Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
The present invention relates to an injèction stretching
and blow molding process and, mor~e particularly, to a process
for stretching and blow molding hollow molded products by
two-layer injection molding using a synthetic resin which is
hard to stretch and blow mold.
As methods of blow molding an injection molded parison
with a bottom into a hollow molded product such as a bottle
or the like, there are known a method which comprises molding
a parison with a bottom by injecting a molten resin into a
closed mold consisting of a core and a cavity mold, and
blow molding the parison with a bottom after transferring it
together with the core to the blow mold; and another method
which comprises injection molding a parison with a bottom,
transferring it to a blow mold after separating it from the
injection core, inserting into it a blow core equipped with
a stretching rod, and blow molding the parison with a bottom
while stretching it in the axial direction in the blow mold.
The former method ig generally called injection blow mold-
ing, and the latter is generally called injection stretching andblow molding. Injection stretching and blow molding can provide
a molded product having a thinner wall, a higher transparency
and a greater strength than products of former method.
Injection stretching and blow molding, however, may not
be applied favorably to any resin. Of stretchable, blow
moldable resins, some resins cannot be molded readily. In
particular, vinyl chloride resins may be stretched and
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molded with extreme ease into hollow molded products having
various shapes. However, sometimes difficulties arise in
molding them into hollow molded ~roducts by means of injection
stretching and blow molding because a layer of the resin may
break before it is sufficiently stretched or before it is
sufficiently expanded to fit a blow mold, and because it is
extremely difficult to mold these resins into hollow molded
products having a uniform wall thickness.
Although there are several possible reasons for this
poor moldability, it has been presumed by the present inventors
that one of the reasons stems from the problem of maintaining
a thermal distortion temperature at the time of stretching
and blow molding, as evidenced by the facts that the resin
can be subjected to injection blow molding and that the
distribution of wall thicknesses in a product produced by
means of stretching and blow molding is not uniform. Therefore,
attempts have been made to find a procedure suitable for
maintaining a thermal distortion temperature appropriate for
stretching and blow mold,ing. As a result, it has been found
that it is very difficult to maintain an optimum thermal
distortion temperature for a parison with a bottom obtainable
by injection molding a resin such as, for example, a vinyl
chloride resin, a styrol resin, an acrylic resin, an acrylo-
nitrile resin, an ABS resin, or an AS resin, as compared
with a parison with a bottom obtainable from a polyethylene
terephthalate resin which is readily subjected to in~ection
stretching and blow molding. Thus, it is apparent that a
further procedure is needed to maintain such a parison with
a bottom at a thermal distortion temperature suitable for
stretching and blow mol~ing.
From the foregoing, the inventors considered a procedure
comprising stretching and blow molding by inserting a temper-
ature-adjustable core into a parison with a bottom and
allowing the parison to be maintained at an appropriate
temperature by means of the core. The procedure was found
to be suitable in the case of a resin extremely moldable by
means of stretching and blowing, such as polyethylene tereph-
thalate or polypropylene, but relatively unsuitable to the
stretcing and blow molding of a resin which is hard to mold
by means of stretching and blowing, such as the above-mentioned
vinyl chloride resin or the like. The reason for this is
that, when the parison with a bottom in which the core is
inserted is stretched in the axial direction by means of the
stretching rod at the center of the core, the inner surface
of the parison is brought into firm contact with the heated
core by tensile strength'and is partially welded, so that
the parison cannot be fully expanded in the mold by blowing
air thereinto.
~ s a result of extensive research on means for maintaining
a parison with a bottom at an appropriate temperature, apart
from using a heated core, it has now been found best to use
a readily stretchable, blow moldable resin as a means for
maintaining the parison at an optimum temperature.
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Summary of the Invention
Accordingly, the object of the present invention is to
provide a process for stretching,and blow molding in which a
parison with a bottom, particularly one made of a resin that
is hard to stretch and blow mold, is maintained at an
appropriate temperature.
Another object of the present invention is to provide a
process for stretching and blow molding without using a
heated core as a means for maintaining the parison with a
bottom at an appropriate temperature.
A further object of the present invention is to provide
a process for stretching and blow molding in which the
readily stretchable, blow moldable resin is used as a means
for maintaining the parison with a bottom at an appropriate
lS temperature.
Brief Description of the Drawings
Fig. 1 is a longitudinal sectional view illustrating a
parison with a bottom in accordance with an embodiment of
the present invention; ahd
Fig. 2 is a longitudianl sectional view illustrating a
hollow molded product prepared in accordance with the embodi-
ment of the process according to the present invention.
Detailed Description of the Invention
The above-mentioned readily stretchable, blow moldable
resins, may be adjusted to a temperature most suitable to
stretching and blow molding without taking any special
measures, as long as they are subjected to molding immediately
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after the adjustment of their temperatures.
While it is possible to maintain a temperature to some
extent even when the readily stretchable, blow moldable
resin is in contact with another resin, the maintenance of a
temperature of a resin which is hard to stretch and blow
mold is achieved by utilizing a procedure of injection
molding a two-layer parison. It has accordingly been found
that a hollow molded product having a desired shape is
produced by injection molding the hard resin and the readily
stretchable, blow moldable resin into a two-layer parison
with a bottom, one layer consisting of the former and the
other consisting of the latter; adjusting the temperature of
the parison with a bottom to a temperature suitable for ,- -
stretching and blow molding; and simultaneously stretching
the parison with a bottom in the axial direction in a blow
mold and blowing air into the stretching parison.
Although the hollow molded products thus produced are
not necessarily satisfactory with respect to the distribution
of wall thicknesses, the~re have been no problems with breakage
as have often occurred when only a resin which is hard to
stretch and blow mold is employed and expansion by air
blowing is effected over its full surface. Further improve-
ments have been developed to finally achieve the present
invention with satisfactory results.
The present invention will be described in more detail
by way of an example to which the present invention is not
intended to be limited in any way.
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A polyethylene terephthalate resin is first injection
molcled into an inner layer 1 having a thickness of 2 mm.
Outside the inner layer 1, a vinyl chloride resin is then
injection molded so as to form an outer layer 2 having a
thickness of 3 mm. The two~layer parison 3 with a bottom
thus prepared is heated to a thermal distortion temperature
appropriate for stretching and blow molding and then trans-
ferred to a blow mold where the parison with a bottom is
subjected simultaneously to stretching in the axial direction
and air blowing to form a two-layer hollow molded product 4.
The thermal distortion temperature may be preferably adjusted
to a temperature most suitable for stretching and blow
molding the polyethylene terephthalate resin. The blow
molding may be carried out by stretching the outer layer 2
together with the inner layer 1 by means of a stretching
rod. This procedure permits the maintenance of the thermal
distortion temperature in the outer layer 2 by means of the
inner layer 1 without causing a remarkable decrease in the
temperature during the blow molding. This procedure also
can provide a hollow molded product having no difference in
stretching and expansion between the two layers after molding
because the outer layer 2 is expanded together with the
inner layer 1 which is readily stretchable and blow moldable
and is uniformly expandable. Consequently, the hollow
molded products 4 thus produced have a high strength and a
uniform thickness in their inner and outer layers.
Although the above-mentioned embodiment employs an
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inner layer 1 of polyethylene terephthalate resin, the vinyl
chloride resin may be employed as the inner layer 1 in place
of the polyethylene terephthalate resin. In this case, some
irregularity in wall thickness may be caused in the inner
layer as compared with the outer layer; however, the degree
of irregularity of the hollow molded products may be considered
negligible.
The thicknesses of the two different layers in a parison
with a bottom may not necessarily be the same and, from an
economic point of view, the cheaper resin layer is preferably
made thicker than the more expensive resin layer.
The following table illustrates the thermal distortion
temperatures of various resins which are hard to stretch and
blow mold; the polyethylene terephthalate resin is used as
15 the readily stretchable, blow moldable resin:
PET 75 - 85C (inner layer)
PVC 84C (outer layer)
Styrol 82C (outer layer)
Acryl ' 88C (outer layer)
ABS 82C (outer layer)
AS 85C (outer layer~
Acrylonitrile74C (oute.r layer)
