The present invention relates to a method for producing hollow articles
including lamp globes and packaging containers by means of stretch blow molding
a preform that is injection molded from a synthetic resin such as polyethylene
terephthalate, polycarbonate, and the like.
Packaging containers including PET bottles are well-known hollow
articles produced by means of stretch blow molding. The stretch blow molding is
a technique that an injection molded preform is stretched in an axial direction
while simultaneously expanding it into bottles of various shapes by blowing air
in a blow mold.
Fig. 5 shows an injection mold that is used to mold preforms for
the subsequent stretch blow molding. The injection mold comprises a cavity mold
1 for use in forming a preform body, a neck mold 2 for use in forming a mouth of
the preform, and a core mold 3 which passes through the neck mold 2 into the cavity
mold 1. These components when combined together form a cavity 4 that extends into
the neck mold 2. A molten resin is injected into the cavity 4 through a gate 5
in the bottom of the cavity mold and is filled therein to form a predetermined
The preforms obtained by such conventional means have a gate mark
on the outer surface of the bottom of the bottle because the molten resin is injected
and filled through the gate 5 in the bottom of the cavity mold. The gate mark
remains on the bottom of a final product when the preform is stretch blow molded
into the product. In most cases, the gate mark is located at a concave portion
of the outer surface of the bottom and is unnoticeable from outside when the final
product is held vertically. The gate mark is thus of little significance for PET
bottles for water and carbonated beverages or the like, in which the container
does not need to have any higher level additional value depending on the kind
of the content.
However, the gate mark on the bottom may reduce the additional value
of the container when it is used for accommodating expensive products such as cosmetics.
Especially, displacement of the gate mark, if any, may often prove fatal to the
container and make it a defective product. Therefore, a yield is a concern for
the molding of the containers adapted to containing expensive products.
When a lamp globe is molded by means of the stretch blow molding,
the gate mark on the preform is located at the center of a spherical surface. The
gate mark cannot be eliminated completely even when it is post-treatable. For
the lamp globes for interior use with the spherical surface facing down, treatment
of the gate mark results in fine irregularity on the surface and deteriorates transmittance
at that portion due to diffused reflection of light, shadowing partially the lamp
globe. In addition, the gate mark tends to be visible on the surface due to difference
in surface luminance after the treatment. Thus, there is also a problem that the
globes for both illumination and decoration purposes are considered less valuable
than those made of glass.
The problem of the gate mark on the bottom of the preform can be
solved by means of providing the gate at a position corresponding to the neck of
the preform, as disclosed in Japanese Patent Publication JP 47 038548B.
In this known method, however, a nozzle touching section is provided
on a parting face of a pair of neck molds. Thus the neck molds are pushed aside
and opened due to the injection pressure, which causes necessity of exclusive
mold clamping means for the neck molds. Such clamping means, which are not present
in conventional systems, causes a further problem of complicating a molding machine.
For the molding, since the molten resin is injected at a single location
on the side, the resin hardly flows uniformly to complete filling and the injection
tends to cause a welding line. Accordingly, an advanced molding technique is required
to mold a uniform preform. It is therefore difficult to solve the problem of the
gate mark by means of the above-mentioned known methods.
The present invention is directed to provide a novel method for injection
stretch blow molding hollow articles that solves the problems in the above-mentioned
known methods and that can prevent a trouble caused during use due to the gate
mark on the outer bottom surface thereof as well as prevent reduction of additional
value, even though it is a method of injection molding preforms having no gate
mark on the bottom surface thereof by means of providing the gate at a position
corresponding to the neck of the preform to stretch blow mold the preform, as in
the above-mentioned known method.
DISCLOSURE OF THE INVENTION
The present invention is to achieve the above-mentioned object by
means of a method for stretch blow molding a hollow article according to the features
of claim 1. This method comprises the steps of closing a cavity mold being for
use in molding a body of the preform and a neck mold being for use in molding a
mouth of the preform, inserting a core mold into the cavity through the neck mold
to form a cavity that reaches inside of the neck mold, injecting a molten resin
from a circular resin reservoir to the cavity to injection mold the preform having
no gate mark on the outer bottom surface thereof, said circular resin reservoir
having a desired outer diameter to form a gate at a parting section between said
cavity mold and neck mold around the cavity, said gate being communicated with
an injection path formed in the parting between the cavity mold and the neck mold;
and stretch blow molding the preform into a hollow article of which the outer bottom
surface has no gate mark and requires no post-treatment while holding the mouth
of the preform by means of the neck mold.
Furthermore, the present invention is to stretch blow, in a blow
mold for lamp globes, the preform obtained by means of injecting the molten resin
from the above-mentioned resin reservoir into the cavity while forming the gate
mark on a flange portion by the above-mentioned resin reservoir to mold it into
a lamp globe of which the outer bottom surface has no gate mark and requires no
post-treatment which otherwise may cause optical defect at the center of an luminous
portion of said globe. The above-mentioned flange portion is integrally formed
with the luminous portion as a flange for fixing the lamp globe.
In addition, the present invention is to stretch blow mold, in a
blow mold for containers, the preform obtained by means of injecting the molten
resin from the above-mentioned resin reservoir into the cavity while forming the
gate mark on a flange portion by the above-mentioned resin reservoir to mold it
into a container of which the outer bottom surface has no gate mark and requires
no post-treatment. The above-mentioned flange portion is integrally formed with
the body as a flange for receiving a cap.
Another feature of the present invention is that the material for
molding the above-mentioned lamp globe is a thermoplastic resin having heat resistance
such as polycarbonate or the like, and that the material for molding the above-mentioned
container is a thermoplastic resin such as polyester resin and polycarbonate or
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a vertical sectional view of an injection mold illustrating
how a preform is injection molded according to the present invention. Fig. 2 is
a vertical sectional view of a blow mold illustrating how a spherical globe is
stretch blow molded. Fig. 3 is a vertical sectional view of a spherical globe
molded according to the present invention. Fig. 4 is a vertical sectional view
of a blow mold illustrating how a packaging container is stretch blow molded according
to the present invention. Fig. 5 is a vertical sectional view of an injection mold
illustrating how a preform for a conventional container such as a PET bottle is
BEST MODE FOR CARRYING OUT THE INVENTION
Figs. 1 through 3 shows an embodiment of a case where a lamp globe
is molded according to the present invention.
In Fig. 1, reference numeral 10 represents an injection mold for
use in molding a preform. The injection mold comprises a cavity mold 11 for use
in forming a preform body, a neck mold 12 for use in forming a mouth of the preform,
and a core mold 13 which passes through the neck mold 12 into the cavity mold 11.
The core mold 13 forms a lengthwise cavity 14 that reaches the inside of the neck
A gate 15 for use in injecting a molten resin into the above-mentioned
cavity 14 is provided at a parting section between the cavity mold 11 and the neck
mold 12 around the cavity 14. A circular resin reservoir 17 having a desired outer
diameter is formed at the parting section. The above-mentioned gate 15 is provided
in the resin reservoir 17. The gate 15 is communicated with an injection passage
18 provided at the parting between the cavity mold 11 and the neck mold 12.
Reference numeral 19 represents a base plate on which the cavity
mold 11 is mounted. Reference numeral 20 represents a transfer plate composed of,
for example, a rotary plate having a back surface where the neck mold 12 is mounted
such that it can be opened and closed in the horizontal direction. The transfer
plate moves upward and downward relative to the cavity mold 11 together with an
elevator plate 21 provided above the transfer plate to open and close the mold.
While not illustrated in the figure, the core mold 13 is connected to a clamping
machine that is provided above the elevator plate 21. Operation of the clamping
machine forces the core mold to be inserted into the central portion of the cavity
mold 11 through the transfer plate 20, the elevator plate 21, and the neck mold
12 after the mold is closed.
In Fig. 3, reference numeral 22 represents a blow mold formed of
a pair of mold halves that travels on a base plate 23 in the direction close to
and away from each other. When the blow mold is closed, a blow cavity 24 is formed
that is used to form a lamp globe 30 having a spherical body. A blow core 26 through
which a stretching rod 25 is passed contractibly is provided at an upper portion
such that it can move upward and downward.
While not illustrated in the figure, the above-mentioned globe 30
is molded in the following manner. First, the injection mold 10 is closed and an
injection machine is nozzle-touched to the gate of the injection passage at the
parting. An opaque-colored molten polycarbonate resin is injected into the mold.
An injection pressure is received by a clamping force which is applied in a vertical
direction. The injection passage 18 provided at the parting between the cavity
mold 11 and the neck mold 12 relieves an urging force against the side of the neck
mold that is generated by the injection pressure. Therefore, no clamping means
is required to prevent forced opening of the neck mold 12 in a horizontal direction.
The molten resin in the above-mentioned resin passage 18 flows into
the resin reservoir 17 through the gate 15. It flows into the cavity 14 from the
periphery thereof while being guided by the resin reservoir 17 and flown in a
ring-shaped flow. Thus the molten resin uniformly flows without turbulence simultaneously
into the cavity on both sides of the cavity mold 11 and the neck mold 12 even when
the position of the gate is only one. Thereby the cavity mold 11 forms a body
16a, a mouth 16b having a threaded outer surface is formed by the neck mold 12,
and a preform 16 is formed while having a flange 31 formed by the resin reservoir
17 on the periphery of the mouth.
The preform 16 as described above has no welding lines because of
uniform injection of the molten resin from the periphery of the cavity which welding
lines otherwise tend to be generated when the molten resin is injected through
a single portion on the side. This in turn prevents non-uniform section of a final
product which otherwise may be caused due to the weld.
Next, the neck mold 12 and the core mold 13 are moved upward to open
the mold. The preform 16 is released from the cavity mold 11 and the core mold
13 and is transferred to the above-mentioned blow mold 22 while the mouth 16b
being held by the neck mold 12. Then the spherical globe 30 is stretch blow molded
by means of an ordinary technique. For the molding, after the mold halves are closed
the body 16a of the preform 16 is stretched in the axial direction by the stretching
rod 25 and is expanded by blowing air fully in the blow cavity to form the spherical
The mold is opened after molding and the lamp globe is transferred
to other location with the mouth being held by the neck mold 12. A part of the
mouth 16b above the flange 31 is removed as a post-treatment. In such globe 30,
the gate mark is present at the outer edge of the flange 31 after sprue formed
by the injection passage 18 is removed. Therefore, it is not necessary to treat
the gate mark at the center of the body, which is required conventionally. The
gate mark, if present, does not deteriorate the appearance because it is at a
portion other than the luminous portion.
The mouth 16b may not be removed. Instead, the thread in the outer
surface of the mouth may be used to threadedly fix the lamp globe to a lamp socket.
When the mouth 16b is removed, the flange 31 is used as a flange to fix the lamp
As described above, there is no shadow at the center of the lamp
globe due to the gate mark. The gate mark is not visible on the surface which otherwise
may be visible due to difference in surface luminance after the treatment. Accordingly,
the lamp globes made of a synthetic resin such as polycarbonate are of comparable
quality to those made of glass. The value of the lamp globes for illumination and
decoration purpose is improved significantly. In addition, it is possible to decrease
a price because they can be mass-produced by using stretch blow molding.
Fig. 4 shows an embodiment of a container 32 having no gate mark
at the center of the outer bottom surface. The container 32 is formed as in the
case of the lamp globe 30 by means of molding the preform 16 and stretch blowing
the preform 16 into the container 32 in a blow mold 34 having a cavity 33 to form
a lengthwise container.
In this container 32, the preform 16 is stretch blow molded in the
blow mold 34 that the preform is formed by means of injecting a molten polyethylene
terephthalate resin into the cavity 14 through the gate 15 of the resin reservoir
17 at the parting section of the cavity 14 of the above-mentioned injection mold
10. Therefore, the gate mark is generated at a flange portion 35 by the above-mentioned
resin reservoir 17. As a result, there is provided the gateless container 32 of
which the outer bottom surface has no gate mark and requires no post-treatment
of the outer bottom surface thereof. In the container 32 described above, the
flange 35 formed by the resin reservoir 17 may be integrally formed with the body
as a flange for receiving a cap.
The center of the preform 16 is displaced out of the center of the
end of the stretching rod 25 when the rate of stretching in the vertical direction
by the stretching rod 25 is larger than the rate of expansion in the radial direction
by blowing air even though the difference between them is quite small. This results
in displacement of the gate mark out of the center of the outer bottom surface
in the conventional container having the gate mark on the bottom, deteriorating
the appearance of it. The container may be discarded as a defect product for containing
a product, such as cosmetics, having a higher level additional value. No such
a problem occurs for the container 32 having the gateless outer bottom surface.
This eliminates defect products due to displacement of the gate mark, increasing
While the above-mentioned embodiments have thus been described in
conjunction with the case where the injection mold 10 is formed of the lower cavity
mold 11 and the upper neck mold 12, the injection mold 10 may be formed of them
with their position reversed. In such a case, the cavity mold 11 may be extended
down to be fixed to the upper base plate. The neck mold 12 is provided together
with transfer means in such a manner that the neck mold is movable upward and
downward relative to the back surface of the cavity mold.
As described above, a gate is provided in the circular resin reservoir
having a desired outer diameter that is formed at the parting section of the cavity
to injection mold a preform having no gate mark on the outer bottom surface, which
is then stretch blown into the hollow article having no gate mark on the outer
bottom surface. For the lamp globes, the optical defect can be eliminated which
otherwise is caused due to the gate mark. For the containers, it is possible to
prevent reduction of the additional value which otherwise is caused due to the
gate mark on the outer bottom surface. Application of it is advantageous in the
industrial field. In addition, no significant modification is made on the injection
mold and the blow mold as compared with those having a conventional structure.
No special technique is required for the molding. Accordingly, the present invention
has a significant industrial applicability.