JPS5838126A - Extruded hollow molding of polyolefin and molding method thereof - Google Patents

Abstract

PURPOSE:To obtain a molded item whose outer surfaces have a gloss value of 75% or more and a surface roughness of 0.2mu or less, by clamping a parison by molds whose inner surfaces are in the state of a mirror and which are heated to a temperature not less than the melting point of a polyolefin minus 30 deg.C, and blowing a gas into the parison so that the parison is brought into firm contact with the inner surfaces of the molds. CONSTITUTION:The parison 1 is clamped by the molds 2, 2 whose inner surfaces are made to mirror surfaces by means of plating or the like and which are heated to a temperature not less than the melting point of a polyolefin minus 30 deg.C, and a gas is blown into the parison 1 so that the outer surfaces of the parison 1 are brought into firm contact with the inner surfaces of the molds 2, 2. Thus the extruded hollow item of the polyolefin whose outer surfaces have a gloss value of 75% or above and a surface roughness of 0.2mu or below.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an extrusion hollow molded product of polyolefin having a smooth and glossy outer surface, and a method for molding the same.

Extrusion blow molding is a type of thermoplastic resin molding, and uses polyethylene, polypropylene, polyvinyl chloride, polyester, ABS, polycarbonate, etc., among which so-called polyolefins such as polyethylene and polypropylene are representative.

However, extruded hollow molded products made of polyolefin, especially high-density polyethylene, have a drawback in that even when a mold with a mirror-like inner surface is used, the outer surface always becomes matte and has low gloss. It's here. Conventionally, the gloss of the outer surface has been approximately 10 to 30%, and as an improvement measure, multilayer products have been seen in which the exterior is coated with a different type of polymer such as nylon, which has excellent gloss.

This invention was arrived at as a result of various studies in order to improve the above-mentioned drawbacks, and the gist of the invention is to provide a polyolefin whose outer surface has a gloss level of 4 cm or more and a surface roughness of 0, &μ or less. The parison is clamped using an extruded hollow molded product with a mirror-like inner surface and a mold heated to a temperature 30°C lower than the melting point of the polyolefin, and gas is blown into the parison to tighten the outer surface of the parison. This is a method for molding an extrusion hollow molded product of polyolefin, which is characterized in that it is brought into close contact with the inner surface of a mold.

The present invention will be described in detail below using the drawings.

During extrusion and during molding, a cylindrical parison melted and extruded from a gouer is sandwiched between the mold clamps, and a gas, usually compressed air, is blown into the parison to expand the parison and bring the outer surface of the parison to the inner surface of the mold. After the molds are brought into close contact and cooled and solidified, the mold is opened and the molded product is removed.

Figure 1.2.3.4 and Figures 1a, 2a, 3a.

FIG. 2 is a cross-sectional view and a longitudinal cross-sectional view of a mold for a parison showing a 4aH extrusion blow molding process. As shown in FIGS. 1 and 1a, the parison 1 is clamped by the mold 2.2, and the scissors are further advanced as shown in FIGS. 3 and 3a, and finally, as shown in FIG.

As shown in Figure 4a, the entire outer surface of parison 1 is covered with mold 2.2.
It comes into close contact with the inner surface of the molded product.

Normally, a normal temperature cooling water pipe is provided inside the mold, and even if the mold's inner surface periodically contacts and closely contacts the parison, the temperature of the mold's inner surface is about 40°C. Compressor set pressure 5~
By blowing compressed air at a rate of 7 kg/cm2, the parison 1 comes into contact with the molds 2, 2 as shown in Figure 2.2a and Figure 3.3a, but among polyolefins, high-density polyethylene is crystalline and Since the solidification rate is high, it is thought that the parison hardens instantly, and the outer surface of the parison becomes matte with fine irregularities, resulting in a gloss level of only about 10 to 30%. Also, the air pressure inside the parison is shown in Figure 2.2.
a, Fig. 3.3a The pressure in the parison is lower than the set pressure of the compressor until it reaches Fig. 4.4a, and the solidification of the parison progresses in this state until the blowing time elapses. Although the air pressure increases and becomes saturated, no improvement in the glossiness of the outer surface of the parison is observed even if the set pressure of the compressor is raised to about 1 kg/cm'.

In response to such a phenomenon, the present invention clamps the parison with a mold whose inner surface is mirror-like by means of plating or the like and is heated to a much higher temperature than usual, and blows gas into the parison.
They have discovered that by bringing the outer surface of the parison into close contact with the inner surface of the mold, it is possible to obtain an extruded hollow molded product whose outer surface has high gloss and low surface roughness. In the present invention, the gloss level is 4T, based on IS z8741, and the surface roughness is based on JISBO601 (center line average roughness: Ra).

The inner surface of the mold of the present invention needs to be finished to a mirror finish, for example 6 in JIS BO601°.
It is preferable to have a polished bottom and chrome plating on top.

Furthermore, the mold is heated by pouring heating oil into the piping inside the mold or by incorporating an electric heater into the mold.This heating delays the solidification of the outer surface of the parison, but the temperature is determined by the temperature of the polyolefin used. The temperature is 30° C. lower than the melting point, and if it is lower than that, high gloss and small surface roughness cannot be obtained. The upper limit of the above-mentioned temperature is sufficient as the melting point of the polyolefin used, and even if it exceeds the upper limit, an effect can be obtained, but it is wasteful in terms of thermal efficiency.

After the outer surface of the parison is brought into sufficient contact with the mirror-like inner surface of the mold, which is heated to a much higher temperature than usual, the mold is cooled down, but this cooling is done by injecting a refrigerant into the piping inside the mold as usual. Just let it flow. Of course, it is possible to alternately heat and cool the same mold, for example, a heating medium and a refrigerant may be alternately flowed through the same pipe, or an electric heater and a refrigerant pipe may be provided together. Even if the refrigerant starts flowing at the same time as or immediately after the start of blowing, due to heat transfer from the parison and deviations in temperature control, the inner surface temperature of the mold will still be 30 degrees lower than the melting point of the polyolefin used even 10 seconds after the start of blowing. ℃
If the temperature is low or above, the refrigerant may start flowing at the same time as or immediately after the start of blowing.

The present invention will be explained in more detail by giving Examples and Comparative Examples below. Extrusion blow molding machine with a screw diameter of 50 mm on each side and mirror finish on the inner surface with chrome plating, JIS
Center line average roughness (Ra) according to BO601 is 0.1
μ, and using a mold that allows heated oil and room temperature oil to flow alternately into the same pipe, the set pressure of the compressor is set to 6 kg /
cm2, the blowing time is 2 minutes, the average wall thickness is 2m, and the internal volume is 4.
A 50 cc square container was molded, the body of the molded product was cut out, and the gloss and surface roughness were measured.

Example 1, Comparative Example 1 Melt index 0.3 g/l 0 μm, density 0
．． 9509/CC, high density polyethylene with a melting point of 132°C, was molded at a maximum resin temperature of 210°C.

When the inner surface temperature of the mold was 110°C, the flow of heated oil was stopped at the same time as blowing was started, and 5 seconds later, room temperature oil was flowed as a coolant. The gloss of the molded product obtained was 86% and the surface roughness was 0.1μ. there were. The temperature of the mold inner surface 10 seconds after the start of blowing is 1
The temperature was 05°C. For comparison, a molded product obtained in the same manner except that oil at room temperature was continued to flow and the mold inner surface temperature was set at 50° C. had a gloss level of 11 inches and a surface roughness of 1.8 μm.

Example 2. Comparative Example 2 Melt 70-index 0.5 El/10 m
Molding was performed using W, a polypropylene homopolymer with a melting point of 159°C, at a maximum resin temperature of 230°C. At the mold inner surface temperature of 140° C., the flow of heated oil was stopped at the same time as blowing was started, and after 3 seconds, room temperature oil was flowed as a coolant. The resulting molded product had a gloss level of 87φ and a surface roughness of 01μ. Note that the temperature of the inner surface of the mold 10 seconds after the start of blowing was 130°C.

For comparison, the mold inner surface temperature was 6.
The gloss of a molded product obtained in the same manner except that the temperature was 0°C was 5.
The surface roughness was 0.8μ.

Example 3, Comparative Example 3 Melt index o, 29710 mm, density 0935
g/cc, medium density polyethylene with a melting point of 125°C was molded at a maximum resin temperature of 220°C. Mold inner surface temperature 1
At 00° C., the flow of heated oil was stopped at the same time as the blowing was started, and 5 seconds later, oil at room temperature was flowed as a coolant. The resulting molded product had a gloss level of 82% and a surface roughness of 0.2 μm. Note that the temperature of the inner surface of the mold 10 seconds after the start of blowing was 100°C. Also, for comparison, the inner surface temperature of the mold was 30° when room temperature oil was continuously flowing.
The glossiness of the molded product obtained in the usual manner as C is 1
The surface roughness was 2.2μ.

[Brief explanation of the drawing]

Figure 1.2.3.4 is a cross-sectional view of the parison and mold showing the extrusion hollow molding process, Figures 1a, 2a, 3a. 4a is a longitudinal sectional view of the same. 1...Parison, 2...Mold. Patent applicant Showa Denko K.K. agent Patent attorney Sei Kikuchi - Figure 1 Figure 2 Figure 3 Figure 4

Claims (2)

[Claims] (1) An extruded blow-molded polyolefin product characterized by an outer surface having a gloss level of 75 or more and a surface roughness of 0.2μ or less. (2) Clamp the parison with a mold whose inner surface is mirror-like and heated to a temperature 30°C lower than the melting point of the polyolefin, blow gas into the parison, and convert the outer surface of the parison to the inner surface of the mold. A method for molding an extrusion hollow molded product of polyolefin, which is characterized in that it is brought into close contact with a polyolefin.