Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION AK-9401
TITLE OF THE INVENTION
METHOD FOR INJECTION MOLDING POLYETHYLENE TEREPHTHALATE
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for
injection molding an uncrystallized or crystallized
polyethylene terephthalate into a desired molded form such as
preform, with pre-drying omitted.
2. Background Art
A pellet of the polyethylene terephthalate (PET)
produced by bulk polymerizing of ethylene glycol with
terephthalic acid or dimethyl terephthalate is an
uncrystallized and transparent material. The PET whose
moisture content does not reach an equilibrium has further
moisture-absorption characteristics and thus when the PET is
molten in a moisture absorbed condition, a hydrolysis
develops to cause the PET not to be molded.
An uncrystallized PET pellet, when heated to a
temperature above its glass transition point (Tg), becomes
softened somewhat to have an adhesive characteristics. The
adhesive characteristics causes the PET pellets to be poorly
bitten into a screw and thus not to be fed when molding. On
the other hand, a crystallized PET pellet, even at a
temperature above its Tg, is not softened, thereby keeping
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hardness and having no adhesive characteristics.
Thus, material manufacturers dry an uncrystallized
PET pellet at a temperature of 150 to 160~C to produce a
dehumidified and crystallized PET pellet which is sold as a
commercially available molding material. This causes an
ordinary and commercially available PET to become more
expensive than an uncrystallized PET. The uncrystallized
PET, which has been sold by some of material manufacturers,
has been dried to effect dehumidification and crystallization
by molding manufacturers before use. Therefore, the
crystallized PET described herein means a dried commercially
available product, while the uncrystallized PET means an
undried product. In molding a product using the PET as a
material, even the crystallized PET has a moisture-absorption
characteristics in nature and thus absorbs moisture content
to some extent varying with control conditions before it is
used as a molding material, thus it is customary to dry
preliminarily the PET immediately before molding.
It takes about four hours to accomplish the
preliminary drying at a temperature of 150~C. Both the
increased usage of material per hour to improve a molding
cycle efficiency, and the consideration to the prevention of
a shortage of fed material even for a long time operation
cause a dryer installed to an injection molding machine to
become inevitably large in size and the power consumption
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CA 02122282 1997-11-26
required for drying to tend to increase. Problems exist in
that it takes a lot of time to make preparations before
molding operation, and that a malfunctioned dryer affects
seriously molding operation.
Then, injection molding means omitting the
preliminary drying can be devised. As such molding means,
there has been known a vent type molding machine in which an
injection screw in a heating cylinder is divided into a first
stage and a second stage, and the moisture content in a molten
resin in the first stage is removed to the outside through a
vent opening provided between both the stages.
In this well-known vent type molding machine, the
screw groove of a feeding zone of the first stage leading from
the rear end at which a hopper is located to a metering zone
at the front end is formed in a deep groove; pellets of the
material resin fed from the hopper in a humid condition is
molten due to shearing heat and compression while being
sequentially fed by screw rotation; and then the groove leads
through the metering zone having a shallow groove to the
second stage.
The screw groove in the second stage is formed in a
deep groove, so that when the molten resin is transferred to
the groove, the pressure of the molten resin is reduced due to
the rapid increase in the groove depth of the screw to cause
the moisture content in the resin to be vaporized, and
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the moisture content wrapped in the resin by screw rotation
to be separated, which moisture content is then sucked and
removed through the vent opening by a vacuum pump. The
molten resin whose moisture content has been removed in this
manner is fed by screw rotation to a metering zone at the
front end of the second stage, and then to a position in
front of the screw to store, in a similar manner to an
injection molding machine having an ordinary mechanism. The
molten resin thus stored is injected and loaded into a cavity
by the forward movement of the injection screw after stoppage
of screw rotation.
Although, generally the employment of such vent
type injection unit may allow even a material having a large
moisture absorption to be molded without the preliminary
drying, such unit is not applied to all of resins, and with
respect to the PET, particularly an uncrystallized one, the
pellet is softened and compressed in a feed zone to cause it
to be accumulated, whereby the PET in the hopper cannot enter
the zone, and the biting by the screw becomes extremely poor
such that the material cannot be fed. As a result, it has
been quite difficult to perform the injection molding without
the necessity of the preliminary drying by employing a
conventional vent type injection unit.
SUMMARY OF THE INVENTION
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The present invention is devised to solve problems
with the above-mentioned conventional PET injection molding,
and it is an object of the invention to provide a method for
injection molding the PET by which although a vent type
injection unit is employed as means for omitting preliminary
drying, the unit does not develop a poor screw biting, and
even when a molding material is an uncrystallized or
crystallized PET, can feed a certain amount of the material
at all times to injection mold a desired transparent molded
form such as a preform, and use the preform to produce a
thin-wall container with a low cost.
The present invention according to the above-
mentioned purpose employs a vent type injection unit in which
an injection screw comprising a first stage and a second
stage is rotatably and movably mounted in a heating cylinder
having a vent, and limits the feed rate of the material resin
to a feed zone of the first stage of the vent type injection
unit in such a manner that the material resin is not
accumulated due to compression even if the material resin is
softened to an elastomeric state before reaching a
compression zone in front of the feed zone. This limitation
is effected by making the sectional area of the screw groove
in a receiving zone at the feed zone rear end at which the
hopper port of the first stage is located smaller than the
sectional area of the screw groove in the feed zone, in which
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condition an uncrystallized or crystallized polyethylene
terephthalate produced with the preliminary drying omitted is
loaded in the hopper as a material resin to plasticate and
inject.
Although it is preferable that the sectional area
of the screw groove in the above-mentioned receiving zone 11
is set to no larger than 3/4 the sectional area of the screw
groove in feed zone, a smaller sectional area thus set causes
the feed rate to become lower and the material metering time
to become longer than is required, so that it is most
preferable that the former accounts for 2/3 to 1/2 the
latter. In order to feed ~aster the PET, the screw is formed
in a double screw to the halfway position of the compression
zone, and then in a single screw from the compression zone to
the metering zone so as to set the screw pitch to a large
value, thereby making sufficient the stretching of the molten
resin to improve the vent effect in a vent opening of the
second zone.
In addition, the molding is performed in such a
manner that the heating temperature on the second stage side
of the heating cylinder is set to a value lower than the
heating temperature on the first stage side, thereby
preventing acetaldehyde occurrence due to an overheated
molten resin.
The removal of the moisture content in the resin
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from the above-mentioned heating cylinder is performed by
being sucked under a reduced pressure by means of a vacuum
pump, which removal is not interrupted even when injecting
during operation, and when injecting, the reduced pressure is
set to a value lower than that when plasticating, thereby
preventing a vent up due to the forward movement of the
injection screw. An injection molded preform is immediately
or later transferred to a blowing mold, where it can be
stretch-blow molded into a packaging container such as a
thin-wall bottle.
BRIEF DESCRIPTION OF THE DRAWINGS
Drawings show an example of a vent type injection
unit capable of performing the injection molding method of
the present invention, in which:
Fig. 1 is a schematic longitudinal sectional and
side view of the vent type injection unit according to the
present invention, and
Fig. 2 is an enlarged side view of a screw provided
in the vent type injection unit of Fig. 1, in which a front
end thereof is omitted.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In Figs., the numeral 1 indicates an injection
screw which is rotatably and movably inserted into a heating
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cylinder 2. The injection screw 1 has a shape such that two
screws having a deep groove section and a shallow groove
section are reconnected to each other to form a single screw,
and has a screw design such that a screw portion 10 located
in the rear portion of the heating cylinder 2 is made a first
stage (10), while a screw 20 located in the front portion
thereof is made a second stage (20).
The above-mentioned heating cylinder 2 includes a
vent opening 3 on the wall portion on which the rear portion
of the second stage 20 is located, which opening is enclosed
and connected with a vacuum pump (whose view is omitted). On
the rear wall portion on which the rear portion of the first
stage 10 is located, there is bored a feed port 4, which is
mounted with a hopper 5 for a PET pellet (hereinafter simply
called a resin). On the outer periphery of the heating
cylinder 2 and a nozzle 6, there are provided band heaters,
though omitted in Figs., for heating a resin fed by screw
rotation from the above-mentioned feed port 4.
Both the above-mentioned vent opening 3 and the
feed port 4 have an opening width with substantially the same
size as the pitch of the screw they face.
Fig. 2 shows a screw design of the above-mentioned
injection screw 1, in which the ratio of the first stage 10
to the second stage 20 in the overall length L is about 6:4,
thus the second stage 20 being formed shorter than the first
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stage 10. The first stage 10 is divided into four zones of a
receiving zone 11, a feed zone 12, a compression zone 13 and
a metering zone 14 from the rear end, while the second stage
20 is divided into three zones of a feed zone 21, a
compression zone 22 and a metering zone 23 from the rear end.
In the first stage 10 and the second stage 20, the
screw is formed in a double screw up to the halfway position
of the compression zones 13 and 22 except for the metering
zones 14 and 23, whereby the feed of the material resin to
the compression zones 13 and 22 by screw rotation is
performed as fast as possible; while the screw in the
metering zones 14 and 23 is formed in a single screw to widen
the screw pitch in order to reduce as small as possible an
un-uniformity of the molten resin.
In the first stage 10, the screw pitch P1 of the
receiving zone 11 is set to a value smaller than the screw
pitch P2 of the feed zone 12, while the screw pitch P3 of the
metering zone 14 is set to a value twice the above-mentioned
screw pitch P2. The screw pitch of the feed zone 21 and the
metering zone 23 of the second stage 20 is set in a similar
manner to the screw pitch of the first stage 10.
The screw groove depth of the first stage 10 is set
at the dimensions calculated from the screw groove depth of
the feed zone 12. In order to limit the feed rate in the
feed zone 12, the screw groove depth in the above-mentioned
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receiving zone 11 is set to a shallow depth such that the
sectional area of the screw groove in relation to the above-
mentioned screw pitch P1 becomes 2/3 the sectional area of
the screw groove of the feed zone 12; while the screw groove
depth in the metering zone 14 is set to a depth shallower
than in the receiving zone 11 in order to stretch the molten
resin by screw rotation.
The screw groove depth in the feed zone 21 of the
second stage 20 is set to a deepest depth in order to
vaporize the moisture content in the molten resin by a rapid
pressure reduction; while the screw groove depth in the
metering zone 23 is set to a value deeper than in the
metering zone 14 of the first zone 10, thereby making easy
the feed of the molten resin by screw rotation.
To injection mold a molded article such as a
preform using an uncrystallized PET by employing the vent
type injection unit having the above-mentioned arrangement,
first the temperature of the heating cylinder 2 that the
second stage 20 holds is set to 270~C, and the temperature of
the heating cylinder 2 that the first stage 10 holds is set
to 280~C. The undried and uncrystallized PET as a molded
material is loaded in the above-mentioned hopper 5, and then
the injection screw 1 is caused to rotate at a high speed
(100 rpm). The atmospheric pressure of the opening 3 is
reduced to about -730 mm Hg by the vacuum pump.
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The PET in the hopper 5 is fed by screw rotation
into the receiving zone 11. The feed rate of the PET from
the receiving zone 11 to the feed zone 12 is limited to a
value accounting for 2/3 the sectional area of the screw
groove of the feed zone 12 determined by the above-mentioned
screw groove sectional area setting, so that the PET in the
feed zone 12 is not excessively accumulated, and thus even if
the PET iS softened to an elastomeric state, pellets are not
massed with each other due to compression and not accumulate
before the resin reaches the compression zone 13.
The PET softened in the feed zone 12 is compressed
by the tapered surface of the compression zone 13 to cause it
to be molten, stretched and temporarily plasticated, so that
the PET is ~ed into the second stage 20 whose temperature is
set to a value lower than the first stage 10. The screw
groove of the feed zone 21 is formed in a deep groove, so
that the pressure of the molten resin is reduced to cause the
moisture content in the PET to be vaporized, and the moisture
content wrapped in the molten PET by screw rotation to be
separated, which moisture content is then sucked and removed
through the vent opening 3 by the vacuum pump. The molten
PET whose moisture content has been removed in this manner is
fed by screw rotation to the metering zone 23 of the second
stage 20.and further plasticated, and then fed to a position
in front of the screw to store, in a similar manner to an
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injection molding machine having an ordinary mechanism. As
the molten PET is fed, the injection screw 1 is moved
backwardly and stopped at a desired position to meter an
amount required for injection. After metering, the molten
resin is injected and loaded into a mold by forwardly moving
the injection screw 1. Then the atmospheric pressure of the
vent opening 3 is changed to about -650 mm Hg and maintained
by the vacuum pump during injection operation. The
atmospheric pressure can be easily controlled by taking the
outer air into a suction line through valve operation.
The preform thus injection molded was transparent
similarly to the case where a preliminary drying is performed
before injection molding, and has no cloudiness due to
hydrolysis, and also has an acetaldehyde occurrence within an
allowable range.
An example of the specifications of the vent type
injection unit of the present invention will be shown
hereinafter:
Injection screw (double screw)
Screw length (L) :1670 mm
Screw outside diameter (D) : 64 mm
L/D of Screw : 26
First stage (Length) (Screw minor dia.) Screw pitch
Screw 998 mm
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Receiving zone 128 mm 52.0 mm 32.0 mm
Feed zone 646 mm 45.0 mm 38.5 mm
Compression
zone 64 mm
Metering zone 160 mm 57.2 mm 77.0 mm
Second stage (Length) (Screw minor dia.) Screw pitch
Screw 672 mm
Feed zone 448 mm 43.2 mm 38.5 mm
Compression
zone 64 mm
Metering zone 160 mm 56.0 mm 77.0 mm
Screw revolution speed : 100 rpm
Injection stroke : 120 mm
When the PET is crystallized one, it is softened in
the feed zone 12 more slowly and transferred to the
compression zone 13 faster than the uncrystallized PET,
thereby tending to be apt to develop an uneven melting.
However, this problem can be solved by setting the
compression zone 13 to a length longer somewhat than the
above-mentioned numerical value.
The employment of the above-mentioned vent type
injection unit allows the injection molded preform to be
molded into a thin-wall packaging container such as a bottle
by the well-known stretch-blow molding. The well-known
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stretch-blow molding is broadly classified into a molding
method called the so-called two stage type in which the
injection molding of the preform and the stretch-blow molding
of the preform into a container are performed in separate
processes, and a molding method called the so-called one
stage type in which the injection molding of the preform
through the stretch-blow molding of the preform into a
container are continuously performed, in either of both the
methods, the PET preform produced by the injection molding
with the preliminary drying omitted can be molded into a
thin-wall container as with the PET preform injection molded
after being dried.
The following shows molding conditions in a case
where with an injection molded preform held at its neck to a
limit in which its shape can be kept, the preform is released
from a cavity and a core, transferred to a blowing mold when
the surface temperature of the preform is being raised due to
an internal potential heat, and immediately stretch-blow
molded to mold a thin-wall container:
Material resin : UNIPET RY523 (manufactured by Unipet
Co.,Ltd.)
Molded form : flat bottle
Dimensions : overall height 250 mm
port inside dia. 23.5 mm
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length under neck 230 mm
body outside dia. 90x50 mm
body wall thickness 0.4 mm
weight 36 g
Preform
Dimensions : Overall height 142 mm
port inside dia. 23.5 mm
length under neck 230 mm
body wall thickness 2.9 mm
body upper portion outside dia 28 mm
body lower end outside dia. 24 mm
draft 2/122
Preform molding conditions
Injection cylinder temperature
First stage : Front 285~C, middle 285~C, rear 280~C
Second stage: Front 270~C, rear 270~C
Nozzle temperature : 270~C
Injection mold temperature (set value)
Cavity mold : 15~C
Core mold : 15~C
Injection pressure (dwell) : 75 kg/cm
Loading dwell time : 6.0 sec.
Cooling time : 2.8 sec.
Releasing temp. : 60 to 70~C (preform surface
temperature)
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Stretch-blow molding conditions
Mold temperature (set value) : 20~C
Stretch-blowing temperature : 88~C (preform surface
temperature)
Blowing pressure (stretching) : 20 kg/cm
Blowing time : 2.0 sec.
Percent of stretch:
Longitudinal (axial direction) 190 %
Lateral (radial direction) 360 x 200 %
As described above, according to the present
invention, the injection molding of an uncrystallized PET and
that of a crystallized PET with the preliminary drying
omitted can be performed by the employment of the vent type
injection unit, thereby allowing a molded article having
little hydrolysis to be obtained. A molded article
developing little acetaldehyde due to overheating is obtained
so that the uncrystallized PET having a lower commercial
price can be utilized as a molding material, whereby the
material cost is reduced and the running cost required for
the preliminary drying can be saved even for the crystallized
PET, thereby allowing the price of a packaging container
using the PET as a material to be reduced. The present
invention has a further advantage in that the injection
molding itself requires no complex operation and can be
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performed by employing conventional molding technologies as
they are, thereby having a significantly high utility value
in industrial field.
