Note: Descriptions are shown in the official language in which they were submitted.
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PARISUN AN'I~ BLOW 1'1U1_17I~'G ~IhTI-fOI~ I=OR I3U~1~°I'I.I: I-
IA~'I~T'G h'~~pFRCI"r
FIELD OF INVENTION
The invention relates to blow molding, and more specifically to a method for
blow
molding a bottle with an undercut shape and a parison used in the blow molding
of a
bottle with an undercut shape.
s BACKGJE~OIJND OF TI3E INVENTION
It is known to produce plastic (or resin-based) bottles by injection blow
molding.
Injection blow molding comprises the steps of first producing an intermediate
resin article
having a shape similar to that of the final resin article. This intermediate
resin article is
often referred to as the parison, or preform. The parison is usually formed by
an
injection molding process whereby molten resin is forced into a cavity defined
by the
space between a moveable core rod and the parison mold into which the core rod
is
placed. Once the injection molding part is complete, the thus-formed parison
is
transferred from the parison mold to the blow mold via the moveable core rod
on which it
rests.
The parison is next transferred, via the core rod, to the blow mold. The
longitudinal axis of the core rod is generally parallel to the plane of
separation of the
blow mold halves. The blow mold cavity has the shape of the outside of the
final bottle
product. The parison is then inflated, often through the introduction of air
through the
core rod, and caused to expand to the shape of the blow mold. Typically,
because the
2o parison wall is effectively stretched during the blowing process, the final
product wall
thickness is less than the wall thickness of the parison.
The blow mold is typically comprised of two halves, each of which moves in a
substantially opposite direction with respect to the other. The contact area
where the two
mold halves meet and later part (after the blowing is complete) is usually
called the
parting line. The parting line is usually substantially parallel to the
longitudinal axis of
the core rod. Where special product shapes are desired, however, it is
sometimes
necessary to have a parting line that is not linear or completely parallel to
the core rod
axis.
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Where special bottle shapes are desired, and thus the parting line is not
linear, it
becomes important to insure that the core rod can be both transferred into the
location
between the open blow mold halves and removed from the same position with the
blown
bottle in place on the core rod. Because of conventional machinery, it is
equally
important is that the blown bottle be able to be stripped from the core rod in
a direction
parallel to the longitudinal axis of the core rod.
Because the blown bottle must be stripped from the core rod in a direction
parallel
to the axis of the core rod, it can be difficult to blow mold bottles having
an undercut
shape. This is especially true where it is important that the product bottle
have a
consistent wall thickness throughout. Where consistent wall thickness is
desired, it is
preferred that the parison be positioned in (or as close as possible to) the
geometric center
of the blow mold cavity. Where the parison is not in the middle of the blow
mold cavity,
the distance that the molten plastic comprising the parison must travel when
blown is
greater in some parts of the mold than in others. This incongruity in the
distance
stretched results in a bottle having varying wall thickness in its different
regions.
SUMMA1ZY OF INVENTION
The present invention provides both a method and a device for the blow mold
production of a bottle having an undercut surface and an off-center vertical
axis. The
method involves the blow molding of a container having a substantially
constant wall
thickness, but having an oblong base, a round top edge defining a round
opening, the
round top edge defining a plane which is not parallel to the plane of the
base, a neck
portion disposed at the same angle as the plane defined by the top edge, a
front outer
surface, and a rear outer surface having a lower portion and an upper portion.
The
method comprises the steps of forming a parison having a cylindrical upper
neck portion
having a cavity extending therethrough, the upper neck having a top surface
edge defining
a top plane. The parison has a hollow, elongated lower body portion having a
closed
lower end and an upper end, the lower body portion having a major axis
extending in a
direction parallel to the longitudinal axis of the cylindrical upper neck
portion. The
parison also has a front wall and a rear wall, the closed lower end defining a
plane which
is parallel to the top plane defined by the top surface edge. Below the top is
a transition
region connecting the upper neck portion and the lower body portion, the
transition
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region having a top end with the same cross section as the cylindrical upper
neck portion
and a lower end with the same cross section as the upper end of the lower body
portion.
Important in this method is the fact that the front wall is thicker than the
rear wall. After
the parison is made according to these characteristics, it is blow molded in a
blow mold to
form the final product shape.
The invention also comprises the parison itself. The parison of the invention
has a
first wall a first distance from the blow mold inner surface and a second wall
a second
distance from the inner surface. The first distance is less than the second
distance and the
first wall is thinner than the second wall. The parison comprises a
cylindrical upper neck
1o portion having a cavity extending therethrough, the upper neck having a top
surface edge
defining a top plane. The parison also has a hollow, elongated lower body
portion having
a closed lower end and an upper end, the lower body portion having a major
axis
extending in a direction parallel to the longitudinal axis of the cylindrical
upper neck
portion. The lower body portion has a front wall and a rear wall and a closed
lower end
defining a plane which is parallel to the top plane defined by the top surface
edge. The
parison also has a transition region connecting the upper neck portion and the
lower body
portion, the transition region having a top end with the same cross section as
the
cylindrical upper neck portion and a lower end with the same cross section as
the upper
end of the lower body portion. The transition region has a sloped wall
extending from
2o the rear wall of the lower body portion out and up from the rear wall of
the lower body
portion to the upper neck portion. The front wall is thicker than the rear
wall. This
parison produces a bottle having a parting line mark on its sides, as opposed
to its front
or rear walls, which is generally more aesthetically pleasing.
It is to be understood that both the foregoing general description and the
following
detailed description are exemplary, but are not restrictive, of the invention.
B7RIEF DESCRIPTI~N OF T>EIE I)RAVVING
The invention is best understood from the following detailed description when
read in connection with the accompanying drawing. It is emphasized that,
according to
common practice, the various features of the drawing are not necessarily to
scale. On the
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contrary, the dimensions of the various features are arbitrarily expanded or
reduced for
clarity. Included in the drawing are the following figures:
FIG. 1 is a schematic view of a bottle blow molded in accordance with the
present
invention;
FIG. 2 is a schematic view of a blow mold which would not work with typical
core rod handling apparati;
FIG. 3 is a cross-sectional sideview of a parison mold according to the
present
invention;
FIG. 4 is top view of the bottom half of the parison mold of FIG. 3;
1o FIG. 4.A shows an isometric view of a parison mold bottom half;
FIG. 5 is a cross-sectional sideview of the parison mold of FIG. 3 with a core
rod
in place;
FIG. 6 is a cross-sectional sideview of the parison mold of FIG. 3 with a core
rod
inserted and a resin injection nozzle in place;
i5 FIG. 6A is a cross-sectional sideview of the apparatus of FIG. 6 showing
the
injection of resin to form the parison;
FIG. 7 is a cross-sectional sideview of a blow mold according to the present
invention;
FIG. 7A shows an isometric view of a blow mold bottom half;
2o FIG. 8 is a top view of the bottom half of the blow mold of FIG. 7;
FIG. 9 is a bottom view of the top half of the blow mold of FIG. 7;
FIG. 10 is a cross-sectional sideview of the blow mold of FIG. 7 with a core
rod
and parison disposed therein;
FIG. 11 is a schematic sideview of a core rod and parison disposed within a
blow
25 mold according to the present invention;
FIGS. 11A and 11B are sections taken from FIG. 11; and
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FIGs. 12A and 12B are the sections of FIGS. 11A and 11B, respectively, with
dimensions indicated thereon.
DETAILED DESCRIPTION OF INVENTION
The present invention relates to the production of a bottle having an off-
center
vertical axis. FIG. 1 shows one example of the type of bottle 100 having an
undercut
region 110 which can be produced through a blow molding process according to
the
present invention. Before the present invention, this type of bottle was
typically made
through an extrusion molding process because its shape presented several
problems to
conventional blow-molding techniques.
FIG. 2 shows a mold shape which would provide product bottle 100 of FIG. 1.
Typically, the parison shape for a bottle of constant wall thickness has
itself a constant
wall thickness. Such a parison is then inserted into the geographic center (or
near
geographic center) of the blow mold to insure consistent wall thickness. In
the case of the
bottle of FIG. 1, this parison would have to be oriented approximately as
shown in FIG.
2, with core rod 200 and parison 210 in place within the near geographic
center of the
mold. The problem with this requirement, however, is that conventional
processing
machinery strips the product bottle from the core rod in a direction parallel
to the
longitudinal axis of the core rod. Thus, the resultant blown bottle of FIG. 2
cannot be
stripped from the core rod in a direction parallel to the axis of the core rod
because the
front part of the bottle blocks its removal. An entirely new machine for
moving the core
rod in an articulated fashion with respect to the bottle during the stripping
step would
have to be created which would cantilever the core rod with respect to the
blown bottle.
Moreover, a traditional machine for stripping the bottle from the core rod
could not be
used.
One alternative to this problem would be to orient the blow mold with respect
to
the core rod such that the product bottle is rotated 90° from that
shown in FIG. 7. This
would result, however, in the parting line (which is visible on the product
bottle) being
oriented on the front and back faces of the bottle, rather than on the sides.
In addition to
being less aesthetically pleasing, the front/back parting line makes label
adhesion more
difficult.
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The present invention solves these problems by providing a novel method and
parison. The parison used in the present invention has varying wall thickness
along its
different sides which overcome the need for the parison to be present in the
physical
middle of the blow mold. The use of this novel parison allows undercut bottles
of the
type shown in FIG. 1 to be blow molded, whereas in the past extrusion molding
would
likely have been used.
FIG. 3 shows a cross-sectional sideview of the parison mold according to the
present invention. FIG. 3 shows parison mold 305 with parison mold top half
300 and
parison mold bottom half 310 which come together along non-linear parting line
320 to
form parison mold cavity 330. FIG. 4 shows a top view of parison mold bottom
half 310
having cavity x.50. FIG. ~.A shows an isometric view of the bottom half 310.
FIG. 5 shows a schematic view of the cross-sectional sideview of parison mold
305 shown in FIG. 3 with core rod 500 oriented between mold halves 300 and
310.
When the mold closes, both core rod 500 and top mold half 300 move toward mold
half
310 as shown by the arrows in FIG. 5. Alternatively, all three components
could move
as the mold closes. FIG. 6 shows core rod 500 in place within parison mold
cavity 330.
Also shown schematically is resin injection nozzle 600 which delivers resin
into parison
mold cavity 330. FIG. 6A shows the space defined between core rod 500 and
parison
mold cavity 330 being filled with resin to form a parison. As will be
discussed more
2o fully below, core rod 500 with its partially cooled resin will be moved to
the blow mold
cavity.
FIG. 7 shows a cross-sectional sideview of a blow mold 705 that can be used in
the present invention. Blow mold top half 700 contacts blow mold bottom half
710 along
blow mold parting line 720 to form blow mold cavity 730. FIG. 7A shows an
isometric
z5 view of blow mold bottom half 710. FIG. 8 shows a top view of blow mold
bottom half
710 having cavity 810; FIG. 9 shows a bottom view of blow mold top half 700
having
cavity 910.
FIG. 10 illustrates a schematic cross-sectional side view of blow mold 705
shown
in FIG. 7, with core rod 500 and parison 610 disposed within blow mold cavity
720. In
30 this position, the parison is ready to be blown, according to known
practices, and air is
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injected through a small passageway within core rod 500 (not shown) which
blows the
parison outward against the blow mold inner wall.
FIG. 11 shows a more detailed view of a parison 110 disposed on care rod 120.
This embodiment is slightly different than those shown in the above figures,
but is
consistent with the present invention. Specifically, parison 110 comprises a
cylindrical
upper neck portion 140 having a cavity extending therethrough. Upper neck
portion 140
has a top surface edge 145 defining a top plane, a hollow, elongated lower
body portion
150 having a closed lower end 155 and an upper end 160. Lower body portion 150
has a
major axis extending in a direction parallel to the longitudinal axis of the
cylindrical
1 o upper neck portion 140, and has a front wall 170 and a rear wall 175.
Closed lower end
155 defines a plane which is parallel to the top plane defined by the top
surface edge
145. Also shown in FIG. 11 is a transition region 180 connecting upper neck
portion 140
and lower body portion 150. Transition region 180 has a top end 181 with the
same cross
section as the cylindrical upper neck portion 140 and a lower end 182 with the
same cross
tS section as the upper end 160 of lower body portion 150. Transition region
180 has a
sloped wall extending from rear wall 175 of lower body portion 150 out and up
from rear
wall 175 to upper neck portion 140.
As can be seen in FIG. 11, front wall 170 is thicker than the rear wall 175.
Sections A-A and B-B are shown in FIGs. 11A and 11B, respectively. FIG. 11A
shows a
2o cross-section of lower body portion 150 near upper end 160. FIG. 11B shows
a cross-
section of lower body portion 150 toward the closed lower end 155. FIG. 11A
shows the
cross-section where the distance from the parison to the blow mold wall is
approximately
the same on either side of the parison. In contrast, FIG. 11B shows the cross-
section
where the distance from the parison to the blow mold wall is much greater on
front wall
25 170 of the parison than on rear wall 175. Where the distance is greater,
the thickness of
the parison itself is also greater.
This change in parison wall thickness is dependent upon the distance the
parison
wall material must travel when blown. In those areas along the parison wall
where little
stretching will occur, there exists no need to supply extra material. On the
other hand, in
;o those areas along the parison wall where a relatively large amount of
stretching will
occur, additional material is needed.
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FIGs. 12A and 12B illustrate a preferred embodiment of the parison shown in
FIG. 11. In FIG. 12A, x = about 0.08 inches (°'about" meaning +/- 0.01
inches, in all
cases herein unless otherwise noted), and y = about 0.07 inches. In FIG. 12B,
x' -
about 0.17 inches, and y' = about 0.07 inches.
Also illustrated in FIGs. 11 and 12 is the fact that along front wall 170 of
the
parison the wall thickness is tapered from closed lower end 155 having a first
thickness
up to a thickest central portion and down to a second thickness at the
transition region.
At the lower end of transition region 182 at front wall 170, the parison wall
thickness is
about equal to the parison wall thickness at the upper end 160 of lower body
portion 150.
1 o This is because the distance that the material comprising the parison at
that point must
travel during blowing is about the same.
This parison also relates to a novel method of blow molding a container having
an
undercut shape. More specifically, the method according to the present
invention
provides a bottle having an undercut shape having a substantially constant
wall thickness.
The bottle also has a base, a round top edge defining a round opening, the
round top edge
defining a plane which is not parallel to the plane of the base, a neck
portion disposed at
the same angle as the plane defined by the top edge, a front outer surface,
and a rear
outer surface having a lower portion and an upper portion. The method used to
blow
mold such a bottle comprises the steps of first forming a particular parison,
as discussed
2o above, and then blowing the parison in a blow mold. The parison has a first
wall which
is a first distance from the blow mold inner surface and a second wall which
is a second
distance from the inner surface of the blow mold. The first distance is less
than the
second distance and the first wall is thinner over most of its length than the
second wall.
More specifically, the method uses a parison where the front wall is about 2.5
times thicker at its thickest point than the rear wall. This aspect of the
invention is
important as the material along the front wall must travel much farther during
blowing,
and thus is stretched to a much greater extent, than the material comprising
the rear wall
of the parison.
Although illustrated and described herein with reference to certain specific
3o embodiments, the present invention is nevertheless not intended to be
limited to the
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details shown. Rather, various modifications may be made in the details within
the scope
and range of equivalents of the claims and without departing from the spirit
of the
invention.
