U.S. patent application number 08/952476 was filed with the patent office on 2001-05-24 for pipe extrusion die and resin pipe.
Invention is credited to MURATA, TAKUMI, NISHIMURA, KATUNARI, YAMADA, MASAHIRO, YOSHIMURA, YOSHIHIRO.
Application Number | 20010001672 08/952476 |
Document ID | / |
Family ID | 13353941 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001672 |
Kind Code |
A1 |
YOSHIMURA, YOSHIHIRO ; et
al. |
May 24, 2001 |
PIPE EXTRUSION DIE AND RESIN PIPE
Abstract
A pipe molding die of the present invention is capable of
rectifying a flow of a molten resin by a resin reservoir and
uniformizing the flow of the resin extruded from the die in
whichever position in a peripheral direction on a flow path within
the die. Consequently, ununiformity in wall thickness of the resin
pipe to be molded can be eliminated. The pipe molding die comprises
a throttle part as one of components thereof, which includes a
core, a shell part fitted to the core and a resin reservoir as a
portion of a flow path which is formed between the core and the
shell part. A molten resin as a pipe raw material flows through the
flow path. The resin reservoir is provided in at least one of the
core and the shell part, and takes a ring-like shape circumscribing
a central axis of the pipe molding die with the central axis
centered. The resin reservoir assumes a recessed shape in
cross-section.
Inventors: |
YOSHIMURA, YOSHIHIRO;
(YAMAGUCHI, JP) ; NISHIMURA, KATUNARI; (YAMAGUCHI,
JP) ; MURATA, TAKUMI; (YAMAGUCHI, JP) ;
YAMADA, MASAHIRO; (YAMAGUCHI, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
13353941 |
Appl. No.: |
08/952476 |
Filed: |
February 13, 1998 |
PCT Filed: |
March 13, 1997 |
PCT NO: |
PCT/JP97/00805 |
Current U.S.
Class: |
425/380 ;
425/382.4 |
Current CPC
Class: |
B29C 48/3001 20190201;
B29C 48/705 20190201; B29C 48/32 20190201; B29C 48/09 20190201 |
Class at
Publication: |
425/380 ;
425/382.4 |
International
Class: |
B29C 047/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 1996 |
JP |
8-067750 |
Claims
What is claimed is:
1. A pipe molding die comprising: a throttle part defined as one of
parts constituting said pipe molding die, said throttle part
including: a core; a shell part, fitted to said core, for forming a
flow path through which a molten resin as a pipe raw material flows
between said core and said shell part; and a resin reservoir
provided in at least one of said core and said shell part as a
portion of said flow path and taking a recessed shape in
cross-section and a ring-like shape circumscribing a central axis
of said pipe molding die with this central axis centered.
2. The pipe molding die according to claim 1, wherein said flow
path has a throttle in a shape constricted thinner than other
portions along said flow path, and said throttle is formed with
said resin reservoir.
3. The pipe molding die according to claim 1 or 2, wherein a
recessed cross section of said resin reservoir is formed in a
curved-surface configuration to prevent a flow of the molten resin
from being stagnated.
4. The pipe molding die according to any one of claims 1 through 3,
wherein said resin reservoir is sem-icircular in cross-section, a
radius of curvature thereof is 10 mm-100 mm, and an angle made by
each of tangential lines at both ends of the semi-circular arc of
said resin reservoir and by the central axis is 15-120.degree..
5. The pipe molding die according to claim 4, wherein the radius of
curvature is 25 mm, and the angle is 75.degree.-90.degree..
6. The pipe molding die according to any one of claims 1-5, wherein
said die is used for molding a polyolefine pipe involving the use
of polyolefine as a pipe raw material.
7. The pipe molding die according to claim 6, wherein the
polyolefine is polyethylene.
8. A resin pipe characterized in that an average wall thickness of
said pipe is set to one of values in a range of 5 mm-50 mm, that a
difference between a maximum wall thickness and a minimum wall
thickness of said pipe is equal to or larger than 0 mm but equal to
or smaller than 1.0 mm, and said pipe is manufactured by said pipe
molding die according to any one of claims 1 through 7.
Description
TECHNICAL FIELD
[0001] The present invention relates to a pipe molding die and a
resin pipe molded by the pipe molding die.
BACKGROUND ART
[0002] A pipe molding die is employed for manufacturing an elongate
resin pipe such as a polypropylene pipe and a polyethylene pipe
that are used as, e.g., gas pipes.
[0003] The pipe molding die is supplied, from a resin extruder,
with a molten kneaded resin (The molten kneaded resin will
hereinafter be termed a "molten resin".) defined as a raw material
for the resin pipe. The supplied molten resin is discharged finally
in the form of a resin pipe as an extrusion molded product from the
die via a flow path within the die. According to procedures
thereof, the molten resin supplied into the die is temporarily
expanded in a cylindrical shape, and thereafter gradually throttled
down into a pipe in the end that has a diameter corresponding to an
application.
[0004] In general, the pipe molding die is basically constructed of
a die part, a throttle part and a land part. The die part forms the
molten resin supplied in from a resin extruder in a cylindrical
shape. The throttle part gives a rectifying effect by throttling
the cylindrical molten resin fed in from the die part. And the land
part uniforms a flow velocity of the resin. Then, those
constructive parts are each concentrically arranged in sequence
from an upstream side to a downstream side in a flowing direction
of the molten resin.
[0005] The resin pipe as an extrusion molded product manufactured
by the pipe molding die described above is required to have no
ununiformity in terms of wall thickness. Namely, it is required
that both of an inner surface configuration and an outer surface
configuration of the resin pipe be concentrically complete rounds
as viewed in cross-section.
[0006] It is because problems as shown in the following items
(1)-(3) might arise if the pipe has the ununiformity in wall
thickness.
[0007] (1) It is undesirable in terms of external appearance.
[0008] (2) A core deviation tends to occur when in a butt seam
fusion to fuse end surfaces of the pipe by butting them with each
other.
[0009] (3) Contaminations and flaw on the surface of the pipe are
undesirable for joining a joint to the pipe by fusion, and
therefore the pipe surface is required to be cut. In that case, an
outer peripheral surface of the pipe is fixed by a jig, and the
pipe surface is cut by a cutting tool while moving the cutting tool
along the pipe. If ununiformity in wall thickness is large,
however, the pipe can not be uniformly held by the jig because of
the ruggedness on the pipe surface, with the result that there
might be a firmly fixed portion and a slackened portion to make the
pipe unstable. Further, since a distance between the outer surface
of the pipe fixed by the jig and the cutting tool is not uniform,
an adhesion is poor, and a complete round can not be obtained even
when cut off. Besides, unevenness in cutting is to appear. A
cutting quantity must increase in order to prevent the unevenness
in cutting, and correspondingly a more extra of pipe raw material
is needed.
[0010] Such being the case, it is a general practice that a flow of
the resin extruded from the die is kept constantly in whichever
position on a flow path within the die to uniformize the wall
thickness of the resin pipe to be molded by the pipe molding die.
Methods of enhancing a rectifying effect and a throttle effect are
effective in terms of keeping constantly the flow of the resin
through the flow path.
[0011] For making an attempt to enhance the rectifying effect and
the throttle effect as well, the die must be increased in size.
When increasing the size of the die, a pressure necessary for
flowing the molten resin has to be risen. Furthermore, if the
pressure rises, a temperature of the molten resin increases enough
to easily deteriorate the resin or to cause an excessive luster on
the pipe surface to such an extent as to be visually undesirable,
resulting in a devaluation of a commercial product. Then, pressure
tightness of the die and of the extruder must be increased.
[0012] Moreover, according to the tests by the present inventors,
it has proved that the ununiformity in wall thickness is to occur
even when making an endeavor to enhance the rectifying effect and
the throttle effect in the technologies contrived so far in the
case of manufacturing a pipe that is equal to or larger than 8 mm
in wall thickness.
[0013] It is an object of the present invention to provide a pipe
molding die capable of simply preventing an occurrence of
ununiformity in wall thickness of a resin pipe irrespective of a
degree of desired dimension of the wall thickness of the resin
pipe, and also a resin pipe molded by this pipe molding die.
DISCLOSURE OF INVENTION
[0014] A pipe molding die according to the present invention
comprises a throttle part defined as one of constructive parts
thereof. This throttle part includes a core, a shell part fitted to
the core, and a resin reservoir as a portion of a flow path. The
flow path being formed between the core and the shell part. A
molten resin as a pipe raw material flows through the flow
path.
[0015] The resin reservoir is provided in at least one of the core
and the shell part and takes a ring-like shape circumscribing a
central axis of the pipe molding die with the central axis
centered. Further, the resin reservoir assumes a recessed shape in
cross-section.
[0016] The thus constructed pipe molding die according to the
present invention, a flow of the resin extruded from the die can be
uniformized in whichever position on the flow path within the die
owing to the resin reservoir, and it is therefore feasible to
restrain a momentum of the flow of the molten resin. Consequently,
the flow becomes smooth to enhance a rectifying effect.
Accordingly, no ununiformity in wall thickness of the resin pipe to
be molded can be seen.
[0017] Moreover, a capacity of the resin reservoir may be varied
corresponding to a dimension of desired wall thickness of the
molded resin pipe, i.e., the resin reservoir may be so formed as to
decrease the capacity thereof in the case of a thin resin pipe but
increase the capacity thereof in the case of a thick resin pipe. A
quantity of the molten resin in a longitudinal direction (The
longitudinal direction means from an upstream side to a downstream
side of the flow path.) at the throttle part is thereby kept
constant regardless of a degree of dimension of desired wall
thickness of the resin pipe. The keeping constantly the quantity of
the molten resion at the throttle part makes it possible to prevent
an occurrence of the ununiformity in wall thickness of the resin
pipe.
[0018] Thus, a size of the resin reservoir provided in the throttle
part as one of the constructive parts of the die, is simply set
corresponding to the wall thickness of the resin pipe to be molded,
whereby the rectifying effect can be enhanced without increasing
the size of the die itself.
[0019] The flow path described above may include a throttle taking
a constricted shape narrower than other parts along this flow path,
and this throttle may be formed with the resin reservoir described
above.
[0020] Moreover, it is desired that the resin reservoir be formed
in such a configuration as not to cause a stagnation and a
residence (The stagnation and the residence are hereinafter
generically termed a "stagnation".) in the flow of the molten
resin. It is desirable that a recess cross section of the resin
reservoir is formed for example in a curved-surface configuration,
espacially is a semi-circular configuration. In that case, it is
preferably that a radius of curvature be 10 mm-100 mm, and an angle
made by the central axis and each of tangential lines at both ends
of a semi-circular arc of the resin reservoir be
15.degree.-120.degree..
[0021] Further, it is more desirable that the radius of curvature
be 25 mm, and the angle be 75.degree.-90.degree..
[0022] The resin pipe according to the present invention is molded
by using the pipe molding die as well as being molded of
polyolefine as a pipe raw material.
[0023] Polyolefine as the pipe raw material is desirably
polyethylene.
[0024] Furthermore, it is preferable that the resin pipe be
manufactured so that an average wall thickness thereof is set to
one of values in a range of 5 mm-50 mm, and that a difference
between a maximum wall thickness and a minimum wall thickness of
the pipe is equal to or smaller than 1.0 mm and, preferably, equall
to or smaller than 0.3 mm.
BRIEF DESCRIPTION OF DRAWINGS
[0025] FIG. 1 is a vertical sectional view of a pipe molding die
according to the present invention;
[0026] FIG. 2 is an enlarged view of the principal portion in FIG.
1;
[0027] FIG. 3 is an enlarged view of the principal portion in FIG.
2;
[0028] FIG. 4 is an example of variation of that shown in FIG. 2,
in which a portion for forming a resin reservoir is different;
[0029] FIG. 5 is another example of variation of that shown in FIG.
2, in which the portion for forming the resin reservoir is
different;
[0030] FIG. 6 is a view showing a comparative example with a resin
reservoir according to the present invention;
[0031] FIG. 7 is a view showing another comparative example with
the resin reservoir according to the present invention;
[0032] FIG. 8 is a diagram showing a distribution of wall thickness
of a polyethylene pipe molded by a pipe molding die according to
the present invention;
[0033] FIG. 9 is a diagram of comparison with FIG. 8, showing a
distribution of wall thickness of a polyethylene pipe molded by a
prior art pipe molding die; and
[0034] FIG. 10 is a view illustrating a hitherto-existing throttle
portion having no resin reservoir, and corresponding to FIGS. 3, 6
and 7.
BEST MODE OF CARRYING OUT THE INVENTION
[0035] Preferred embodiments of the present invention will
hereinafter be described with reference to the drawings.
[0036] FIG. 1 is a vertical sectional view showing one example of a
pipe molding die.
[0037] In this pipe molding die 1, a molten resin is supplied from
a left end in FIG. 1, and similarly a resin pipe P having a desired
diameter as an extrusion molded product, is discharged from the
right end. More specifically, the left end side of FIG. 1
corresponds to an upstream side in a flowing direction of a molten
resin p, while the right end side of the same corresponds to a
downstream side. Hereinbelow, the upstream and downstream sides in
the flowing direction of the molten resin p are simply termed an
"upstream side" and a "downstream side".
[0038] The die 1 is constructed roughly of three parts. Videlicet,
they are a die part 2, a throttle part 3 and a land part 4. These
parts are arranged in sequence from the upstream side to the
downstream side of the molten resin p. Inside the individual parts
2, 3 and 4, consecutive flow paths which will be mentioned later on
are provided respectively. Then, the molten resin p flows
sequentially through those flow paths.
[0039] The die part 2 is a part located on the upstream side of the
die 1 and is a part for receiving the molten resin p supplied from
an unillustrated resin extruder. The molten resin p supplied to the
die part 2 passes through the die part 2 and arrives at the
throttle part 3.
[0040] Further, the die part 2 consists of a core 2a and a shell
part 2b into which the core 2a is inset.
[0041] The core 2a is located on a central axis L of the die 1 and
is a cylindrical body, an upstream-side end portion of which is
conically pointed. Note that the throttle part 3 and the land part
4 respectively have a core 3a and a core 4a, which are also located
on the central axis L.
[0042] The shell part 2b, which takes a cylindrical shape on the
whole with one end opened and the other end closed, includes a
disk-like proximal seat member 2b.sub.1 located at an upstream-side
end and an external cylindrical portion 2b.sub.2 occupying other
portions of the shell part 2b. The external cylindrical portion
2b.sub.2 is a hollow cylindrical member extending from a peripheral
edge of the proximal seat member 2b.sub.1 toward the downstream
side.
[0043] Further, the proximal seat member 2b.sub.1 has such a flat
recessed portion as being hollowed out in a conical shape at the
center thereof enough to receive the above upstream side end of the
core 2a, and the external cylindrical portion 2b.sub.2 covers the
cylindrical portion of the core 2a.
[0044] A flow path 2c and a flow path 2d of the die part 2 are
respectively formed between the core 2a and the proximal seat
member 2b.sub.1 and between the core 2a and the external
cylindrical portion 2b.sub.2.
[0045] The flow path 2c looks like a generally trigonal pyramid
shape as the whole (The flow path 2c appears a forked shape
gradually expanding as it approaches the downstream side in FIG.
1.). Then, an apex side of the flow path 2c, which is located on
the upstream side, is opened. Note that the aperture of the flow
path 2c serves as a molten resin receiving port for receiving the
molten resin into the pipe molding die 1 from the resin extruder,
and is designated by the numeral 7.
[0046] The flow path 2d is continued with the right side of the
flow path 2c, and takes a cylindrical configuration on the whole
(In FIG. 1 the flow path 2d appears to be two lines of parallel
passageways continuous from the forked flow path 2c.). A diameter
of the flow path 2d is larger than a diameter of the resin pipe P,
and a screw groove 8 is formed in an internal peripheral portion of
the flow path 2d.
[0047] The molten resin p entering the flow path 2d from the flow
path 2c goes on flowing toward the throttle part 3 while changing
its flowing shape into a cylindrical shape from a triangular
conical shape as the flow thereof advances.
[0048] Note that the die part 2 has the screw groove 8 as stated
above, and hence the die 1 is termed a spiral die. The die is
classified, in addition to this, into a spider die, a cross head
die, a basket die and other types of dies. In the great majority of
cases, the spiral die is used, however, the variety of dies are
separately employed according to the applications and
necessities.
[0049] The throttle part 3 is a part located between the die part 2
and the land part 4, and is a part for giving a so-called
rectifying effect. More specifically, as already mentioned, the
throttle part 3 is a part which admits a passage of the molten
resin p flowing in from the flow path 2d in the cylindrical shape
on the whole while being temporarily expanded larger in terms of
its diameter than the resin pipe P, and gradually throttles the
diameter of the cylindrical molten resin p down to the diameter of
the resin pipe P.
[0050] The throttle part 3 described above is constructed of a core
3a and a shell part 3b fitted to an outer portion of the core
3a.
[0051] The core 3a comprises a head-cut conical part 3a.sub.1 and a
cylindrical part 3a.sub.2. The head-cut conical part 3a1 occupies a
half on the upstream side and taking a head-cut conical shape, and
a cylindrical part 3a.sub.2 similarly occupies a half on the
downstream side and taking a cylindrical shape.
[0052] The shell part 3b has a fitting seat member 3b.sub.1 and a
throttle element 3b.sub.2. The fitting seat member 3b.sub.1
corresponds to the head-cut conical part 3a.sub.1 of the core 3a,
and the throttle element 3b.sub.2 corresponds to the cylindrical
part 3a.sub.2 of the core 3a, respectively.
[0053] The fitting seat member 3b.sub.1 assumes a channel-like
configuration in cross-section, and a side wall part 3b.sub.1-1
located on the upstream side thereof is formed with a hole
3b.sub.1-2, into which the head-cut conical part 3a.sub.1 is
inserted, taking the same configuration as this. Further, the
cylindrical part 3a.sub.2 of the core 3a is located on the central
axis L in a space 10 surrounded by a peripheral wall part
3b.sub.1-3 of the fitting seat member 3b.sub.1. Then, the ring-like
throttle element 3b.sub.2 is fitted to an outer periphery of this
cylindrical part 3a.sub.2.
[0054] When the core 3a and the throttle element 3b.sub.2 are
fitted to the fitting seat member 3b.sub.1, a flow path 3c is
formed between the hole 3b.sub.1-2 of the side wall part 3b.sub.1-1
of the fitting seat member 3b.sub.1 and the head-cut conical part
3a.sub.1 of the core 3a, and a flow path 3d is formed between the
throttle element 3b.sub.2 and the cylindrical part 3a.sub.2 of the
core 3a.
[0055] The flow path 3c which is a flow path is continued with the
right side of the flow path 2d of the die part 2 and formed a
head-cut conical shape on the whole, and the head-cut side thereof
is directed toward the downstream side (The flow path 3c appears to
be two lines of passageways narrowing down on the downstream side
in FIG. 1.).
[0056] The flow path 3d is continued with the right side of the
flow path 3c and assumes the cylindrical shape on the whole.
Further, the flow path 3d is smaller in diameter than the flow path
2d similarly taking the cylindrical shape in the die part 2 (The
flow path 3d appears two lines of parallel passageways continuous
from the flow path 3c in FIG. 1.). The flow path 3d is wide both at
the upstream end and at the downstream end thereof, but is
constricted narrowly at the central portion. This constricted part
is referred to as a throttle designated by the numeral 11. The
throttle 11 is formed in such a manner that the central part of an
inner peripheral surface 3b.sub.2-1 of the throttle element
3b.sub.2 is protruded in a trapezoidal shape on the side of the
central axis L.
[0057] The throttle 11 is, as obvious from FIG. 3, provided with a
resin reservoir 13 serving as a portion of the flow path 3d at the
central portion thereof. The resin reservoir 13, which is a
recessed portion provided in the throttle 11 and opened on the side
of the central axis L, takes a ring-1ike shape about the central
axis L. Further, the resin reservoir 13, as obvious from FIGS. 2
and 3, is semi-circular in cross-section and is 25 mm in curvature
radius R. The curvature radius R is not, however, limited to 25 mm
and, though an acceptable range may be 10 mm-100 mm, desirably
falls within a range of preferably 10 mm-50 mm in terms of
enhancing the rectifying effect and preventing so-called black
burning by decreasing a residence time of the molten resin within
the resin reservoir 13.
[0058] Set, also as shown in FIG. 3, within a range of
75-90.degree. is an angle .alpha. made by a tangential line t drawn
at the upstream-side end 13a and at the downstream-side end 13b of
the resin reservoir 13 (Only one tangential line drawn at the
upstream-side end 13a is shown.), in other words, drawn at the two
ends 13a, 13b of a semi-circular arc 13c in cross-section of the
resin reservoir 13, and by the central axis L (in other words, a
wall surface 3a.sub.2' of the cylindrical part 3a.sub.2 parallel to
the central axis L). The angle .alpha. is not, however, confined to
the range of 75-90.degree. and may fall within a range of
15-120.degree.. In short, this range may be the one enough to
enhance the rectifying effect by the resin reservoir 13, not to
cause a stagnation and to prevent the black-burning, and preferably
the one of 75-90.degree. in terms of the effect. The numerical
values given above are calculated based on an endorsement through
the tests implemented by the present inventors.
[0059] Note that the angle .alpha. is shown in the Figures in the
two cases of its being made by the tangential line t and the
central axis L, and by the tangential line t and the wall surface
3a.sub.2' of the cylindrical part 3a.sub.2 parallel to the central
axis L.
[0060] Moreover, the two ends 13a, 13b of the circular arc are so
formed as to be curvilinearly bent enough not to hinder a small
influx of the molten resin. Then, gaps "a", "b" between the two
ends 13a, 13b and the cylindrical part 3a.sub.2 are set to 2 mm.
The gaps are not, however, limited to 2 mm and are, though an
acceptable range may be 0.5 mm-5 mm, desirably set to a range of
preferably 1 mm-3 mm.
[0061] Then, the resin reservoir 13 is not provided in the fitting
seat member 3b.sub.1 but may be, as illustrated in FIG. 4, provided
in the cylindrical part 3a.sub.2 of the core 3a. Further, as shown
in FIG. 5, the resin reservoir 13 may be provided in both of those
parts.
[0062] Moreover, the throttle 11 formed with the resin reservoir 13
may be provided at the cylindrical part 3a.sub.2 of the core 3a and
may be provided both at the throttle element 3b.sub.2 and at the
cylindrical part 3a.sub.2.
[0063] Further, the cross-sectional shape of the resin reservoir 13
may be, in addition to the semi-circular shape, shapes of smoothly
curved surfaces such as circular arcs in other forms, a part of
elliptical shape parabolic shape and so forth causing no stagnation
of the flow of the molten resin p. However, the semicircular shape
in cross section is the best in terms of causing no stagnation of
the flow of the molten resin, and is easy to work.
[0064] The resin reservoir 13 may be formed in other places than
the throttle 11 in the flow path 3d.
[0065] Note that FIGS. 6 and 7 show a comparative examples with the
resin reservoir 13 according to the present invention. If the resin
reservoir 13 takes a rectangular shape in cross-section with
corners rounded as illustrated in FIG. 6 or an isosceles triangular
shape in cross-section with an apex rounded as shown in FIG. 7, it
might happen that the molten resin is stagnated at the corners and
the apex thereof. As the result, the resin is burned black and the
burned substances are adhered to the corners and the apex as well.
Accordingly, it is of importance how the configuration of the resin
reservoir 13 is selected.
[0066] The land part 4 is a part, located on the downstream side of
the die 1, for uniformizing a flow velocity of the molten
resin.
[0067] Such land part 4 is constructed of a core 4a and a shell
part 4b fitted to an outer portion of the core 4a.
[0068] The core 4a has a configuration similar to the core 3a of
the throttle part 3, and is constructed of a head-cut conical part
4a.sub.1 corresponding to the head-cut conical part 3a.sub.1 and a
cylindrical part 4a.sub.2 corresponding to the cylindrical part
3a.sub.2 of the core 3a. The head-cut conical part 4a.sub.1 is,
however, by far smaller in difference between the upstream side and
the downstream side than in the head-cut conical part 3a.sub.1.
Further, the head-cut conical part 4a.sub.1 is hollow.
[0069] The shell part 4b includes a flange member and takes a
cylindrical shape on the whole. The shell part 4b comprises a
flange part 4b.sub.1 having the same major diameter as that of the
throttle element 3b.sub.2 of the core 3a and being contiguous to
the throttle element 3b.sub.2, and an outer cylindrical part
4b.sub.2 extending from a portion, closer to the central axis L, of
the flange part 4b.sub.1 toward the downstream side.
[0070] A flow path 4c and a flow path 4d of the land part 4 are
respectively formed between the head-cut conical part 4a, of the
core 4a and the flange part 4b.sub.1 of the outer shell part 4b,
and between the cylindrical part 4a.sub.2 of the core 4a and the
outer cylindrical part 4b.sub.2 of the outer shell part 4b.
[0071] The flow path 4c is a flow path continuous on the right side
of the flow path 3d of the throttle part 3 and takes an extremely
gently slant head-cut conical shape, and a head-cut side thereof is
directed rightward in FIG. 1 (The flow path 4c appears to be two
lines of passageways in which a spacing therebetween is narrowed
down extremely gently from the flow path 3d of the throttle part 3
as it approaches toward the downstream side in FIG. 1.).
[0072] The flow path 4d is continued with the right side of the
flow path 4c and assumes a cylindrical configuration on the whole.
The flow path 4d is smaller in diameter than the flow path 3d
similarly taking the cylindrical shape (The flow path 4d appears
two lines of parallel passageways continuous from the flow path 4c
in FIG. 1, wherein a width dimension "w" of each of the passages
appearing parallel is so set as to be a wall thickness of the resin
pipe P defined as an extrusion molded product.). A diametrical
dimension "W" of the flow path 4d is, i.e., a diametrical dimension
of the resin pipe P.
[0073] Incidentally, what is indicated by the numeral 15 is an
outlet of the flow path 4d, in other words, a pipe discharge port
of the die 1, from which the resin pipe P defined as the extrusion
molded product is finally discharged.
[0074] According to the thus constructed die 1, when the molten
resin p from the resin extruder is supplied into the die 1 through
a molten resin receiving port 7, this molten resin p is led
inwardly of the die 1 along a route such as flow path
2c.fwdarw.flow path 2d.fwdarw.flow path 3c.fwdarw.flow path
3d.fwdarw.flow path 4c.fwdarw.flow path 4d, and thereafter
discharged, in the form of the resin pipe P as the extrusion molded
product, out of the pipe discharge port 15 of the die 1.
[0075] FIG. 8 shows a distribution of wall thickness of a
polyethylene pipe molded by the pipe molding die according to the
present invention. It can be understood from FIG. 8 that a
difference between a maximum wall thickness and a minimum wall
thickness of the pipe, viz., an ununiformity in wall thickness is
extremely equal to or smaller than 0.3 mm, and therefore a neat
circle is depicted.
[0076] Set conditions in this case are as follows:
[0077] Curvature radius R=25 mm
[0078] .alpha.=75-90.degree.
[0079] a, b=2 mm
[0080] Nominal dimension=200 mm (major diameter: 216 mm.phi., and
average wall thickness: 17 mm).
[0081] FIG. 9 is a diagram compared with FIG. 8, and shows a
distribution of wall thickness of the polyethylene pipe molded by
the pipe molding die in the prior art, wherein the polyethylene
pipe having the same nominal dimension of 200 as the one described
above is manufactured by the die with the throttle 11 including no
resin reservoir 13 as shown in FIG. 10. As can be understood from
FIG. 9, the difference between the maximum wall thickness and the
minimum wall thickness of the pipe, i.e., the ununiformity in wall
thickness is 1.6 mm, and ruggedness on a pipe surface (a pipe
internal surface) can be seen.
[0082] Thus, the die 1 according to the present invention includes
the resin reservoir 13, whereby there could be obtained the
polyethylene pipe with a remarkably reduced ununiformity in wall
thickness, which pipe is substantially a complete round in
cross-section. Note that the pipe is not confined to the
polyethylene pipe using polyethylene as a pipe raw material but may
embrace a polyolefine pipe using polyolefine as a pipe raw
material. The die 1 according to the present invention is, however,
optimal to the molding of the polyethylene pipe.
[0083] As a result of generalizing the tests performed by the
present inventors, the major diameter (diameter) and the average
wall thickness of the resin pipe P formed by the pipe molding die 1
applied thereto are set respectively within a range of 60 mm-500 mm
and a range of 5 mm-50 mm, however, it could be recognized that the
resin pipe becomes preferable by setting the major diameter of the
resin pipe P within a range of 80 mm-220 mm and the average wall
thickness of the pipe P within a range of 8 mm-20 mm.
[0084] Then, in the distribution of wall thickness of the resin
pipe P, the wall thickness ununiformity conceived as the difference
between the maximum wall thickness and the minimum wall thickness
can be set within a range of 0 mm-1.0 mm in the normal setting
described above and a range of 0 mm-0.3 mm in the preferable
setting described above, and therefore it proved that the
preferable resin pipe can be manufactured.
[0085] When the resin pipe P is thus manufactured by use of the die
1 according to the present invention, the resin p extruded from the
die 1 can be flowed uniformly in whichever position on the flow
path 3d within the die 1 owing to the resin reservoir 13, and it is
therefore feasible to restrain a momentum of the flow of the molten
resin p. Consequently, the flow of the molten resin p gets smooth
enough to enhance the rectifying effect. Accordingly, it is
possible to prevent the occurrence of the ununiformity in wall
thickness of the resin pipe P to be molded, and also it is possible
to obtain the resin pipe P with the minor and major diameters that
are both substantially the complete rounds.
[0086] Incidentally, it can be expected that the above effect is
enhanced all the more in combination with the throttle effect.
[0087] Further, the ununiformity in wall thickness can be reduced,
and hence the problems described in the items (1)-(3) in the
description of the prior art can be obviated.
INDUSTRIAL APPLICABILITY
[0088] As discussed above, in the pipe molding die according to the
present invention, the flow of the molten resin is rectified by the
resin reservoir, and the flow of the resin extruded from the die
can be uniformed in whichever position in the peripheral direction
on the flow path within the die. Therefore, the pipe molding die is
applicable as the one capable of preventing the ununiformity in
wall thickness of the resin pipe to be molded. Further, the resin
pipe manufactured by using this pipe molding die has no
ununiformity in wall thickness, and therefore a utility value
thereof becomes higher correspondingly.
* * * * *