JP4113026B2 - Thermoplastic resin composition foamed molding method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an injection molding method for obtaining a foamed molded article by supplying foaming gas to an injection molding machine, kneading and dissolving in a molten thermoplastic resin, and then injecting it into a cavity.
[0002]
[Prior art]
In an injection molding method using a thermoplastic resin, foaming has been studied for the purpose of reducing material, reducing weight, etc., and as a method of performing injection foam molding, the material thermoplastic resin is thermally decomposed. A method of using a chemical foaming agent of a mold as a master batch and a method of injecting a physical foaming agent, that is, a supercritical fluid or a foaming gas, into a cylinder of a molding machine are known.
As a method for injecting foaming gas, Japanese Patent Application Laid-Open No. 2002-79545 proposes an injection foaming method in which a foamed gas is supplied to an injection molding machine at a low pressure to obtain a molded article having a good appearance and a high expansion ratio.
[0003]
However, the conventional method of injecting the foaming gas uses a screw of the same length as that of a normal injection molding machine, and the length of the second-stage metering / plasticizing part for kneading and dissolving the foaming gas is shortened. Since the injection gas and the molten resin were not sufficiently dissolved and kneaded, the gas and the molten resin were separated, and the intended foamed molded product could not be obtained.
If you try to add a gas-impregnated part to the tip of the screw as in the conventional physical foam injection molding device, the total length of the screw will be increased by the added amount, and not only can the existing cylinder be used, but the total length of the molding machine It was necessary to make a major remodeling to extend. Moreover, in such a physical foam molding machine, in order to dissolve more gas, the back pressure is set lower than the pressure of the supplied foam gas.
However, in a physical foam injection molding machine with a conventional length screw using an existing cylinder, if the back pressure is set lower than the supply gas pressure, the metering is performed in a state where the kneading and melting are not performed completely, and the molten resin Injecting is performed with the foaming gas separated from each other, resulting in a phenomenon that a molded product cannot be produced.
[0004]
[Problems to be solved by the present invention]
An object of the present invention is to provide an injection foam molded article for obtaining an injection molded article having a good appearance in an injection molding machine having a screw having an existing screw length, which is a problem of the prior art of physical foaming. It is in.
[0005]
[Means for Solving the Problems]
The apparatus for molding a thermoplastic resin molded article of the present invention includes a plasticizing step for bringing a thermoplastic resin into a molten state in a cylinder of a molding machine, and a foaming gas from a gas supply port into the molten thermoplastic resin. The injection foam molding machine has a gas supply and dissolution process for supplying and kneading and dissolving the gas, and a molding process for obtaining a foamed molded product from the obtained gas-dissolved molten resin. A compression structure is formed, the thermoplastic resin is plasticized in the first stage, and the volume is expanded downstream to reduce the molten resin pressure below the gas supply pressure, and the supplied foam gas and molten resin are kneaded. -Using a molding machine to be melted, the pressure is higher than the gas supply pressure from the second stage compression section to the measuring section.
As a result, even when an existing cylinder and an existing length screw are used, the back pressure is increased by 0.1 to 5.0 MPa higher than the gas pressure to be supplied without plasticizing the entire length of the existing screw. This makes it possible to improve the gas kneading / dissolving property, prevent separation of the foamed gas and the molten resin, and obtain a good molded product.
[0006]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.
In the embodiment of the present invention, each surveying was performed by the following method.
(1) Maximum cell diameter: The cross-section of the foamed part of the foam was observed with a microscope, the cell diameter of 50 cells was determined, and the average value of 10 cells selected in descending order was taken as the maximum cell diameter. When the cell is elliptical, (major axis + minor axis) / 2 was defined as the cell diameter.
(2) Flatness: The light source was reflected on the surface of the molded product, and the presence or absence of irregularities on the surface of the molded product was confirmed.
(3) Silver streak: The appearance was visually observed to confirm the presence or absence of a white streak pattern generated on the surface of the molded product.
[0007]
[Example 1]
When the screw diameter D = φ112 mm as the injection molding machine and the length of the flight part of the screw L, a molding machine with L / D = 18 was used. Carbon dioxide provided by a commercially available cylinder is used as the foaming gas, and a mechanism is provided that can be injected into the cylinder through a pressure reducing valve and a check valve attached to the cylinder. The pressure of the cylinder of carbon dioxide was 7.2 MPa, and the pressure supplied to the injection molding machine was kept at 2 MPa by a pressure reducing valve. The back pressure was kept at 4 MPa. The amount of carbon dioxide injected was 0.7% by weight based on the resin.
[0008]
As a mold for injection molding, the product dimensions are 900mm x 440mm, and the cavity interval (molded product thickness) can be finely adjusted by the motor of the molding machine, and injection is performed by a five-point direct gate with molten resin arranged in the mold. A mold having such a structure was attached to a molding machine, and the mold temperature was set to 50 ° C., the cooling time was 40 seconds, and the resin temperature at the time of injection was 190 ° C. As the molding material, block polypropylene [manufactured by Sumitomo Mitsui Polyolefin Co., Ltd., MFR: 45 g / 10 min (230 ° C.)] was used.
At the time of injection, the mold was set so that the molten resin was not foamed and the cavity gap was fully filled. The time from the end of injection to the start of the core back was 0.5 seconds, and the core back speed was 20 mm / sec. The gap in the mold cavity at the time of injection, at the start of the core back and at the end of the core back was 1.2 mm.
Under the above conditions, the molded product obtained by injection foaming had a cell diameter of 200 to 300 μm, and there was no problem in silver streak generation or product shape. Table 1 shows the injection pressure of the foaming gas, the product thickness of the foamed product, the maximum cell diameter, the flatness, and the occurrence of silver streak.
[0009]
[Example 2]
The same operation as in Example 1 was performed except that the back pressure was set to 3 MPa. The results are shown in Table 1.
[0010]
[Comparative Example 1]
The same procedure as in Example 1 was performed except that the back pressure was set to 2 MPa. The results are shown in Table 1. Although molding was possible, partial foaming failure occurred and the flatness was poor.
[0011]
[Comparative Example 2]
The same operation as in Example 1 was performed except that the back pressure was set to 1 MPa. The results are shown in Table 1. The gas separated from the resin was injected into the cavity, and the molded product was not shaped.
[0012]
[Table 1]

Claims (3)

This is an injection molding method for a thermoplastic resin molded article comprising a gas impregnating and dissolving step of supplying and impregnating and dissolving a foamed gas to a molten thermoplastic resin, and a molding step of obtaining a foamed molded product from the obtained gas-impregnated molten resin. And
The carbon dioxide used as a blowing gas, the supply pressure of the blowing gas during the molten resin metered 1.0
An injection molding method characterized by plasticizing and measuring at a pressure higher by 0.1 to 5.0 MPa than a foaming gas pressure to which a back pressure is supplied at ~ 2 MPa .   The injection port provided with a two-stage compression screw is used, The supply port for supplying foaming gas to a compression part from the supply part of the screw downstream 2nd stage is provided, It is characterized by the above-mentioned. Injection molding method. The injection molding method according to claim 1 or 2, wherein the foaming gas is decompressed to a storage pressure or less and supplied to the molten resin.