Resin material]
-Polypropylene (manufactured by SunAllomer Ltd., PM940M, random copolymer, melt flow rate: 30 g / 10 minutes)
[Hinge cap configuration]
・ Thickness of cap body: 1.0 to 1.4 mm
・ Thickness of top lid: 1.7 to 3.2 mm
-Hinge thickness: 0.1-0.5 mm
-Weight: 12.6 g

(Example 1)
A molten resin composition was prepared by adding 3 parts by mass of carbon dioxide in a supercritical state to 100 parts by mass of the resin material. Using this molten resin composition, a hinge cap having the same shape as that of the comparative example was produced. The injection speed was 10 mm / sec. The weight of the hinge cap according to Example 1 was 11.8 g, and a weight reduction rate of 6.3% was achieved based on the comparative example. In this example and the following examples, a MuCell injection molding machine (“MuCell” is a registered trademark of Trexel.Co.Ltd) was used.

When the inside of the hinge portion of the hinge cap according to Example 1 was confirmed by a cross-sectional image (magnification: 50 times) by CT scan, no foaming was observed. On the other hand, when the inside of the thick portion of the hinge cap was confirmed by a cross-sectional image by CT scan, a uniform void was widened. Further, although a slight swirl mark was observed on the surface of the molded product, the appearance was good.

(Example 2)
A molten resin composition was prepared by adding 1.5 parts by mass of carbon dioxide in a supercritical state to 100 parts by mass of the resin material. Using this molten resin composition, a hinge cap having the same shape as that of the comparative example was produced. The injection speed was 10 mm / sec. The weight of the hinge cap according to Example 2 was 11.6 g, and a weight reduction rate of 8.5% was achieved based on the comparative example.

When the inside of the hinge portion of the hinge cap according to Example 2 was confirmed by a cross-sectional image (magnification: 50 times) by CT scan, no foaming was observed. On the other hand, when the inside of the thick portion of the hinge cap was confirmed by a cross-sectional image by CT scan, the voids were unevenly expanded as compared with Example 1. In addition, dents were observed on the surface of the molded product.

(Example 3)
A molten resin composition was prepared by adding 1.0 part by mass of nitrogen in a supercritical state to 100 parts by mass of the resin material. Using this molten resin composition, a hinge cap having the same shape as that of the comparative example was produced. The injection speed was 10 mm / sec. The weight of the hinge cap according to the second embodiment was 11.7 g, and a weight reduction rate of 7.4% was achieved based on the comparative example.

When the inside of the hinge portion of the hinge cap according to Example 3 was confirmed by a cross-sectional image (magnification: 50 times) by CT scan, no foaming was observed. On the other hand, when the inside of the thick portion of the hinge cap was confirmed by a cross-sectional image by CT scan, a uniform void was widened. However, a linear swirl mark was observed on the surface of the molded product.

(Example 4)
A molten resin composition was prepared by adding 0.5 parts by mass of nitrogen in a supercritical state to 100 parts by mass of the resin material. Using this molten resin composition, a hinge cap having the same shape as that of the comparative example was produced. The injection speed was 30 mm / sec. The weight of the hinge cap according to Example 4 was 11.9 g, and a weight reduction rate of 5.6% was achieved based on the comparative example.

When the inside of the hinge portion of the hinge cap according to Example 4 was confirmed by a cross-sectional image (magnification: 50 times) by CT scan, no foaming was observed. On the other hand, when the inside of the thick portion of the hinge cap was confirmed by a cross-sectional image by CT scan, the voids were sparsely expanded as compared with Example 1. In addition, dents were observed on the surface of the molded product.

1 ... Cap body (thick part), 2 ... Top lid (thick part), 3 ... Hinge part (thin wall part), 5 ... Hinge cap (molded body), 6 ... Cap (thick part), 7 ... Tamper ( Thick part), 8 ... Connecting part (thin wall part), 10 ... Tamper evidence cap (molded body), C ... Cavity, D1 ... Upper mold, D2 ... Lower mold, R1 ... Region corresponding to thick part, R2 ... Area corresponding to thin-walled parts

Claims (9)

(A) A step of preparing a molten resin composition containing a resin material and a supercritical fluid, and
(B) A step of injecting the molten resin composition into the cavity of the mold, and
(C) A step of foaming the molten resin composition by reducing the pressure in the cavity.
(D) A step of recovering a molded product having a plurality of voids derived from the foam from the mold.
Including
Manufacture of an injection-molded article, wherein the molded article has a thick portion having a thickness of 1.8 to 10 mm and containing a plurality of voids, and a thin-walled portion having a thickness of 0.3 to 1.7 mm. Method. The molded body is a hinge cap including a cap main body, an upper lid, and a hinge portion connecting the cap main body and the upper lid.
The cap body and the upper lid are the thick portions.
The method for manufacturing an injection-molded article according to claim 1, wherein the hinge portion is the thin-walled portion. The molded product is a tamper evidence cap including a cap, a tamper, and a connecting portion between the cap and the tamper.
The cap and the tamper are the thick portions, and the cap and the tamper are the thick portions.
The method for manufacturing an injection-molded article according to claim 1, wherein the connecting portion is the thin-walled portion. The method for manufacturing an injection molded product according to any one of claims 1 to 3, wherein in the mold, the distance from the gate of the mold to the portion corresponding to the thin wall portion is 40 mm or more. The method for producing an injection-molded article according to any one of claims 1 to 4, wherein in the mold, a portion different from the portion corresponding to the thin-walled portion is a flow end. The resin material is polypropylene resin,
The supercritical fluid is carbon dioxide,
(B) The method for producing an injection-molded article according to any one of claims 1 to 5, wherein the injection speed in the step is 5 to 30 mm / sec. A thick portion having a thickness of 1.8 to 10 mm and containing a plurality of voids,
A thin part with a thickness of 0.3 to 1.7 mm and a thin part
Injection molded body. With the cap body,
With the top lid
A hinge portion connecting the cap body and the upper lid,
Equipped with
The cap body and the top lid have a thickness of 1.8 to 10 mm and contain a plurality of voids.
A hinge cap having a hinge portion having a thickness of 0.3 to 1.7 mm. With a cap,
With a tamper,
The connecting portion between the cap and the tamper,
Equipped with
The cap and the tamper have a thickness of 1.8-10 mm and contain a plurality of voids.
A tamper evidence cap having a connecting portion having a thickness of 0.3 to 1.7 mm.

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