KR20230002202A - Plastic injection molding method - Google Patents

Abstract

A method comprises: a filling step of completely filling a molten foamable plastic composition into a cavity by reducing the volume of the cavity during or immediately after injection; a cooling step of cooling the composition to a state where a solidified layer in contact with a mold surface is mixed with the internal melted layer; an expansion step of expanding the volume of the cavity to the volume of a desired molded product; and a taking out step of taking out the molded product after further cooling. The molded product includes a non-foaming or low-foaming layer having a dense structure on the surface and an internal highly foaming layer, and is lightweight and has a good appearance and rigidity.

Description

Plastic injection molding method {Plastic injection molding method}

The present invention relates to a method for injection molding of a foamable plastic composition. In particular, the present invention relates to a method for injection molding a molded plastic foam product which is lightweight and has good thermal insulation and rigidity, wherein the surface of the molded product includes a non-foaming or low-foaming layer having a dense structure, and the inside of the molded product is It contains a fine and uniform highly foamed layer.

Regarding the injection molding method of foam plastic, Japanese Patent Laid-Open No. 62-246710 discloses an injection molding method, wherein a mold to be coupled to an ejection port of an injection molding machine consists of a fixed mold part and a movable mold part, and between them The formed cavity can be enlarged and contracted by moving the movable mold part, and before the foaming agent is injected into the mold from the injection molding machine, the cavity is enlarged to a predetermined size by retracting the movable mold part.

In addition, Japanese Patent Laid-Open No. 4-214311 discloses an injection compression molding method, wherein a mold relatively moving in a direction of expanding or reducing the cavity volume is positioned at a predetermined cavity volume reduction position, and the cavity is filled with resin. While injecting the expandable molten resin into the cavity by maintaining pressure to prevent foaming, it is moved in the direction of expanding the cavity volume, the surface of the resin is cooled for solidification, and foaming by moving the mold again in the direction of expanding the cavity volume. is lowered, causing the resin in the cavity to foam, and after cooling, the molded product is taken out.

However, the conventional method has a problem in that a molded product with good appearance is not produced. The former method causes serious problems as follows. Since the cavity volume expands simultaneously with injection, cell rupture occurs in the surface portion of the molded product, so that the surface has no or low foaming, and a dense cell structure is not obtained. When using a mold having a thin-walled cavity structure away from the entrance, it is difficult to completely fill the mold. The latter method causes serious problems as follows. In the case of using molds with thin-walled cavities away from entrances (e.g., molds for molded products with large flat areas, ribs and bosses, etc.) and conventional molding machines, it is necessary to prevent foaming of the foamed plastic. It is difficult to maintain pressure for When injection is performed at high speed and high pressure using a large molding machine, or when a back pressure method is used together, the foamable plastic can be held at a pressure that prevents foaming. However, it is economically problematic.

In addition, the conventional method, in the case of manufacturing a molded product having a wide plane area and thin-walled ribs and walls or bosses at the end, the resin cannot be completely filled in the end of the molded product (so-called "short"). It has a problem that a molded article having a short shot") dense surface and a good appearance cannot be produced and defects occur.

This method causes the following serious problems. In either of the two conventional methods, the cavity is enlarged simultaneously with or during injection. That is, the volume of the cavity to be filled is increased during resin supply. When the above operation is performed in a cavity having a large planar area, there may be cases where the supply rate of the resin does not follow the rate of increase in the volume of the cavity. In this state, it is very difficult to keep the pressure of the resin to be supplied into the cavity constant, and the initiation of foaming of the resin cannot be prevented. A resin supply pressure of more than a predetermined value inside the above-described cavity is required to fill the end portion, and short shots occur. Even if the cavity is expanded and then compressed again, the foamability of most resins is already completed simultaneously. Therefore, in the step of compressing the cavity, the air bubbles generated by foaming are simply compressed, so that the fluid filling effect of the resin to the ends cannot be expected.

For the same reason as above, since the decrease in the pressure of the resin in the cavity proceeds to foaming in the middle of supplying the resin, the bubbles generated by the foaming continuously burst at the front part of the supplied resin, leaving traces on the surface of the molded product, thereby forming a good quality. It is not possible to produce molded articles with external appearance.

To prevent such a decrease in pressure, a back pressure method has been used together, in which compressed gas is pre-sealed into the mold to keep the pressure constant at the front of the flowing resin. Even in the above case, there is a phenomenon in which the flow of the resin stops or recedes in the plane region of the cavity in relation to the rate of increase of the cavity volume and the supply rate of the resin, so that traces or marks are formed on the surface of the molded product, making the appearance of the product bad. .

In order to eliminate the drawbacks of the above conventional method, the shape of the molded product must be limited to a rod shape and a column shape or a shape having sufficient thickness in the thickness direction with respect to the projected area of the flat portion. That is, the manufacture of a good molded article by a method comprising a step of enlarging the cavity simultaneously with or during injection is subject to restrictions in which the shape of the molded article is limited to a narrow range.

Accordingly, the present invention has been made in view of the deficiencies of the prior art, and an object of the present invention is to provide an injection molding method for producing a molded plastic foam product that is light in weight and has good appearance and rigidity, the surface of which is free. It has a foamed or low-foamed and dense structure, and the inside of the molded product is located away from the entrance.

A high foam layer is incorporated by a conventional injection molding machine using a mold having a thin-walled cavity structure.

The foamable plastic composition used in the present invention includes a thermoplastic and a foaming agent. Thermoplastics include polystyrene, polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, ABS resin, methacrylate resin, polyvinyl alcohol and the like. The blowing agent includes inorganic compounds such as ammonium carbonate and sodium bicarbonate and organic compounds such as azo compounds, sulfohydrazide compounds, nitroso compounds and azide compounds. Azo compounds include azodicarbonamide (ADCA), 2,2'-azobisisobutyronitrile, azohexahydrobenzonitrile, diazoaminobenzene and the like. Sulfohydrazide compounds include benzene sulfonyl hydrazide, benzene-1,3-disulfonyl hydrazide, diphenylsulfone-3,3'-disulfonyl hydrazide, diphenyl oxide 4,4'-disulfonyl hydrazide, and the like. include Nitroso compounds include N,N'-dinitroso-pentamethylenetetramine (DNPT), N,N'-dimethyl terephthalate, and the like. Azide compounds include terephthalazide, p-tertiary butyl-benzamide, and the like.

The foamable plastic composition used in the present invention can be prepared, for example, by dry blending a thermoplastic plastic and a foaming agent in an amount of 0.5 to 5% by weight based on the weight of the thermoplastic plastic using a rotary drum or the like.

The reduction of the cavity volume carried out in the present invention is such that the volume of the cavity at the moment when injection starts is A, the volume of the cavity at the moment when the volume is reduced during or immediately after injection (completion of reduction) is B, and the desired molded product The volume (size) of the cavity at the moment the volume is expanded (completion of expansion) is indicated by C and explained. Practically, the volume (B) is equal to the value obtained by dividing the volume (C) by the average expansion ratio. Volume A is equal to 1.1 to 5 times, preferably 1.5 to 3 times, volume B. Since the pressure gradient on the foamable plastic composition during filling increases, the molding method using a volume smaller than 1.1 times the volume B is not good, so that uniform filling cannot be achieved and a molded plastic foam product having a good appearance. cannot be manufactured In addition, since the pressure on the expandable plastic composition at the time of injection temporarily drops and foaming starts before completion of filling, the molding method using a volume exceeding 5 times the volume B is not good, and molding with good appearance Plastic foam products cannot be manufactured.

Enlargement of the cavity volume can be achieved by movement of the mold, for example by the clamping mechanism of the injection molding machine, or by movement of a slide core mounted on the mold. This change in cavity volume can be manually manipulated, but is preferably controlled by the molding machine or the mold itself. If the increase in cavity volume is carried out by movement of the mold by the clamping mechanism of the injection molding machine, the injection molding machine preferably has such a control mechanism by which the mold can be moved and stopped arbitrarily during the molding operation. If the increase in cavity volume is carried out by moving the slide core, the injection molding machine preferably has a mechanism capable of arbitrarily controlling the movement of the slide core.

One way to completely fill the cavity into the cavity is to inject the composition into the cavity in an amount of 50 to 100% by volume, preferably 80 to 100% by volume, particularly preferably 90 to 100% by volume based on the volume (B). Afterwards, a method of reducing the cavity volume may be included.

In the method of the present invention, the cooling of the cavity to a state in which the solidified layer in contact with the mold surface is mixed with the inner melted layer is performed after reduction of the cavity volume and completion of injection filling, for example, by using a cooling mechanism for an injection mold, It may be performed by maintaining the volume (B) state for 1 to 15 seconds.

The foamable plastic composition used in the present invention may contain antioxidants, weathering agents, ultraviolet absorbers, antistatic agents, coloring agents, olefin elastomers, inorganic fillers such as talc, etc., as needed, within a range that does not impair the object of the present invention. can be combined.

In the process of injecting the molten foamable plastic composition into the cavity, the method of the present invention maintains the cavity in a collapsed position, and operations of cooling/expanding/recooling the cavity are performed after completing filling of the composition into the cavity in the collapsed position. characterized by Therefore, the molded article produced by the above method has good appearance, heat insulation, etc., and the surface thereof includes a layer having a dense structure that is non-foaming or low-foaming, and the inside thereof includes a high-foaming layer.

The present invention has been made in view of the deficiencies of the prior art, and an object of the present invention is to provide an injection molding method for producing a molded plastic foam product having a light weight and good appearance and rigidity, wherein the surface of the molded product is foam-free or It has a low foaming and dense structure, and the inside of the molded product contains a high foaming layer by a conventional injection molding machine using a mold having a thin-walled cavity structure at a location away from the entrance and exit.

The injection molding method according to the present invention does not require high injection speed and injection pressure during injection, and can produce a molded product with a good appearance having a non-foaming or low-foaming layer on the surface and a high-foaming layer inside. . The molded product produced by the above method is lightweight and has good thermal insulation properties, so it is suitable for use in automobile parts, home appliances, industrial parts, and the like.

1 is a perspective view showing a bottom portion of a molded product produced in an embodiment of the present invention;
Fig. 2 is a perspective view showing the inside of a molded product manufactured in an embodiment of the present invention;
Fig. 3 is a partial view of a molded article for use in the evaluation of cell state in an embodiment of the present invention.

Molding was carried out using the above composition, injection molding machine and mold, and the cylinder temperature of the molding machine was set to 230°C and the cooling water temperature of the mold was set to 70°C according to the following procedure.

(i) The space between the mold surface (movable mold) in contact with the position indicated by reference numeral 7 in FIG. 1 and the mold surface (fixed mold) in contact with the position indicated by reference numeral 6 in FIG. 420 g of the composition was injected into a cavity having a diameter of 2.7 mm and a volume of 590 cc. Injection was completed after 3 seconds.

(ii) During the injection, the reduction of the cavity volume started 1.5 seconds after the start of injection and was completed after 2 seconds, so that the upper thickness of the cavity was 1.7 mm and the volume was 470 cc.

(iii) After completion of cavity debulking, cooling was performed for 10 seconds. Subsequently, the expansion of the volume started and was completed after 1.5 seconds, so that the upper thickness of the cavity became 5 mm and the volume became 869 cc.

(iv) After completion of cavity volume expansion, cooling was carried out for 60 seconds to take out a molded product having a size of 410 mm in length x 295 mm in width x 52.8 mm in height (volume 869 cc) as shown in FIG. The mold was opened for

Table 1 shows the characteristics of the molded product obtained and the maximum pressure (measured value) during molding.

Claims (1)

In the injection molding method of the foamable plastic composition,
a) a filling step of completely filling the cavity with the molten foamable plastic composition by reducing the volume of the cavity during or immediately after injection;
b) a cooling step of cooling the composition to a state where the solidified layer in contact with the mold surface is mixed with the inner melted layer;
c) an enlargement step of expanding the volume of the cavity to the volume of the required molded article;
d) A process for injection molding of a foamable plastic composition comprising a taking-out step of taking-out the molded product after further cooling.

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