Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Controlling fluid pressure in direct contact with molding...
Reexamination Certificate
1998-10-26
2001-09-04
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Controlling fluid pressure in direct contact with molding...
C264S523000, C264S540000, C264S526000, C264S528000, C425S526000
Reexamination Certificate
active
06284171
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a blow molding process of a plastic product, and more particularly to a blow molding process in which a cooling is performed from the insides of a mold and a hollow molded product.
2. Description of the Invention
In a blow molding, after a sleeve-like molten resin called a parison extruded from an extruder is sandwiched between mold halves of a split mold, air is blown into the parison to inflate it so that a configuration of the mold cavity is applied to the inflated parison, and then the parison is cooled and hardened within the mold to obtain a desired hollow molded product.
In the blow molding, although the outside of a hollow molded product such as a bottle, or the like is cooled because it contacts the mold through which a cold water circulates, the inside of the hollow molded product is not cooled because the inside is in a heat insulated state. In case the inside of the hollow molded product is not cooled as just discussed, the pressure inside the hollow molded product becomes an original pressure of air to be introduced and the hollow molded product can be cooled by being intimately contacted with the mold. However, since only the cooling effect is obtained from the mold, cooling efficiency is bad.
A cooling method is disclosed in Japanese Patent Laid-Open Publication (Tokkai) No.3(1991)-13313, in which the pressure inside the hollow molded product is, in order to reduce the time for cooling, maintained at a prescribed pressure level and then air is discharged while blowing air into the interior of the hollow molded product, so that the hollow molded product is cooled also from the inside.
Japanese Patent Laid-Open Publication (Tokkai) No.3(1991)-222714 discloses a device in which an air blowing tube for introducing air, and a discharge tube for discharging air within a hollow molded product are, in order to enhance the inside cooling efficiency, located away from each other so that the air flows the entire hollow molded product. Further, Japanese Patent Laid-Open Publication (Tokkai) No.5(1993)-104615 discloses a device in which an air blowing port is rotated under air pressure so that the air is blown to the entire hollow molded product.
However, in case air is discharged while blowing air into the hollow molded product, the pressure within the hollow molded product becomes lower than the original pressure of the air. For this reason, the force for urging the parison against the mold becomes small compared with a case where no inside cooling is performed, and the cooling effect from the mold is lower than in the case where no inside cooling is performed.
That is, in the conventional inside cooling process, the cooling effect from the inside is available but on the other hand, the cooling effect from the mold is lowered. As a consequence, it is unable to obtain a sufficient cooling effect.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a blow molding process in which the cooling effect from a mold and the cooling effect from the inside of a hollow molded product are effectively obtained and therefore, the hollow molded product can be blow molded under a large cooling effect.
As a result of a hard study for achieving the above object, the present applicant found out the fact that by setting the inside gas pressure of the hollow molded product within a certain range, the cooling effect from both the inside and outside of the hollow molded product can be efficiently and sufficiently exhibited without lowering the cooling effect from the mold and by cooling the inside with a prescribed quantity or more of cooling gas such as air, or the like.
The present invention has been made based on the above finding. According to the present invention, there is provided in a blow molding process in which a gas is blown into a parison within a mold to inflate the parison so that the parison is intimately contacted with a mold shaping portion to form a hollow molded product, and the gas within the hollow molded product is discharged for the purpose of cooling, the blow molding process being characterized in that an internal gas pressure of the hollow molded product is set to 5 to 8 kgf/cm
2
(G) and the gas satisfying the expression F≧10×W where F is a gas flow (N 1/min) and W is a hollow molded product weight (g) is discharged while blowing.
The inside gas pressure of the hollow molded product and the flow rate of the gas are defined within the above-mentioned ranges based on the following test result.
In
FIG. 2
, the temperature for removing the hollow molded product is measured without performing an inside cooling but by changing the original pressure (blowing pressure) of air. The molding conditions are as follows.
Bottle capacity
200 ml
Bottle weight
20 g
Resin
high density polyethylene (HDPE)
Resin thickness
body portion 0.5 to 1.5 mm
bottom portion 1 to 3 mm
Blowing time
6 sec.
Discharging time
1 sec.
Blowing air temperature
20° C.
Mold cooling water temperature
17° C.
As shown in
FIG. 2
, the lower the pressure within the bottle is, the more the force for urging the parison against the mold is decreased. For this reason, the effective contact area with the mold is reduced. Further, since the molded product is contracted with the progress of cooling, a gap between the molded product and the mold is enlarged. As seen, when the original pressure of air is decreased, the cooling effect from the mold tends to be decreased and the temperature for removing is increased.
The present applicant found out from this result the fact that if the original pressure (blowing pressure) of air is set to 5 to 8 kgf/cm
2
(G) or more, the cooling effect from the mold becomes constant. The reason seems to be as follows. If the force for urging the resin against the mold is 5 to 8 kgf/cm
2
(G) or more, the effective contact area of a molten resin with the mold is unchanged and the molded product is not easily removed from the mold even if the molded product is contracted. As a consequence, the cooling effect from the mold becomes constant.
In
FIGS. 3 and 4
, in case the inside cooling is performed, when the original pressure of air is 5 kgf/cm
2
(G) or 10 kgf/cm
2
(G) and the removing temperature is measured by varying the pressure within the bottle. In
FIGS. 3 and 4
, in case the pressure within the bottle is 5 kgf/cm
2
(G) when the original pressure of air is 5 kgf/cm
2
(G), it indicates a state that no inside cooling is performed. In case the pressure within the bottle is 10 kgf/cm
2
(G) when the air original pressure is 10 kgf/cm
2
(G), it indicates a state that no inside cooling is performed.
First, let's review the case where the original pressure of the bottle is 5 kgf/cm
2
(G).
Up to 2 to 5 kgf/cm
2
(G) of the pressure within the bottle, the removing temperature is decreased because the cooling effect from the inside overcomes the decrease of the cooling effect from the mold. For example, in case the pressure within the bottle is set to 4 kgf/cm
2
(G) and the flow rate of the air is set to 100 N1/min, the removing temperature is lowered by 10 to 15° C., compared with the case where no inside cooling is performed.
If the pressure within the bottle is lowered to 2 kgf/cm
2
(G) or less, an apparent removing temperature is not changed in spite of the fact that air of the flow rate of 200 to 230 N1/min flows because the cooling effect from the inside is offset by the decrease of the cooling effect from the mold. In this case, the removing temperature is lowered by 15° C. compared with the case where no inside cooling is performed.
However, in case the air was flowed at a flow rate of 300 N1/min by increasing the original pressure of air to 10 kgf/cm
2
(G) while maintaining the pressure within the bottle at the level of 6 kgf/cm
2
(G), the removing temperature could be lowered by 20° C. to 30° C. compared with the case where no inside cooling was performed.
It should be noted, however, that in case the pressure within the bottle is maintained at
Ichinohe Kenji
Nonomura Akira
Ootani Ken-ichi
Kao Corporation
McDowell Suzanne E
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Silbaugh Jan H.
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