Extrusion-blow molded container having cylindrical body and metal mold for producing the same

There is provided a cylindrical extrusion-blow molded container, or a cylindrical delaminatable blow-molded container in particular, having an inner layer made of a poorly fusion-bonded resin material such as nylon, in order to improve the fusion-bond of the bottom seal section of the container, to prevent the bottom from cracking. In a cylindrical extrusion-blow mold container, or a cylindrical delaminatable blow-molded container having an air inlet hole arranged at a neck of the container, a bottom seal section in a bottom wall of a bottom is formed with an elliptic thick and raised section arranged in and near the bottom seal section, said elliptic thick and raised section approximately showing an ellipsoid having an elliptic contour with its major axis running along a parting line.

BACKGROUND OF THE INVENTION 
This invention relates to a technology for preventing a cracked bottom of 
an extrusion-blow molded container having a cylindrical body. 
There is proposed an extrusion-blow molded container, particularly a 
delaminatable blow-molded container having an air inlet hole arranged at a 
neck of the container. In order to prevent a bottom from cracking, such 
container is formed with a bottom rib in a bottom seal section. The bottom 
seal section includes fusion-bonded sections, each of which is provided 
with projections to be inserted into the other member for mutual 
engagement of the fusion-bonded sections (Japanese Patent Laid-Open No. 
8-216238). 
In the above described prior art, the bottom rib is formed in the bottom 
seal section of the blow-molded container by fusion-bonding a pair of 
fusion-bonded sections projecting from a bottom wall of the container at 
inner surfaces thereof. The fusion-bonded sections of the bottom rib are 
provided with projections and recesses alternately, so that the 
fusion-bonded sections may be held in mutual engagement with each other, 
so as to make the bottom seal section show an improved cohesive force and 
an enhanced strength against shearing force that can be applied in 
parallel with a parting line (along X-X' line in FIG. 2). With such an 
arrangement, the crack of the bottom of the extrusion-blow molded 
container having large volume is prevented. 
Particularly, in the case of a delaminatable blow-molded container, such 
projections and recesses are very advantageous, because inner layers of 
the container that are typically made of a poorly adhesive resin material 
such as nylon can be tightly bound together due to the projections and 
recesses, and hence the bottom of the container is practically free from 
cracking. 
However, containers having an elliptic cylindrical body and an elliptic 
bottom (hereinafter referred to as "elliptic cylindrical containers") and 
those having a column-shaped body and a circular bottom (hereinafter 
referred to as "cylindrical containers") differ significantly from each 
other in terms of the phenomenon of cracked bottom. In the case of the 
delaminatable blow-molded container having the inner layers typically made 
of a poorly adhesive resin material, the bottom can easily become cracked 
when the container is filled with liquid or accidentally dropped, if the 
container is a not an elliptic container but a large cylindrical 
container. 
SUMMARY OF THE INVENTION 
In view of the above identified problem, it is therefore an object of the 
present invention to provide a large cylindrical blow-molded container, 
particularly a large cylindrical delaminatable blow-molded container 
having the inner layers made of a poorly adhesive resin material such as 
nylon, in order to improve the adhesiveness of the bottom seal section of 
the container and to prevent the bottom from cracking. 
As a result of intensive research efforts and comparative experiments on 
the phenomenon of cracked bottom of elliptic cylindrical containers and 
cylindrical containers, the inventors of the present invention found that 
the phenomenon of cracked bottom can appear when the bottom seal section 
of the blow-molded container shows a low adhesiveness due to a contraction 
with time of the container immediately after the blow-molding. 
It is known that an extrusion-blow molded container contracts with time 
after the blow-molding along the parting line (in the direction of X-X' 
line in FIG. 2) if it is a cylindrical container, whereas the container 
contracts along a direction perpendicular to the parting line (in the 
direction of Y-Y' in FIG. 2) if it is an elliptic cylindrical container. 
While the above described conventional blow-molded containers are provided 
with a bottom rib arranged in the bottom seal section in order to prevent 
the bottom from cracking, the fusion-bonded interface of the bottom rib of 
the container is broadened to make the bottom apt to be cracked if the 
container is a cylindrical container, because, unlike an elliptic 
cylindrical container, it contracts in a direction different from that of 
the parting line. 
Thus, the inventors of the present invention invented that the 
fusion-bonded interface of the bottom of the cylindrical blow-molded 
container can be made to show an improved adhesiveness and prevented from 
cracking, if the bottom is made to contract in a direction perpendicular 
to that of the parting line (the direction same as the one along which an 
elliptic cylindrical container contracts). 
As a result of the above described intensive research efforts and 
comparative experiments on the phenomenon of cracked bottom of elliptic 
cylindrical containers and cylindrical containers, the inventors of the 
present invention also found 
that a cylindrical container can be made to contract less along the parting 
line and effectively prevent the bottom from cracking if the bottom rib is 
made have a small width; 
that, while the delaminatable blow-molded container having an elliptic 
cylindrical profile is conventionally made to have engaging sections 
arranged in two rows disposed one on the other in the bottom rib, the 
cylindrical delaminatable blow-molded container can effectively prevent 
the bottom from cracking when the engaging sections are arranged in a 
single row; 
that the pins for forming the sections of the bottom rib are preferably 
made to show a small taper angle to make them nearly as long as the width 
of the rib in order to make the bottom more fusion-bondable; and 
that the bottom can be prevented from deforming downwardly with time when 
the bottom and the bottom rib are made to show an arch-like profile. 
Thus, on the basis of the above findings, the object of the present 
invention is achieved by providing a cylindrical blow-molded container in 
which an elliptic thick and raised section is formed along a bottom seal 
section, a direction of the contraction is shifted by the elliptic thick 
and raised section by utilizing the contraction with time after removing 
the blow-molded product, so that the container may contract in a direction 
perpendicular to the parting line as in the case of an elliptic 
cylindrical container. 
In order to make the effect of contraction with time more effectively, a 
metal mold is made of aluminum which is quickly cooled, except pinch-off 
sections of the mold. Additionally, a flow path of the cooling medium is 
located remotely from the pinch-off sections. 
With this arrangement, the elliptic thick and raised section and peripheral 
areas of the bottom rib are made to maintain heat after the molded product 
is taken out of the mold, so that the elliptic thick and raised section 
may contracts with time. 
Additionally, the object of the present invention can be achieved more 
effectively when the wall of the bottom seal section and the shape of the 
bottom rib are made to show an arch-like profile, when the bottom rib is 
made to show a low profile (a small length in the axial direction of the 
container), when the pins are made to show no stepped sections, and when 
the pins are made to show a small taper angle so as to make them nearly as 
long as the width of the rib. 
Specifically, according to the invention, there is provided a cylindrical 
extrusion-blow mold container, or a cylindrical delaminatable blow-molded 
container having an air inlet hole arranged at a neck of the container, 
characterized in that a bottom seal section in a bottom wall of a bottom 
is formed with an elliptic thick and raised section arranged in and near 
the bottom seal section, said elliptic thick and raised section 
approximately showing an ellipsoid having an elliptic contour with its 
major axis running along a parting line. Additionally, the bottom seal 
section may be formed with an arch-shaped bottom rib. 
Furthermore, the bottom rib of said bottom seal section may have a 
thickness substantially equal to or less than a thickness of the bottom 
wall of the seal section, fusion-bonded sections of the bottom rib may be 
formed with engaging sections to be forcibly pressed one into the other, 
and said engaging sections may be alternately arranged along a center line 
of the bottom rib. 
According to the invention, there is also provided a metal mold for 
extrusion-blow molding a cylindrical container, characterized in that an 
elliptic thick and raised section is arranged in and near a pinch-off 
section along a parting line, an area in and near the elliptic thick and 
raised section is made of a material having low thermal conductivity, so 
that the elliptic thick and raised sections of the molded container can 
maintain heat after the container is removed out of the mold.

PREFERRED EMBODIMENT OF THE INVENTION 
Now, the present invention will be described in greater detail in terms of 
a preferred embodiment of the invention. 
Firstly, the embodiment of the blow-molded delaminatable container will be 
described by referring to the accompanying drawings. 
A container 1 illustrated in FIGS. 1 through 5 is made of a laminate 
comprising an outer layer 2 and an inner layer 3, and is molded by 
lamination-extrusion-blow-molding. However, the present invention is not 
limited to a container made of a laminate or multilayer, and is applicable 
to a container made of a single layer body. 
Said outer layer 2 may be made of any material that can maintain the 
appearance of the container, for example, high density polyethylene resin, 
although the present invention is not limited thereto. The inner layer 3 
is laid or laminated on the outer layer 2, and can be delaminated or 
peeled off from the outer layer. The inner layer can change its shape 
freely, and is made of nylon and ethylenevinylalcohol copolymer (EVOH 
resin), although the present invention is not limited thereto. 
The blow-molded delaminatable container 1 comprises a cylindrical body 
section 4, a neck section 5 and a bottom section 6. The outer layer 2 of 
the neck section 5 has an opening 7 bored through the outer layer 2 in 
order to introduce air between the outer layer 2 and the inner layer 3, so 
that the inner layer 3 is delaminated or peeled off from the outer layer 2 
as air is taken in through the opening 7. 
As shown in FIGS. 2 through 4, the bottom section 6 comprises a bottom end 
wall 8 arranged along an outer periphery of the container and a bottom 
wall 9 which is located inside the bottom end wall 8 and curved inwardly 
(upwardly) from the bottom end wall 8. The bottom wall 9 has a bottom seal 
section 10 along a parting line running through an axial center of the 
container. The bottom seal section 10 is provided with a bottom rib 11. 
Said bottom wall 9 includes a flat wall section 12, an elliptic thick and 
raised section 13 and a container aligning stepped section 14. The 
elliptic thick and raised section 13 is formed along the bottom rib 11, 
and has a width increasing toward the axial center of the bottom and a 
level rising into an inside of the container (upwardly) also toward the 
axial center thereof. 
Said bottom rib 11 runs through the axial center of the bottom, and is 
formed on the bottom wall 9 along a parting line. The bottom rib 11 is 
suspending outwardly (downwardly) from a lower surface of said elliptic 
thick and raised section 13, and its level is raised upwardly as toward 
the center to show an arch-like profile. The bottom rib 11 is provided 
with engaging sections 15 arranged along a center line of the bottom rib 
11 at regular intervals. Preferably, a height (an axial dimension) of the 
bottom rib 11 is made small. 
As shown in FIG. 5, the bottom rib 11 of the bottom seal section 10 is 
formed by fusion-bonding a parison in a manner as will be described 
hereinafter, and fusion-bonded sections 16a, 16b are made to have 
respective projections 17a, 17b radially projecting from a fusion-bonded 
interface 10a and respective recesses 18a, 18b radially recessed. The 
projections and the recesses are arranged alternately. The projections 17a 
and the recesses 18a formed on the fusion-bonded section 16a are located 
vis-a-vis the corresponding recesses 18b and the corresponding projections 
17b formed on the fusion-bonded section 16b. The projections 17a, 17b are 
forcibly pressed into and engaged with the corresponding respective 
recesses 18b, 18a, so that the fusion-bonded sections 16a, 16b are 
integrated to form the bottom rib 11, as they are fusion-bonded and 
engaged with each other. 
In the drawings, reference symbols 2a and 2b respectively denote the outer 
layers of the fusion-bonded sections 16a, 16b, and reference symbols 3a 
and 3b respectively denote the inner layers of the fusion-bonded sections 
16a, 16b. 
Now, a blow metal mold for molding the above-described container will be 
described by referring to FIG. 6. 
As shown in FIG. 6, a blow metal mold 20 comprises a pair of split molds 
20a, 20b. Each of the split molds is formed with a cavity 21a, 21b and a 
pinch-off section 22a, 22b. 
Said pinch-off sections 22a, 22b respectively comprise pinch-off edges 23a, 
23b provided with respective lower resin escape sections 24, 24b, stepped 
sections 25a, 25b and pins 26a, 26b. The stepped sections 25a, 25b are 
located respectively on a cavity side of the pinch-off edges 23a, 23b. 
Additionally, the stepped sections 25a, 25b are located radially outside 
tips of the respective pinch-off edges along a direction perpendicular to 
the parting line, so as to define a thickness of the bottom rib 11, which 
thickness runs in a radial direction of the container. The pins 26a, 26b 
are projected inwardly from the respective stepped sections 25a, 25b. The 
pins 26a and pins 26b are arranged alternately. 
The pins 26a arranged on the stepped section 25a and the pins 26b arranged 
on the oppositely disposed stepped section 25b are arranged alternately at 
regular intervals. Each of the pins 26 has a frusto-conical front end. 
Preferably, the front end of each of the pins 26 shows a small taper 
angle. The small taper angle makes the engagement deep. 
The bottom rib 11 of the container is formed by said pinch-off edges 23a, 
23b and the stepped sections 25a, 25b, and the engaging sections 15 
(projections 17a, 17b and recesses 18a, 18b) are formed by said pins 26a, 
26b. 
The distance separating the pinch-off edges 23a, 23b from the respective 
stepped sections 25a, 25b is shorter than the height of the parison for 
forming the fusion-bonded sections. 
The cavity 21a, 21b has a bottom wall which is provided with an elliptic 
raised section 27a, 27b having an elliptic contour with its major axis 
running along the parting line and its level rising into the inside of the 
container (upwardly) toward the center thereof. The pinch-off edges 23a, 
23b have an arch-shaped upper surface. 
The metal mold is preferably made of aluminum because aluminum can be 
cooled quickly. However, pinch-off peripheral sections 28a, 28b are 
preferably made of a material having a low thermal conductivity. A flow 
path (not shown) of a circulating cooling medium such as water is arranged 
remotely from the elliptic raised section 27a, 27b. 
Parisons are placed in the metal mold in the state as shown in FIG. 6, and 
then the split molds 20a and 20b are tightly put together. The projections 
17 and recesses 18 are formed by the pins 26. The radially and oppositely 
disposed walls of the parisons are fusion-bonded together as the split 
molds are tightly put together. The bottom rib 11 is formed by the stepped 
sections 25. A lower end portion of the parisons is cut off by the 
pinch-off edges 23. Subsequently, compressed fluid is blown into the 
parisons through a nozzle (not shown), so that the container is formed by 
blow-molding within the cavities. 
The bottom wall of the container can be made thick along the parting line, 
because the parisons are pressed and raised upwardly by the stepped 
sections 25 of the pinch-off sections 22 during the blow-molding 
operation. The elliptic thick and raised section 13 is formed by the 
raised sections 27a, 27b of the metal molds. 
The thin bottom rib 11 is formed by the stepped sections 25 of the 
pinch-off sections. The projections 17 and recesses 18 are alternately 
formed by the pins 26 of the pinch-off sections, to form the bottom rib 11 
with rigid and secure engagement. 
Since the pinch-off peripheral sections 28a, 28b are made of a material 
having a low thermal conductivity and the flow path is located remotely 
from the elliptic raised section 27a, 27b, only the areas in and near the 
elliptic thick and raised section 13 and the bottom rib 11 are heated 
sufficiently. Therefore, the areas of the elliptic thick and raised 
section 13 will be contracted with time in a direction perpendicular to 
the fusion-bonded interface. 
Since the bottom seal section 10 is provided with the thick raised section 
13 having the elliptic shape, the contraction with time is conducted in 
the direction perpendicular to the parting line. 
Additionally, since the metal mold is made of thermally conductive aluminum 
except the pinch-off sections 22 and cooled quickly by the cooling system, 
the areas in and near the elliptic thick and raised section and the bottom 
rib are made to remain heated sufficiently after the molded product is 
removed from the metal mold. Thus, the contraction with time of the 
elliptic thick and raised section 13 along the parting line is further 
enhanced. 
Since the bottom rib 11 has the low height or thickness, it contracts less 
along the parting line to increase the bonding stability of the 
fusion-bonded sections. 
Since the elliptic thick and raised section 13 and the bottom rib 11 are 
formed to raise upwardly to show the arch-like profile, the central area 
of the bottom of the container is prevented from lowering with time. 
Since the pins 26 are made to show a small taper angle to increase the 
projecting length of the engaging sections 15 of the bottom rib 11 from 
the fusion-bonded interface 10a, the bottom is more resistant to the 
shearing force along a direction perpendicular to the fusion-bonded 
interface. 
Since the engaging sections are arranged into a curved single row, they are 
more resistive against shearing force than when they are arranged in two 
rows disposed one on the other. 
As the above functions or effects are combined, the fusion-bonded interface 
is remarkably improved, so that a cylindrical delaminatable blow-molded 
container having a large size can be produced according to the invention. 
Now, the following is a specific example of the bottom seal section of a 
cylindrical delaminatable blow-molded container according to the 
invention. 
A cylindrical delaminatable blow-molded container having a volume of 500 ml 
was formed by extrusion blow-molding, using high density polyethylene for 
the outer layer and nylon for the inner layer. 
The thus formed blow-molded container had a bottom diameter of about 86 mm 
and the fusion-bonded interface section had about 52 mm width. The 
elliptic thick and raised section was formed in the bottom seal section. 
It approximately showed an ellipsoid raised from an ellipse having a 52 mm 
long major axis and a 20 mm long minor axis. It had a radius of curvature 
of about 300 mm at the bottom surface. Its wall thickness was about 3 mm. 
The bottom rib had 2 mm width, and engaging sections were arranged 
alternately at regular intervals of 4 mm along a curved line with a radius 
of curvature of 246 mm. 
The recesses had a diameter of 1.4 mm at the opening and that of 0.6 mm at 
the bottom, and had 1.5 mm deep. 
For comparison example, an elliptic cylindrical blow-molded container was 
formed according to Japanese Patent Laid-Open No. 8-216238. It had a flat 
bottom wall with a straight bottom rib that had about 3 mm width. It had 
engaging sections arranged in two rows disposed one on the other. 
The cylindrical blow-molded container according to the invention showed a 
level of adhesiveness at the bottom seal section that is substantially 
same as that of the cylindrical blow-molded contains having a 
substantially same volume. 
Now, a further embodiment of single layer or multilayer cylindrical 
extrusion-blow molded container according to the invention will be 
described. 
This embodiment is realized by applying the construction of the bottom seal 
section according to the invention to a single layer or multilayer 
cylindrical extrusion-blow molded container. 
While an ordinary single layer or multilayer blow-molded container differs 
from a delaminatable blow-molded container as shown in FIGS. 1 through 5 
in that the former is not provided with an air inlet hole and the layers 
cannot be peeled off from each other, the bottom seal section of such 
container can be formed by an ordinary blow-molding process by using the 
above described metal mold. 
FIG. 7(a) shows the bottom seal section of a single layer blow-molded 
container. In this Drawing, it comprises fusion-bonded sections 30a, 30b, 
a fusion-bonded interface 31, recesses 32 and projections 33. 
According to the invention, since the bottom seal section shows an improved 
level of adhesiveness, the bottom is prevented from cracking even if it is 
made of a material such as polyethyleneterephthalate or polypropylene that 
is poorly fusion-bonded and that shows a poor strength when dropped, so 
that a large cylindrical blow-molded container can be produced. 
FIG. 7(b) shows the bottom seal section of a multilayer or lamination 
blow-molded container. In this Drawing, it comprises fusion-bonded 
sections 40a, 40b, a fusion-bonded interface 41, outer layers 42a, 42b, 
inner layers 43a, 43b, an adhesive layer 44, recesses 45 and projections 
46. 
The outer layers 42 of a multilayer blow-molded container are typically 
made of a reinforcing material such as high density polyethylene or 
polypropylene etc., and the inner layers 43 are made of a functional 
material such as nylon, EvOH or polyethyleneterephthalate etc., although 
the present invention is not limited to these materials. 
According to the invention, since the bottom seal section shows an improved 
level of adhesiveness, a large cylindrical blow-molded container can be 
produced by using such a bottom seal section even if it is made of poorly 
fusion-bonded resin such as nylon. 
The present invention provides the following advantages. 
An elliptic thick and raised section is formed along the fusion-bonded 
interface of the bottom seal section of a cylindrical blow-molded 
container, so that the contraction with time immediately after the molding 
operation is conducted along a direction perpendicular to the parting 
line. Thus, the bottom of the blow-molded container is remarkably free 
from cracking. Therefore, according to the invention, it is possible to 
form a cylindrical blow-molded container with large size. 
Particularly, the present invention can advantageously be used to improve 
the fusion-bonding of the bottom seal section of a delaminatable 
blow-molded container where the inner layer is made of poorly 
fusion-bonded resin such as nylon. Therefore, according to the invention, 
it is possible to form a cylindrical container with large size.