JPS5948114A - Press-molding of polyolefin resin - Google Patents

Abstract

PURPOSE:To prevent cold mark from generating by a method wherein the resin is press-molded under the conditions that the temperature of the molten resin is kept between 250 deg.C or above and 295 deg.C or below and yet the temperature of metal mold is kept between 70 deg.C or above and 130 deg.C or below. CONSTITUTION:The resin is press-molded under the conditions that the temperature of the molten resin is kept at 250-295 deg.C, preferably at 260-290 deg.C and the temperature of the metal mold is kept at 70-130 deg.C, preferably at 80-120 deg.C. Molded pieces with excellent physical properties and external appearance such as with no cold mark can be obtained efficiently. If the temperature of resin exceeds 295 deg.C, the deterioration of the resin starts, while if the temperature of the resin is below 250 deg.C, the temperature of the metal mold must be set higher so as not to eliminate the generation of cold mark, resulting in allowing to prolong the pressing time necessary for molding and rendering to be impracticable.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to press molding of polyolefin resin. Compared to injection molded products, molded products obtained by melting and plasticizing polyolefin resin (hereinafter referred to as resin) using an extruder, supplying it between Hio molds for presses, and cooling it under pressure with a press, There is no need to provide a gate on the mold, and the molten resin flows uniformly within the mold during pressing, so there is little orientation of the resin, there is no weak point near the gate in the molded product, and there is no gap for the web (or Not only can a molded product with good physical properties be obtained because of the low t4 < possible), but also because there is no orientation or residual strain, a molded product without problems such as twisting or deformation can be obtained.

17 However, when molding is performed using this method, the molten resin placed in the mold I flows due to the press pressure and fills every corner of the mold cavity to form a molded product, but
．． The part of the molded product where the molten resin first comes in contact with the mold is noticeably less shiny than the other parts, causing problems such as 1 stiffness and small wrinkles (see below for cold marks). ) occurs, making it impossible to obtain a molded product with a good appearance.

Such cold marks occur (4) The molten resin is placed on the mold until press pressure is applied and the molten resin begins to flow 17 inside the mold. This is thought to be because the temperature of the part of the molten resin in contact with the mold is lower than that of the resin that flows and is molded by the press pressure in the surrounding area as heat is removed from the mold.

As a result of intensive research, the present inventors solved this problem using the method described below.

That is, in the present invention, the temperature of the molten resin supplied onto the mold is set to 2.
50°C or higher, 295°C or lower, preferably 260°C or higher,
The temperature of the mold surface to which the molten resin is supplied is 70°C or higher and 130°C or lower, preferably 80°C or higher, at 290°C or lower.
By press-molding at 120° C. or lower, molded products with good physical properties and appearance without cold marks can be efficiently obtained. The reason why the temperature is defined in this way is as follows. That is, even if the temperature of the molten resin is within the above range, there is no effect unless the molding temperature is within the range specified above. The reason why the upper limit of the resin temperature is set to 295° C. or less is because if it exceeds this temperature, the resin starts to deteriorate and the physical properties deteriorate.

If the resin temperature is lower than 250°C, the mold temperature must be set higher to avoid cold marks; if the mold temperature is too high, it will be close to the temperature at which resin assimilation begins. , the time required for the molten resin to cool and solidify in the mold, that is, the required pressing time, becomes longer, which is not practical.

In this way, by taking into account the solidification temperature of the molding resin used, setting the molten resin temperature within the above temperature range, and setting the mold temperature lower within the above range for press molding, there will be no cold marks. Good molded products can be obtained efficiently.

The polyolefin resin referred to herein refers to homopolymers of olefins such as ethylene and propylene, or copolymers thereof, or copolymers of α-olefins and other monomers, and refers to the polymers of these polymers. It is a mixture.

Furthermore, it also includes blends with appropriate amounts of fillers such as talc and calcium carbonate, and elastomers.

Next, it will be explained in detail using examples.

Examples 1 to 6 A press machine was used in which the door platen was fixed and the platen was moved up and down by hydraulic pressure.

The mold has a bottom of 200m, a height of 20mm, and a thickness of 1゜2.
This is a device for molding a box molding area of mm.The female mold is attached to the lower platen of the press machine, the male die is attached to the upper platen, the upper platen is raised to open the mold, and the lower platen is placed on the lower platen. A device was used in which a molding resin that had been melted and plasticized at a constant temperature by an extruder was supplied to the center of the mold, and the upper platen was lowered using hydraulic pressure to tighten the male and female molds and apply pressure to form the mold.

The male and female molds each have an electric heater embedded in them, allowing the temperature of the mold surface to be adjusted.

As molding resins, (^) polypropylene with MFRIQ, (B) polypropylene copolymer with MFR of 30 and made by copolymerizing 5% ethylene and 95% propylene, and (CIMFR14, high-density polyethylene with a density of [0,965]) Three types of resins were used.These resins were molded at the molten resin temperature and mold temperature shown in Table 1.The pressing time from the time the mold was tightened to the start of mold opening is also shown in Table 1.

When molded in this manner, cold marks appear at the center of the outside bottom of the box molding field.

The molding results are shown in Table 1. Items marked with an ○ mark in the table indicate that a box forming field with a good appearance and no cold marks was obtained.

Comparative Examples 1 to 6 Molding was performed in the same manner as in the example except that the mold temperature was changed from that in the example. Molding conditions and molding results are shown in Table 2. An X mark indicates that a cold mark was generated at the center of the outside bottom of the box molding field.

Claims (1)

[Claims] When the polyolefin resin in the molten state (4) is supplied to the mold (2) and the blur is reduced to obtain a molded product (17), the mold is removed.
(2) the temperature of the resin supplied to the mold is 250°C or higher and 295°C or lower, and the temperature of the mold to which the molten resin is supplied is set to 70°C or lower.
A method for press molding polyolefin resin, characterized by molding at a temperature of 130° C. or higher and 130° C. or higher.