JPH0459211A - Molding method for injection molding - Google Patents

Abstract

PURPOSE:To apply sufficient dwell to resin in a cavity and prevent the generation of sink marks by increasing the clamping pressure, providing the compression process for closing a mold which is slightly opened in the gas injection process and molding. CONSTITUTION:The molding method for the subjection injection molding is first injecting into a cavity of a mold by the short shot and filling therein. Then, pressurized gas is injected into the molten resin filled in the cavity through an injection nozzle of a molding machine or a suitable support in the mold. At that time, the mold is opened slightly by the injection of pressurized gas. Then, the slightly opened mold is closed completely by increasing the mold clamping force putting up resistance to the pressure of pressurized gas. The pressure of pressurized gas and the molding clamping pressure are applied to the resin in the cavity by said arrangement, and sufficient dwell can be applied even to the sites where the gas is not infiltrated by the action of both pressures. The generation of sink marks caused by the heat shrinkage can be prevented by said dwell.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention provides injection molding into thick-walled or uneven-walled parts of a molded product by injecting pressurized gas into a molten resin filled in a cavity. This invention relates to a molding method that prevents sink marks and the like.

(Prior art) Conventionally, a gas inlet for injecting gas is provided in a part of the cavity, and after molten resin is injected into the cavity, gas is injected into the molten resin filled in the gas inlet. Thus, a method is provided for preventing sink marks occurring in thick-walled parts or uneven-walled parts of molded products.

(Problems to be Solved by the Invention) However, in the conventional molding method described above, it is very difficult to completely flow the gas to the tip of the gas inflow portion. If the gas does not flow to the tip of the gas inflow portion, there is a problem in that sink marks occur in the parts of the molded product where the gas does not flow.

(Means for Solving the Problems) The injection molding method of the present invention includes an injection process in which a certain amount of resin is injected into a cavity, and a pressurized gas is injected into the injected molten resin to slightly tighten the mold. Molding is performed by a gas injection step in which the slightly opened mold is opened, and a compression step in which the slightly opened mold is closed by increasing mold clamping pressure.

(Function) In the gas injection process, pressurized gas is injected into the molten resin in the cavity, and after the pressure causes the mold to open slightly, the mold clamping force of this mold is increased in the compression process. Close the slightly open mold. As a result, sufficient pressure can be applied to the resin in the cavity through both the pressure from the pressurized gas and the clamping pressure of the mold, even in areas where the gas cannot reach, thereby preventing sink marks. Prevent occurrence.

(Example) Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

The injection molding method according to the present invention includes an injection process in which a certain amount of resin is injected into a cavity, a gas injection process in which pressurized gas is injected into the injected molten resin, and the mold is slightly opened. The method includes a compression step of increasing mold clamping pressure to close the slightly opened mold.

In the injection process, resin is injected and filled into the cavity of the mold using short shots.

In the gas injection step, pressurized gas is injected into the molten resin filled in the cavity from an injection nozzle of the molding machine or from a suitable position in the mold immediately after the injection step is completed or after a predetermined period of time has elapsed. The pressurized gas injected in this manner selectively flows through the hot spots in the molten resin (low clay spots). At this time,
The mold is closed with a rather low clamping force so that it opens slightly due to the injection of pressurized gas. In other words, the mold opens slightly due to the injection of pressurized gas.

In the compression step, the mold, which was slightly opened in the gas injection step, is completely closed against the pressure of the pressurized gas by increasing the mold clamping pressure. As a result, the pressure of the pressurized gas and the mold clamping pressure are applied to the resin in the cavity, and the action of both pressures makes it possible to apply sufficient holding pressure even in places where the gas cannot reach. This holding pressure can prevent the occurrence of sink marks due to thermal contraction of the resin.

After this pressure is maintained, the pressurized gas is removed and the product is cooled in a cooling process, the mold is opened, and the molded product is taken out.

Next, the results of measuring the sink mark of the molded product 1 when the molded product 1 shown in FIGS. 1 and 2 was molded under various conditions by the method described above will be described. In addition,
The code 2 in Fig. 2 indicates the molded product 1 by injection of pressurized gas.
This is the hollow part that occurs in the

Measurement of molded product l when molded using an injection molding machine with a mold clamping force of 240 tons, using polypropylene (PP) as the resin, molding conditions of resin temperature 250 degrees, mold temperature 45 degrees, and gas pressure 100 kg/-. The sink mark measured at point P was 5 μm.

In addition, using an injection molding machine with a mold clamping force of 240 tons, the resin is polycarbonate (PC), and the molding conditions are resin temperature 3.
When molding at 20 degrees Celsius, mold temperature 90 degrees Celsius, and gas pressure 100 kg/-, the sink mark at measurement point P is 3.5.
It was μm.

Furthermore, a molded product 1 similar to the above was molded using an injection molding machine with a mold clamping force of 240 tons, using polypropylene as the resin, molding conditions such as resin temperature of 250 degrees, mold temperature of 45 degrees, and gas pressure of 10 degrees.
When molding at 0 kg/cd, measurement point P
The sink mark was 50 μm.

This result shows that almost no sink marks occur in the molded products molded by the method according to the present invention.

(Effects of the Invention) As described above, according to the method of the present invention, sufficient holding pressure can be applied to the resin in the mold by the action of the pressurized gas pressure and mold clamping pressure. Sufficient holding pressure is applied even in areas that cannot be reached, and it is possible to prevent sink marks caused by heat shrinkage of the resin.

[Brief explanation of drawings]

1 and 2 show a molded product experimentally molded by the method of the present invention, FIG. 1 is a plan view showing the molded product, and FIG. 2 is a sectional view taken along line nn in FIG. 1. . 1... Molded product P... Measurement point Patent applicant: Sekisui Chemical Co., Ltd. Representative Hiroshi 1) Kaoru

Claims (1)

[Claims] 1) An injection process in which a certain amount of resin is injected into the cavity, a gas injection process in which pressurized gas is injected into the injected molten resin to slightly open the mold, and a gas injection process in which the mold is slightly opened by increasing mold clamping pressure. A molding method in injection molding, comprising a compression step of closing a mold that has been opened.