Note: Descriptions are shown in the official language in which they were submitted.
- 1- 11535ZZ
~ his application is a division of Canadian Serial
No. 317,873, filed December 13, 1~78.
FIELD OF INVENTION AND BACKGRO~ND
This invention is directed to a method and apparatus
for molding plastic bottles. More particularly, it is
directed to a parison mold and core rod combination which
forms a parison having a flat bottom wall with sharply
tapered corners, and a shoulder having a substantially
straight outer wall and additional plastic at the inner
wall thereof which permits a deeper or longer stretch
without having the stretch rod damage the parison bottom
and/or without deformation in the shoulder of the fin-
ished bottles. The shoulder of the finished bottle is
slightly thicker than the sidewall.
In recent years substantial effort has been directed
to the formation of molecularly oriented plastic bottles
as a replacement or partial replacement for glass bottles.
According to the prior art, a plastic parison is first
injection molded in a parison mold and the parison there-
after stretch/blown into the finished bottle using eitherof two commonly employed processes, the first being the
so-called reheat or two-stage process, and the second
being the so-called hot blow or one-stage process. In
the reheat or two-stage process the parison, after for-
mation in the parison mold, is removed from the parison
mold, cooled to room temperature, and stored for sub-
sequent stretch/blowing in a blow mold into the finished
bottle. At the time of the stretch/blowing operation,
the parison is reheated and brought to the stretch/blow
temperature of the plastic by means of heaters prior to
blowing. In the reheat system, if a particular section
of the parison is to be at a higher heat, more heat is
put into that particular area by using hotter heaters, or
the like. In the hot blow or one-stage process after
the parison is injection molded in a parison mold it is
transferred to a blow mold substantially immediately
after formation and while still retaining the heat neces-
sary for the parison to be stretch/blown into the fin-
ished bottle. In the hot blow or one-stage process, the
cooling of the parison is essentially uniform throughout
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the parison and, accordingly, provided the thickness of
all areas of the parison is the same, all areas of the
parison will be at substantially the same temperature.
Various plastics have been suggested for use in the
formation of molecularly oriented plastic bottles. The
plastics most commonly suggested are polyacrylonitrile
or polyethylene terephthalate (P~T). Basically the steps
of the bottle formation are the same with all plastics.
However, each individual plastic will have its own char-
acteristics, and certain modifications -- as known to one
skilled in the art -- are required with respect to han-
dling including modification of temperature conditions,
and the like.
As known in the art, the parison to a substantial
extent controls the shape and size of the finished bottle.
Accordingly, it is essential in order to obtain a bottle
without deformations or other imperfections to carefully
control the thickness of the parison walls and bottom in
addition to controlling the neck finish. Control of the
parison during formation has permitted the manufacture of
high quality bottles of small and intermediate size with
acceptable quality control being possible. However, it
has been found in the manufacture of large size bottles,
i.e., up to 64 fluid ounces, which are becoming increasingly
popular, that problems are encountered including tearing
and/or puncturing of the parison bottom by the stretch
rod during stretching; deformation at the shoulder of the
bottle, and overall poor quality control due primarily to
the longer or deeper stretch operation necessary in the
formation of bottles of large size.
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SUMMARY OF THE INVENTION
The invention as disclosed comprehends a method and
apparatus for formation of parisons capable of being stretch/
blown into bottles of large size with improved and consistent
quality and more particularly seeks to provide a method and
apparatus for the formation of a parison capable of being
stretch/blown into a large size bottle without substantial
deformation or imperfections in the bottom of the bottle or
in the shoulder of the bottle and wherein the shoulder is of
slightly greater thickness than the sidewall.
The invention to which this divisional application is
directed pertains to a parison mold-core rod combination for
forming an injection molded plastic parison comprising a parison
mold having a sidewall, a flat bottom surface and a sharply
tapered annular corner, and a core rod having a flat end portion
and a tapered wall. The flat end portion and tapered wall of
the core rod and the flat surface and annular corner of the
mold are constructed and arranged to mate and form the cavity
for molding a parison to provide a parison having a thin flat
bottom portion, a thin sharply tapered annular corner and a
relatively thick sidewall, there being a rapid transition from
the thickness at the flat of the parison bottom and the parison
sidewall.
These and other aspects of the present invention will be
apparent from the following description, particular reference
being made to the drawing.
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Briefly, the aforesaid and other objects of the
invention are accomplished based onthe recognition that
in a continuous or hot blow process a particular area of
the parison can be made hotter by increasing the thick-
ness of that area of the parison, coupled with the dis-
covery that the bottom thickness of the parison is criti-
cal in determining the stretch characteristics of the
parison. The thinner the bottom wall, the less the
amount of heat contained in the bottom of the parison
which permits a longer or deeper stretch of the shoulder
and sidewall portions without having the stretch rod
puncture, tear, or deform the bottom of the parison. Ac-
cordingly, formation of a parison with a bottom thinner
than the sidewall permits the manufacture of a bottle of
larger size with a relatively thinner sidewall.
! It was further discovered that deeper and longer
stretch of the parison without tear or deformation is
possible by providing a parison having less or an equal
amount of plastic material deposited at the corner of the
parison bottom than at the middle of the parison bottom.
The latter is accomplished, for example, by utilizing a
core rod having a flat end mated with a parison mold
' having a sharply tapered annular corner. The result is
a parison bottom having an annular corner thickness equal
to or thinner than the parison bottom at the middle of
the bottom. The extra plastic in the sidewall of the
parison relative to the parison bottom provides additional
heat, permitting a greater stretch. The parison bottom,
being cooler in relation to the sidewall, is not punctured
or otherwise deformed by the stretch rod.
It was also discovered that deformation and wrinkling
of the shoulder portion of the finished bottle can be
prevented by increasing the amount of plastic at the
shoulder. Preferably, the outer wall of the shoulder
will be substantially straight, with the additional plas-
tic being at the inner wall of the shoulder. Further, it
was found that with the increased amount of plastic in the
shoulder area it was possible to provide a bottle wherein
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the thickness of the shoulder is substantially equal to
or preferably slightly greater than the side~lall. With-
out having the parison shoulder of a greater thickness,
it was found that the shoulder of the finished bottle was
thinner than the sidewall of the bottle, leading at times
to deformation or wrinkling during manufacture.
Finally, it was discovered that there is a direct
relationship in the thickness desired in the bottom,
shoulder, and sidewall areas of the parison in order to
provide uniformity in the finished bottle and to provide
the essential tensile yield strength for a given bottle
diameter at the bottom, shoulder, and sidewall areas. The
bottom of the parison is preferably thinner in comparison
to the sidewall and shoulder portions to permit a greater
stretch without puncture of the bottom wall. It is also
desirable to have the shoulder area thicker in comparison
to the sidewall to compensate for the flow of material
in the shoulder area. The desired characteristics are
provided by programming the core rod and parison mold
design to obtain the desired bottom thickness relative to
the sidewall portion of the parison and thereafter shaping
the core rod adjacent to the mouth of the parison mold
to provide the desired thickness at the shoulder area.
Final adjustment is achieved by moving the core rod
; within the mold.
DRAWING AND DETAILED DESCRIPTION OF INVENTION
In the drawing which illustrates preferred embodi-
ments and mode of operation of the invention,
FIGURE 1 is a fragmentary cross-sectional
view of a split parison mold including a split neck
ring finish in closed position with the core rod in
place;
FIGURE 2 is a cross-sectional view of a molded
parison with a shaped core rod in place, showing the
relationship between the thickness of the bottom,
sidewall, and shoulder areas of the parison;
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FIGURE 3 is a fragmentary cross-sectional view
of a conventional or prior art parison with the core
rod in place;
FIGURE 4 is a fragmentary enlarged cross-section
of the shoulder of a parison made in accordance with
this invention;
FIGURE 5 is a fragmentary enlarged cross-section
of the bottom of a parison made in accordance with this
invention;
FIGURE 6 is a fragmentary cross-sectional view
of a solid parison mold including a split neck finish
in a closed position with the core rod in place; appear-
ing with FIGURES 1, 2 and 3;
FIGURE 7 is a broken-away view of a finished
bottle partly in cross-section; and
FIGURE 8 is a fragmentary enlarged cross-section
of the bottom of another embodiment of a parison made
in accordance with this invention.
Referring primarily to FIGURES 1 and 2, there is illus-
trated a partable injection mold 10 having a parison cavity
12 for molding a parison in a conventional manner through
gate opening 14. The illustrated mold 10 is designed for
molding a tubular parison, generally identified by numeral
20, for a container which includes a threaded neck finish 22
as shown in FIGURE 2 wherein the core rod remains in place.
For convenience of handling the parison, the mold 10 is
provided with a split neck ring 18 to provide the threaded
neck finish. Cooperating with mold 10 and split neck ring
18 for the formation of the parison is a core rod generally
identified by the numeral 30. The core rod includes a
base portion 32 and a core rod proper 34 having a flat end
36 which in cooperation with flat mold section 11 forms
the bottom 24 of the parison 20. Additionally, the core
rod includes a shoulder 38 which in cooperation with curved
mold section 13 forms the shoulder portion 26 of the pari-
son. The bottom thickness of the parison is controlled by
the upward or downward adjustment of the core rod. Addi-
tionally, the shape of and amount of plastic deposited at
the parison shoulder 2~ is controlled hy machining the cor~
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rod shoulder 38. As apparent, however, movement of the
core rod will affect the shoulder and bottom thickness and,
accordingly, bottom thickness, sidewall thickness, and
shoulder thickness must be programmed for each parison,
as determined by the dimensions of the desired finished
bottle, and the parison mold and core rod mated accordingly.
FIGURE 2 illustrates a parison having a thin bottom,
a sharply tapered bottom corner wall X, and a thick shoulder
wall Y relat~ve to sidewall Z. It has been found, in accor-
dance with the present invention, that the relative thick-
ness of the bottom of the parison and the thickness of the
shoulder to form a bottle having substantially uniform wall
thickness with the essential tensile yield strength and
without imperfections is at a substantially direct ratio.
In other words, the thinner the bottom, the thicker the
shoulder to provide a parison sidewall of intermediate
thickness~ As will be apparent, the thickness of the shoul-
der relative to the bottom and sidewall of the parison is
determined by machining of the core rod to the desired shape
after the thickness of the bottom and sidewall has been de-
termined.
As is further apparent, by using a mold 10 having the
flat and sharply tapered cornered bottom 11 and a core rod
30 with a flat end 36, the molded parison will have a cor-
ner wherein there is a rapid transition from bottom to side-
wall. The enlarged fragmentary cross-section of the bottom
of the parison as shown in FIGURE 5, made with a parison
mold assembly shown in FIGURE 1, more clearly illustrates
the bottom-to-sidewall relationship. FIGURE 8 is a view
showing the same relationship. However, in the view of
FIGURE 8, the portion of the bottom wall which is flat is
substantially reduced from the embodiment shown in FIGURE 5.
Accordingly, the taper from the flat bottom wall section
to the sidewalls, while still being sharply tapered, is
substantially longer. As is apparent, the bottom wall in-
cluding the flat section and the tapered section, in both
embodiments, has a thickness less than the thickness of the
parison sidewall. This rapid transition is highly advan-
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tageous during a stretching operation to provide a bo-ttle
of large size, such as a 64 fluid ounce or the like bottle,
since the bottom, including the annular corner, will be
cooler, permitting rapid anddeep stretch without puncture
or deformation. Moreover, as above stated, in order to
provide the increased plastic needed to prevent deformation
at the bottle shoulder, the core rod is to be machined so
as to provide the additional plastic deposit S as best seen
from the enlarged fragmentary view shown in FIGURE 4. Also,
as illustrated, preferably the outer wall of the shoulder
will be substantially straight with the additional plastic
S deposited on the inner wall, as shown in FIGURE 4.
After the stretch/blow operation, the bottle 40 will
have the shape as shown in FIGURE 7 wherein the wall thick-
ness at the shoulder A is slightly greater than the side-
wall thickness as shown at B, providing a bottle which
will have the essential strength at the sidewall and at
the shoulder areas.
FIGURE 6 illustrates a parison mold-core rod assembly
substantially similar to that shown in FIGURE l. However,
in FIGURE 6 the assembly split or partable mold lO has
been replaced with a solid mold lO. The solid mold permits
better and more uniform cooling in that there is no split
in the mold. Further, the solid mold requires less clamping
pressure. In order to remove the parison and core rod
assembly from the solid mold wihtout damage to the parison,
the mold has a slight taper from the gate end to the neck
ring end. Moreover, the split neck ring assembly 18 is
modified in order that the neck ring assembly can be directly
withdrawn from association with the parison mold 10 without
splitting of mold 10. The slight taper in the mold is not
sufficient to impart noticeable variation in the finished
bottle.
To contrast the parisons of the present invention
from the prior art, FIGURE 3 is a fragmentary illustration
of a conventionally employed or prior art core rod 30a
having parison 20a formed thereon. As apparent, the
rounding of the core rod in conjunction with the mold pro-
vides a uniform thickness to the parison sidewall and bottom.
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With the configuratlon of EIGURE 3 in a deep stretch or
long stretch operation to provide a large sized bottle,
rupture of the bottom often occurs, the disadvantages of
such occurrence being apparent.
Although the parison mold-core rod assembly can be
used in the formation of parisons of any biaxially oriented
plastic material, it is of particular value when working
with polyethylene terephthalate (PET). It has been found
that with PET it is essential to have a definite relation-
ship between the parison diameter and the average bottlediameter particularly to obtain the essential tensile yield
strength in the hoop direction. Accordingly, since it is
normally not possible or desirable to control the bottle
diameter to obtain a bottle of a desired volume, in most
instances it is essential to control the diameter of the
parison through an adjustment in the diameter of the core
pin used to produce the parison for forming the bottle.
Further, it has been found that PET, when stretching the
parison longitudinally, requires substantial heat in the
sidewall and shoulder areas relative to the bottom and,
thus, requires a parison with a thin bottom relative to the
shoulder and sidewall areas. Furthermore, although the
present invention is primarily concerned with parisons for
use in a continuous or one-step process, the parison of
this invention can also be of value in the so-called reheat
or two-stage process. As will be apparent to one skilled
in the art, various modifications can be made within the
hereinbefore described apparatus and process with respect
to making improved bottles. The preferred embodiments
described are not to be construed as a limitation of the
invention.
