JP2000210969A - Foam injection molding method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily discriminate a recycling time by making the skin layer of a molded product thin and forming cells in a thick wall portion uniformly in the same size or forming the cells on the side of the skin layer small or relaxing an external impact without using a different kind of a material. SOLUTION: At the injection molding of a molten resin mixed with a blowing agent, the vol. of a cavity 3 in a mold is set to a final objective vol. or less and the surface temp. of the mold is heated to the Tg of the resin or higher and the mold is filled with the resin 4 and, subsequently, the mold is opened so that the vol. of the cavity 3 becomes the final objective vol. and cooled. By this constitution, the skin layers 6 of a molded product are formed thinly and hollow parts 5 are uniformly formed in the thick wall portion between the skin layers. By opening the mold stepwise, the diameter of the hollow parts 5 is made large at the center portion between the skin layers 6 and made small at the portions near to the skin layers 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a fine bubble layer in the thickness of a plastic product and controlling the size of the bubbles within the thickness of the molded product according to the use of the molded product. The present invention relates to a molding method and an apparatus therefor.

[0002]

2. Description of the Related Art Conventionally, there is a method of forming a hollow portion inside one of injection molded products. This method is a method of injecting an inert gas such as nitrogen gas into a molten resin to obtain a hollow portion. However, inside the molded article obtained by this molding method, a continuous hollow portion is formed between the surface layers of the molded article from the portion into which the gas is injected. For this reason, since the inside of the molded article is hollow, the impact resistance is inferior. Furthermore, when the thickness of the molded product is relatively thick, a hollow portion is easily formed inside the molded product. However, when the thickness of the molded product is reduced, the internal pressure of the cavity increases, so that the hollow portion is not formed in the entire interior of the molded product. is there.

[0003] Furthermore, there are several techniques for obtaining a lightweight molded article.
In some cases, a foaming agent or a resin mixed with the blowing agent is added to a molding material to perform injection molding. According to this method, there is an advantage that a molded article having a hollow portion formed relatively uniformly inside the molded article can be obtained even if the thickness of the molded article is relatively thin.

[0004]

However, in the above-mentioned foam molding method, when the thickness of the molded product is reduced as in the case of gas injection molding, the internal pressure of the cavity is increased, so that bubbles are not uniformly formed inside the molded product. As a result, there is a problem that the shape of a molded product cannot be obtained, and further, a jetting-like appearance defect occurs.

[0005] The size and number of foamed cells in a molded article can be changed to some extent by the amount and type of a foaming agent to be added. There is a problem that foaming is limited by mold temperature or the like, and desired bubbles are not formed in the thickness of the molded product. In addition, when the predetermined strength cannot be obtained due to non-uniform air bubbles in the molded article, the molded article may be reinforced with a different material, and in this case, there is an obstacle to separation during recycling. .

An object of the present invention is to uniformly form air bubbles of a uniform size in a molded article, to increase the thickness of air bubbles at the center of the wall thickness and to reduce the air bubbles at the skin layer side, and to make the skin layer thin. An object of the present invention is to provide a foam injection molding method and an apparatus thereof that can be formed or obtain a molded product suitable for recycling.

[0007]

In order to achieve the above object, according to the first aspect of the present invention, in a foam injection molding method, when injection molding is performed using a molten resin to which a foaming agent has been added, a mold inside a mold is required. The volume of the cavity is set to be equal to or less than the final target volume, and the surface temperature of the mold is heated to a temperature equal to or higher than the glass transition temperature of the resin. Subsequently, the resin is filled in the mold, and then the mold is placed in the cavity. By opening the inner volume to the final target volume and then cooling the mold, it is possible to form the skin layer of the molded product thinly and to form a uniform bubble layer within the thickness between the skin layers. It is a feature.

Further, in the invention according to claim 2,
According to the first aspect of the present invention, the surface temperature of the mold is controlled to be high when the resin is filled, and thereafter to be gradually lowered.

Further, in the invention according to claim 3,
In the invention described in claim 1 or 2, when the mold is opened after filling the resin until the volume in the cavity reaches the final target volume, the opening is performed stepwise.

Further, in the invention according to claim 4,
The invention according to claim 1, 2 or 3 is characterized in that the surface temperature of the mold is partially or entirely heated to the glass transition temperature of the resin or higher.

Further, in the invention according to claim 5,
In a foam injection molding apparatus, when filling a mold with a molten resin added with a foaming agent, the core can be advanced to control the volume in the cavity to be smaller than the final target volume, and the final target volume can be controlled. After filling the resin into a controlled cavity whose volume is smaller than the volume, the mold can retreat the core at a stroke or in a stepwise manner so that the volume in the cavity becomes a final target volume; and A heating device capable of heating the temperature of part or all of the surface of the mold to a temperature equal to or higher than the glass transition temperature of the resin, a cooling device for cooling the mold, filling of the cavity with the resin, the mold And a controller for controlling the surface temperature of the core and the advance and retreat of the core.

[0012]

According to the first to fifth aspects, the entire or at least one of the mold wall surfaces on which a molded article is to be formed in advance, that is, the mold surface temperature is equal to or higher than the glass transition temperature (hereinafter referred to as "Tg") of the resin. The cavity volume is first reduced, and a resin containing a foaming agent is injected and filled therein.

At this time, on the surface of the cavity, a skin layer serving as the surface of the molded product is formed as in the case of general injection molding, and the cavity is eventually filled with resin. At this time, since the cavity internal pressure increases in proportion to the filling, a sufficient pressure is applied to the mold surface. On the other hand, since the mold surface is maintained at Tg or more, the development of the skin layer is suppressed.

After the resin is sufficiently filled in the cavity,
By expanding the cavity volume to the final target molded article position, the resin pressure inside the molded article is alleviated, and at the same time, bubbles are formed between the skin layers by the foaming agent added into the resin. At this time, the mold surface temperature becomes Tg
Since the temperature is maintained at or above the temperature, foaming proceeds even at a position near the mold surface, and as a result, bubbles are formed up to the vicinity of the skin layer.

Next, after the cavity volume is expanded to the position of the final molded product, the surface of the molded product is cooled to the resin Tg or less, and then the molded product is removed from the mold. As a result, an injection-molded article having bubbles of a relatively uniform size up to near the surface of the molded article is obtained.

Further, in claim 3, after the cavity is filled with the resin, the expansion of the cavity is stopped on the way to the final molded article position. Thereby, since the surface temperature of the mold is maintained at Tg or higher inside the molded product, minute bubbles are formed up to the vicinity of the skin layer.

Next, after the minute bubbles are formed in the cavity, the volume of the cavity is further increased to a required final molded article position. At this time, in the inside of the molded article, a difference in cooling rate between the vicinity of the skin layer surface and the vicinity of the center of the thickness occurs due to the formation of bubbles. As a result, the microbubbles near the skin layer tend to solidify relatively quickly, but since the inside of the foam is harder to solidify than the inside of the molded product and it is not completely solidified, the cavity volume increases and the inside of the molded product becomes the center. Bubbles that have become larger are enlarged.

By increasing the mold surface temperature at the time of filling at Tg or higher, the cavity volume may be expanded in two or more stages or in plural stages. Hereinafter, the present invention will be described in more detail with reference to examples.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION In a foam molded article using a foaming agent, in order to uniformly foam the inside of the molded article, the internal pressure of the cavity at the time of filling may be set to be equal to or less than the foaming pressure of the foaming agent. Therefore, if the pressure inside the cavity is reduced when the foaming agent foams, foamed bubbles are formed inside the molded article regardless of the pressure at the time of filling.

In general, in a foam molded article, foaming in the resin is suppressed by the skin layer and its development, and even if the core is retracted and the inner pressure of the mold is reduced, the center of the wall having a relatively high resin temperature is obtained. Bubbles are concentrated in the portion. This is because the reaction of the foaming agent is performed in a portion where the temperature is high.

On the other hand, the present invention suppresses the development of the skin layer when the in-mold pressure is reduced by the retreat of the core, thereby suppressing the formation of air bubbles in the molded article, the cooling rate of the resin and the timing of expanding the cavity. And the size of the bubbles in the molded article can be changed.
In the present invention, the temperature of the mold is partially changed to control the foaming and the size of the bubbles at a specific portion,
The density can be changed arbitrarily.

[0022]

Embodiment 1 This embodiment corresponds to claims 1 to 5,
FIG. 1 is a molded product drawing schematically showing a molded product. The molded product has a length a = 300 mm and a width b = 30 m before filling.
m, and the molded product thickness c = 4.0 mm. The gate is a pin side gate, which is part B of the molded product. FIG. 2 shows a cross section of a mold cavity corresponding to a portion XX 'in the molded product of FIG. 1, and schematically shows a process of forming the molded product in three steps.

A is a cross-sectional view of the cavity before the start of filling. Here, the movable-side core 1 advances in the direction in which the thickness of the molded product is reduced, and is maintained at a cavity gap of 1.0 mm with the cavity-fixing-side main mold 2. Here, the mold surface temperature of the movable side core 1 and the fixed side main mold 2 is set to a high-frequency temperature controller (10 ° C./sec as a molded product surface temperature in-house developed product).
The temperature of the mold having the cavity is heated to 95 ° C., and the polypropylene resin (MK81 manufactured by Showa Denko KK)
1) 100 parts by weight of an azodicarponamide-based blowing agent (Polyslen EE-5, trade name, manufactured by Eiwa Chemical Industry Co., Ltd.)
06) Using a resin to which 5 parts by weight were added and mixed, injection molding was performed using a molding machine (IS-350EW manufactured by Toshiba Machine Co., Ltd.). The molding condition is a resin temperature of 225 ° C., and the filling time in the cavity 3 is 0.3 seconds. B of FIG.
FIG. 4 is a cross-sectional view of the cavity 3 immediately after filling is completed, and no air bubbles are formed in the resin 4 in the cavity 3.

FIG. 2C shows 0.2 seconds after the completion of resin filling.
Later, the movable core 1 was enlarged by 3.0 mm so that the shape in the cavity 3 formed the final molded product shape, and the gap between the fixed main mold 2 and the movable core 1 was set to 4.0 mm.
As a result, a hollow portion (bubble) 5 was formed in the molded product cavity 3 (resin 4). At the same time, the mold surface temperature of the fixed-side main mold 2 and the movable-side core 1 forming the molded product is 40
The mold was cooled by a cooling device so as to reach a temperature of ℃, and after cooling the molded product, the molded product was taken out. As a result, a molded article in which the hollow portion 5 was formed relatively uniformly inside the molded article and near the surface of the molded article was obtained. At this time, in the cross section of the molded product, the distance between the surface of the molded product and the hollow portion 5 is 0.04 mm.
Met. The filling of the resin, the heating of the mold, the advance and retreat of the core, and the cooling were all performed automatically by a controller. Reference numeral 6 in the drawing denotes a skin layer (solidified layer).

[0025]

Comparative Example 1 In the same manner as in Example 1, except that the mold surface temperature of the movable-side core 1 and the fixed-side main mold 2 was set to 40 ° C. while the cavity 3 was enlarged from the start of filling. Injection molding was performed. As a result, as shown in FIG. 3, in the molded product cavity 3, hollow portions 5 having different sizes were randomly formed in the molded product cross section. Moreover, the shape of the hollow portion 5 is circular,
It became elliptical. No hollow portion 5 was formed near the surface of the molded product, and the distance between the surface of the molded product and the hollow portion 5 was 0.3 mm.

[0026]

Embodiment 2 This embodiment corresponds to claim 3. In embodiment 1, the mold surface temperature of the movable core 1 and the fixed main mold 2 is set to 95 ° C. at the start of filling. Until filling is completed, injection molding is performed in the same manner as in Example 1.
0.2 seconds after the completion of filling, as shown in FIG.
The mold temperature was 80 ° C. At this time, inside the molded product,
A minute hollow portion 5 is formed near the surface of the molded product. After cooling for 2.5 seconds in this state, the cavity 3 was enlarged by 3.0 mm and extended to the final molded product shape position as shown in FIG. 4B.

As a result, the hollow portion 5 at the center of the molded product
Is further expanded, but the hollow portion 5 near the surface of the molded product is cooled in the state of the minute hollow portion 5, so that the hollow portion 5 does not expand. As a result, the diameter of the hollow portion 5 inside the molded product became three times or more, and two layers of the hollow portion 5 were formed.

[0028]

Comparative Example 2 Injection molding was performed in the same manner as in Example 2 except that the surface temperature of the mold of the molded product was set to 40 ° C. As a result, as shown in the cross-sectional view of the mold when the cavity volume is enlarged in FIG. 5, the hollow portion 5 is not formed near the surface of the molded product due to the developed skin layer 6 when the cavity 3 is primarily enlarged. Furthermore, at the time of secondary enlargement, since the hollow portion 5 is not formed uniformly near the thickness of the molded product, the expansion of the internal hollow portion 5 becomes uneven. As a result, the hollow portion 5 was not formed near the surface of the molded article, and sink was generated on the surface of the molded article due to uneven internal foaming, so that a smooth molded article could not be obtained.

[0029]

According to the present invention, in molding an injection-molded article using a foaming agent, by controlling the volume in the cavity and the temperature of the mold, a hollow portion (bubbles) and a foam according to the demand of the molded article can be obtained. Not only the formation of the hollow ratio, but also the formation of the hollow portion according to the use in one molded product becomes easy. As a result, product design expands,
Since external impact can be reduced without using different kinds of materials, separation at the time of recycling becomes easy.

[Brief description of the drawings]

FIG. 1 is a schematic view of a molded article.

FIG. 2 is a cross-sectional view of a mold during a molding step at a section XX ′ of the molded article in Example 1. A Before resin filling B Immediately after resin filling C Shape of molded product cavity expansion

FIG. 3 is a cross-sectional view of a mold when a cavity volume is increased in Comparative Example 1.

FIG. 4 is a cross-sectional view of a mold during a molding step at a section XX ′ of a molded article in Example 2. A A cross-sectional view of the mold at the time of the first cavity volume expansion. B Cross-sectional view of the mold at the time of the second cavity volume expansion.

FIG. 5 is a cross-sectional view of a mold at the time of the second expansion of the cavity volume in the XX ′ portion of the molded product in Comparative Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Movable core 2 Fixed main mold 3 Cavity 4 Resin 5 Hollow part (bubble) 6 Skin layer (solidified layer)

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) B29C 45/78 B29C 45/78 // B29K 105: 04

Claims (5)

[Claims] When performing injection molding using a molten resin to which a foaming agent has been added, the volume of a cavity in a mold is set to be equal to or less than a final target volume, and the surface temperature of the mold is set to a glass transition temperature of the resin. By heating to a temperature above the temperature, then filling the mold with resin, then opening the mold until the volume in the cavity reaches the final target volume, and then cooling the mold, the skin layer of the molded article is cooled. And a foam layer is formed uniformly within the thickness between the skin layers. 2. The mold has a high surface temperature when filled with a resin,
2. The foam injection molding method according to claim 1, wherein the step is performed while controlling the temperature stepwise. 3. The foam injection molding method according to claim 1, wherein when the mold is opened until the volume in the cavity reaches the final target volume after filling the resin, the opening is performed in a stepwise manner. 4. The method according to claim 1, wherein the surface temperature of the mold is partially or entirely heated to the glass transition temperature of the resin or higher.
Or the foam injection molding method according to 3. 5. When filling a mold with molten resin to which a foaming agent has been added, the core can be advanced to control the volume in the cavity to be smaller than the final target volume, and the final target volume can be controlled. After filling a resin into a cavity whose volume is controlled to be smaller than that of the mold, the core can be retracted at once or stepwise so that the volume in the cavity becomes a final target volume; A heating device capable of heating the temperature of part or all of the surface of the mold to a temperature equal to or higher than the glass transition temperature of the resin, a cooling device for cooling the mold, and filling the cavity with the resin, A foam injection molding apparatus, comprising: a controller for controlling the surface temperature and the advance and retreat of the core.