JPS585772B2 - Manufacturing method of transparent 2D high-temperature composite products - Google Patents

Description

DETAILED DESCRIPTION OF THEINVENTION The present invention relates to a blow molding method for producing a biaxially oriented polypropylene resin hollow molded article having excellent transparency.

The present invention particularly relates to a method for blow molding a cylindrical preform (parison) of the same resin below its melting point.

When a crystalline synthetic resin is stretched below its melting point, molecular orientation occurs in that direction, increasing its strength.

A material such as polypropylene will not become transparent even if it is blown at a temperature above its melting point, but when it is biaxially stretched, not only does it gain increased mechanical strength but it also becomes very transparent.

Therefore, when a cylindrical parison is blown below its melting point,
In particular, at temperatures where the yield point exists, it is difficult for expansion to begin with the gas pressure used in normal blow molding (where the parison is pulled above its melting point); however, if gas is blown in at excessive pressure, the parison will only expand sideways and will not become a product.

Therefore, a method has been proposed in which one end of a cylindrical parison is clamped or its bottom is sufficiently stretched in the axial direction with a rod or the like, and then compressed gas is pumped in to inflate it.

When a large number of bottles were molded from polypropylene resin using this method, a large number of fine vertical cracks were formed in the bottle walls, which is undesirable in itself and also reduces transparency.

As a result of attempts by the present inventors to improve the molding method itself, including the above-mentioned drawbacks, they have developed a new biaxially stretched blow molding method for polypropylene resin that allows easy blowing and expansion of parisons, has a good yield, and eliminates the above-mentioned drawbacks, resulting in a transparent, elegant, and strong product.

The present invention is a biaxially oriented blow molding method for polypropylene resin, in which a cylindrical polypropylene resin hollow molding preform below its melting point is clamped between split blow molding dies and the parison is subjected to mechanical stretching in the axial direction for blow molding, in which the mechanical stretching in the axial direction applied to the parison is stopped before it exceeds the yield point of the resin and necking begins, and during or after that, blow molding is performed using compressed gas.

Even if pressurized gas is injected into a parison below its melting point, the component of force that stretches it in the axial direction is small, so it must be stretched mechanically.

However, while there is no problem with polyvinyl chloride resin or vinylidene chloride resin, which have good transparency, when the molded product is extended in the axial direction to the bottom end of the mold as in the past, the above-mentioned drawback occurs with polypropylene resin.

The mechanical stretching force applied to the parison in the axial direction supplements the stretching force due to the axial gas, but mechanical means alone may be used up to the start point of necking stretching, and when pressurized gas is supplied at the same time, the resultant force is made to exceed the yield point of the material.

The biaxial orientation of the parison by the pressurized gas is preferably carried out at an initial stage just past the yield point, including simultaneously with the axial elongation of the parison.

It is preferable that the parison undergoes a transition to biaxial orientation mainly caused by the pressurized gas before necking occurs due to mechanical stretching in the axial direction.

An investigation was carried out into the relationship between axial stretching and the occurrence of fine cracks in molded products that were blown after axial stretching of a polypropylene parison. It was found that when the parison was stretched beyond the point at which necking began, cracks would occur even if the parison was then blown in all directions with compressed gas, and that the occurrence of cracks became more severe as the ratio of stretching to the original length of the parison increased.

As shown in the graph in Figure 3, when the relationship between tensile force and elongation of the material used in molding at the same temperature as during molding is shown, there is initially a section a where the material elongates in proportion to the tensile force, and after passing a peak called the yield point A, the cross-sectional area of the material decreases and eventually a constriction called a neck appears, the force required for pulling decreases slightly, and thereafter only the test piece elongates with the tensile force that describes the gentle line portion C.

The necking starting point refers to a point C where the tensile force vs. elongation curve suddenly changes from the section b, which is past the yield point, to the gentle necking extension section C.

After passing the yield point, molecular orientation of the material begins, and at the necking starting point, molecular orientation of the material is almost completed locally, and at the necking stretching portion C, the neck of the material does not become tight, and the neck region stretches and necking stretching is carried out.

It is believed that oriented and non-oriented molecules are mixed in the region b between the yield point and the necking onset point.

The present invention is characterized in that the insufficient axial elongation of the polypropylene parison due to the pressurized gas is not compensated for by mechanical means to an extent that exceeds the yield point of the material, and unoriented molecules remain in this section, and mechanical elongation is limited before it reaches the starting point of necking elongation where the unoriented molecules disappear, but is stopped to avoid excessive axial mechanical elongation that would cause necking elongation, and the transition to blow-expansion molding mainly using pressurized gas is completed before the stage where the unoriented molecules disappear.

The present invention will now be described with reference to examples.

FIG. 1 shows a bottomed parison of injection-molded polypropylene resin heated to a temperature below the melting point at which molecular orientation is facilitated,
1 is a cross-sectional view of the blow molding split molds 6, 6 sandwiched therebetween.

1 is a parison, 2 is a parison holding mold, 3 is a blowing core mold that holds the parison from the inside, 4 is an extension rod that extends the parison in the axial direction, 5 is a passage for the pressurized gas for blowing, 6
, 6 are split molds for blow molding engraved with the shape of the desired product.

A parison, heated to a desired temperature and clamped in a blow mold, is initially in position l.

Next, the extension rod 4 is extended, and while the parison is in the position 1' where the molecular orientation is incomplete in that direction, compressed gas is fed in, and the parison is inflated in all directions until it is pressed against the inner walls of the blow molds 6, 6, as shown in the explanatory diagram of FIG. 2, to produce a container T such as a bottle.

At this time, in order to prevent the parison from becoming misaligned, it is preferable to lead the leading end of the parison slightly with the extension rod until it reaches the bottom end 8 of the mold.

Polypropylene (MFI-1, density -0.90 to 0.
A parison having the shape shown in FIG. 1 was injection molded using a resin having a viscosity of 1.91 g/cm, melting point of about 150° C., trade name Showaromer EGI10 manufactured by Showa Yuka Co., Ltd.

The diameter was 20 mm, the height was 70 mm (2 mm of which was the neck and the remaining 50 mm was blown in), and the wall thickness was 3.0 mm.

The injection molding conditions were 210°C and injection pressure 970v/cm'.
, Injection time 10 seconds, cooling time 10 seconds, injection mold is 18
The mixture was cooled with 0° C. water at 1,000 cc/min.

This parison is cooled to about 80° C., and after the completion of injection molding, the injection core mold (injection molding system not shown) is removed from the parison, and then the parison is removed from the injection mold together with the parison holding mold 2.

Next, the blowing core mold 3 is inserted from the mouth of the parison 1,
After the parison is heated to 32° C. (heating mechanism not shown), it is clamped between split blow molds 6, 6.

Here, compressed air was supplied experimentally, but the pressure was 1.2 kg/cm
The expansion of the parison did not occur until about 100°C.

When air with a pressure of 2.0 kg/cm or more was blown into the parison, the parison underwent localized lateral expansion at the abdominal portion, but did not expand in the axial direction and did not become a product.

Next, the parison 1 having the shape shown in FIG. 1, which had been heated to 132° C., was clamped between dies 6, 6, stretched using the stretching rod 4, and air was blown in at a pressure of 2.0 kg/cm at various stretch rates of the parison to expand it in all directions to form a bottle having a height of 80 mm, a diameter of 50 mm, and a capacity of 150 cc, as shown in FIG. 2.

The relationship between tensile strength and elongation at 132° C. of the material used in this experiment is approximately as shown in FIG.

The stretched portion of the parison was 50 mm, and each sample was stretched by changing the length of the rod 4 in steps of 5 mm, and the blowing of compressed air was started at each step.

At this time, as shown in the table below, the haze value of the molded product increases gradually after passing the yield point, and as it approaches the point at which necking begins, even small cracks begin to appear. After it is stretched axially beyond the point at which necking begins, it can no longer be called a transparent product, even if it is inflated with compressed air.

The experimental results are shown in the following table together with examples and comparative examples.

The parison is stretched in the axial direction. The elongation of the parison relative to its original length when the blowing of compressed gas begins is 0.38 mm.
10 3.6//3
20 8.6//4 30
17.3 Slightly fine cracks occur // 5 40 21.0 Fine cracks begin to increase.Comparative Example 1 50 (Necking 28.7 Beyond the starting point of fine cracks 4) Many cracks and haze, not suitable for use as a transparent product.Comparative Example 2 Blowing pressure 2.0 kg Without axial stretching, the parison expands laterally but does not stretch in the axial direction, and a product cannot be made.

If the axial stretch is delayed, the part that first comes into contact with the die will become thicker, and the part that stretches later will become thinner.

[Brief description of the drawings]

Fig. 1 is a cross-sectional view of a parison held in a blow molding die for explaining the present invention, Fig. 2 is a cross-sectional view of the blow molding completed, and Fig. 3 is a graph showing the relationship between the tensile force and elongation of the resin used in molding at the molding temperature. 1...Parison, 2...Parison holding die, 3...Blow core die, 4...Extension rod, 5...Blow pressure gas passage, 6, 6
...Split mold for blow molding, 7... Molded product, 8... Bottom end of the mold.

Claims (1)

[Claims] A biaxially oriented blow molding method for polypropylene resin in which a parison made of polypropylene resin is blow-molded by mechanically stretching it in the axial direction below its melting point, the mechanical stretching in the axial direction applied to the parison is stopped before it exceeds the yield point of the resin and necking stretching begins, and during or after that, blow molding is performed using compressed gas.