Note: Descriptions are shown in the official language in which they were submitted.
~1~6~'~5
This invention relates to a method for molding hollow objects of
thermoplastic resins and an apparatus for making such hollow objects, and
more particularly, to a method and apparatus for molding hollow objects of
thermoplastic resins wherein each of solid parisons is heated in a manner as
to have proper temperature differences in the respective axial portions of
the solid parison so that the resultant hollow object is suitably controlled
in thickness in a lengthwise direction.
In general, methods of blow-molding thermoplastic resins can be
broadly classified into three categories: cold parison molding, hot parison
molding and cold sheet molding. Most hollow materials or objects are now
produced by a cold or hot parison molding method using resin tubes ~so-called
"parisons"). In this sense, this invention concerns a cold parison molding
method or a biaxially stretching and blow-molding method (hereinafter referred
to simply as stretch and blow molding) which comprises a parison producing
step and a heating~ stretching and blowing step.
In the case where plastic containers which have a flat or irregular
form in section, or the diameter and sectional area of which vary to a great
degree in a lengthwise direction, are produced by a hot parison molding
method, the thickness of the final, desired, moldings is made uniform by suit-
ably controlling the thicknesses of portions of a molten parison. This
O~
parison is~o~ttonded-by means of an extruder or is produced by the use of an
injection molding machine, according to the shape of the ultimate moldings.
That is, upon the extrusion of a molten parison, a die or core which has an
angle with respect to an extruding direction is moved vertically for changing
the clearance between dies and cores to control or change the thicknesses of
a molten parison in a lengthwise direction ~i.e., a parison-thickness control
method). Furthermore, the control of thickness of a molten parison in the
circumferential direction can be achieved by the use of a flat or deformed
extrusion die or coreJ so that it is possible to produce a container which
bo~
has greatly different diameters in the~Xoll portions thereof without unnec-
*
.
lZS
essarily incre&sing the weight of container, or to form a container, which is
flat or deformed in section, with a uniform thickness in all portions of the
container, or to make uniform the bottom thereof. In many instances the
bottoms tend to be thin.
In a cold parison molding system which comprises a parison producing
step and a blowing step, the two steps being independent each other, it is
usual to produce parisons by continuously extruding cylindrical pipes,
followed by cutting, so that it is almost impossible to change or control in
a lengthwise direction the thickness of parison in accordance with the shape
of an ultimate container product. Furthermore, the control of thickness in
the circumferential direction does not prevail to such an extent as in the
hot parison molding system since it is difficult to place a cut parison in
correct position in a metal mold when the parison has a thickness which varies
in the circumferential direction. A complicated apparatus is required.
Accordingly, where containers have irregular cross-sections are molded by a
cold parison molding method without controlling the thickness of parison,
extremely thin wall portions may be formed. To avoid this, it is necessary
to increase the entire thickness of the staring parisons. This is undesirable
as the ultimate containers disadvantageously increase in weight, resulting in
increase in production costs and reduction in transparency of the containers.
Especially with a stretch and blow molding system wherein the blow
molding temperature is low, a parison becomes poor in moldability. Accord-
ingly, upon expansion by blowing, the thickness of portions of the parison
not in contact with a metal mold at an initial stage of the expansion are
made extremely thin in comparison with the remainder of the container, i.e. a
small diameter body portion and/or neck portion. These portions include long
span portions of a container with a flat or deformed section and a great
diameter body portion and a bottom portion of a container which has different
body diameters therein or which is formed by a wide range of expansion ratios.
The resultant containers have extremely non-iniform thickness distribution.
~,~
.h . ~
1~61Z5
In accordance with one aspect of this invention, there is provided
a method of forming biaxially oriented hollow articles having an irregular
configuration with respect to a particular axis comprising:
a) introducing a tubular parison having a uniform wall thickness into
a closed heating zone and conducting said parison in a predetermined path
through said heating zone for a time sufficient to impart heat to elevate the
temperature of the entire parison to a first orientation temperature within
1C temperature tolerance;
b) subjecting the entire circumference of an axially extending portion
of said parison which corresponds to a portion of a biaxially oriented hollow
article having a rotatably smaller expansion ratio than the remaining portion
to the influence of an auxiliary source of heat within said closed heating
zone during a concluding portion of said path to elevate said axially extend-
ing portion only to a second temperature which is higher than that of the
remainder of the parison by 1 to 5C;
c) removing said heated parison from said closed heating zone and
axially stretching said heated parison;
d) placing said stretched and heated parison into a molding zone having
an irregular circumferential configuration with respect to the longitudinal
axis of said parison when disposed therein, said axially extending portion
of the parison being disposed in axial alignment with a portion of the molding
zone having a circumferential dimension which is less than the maximum circum-
ferential dimension of the molding zone; and
e) creating a pressure differential to expand said parison out into con-
formity with said molding zone.
Another aspect of the invention provides apparatus for heating
parisons comprising: (a) a furnace; (b) conveyor means including chain means
for transporting tubular parisons along a circuitous path in said furnace; (c)
said conveyor means including a series of support means for holding each of
said parisons in positions for rotation about their respective central axes
,;~. ~ '
lZS
while being transported along said path; (d) said furnace also including
auxiliary heating means disposed along a predetermined length of said
circuitous path for heating an axially extending portion of a parison to a
temperature different from that of the remainder of the parison, said
auxiliary heating means comprising an elongated fixed source of heat disposed
parallel to said predetermined length of said circuitous path with said chain
means guided to move in one direction spaced horizontally from one side of
said auxiliary heating means and in the opposite direction spaced horizontally
from the other side of said heating means; ~e) and means for rotating a
support means for a parison while exposed to said auxiliary heating means,
including a shaft including a gear rotatably journaled on said chain means,
and fixed sprocket means engaging with said gear along said predetermined
path.
Thus the apparatus of this inveniton is characterized in that the
auxiliary heating means is provided within the parison heating device, the
parison being first uniformly heated by means of the parison heating furnace
and thereafter the certain portions with respect to the axial direction of
the parison being further heated by the auxiliary heating means while the
parison is rotated for producing predetermined temperature differences in
the parison which are uniform in the circumferential direction while varying
with respect to each other in the axial direction. A known stretcher and a
fluid-blowing means are provided for axially stretching the heated parison,
followed by blowing thereinto a pressurized fluid or air mold.
The thermoplastic resins useful in the present invention are all
thermoplastic resins capable of blow-molding by the cold parison molding -
method. In particular these resins include high density polyethylene, low
density polyethylene, polypropylene, polyvinyl chloride, polyvinylidene
chloride and the like~ These thermoplastic resins may be laminated with a
thermoplastic resin of a different type.
This invention is very effective when applied to the production of
~ - .
11~ lZ5
containers whose body diamcter varies in a lengthwise direction, e.g. a con-
tainer having a conical form or a container with a narrow body part. However,
this invention is not limited to the production of containers with such speci-
fic forms or shapes. It is also effective for the production of general
cylindrical containers with the bottom being made strong and uniform in thick-
ness. The heating source of the auxiliary heating means is not limited to a
specific type but preferably may be a radiant heating means such as an infra-
red heater, a silica tube heater, a far infrared heater, a hot air generator,
a steam heating device and the like.
The heating source of the heating device which includes therein the
auxiliary heating means is not limited to those of a specific type and may be
- 4a -
S
the same type of heating source used in the auxiliary heating means. In
either case, the heating source is required to control the temperatures with-
in 1 C.
Where the temperature difference in the axial direction of the
parison attained by using the auxiliary heating means is smaller than 1 C, it
is difficult to make uniform the thickness of a final product and the mainten-
ance of such small temperature difference will be almost impossible. However,
with the temperature difference in excess of 5 C, good molding products can
not be obtained since the portions heated by the auxiliary heating means are
expanded to an excessive extent and so-called "necking" will take place.
The heating device is required to be so designed as to vertically
support parisons upon respective supporting pins and to be capable of circul-
atingly feeding or conveying the parisons through the heating device. Further-
more, when the parisons pass by the auxiliary heating means, it is necessary
to permit the parisons to rotate together with the respective supporting pins
so that the parisons are uniformly heated in a circumferential direction.
This invention will be particularly illustrated by the following
detailed description and accompanying drawings, in which:
Figure 1 is a plan view of a preferred form of heating device
embodying the present invention;
Figures 2 and 3 are vertical sectional views t~Xen along the lines
A-A and B-B in Figure l;
Figure 4 is a plan view taXen along the line C-C in Figure 3;
Figure 5 is a front view of a hollow object obtained in accordance
with the invention; and
Figure 6 is a bottom view of the container of Figure 5.
~; Figure 1 is a plan view of a heating device and an auxiliary means
embodying the present invention, in which indicated at 1 is the auxiliary
heating means, at 2 a hot air port of the heating device, at 3 a parison
conveying or feeding chain, at 4 and 4' axes for returning the conveying chain
3, at 5 a heating furnace and at 6 means for introducing a parison into the
,v _ 5 _
11~;61~S
heating furnace 5;
Figures 2 and 3 are vertical sectional view taken along the lines
A-A and B-B in Figure 1, respectively, in which views indicated at 7 is a
parison supporting pin, at 8 a gear wheel for rotating the supporting pin 7,
at 9 a parison, at 10 a fixed sprocket mating with the gear 8, at 11 a shaft
supporting the pin 7 and at 12 a chain guide,
Figure 4 is a plan view taken along the line C-C in Figure 3,
Figure 5 is a front view of a container obtained in accordance with
the present invention; and
Figure 6 is a bottom view of the container of Figure 5.
Referring now to Figures 1, 2, 3 and 4, the chain 3 which carries
the parison supporting pins 7 is endless and runs in the direction of the
arrows shown in Figure 1. The heating furnace 5 is entirely uniformly heated
up to a predetermined molding temperature T by hot air which is blown through
the hot air ports 2. The parison 9 is put on the supporting pin 7 with the
aid of the parison introducing means 6. The parison 9 is fed through the heat-
ing device to heat it entirely and uniformly and comes to the position where it
encounters the auxiliary heating means 2 on its next to last pass of the con-
veyor means. The auxiliary heating means 1 is, for example, sets of three
far infrared heaters which are set in position to heat the upper, middle and
lower portions of the parisons, respectively, and which are ad~ustably mounted
to be movable up and down in a vertical line. Moreover, the auxiliary heaters
are respectively connected to voltage regulators to suitably control the quan-
tity of heat. The parison 9 which is mounted on the supporting pin 7 is fed
through the furnace in the direction of the arrows while allowing it to rotate
at a constant rotational frequency by means of the gear 8 mating with fixed
sprocket, during which certain portions of the rotating parison are further
heated for a predetermined period of time by the auxiliary heating means. As
a result, the upper, middle and lower portions or parts of the parison is
uniformly heated in a circumferential direction up to temperatures of (T + tl) C,
- 6 -
~'~d~,``'
~1~61;~S
(T + t2) C and (T ~ t3) C, respectively. The thus heated parison is then fed
to a subsequent blowing step or a stretching and blowing step to shape the
same into a desired form.
EXAMPLE
Polypropylene having a density of 0.900 g/cc (at 20 C) and a melt
index of 0.5 g/10 min (ASTM-D-1238-62T) was used as a raw material for pro-
ducing by means of an extruder and a parison cutter each having an outer
diameter of 22 mm, a thickness of 5 mm, an inner diameter o~ 12 mm and a
length of 120 mm. The resultant parison was heated to 155 C (hereinafter
referred to as T C) by the use of the heating device of Figure 1. The upper
portion (i.e., A portion (expansion ratio 1.6) in Figure 5) of the parison
was further heated by the far infrared heaters up to T + 4 C, the middle
; portion (i.e., B portion (expansion ratio 2.3) in Figure 5) up to T + 2 C,
but the lower portion (i.e., C portion (expansion ratio 3.0) in Figure 5)
- 6a -
,~
~3
11~61Z5
was not supplementarily heated. The thus heated parison was withdrawn from
the furnace and the ends of the parison were grasped by conventional apparatus
for stretching the parison by about 2.5 times as long at a constant rate in
the direction of a tube axis. The thus stretched parison was immediately
placed in a metal mold having an internal configuration as shown in Figures
5 (front view) and 6 (bottom view), and a pressurized fluid, such as air,
was blown into the parison to mold a container. The thickness distribution
of the container which was obtained by the use of the auxiliary heating means
in accordance with the present invention is shown in the Table below in com-
10 parison with that of a container obtained as a result of conventional uniform
heating. The measured points are indicated by numerals corresponding to those
: of Figure 5.
'. -
Measured Thickness (mm)
point _ _
, ~ Container of invention Container of prior art
Front Side Front Side
~0 ~; ~ ~ .
It wi.ll be apparent from the Table that the thickness distribution
of the container of this invention is markedly uniform when compared with the
` 11~;6~;25
container which is obtained from the same kind of parison uniforrnly heated by
a conventional manner.
As for the molding of containers of different shapes, the almost
same results can be obtained by suitable control of heating intensity, heat-
ing position and heating time. It should be noted that the above-described
specific apparatus is intended to merely illustrate one preferred embodiment
of the present invention. Various modifications may be made without depart-
ing from the scope of this invention.
,
