Note: Descriptions are shown in the official language in which they were submitted.
-` ~23~666 28808CAl
~ETHOD AND APPARATUS FOR MOLDING HOL~OW ARTICIES
The present invention relates generally to the molding of
hollow articles. In one aspect the invention relates to a method of
rotationally molding cross-linkable polymeric material into a hollow
article. In another aspect the invention relates to apparatus for
rotationally molding cross-linkable polymeric material into a hollow
article. In still another aspect the invention relates to the product
produced by a method of rotationally molding cross linkable polymeric
material into a hollow article.
In recent years tubular articles such as pipes and pipe
fittings have increasingly been manufactured from synthetic polymeric
materials. Such materials include polyolefin materials such as high
density polyethylene. Pipe can be economically manufactured from such
materials by means of extrusion. Pipe Eittings, such as, for example,
tees, wyes, elbows, valve bodies and the like, have been produced by
injection-molding in order to produce the relatively complex shapes of
such fittings. It is well known, however, that injection-molding is an
expensive process due to the tooling costs involved in designing and
machining the dies required to form such injection-molded parts.
~conomics generally dictates that a large number of injection-molded
parts must be produced by a particular die in order to satisfactorily
amortize the tooling costs incurred in its design and construc-tion. The
size of a part to be injection-molded is also o~ primary importance since
tooling costs ~or injection-molding dies increase greatly as the size of
the article to be molded increases. In the specific case of molded
plastic pipe fittings, fittings having a nominal diameter of up to about
~ inch can be economically produced by injection-molding. ~lowever, those
~346~6 28808C~l
fittings having nominal diameters from about ~ inch up to about 48 inch
and larger, and having nominal wall thicknesses oE up to about 1 1/2 inch
and larger, are generally produced by other, more economical means.
One technique which can be used to produce large hollow or
tubular articles from synthetic polymeric material is known as rotational
molding. In this technique, a mold having a cavity defining the outer
surface of the article to be formed therein is mounted for ro-tation
and/or revolution about one or more axes of rota-tion. Cross-linkable or
thermosetting polymeric material is in-troduced into the mold and it is
simultaneously heated and rotated and/or revolved about -the one or more
axes of rotation to fuse the polymeric material and distribute it more or
less equally over the inside surface of the mold cavity to form the wall
of the hollow article.
In the rotational molding of hollow articles from
cross-linkable polymeric materials, it has been found that the interior
surfaces of such articles so molded often do not exhibit the smoothness
desired for the required utility of the article. ~xamples of articles
which require maximum smoothness of the inner surfaces thereof are molded
pipe fittings such as tees, wyes, elbows, valve bodies and the like. It
is believed that the lack of desired inner surface smoothness in such
articles, especially those articles having relatively thick walls, is
attributable to the difficulty in achieving -the desired uniformity of
cross-linking or curing from the outer surface to the inner surface of
the article from an externally heated rotational mold.
Accordingly, it is an object of the present invention to
provide method and apparatus for rotationally molding large-si~e hollow
articles having relatively thick walls.
Another object of the present invention is to provide improved
smoothness of the interior surfaces of rotationally molded tubular
articles.
Yet another object of the present invention is to enhance the
curing of cross-linkable polymeric material in the rotational molding of
tubular articles.
Still another object of the present invention is to produce
pipe fit-tings of rela-tively large diameter and relatively thick walls.
~23~66 28808CAl
Another object of the present invention is to provide method
and apparatus for rotationally molding relatively large, generally
tubular articles having relatively thick walls, which method and
apparatus are simple, reliable, efficient and economical in operation.
A further object of the present invention is to provide a
molded tubular article of moldable plastic material which is
characterized by smooth interior wall surfaces and relatively low cost.
The present invention contemplates a method of molding an
article from melt-fusible and cross-linkable plastic material, which
article is characterized by at least one general].y tubular sidewall. The
method includes providing a mold having a cavity generally corresponding
to the shape of the article and defining at least one generally tubular
sidewall. The mold is further characterized by at least one removable
plug closing the mold at at least one extremity of at least one of the
thus defined generally tubular sidewalls. The me-thod further include
depositing a quantity of the plastic material within the mold, and
placing the at least one removable plug in the mold to close the mold at
the respective extremity of the thus defined tubular sidewall. The mold
is then revolved about at least two nonparallel rotational axes. The
mold and the material contained therein are heated to a tempera-ture
suitable for fusing the material and for a time sufficient to partially
cross-link the material in the desired form of the hollow article. The
at least one removable plug is thereafter removed from the mold to expose
the interior of the mold to the heating, and the heating of the mold and
the material contained therein is continued after removing the at least
one removable plug for an additional time sufficient to achieve the
desired degree of cross-linking of the plastic material in the desired
form of the article. The thus cross-linked hollow article is allowed to
cool and the thus cooled hollow article is recovered from the mold. The
present invention further contemplates articles manufactured in
accordance with the method.
The present invention additionally contemplates apparatus for
producing a hollow article which includes a mold assembly having a cavity
therein generally corresponding to the exterior shape of the article to
be produced and having at least one portion defining a generally tubular
sidewall of the article. The mold assembly is characterized by an
~,~3~66 28808CA1
aperture therein communicating with the interior thereof at the extremity
of the portion defining the generally tubular sidewall of the article.
The apparatus further includes plug means for closing the aperture in the
mold assembly, and, alternately, for opening the aperture, as well as
means for securing the plug means in pos:ition closing the aperture in the
mold assembly. The apparatus is additionally provided with means secured
to the mold assembly for revolving the mold assembly about at least two
nonparallel axes of rotation. The apparatus also includes means for
heating the mold assembly and the contents of the cavity therein.
Other aspects, objects, advantages and features of the present
invention will become apparent to those skilled in the art to which the
present invention pertains upon a reading of the remainder of this
specification and the claims appended thereto with reference to the
accompanying drawings in which:
FIG. 1 is an isometric view of a mold carried by a
mold-supporting device and apparatus for simultaneously revolving the
mold about two nonparallel axes;
FIG. 2 is a cross sectional view of the mold of the present
invention showing the molded article in place therein and showing the
removable insulated plugs positioned within extremities of the tubular
sidewalls of a molded article, and
FIG. 3 is a top plan partially in cross-section view
illustrating the apparatus of FIG. 1 with a mold and mold-supporting
device disposed within an oven, with the top portion of the oven broken
away to more clearly illustrate the construction of the apparatus.
Referring now to the drawings, the apparat~ls of the present
invention is generally designated by the reference character 10. The
apparatus 10 includes a mold support and drive assembly 12, a mold
assembly 14 and a mold heating assembly 16.
The mold support and drive assembly 12 includes a support
pedestal 18, a drive unit 20 mounted on the upper end of the support
pedestal 18, and an output shaft subassembly 22 drivingly secured to and
supported by the drive unit 20. The outer end portion of the output
shaft subassembly 22 is drivingly secured to and provides support for a
power transmission unit 24. A power output shaft 26 is drivingly secured
i~3466~ 28808CA1
to the power transmission unit 24 and is supported by the power
transmission unit.
The mold assembly 14 includes a lower mold section 28 and an
upper mold section 30. The lower mold section 28 is secured to and
supported by a lower mold support frame 32 which is fixedly secured to
the outer end of the power output shaft 26 by means of a mounting plate
34 which is fixedly secured to the lower mold support frame 32. The
upper mold section 30 is secured to and supported by an upper mold
support frame 36. The upper and lower mold sections 30 and 28 are
secured together to define the cavity therein generally corresponding to
the ex-terior shape of the article to be produced by means of a plurality
of hook-type, toggle action pull clamps 38 located at space locations
about the exterior of the thus joined mold sections 30 and 28. With the
upper and lower mold sections 30 and 28 thus secured together, the mold
assembly 14 further includes a mold end plate 40 positioned about each
open extremity of the cavity defined by the joined upper and lower mold
sections. Each of the mold end plates 40 is provided with an aperture 42
therein. Mold assembly 14 additionally includes removable mold plugs 44
and 46, each disposed in a respective aperture 42. The mold plug 44
differs from the mold plug 46 in the inclusion of a vent hole 48 -therein
to vent the interior of the mold cavity during the molding process.
Referring now to ~IG. 2, the mold assembly 14 is partially
shown in cross-section along the parting line bet.ween the lower mold
section 2B and the upper mold section 30, the upper mold section being
removed to more clearly illustrate the structure of the mold assembly.
It will be seen that the mold assembly 14 forms a cavity which defines a
hollow article 50 in the form of a pipe tee. Such an article is
characterized by a pair of tubular sidewalls 52 and 54, the axes of which
are nonparallel and are actually perpendicular to one another. The mold
end plates 40 define flanges at the extremi-ties of the side-walls 52 and
54. The removable mold plugs 44 and 46 are sized and shaped to be
closely received within the apertures 42 and to direct the material to be
molded into the adjacent flange portions of the cavity during the molding
process. The upper and lower mold sections 30 and 28 are preferably made
of metal having relatively high thermal conductivity and having a minimum
wall thickness commensurate with the strength required to support the
1~3~66~ 28808CA1
material to be molded within the cavity. Similarly the mold end plates
40 are also preferably formed by the sarne type of metallic material. The
removable mold plugs 44 and 46 are preferably formed of relatively thick,
thermal insu]ating material which does not readily conduct heat from the
exterior of the mold to the material to be molded within the cavity.
Suitable materials for the construction of the mold sections 20 and 30
include steel and aluminum with aluminum being presently preferred.
Suitable material for the construction of the mold plugs 44 and 46
include various synthetic resinous materials which maintain their
physical intregity when subjected to the molding temperatures involved in
the practice of the instant process. A presently preferred material for
construction of the mold plugs is poly(phenylene sulfide) which is sold
under the registered trademark RYTON by Phillips Petroleum Company.
The mold assembly 14 further includes a plurality of mold end
support frames 56 each disposed adjacent a respective mold end plate 40.
The mold end support frames 56 are each supported on the upper and lower
mold support frames 36 and 32 by means of a pair of hook-type, toggle
action pull clamps 58 mounted on respective mold support frames and
engagable with a corresponding lug 60 mounted on the corresponding mold
end support frame 56. A plurality of spring-loaded bolts 62 are
threadedly secured through each mold end support frame 56 and yieldably
engage the corresponding adjacent mold end plate 40 to secure the mold
end plate 40 to the open extremity of the joined upper and lower mold
sections 30 and 28. Each mold end support frame 56 further includes a
mold plug retainer plate 64 which is secured across the central portion
of each mold end support frame 56 by means of a pair of hand-operated
-threaded retainer bolts 66. The mold plugs 44 and 46 are removably
retained in position within the apertures 42 of corresponding end plates
40 by means of a pair of spring loaded threaded bolts 68 which are
threadedly secured through the respective mold plug retainer plate 64 to
yieldably urge the mold plugs 44 and 46 into the respective apertures 42.
The hand-operated threaded retainer bolts 66 facilitate the quick and
convenient removal of the mold plug retainer plates 64 and the
corresponding mold plugs 44 and 46 from the mold assembly 14 during the
operation of the apparatus 10.
~23~66 288n8cAl
The mold heating assembly 16 includes an oven housing 70 which
fully encloses the mold assembly 14, the power transmission unit 24, the
power output shaft 26, and the outer end portion of the output shaft
subassembly 22. The oven housing 70 is provided with a makeup air inlet
72 and an exhaust outlet 74 which is connected via conduit 76 to a
suitable exhaust blower 78. The mold heating assembly 16 additionally
includes a circulation blower 80 disposed wi-thin the oven housing 70 for
vigorously circulating the air within the mold heating assembly 16 to
circulate hot air within the oven housing 70 and thereby increasing the
efficiency of the molding process of the present invention~
In the operation of the apparatus 10, the mold assembly 14 is
assembled as described above and is charged with a predetermined amount
of particulate thermosetting or cross-linkable polymeric material
sufficient to provide a molded article of the desired wall thickness.
15 One of -the mold plugs 44 or 46 can be removed from the mold assembly 14
to facilitate the charging of the particulate molding material and then
can be conveniently replaced after the mold assembly is fully charged.
The oven housing 70 is then closed about the thus charged mold assembly
14 and oven heating is initiated along with the operation of the blowers
20 78 and 80. The mold support and drive assembly 12 is also actuated to
begin revolving the mold assembly 14 about the axis of rotation of the
output shaft subassembly 22 in the direction of the arrow 82, as shown in
FIG. 1. Through the action of the power transmission unit 24, the thus
revolving mold assembly 14 is rotated about the axis of rotation of the
power output shaft 26 in the direction of the arrow 84. It will be seen
that this action by the mold support drive assembly 12 causes the mold
assembly 14 to be simultaneously revolved about the two nonparallel axes.
In a preferred embodiment, illustrated in the drawings, the axes of
rotation about which the mold assembly 14 is revolved are parallel to the
axes of the corresponding tubular sidewalls 52 and 54 of the hollow
article 50 to be molded. The mold heating assembly 16 can be operated at
any temperature which will provide the desired melt fusion and
cross-linking of the polymeric material disposed within the mold assembly
14. In general, -~he operating temperature of the mold heating assembly
35 is in the range from about 350F to about 500F.
1~3~666 28~08C~l
The mold assembly 14 iS preferably simultaneously revolved
about the two nonparallel axes in the circulating hot air within the mold
heating assembly 16 at -the desired temperature until such time as -the
polymeric material within the mold assembly 14 has fused and formed the
molded article while achieving a degree of cross-linking which varies
Erom a higher degree in the material near the mold surface to a lesser
degree in the material toward the interior of the sidewall. This
condition is typically achieved when the polymeric material has gained
such a degree of physical stability that the motion of the mold assembly
14 can be stopped for a short period of time to remove the mold plugs 44
and 46 from their respective apertures 42 in the mold end plates 40. The
heating prior to the removal of the mold plugs 44 and 46 can be performed
for any period of time adequate to achieve the desired partial
cross-linking. However, the period of time generally falls in the ran8e
from about 2 hours to about 5 hours. Upon removal of the mold plugs 44
and 46, the revolution of the mold assembly 14 within the heated mold
heating assembly 16 is continued with hot air being circulated both
around and through the open mold assembly 14 until the desired complete
cross-linking of the polymeric material is achieved at which time heating
by the mold heating assembly 16 is stopped and the molded article within
the mold assembly 14 is allowed to cool for a time sufficient to allow
the mold to be disassembled and the article to be removed therefrom in
its completed form. The cooling step will continue for a period of time
sufficient to achieve the desired integrity of the molded article,
however, cooling of the molded article within the mold assembly 14 is
generally performed for a time in the range from about 17 minutes to
about 25 minutes depending on the size of the molded article and other
parameters. Cooling can be achieved by circulating ambient or cooled air
through the mold assembly 14 while maintaining the simultaneous rotation
of the mold assembly 14 about the nonparallel axes of rotation. Cooling
can also be achieved by circulating water-laden air or a fog through the
revolving mold assembly 14. It is also within the scope of this
invention to perform cooling by the application of water to the molded
article to facilitate the cooling.
In a variation of the process of the present invention, the
mold assembly 14 can be initially charged with a fractional portion of
~23~666 28808CA1
polymeric material necessary to mold a comp].eted hollow article. ~pon
sufficient fusion and partial cross-linking of the initial charge of
polymeric material, an additional charge of polymeric material can be
added to the interior of the mold assembly 14 and the heating to fusion
and cross-linking can be continued. The addition of fractional portions
of the polymeric material followed by additional fusion and cross-linking
can be continued until the full desired amount of polymeric material has
been charged to the mold assembly. Upon the desired partial
cross-linking of the last charge of polymeric material, the mold plugs 44
and 46 can be removed as described above followed by the previously
described additional heating or curing of the polymeric material and
cooling of the thus molded hollow article. Upon the desired cooling of
the molded hollow article, the completed article is then recovered from
the mold assembly 14 by releasing the pull clamps 38 and 58 and parting
the mold sections 30 and 32.
While it is not desired to limit the scope of the present
invention to the employment of any particular molding material, the
method and apparatus of the present invention are advantageously employed
in the molding of cross-linking or thermosetting polymeric materials.
Such materials are generally heat fusible, and are preferably charged to
the mold assembly 14 in a powdered condition to facilitate heat transfer
to the polymeric materials and to eliminate or sharply reduce the
possibility of the incidence of bubbles in the molded article. The
presently preferred heat fusible, cross-linkable polymeric materials for
use in the method and apparatus of the present invention include
cross-linkable polyolefins, and mixtures or copolymers thereof. The
presently preferred cross-linkable polyolefin for use in the present
invention is cross-linkable polyethylene. For example, Marlex CL-lOO, a
cross-linkable polyethylene sold by Phillips Chemical Company of
Bartlesville, Oklahoma, is a suitable material.
Example I
A series of fifteen runs was made in which five pipe tees, each
having a nominal diameter of 18 inches, and 10 pipe tees, each having a
nominal diameter of 8 inches, were produced using Marlex CL-100 in mold
apparatus as described above and illustrated in the drawings. The 1~
inch pipe tees produced in runs 1-4 and the 8 inch pipe tees produced in
~34666 28808CA1
runs 6 and 7 did not employ the removable mold plugs of the present
invention and all heating was applied to the exterior of the mold. The
18 inch pipe tee of run 5 and the 8 inch pipe tees of runs 18-15 were
produced utilizing the removable mold plugs o-f the present invention with
circulating heated air being applied to the tees both before and after
removal of the mold plugs. Runs 3 and 4 were each characterized by two
separate changes of cross-linkable polyethylene to build up $he desired
wall thickness of the pipe tees. Oven temperatures ranged from 350F to
500F during molding of the tees. After completion of each heating cycle
the mold containng the cross-linked polymer tee was removed from the
heating oven and rotated about its nonparallel axes in a cooling chamber
where it was subjected -to air cooling for from about 10 to about 15
minutes, fog cooling with water vapor laden air for from 0 to about 15
minutes, and liquid water cooling from 0 to about 10 minutes. Each thus
cooled pipe tee was then removed from the mold apparatus and subjected to
a destruc-tive burst test and the observed burst pressure recorded. The
results of these 15 runs are shown in Table I.
1 1
0 C~ ~ ~ ~I O
1 ~ c~ o~ û I c~ u û ru
a 3u 'u u 'u
~ ~ o o o o U~ U~ U) U~ U 'u
O ` ¦ O G O O U~ O O 1~ Lo
e . R 'a a
~ ~0 ~ ~ ~ ~
d ~1 .d d d d d d a a a 4
`I - ~ ~ In ~ o~ ~ ~ u7 ~ ~ ~1 ~1 ~
o~l
d o o o o u~o u~o o o o o o o o I o o o o o o o
3.1 O o o ") o c~ o o o o o o o o o o o o o o o o
~ ol~ In~ ~ n ~ Lr)~ ~ ~ ~ ~t~ ~ ~
~ ~ ~ r ,r~
O O U~ O U~ U~ O ~ ~ ~ ~ ~
~I 00~ 00~ <~ C~ C~ ~1 ~`1 ~`I
~,~ Z ~0 ~Z; 'Z ~1 ZO Z ~ ~ t~
_1~ 00 oO 0~ 0~ 0~ ~0 0~ 00 00 0~
c~ ~ ~ ~ o 1~ o~ a~ ~l
n o o o r) o u~1
~23~666
12
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¢ ~ o o o o o
~0 ~ ~ ~D ~ ~ ~ O
oo
d ~1 U _~ 1, u
e I ~ u u u u
d 1~ u) n u~ u~
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~ ¢1 ~~ ,~ ~ o, o~
a d d a d
Pl 3 ~ ~n ~.9
d ~ L~ 3 u~ o
3 ~ ~`I ~I ~I ~1 ~<~
d ~ O O O O O o O O ~:
U ~ C~ O O O O O O O O t~
~_ ~ ~ ~ ~ ~ ~ ~ 'o ~0 ~
H E-l ~ _
~ ~ 24)S(~
~$ ~ ~ ~ ~ ~ O
~3' c~l c~ ~ c~ c~ u3
~ ~ Q) ~ Q~ 3) ~U O O ~
~1 ~ ~ ~ ~ ~
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~ ~o~o~
d I c~J ~ ~:t ,1 ~a ~o u
o u~ o ~n
:1~34666 28808CAI
13
From the data in Table I it will be seen that the use of the
removable mold plugs reduced the hea-ting cycle time of inventive runs
8-15 for production of 8 inch pipe -tees :in comparison -to control run 7,
and reduced the heating cycle time of inventive runs 8-10 and 13-lS in
comparison with control run 6. The heatin~ cycle time of each inventive
runs 11 and 12 was equal to the heating cycle time of control run 6.
Inventive runs 8, 10, 11 and 15 all exhibited burst pressures exceeding
either of the burst pressures exhibited by control runs 6 and 7.
Inventive run 5 for the production of an 18 inch pipe tee produced a
percentage increase in burst pressure over control runs 1-4 in the range
from about 23.3% to about 42.3%. All pipe tees produced in inventive
runs 5 and 8-15 exhibited smooth interior surfaces and little or no out
of roundness.
It will be understood that the method and apparatus of the
present invention are broadly applicable to the molding of any hollow
article in which the finished article has at least one aperture therein
through which heated air can be circulated to facilitate the curing or
cross-linking of the interior surface of the tubular article during the
molding thereof. The method and apparatus of the present invention
provide the advantages of more uniform curing or cross-linking of the
sidewalls of the molded hollow article, smoother interior surfaces of the
molded hollow articles and reduced heating time to achieve the necessary
cross-linking of the polymeric material -through the full thickness of the
sidewalls of the hollow article.
Changes may be made in the combination and arrangement of
parts, elements or method steps as heretofore set for~h in the
specification and shown in the drawings without departing from the spirit
and scope of the invention as defined and limited only in the following
claims.
