JPH0994842A - Manufacture of hollow injection molded article - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for making a hollow injection molded article which has a large hollow part inside and an embossed surface without a sink mark and gloss unevenness on the surface. SOLUTION: The volume of a cavity can be increased or decreased by moving a mold cavity surface, a mold in which at least a part of its inner surface has an embossed pattern is used, and an enough amount of a molten resin is injected into the cavity to fill up the cavity with the mold cavity contracted. When the molten resin, the volume of which is 87-97% of that of the cavity, is injected, the injection of pressure fluid into the molten resin is started, and after the molten resin being closely contacted with the cavity surface, the volume of the mold cavity is increased while the resin being pressurized by the pressure fluid.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a hollow injection molded article. More specifically, the present invention relates to a method for producing a hollow injection-molded article having a large hollow portion inside the injection-molded article and having an embossed surface with no gloss on the surface.

[0002]

2. Description of the Related Art Conventionally, when a molded product is manufactured by injection molding, a molten resin in an amount smaller than its volume is injected into a mold cavity, and at the same time, a pressurized fluid (mainly a gas body) is injected to melt it. A technique has been proposed and put into practical use in which a resin is cooled while being pressed against the inner surface of a cavity to form a molded product having a hollow portion inside. At this time, in order to improve the strength of the molded product, many molding methods have been proposed in which a reinforcing rib is provided in the hollow portion inside the molded product.

For example, in US Pat. No. 4,247,515, a protrusion protruding from the mold cavity surface into the cavity space is provided, and the viscosity of the molten resin, the elastic density, the interfacial tension between the molten resin and the pressurized fluid, etc. are determined. Techniques for adjusting are described. The space where the protrusion is provided is narrower than the facing mold cavity surface, and the molten resin cools faster in this part than other parts, so the pressurized fluid does not flow easily, and the molten resin is the protrusion. The solidified material surrounds the place where is provided and remains as a reinforcing rib connecting both surfaces of the molded product.

In Japanese Patent Publication No. 61-53208, the fluidity and amount of the molten resin injected into the mold cavity are made non-uniform, and the molten resin portion having a high fluidity and the molten resin portion having a small fluidity are provided. Pressurize the pressurized fluid into the hollow part,
A method of leaving the other portion as a solid reinforcing rib having no hollow portion is described. Further, in this publication, the molten resin and the pressurized fluid are simultaneously pressed into the mold cavities in which the intervals between the facing cavity surfaces are different, or the molten resin and the pressurized fluid are simultaneously pressed into the mold cavity and the mold cavity is pressed. By partially expanding the space between the mold cavity surfaces, the pressurized fluid is preferentially introduced into the space where the space between the cavity surfaces is large or the space where the space between the mold cavities is widened to form a hollow part. It is described that a hollow portion is not formed and a rib is left as a solid rib in a portion where the distance is small or where the distance between the mold cavity surfaces is not enlarged.

Further, in Japanese Patent Publication No. 7-12621, there is provided a movable core capable of moving from the mold cavity surface to the cavity space to a protruding position and retracting from this position, and at the same time, the mold cavity volume Using an injection molding die that can be reduced or expanded, a sufficient amount of molten resin is injected to fill the cavity in the reduced volume, and the pressurized fluid is injected with the movable core protruding into the cavity space. A method of press-fitting into a molten resin and then retracting the movable core and expanding the mold cavity volume is described. According to the method described in this publication, the resin portion surrounding the movable core forms a reinforcing rib.

[0006]

Recently, the required performances of products have become diversified and sophisticated, and the molded products produced by the above-mentioned molding techniques are no exception. Regarding the appearance of molded products such as high gloss, matte, and embossing. The demands for diversification and sophistication are becoming more diverse, and of course, it is required that they are uniform over the entire molded product. When an embossed pattern is applied to the surface of the molded product according to the method for producing a molded product having a hollow portion, the surface of the molded product corresponding to the rib-provided portion and the rib-unprovided portion respectively has a glossy embossed pattern. The unevenness occurs. This is because there is a difference in the adhesiveness of the molten resin to the cavity surface during molding. Further, the flowability of the molten resin in the portion provided with the rib is worse than that in the portion not provided, and a weld line is generated on the surface portion of the molded product corresponding thereto.

According to a study by the present inventors, US Pat.
When producing a molded product having a hollow portion inside by the method described in Japanese Patent No. 247515 or Japanese Patent Publication No. 61-53208, when the protrusion of the protrusion protruding from the mold cavity surface to the cavity space is low, The molten resin does not cool faster than other parts and flows by being pressed by the pressurized fluid, so it does not remain as a solid rib, and the molten resin is pressed on the outer peripheral side of the molded product, and the outer peripheral side becomes extremely extreme. The thickness of the wall becomes large and the contact time with the cavity surface is different when the thick part is thermally contracted, which causes sink marks and uneven gloss.

Further, in the case where a molded product having a hollow portion inside is manufactured according to the method described in Japanese Patent Publication No. 7-12621, a portion where the movable core projects into the mold cavity space and a portion where this does not exist. Due to the provision of and, uneven gloss occurs for the same reason as above. Furthermore, since the fluid for pressurizing the molten resin injected into the mold cavity space is used after the injection of a predetermined amount of molten resin is completed, the resin that has adhered to the mold cavity surface once leaves the cavity surface due to volume contraction. However, since it is brought into close contact with the cavity surface again by the pressurized fluid, there is a problem that the embossed pattern is not sufficiently transferred and the glossiness becomes more remarkable.

As described above, conventionally, a molded product having a reinforcing rib in the hollow portion having an embossed pattern on the surface is inferior in transferability of the embossed pattern to the surface as compared with a general injection molded product, and therefore, a coating treatment, etc. It was necessary to improve the appearance. The present invention is a method for producing a molded article that solves the above-mentioned problems existing in the prior art, that is, a method for producing a molded article having an embossed pattern on the surface and reinforcing ribs in the hollow portion by an injection molding method. The purpose is to provide.

[0010]

In order to solve the above-mentioned problems, according to the present invention, the cavity volume can be reduced / expanded by the movement of the die cavity surface, and at least a part of the inner surface thereof. Using a mold with an embossed pattern on it, inject a sufficient amount of molten resin into the mold cavity to fill the cavity with the mold cavity volume reduced to 87-97% of the cavity volume. When the molten resin is injected, injection of pressurized fluid into the molten resin is started to bring the molten resin into close contact with the mold cavity surface, and then continue to pressurize the molten resin with the pressurized fluid. A method of manufacturing a hollow injection-molded article for expanding the mold cavity volume is characterized.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be described in detail with reference to the drawings. 1 is an example of a hollow injection-molded article obtained by the method of the present invention, a plan view of a plane cut parallel to both upper and lower planes, and FIG. 2 is a front view of a cut plane of AA portion in FIG. FIG. 3 is a schematic view showing a process for producing a hollow injection-molded article by the method of the present invention. (A) is a vertical cross-sectional view of a state in which injection of molten resin into a mold cavity is almost completed, and (b) is melting. A vertical cross-sectional view of a state in which a pressurized fluid is injected into the resin and the molten resin is pressed and brought into close contact with the mold cavity surface, (c) shows the volume of the mold cavity while injecting the pressurized fluid into the molten resin. FIG. 4 is an enlarged vertical cross-sectional view, FIG. 4 shows the amount of molten resin in the mold cavity when performing the method of the present invention, injection of pressurized fluid, pressure, change in cavity volume, etc., and molding cycle (time). And the figure which showed the relationship of.

According to the method of the present invention, as shown in FIGS. 1 and 2, there can be obtained a hollow injection-molded article having upper and lower surfaces which are flat and having a hollow portion and a reinforcing rib in the middle portion. Figure 1,
In FIG. 2, 1 is an upper surface, 2 is a lower surface, 3 and 4 are side surfaces, 5
Is a hollow portion, and 6 is a reinforcing rib. A mold is used in which an embossed pattern is engraved on at least a part of the cavity-corresponding portions forming the surfaces of the upper surface 1, the lower surface 2, and the side surfaces 3 and 4. At least a part means at least a part of the cavity-corresponding part forming the surface of the molded product, and (i) the upper surface 1
Alternatively, the whole surface of one surface of the lower surface 2, (ii) a part of these surfaces, (iii) all of the upper surface 1 and side surfaces, (iv) a combination of (ii) and (iii), and the like are included.

The hollow injection-molded article as shown in FIGS. 1 and 2 is molded by the procedure shown in the schematic view of FIG. 3 with the steps (a) to (c). In FIG. 3, 7 is a fixed mold, 8 is a movable mold, 9 is a mold cavity, 10,
11 is a cavity surface, 12 is a runner, 13 is a molten resin, 14 is a side gate, 15 is a pressurized fluid press-fitting device, 1
Reference numerals 6 and 17 are hollow portions.

In manufacturing a hollow injection-molded article by the method of the present invention, first, a sufficient amount of molten resin is injected into a mold cavity that is clamped with its volume reduced {FIG. )reference}. Here, "sufficient amount of molten resin" means a sufficient amount of molten resin to fill the mold cavity volume in the contracted state, and an excess amount within 10% of the mold cavity volume in the contracted state. Means forgiveness.

According to the method of the present invention, the mold cavity has a volume of 87 to 97 of the reduced size of the mold cavity.
The injection of the pressurized fluid into the molten resin is started at the time when the molten resin in an amount corresponding to% is injected. When the molten resin that exceeds 97% of the mold cavity volume is injected, the resin that adheres to the mold cavity surface separates from the cavity surface due to volumetric contraction, and thereafter, the molten resin of the pressurized fluid is injected from the pressurized fluid pressurizing device 15. When the injection into the cavity is started, the resin surface separated from the cavity surface is again brought into close contact with the cavity surface by the pressurized fluid, and it is unavoidable that the surface of the molded article becomes uneven. On the other hand, when the injection of the pressurized fluid into the molten resin is started at the time of injecting the molten resin of less than 87% of the mold cavity volume, the molten resin is pressed against the side surfaces 3 and 4 and the upper surface of the molded product. , The bottom surface is not thick enough,
Reinforcing ribs also become thin and a molded product with excellent strength cannot be obtained, which is not preferable.

The injection pressure of the molten resin into the mold cavity is selected in the range of 50 to 150 kg / cm ² ,
The embossed pattern engraved on the surface of the cavity can be transferred to the surface of the molded product. Even if the injection pressure is 50 kg / cm ² or less, the molten resin can be injected into the mold cavity, but the embossed pattern on the surface of the molded product is inferior in transferability.
Not preferred. If the injection pressure exceeds 150 kg / cm ² ,
The mold cavity expansion mechanism must be able to withstand this pressure and is too large, which is not desirable.

FIG. 3B shows a pressurizing fluid press-fitting device 15 for the molten resin injected into the mold cavity in a contracted state.
It shows a state in which a pressurized fluid is injected and the molten resin is completely adhered to the mold cavity surface. In the case of the method of the present invention, the mold cavity volume is expanded by a mold cavity volume expansion mechanism (not shown) while continuing to pressurize the molten resin with the pressurized fluid (FIG. 3 (c)).
reference}. The expansion of the mold cavity volume is started after the injection of the pressurized fluid into the molten resin is started, and the mold cavity volume is increased while continuing to pressurize the molten resin with the pressurized fluid to the mold cavity surface. By enlarging, even if the volume of the molten resin contracts, it can be prevented from separating from the cavity surface.

The expansion rate of the mold cavity volume, that is, the expansion rate of the interval between the opposing mold cavity surfaces, depends on the size and thickness of the molded product, the type of resin used for molding, the melt viscosity, the resin additive. , The molding temperature, the temperature of the mold cavity surface, etc., but the speed should be such that the molten resin does not separate from the mold cavity surface in the process of expanding the mold cavity volume, select from 10 to 30 mm / min. You can If the expansion rate of the volume is less than 10 mm / min, the molten resin will be cooled during the expansion of the die cavity volume, and it will be difficult for the resin to follow the die cavity surface, making it difficult to form a hollow portion. Absent. On the other hand, 30
If the velocity exceeds mm / min, the molten resin peels off from the mold cavity surface, and the pressurized fluid injected into the molten resin presses the softest part of the molten resin (usually the hottest part). In some cases, the molten resin locally expands excessively, holes are formed, and the pressurized fluid leaks out, so that a molded product having a desired hollow portion cannot be obtained, which is not preferable.

The molten resin injected into the mold cavity is
Expand the mold cavity volume while continuing to pressurize with pressurized fluid, hold the molded product in the cavity with the mold cavity at its maximum volume, and keep the molded product at a temperature at which it does not deform even if it is removed from the mold. After cooling, after the completion of cooling, the pressurized fluid remaining in the hollow portion of the molded product is purged to bring the pressure to about normal pressure, the mold is opened, and the molded product is released to obtain the target product. After releasing the molded product, the mold is clamped by a mold clamping mechanism (not shown) to reduce the mold cavity volume.
It is possible to reduce the volume to the minimum volume by a mold cavity volume reduction mechanism (not shown), start the next molding cycle, and subsequently perform continuous molding in the same procedure.

FIG. 4 shows the relationship between the amount of molten resin in the mold cavity, injection of pressurized fluid, pressure, change in cavity volume, etc. and the molding cycle (time) when the method of the present invention is carried out. It is a conceptual diagram. The vertical axis represents relative values such as the amount of molten resin injected into the mold cavity, the volume of the mold cavity, and the pressure of the pressurized fluid injected into the molten resin, and the horizontal axis represents the time of molding. Elapsed time is taken.

First, the injection of the molten resin into the mold cavity whose volume has been reduced is started, and then the injection amount of the molten resin into this mold cavity is reduced to 87 of the die cavity volume. At the time of injecting the molten resin in an amount corresponding to ˜97%, the injection of the pressurized fluid into the molten resin is started. The pressure of the pressurized fluid in the mold cavity continues to rise rapidly until the volume of the mold cavity is expanded, and when the volume of the mold cavity is expanded, it is rapidly reduced to a constant pressure. Even after the mold cavity volume has reached the maximum volume, this pressurized state is maintained until cooling of the molded product in the mold cavity is completed and until pressurized fluid remaining in the hollow part of the molded product is purged. To maintain.

The resin that can be used when carrying out the method of the present invention is not particularly limited, and generally includes all thermoplastic resins to which injection molding can be applied. In the present invention, the pressurized fluid means a gas or liquid at room temperature and atmospheric pressure that does not chemically change when pressurized. Specific examples include air, gases such as nitrogen gas, carbon dioxide gas, helium gas, and argon gas, and liquids such as water, glycerin, and liquid paraffin. Among these, inert gases such as nitrogen gas and helium gas are preferable.

According to the method of the present invention, an outer box such as a radio or a television, an electric product such as a television mount, an OA such as a housing of a personal computer or an office computer, a copying machine or a housing of a printer.
Equipment products: Various molded products such as automobile door knobs and automobile parts such as instrument panels, which have a hollow structure inside and rub an embossed pattern on a part of the surface, can be manufactured.

[0024]

EXAMPLES The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. In the example described below, the mold is a rectangular flat plate having a size of 200 mm × 400 mm × thickness (changeable), and a molded product having the reinforcing rib shown in FIG. And longitudinal 2
On one side and each of the two sides in the short direction,
With an embossed pattern with a roughness of about 90 μm, the distance between the facing surfaces at the minimum volume of the cavity is 5 mm, the distance between the facing surfaces at the maximum volume is 20 mm, and the expansion speed of the volume is 10 to 30 mm.
A mold that can be selected in the range of / min was used. Details of the movable mechanism of this mold are described in the prior application of the applicant of the present invention (JP-A-6-339950).

[Example 1] Impact-resistant polystyrene (manufactured by Mitsubishi Chemical Corporation, brand name: Dialex HT88) was melted at a cylinder temperature of 220 ° C with an injection molding machine (manufactured by Mitsubishi Heavy Industries, Ltd., model: 850MG). The above mold is mounted on this molding machine, the mold cavity surface temperature is set to 50 ° C.,
It was injected into a cavity with a minimum cavity volume. When the filling rate of the molten resin into the mold cavity reached 95%, the injection of 50 kg / cm ² of pressurized air was started. The injection amount of the molten resin into the mold cavity was 102%, and the molten resin pressure caught by the pressure sensor installed on the mold cavity surface when the injection of the molten resin was completed was 80 kg / cm ² . 0.5 seconds after the injection of the molten resin into the mold cavity was completed, the mold cavity volume was expanded at a speed of 30 mm / min. The pressure on the cavity surface caught by the pressure sensor installed on the die cavity surface is 4 when the cavity volume is expanded.
It was 8 kg / cm ² .

The molding cycle in this example was as follows: injection of molten resin into the mold cavity (95%) for 10 seconds, expansion of cavity volume for 30 seconds, cooling for 90 seconds, and mold opening and releasing for 20 seconds.
Seconds are required, and the total cycle is 150 seconds. The molded product obtained in this example has a thickness of 20 mm, a weight of 480 mm, a hollow ratio of 61%, has a reinforcing rib as shown in FIGS. 1 and 2 inside the molded product, and has its surface The embossed pattern of about 90 μm engraved on the mold cavity surface was transferred as it was, and no weld line, sink mark, or gloss was observed on the entire surface of the molded product, and the appearance was excellent.

[Example 2] In the example described in Example 1, except that 50 kg / cm ² of air was started to be injected when the mold cavity filling rate of the molten resin reached 90%. A molded article having a hollow portion was manufactured by an injection molding method in the same procedure as in the same example. The molded product obtained in this example was as excellent as the one obtained in Example 1 in terms of reinforcing ribs, appearance and transfer of embossed pattern.

[Embodiment 3] In the embodiment described in Embodiment 1, except that the expansion rate of the cavity volume is changed to 10 mm / min, a molded product having a hollow portion is injection-molded by the same procedure as in the same embodiment. Manufactured by. The molded product obtained in this example was as excellent as the one obtained in Example 1 in terms of reinforcing ribs, appearance and transfer of embossed pattern.

[Embodiment 4] A molded product having a hollow portion in the same procedure as in the embodiment 1 except that the injection amount of the molten resin into the mold cavity is changed to 107%. Was manufactured by an injection molding method. In this example,
The pressure of the molten resin installed on the mold cavity surface when the injection of the molten resin into the mold cavity was completed was 150 kg / cm ² , and the pressure caught by the mold cavity surface after expanding the cavity volume and ending. Was 48 kg / cm ² . The molded product obtained in this example was as excellent as the one obtained in Example 1 in terms of reinforcing ribs, appearance and transfer of embossed pattern.

[Comparative Example 1] In the example described in Example 1, except that the injection of 50 kg / cm ² of air was started when the filling rate of the molten resin into the mold cavity reached 100%. A molded article having a hollow portion was manufactured by the injection molding method in the same procedure as in the example. The molded article obtained in this example had uneven gloss on the surface of the molded article, and the roughness of the embossed pattern in the high-gloss portion was 30 μ, and the transfer of the embossed pattern was poor.

[Comparative Example 2] A molded article having a hollow portion in the same procedure as in Example 1 except that the injection amount of the molten resin into the mold cavity was changed to 98% in the example described in Example 1. Was manufactured by an injection molding method. In the molded product obtained in this example, the wall surface of the four circumferences is thicker than the upper and lower planes,
As for the appearance, the wall surface on the four edges was glossy and the transfer of the embossed pattern was poor.

[0032]

INDUSTRIAL APPLICABILITY The method of the present invention has the following special advantageous effects, and its industrial utility value is extremely large. 1. According to the method of the present invention, a molded product having a large hollow portion inside the injection molded product and having reinforcing ribs for reinforcing both surfaces can be manufactured. 2. According to the method of the present invention, since the injection timing of the pressurized fluid to be injected into the molten resin injected into the mold cavity is adjusted, it is possible to manufacture a molded product having an embossed pattern with no gloss on the surface of the molded product. You can

[Brief description of drawings]

FIG. 1 is a plan view of a hollow injection-molded article obtained by the method of the present invention, in which a hollow portion is cut in parallel with a flat portion.

FIG. 2 shows a front view of a cross section taken along the line AA in FIG.

FIG. 3 shows a schematic view of a process for producing a hollow injection-molded article by the method of the present invention.

FIG. 4 is a conceptual diagram showing the relationship between the amount of molten resin in the mold cavity, injection of pressurized fluid, pressure, changes in cavity volume, etc. and the molding cycle (time) when the method of the present invention is carried out. Is.

[Explanation of symbols]

 1: Upper surface 2: Lower surface 3, 4: Side surface 5: Hollow part 6: Reinforcement rib 7: Fixed side mold 8: Movable side mold 9: Mold cavity 10, 11: Cavity surface 12: Runner 13: Molten resin 14 : Side gate 15: Pressurized fluid press-fitting device 16, 17: Hollow part

Claims (3)

[Claims] 1. A mold cavity in which the cavity volume can be reduced / expanded by moving the mold cavity surface and at least a part of the inner surface of which has an embossed pattern is used. Injecting a pressurized fluid into the molten resin at the time when a sufficient amount of molten resin is injected to fill the cavity with the mold cavity volume reduced, and 87 to 97% of the cavity volume of the molten resin is injected. Of the hollow injection-molded product, which is characterized by expanding the mold cavity volume while continuing to pressurize the molten resin with the pressurized fluid of the molten resin after completely initiating the molten resin on the mold cavity surface. Production method. 2. The pressure in the mold cavity immediately after the molten resin is injected into the mold cavity is 50 to 150 kg /
The method for producing a hollow injection-molded article according to claim 1, wherein the method is selected in the range of cm ² . 3. The speed at which the mold cavity volume is expanded is selected in the range of 10 to 30 mm / min.
The method for producing the hollow injection-molded article according to claim 1 or 2.