JPS5842823B2 - Injection molding method for thermoplastic resin molded products - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a technique for preventing mold corrosion during injection molding of a thermoplastic resin that generates corrosive gas when heated and melted.

Conventionally, when injection molding thermoplastic resins that generate corrosive gaseous substances through sublimation, evaporation, decomposition, etc., there is a problem in that the mold corrodes due to the gaseous substances, and once corrosion begins, the mold must be repolished before use. However, it is not possible to polish down to the finest details, and as a result, corrosion progresses and the mold becomes unusable.

Conventionally, methods to solve this problem include applying a rust preventive agent to the inner surface of the mold cavity after molding, treating it with a neutralizing solution, plating finishing of chrome steel, and more recently stainless steel. Measures such as using steel are being taken.

However, none of these methods is a fundamental solution.

Corrosive substances are already adhered to the inner surface of the mold cavity, and the rust preventive must be applied after these have been completely removed.

However, it is impossible to completely remove the details of injection molds, which often have complex shapes, and the shelf life of the rust preventive agent is not very long.

The treatment method using a neutralizing liquid requires the steps of neutralization → washing with water → water removal, and it is difficult to completely remove moisture, and corrosion may occur due to residual moisture.

Furthermore, as in the case of applying a rust preventive agent, it is difficult to neutralize the details of the cavity.

The method of plating the inner surface of the mold cavity with chrome steel, etc. is a widely used method, and since single-layer plating has extremely little effect on corrosion, three-layer plating is usually used. It is being done.

Even in this case, pinholes cannot be completely avoided, and corrosion from them can naturally occur.

In addition, many products have complex designs, and even cores, slide parts, etc. that do not directly touch the product surface must be plated, and although the processing is expensive, its effectiveness is not always guaranteed. I couldn't say it was enough.

Recently, stainless steel, which has excellent corrosion resistance, is sometimes used as a material for injection molding molds, but it has extremely poor workability in mold manufacturing and is very expensive. As a material, it is extremely versatile.

All of these corrosion prevention measures inject gaseous substances that cause corrosion into the mold cavity together with heated molten resin, so they are only symptomatic treatments based on the assumption that corrosion is occurring and cannot be called a fundamental solution. It is something.

The inventors have completed this invention as a result of intensive research into a new technology that can completely solve the above problems.

That is, the present invention involves injecting a thermoplastic resin that generates corrosive gas when heated and melted into a substantially sealed mold cavity that is previously pressurized with pressurized gas above atmospheric pressure. It is an injection molding method characterized by
More specifically, vents are provided through the product cavity and pores of the injection mold to allow gas to flow into the product cavity at least at a pressure sufficient to prevent the generation of corrosive gaseous substances from the thermoplastic resin during injection molding. After injecting resin that can generate corrosive gaseous substances when heated and melted in a state filled with
This is an injection molding method that completely prevents corrosion of the mold by releasing pressurized gas to a diameter of about 10 sand and sealing the gaseous substance in the injected resin.

In this invention, the dimensions of the pores need to be of such thickness, width, and distance that the resin will not flow during injection.

Generally 5 to 107 with a thickness of 0.01 to 0.05 mm
If the distance is 10 to 20 degrees with a width of nn, the resin will not flow into the pores.

Furthermore, the most effective location for the pores is at the flow end of the resin within the product cavity.

That is, the pressure of pressurized gas is required until the product cavity is filled with resin and the product surface is solidified.

Next, an example of the implementation of the present invention will be explained using the drawings in accordance with the order of molding.

Valve 2 is connected to the closed mold 1 from a pressure source (not shown).
When opened (at this time, the pressure release valve 3 is closed).

), the pressurized air passes through the ventilation holes 4 and pores 5 and fills the product cavity 6 of the desired product shape.

The mold 1 is cooled with a coolant such as water.

In this state, a resin that generates corrosive gaseous substances is injected.

The resin 7 fills the product cavity 6 of the mold through the sprue 8, runner 9, and gate 10.

Next, the valve 2 leading to the pressure air source is closed until the surface layer of the product has solidified, and the pressure gas remaining in the vent hole 4 is released.

After cooling and solidifying the resin 7 in the product cavity 6, the resin 7 is transferred to the mold 1.
It is opened and taken out as a product.

This completes one process. Note that the thermoplastic resin to which this invention is applied is one that can generate corrosive gas when heated and melted.

Examples include hard and soft polyvinyl chloride, blend resins of acrylonitrile-butadiene-styrene copolymer and polyvinyl chloride, and halogen-based, phosphorus-based,
Polystyrene, rubber-reinforced polystyrene, acrylonitrile containing various flame retardants such as metal hydroxide and sulfur compounds.
Examples include flame-retardant thermoplastic resins such as butadiene-styrene copolymer resin, acrylonitrile styrene copolymer resin, polypropylene, high-pressure, medium-low-pressure polyethylene, nylon, polycarbonate, and polyphenylene oxide-polystyrene blends.

Further, the pressurized air used in the present invention may be air, nitrogen, carbon dioxide, or any other gas that does not substantially adversely affect the plasticized resin.

The pressure applied to the mold cavity is determined based on the relationship between the type and amount of corrosive gaseous substances that may be generated, molding temperature, speed, etc.

The pressure is such that corrosive gaseous substances cannot be generated from the resin in the plasticized state, and the gauge pressure is generally 0.1 to 8.
0KP/cIF¥, preferably 0.5 to 10 hours/d.

Naturally, if the conventional corrosion prevention method described above is added to this invention, the effect will be even more remarkable.

The injection molding method for thermoplastic resin molded products of the present invention will be specifically and comparatively explained below using Examples and Comparative Examples.

Example 1 For the purpose of observing corrosion of metal, a test was conducted using a mold in which a copper plate with a well-polished surface was placed at the position shown at 11 in FIG. 11.

The pressure air used was air with a gauge pressure of 7.5 KS'/i, and the raw material used was Styron 492 (Asahi Dow ■).
, registered trademark) and 5.0 parts by weight of a commercially available bromine-substituted organic compound as a flame retardant were used.

This thermoplastic resin was injected into a product cavity previously filled with pressurized air at 210° C., and at the end of the injection, valve 2 communicating with the pressurized air source was closed, and release valve 3 was opened to release the pressurized air.

This was regarded as one cycle, and when the copper plate 11 was observed after 5,000 cycles, no discoloration was observed and no corrosion was observed.

Also, the copper plate was left indoors for more than 72 hours,
No changes were observed.

Comparative Example 1 A mold having the same structure as in Figure 1 was used, except that the ventilation holes 4 and pores 5 were not provided, no pressurized air was used, and the molding conditions and raw materials used were exactly the same as in Example 1. The test was conducted as follows.

As a result, the steel plate 11 began to discolor around 250 cycles, and the discoloration became even darker after 500 cycles.After that, when the copper plate was left indoors, it completely turned black in about 48 hours, and corrosion was clearly observed.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an injection mold used in carrying out the present invention. 1...Mold, 2...Valve, 3...
・Pressure release valve, 4...Vent hole, 5...
Pore, 6...Product cavity, 7...
Resin, 8...Sprue, 9...Runner, 10...Gate, 11...Copper plate.

Claims (1)

[Claims] 1. Injection characterized by injecting a thermoplastic resin that generates corrosive gas when heated and melted into a substantially sealed mold cavity that is previously pressurized with gas above atmospheric pressure. The molding method