JPH0318570B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to improvements in stretch blow molding methods.

In recent years, when blow molding resins that have a stretching effect such as polyethylene terephthalate or vinyl chloride into containers, etc., stretching the parison in the longitudinal direction increases mechanical strength, making it possible to create thin and lightweight containers. Stretch blow molding tends to be widely adopted because it enables molding.

In this conventionally known stretch blow molding method, a parison (preformed product) that has been formed by a hot parison method or a cold parison method is soaked or heated to a temperature suitable for stretching and then transferred to a blow mold. After stretching or stretching the parison in the longitudinal direction with a stretching rod of approximately 1.5 to 3.0 times the length of the parison, a pressurized fluid such as compressed air is blown into the final molded product.

However, in general, containers formed by the stretch blow molding method have a threaded part for the cap or a sealing surface for the seal in the neck, so it is necessary to prevent the neck from deforming during stretch blow molding or to slowly cool it. The neck needs to be cooled quickly to prevent bleaching, etc.
Since the tip of the stretching rod comes into direct contact with the bottom of the parison, it must be cooled to a certain temperature to ensure a certain level of mechanical strength during stretching to prevent breakthrough during stretching. .

For this reason, it is almost inevitable that the neck and bottom of the parison will already be at a lower temperature than the body of the middle part before the stretch blowing process, and when such a parison is stretched, , only the body part, which is easy to stretch, will stretch, and the other parts will hardly stretch. Even if you mold this, the wall thickness will be uneven, unnecessary parts will be thick, and in severe cases, it will be difficult to stretch during molding. Problems such as blow-off and molding defects were inevitable.

This invention was made in order to solve the above-mentioned problems, and the present invention was made in order to solve the above-mentioned problems.
During blow molding, the above-mentioned problems are solved by varying the moving speed of a stretching rod that stretches the parison in the longitudinal direction.

The following will explain the embodiment shown in FIG.
1 is a neck type, 2 is a blown split type, 3 is a blown bottom type,
These molds 1, 2, and 3 form a blow molding mold 4 .

Reference numeral 5 denotes a stretching rod, to which a variable speed drive device such as a cylinder or the like controlled by a cam or servo valve (not shown ) is connected. The rod 5 moves forward and backward at a speed to stretch the parison, which will be described later.The rod 5 is usually provided with a blow hole, through which a pressurized fluid for blow molding such as compressed air is supplied, and the rod 5 is teeth,
A seal is provided to prevent the pressurized fluid from escaping from the mouth.

6 is a preformed parison, 6' is a parison in the middle of stretching, 6'' is a parison that has been stretched, and 7 is a molded product.

Next, the present invention will be explained step by step.

Figure 2a shows a parison (preformed product) whose temperature has been controlled in advance to a temperature suitable for stretching in the blow molding mold 4 .
6 is inserted, from this state, the stretching rod 5 is passed through the neck mold 1 and inserted into the parison 6, and as shown in FIG.
The stretching rod 5 is moved to position 2 at the same moving (lowering) speed as in the conventional method.

Therefore, up to approximately 1/2 of this amount of stretching, the trunk, which is the easiest to stretch, is mainly stretched and becomes thinner.

From the state shown in Figure 2 b (approximately 1/2 extended state), the moving speed of the stretching rod is suddenly reduced (1/3 of the initial speed).
The following speed) or move from this state (approximately 1/2 extended state as shown in Figure 2 b) for an appropriate time (depending on the conditions such as the material resin to be processed and the shape of the molded product, for example) After rapidly decelerating (several seconds) (to 1/10 or less of the initial speed), the speed is increased again to the initial speed and stretched.

During this deceleration stretching, the thinner part that was stretched in the previous process is hardly stretched and is cooled more rapidly than the thicker part, so the temperature of the thinner middle part (body part) is The temperature approaches or is lower than that of the upper and lower parts (near the neck and bottom where they cooled earlier), and the strength also increases due to being stretched.

Therefore, in the deceleration stretching from this state to the final stretching position shown in FIG. Due to the increase in tensile strength due to the stretching effect caused by the stretching process, the parts near the neck and bottom, which had hardly elongated in the previous process, remain relatively high temperature and have low tensile strength. , since the part is mainly stretched,
As shown in FIG. 2c, the thickness of the parison after stretching is almost uniform over its entire length.

From this state, pressurized fluid is blown into the blowing hole provided in the drawing rod 5 (not shown), and blow molding is carried out as shown in FIG. 2d.
As shown in Figure 2, the molded product is molded into the shape of the final molded product, the blow molding mold is opened and the mold is released, and one step is completed.

In addition, in this example, an example was explained in which blow molding is performed after the completion of the stretching process, but blow molding may be performed simultaneously with the stretching process, and in this case,
When reducing the moving speed of the drawing rod, the blowing amount is also reduced in accordance with the moving speed.

In addition, as shown by line a in Figure 3, when the stretching rod is stretched at a constant speed that is reduced from approximately the middle part of the stretching stroke to the final stroke, or depending on conditions such as the stretching length or the molding material, After rapidly reducing the moving speed of the drawing rod for a certain period of time as shown in line b in the figure,
Although the case where the speed is increased again is illustrated, it goes without saying that the deceleration start position, deceleration time, speed, etc. can be changed within a range that does not depart from the gist of the present invention depending on the molding conditions.

As described above, according to the present invention, by continuously changing the stretching speed of the stretching rod, the wall thickness of the parison after stretching can be made approximately uniform, so that the molded product can also have a uniform thickness distribution. At the same time, the stretching effect is also approximately the same, so that there is no local decrease in mechanical strength due to overstretching or insufficient stretching.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the conventional stretch blow molding process, Fig. 2 is a schematic diagram showing the stretch blow molding process of the present invention, and Fig. 3 is a line showing the relationship between the stretching stroke and time of the present invention. It is a diagram. 5: Stretching rod, 6: Parison.

Claims (1)

[Claims] 1. A stretch blow molding method in which a preformed parison is stretched in the longitudinal direction or after being stretched in the longitudinal direction, and then a pressurized fluid such as compressed air is blown into the shape of the final molded product. A stretch blow molding method characterized in that the parison is stretched by changing the moving speed of a stretching rod during the process of stretching the parison in the longitudinal direction. 2. The stretch blow molding method according to claim 1, wherein the stretch rod is moved at high speed to approximately 1/2 of the amount of stretching, and then decelerated to 1/3 or less of this speed. 3. A patent for moving a drawing rod at high speed to approximately 1/2 of the amount of drawing, then decelerating to 1/10 or less of this speed, and then increasing the speed to less than the initial speed. A stretch blow molding method according to claim 1.