BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to a method for stretch blow molding
a wide-mouthed container which is used as a packaging container for processed food,
confectionery, or the like and which is made of thermoplastic resins.
2. Detailed Description of the Related Art
Stretch blow molded packaging containers which is made of thermoplastic
resin and which have a thin wall thickness of a body portion are known. Examples
of the thermoplastic resin used therefor includes polyesters, polypropylene and
polycarbonate, and particularly bottles for beverages made of polyethylene terephthalate
are generally and widely known as so-called "PET bottle".
One of the stretch blow molding methods is a method in which a preform
having a mouth portion, a body portion and a bottom portion is injection molded
within an injection mold, the perform is released from the injection mold in a
state where the inside thereof is still in a high temperature while holding the
mouth portion of the preform, which is cooled and solidified, with a lip mold,
the preform is transferred to a blow mold as it is, and the preform is stretch
blow molded within the blow mold into a container such as a bottle having a thin
wall of the body portion.
In such a molding method, the temperature of the outer and inner
surfaces of the preform is lowered below an orientation temperature due to cooling
during injection molding. After releasing the preform, the outer and inner surfaces
of the preform are heated by utilizing the heat inside the preform till performing
stretch blow molding to increase the surface temperature to the orientation temperature.
At the same time, the temperature inside the preform body portion except the mouth
portion held by the lip mold is lowered to the predetermined orientation temperature.
After then, stretch blow molding is performed.
By the way, a wide-mouthed container used for a packaging container
for jam, peanut butter, candies or the like has a wider mouth portion than a bottle
for beverages. Further, because of the shape of the container, outside diameter
of the body portion thereof is limited so as to be a slightly larger than that
of the mouth portion to some extent. In case where the body portion of the preform
for a wide-mouthed container is formed similarly to that for a bottles in a longitudinal
direction, since the radial stretching ratio is small, it is difficult to obtain
a container of which body portion is made thinner wall by air blowing. In order
to solve this problem, a preform for a wide-mouthed container is formed to have
a conical-shaped body portion or a truncated-conical-shaped one, thereby enabling
of setting the stretching ratio relatively large.
When the preform for such a wide-mouthed container is stretch blow
molded after lowering the preform temperature to a set temperature after releasing
in a high temperature state, a connecting portion between the mouth portion and
the body portion becomes thicker than other portions of the body portion. As a
result, thinning of the body portion just below the mouth portion cannot be achieved,
and in some cases a ring-shaped thicker portion appears just below the mouth portion.
This makes the appearance of the resulted container worse as a packaging container,
and it would be considered as a reject.
The reason why this connecting portion becomes thicker is that heat
at the connecting portion is transferred to lip mold or blow core inserted into
the mouth portion during stretch blowing, resulting in heat loss at the connecting
portion. After releasing the preform, it is transferred to the blow mold while
holding the mouth portion of the preform with the lip mold. Accordingly, during
the transferring after releasing and after transferring the preform within the
blow mold, heat exchange between the lip mold and the connecting portion of the
body portion which is still in a high temperature state via the mouth portion
is still continued. As a result, temperature at the connecting portion is lowered
below the temperature of the other body portion which is exposed to the atmosphere,
and it is difficult to stretch the connecting portion more than the other body
In order to solve the problems, in the conventional method, the heat
loss due to the heat exchange with the lip mold is compensated by increasing the
thickness of the associated body portion to increase the heat amount to be held
thereby. However, there is a certain limit to increase the wall thickness. For
example, if the wall thickness of a preform made of polyethylene terephthalate
is 4.0 mm or more, the outer surface where the wall thickness thereof is thickened
is whitened during its transferring to the blow mold. This results in a light whitened
stain on the container molded by stretch blow molding. Particularly, in such a
case where the wide-mouthed container for packaging, as shown in Fig. 4, is formed
to have a tapered surface at the upper body portion to be capable of visual inspection
of contents from outside therethrough, such a defect would be a fatal damage and
deteriorate commodity value thereof. Thus, there is a problem that such a container
cannot be commercialized.
SUMMARY OF THE INVENTION
The present invention has been deviced to solve the above mentioned
problems for the wide-mouthed container by stretch blow molding, and an object
of the present invention is to provide a novel method for stretch blow molding
a wide-mouthed container which is capable of stretching the entire body portion
including the connecting portion with the mouth portion to a thinner wall thickness
by employing a preliminary stretching within a blow mold before stretch blowing
without making the upper body portion be thicker.
In order to achieve the above-mentioned object, the present invention
is to provide a method for stretch blow molding a wide-mouthed container, comprising
the steps of injection molding a preform having a large diameter mouth portion,
a body portion and a bottom portion within an injection mold, releasing the perform
from the injection mold in a state where the inside thereof is still in a high
temperature while holding the mouth portion of the preform, which is cooled and
solidified, with a lip mold, transferring the preform to a blow mold as it is,
and stretch blow molding the preform into a desired container, wherein before the
stretch blowing step of the preform a stretching rod is descended to a predetermined
length to preliminary stretch the body portion in some extent, thereby thinning
a connecting portion of the upper body portion between the body portion and the
mouth portion, and after then the entire body portion is stretch blow molded.
Further, in the present invention, the body portion of the preform
is formed in a conical shape in which the body portion is made thicker than the
mouth portion and the diameter of the body portion is gradually decreased towards
the bottom portion, and the body portion is preliminary stretched to about 10 mm
length to thin the wall thickness of the upper body portion.
According to the present invention, the wall thickening phenomenon
at the connecting portion between the mouth portion and the body portion of a wide-mouthed
container can be improved, thereby enabling to obtain a wide-mouthed container
for packaging which has a uniform body wall thickness and a well finished connecting
portion. As to the molding step therefor, it is not required to use any additional
special means or the like, but a preliminary stretching by a stretching rod in
a vertical direction is simply carried out as an additional step before the stretch
blow molding step. Accordingly, the present invention is directly applicable to
the above-mentioned conventional molding method without increasing the molding
cycle, thereby solving the problems as above.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will be apparent from the following detailed description of the preferred
embodiments of the invention in conjunction with the accompanying drawings, in
PREFERRED EMBODIMENTS OF THE INVENTION
- Fig. 1 is an explanatory view showing a step of a method for stretch blow molding
a wide-mouthed container according to the present invention, in which a preform
is set within a blow mold before preliminary stretching shown as a longitudinal
- Fig. 2 is a longitudinal cross section of the preform within the blow mold
after preliminary stretching;
- Fig. 3 is a longitudinal cross section of the preform showing its wall thickness
change by the preliminary stretching; and
- Fig. 4 is a half of a longitudinal cross sectional front view of a wide-mouthed
container prepared by the stretch blow molding method according to the present
Preferred embodiments of the present invention will now be described
in detail below with reference to the accompanying drawings. Besides, the present
invention is not limited thereto.
In the drawings, reference numeral 1 denotes a preform for a wide-mouthed
container. The preform 1 includes a mouth portion 2, and a body portion 3 having
a bottom portion. The diameter of the mouth portion 2 is about 70 mm and the thickness
thereof is about 1.5mm. The body portion 3 is integrally formed with the mouth
portion 2 from its lower edge to have a height of 50 mm and the thickness of the
body portion is about 3.4 to 3.9 mm. In the present embodiment, polyethylene terephthalate
is used as a thermoplastic resin.
This body portion 3 is formed to have a thin wall from a connecting
portion 3a of the mouth portion 2 to the bottom portion 4. The body portion 3 is
made in a conical shape by gradually decreasing its diameter toward the bottom
portion. The inside bottom surface of the bottom portion 4 is made in a circular
plane to abut a tip end member 6 of a stretching rod 5.
The preform 1 having a configuration above is formed by injection
molding similar to the prior art. During injection molding, a skin layer is formed
inside and outside surfaces of the preform by cooling by means of an injection
mold. The preform is released in a high temperature state while the formed skin
layer maintains the shape of the preform. The preform is transferred into a blow
mold 8 while the neck portion (or mouth portion 2) which is cooled and solidified
is held with the neck mold 7. After closing the mold, a blow core 9 equipped with
the stretching rod 5 is hermetically inserted above into the mouth portion, thereafter
stretch blow molding is carried out to form a wide-mouthed container 11 shown in
The stretch blow molding is carried out after waiting (for about
2 seconds) till the temperature of the body portion except for the mouth portion
2 of the preform 3 be lowered to a predetermined orientation temperature. During
the stand-by, the heat at the upper body portion is decreased due to cooling by
the lip mold 7 and the blow core 9 and the temperature at the associated portion
is lower than the other body portion 3. As a result, it would be difficult for
the portion to be stretched. In order to solve this problem, during the stand-by
time interval the stretching rod 5 is descended so as that the body portion 3
is preliminary stretched longitudinally before the upper body portion is excessively
cooled. The length for preliminary stretching may be sufficiently about 10 mm,
thereby undesirable thickened wall thickness at the upper body portion can be
Fig. 3 shows the change in wall thickness before stretching (right
side drawing) and after stretching (left side drawing). The below-mentioned table
shows the change amount of the wall thickness at respective measuring positions
(four positions) of A-D at a 5mm interval from the inner lower end of the mouth
portion 2. Besides, the stretching length (S) is 10 mm and the length at each of
the measuring positions is measured from the inner lower end of the mouth portion
Measuring position (mm)
Thickness before stretching (mm)
Thickness after stretching (mm)
Decreasing rate (%)
According to the results, preliminary stretching the preform to 10
mm does not affect the entire body portion to be stretched uniformly, but the stretched
portion is concentrated at the upper body portion from the inner lower end of the
mouth portion 2 to the portion below about 10 mm in length, thereby thinning the
After performing such a preliminary stretching, successively or after
a predetermined time has passed, the stretching rod 5 is extended to the bottom
surface of the blow mold 8 while a high pressure air is blown from the blow core.
As a result, since the heat amount held by the lower body portion is large due
to the difference in the above-mentioned thickness, longitudinal stretching of
the lower body portion precedes to be thinned. The temperature at the stretched
portion is lowered, resulting in disappearing of the temperature difference between
the preliminary thinned upper body portion and the lower body portion. Accordingly,
during the stretching process the upper body portion is also longitudinally stretched.
Radial expansion stretching is achieved by introducing blowing air
at the same time of the longitudinal stretching, and the upper body portion is
further stretched from the connecting portion of the mouth portion to be thinned.
This expansion stretching is expanded from the upper body portion to the lower
body portion so as to go along with the longitudinal stretching by the stretching
rod 5, and finally, the entire body portion is thinned in a predetermined thickness
distribution to form a container body portion 13.
As a result, the wall thickening phenomenon at the connecting portion
13a in a wide-mouthed container 11 which conventionally tends to occur can be improved,
thereby enabling to form a container body portion 13 having an almost uniform
thickness from the lower end of the container mouth portion 12 to the container
bottom portion 14 and also to obtain a wide-mouthed container 11 for packaging
which has a uniform body wall thickness and a well finished connecting portion.
Further, there is no need to use any specific means or device in the molding process,
and it is sufficient for the purpose to preliminary stretch a preform in a longitudinal
direction by a stretching rod before stretch blowing. Accordingly, the present
invention can be directly applied to a conventional molding method without increasing
a molding cycle.
While there has been described what are at present considered to
be preferred embodiments of the invention, it will be understood that various modifications
may be made thereto, and it is intended that the appended claims cover all such
modification as fall within the true spirit and scope of the invention.