JPH06254924A - Injection molding method - Google Patents

Abstract

PURPOSE:To improve gloss and luster by preventing sink mark and to prevent irregularities in the glass and luster which will be easily generated on a face of a thick part opposite to a molding side corresponding to a molding position of the thick part, in molding a molded piece having a thick part partially protruding on the rear face side. CONSTITUTION:After a cavity face 2 at a top face side is heated, a melted resin is injected. Between a rear face side cavity face 4 and the resin, pressure gas is injected so as to press the resin to the top face side cavity face 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an injection molding method, and more particularly to an injection molding method involving pressurization of a pressurized gas between a resin in a cavity and a cavity surface.

[0002]

2. Description of the Related Art Conventionally, it has been known to pressurize a pressurized gas between a resin in a cavity and a cavity surface at the time of injection molding (Japanese Patent Publication No. Hei 4-501090). It is also known to pressurize a pressurized gas between a resin and a core surface forming the inner peripheral side of the molded product when molding the thick-walled tubular molded product (JP-A-53-47457). ).

In the above-mentioned conventional technique, the volumetric shrinkage of the resin caused by cooling is suppressed by the pressurization of the pressurized gas to prevent the occurrence of so-called sink marks (recesses formed on the surface of the molded product).
The resin is pressed against the cavity surface by the pressurized gas to improve the mold reproducibility.

[0004]

By the way, recently, there is a great demand for large molded products such as casings of automobile parts and household electric appliances, and cost reduction can be achieved by omitting secondary processing such as painting and plating. As described above, there is a demand for a large-sized molded product which is not only free from sink marks but also has a good gloss and gloss and a good surface condition.

However, even the above-mentioned conventional molding methods have not sufficiently met the above requirements.

In particular, in the above-mentioned large-sized molded article, ribs for reinforcement, bosses for assembly, and the like are often projected to the back side, and the locations where the ribs, bosses, etc. are formed are partially thick parts. In the case of a molded article having such a partially thick portion, the surface opposite to the molding side of the thick portion and corresponding to the molding position of the thick portion (hereinafter referred to as "thick portion corresponding surface") However, there is also a problem that the gloss and gloss of the surrounding area tend to be different.

The present invention has been made in view of the above problems, and in the molding of a molded product having a thick portion that partially projects on the back side, while improving the gloss and luster while preventing sink marks, The purpose of this is to prevent uneven gloss and gloss that are likely to occur on the surface corresponding to the thick portion.

[0008]

For this reason, according to the present invention, when molding a molded product having a partial thick-walled portion protruding to the back side, the surface side cavity surface for molding the surface side of the molded product is heated. After that, the molten resin is injected, and a pressurized gas is injected between the resin in the cavity and the back side cavity surface for molding the back side of the molded product.

The present invention will be further described.

As shown in FIG. 1, the mold 1 used in the present invention is composed of a fixed mold 1a and a movable mold 1b which are assembled with each other with a sealing material 8 interposed therebetween. The front surface side cavity surface 2 for molding the front surface side of the molded product is formed on the fixed mold 1a side, and the rear surface side cavity surface 4 for molding the rear surface side of the molded product is formed on the movable mold 1b side. However, the front side cavity surface 2 may be the moving die 1b side and the back side cavity surface 4 may be the fixed die 1b side.

A space 7 surrounded by the spacer block 5 and the moving side mounting plate 6 is formed on the back side of the movable die 1b. The space between the movable die 1b and the spacer block 5 and the space between the spacer block 5 and the moving side mounting plate 6 are sealed with a sealing material 8, while the pressurized gas pressure inlet 12 leading to the space 7 is provided. It is provided.

A plurality of ejector pins 9 are provided through the movable die 1b. The molten resin is not allowed to enter between the movable die 1b and the ejector pin 9, but a clearance through which gas can pass is left. As shown in FIGS. 2A and 2B, this clearance can be easily provided by making the cross section of the ejector pin 9 not a circle but a shape in which a part of the circle is scraped off.

The rear end of the ejector pin 9 is provided with two ejector plates 10 provided in the space 7.
It is held between a and 10b.

The tip of the protruding rod 11 penetrating through the moving side mounting plate 6 is located at substantially the center of the moving side mounting plate 6, and the sealing material 8 is provided on the moving side mounting plate 6 around it. It is provided. The ejecting rod 11 moves forward to push up the ejector plates 10a and 10b, and thereby ejects the ejector pin 9 into the cavity.

In the figure, 13 is a sprue bush, 14 is a fixed side mounting plate, and a sealing material 8 is also interposed between the fixed die 1a and the fixed side mounting plate 14.

In the present invention, prior to injection of molten resin,
The front side cavity surface 2 is heated. This heating can be performed by sandwiching the inductor 3 for high-frequency induction heating between the fixed die 1a and the movable die 1b, and oscillating the inductor 3 in the sandwiched state, as shown in FIG.

When the heating is performed by such high frequency induction heating, it is possible to selectively heat only the front surface side cavity surface 2 without heating the entire mold 1, and to improve the cooling efficiency after the molten resin injection. There is no major loss.

The surface side cavity surface 2 is preferably heated to a temperature at which the elastic modulus of the resin used decreases to 1/3 or less of room temperature.

In addition to the sandwiching of the inductor 3 as described above, for example, a small inductor 3 is provided from the back side of the fixed die 1a toward the front surface side cavity surface 2.
It is also possible to embed it inside and oscillate the inductor 3 before injecting the molten resin.

The above heating may be performed only on the front side cavity surface 2, but may be performed on the back side cavity surface 4 as well. Further, particularly when the inductor 3 is embedded, only the vicinity of the region where the surface corresponding to the thick portion is formed may be heated.

In the present invention, the molten resin is injected into the cavity of the mold 1 that has been heated as described above to fill the cavity, and then the pressurized gas is injected into the space 7 from the pressurized gas inlet 12. To do. The pressurized gas may be air or carbon dioxide gas, but is preferably an inert gas such as nitrogen.

The pressurized gas pressed into the space 7 flows into the cavity through the clearance left between the movable die 1b and the ejector pin 9. That is, the pressurized gas enters between the resin in the cavity and the back surface side cavity surface 4, and presses the resin in the cavity toward the front surface side cavity surface 2.

By the way, the surface side cavity surface 2 in the present invention is preheated so that the molten resin injected into the cavity is not cooled rapidly even if it contacts the surface. Therefore, when the resin is pressed in the direction of the front surface side cavity surface 2 by the pressurized gas, the fluidity of the resin in the vicinity of the front surface side cavity surface 2 is maintained, and the resin is surely pressed against the front surface side cavity surface 2 for mold reproducibility. And the pressing can be performed uniformly on the entire surface side of the molded product.

After pressurizing gas is pressed in this way, the resin is cooled, the pressurizing gas is discharged, and the molded product is taken out from the mold.

The object to be molded according to the present invention, which is molded in this manner, is a molded product having a partial thick-walled portion protruding to the back side. The molded product having a partial thick-walled portion protruding to the back side refers to a molded product having ribs, bosses, or pressure transmission channels on the back side. Here, the pressure transmission channel is a rib-like thick portion provided to guide the flow of the pressurized gas to be pressed and to facilitate the pressurization of the pressurized gas from the position where the pressurized gas is pressed. Say.

Further, the present invention, as shown in FIG.
When forming the thick portion 17 such that a ≧ (2/3) b, where b is the width of the partially formed thick portion 17 and a is the thickness around the thick portion 17. Especially effective. That is, in the case of such a thick portion 17, the thick portion corresponding surface 18 is likely to have a gloss or gloss different from that of the peripheral portion, and the present invention is effective for preventing it.

Further, instead of providing a clearance between the movable die 1b and the ejector pin 9, as shown in FIG. 5, the ejector pin 9 is formed into a hollow pipe shape so that the molten resin does not enter the tip but gas can pass therethrough. A plug 15 made of sintered metal or the like is provided, and the ejector plate 1 is further provided.
A communication passage 16 for guiding the pressurized gas to the rear end portion of the ejector pin 9 may be provided in 0a and 10b.

[0028]

【Example】

Example 1 Using a mold as shown in FIG. 1, a molded product having a partial thick-walled portion on the back side as shown in FIGS. 6 and 7 was molded by the method of the present invention. . The heating was performed by sandwiching an inductor for high-frequency induction heating between a fixed mold and a movable mold to oscillate, and heating only the surface-side cavity surface to 60 ° C.

The results are shown in Table 1.

Other conditions and evaluation criteria are as follows.

(1) Resin used Polystyrene ("# 400" manufactured by Asahi Kasei Kogyo Co., Ltd.) was dry blended with a carbon masterbatch and colored black.

(2) Molding conditions The above polystyrene was plasticized at 220 ° C. and the injection speed was 6
It was set to 0 to 70 mm / sec. Pressure of pressurized gas is 40k
Pressing was applied at g / cm2 for 30 seconds and the pressure was maintained for 40 seconds. The cooling time was 60 seconds.

(3) Evaluation Criteria As for sink marks, those in which distortion is observed in the copied image are marked with Δ, and those in which there is no distortion in the transferred image are marked with ◯. Regarding the gloss unevenness, the one having a difference in gloss or the occurrence of flow marks on the surface corresponding to the thick portion and the periphery thereof was evaluated as Δ, and the one having no difference in gloss as a whole was evaluated as ○.
A gloss meter (gas tester) was used to measure the glossiness.

Comparative Example 1 Molding was performed in the same manner as in Example 1 except that the front side cavity surface was not heated, and the same evaluation was performed.

The results are shown in Table 1.

[0036]

[Table 1]

[0037]

EFFECTS OF THE INVENTION The present invention is as described above, and not only does not cause sink marks, but also has excellent gloss and luster and does not cause unevenness, so that a molded product having an excellent appearance can be obtained. It is a thing.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a mold used in the present invention.

FIG. 2 is a transverse cross-sectional view near an ejector pin.

FIG. 3 is an explanatory diagram of a method of heating the front surface side cavity surface.

FIG. 4 is an explanatory diagram of a thick portion.

FIG. 5 is a vertical sectional view around an ejector pin showing another example for guiding a pressurized gas to a cavity.

FIG. 6 is a plan view of molded products molded in Examples and Comparative Examples.

7 is a cross-sectional view taken along the line I-I in FIG.

[Explanation of symbols]

 1 Mold 1a Fixed Type 1b Moving Type 2 Front Side Cavity Surface 3 Inductor 4 Back Side Cavity Surface 5 Spacer Block 6 Moving Side Mounting Plate 7 Space 8 Sealing Material 9 Ejector Pins 10a, 10b Ejector Plate 11 Extruding Rod 12 Addition Pressure gas pressure inlet 13 Sprue bush 14 Fixed side mounting plate 15 Plug 16 Communication passage 17 Thick wall 18 Thick wall corresponding surface

Claims (1)

[Claims] 1. When molding a molded product in which a thick portion is partially projected to the back side, a molten resin is injected after heating the surface side cavity surface for molding the surface side of the molded product, and the back surface of the molded product. An injection molding method comprising pressurizing a pressurized gas between a back side cavity surface for molding the side and a resin in the cavity.