JP2001121581A - Method for gas pressurized injection molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for gas pressurized injection molding for obtaining a molding having an excellent appearance level by easily designing a mold. SOLUTION: The method for gas pressurized injection molding an uneven section thickness molding having a base plate, a side wall 3 and a thick rib 4 provided at the rear surface side of the plate comprises the steps of filling a resin in a mold having a molding cavity constituted of a design surface side mold 1a and a rear surface side mold 1b in such a manner that parting line of a mold insert at a mold cavity surface 2b corresponding to the rear surface of the molding of a projected side of the rib 4 exists on a surface 19 including the vertex of the rib 4, filling a pressurized gas between the filled and the cavity surface 2b, and pressing the design surface of the molding to the cavity surface 2a corresponding to the design surface of the molding.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas pressure injection molding method for a plastic material, and more particularly, to a method for pressurizing gas between a resin injected into a mold cavity and a mold cavity surface. The present invention relates to an injection molding method involving injection.

[0002]

2. Description of the Related Art Generally, when performing injection molding of an uneven thickness molded product having a protruding thick portion such as a rib or a boss on one surface, the thick portion is thickened due to volume shrinkage accompanying cooling and solidification of a molten resin. On the design surface opposite to the protruding side of the portion, a concave portion called sink occurs, which significantly impairs the appearance of the molded product.
Therefore, as a method to prevent sinks in this thick part,
2. Description of the Related Art Conventionally, there has been known a hollow injection molding method in which a pressurized gas is injected into a mold cavity filled with a molten resin to form a hollow portion in a thick portion.

[0003] However, molded articles produced by the hollow injection molding method are:
In the thick part where gas is injected and the thin part where gas is not injected, there is a difference in pressing force against the mold cavity surface,
As a result, uneven gloss may be generated on the design surface side of the thick part, which causes a problem of poor appearance of the molded product. On the other hand, Japanese Patent Application Laid-Open No. 50-75247 discloses that a pressurized gas is injected between a resin injected into a mold cavity and a mold cavity surface, and is opposite to the injection side of the pressurized gas. A molding method is described in which the design surface of a molded product is pressed against a corresponding mold cavity surface.

[0004]

In recent years, manufacturers of household electrical appliances and office automation equipment have omitted the secondary processing such as painting and plating after molding of housings of these products in order to reduce product costs. There has been an increasing demand for the development of molding techniques that can obtain molded articles having excellent appearance and capable of forming. However, in order to prevent sink marks and uneven gloss on the design surface of uneven thickness molded products, only the gas pressure injection molding method described in Japanese Patent Application Laid-Open No. 50-75247 is practiced to prevent sink and uneven gloss. Is not enough.

Accordingly, the present inventor has disclosed a gas pressure injection molding method for preventing sink marks and uneven gloss on the design surface of an uneven thickness molded product and obtaining a molded product having an extremely good appearance level. 31532 and Japanese Patent Application No. 10-125651. By the way, in the gas pressurized injection molding method proposed by the present inventors, the injected pressurized gas leaks to the outside of the mold, which is caused by a decrease in the pressing force of the design surface against the mold cavity surface. In order to prevent appearance defects, it is necessary that the portion into which the pressurized gas is injected on the mold cavity surface corresponding to the back surface of the molded product should not be divided by the mold nest.

Alternatively, when the pressurized gas injection portion is divided by a mold nest, a sealing material is provided on the mating surface of the mold components to prevent the pressurized gas from leaking from the dividing line, and the mold is provided. It was necessary to cut off all routes to the outside. However, the above-described restrictions on the mold structure not only reduce the degree of freedom in mold design, but also increase the mold cutting period and, in some cases, increase the mold manufacturing cost, which is not preferable. .

Therefore, the inventor of the present invention divides the pressurized gas injection portion by a mold insert, and eliminates sink marks and unevenness in the design surface without installing a sealing material on the mating surface of the mold components. The present inventors have found a method for preventing the occurrence of a molded article having an extremely good appearance level and solving the above-mentioned problem.

[0008]

That is, the present invention is a gas pressure injection molding method for a molded article having a thick rib, wherein the molded article cavity is constituted by a design surface side mold and a back side mold, The dividing line of the mold nest on the mold cavity surface corresponding to the rear surface of the molded product that is the projecting side of the thick rib fills the mold with the resin existing on the surface including the top of the thick rib, A pressurized gas is injected between the filled resin and the mold cavity surface corresponding to the back surface of the molded product, and the pressure of the pressurized gas causes the molten resin to come into close contact with the dividing line of the mold nest. An object of the present invention is to provide a gas pressure injection molding method characterized in that a design surface of a molded product is pressed against a corresponding mold cavity surface by preventing gas from leaking from the nested parting line.

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an example of a mold 1 used in the present invention. First, an outline of the mold 1 will be described. As shown in the figure, the mold 1 is composed of a fixed mold 1a and a movable mold 1b, and the uneven thickness molded product molded by the mold 1 has a back surface where a thick part protrudes, and the opposite. The design surface is formed by the mold cavity surface 2b on the movable mold 1b side, and the design surface is formed by the mold cavity surface 2a on the fixed mold 1a side. I have. However, the design surface may be molded with the mold cavity surface 2b on the movable mold 1b side, and the back surface may be molded with the mold cavity surface 2a on the fixed mold 1a side.

The movable mold 1b is composed of a plurality of mold nests, and the movable mold side cavity surface 2b is nested and divided on a surface 21 including the top of the side wall 3 and a surface 19 including the top of the rib 4. ing. A gas injection path 6 is formed in the movable mold 1b. A pressurized gas source (not shown) is connected to the gas injection path 6, and the pressurized gas is supplied from the pressurized gas source.

The movable mold 1b is provided with a gas injection pin 5 whose tip faces the inside of the mold cavity 2. As shown in FIGS. 2A and 2B, the cross section of the gas injection pin 5 has a shape obtained by cutting off a part of a circle, thereby forming a clearance 8 between the gas injection pin 5 and the movable mold 1b. ing. This clearance does not allow the molten resin to flow backward, but has a size that allows the pressurized gas to pass through. The pressurized gas supplied from the pressurized gas source to the gas injection path 6 passes through the clearance 8 between the gas injection pin 5 and the movable mold 1b, and is injected into the mold cavity 2.

In order to prevent pressure loss at the connection between the gas injection path 6 and the gas injection pin 5, a sealing material 7 is provided.
Is installed. As the sealing material 7, it is desirable to use an O-ring made of nitrile-butadiene rubber, but if there is no problem with the heat resistance at the applied mold temperature,
There is no particular limitation. Further, in order to prevent the pressurized gas injected into the mold cavity 2 from leaking, the sealing material 10 is also provided for the clearance between the ejector pin 9 and the movable mold 1b.
Is provided. The sealing material 10 is also used as the sealing material 7.
Is similar to

In FIG. 1, 11 and 12 are ejector plates, 13 is a spacer block, 14 is a fixed side mounting plate, and 15 is a movable side mounting plate. Next, the mold 1
The method of the present invention using is described. First, a molten resin is injected into the mold cavity 2 with the mold 1 closed. The injection amount of the molten resin is not particularly limited.
It is preferable that the amount is sufficient as compared with the volume of, and it is more preferable that the amount is excessive. That is, by injecting an excessive amount of resin into the mold cavity 2, the adhesion between the resin and the mold cavity surfaces 2a, 2b is increased,
This is because the pressure retention of the gas layer described later is improved.

After the injection of the molten resin, when a pressurized gas is injected from the clearance 8 between the gas injection pin 5 and the movable mold 1b, the molten resin injected into the mold cavity 2 and the movable mold side cavity surface 2b A gas layer is formed by the pressurized gas in the gap. A seal member 10 provided in a clearance between the ejector pin 9 and the movable mold 1b helps maintain the pressure of the gas layer.

During the cooling and solidification of the molten resin, the gas layer formed on the projecting side of the ribs 4 of the uneven thickness molded product has its opposite surface, that is, the design surface side of the uneven thickness molded product facing the mold cavity surface. 2a is continuously pressed down. At this time, as shown in FIG. 3, the root of the rib 4 is narrowed down by the pressure of the pressurized gas, whereby the molten resin near the rib 4 flows, and the design surface corresponding to the rib 4 further moves to the mold cavity surface 2.
Therefore, the design shrinkage of the design surface due to the volume shrinkage of the rib 4 affecting the design surface can be prevented.

Therefore, it is very important to maintain the pressure of the gas layer formed in the gap between the molten resin and the movable mold side cavity surface 2b in order to obtain a molded article having an excellent design surface appearance. When the surface 2b is divided by a plurality of mold nests, the injected pressurized gas instantaneously passes through the dividing line of the mold nest and passes through the mating surfaces of various mold components to the outside of the mold. To leak. As a result, the pressing force of the design surface against the fixed mold side cavity surface 2a is weakened, and the appearance of the design surface of the molded product is adversely affected.

Therefore, the present inventor has disclosed in Japanese Patent Laid-Open No. 9-3
In the gas pressure injection molding method proposed in US Pat.
The part where the gas layer is formed on b is premised on a structure that is not divided by mold nesting, that is, an integral mold structure. Alternatively, in the case where the above-mentioned gas layer forming portion is divided by nesting of a mold, in order to prevent leakage of the pressurized gas from the nesting dividing line, a sealing material is provided on the mating surface of the mold constituent members, and the outside of the mold. It was necessary to block all routes leading to it.

However, in actual die manufacturing, the die structure as described above increases the die cutting process, which leads to an increase in die manufacturing time and die manufacturing cost. However, the present inventor has found a relationship between the method of dividing the movable mold side cavity surface 2b by nesting the mold and the pressure retention of the gas layer. It was confirmed that the molded article with uneven thickness could be obtained and the appearance of the design surface was excellent.

That is, as shown in FIG. 4, the width of the mold cavity surface 19 corresponding to the top of the thick rib 4 is Wα, and the distance from the end of the mold cavity surface 19 to the mold nest dividing line 18 is Assuming that L, division by mold nesting may be performed at a position satisfying the following expression (1). 0 <L ≦ (1/2) Wα (1)

At this time, as shown in FIG. 5, when the dividing line 18 of the mold insert is at the end of the mold cavity surface 19,
The gas layer formed on the mold cavity surface 20 corresponding to the side surface of the thick rib 4 instantaneously leaks to the outside of the mold from the dividing line 18 of the mold nest, which is a gas outflow path, and becomes thick. The sink generated on the design surface side of the rib 4 cannot be sufficiently prevented.

However, when the dividing line 18 of the mold insert is located at a position satisfying the expression (1), when the pressurized gas narrows the root of the thick rib 4, the design surface side of the thick rib 4 is changed. At the same time as pressing against the fixed mold side cavity surface 2a, the back side is also pressed against the mold cavity surface 19 corresponding to the top of the thick rib 4, so that the dividing line 18 of the mold insert is made of resin. Closed.

The same applies to the division by the mold insert near the side wall 3. As shown in FIG. 6, the dividing line 17 of the mold nest corresponds to the mold cavity surface 2 corresponding to the back side of the side wall 3.
If it does not exist on the same plane as the side wall 2, the pressurized gas narrows the side wall 3 and the molten resin is pressed against the mold cavity surface 21 corresponding to the top of the side wall. Leakage of pressurized gas can be prevented.

Therefore, the leakage of the pressurized gas in the initial stage of cooling and solidifying the molten resin can be prevented, and a molded article having an excellent design surface can be obtained. In practicing the present invention, the present inventor first disclosed in JP-A-9-31.
As proposed in 532 and Japanese Patent Application No. 10-125651, when the temperature of the cavity surfaces 2a and 2b of the mold 1 is kept relatively high, the pressure of the gas layer is more excellently maintained.

This is because the rapid cooling of the non-design surface side is suppressed, and the thickness of the skin layer on the non-design surface side is reduced when the gas layer is formed. Since the fluidity of the resin is maintained well, the narrowing of the base of the thick rib 4 by the gas pressure as shown in FIG. 3 is promoted, and the molten resin is effectively pressed against the mold cavity surface 19. This is because the dividing line 18 of the mold nest is closed.

In addition, since the narrowing of the base of the thick rib 4 is promoted, the design surface side of the thick rib 4 is effectively pressed down against the fixed mold side cavity surface 2a. Combination with the gas pressure injection molding method proposed by the inventor in Japanese Patent Application Laid-Open Nos. 9-31532 and 10-125651, and the appearance level of the design surface is further improved at a relatively low gas pressure. Goods can be obtained.

According to the method of the present invention, the gas pressure does not concentrate on the thick ribs 4 unlike the hollow injection molding method of injecting a pressurized gas into the thick ribs 4 and the vicinity of the thick ribs 4 Since the difference in the transferability of the design surface in the thin portion is small, uneven gloss can be prevented. Therefore, the molded product obtained after the completion of cooling has a very excellent appearance without sink marks and uneven gloss on the design surface.

The mold mating surface 16 of the mold 1 used in the present invention
If the gas discharge efficiency from the mold is low, air in the mold cavity 2 before the injection of the molten resin, a volatile component gas generated from the molten resin, a part of the pressurized gas, etc. stagnate on the design surface side, resulting in a thick wall. In some cases, sink marks may be induced on the design surface side corresponding to the rib 4.

In this case, as shown in FIG. 7, a gas discharge path 2 to the outside of the mold is provided around the fixed mold side cavity surface 2a.
3 and the cavity surface 2a and the groove 24 are provided.
If the vent slit 25 is provided in between, the gas remaining between the molten resin and the cavity surface 2a is quickly discharged to the outside of the mold, and the above-mentioned problem can be solved. The width and depth of the groove 24 depend on the volume of the molded product.
It is preferable to make it as large as possible, specifically, width,
It is preferable that both the depths be about 3 mm or more and 7 mm or less.

The depth of the vent slit 25 is as follows.
It is preferable to set the size as large as possible within a range where the molten resin does not enter. Depending on the type of the resin to be used and the injection pressure of the molding machine, it is specifically 2/100.
It is preferable that the thickness be not less than mm and not more than 5/100 mm. The width of the vent slit 25 is also preferably as large as possible. May be provided.

The pressure of the pressurized gas necessary to prevent sinking of the design surface of the uneven molded product varies depending on the thickness of the thick rib 4 and the like, but is generally in the range of 1 to 25 MPa, preferably 3 to 25 MPa. 15 MPa. The timing of injecting the pressurized gas into the mold cavity 2 may be before or after the completion of resin injection, but preferably the molten resin is sufficiently filled in the mold cavity 2. Immediately after is desirable.

Further, in order to prevent leakage of the pressurized gas, it is effective to press the molded product against the design surface side with the pressurized gas and simultaneously apply a resin holding pressure to replenish the resin. As shown in FIG. 8, the thickness of the thick portion of the molded product effective for preventing sink marks in the present invention is w at the base of the thick portion and t at the thin portion around the thick portion. In the case, w ≧ (3 /
5) This is the case of an uneven thickness molded product having a thick portion such as t. The present invention is particularly effective when it is desired to omit secondary processing such as painting with an uneven thickness molded product having such a thick portion.

As the pressurized gas that can be used in the present invention, an inert gas such as nitrogen is preferable, but air or carbon dioxide gas may be used. The resin that can be used in the present invention is not particularly limited as long as it is generally called a thermoplastic resin.

For example, rubber-reinforced styrene resins such as polystyrene (PS), high-impact polystyrene (HIPS), and medium-impact polystyrene (MIPS), and styrene-acrylonitrile copolymer (SA)
N resin), acrylonitrile-butyl acrylate rubber-styrene copolymer (AAS resin), acrylonitrile-ethylene propyl rubber-styrene copolymer (AE
S),

Acrylonitrile-polyethylene chloride-styrene copolymer (ACS), ABS resin (for example, acrylonitrile-butadiene-styrene copolymer, acrylonitrile-butadiene-styrene-alpha-methylstyrene copolymer, acrylonitrile-methyl methacrylate-butadiene- Styrene resins such as styrene copolymer) and modified polyphenylene ether (m-PPE).

Acrylic resin such as polymethyl methacrylate (PMMA). Low density polyethylene (LDPE),
High density polyethylene (HDPE), polypropylene (P
Olefin resins such as P). Polyvinyl chloride (PV
C) and vinyl chloride resins such as polyvinylidene chloride. Vinyl chloride resins such as ethylene-vinyl chloride copolymer and ethylene-vinyl chloride-vinyl acetate copolymer;

Polyethylene terephthalate (PET),
Polyester resin such as polybutylene terephthalate (PBT). Polycarbonate resins such as polycarbonate (PC) and modified polycarbonate. Polyamide 66,
Polyamide-based resins such as polyamide 6 and polyamide 46.
Polyacetal (POM) resins such as polyoxymethylene copolymers and polyoxymethylene homopolymers.

Other engineering resins, super engineering resins. For example, polyethersulfone (PES), polyetherimide (PEI), thermoplastic polyimide (TPI), polyetherketone (PE
K), polyether ether ketone (PEEK), polyphenylene sulfide (PPS) and the like. Cellulose derivatives such as cellulose acetate (CA), cellulose acetate butyrate (CAB), and ethyl cellulose (EC). Liquid crystal polymers such as liquid crystal polymers and liquid crystal aromatic polyesters.

Thermoplastic polyurethane elastomer (TP
U), thermoplastic styrene butadiene elastomer (SB
C), thermoplastic polyolefin elastomer (TP
O), thermoplastic polyester elastomer (TPE)
E), thermoplastic vinyl chloride elastomer (TPVC),
Thermoplastic elastomers such as thermoplastic polyamide elastomer (TPAE); Also, a blend of one or more of the above thermoplastic resins or a thermoplastic resin called a polymer alloy may be used. Thermoplastic resin, filler,
It may contain additives and the like.

[0039]

Next, embodiments of the present invention will be described.

[0040]

Example 1 An uneven thickness molded article having a shape as shown in FIG. 9 was molded by a gas pressure injection molding method according to the present invention. The mold of this molded product has almost the same structure as that of FIG. 1, and the thickness of the uneven thickness molded product is 2.0 mm at the base of the substrate portion 26 and the side wall 27, and the thickness of the root of the ribs 28 to 30 is the same. 4.0 mm.

The movable mold side cavity surface of the present mold corresponds to the mold cavity corresponding to the center of the top of the ribs 28 to 30 and the outer periphery of the top of the side wall 27 (the dotted line portion in FIG. 10). The surface is divided by mold nesting, and no sealing material is provided on the divided surface.

A portion 31 on the back surface of the substrate portion 26 is a contact point with a gas injection pin, and a gas injection pin is provided on the mold cavity surface on the movable mold side corresponding to this position. Four gas injection pins are provided, one each in each of four areas surrounded by the side wall 27 and the ribs 28 to 30. Immediately after filling the resin, pressurized gas was injected from the gas injection pins in each of the above areas. Note that nitrogen gas was used as the pressurized gas.

The resin used was ABS resin, "Styrac 191F" manufactured by Asahi Kasei Corporation. Evaluation is based on the following conditions: In injection molding under the following conditions, when the weight of the molded product when the mold cavity is just filled with resin is defined as 100%, the weighing value of the molding machine is changed under the same conditions, and the weight of the molded product is reduced. The test was performed by recording the change over time in the pressure of the pressurized gas pipe when the pressure became 101%. Table 1 shows the results.

The sink of the design surface of the obtained uneven thickness molded product was evaluated by a three-dimensional surface roughness measuring instrument, and the design surface on the rib 29 was used as a measurement object. Table 1 also shows the results. The measuring machine and the measuring conditions are shown below.

(Molding conditions) Molding machine: SG220 manufactured by Sumitomo Heavy Industries, Ltd. Mold temperature Cavity surface on movable mold side: 61 ° C. Cavity surface on fixed mold side: 60 ° C. Pressure of pressurized gas: 18 MPa Pressurized gas injection time: 3 sec

Time until the pressurized gas in the mold is released to the atmosphere: 25 seconds Cylinder temperature: 240 ° C. Injection speed: 100 mm / s Injection pressure: 14 MPa (hydraulic oil pressure) Resin holding pressure: 28 MPa (hydraulic oil) Pressure) × 5sec

(Measurement machine)-Equipment name: "SURFEST 500" manufactured by Mitutoyo Corporation-Detector: Diamond needle contact type sensor (conical 90
° · Radius of tip curvature 5 µm) (Measurement conditions) · Scanning speed: 2 mm / sec · Sampling pitch: 10 µm · Measurement length: 40 mm

[0048]

[Comparative Example 1] Using a mold having a shape as shown in FIG. 11, molding was performed in the same manner as in Example 1, and the same evaluation (measurement of design surface sink was on rib 34) was performed. Was. Table 1 shows the results. In addition, the movable mold cavity surface of the present mold is divided by a mold insert at a mold cavity surface corresponding to the top end portion (the dotted line portion in FIG. 12) of the ribs 34, 35, and 36.

The mold of the molded product is obtained by replacing the movable mold nest of the mold used in the first embodiment.
2 and the side wall 33 have the same dimensions as the molded product of the first embodiment. The rib shape is different from that of the molded product of Example 1, and the rib thickness is 4 mm for the rib 34 and 2 m for the rib 35.
m, the rib 36 is 3 mm, and the rib 37 is 2 mm. A portion 38 on the back surface of the substrate portion 32 is a contact point with a gas injection pin, and a gas injection pin is provided on a mold cavity surface on the movable mold side corresponding to this position.

[0050]

Comparative Example 2 The same molded article as in Comparative Example 1 shown in FIG.
The movable mold cavity surface is not divided by the mold insert, that is, the integrated mold insert is incorporated into the movable mold of the mold of Comparative Example 1, molded in the same manner as in Example 1, and subjected to the same evaluation. Was. Table 1 shows the results.

[0051]

[Table 1]

[0052]

The present invention is as described above. The portion of the mold cavity surface into which the pressurized gas is injected is divided by the mold nest, and the sealing material is added to the mating surface of the mold components. Even without installation, leakage of pressurized gas can be prevented, and a molded product excellent in appearance without sink marks or uneven gloss on the design surface can be obtained. In addition, the conventional restriction on the mold structure is relaxed, and effects such as improvement of the degree of freedom in mold design and suppression of mold production cost are brought.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a mold used in the present invention.

FIG. 2 is a sectional view showing an example of a gas injection pin.

FIG. 3 is an explanatory diagram of the principle of preventing sink marks.

FIG. 4 is a cross-sectional view showing a part of a mold in which a fixed cavity surface is divided at a top of a rib.

FIG. 5 is a cross-sectional view showing a part of the mold in which the position of the dividing line of the mold nest at the top of the rib is not preferable.

FIG. 6 is a cross-sectional view showing a part of a mold in which a fixed mold cavity surface is divided on a design surface side at a top of a side wall.

FIG. 7 is a diagram showing an example of a gas discharge path on the surface of a fixed mold cavity.

FIG. 8 is a diagram showing dimensions of a thick portion.

FIG. 9 is a view showing an uneven thickness molded article molded in an example.

FIG. 10 is a view showing an uneven thickness molded product molded in an example.

FIG. 11 is a view showing an uneven thickness molded product molded in a comparative example.

FIG. 12 is a view showing an uneven thickness molded product molded in a comparative example.

Claims (2)

[Claims] 1. A gas pressure injection molding method for an uneven thickness molded product having a substrate portion, a side wall portion, and a thick rib provided on a back surface side of the substrate portion, wherein the molded product cavity is formed with a design surface side mold. The dividing line of the mold nest on the mold cavity surface corresponding to the back surface of the molded product, which is formed by the back side mold, and which is the projecting side of the thick rib exists on the surface including the top of the thick rib. The mold is filled with resin, and pressurized gas is injected between the filled resin and the mold cavity surface corresponding to the back of the molded product, and the design surface of the molded product is pressed against the corresponding mold cavity surface. A gas pressure injection molding method characterized by the above-mentioned. 2. The mold nesting parting line is present on a surface including the top of the side wall, and the mold cavity surface corresponding to the molded product back surface of the side wall and the mold nesting parting line are on the same plane. 2. The gas pressure injection molding method according to claim 1, wherein there is no gas.