Injection molded body and manufacturing method thereof

Abstract

To provide a method for producing an injection-molded article that is reduced in weight by foaming and is kept opaque even with no pigment used or with the amount of use of pigment significantly reduced.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. In the molded article, an area to have voids formed therein has a thickness of 0.7 mm or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an injection molded article and a method for producing the same. [Background technology]

[0002] In recent years, plastic molded products are used in all kinds of daily necessities and industrial products around us. The demand for plastic molded products is increasing as their quality improves and costs go down, making them more versatile. On the other hand, attention has been focused on the impact of microplastics on the environment, as seen in the problem of marine plastic waste, and there is a growing trend to move away from plastic and to refrain from using plastic products.

[0003] Regarding disposable plastic containers for food and daily necessities, there is a growing demand from users to reduce the amount of petroleum-based plastic used. In response to such demands, efforts are being made to reduce the amount of plastic used by using plant-based resins as part of the raw materials, utilizing recycled plastic materials, and improving dimensions and shapes.

[0004] Foam molding is known as a means for reducing the weight of plastic molded products. Foam molding can be broadly divided into chemical foam molding and physical foam molding. Chemical foam molding uses a foaming agent, while physical foam molding uses a fluid in a supercritical state (hereinafter referred to as "supercritical fluid molding"). Chemical foam molding has concerns about the negative impact of the foaming agent on the environment and issues such as mold contamination. Supercritical fluid molding does not have such concerns. Supercritical fluid molding has traditionally been applied to relatively large industrial products such as automobile part molding and office equipment. In recent years, with the improvement of supercritical fluid generation technology and resin composition kneading technology, the application of supercritical fluid molding to high-cycle injection molding has been considered. Patent Documents 1 to 3 disclose food containers manufactured by supercritical fluid molding. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6085729 [Patent Document 2] Patent No. 6430684 [Patent Document 3] JP 2020-040690 A Summary of the Invention [Problem to be solved by the invention]

[0006] In order to broaden the scope of application of supercritical fluid molding, the present inventors carried out supercritical fluid molding using various molds. As a result, they discovered that the voids formed inside the molded body make the molded body white. This is presumably due to the diffuse reflection effect of light caused by the voids. If it were possible to uniformly disperse voids of uniform size in the region to be foamed, it would be possible to obtain a molded body that is white throughout and has no unevenness, even without the use of pigments.

[0007] The present disclosure provides an injection-molded article that is lightweight due to foaming and is opaque even without the use of pigments or even if a sufficiently small amount of pigment is used, and a method for producing the same. [Means for solving the problem]

[0008] One aspect of the present disclosure relates to a method for producing an injection-molded article, which includes the steps of (A) preparing a molten resin composition containing a resin material and a supercritical fluid, (B) injecting the molten resin composition into a cavity of a mold, (C) reducing the pressure in the cavity filled with the molten resin composition, and (D) recovering a molded article having a plurality of voids derived from the supercritical fluid from the mold, wherein the thickness of the region in which the voids are to be formed in the molded article is 0.7 mm or less.

[0009] According to the study by the present inventors, the size of the voids depends on the thickness of the injection molded article. That is, large voids are formed in the thick regions of the injection molded article, and small voids are formed in the thin regions. By making the thickness of the region where voids should be formed 0.7 mm or less, voids with an average diameter of, for example, 1 to 50 μm can be stably formed in this region. The smaller the voids of this size, the higher the diffuse reflection effect of light, so that this region can be brightly whitened evenly. The content of the pigment in the molten resin composition prepared in the step (A) may be, for example, 2 mass % or less based on the mass of the molten resin composition.

[0010] In the above manufacturing method, it is preferable to select the type of supercritical fluid according to the resin material used and set the injection speed in step (B). For example, when the resin material is a polyethylene resin, the supercritical fluid is preferably nitrogen, and the injection speed in step (B) is preferably 150 to 200 mm / sec.

[0011] One aspect of the present disclosure relates to an injection molded body. The injection molded body has a region containing a plurality of voids derived from a supercritical fluid, and the thickness of the region is 0.7 mm or less. The injection molded body can be manufactured by the above-mentioned manufacturing method for an injection molded body. The injection molded body is lightweight due to foaming, and is opaque even without using a pigment or even if the amount of pigment used is sufficiently small. The injection molded body is, for example, a container having a substantially cylindrical side portion. In this container, for example, at least the side portion corresponds to the region containing a plurality of voids derived from a supercritical fluid.

[0012] It is preferable that the thickness of the region in the injection molded article where the voids are to be formed is sufficiently uniform. Specifically, the ratio B / A of the thickness B of the thickest part of the region to the thickness A of the thinnest part of the region is preferably 1 to 1.5. With this configuration, the region can be whitened more evenly and vividly. Effect of the Invention

[0013] According to the present disclosure, there is provided an injection-molded article that is lightweight due to foaming and is opaque even without using a pigment or using a sufficiently small amount of pigment, and a method for producing the same. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a cross-sectional view showing a schematic state in which bubbles are sufficiently uniformly generated within the cavity. [Diagram 2] FIG. 2 is a perspective view showing a container produced in a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present invention is not limited to the following embodiments.

[0016] <Manufacturing method of injection molded article> The method for producing an injection molded article according to this 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 a molten resin composition into a cavity of a mold. (C) A step of foaming the molten resin composition in the cavity by reducing the pressure. (D) A step of recovering from the mold a molded article having a plurality of voids resulting from the foaming of the molten resin composition. In the molded article produced by this method, the thickness of the region where the voids are to be formed is 0.7 mm or less, and may be 0.3 to 0.7 mm or 0.5 to 0.65 mm. Note that the series of steps (A) to (D) can be carried out, for example, using a MuCell injection molding machine ("MuCell" is a registered trademark of Trexel Co. Ltd.) (see Patent Documents 1 and 2).

[0017] [(A) Process] 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 the supercritical fluid include carbon dioxide, nitrogen, argon, and helium. According to the study by the present inventors, when the supercritical fluid is carbon dioxide, the molten resin composition is prepared by adding preferably 1 to 4 parts by mass, more preferably 2 to 3 parts by mass of carbon dioxide in a supercritical state to 100 parts by mass of the resin material. When the carbon dioxide content of the molten resin composition is 1 part by mass or more, it is easy to form a sufficiently uniform foam layer over the entire molded body, and weight reduction by foaming tends to be realized. On the other hand, when the carbon dioxide content of the molten resin composition is 4 parts by mass or less, excessive foaming tends to be suppressed.

[0018] When the supercritical fluid is nitrogen, the molten resin composition is prepared by adding preferably 0.1 to 1.2 parts by mass, more preferably 0.3 to 0.7 parts by mass of supercritical nitrogen to 100 parts by mass of the resin material. 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 foamed layer over the entire molded body, and weight reduction by foaming tends to be realized. On the other hand, when the nitrogen content of the molten resin composition is 1.0 part by mass or less, excessive foaming tends to be suppressed.

[0019] When the resin material used is a 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 a polyethylene resin, this temperature is preferably about 220 to 240° C. When this temperature is equal to or higher than the lower limit, the resin easily flows in the cavity, while when the temperature is equal to or lower than the lower limit, for example, scorching of the resin tends to be suppressed.

[0020] 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, an antistatic agent, an antioxidant, etc., as necessary. According to this embodiment, since the molded body can be made white by the voids derived from foaming, the molten resin composition may contain a white pigment of 2 mass% or less based on the mass of the molten resin composition, or may not contain a white pigment.

[0021] [(B) process and (C) process] The molten resin composition prepared in the (A) step is injected into the cavity through the gate of the mold. When the molten resin composition is introduced into the cavity, the pressure is reduced, and the cells grow, causing independent bubbles to form in the resin composition. The injection speed is preferably 100 to 300 mm / sec, and more preferably 150 to 200 mm / sec. When the injection speed is 100 mm / sec or more, the resin can easily 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 300 mm / sec or less, the occurrence of burr defects in the molded product tends to be suppressed. According to the study by the present inventors, when the resin material is a polyethylene resin and the supercritical fluid is nitrogen, it is preferable to set the injection speed in the (B) step in multiple stages. That is, it is more preferable to set the initial injection speed in the (B) step to 250 to 300 mm / sec, and to reduce the second-stage injection speed to 150 to 200 mm / sec.

[0022] 1 is a cross-sectional view showing a state in which a large number of voids 2 are uniformly generated in a resin material 1 in a cavity C for producing a thin molded body (with a thickness of about 0.4 to 0.7 mm). The cavity C is formed by an upper mold D1 and a lower mold D2.

[0023] In this embodiment, from the viewpoint of sufficiently promoting foaming in the cavity, it is not necessary to carry out a step of applying pressure to the cavity (pressure holding) after filling the cavity with the molten resin composition. In this embodiment, from the viewpoint of obtaining a sufficiently thin molded body, it is preferable not to carry out a step called "core back" for reducing the pressure in the cavity. Core back is a step of moving the movable part of the mold to expand the volume of the cavity before the molten resin filled in the cavity finishes solidifying (see Patent Document 1). In this embodiment, as described above, foaming can be caused in the cavity by the pressure reduction caused by the introduction of the molten resin composition into the cavity.

[0024] [(D) Process] When the temperature of the molded body drops to about 30 to 60°C, the molded body is removed from the mold. Since voids resulting from foaming are formed in the molded body, the weight is reduced and the amount of plastic material used is reduced. Since voids are formed, it is preferable that the weight of the molded body is reduced by 3% by mass or more compared to a molded body without voids (a normal injection molded body). In addition, it is preferable that the surface of the molded body is free of appearance defects such as minute sink marks or foaming marks (swirl marks).

[0025] <Molded body> The molded article produced by the above method has a region containing a plurality of voids derived from the supercritical fluid, and the thickness of the region is 0.7 mm or less, and may be 0.3 to 0.7 mm or 0.5 to 0.65 mm. Voids having an average diameter of 1 to 50 μm are stably formed in the region of the molded article having a thickness of 0.7 mm or less. Since voids of this size have a high diffuse reflection effect of light, the region can be uniformly and vividly whitened. The size of the voids can be controlled by adjusting the injection amount, injection speed, resin temperature, or pressure in the cavity of the molten resin composition. The average diameter of the voids can be obtained from a microscope image. When the molded article is white, it tends to be difficult to observe the voids with a microscope, so it is preferable to observe a colored molded article with a microscope.

[0026] In the molded product, the thickness of the region where the voids are to be formed is preferably sufficiently uniform. Specifically, the ratio B / A of the thickness B of the thickest part of the region to the thickness A of the thinnest part of the region is preferably 1 to 1.5, and may be 1.2 to 1.6 or 1.5 to 2.3. With this configuration, the region can be whitened more evenly and vividly. An example of the molded product according to this embodiment is a container having a substantially circular opening. Such a container has, for example, a bottom and a substantially cylindrical side portion extending in a direction away from the bottom. In this container, the voids derived from foaming may be formed in at least one of the bottom portion and the side portion. EXAMPLES

[0027] The present disclosure will be described below based on examples and comparative examples. Note that the present invention is not limited to the following examples.

[0028] (Comparative Example) The following resin material was used to manufacture a container having the shape shown in Fig. 2 by normal injection molding. Since no white pigment was mixed into the resin material, the container according to the comparative example was translucent. [Resin material] Polyethylene (Japan Polyethylene Co., Ltd., HJ590, HDPE, melt flow rate: 40g / 10min) [Container configuration] -Thickness at thinnest part: 0.53mm Thickness at thickest part: 0.65mm ·Weight: 10.2g

[0029] Example 1 A molten resin composition was prepared by adding 0.5 parts by mass of supercritical nitrogen to 100 parts by mass of the resin material. This molten resin composition was used to manufacture a container of the same shape as in the comparative example. The injection speed was 200 mm / sec. The weight of the container in Example 1 was 9.8 g, achieving a weight reduction rate of 4.1% 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. As in the comparative example, no white pigment was added to the resin material, but the entire container was able to be made white due to fine voids caused by foaming. Although slight swirl marks were observed on the surface of the molded body, the appearance was good.

[0030] Example 2 A molten resin composition was prepared by adding 0.3 parts by mass of supercritical nitrogen to 100 parts by mass of the resin material. This molten resin composition was used to manufacture a container of the same shape as in the comparative example. The injection speed was 110 mm / sec. The weight of the hinge cap according to Example 2 was 10.1 g, and a weight reduction rate of 1.0% was achieved based on the comparative example. As in the comparative example, no white pigment was mixed into the resin material, but the entire container could be made white due to fine voids caused by foaming. However, compared to Example 1, there were unevenness in the white color. [Explanation of symbols]

[0031] 1...resin material, 2...space, 10...molded body, C...cavity, D1...upper die, D2...lower die

Claims

1. (A) preparing a molten resin composition containing a resin material and nitrogen in a supercritical state; (B) injecting the molten resin composition into a cavity of a mold; (C) foaming the molten resin composition by reducing the pressure caused by introducing the molten resin composition into the cavity; (D) recovering the molded body having a plurality of voids resulting from the foaming from the mold; Including, the thickness of the region including the plurality of voids derived from the nitrogen in the molded body is 0.7 mm or less; (B) The injection speed in step is 150 to 200 mm / sec; A method for producing an injection molded article, wherein a ratio B / A of a thickness B of the thickest part of the region to a thickness A of the thinnest part of the region is 1.2 to 2.

3.

2. The method for producing an injection molded article according to claim 1, wherein the average diameter of the voids is 1 to 50 μm.

3. The method for producing an injection molded article according to claim 1 or 2, wherein the content of the pigment in the molten resin composition is 2 mass % or less based on the mass of the molten resin composition.

4. The method for producing an injection molded article according to any one of claims 1 to 3, wherein the resin material is a polyethylene resin.

5. The method for producing an injection molded article according to any one of claims 1 to 4, wherein the molded article is a container having a substantially cylindrical side portion.

6. An injection molded article by supercritical fluid molding using nitrogen as a supercritical fluid, A region including a plurality of voids derived from the nitrogen, The thickness of the region is 0.7 mm or less; The injection molded article has a ratio B / A of a thickness B of the thickest part of the region to a thickness A of the thinnest part of the region, which is 1.2 to 2.

3.

7. The injection molded article according to claim 6, wherein the thickness of the region is 0.3 to 0.7 mm.

8. 8. The injection molded article according to claim 6, wherein the pigment content is 2% by mass or less.

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