JPS59120429A - Molding method of molded foam article - Google Patents

Abstract

PURPOSE:To obtain the titled product having a better smooth surface at a desired part by forcing a surfficient gas body into a mold to check the foaming of a foaming synthetic resin when it is injected and releasing pressure after the injection thereof. CONSTITUTION:After or while with the advance of a screw 2, a foaming synthetic resin 3 plastically melted in an injection cylinder 1 is injected into a cavity 7 unfoamed in the amount of under the capacity thereof, a gas body (e.g. air) preheated to about softening temperature thereof 3 is forced into a cavity at an injection port 8 under a pressure of 25-80kg/cm<2> to form a hollow section 12 in the perimiter 11 of an opening of a pressure air hole 10 before the surface layer of the melted resin is hardened. Thereafter, the gas body forced is released outside a mold and the foaming synthetic resin is foamed into the hollow section to obtain an intended molding. To obtain better results, the contents in the cavity is pressurized with the as body prior to the injection to retain the pressure during the injection.

Description

[Detailed description of the invention] The present invention relates to an injection molding method and apparatus for synthetic resin molded articles.

Conventionally, Mod
In the method described in Ern Plastics magazine, May issue, 1969, p. 69, a thermoplastic resin containing a blowing agent is heated and melted in an unfoamed state, and a mold is pressurized in advance to a pressure sufficient to suppress foaming of the resin. After the surface layer of the resin has solidified, the pressurized gas is depressurized and the injection screw is retracted at the same time, allowing the excess resin to flow back into the injection cylinder, causing only the inside of the molded product to foam. However, in this method, the resin returned into the cylinder is foamed once during the process, so it is difficult to obtain a completely smooth surface in the next shot.

Further, since the backflow of the resin is performed through the nozzle portion of the molding machine, the range and amount of backflow are limited, making it difficult to obtain a uniform foam.

In addition, in Japanese Patent Publication No. 55-25552, after injecting a foamable synthetic resin into a mold, a gas body is press-fitted into the cavity to form a hollow body, and then the gas body is discharged outside the mold to cause foaming to occur in the hollow part. It describes how to make it happen. in this way,
Since the process involves forming a hollow body with one hole, the overall thickness of the molded product cannot be reduced, and the shape of the molded product is also limited.

Furthermore, in the method described in Japanese Patent No. 497133, the cavity is completely filled with a foamable synthetic resin, and after the surface layer has solidified, the volume of the cavity is increased and only the inside is foamed. This method requires a mechanism to expand the cavity volume, making the mold structure complicated.

The present invention provides a method for molding a foamed product having a smooth surface that does not have the drawbacks of the various molding methods described above. That is, after injection of the molten foamable synthetic resin into the cavity in an amount less than the internal volume of the cavity is completed, or during injection, a hole is opened in the cavity to allow only the gas to pass through and not the molten resin. Gas is pressurized through the injection port to form a cavity between the injection port and the molten resin that has already reached the injection port, and after the resin is brought into close contact with the cavity wall other than the cavity, the gas is injected into the injection port. The body pressure is released and the foamable synthetic resin is foamed into the cavity.

At this time, preferably, the gas to be press-injected is previously heated to around the softening temperature of the resin, thereby facilitating the formation of cavities and foaming into the cavities, resulting in a good molded product.

Furthermore, prior to injection, the inside of the cavity is pressurized with an inert gas to a pressure sufficient to suppress foaming of the foamable molten resin.

The surface smoothness is further improved.

Next, aspects of the present invention will be explained in more detail using the accompanying image 1 drawing as an example.

First, in Fig. 1, the foamable resin (3), which is plasticized and melted under such pressure that it is kept in an unfoamed state by the screw (2) in the injection cylinder (1), is inserted into the non-return valve (4).
is injected into the cavity (7) in an unfoamed state and in an amount less than the internal volume of the cavity by opening the and advancing the screw (2). After injection is completed, the check valve (4) closes.
At the same time, a pressure hole (1) provided in the cavity (7)
0) After injecting high-pressure gas to compress the resin and push it to the end of the cavity, the pressure in the pressure hole (1) is released and the inside of the cavity (7) is filled with the foaming pressure of the resin (3).

Figures 2-1 to 2-2 show this process in more detail.
Figure 3. Figure 2-1 shows a state in which injection has been completed.

Figure 2-2 shows a state in which a cavity (12) is formed around the opening (11) of the pressure hole (1Φ) due to high pressure gas from the pressure hole (1), and the resin has reached the end of the cavity. By injecting high-pressure gas, the resin is completely spread to every corner of the cavity and is pressed against the cavity wall, forming a non-foamed surface layer. This shows a state in which foaming has occurred, the surface layer is non-foamed, and the inside is foamed.The cavity (12) disappears as the resin foams due to the depressurization of the high-pressure gas. do.

The pressure of the gas injected into the cavity varies depending on the type of resin used and the shape of the molded product, but is usually between 25 and 80.
In particular, about 40 to 60 kg and 7 cm2 is sufficient. In addition, the closer the press-fitting part is to the end of the cavity, the better the pressure transmission to the resin will be, and the easier it will be to push the resin to the end of the cavity, but the gas inside the cavity will flow from between the resin and the cavity wall to the end of the cavity. It is desirable for the surface distance to be at least about 50 VTM away from the leading edge of the flow of the resin because it will easily leak into unfilled areas and it will be difficult to form the intended cavity.

The timing of injecting and releasing the gas must be selected so that the cavity can be created and recovered before the surface layer of the resin solidifies. For example, it is preferable that the resin be press-fitted within 10 seconds after reaching the opening (11) and then released for at least 5 seconds after dyeing.

However, by pressurizing the inside of the cavity in advance to a level higher than the foaming pressure of the resin, an unfoamed surface layer can be better formed, and a foamed product with a surface equivalent to non-foamed molding can be obtained. .

The gas pressure applied within the cavity to suppress foaming may normally be 201 cg/cm2 or less.

As the resin applied to the present invention, any resin that can be generally injection molded can be used. As the blowing agent, organic or inorganic chemical blowing agents that generate gas upon decomposition, physical blowing agents that are gaseous at the molding temperature, and combinations thereof are used.

The injection molding machine used may be a screw in-line type, a pre-plastic type, a plunger type, etc., but is not limited to these.

Next, examples will be shown.

Example 1 A foamed product was molded according to the process shown in FIGS. 1 and 2. Using a screw in-line injection molding machine, 0.5% by weight of azodicarbonamide as a foaming agent was blended with modified polyphenylene oxide resin (Noryl FN-215: ■ Engineering Plastic Manufacturing), and after plasticizing at 250°C, the short side was 150%. Injection was carried out in 4 seconds from the central direct gate into a rectangular cavity with a long side of 525 mm and a wall thickness of 5 mm. Prior to injection, fill the cavity with air for 12 hours.
The pressure was increased to 1 kg/cm2 and kept constant during injection.

After injection is completed, move 50 m closer to the gate on the movable side cavity surface.
After nitrogen gas was injected for 2 seconds at a pressure of 501 cg/crrL2 through a slit-shaped pressure hole provided concentrically at position m, all gas pressure was released. The gas was released within 1 second, and the molded product was entirely covered with an unfoamed surface layer equivalent to that of a non-foamed molded product, and a foamed molded product having a foamed structure inside was obtained.

Example 2 Molding was performed in the same manner as in Example 1 using the same mold and the same resin. However, the inside of the cavity was not pressurized prior to injection. After the injection was completed, nitrogen gas was injected for 5 seconds at a pressure of 50 kg 7 cm2 as in Example 1, and then
The pressure was released.

A foamed molded product was obtained whose surface was smooth and whose interior had a foamed structure. This is thought to be due to the fact that the scars once foamed on one surface are compressed during the formation of cavities.

Comparative Example 1 Molding was carried out in the same manner as in Example 1 using the same mold and the same resin. However, the experiment was carried out in the same manner as in Example 1 except that the interior of the chamber was not pressurized prior to injection, and the gas was not injected after injection was completed.

The surface of the molded product had silver marks, which are traces of foaming, and was rough.

As described above, by carrying out the method of the present invention, a molded article having a smooth surface and foamed only inside can be manufactured at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an example of an apparatus for carrying out the invention. In the figure, the cylinder 2 of the single molding machine is the screw 3, the molten foamed resin 4, the check valve 5, the moving side mold 6, the sealing material 7, the cavity 8, which communicates with the cavity, and the gas injection hole 9, which is only for the gas body. The molten resin passes through the gas passage 10 and shows a pressure hole opening in the V-bite. 2-1, 2-2, and 2-3 are explanatory diagrams of the present invention. Reference numeral 10 in the figure indicates a pressure hole 11, and a periphery 12 of the opening of the pressure hole indicates a cavity. Agent Mitsuyoshi Ezaki Agent Mitsushi Ezaki Figure 1 Figure 2 Publication Figure 2-2 Figure 2-3

Claims (1)

[Claims] 1. After or while injecting the molten foamable synthetic resin into the mold cavity, a gas is pressurized into the cavity, and before the surface layer of the molten resin has solidified, the gas injection port is closed. 1. A method for molding a foamable synthetic resin molded article, which comprises forming a cavity around the mold, and then releasing a molded gas body outside the mold to foam the foamable synthetic resin into the cavity. 2. The method according to claim (1), wherein the gas to be pressurized is preheated to about the softening temperature of the resin. 6 Prior to injection, the gas body pressure in the cavity is sufficient to suppress the foaming of the foamable synthetic resin. Gas pressure as described in section 5), or either one of the gas bodies