BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of molding large containers
having a thin-wall body from an injection-molded, thick-wall closed-end parison
by the process of stretch blow molding.
2. Background Art
It is extremely difficult, because of the desired degree of orientation
and other reasons, to mold a large container (for example, a 20-liter bottle) having
a thin-wall body from an injection-molded, closed-end parison by direct stretch
blow molding. A conventional method thus prepares a relatively large preform as
an intermediate from the closed-end parison before finally forming a large container.
In actual procedures, when the thickness of the wall of the closed-end
parison is remarkably greater than usual (= not greater than 4 mm), conventional
air blowing tends to cause a non-uniform section in the preform. This tendency
is especially observed in resins which do not have self-healing properties, which
polyethylene terephthalate has. Stretch blow molding the preform having such a
non-uniform section into a large container results in a significant un-uniformity
of the wall thickness distribution of the resulting large container, which may
cause critical damage in the molding or forming process.
Polyethylene terephthalate having a wall thickness of greater than
5 mm suffers from whitening, and injection molding a closed-end parison having
the greater wall thickness is thus not suited to form a large container by the
stretch blow molding. The conventional method thus requires a rather complicated
process including the steps of: preparing a dual-structured, thick-wall closed-end
parison by two-step injection molding; and stretch blow molding the thick-wall
closed-end parison to a large container after temperature control.
The conventional method is thus not applicable to form a large container,
for example, a 20-liter bottle, by the process of stretch blow molding, although
it is possible to form a relatively large container from an injection-molded closed-end
From TW-229 174 A a method and apparatus for forming a plastic container
having an enhanced level of crystallinity for improved thermal stability are known.
A preform is expanded by a pulse-blow process one or more times to form an intermediate
article, which is finally expanded to the full container dimensions. The pulse-blow
step is conducted at a relatively high-strain rate to maximize the formation of
crystal nucleation sites, followed by deflation to relax the amorphous orientation.
The final expansion step is conducted at a low-strain rate to minimize the amorphous
orientation. The hereby produced container can be used as a refillable or hot-fill
beverage container. Furthermore, a blow-mold and fluid supply apparatus is provided
for alternatively supplying the high and low-strain rate inflations.
SUMMARY OF THE INVENTION
One object of the present invention is thus to provide a method of
preparing a preform having a uniform wall thickness distribution from a closed-end
parison and easily forming a large container form the preform by the process of
stretch blow molding as well as to provide an apparatus for the same.
Another object of the present invention is to provide a method of
forming a large container by the process of stretch blow molding, which is not
significantly different from the conventional method but only adopts a different
way of air blowing.
The above and the other related objects are realized by a method
including the steps of: preparing a preform from an injection-molded, thick-wall
closed-end parison by repeatedly and intermittently applying an air blow pressure
to the parison and releasing the air blow pressure therefrom, where the preform
has a greater diameter and a thinner wall thickness than the closed-end parison;
and stretch blow molding the preform to a thin-wall large container.
The closed-end parison is released from a mold immediately after
a skin layer keeping a parison shape is formed on the surface of the closed-end
parison having a high inner temperature. The closed-end parison is then molded
to a preform with a preform mold heated to a predetermined temperature, while
the surface temperature of the parison is increased by an internal heat of the
parison. The preform kept at a stretch blow molding temperature is stretch blow
molded in a blow mold to produce a large container.
According to the method of the present invention, part of the closed-end
parison is expanded by application of an air blow pressure, which results in reducing
the wall thickness and decreasing the internal heat of the expanded part. Release
of the air blow pressure gets rid of the internal resistance and accordingly causes
a contraction, which is less than the expansion but returns the wall thickness
of the expanded part to an extent corresponding to the degree of contraction. The
expanded and subsequently contracted part has less internal heat than that of the
residual part. Thus next application of air blow pressure expands another part
by a greater degree than that of the expanded and subsequently contracted part.
Repeated application of air blow pressures successively expands respective
parts of the closed-end parison to finally expand the whole parison. This effectively
prevents critical damage and significant deformation of the parison due to the
partial expansion and gives a preform which is easily molded to a large container.
Repeated expansion and contraction of respective parts of the parison
through the intermittent application and release of air blow pressures easily gives
a preform, even when material applied is a resin which does not have self-healing
properties, which polyethylene terephthalate has. The preform is then blown to
a large container having a uniform wall thickness distribution.
These and other objects, features, aspects, and advantages of the
present invention will become more apparent from the following detailed description
of the preferred embodiment with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
- Fig. 1 is a partially cross-sectional, front view illustrating a molding device
in the open state, which is applicable to stretch blow molding of a large container
according to the method of the present invention;
- Fig. 2 is a plan view illustrating an essential part of the molding device,
where a preform mold is inserted between halves of blow mold;
- Fig. 3 is a plan view illustrating the essential part of the molding device,
where the halves of blow mold are closed while inserting the preform mold;
- Fig. 4 is a vertical cross sectional view schematically illustrating a process
or molding a preform;
- Fig. 5 is a vertical cross sectional view schematically illustrating the blow
mold after the preform mold is removed; and
- Fig. 6 is a vertical cross sectional view schematically illustrating a process
of molding a large container with the blow mold.
Figs. 1 through 6 illustrate an embodiment of molding device applicable
to stretch blow molding of a large container according to the method of the present
invention. A blow mold 1 includes mold halves 12, 12 attached to inside of freely
movable clamping plates 11, 11 and a vertically movable bottom mold 13. The mold
halves 12, 12 have mating surfaces to conform to the shape of a cavity 14 of a
resulting large container. The mold halves 12, 12 are combined to form, on their
upper end, a receptacle 19 for a lip die 3. A support rod 15 for a preform (described
later) is projected from the center of the bottom mold 13 in such a manner as
to allow vertical movement for protrusion and withdrawal. The support rod 15 is
held by a spring member 18 incorporated in a plunger 17 of an elevating mechanism
16 as shown in Figs. 4 through 6.
A closed-end parison 2 prepared by the process of injection molding
is shifted to be positioned above the blow mold 1 while a neck of the closed-end
parison 2 is held by the lip die 3. The closed-end parison 2 is lifted down with
a transfer plate 4 to be placed on the center of the mold halves 12, 12 in the
open state. A blow core 5 and a stretch rod 6 run through the lip die 3 and the
transfer plate 4 received in the receptacle 19 on the upper end of the blow mold
1 in the closed state and are inserted downward into the closed-end parison 2 as
illustrated in Fig. 4.
A preform mold 7 used for molding the closed-end parison 2 into a
preform is attached to a movable board 81 of a horizontally reciprocating unit
8 placed on a base on the side of the blow mold 1. The movable plate 81 is disposed
upright on a front end of a movable bottom plate 83, which is guided by a guide
rail on a table 82 for advance and retreat. The movable plate 81 is linked with
an end of a movable rod 86 of a hydraulic driving unit 85 via a member 84 on the
movable bottom plate 83. Support shafts 87, 87 are horizontally extended over
upper and lower ends of front surface. A pair of upright support plates 88, 88
are attached to both ends of the support shafts 87 to allow a pivotal movement
about a base end, and continuously pressed outward by a spring member 89 spanned
across the support plates 88, 88 and in the vicinity of the support shafts 87.
Frames 90, 90 are further arranged inside the pair of support plates
88, 88. Mold halves 71, 71 of the preform mold 7 are fitted in the frames 90, 90,
where a pair of thermal insulating boards 91, 91 are positioned between the mold
halves 71, 71 and the pair of support plates 88, 88.
The mold halves 71, 71 have mating surfaces to conform to the shape
of a cavity 72. The mold halves 71, 71 are combined to form, an their upper end,
a receptacle 73 for receiving the lip die 3, where the receptacle 73 is placed
at the same height as the receptacle 19 of the blow mold 1. An aperture 74, in
which the support rod 15 is inserted and fitted, is formed on the bottom center
of the mold halves 71, 71.
As shown in Fig. 2, the mold halves 71, 71 of the preform mold 7
in the open state are moved horizontally with the pair of support plates 88, 88
and inserted between the mold halves 12, 12 of the blow mold 1 in the open state.
The mold halves 71, 71 of the preform mold 7 are pressed inward against the force
of the spring member 89 and closed by taking advantage of the force applied to
close the blow mold 1 and transmitted to press the support plates 88, 88 inward.
The mating surface of the mold halves 71, 71 accordingly form the shape of the
cavity 72 of a preform 10.
Release of the closing force by opening the blow mold 1 allows the
mold halves 71, 71 of the preform mold 7 to be automatically opened by means of
the spring member 89 and to be movable, in the open state, outside the blow mold
1 by the reciprocating unit 8.
A typical process of stretch blow molding a large container with
the molding device thus constructed is described hereinafter.
A closed-end parison 2 having a wall portion to be stretched of approximately
10 mm in thickness and a shorter length than a final large container is prepared
by injection molding a polycarbonate. The closed-end parison 2 under the high-temperature
condition is released from an injection mold and a core mold (not shown) while
a neck of the parison 2 is held by the lip die 3. Immediately after the release,
the closed-end parison 2 is shifted with the lip die 3 by means of the transfer
plate 4 to be positioned above the blow mold 1 in the open state.
After completion of the shift, the preform mold 7 heated to 140°
to 150°C is horizontally advanced with the pair of support plates 88, 88 and inserted
between the mold halves 12, 12 of the blow mold 1 (see Fig. 2).
The closed-end parison 2 is lifted down with the lip die 3 and the
transfer plate 4 to be positioned between the mold halves 12, 12. The blow mold
1 is temporarily closed, so that the preform mold 7 is closed and pressed via
the mold halves 12, 12 of the blow mold 1 as illustrated in Fig. 3. The closing
force allows the support rod 15 projected from the bottom mold 13 to be fitted
and received in the aperture 74 formed on the bottom of the preform mold 7.
After the closing of the preform mold 7, the blow core 5 and the
stretch rod 6 are inserted downward through the transfer plate 4 and the lip die
3 into the closed-end parison 2 as illustrated in Fig. 4. This allows the closed-end
parison 2 to be stretched in an axial direction by the stretch rod 6 and expanded
by application of air blow pressures. Air is blown in repeatedly and intermittently,
whereas the stretch in the axial direction is implemented only in the process
of first air blow. The stretch in the axial direction allows the bottom of the
closed-end parison 2 to be securely held between the stretch rod 6 and the support
The air pressure of approximately 2 kg is blown intermittently 4
through 6 times. After each application of air blow pressure, the blown air is
released for the removal of internal pressure and each expanded part is accordingly
contracted back to some extent. It is essential to set an extremely short air
blow time for preventing localized extreme expansion. The final air blow time is
set relatively long, for example, approximately 4 seconds, in order to expand
the whole parison 2 to the shape of the cavity 72. The cylindrical preform 10 having
a wall thickness of approximately 5 mm subsequently undergoes the stretch blow
molding process to form a final large containers.
After the molding of the preform 10 is concluded, the blow mold 1
is opened and the pressing force to the mold halves 71, 71 of the preform mold
7 is removed. The mold halves 71, 71 automatically open by means of the spring
pressure while the preform 10 is kept at its molding position. The preform mold
7 is then retreated horizontally by means of the reciprocating unit 8 and returned
to the position on the side of the blow mold 1. In this state, the preform 10
is supported by the support rod 15 to be positioned on the center of the blow mold
1 as illustrated in Fig. 5. This effectively prevents a draw-down of the preform
10 due to the heat held in the preform 10.
The blow mold 1 is closed and pressed again, so that the preform
10 is stretched in the axial direction by the stretch rod 6 and expanded by application
of air blow pressures. The air blowing process includes a primary low-pressure
step and a secondary high-pressure (8 kg or higher) step. Simultaneously with
or otherwise after the expansion of the preform 10 by the secondary high pressure,
the bottom mold 13 is lifted up to define a bottom mold 13 allows the support
rod 15 to be accommodated in the bottom mold 13 against the force of the spring
member 18 and gives a large container 100 having a wall thickness of approximately
1.2 mm as the final product as illustrated in Fig. 5.
The above embodiment is only illustrative and not restrictive in
any sense. There may be many modifications, alterations, and changes without departing
from the scope of essential characteristics of the present invention. The scope
of the present invention are limited only by the terms of the appended claims.