Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Including application of internal fluid pressure to hollow...
Reexamination Certificate
2000-05-24
2003-04-01
McDowell, Suzanne E. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Including application of internal fluid pressure to hollow...
Reexamination Certificate
active
06540956
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention claims priority from Japanese Patent Application No. 9-277616 filed Oct. 9, 1997, which is incorporated herein by reference. It relates to the molding of plastics, particularly to the shape of the mold used in molding techniques, wherein the deformation occurring during cooling is predicted and the plastic molded product is removed from the mold at high temperature.
2. Description of Related Art
A widely practiced method of molding plastic is to extrude high-temperature molten plastic in a tubular shape, enclose this in a mold and cause it to expand by blowing air into the tube. A conventional example of this will be explained with reference to
FIG. 3
to
FIG. 6
, which show the process of manufacturing a container by blow molding.
As shown in
FIG. 3
, molten plastic in tubular shape (the parison) is extruded into the middle of a split mold, and as shown in
FIG. 4
, the mold is then closed. As shown in
FIG. 5
, when air is blown into the molten plastic, the plastic adheres to the inner wall of the mold and assumes the same shape as this inner wall. At this point in time the molten plastic is at a high temperature of for example 200° C., and it is cooled and solidified by keeping it in the mold while continuing to blow in high-pressure air.
The time required for this cooling varies according to the type of plastic and the form of the molded product, and cooling time has hitherto been determined on the basis of the criterion that deformation due to thermal shrinkage of the resin after removal from the mold is linear. As shown in
FIG. 6
, when the mold is opened, the molded product is removed.
The molded product shown in
FIG. 6
is a container (a bottle) which will be marketed after being filled with a liquid, and the resin temperature at which a mold is opened is usually about 50° C. A dozen or so seconds are required for this cooling.
It thus takes time to cool the high-temperature molten plastic to a point at which the mold can be opened. Production per unit time and production cost are in inverse proportion, and in a manufacturing process in which time management is carried out in units of seconds, even a short cooling time of a dozen or so seconds should be shortened in order to achieve lower production cost.
In order to obtain data relating to the shortest practical cooling time, the inventors performed repeated experiments in which a molded product was released from a mold while still at a high temperature. These experiments showed that if a mold is opened up before the conventionally employed cooling time has elapsed, the high-temperature molten plastic shrinks greatly and undergoes nonlinear deformation, so that the target molded product shape is not obtained. In other words, it was found that cooling time could not be shortened.
It would therefore be desirable to predict the nonlinear deformation of a high-temperature molded product after it has been released from a mold, and to develop a mold which enables molten plastic to be molded in such a way that the shape after deformation is the desired shape.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a mold configured such that a target molded product shape can be obtained even though the cooling time in the plastic molding process is shortened.
Plastic which has been released from a mold undergoes shrinkage deformation in the course of cooling to ordinary temperature. Conventionally, removal of the plastic at a low temperature ensures a regularity in the resulting deformation, i.e. the deformation is linear, and therefore by taking the shrinkage factor into account when designing the mold size so as to make the mold suitably larger, the manufacturability of the target molded product shape (i.e. its design dimensions) can be guaranteed.
However, the broad regularity mentioned above is not found in the course of the nonlinear shrinkage which occurs when a plastic molded product is removed from a mold at high temperature. The inventors have therefore invented a method and apparatus for mold design whereby a target molded product shape is obtained even when a molded product is removed from a mold at high temperature. This is achieved by using the finite element method to simulate deformation behavior, and by taking this deformation into account beforehand when fabricating the shape of the mold (Japanese Patent Application Laid-open No. No. 9-277260). This method and apparatus for mold design ensure that a mold shape which takes nonlinear deformation into account in advance is achieved.
According to a first aspect of the present invention, a mold for blow molding a blow thermoplastic molded product has a mold cavity formed so that the external volume of the molded product obtained by blow molding in this cavity ultimately shrinks by at least 5.8% and at most 13.5% of the volume of the cavity.
High-density polyethylene (HDPE) can be used for the aforementioned thermoplastic, and in this case the mold cavity is preferably formed so that the external volume of the molded product obtained by blow molding in this cavity ultimately shrinks by at least 7.0% and at most 13.5% of the volume of the cavity. This volume shrinkage factor is measured when the temperature has decreased to a normal temperature (20° C.) from the temperature at which the HDPE begins to crystallize (130° C.), and assumes a coefficient of linear expansion of 5.98×10
−4
/° C.
Polypropylene (PP) can also be used for the aforementioned thermoplastic, in which case the mold cavity is preferably formed so that the external volume of the molded product obtained by blow molding in this cavity ultimately shrinks by at least 5.8% and at most 8.5% of the volume of the cavity.
The mold cavity can also be formed so that the external volume of the molded product obtained by blow molding in this cavity ultimately shrinks by at least 5.8% and at most 13.5% of the volume of the cavity, and so that in the region of maximum shrinkage the external size of the body portion of the molded product shrinks by at least 1.0% and at most 10.5% of the size of the corresponding portion of the mold cavity.
If the aforementioned thermoplastic is HDPE, the mold cavity is preferably formed so that the external volume of the molded product obtained by blow molding in this cavity ultimately shrinks by at least 7.0% and at most 13.5% of the volume of the cavity, and so that in the region of maximum shrinkage the external size of the body portion of the molded product shrinks by at least 1.5% and at most 10.5% of the size of the corresponding portion of the mold cavity.
Alternatively, if the aforementioned thermoplastic is PP, the mold cavity is preferably formed so that the external volume of the molded product obtained by blow molding in this cavity ultimately shrinks by at least 5.8% and at most 8.5% of the volume of the cavity, and so that in the region of maximum shrinkage the external size of the body portion of the molded product shrinks by at least 1.0% and at most 10.5% of the size of the corresponding portion of the mold cavity.
According to a second aspect of the present invention, a blow molded product formed using the aforementioned mold, and a method using the aforementioned mold, are provided.
According to a third aspect of the present invention, a blow molded product on the body portion of which an in-mold label has been affixed during blow molding using the aforementioned mold is provided, said in-mold label excluding labels which do not thermally shrink after removal from a mold.
As has been explained above, the present invention is capable of providing a mold such that a target molded product shape can be obtained even though the cooling time in the plastic molding process is shortened.
REFERENCES:
patent: 4482518 (1984-11-01), Brady, Jr.
patent: 4631159 (1986-12-01), Maeda et al.
patent: 5378421 (1995-01-01), Salame
patent: 5405667 (1995-04-01), Heider
U.S. patent application Ser. No. 09/169,129, filed Oct. 9, 1998, abandoned.
Hata Masaharu
Iwamoto Hiroshi
Nishimine Naohide
Tamiya Yuko
Kao Corporation
McDowell Suzanne E.
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