JPH0258089B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to press molding of polypropylene resin. Compared to injection molded products, molded products obtained by melting and plasticizing polypropylene resin (hereinafter referred to as resin) using an extruder, supplying it between male and female press molds, and cooling it under pressure in the press are faster than injection molded products. There is no need to provide a gate, and the molten resin flows uniformly within the mold during pressing, so there is little orientation of the resin, there are no weak points near the gate in the molded product, and there are no (or fewer) welds. Therefore, not only can a molded product with good physical properties be obtained, but also because there is no orientation or residual strain, a molded product without problems such as twisting or deformation can be obtained. However, when molding with this method, the molten resin placed on the mold flows due to the press pressure and fills every corner of the mold cavity, forming a molded product. The parts that come into contact with the mold have a significantly lower gloss than other parts, and problems such as small wrinkles (hereinafter referred to as cold marks) occur, resulting in a molded product with a good appearance. I can't do that. Such cold marks occur because the molten resin is placed on the mold from the time the molten resin is placed on the mold until the molten resin begins to flow inside the mold due to press pressure. The part in contact with the mold loses heat from the mold, so the temperature of the resin flowing and molding due to the press pressure in the surrounding area is lower than that of the mold.
This is thought to be due to a decrease in temperature. As a result of intensive research, the present inventors solved this problem by the method described below. That is, in the present invention, the temperature of the molten resin supplied onto the mold is 260°C or more and 295°C or less, and the temperature of the mold to which the molten resin is supplied is 70°C or more and 120°C or less. , it is possible to efficiently obtain molded products with good physical properties and appearance without cold marks. 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, it will not be effective unless the mold 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 begins to deteriorate and its physical properties deteriorate. If the resin temperature is lower than 250℃, the mold temperature must be set higher to avoid cold marks, because if the mold temperature is too high, it will be close to the temperature at which the resin starts solidifying. The time it takes for the molten resin to cool and solidify within the mold, ie, the required pressing time, becomes longer, which is not practical. In this way, taking into account the solidification temperature of the molding resin used,
By setting the molten resin temperature within the above temperature range and setting the mold temperature lower within the above range for press molding, a good molded product without cold marks can be efficiently obtained. The polypropylene resin referred to herein is a propylene homopolymer, a copolymer thereof, or a mixture of these polymers. 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 4 A press machine was used in which the lower platen was fixed and the upper platen was moved up and down by hydraulic pressure. The mold is for molding a box molded product with a bottom opening of 200 mm, a height of 20 mm, and a thickness of 1.2 mm.The female mold is installed on the lower platen of the press machine, and the male mold is installed on the upper platen. , the upper platen is raised to open the mold, the molding resin melted and plasticized at a constant temperature by an extruder is supplied to the center on the lower platen, and the upper platen is lowered using hydraulic pressure to tighten the male and female molds and apply pressure. , using a molding device. Furthermore, an electric heater is embedded in each of the male and female molds, so that the temperature of the mold surface can be adjusted. The molding resins are (A) polypropylene with an MFR of 10, (B) a polypropylene copolymer with an MFR of 30 and made by copolymerizing 5% ethylene and 95% propylene, and (C) a high density MFR of 14 with a density of 0.965. Three types of polyethylene resins were used. These resins were molded at the molten resin temperature and mold temperature shown in Table 1.
Table 1 also shows the press time from when the mold is tightened to when the mold begins to open. If a cold mark appears when molded in this way, it will appear at the center of the outside bottom of the box molded product. The molding results are shown in Table 1. Items marked with a circle in the table indicate that a molded box with no cold marks and a good appearance 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 the example.The molding conditions and molding results were
Shown in the table. A box molded product with a cold mark on the center of the outside bottom is marked with an x.

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Claims (1)

[Claims] 1. When supplying molten polypropylene resin onto a mold, pressing, and cooling to obtain a molded product, the temperature of the resin supplied onto the mold is set at 260°C or higher and 295°C.
A method for press molding polypropylene resin, characterized in that the temperature of the mold to which the molten resin is supplied is 70°C or higher and 120°C or lower.