Note: Descriptions are shown in the official language in which they were submitted.
PROCESS FOR INJECTION MOLDING AND
HOLLOW PLASTIC ARTICLE PRODUCED l~K~Y
TECHNICAL FIELD
The present appllcatlon ls a dlvlslon of appllcatlon
Serlal No. 605,037 flled July 7, 1989.
Thls lnventlon relates to plastlc ln~ectlon moldlng and
artlcles produced thereby, and more partlcularly, to plastlc ln-
~ectlon moldlng and plastlc artlcles havlng hollow lnterlor por-
tlons produced thereby.
CROSS ~ CE TO RELATED APPLICATIONS
Thls appllcatlon ls related to U.S. patent appllcatlons
entltled ~Apparatus and Method for the In~ectlon Moldlng of
Thermoplastlcs", S.N. 071,363 flled July 9, 1987 (now U.S. Patent
No. 4,781,554 (James W. Hendry) lssued on November 1, 1988);
"Method and Apparatus for the In~ectlon Moldlng of Plastlc
Artlcles", S.N. 098,862 flled September 21, 1987 (now U.S. Patent
No. 4,855,094 (James W. Hendry) lssued on August 8, 1989) and
"Method and System for Locallzed Fluld-Asslsted In~ectlon Moldlng
and Body Formed Thereby", S.N. 133,900, flled December 16, 1987
(now U.S. Patent No. 5,069,859 (Norman Loren) lssued on December
3, 1991), all of whlch have the same Asslgnee as the Asslgnee of
the present lnventlon.
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BACKGROtT~D ART
In the plastic injection molding art, the usual
challenges facing a product designer is to design an article
having requisite strength for the product application and
uniform surface quality for satisfactory appearance, but to
avoid excessive weight, material usage and cycle time. A
design compromise must often be made between strength and
plastic thickness. A relatively thicker plastic section in
the article, such as a structural rib, will incur greater
weight, material usage, cycle time and induce sink marks and
other surface defects
due to thermal gradients in the area of the
thickened section
It is known in the plastic molding art to use
pressurized fluid in conjunction with the plastic
molding of articles The pre~urized fluid is
typically nitrogen gas which is introAl~ceA into the
mold cavity at or near the completion of the plastic
injection The pressurized fluid serve several
pu~ First, it allows the article so formed to
have hollow interior portion~ which ~GLLe_~O~d to
weight and material avings Se~ nA, the pres-
~urized fluid within the mold cavity applie~ outward
pL~ re to force the pla~tic against the mold
surfaces while the article ~olidifies Third, the
cycle time is re~l~ceA as the gas migrates through
the most fluent inner volume of the plastic and
replaces the plastic in these area~ which would
otherwise require an extended cooling cycle
Fourth, the gas yL~ lre r~ the plastic against
the mold surfaces, thereby obta ~ n ~ ~g the maximum
coolant effect from the mold
Howev-r, as the dimensions of the molded
article increasQ, the gas must do more work to
~lgrat- through the volume of the mold cavity to
assist in setting up the article within the cavity
If the prQs-urQ of the ga~ i~ too great as it enters
the mold cavity, there is a risk that it may u~ e
or blow out th- pla~tic within th~ mold cavity,
i e , th- gas is not containeA within the plastic
Thus, there have been practical limitations in the
adaptation of gas in~ectlon in the plastic molding
field
DISCLOSURE OF THE INVENTION
Accordlng to one broad aspect of the present lnventlon,
there ls provlded a process for ln~ectlon molding a hollow plastlc
artlcle lncludlng the steps of ln~ectlng a quantlty of fluent
plastlc lnto a mold cavity of a mold havlng a shape deflnlng at
least a portlon of the artlcle, lntroductlon of a charge of pres-
surlzed gas lnto the mold cavlty upon substantlal completlon of
plastlc ln~ectlon, permlttlng the ln~ected plastlc to solldify by
supportlng the ln~ected plastlc ln the mold, ventlng the gas from
the mold cavity, and removlng the plastlc artlcle from the mold,
whereln the lmprovement comprlses: a portlon of the ln~ected
plastlc ls supported wlthln the mold by a movable support means of
the mold durlng plastlc solldlflcatlon and whereln the step of
ventlng ls accompllshed by movlng the support means to a non-
support posltlon to allow the gas to burst through the thereby
unsupported plastlc portlon.
Wlth reference to preferred embodiments, one aspect of
the present lnventlon ls a method for ln~ectlon moldlng hollow
plastlc artlcles wlth pressurlzed gas whlch provldes for dlsplace-
ment by the gas of a portlon of plastlc from the mold cavlty lnto
a flow coupled splll cavlty. Thls feature enables plastlc artl-
cles of relatlvely greater dlmenslons to be successfully molded
wlth the advantages of establlshed gas ln~ection moldlng tech-
niques.
More speclflcally, the process lnvolves the lnltlal ln-
~ectlon of a quantlty of fluent plastlc lnto a mold cavlty havlng
a shape deflnlng at least a portlon of the plastic artlcle to be
3a
molded. At or near the completlon of the plastic in~ection, a
charge of pressurized gas is introduced into the mold cavity to
displace a portlon of the still fluent plastic. The displaced
plastic flows through a passage from the mold cavity into a con-
nected spill cavity or reservoir. The reservoir may alternatively
serve as: (i) an appendage of the complete article; (ii) a separ-
ate article; or (iii) a cavity to receive spilled plastic for re-
grinding. The plastic which is displaced is generally the hottest
and most fluent. In this regard, the introduction of the charge
of pressurized gas into the mold cavity can be timed to modulate
the amount of plastic displaced, i.e., the longer the delay in
introduction, the cooler and less fluent the plastic in the mold
cavity.
In another feature of the invention, the hollow plastic
article may be formed with an lntegral internal wall by introduc-
tion of two or more charges of pressurized gas. Each gas charge
tends to form a cell within the article, and the
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cells are divided by membranes which serve as
integral internal walls to enhance the structural
properties of the article.
In yet still another feature of the invent-
ing, venting of the gas from the mold cavity is
accomplished by moving a support mechanism for a
portion of the solidified injected plastic to allow
the gas to burst through the unsupported plastic
portion at the reservoir, the runner or an incon-
spicuous part of the article itself.
The present invention admits to molding of
relatively large size structural articles for use in
diverse product fields, such as a box-sectioned
frame member for an automobile or refrigerator door
or the hood of a car having a reenforcing beam.
Other advantages and features of the present
invention will be made apparent in connection with
the following description of the best mode for
carrying out the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a flow chart indicating the basic
steps involved in practice of the process of the
present invention;
FIGURE 2 is a schematic side view of a
plastic injection molding apparatus adapted to carry
out the process of the present invention;
FIGURE 3 is a top plan view of the apparatus
of FIGURE 2;
FIGURE 4 is another schematic view of a
plastic injection molding apparatus illustrating an
alternative arrangement for practicing the process
of the present invention;
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FIGURE 5 is a side schematic view in cross-
section showing still another plastic injection
molding apparatus adapted to mold a hollow plastic
article having an integral internal wall in
accordance with the process of the present inven-
tion;
FIGURE 6 is a plan view of the apparatus
shown schematically in FIGURE 5:
FIGURE 7 is a view similar to FIGURE 5 wherein
the internal wall is displaced from the central location
of FIGURE 5;
FIGURE 8 is a plan view of the apparatus of
FIGURE 7;
FIGURE 9 is a plan view of yet still another
plastic injection molding apparatus;
FIGURE 10 is an enlarged side schematic view
of the apparatus of FIGURE 9, partially broken away and
illustrating one embodiment of a venting step;
FIGURE 11 is a view similar to FIGURE 10
illustrating a second embodiment of the venting step;
FIGURE 12A is a view similar to FIGURE 10
illustrating a third embodiment of the venting step;
FIGURE 12B is a view of the third embodiment
after venting;
FIGURE 13A is a view similar to FIGURE 10
illustrating a fourth embodiment of the venting step;
and
FIGURE 13B is a view of the fourth embodiment
after venting.
BEST MODE FOR CARRYING OUT THE INV~N'1'10N
FIGURE 1 is a flow chart of the steps
involved in practicing the process of the present
invention.
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In step 10, a quantity of molten plastic is
injected from an injection molding machine into a mold
cavity. The plastic is any thermoplastic and works
particularly well with glass or mineral filled ther-
moplastic polyester, commonly known by the trademark
Valox of General Electric Co. The quantity is suffi-
cient to provide the mass of the article to be molded,
but desirably less than the quantity which would fill
the mold cavity.
In step 12, a charge of pressurized gas is
introduced into the mold upon substantial completion of
the injection of the quantity of molten plastic.
In step 14, the gas flow into the mold is
maintained in pressure and duration in amount and time
sufficient to displace a controlled quantity of plastic
from the mold cavity into a spill cavity which is flow
coupled to the mold cavity. The gas tends to displace
the hottest, most fluent plastic in the central portion
of the mold cavity. Consequently, the molded plastic
article has a hollow interior where the least viscous
plastic has been displaced. The p~s-ence of the gas
affords savings in weight and material usage. Added
benefits include enhanced surface quality due to the
outward pressure exerted by the gas, and reduced cycle
time due to displacement of the relatively hot plastic
from the central portion of the article.
In step 16, the article is permitted to
solidify within the mold cavity while the internal gas
pressure is maint~
In step 18, the pressurized gas is vented from
the interior of the molded article preparatory to
opening the mold. Numerous ways of venting are possible
such as described in the U.S. Patent to Friederich
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4,101,617 or as described in co-pending patent application
Serial No. 071,363 noted above (now U.S. Patent No.
4,781,554 (James W. Hendry) issued on November 1, 1988).
In step 20, the plastic article is removed from
the mold.
In step 22, the purged or displaced plastic is
removed from the spill cavity or reservoir. In certain
cases, steps 20 and 22 can be the common operation of
ejecting the moldings so formed from the article cavity and
the spill cavity.
FIGURES 2 and 3 are schematic side and plan
views, respectively, of a plastic injection molding
apparatus, generally indicated at 24, adapted to carry out
the process of the present invention.
A nozzle 26 of a plastic injection molding
machine is brought into registering position with a modified
sprue bushing 28 associated with a mold. The sprue bushing
28 may be of the type disclosed in the above-noted co-
pendin-g application Serial No. 098,862 filed September 21,
1987, (now U.S. Patent No. 4,855,094 (James W. Hendry)
issued on August 8, 1989). The sprue bushing 28 has a
plastic flow path 30 formed at its center to permit the
passage of molten plastic through a sprue 34 into a mold
cavity 36.
The modified sprue bushing also includes a gas
path 32 to permit the introduction and venting of a charge
of pressurized gas.
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The mold cavity 36 is flow coupled through a
runner segment 38 to a spill cavity 40. The volume of spill
cavity 40 may be varied by any well-known means to control
the quantity of displaced plastic such as by a lead screw
42.
A molded article 46 produced by the process
described in reference to FIGURE 1 includes an interior void
44 formed by the presence and influence of the pressurized
gas. The spill cavity 40 may be formed to
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mold an integral appendage of the article 46, or a
separate article, or simply scrap for regrinding.
FIGURE 4 is another schematic view of a
plastic injection molding apparatus, generally indicated
at 50, illustrating an alternative arrangement for
practicing the process of the present invention. In
this case, the apparatus 50 employs first and second
spill cavities 54 and 56 which are flow coupled through
runners 58 and 60, respectively, to a mold volume 52.
Again, a nozzle 26 from an injection molding machine
registers with the sprue bushing 28 to inject a quantity
of molten plastic into the mold cavity. A charge of
pressurized gas flows along the gas -path 32 in the
modified sprue bushing 28 and into the cavity 52 to
displace the least viscous plastic from the mold cavity
52 into the first and second spill cavities 54 and 56.
This process, when performed in accordance with the
steps of FIGURE 1, will yield a molded article 64 having
a central void 62 due to the displacement of plastic by
the pressurized gas.
FIGURES 5 and 6 are side and plan schematic
views, respectively, of still another plastic injection
molding apparatus, generally indicated at 70, adapted to
mold a hollow plastic article 78 having an integral
internal wall 80. In this case, the injection molding
machine nozzle 26 aligns with a sprue 72 which divides
into a pair of runners 74 and 76. Each of the runners
74 and 76 connects to a bushing 28', which is modified
from the sprue bushing 28 of FIGURE 1 only to the extent
required to remove it to the ends of the runners 74 and
76. In this example, the pair of bushings 28' are
situated at opposite lateral extremes of the mold
cavity 52 to produce a molded article 78 with an
integral internal wall 80 at the center. The position-
ing of the bushings 28', as defining the gas entrypoints, will determine the resulting position of the
integral internal wall 80.
In the apparatus 70 of FIGURES 5 and 6, the
gas charges introduced through the paths 32 in the
bushings 28' are simultaneous. Each gas charge tends to
form a cell, as shown by voids 82 and 84, within the
article 78. The cells are divided by a membrane which
serves an integral internal wall 80.
In other respects the apparatus 70 of FIGURES
5 and 6 is essentially similar to the apparatus 50 of
FIGURE 4. Specifically, the apparatus 70 likewise
employs first and second spill cavities 54 and 56 flow
coupled to the mold cavity 52 through runners 58 and 60,
respectively.
In the apparatus 70 of FIGURES 7 and 8, the
gas charges introduced through the paths 32 in the
bll-c~inqs 28' are sequential so that the membrane which
serves as an integral wall 80' is displaced to one side.
For example, the interval between the gas charges may be
between .25 and 1.0 seconds apart.
FIGURES 9 and 10 are plan and side schematic
views, respectively, of another plastic injection
molding apparatus, generally indicated at 70', adapted
to mold a hollow plastic article. The apparatus 70'
employs spill cavities 54' and S6' coupled to the mold
cavity through runners 58' and 60', respectively.
The plastic in at least one of the spill
cavities 54' and 56' is supported during plastic
solidification by a movable mold part such as a pin 86
supported within the mold of the apparatus 70'. The gas
is vented by moving the pin away from the supported
plastic prior to opening the mold to the atmosphere so
that the pressurized gas bursts through the now
--10--
unsupported plastic within the spill cavity. The gas
then travels around the pin 86, through the mold and to
the atmosphere in a controlled fashion. The pin 86 may
be moved relative to the mold in any well-known fashion
and is supported in a bore 87 in the mold leaving
approximately 0.005 inches clearance around the pin 86
to permit the gas to travel around the pin 86.
FIGURES 11, 12A and 13B show alternate embodi-
ments of a pin, generally indicated at 86 ', 86~ and
86'~, respectively, for venting the gas from the
article. FIGURES 12B and 13B show the pins 86~ and
86'~ in their venting positions, respectively. Each of
the pin~ 86 ', 86~ and 86 ' ~ include an angled end portion
88 ', 88~ and 88 ' ~ , respectively, for receiving and
retaining a portion of the solidified injected plastic
therein at a plastic reservoir, a runner segment, a
sprue portion or an inconspicuous part of the article
itself. Movement of the pins 86 ', 86~ and 86 ' ~ away
from their Le_~ective supported portions of injected
plastic causes their respective end portions 88 ', 88~
and 88~ ' to carry a portion of plastic therewith to
facilitate the venting step, as illustrated in FIGURES
12B and 13B.
In the embodiment of FIGURE 11, the pin 86'
includes a central ejector 90 which can be operated in
any well-known fashion to subseguently eject the
solidified plastic from the end portion 88 ' after
venting and prior to the next cycle.
The invention has been described in illustra-
tive embodiments, but it will be evident to those
skilled in the art that variations may be made from the
foregoing teachings without departing from the scope of
the following claims.
