Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF I'HE INVENTION
This invention relates to injeckion molding and more
particularly to an improved electrically heated nozzle bushing
which imparts a curving motion -to the melt en-tering the cavity.
It is well known that molecular orientation effec-ts
the strength of injec-tion molded plastic products. Orientation
is generally thought to be caused by flow of polymer in the
direction of least resis-tance where stresses induced are
generally parallel to the flow direction. This resulting uni-
directional orientation causes the molded product to be strongerto resist bending forces along the direction of orientation and
weaker to resist bending forces across the direction of orien-
tation. For instance, a center-gated coffee cup would be
stronger in the vertical direction, but very weak in the hoop
direction. On the other hand, it is well known to form plastic
film with a biaxial molecular orientation to improve its strength
characteristics.
More recently, rotation of the mold has been used in
injection molding to form the product with a biaxial or multi-
axial molecular orientation. While this has been successfulin achieving increased product strenyth, it is not a practical
solution to the problem because of diFficul-ties in constructing
and operating the spinning molds. It is not suitable for
multicavity application, and is very difficult for irregular
shaped products.
Even more recently, attempts have been made to achieve
biaxial or multiaxial molecular orientation by designing the mold
cavity to provide the inflowing melt with an irregular flow
pattern. This has usually been in the form of ribs or other
shapes against which the incoming melt impinges to spread or
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1 disperse it in different directions. While having better resulks
in terms of increased product strength, this method or concept,
of course, has the disadvantage that the product must include
the ribs or other shapes introduced into the design to provide
this ~illing action. Needless to say, this is not satisfactory
for a large range or products.
~ he applicant's copending Canadian application serial
number 35~,3~9 filed May 21, 1980 discloses an injection molding
system with an improved nozzle tip portion to overcome these
problems, but it is for valve-gating. Sprue gated systems have
traditionally had the problem or concern o~ reducing or elimi-
nating str.inging of the melt from the gate when the mold opens
to eject the product from the cavity~
SU~MARY OF THE INVENTION
.
Accordingly, it is an object of the present invention
to at least partially overcome these disadvantages by provlding
an injection molding nozzle system wherein a swirling motion is
imparted to the melt entering the cavity to reduce unidirectional
molecular orientation of the molded product.
To this end, in one o~ its aspects, the invention
provides an injection molding nozzle bushing comprising: a
hollow elongated body having a runner passage extending longi-
tudinally therethrough fxom an inlet to an outlet at opposite
ends thereof; elongated heating means adapted to heat the
elongated body; and a spiral blade member fixed in position in
the runner passage to extend to the outlet thereof.
In another of its aspects, the invention provides an
injection molding nozzle bushing comrpising: a hollow elongated
inner core portion having an inn~r surface defining a generally
3~ cylindrical central runner passage extending longitudinally
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1 therethrough from an inlet to an outlet at opposite ends thereof,
the core portion being formed of a high strenyth, corrosion
resistant thermally conductive material; an elongated electric
heating element extending around the inner core portion; an
elongated conductive portion cast on the i.nner core portion and
the heating element to be bonded to them a:Long at least a portion
of their lengths; and a spiral blade member fixed in position in
the central runner passage to extend to the outlet thereof.
In yet another of its aspects, the invention further
1~ provides an injection molding nozzle bushing comprising: a
hollow elongated inner core portion having an inner surface
defining a central runner passage exten~ing longitudina:Lly
therethrough from an inlet to an outlet at opposite ends thereof,
the centxal runner passage being generally cylindrical with a
tapered portion which gradually decreases in diameter towards
the outlet, the inner core portion being formed of a high
strength, corrosion resistant, thermally conductive metal; an
electrically insulated helical heating element having a plurality
of spaced coils encircling the inner core portion and terminal
means adapted to receive electric power from an external source;
an elongated conductive portion cast on the inner core portion
and the heating element to be bonded to them along their lengths,
the conductive portion being formed of a metal having a high
thermal conductivity; a spiral blade member extending in the
central runner passage from the inlet to the outlet, the sprial
blade member being attached along its edges to the inner surface
of ~he inner core portion, the spiral blade member being formed
of corrosion resistant material and gradually decreasing in
pitch and thickness adjacent the outlet; an elongated outer
sleeve portion formed of a corro~ion resistant metal extending
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1 around the conductive portion; and retaining means adapted to
retain the nozzle bushing against rotation.
Further objects and advantages of the invention will
appear from the ~ollowing description, taken together with the
accompanying drawings~
BRIEF DESCRIPTION OF THE DR~WINGS
.
Figure 1 is a partial cut-away perspective view of an
injection molding system having a nozzle bushing according to a
preferred embodiment of the invention; and
Figure 2 is a sectional view of the same embodiment.
DESCRIPTION OF THE PREFERRED E~BODIMENT
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Referring to the drawings, the injection molding
system has a nozzle bushing 10 which is seated in a cavity plate
12 and a back plate 14. The nozzle bushing 10 provides a hot
runner passage 16 through which hot pressurized melt flows from
a molding machine (not shown) to a cavity 18.
The nozzle bushing 10 has an inner core portion 20
with an inner surface 22 which forms the hot runner passage 16
extending from an inlet 24 to an outlet 26 which is the gate to
the cavity 18. As may be seen, the passage 16 is generally
cylindrical except for a beveled portion 28 at khe inlet 24 to
receive the molding machine and a tapered portion 30 at the
outlet 26. Surrounding the inner core portion 20 is an electric
helical heating element 32 which is cast in a conductive portion
34. The heating element 32 is insulated from the surrounding
material and in this embodiment is shown as of double core
construction extending to an external lead 36 for connection
to a source.of electric power (not shown). The coils of the
heating element 32 are separated from each other to provide for
maximum bonding of the conductive material forming conductive
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1 portion 34 to the surface of the coils as well as to the outer
surface 38 of the inner core portion 20. An outer slee~e portion
40 around ~he conductive portion 34 provides a corrosion
resistant outer finish.
The nozzle bushing lO also includes a spiral blade
member 42 which extends through the hot runner passage 16 from
the inlet 24 to the outlet or gate 26~ The spiral blade member
42 is brazed along its outer edges 44 to the inner surface 22
of the inner core portion 20 to securely fix it in position. The
1~ blade member is formed of sufficient thickness to ensure it
does not rupture during use, but its thickness is gradually
reduced adjacent the outlet 26 to minimize the restriction when
the melt passes through the tapered portion 30 and the gate 26.
Furthermore, the blade member spiral gradually decreases in
pitch in this same area to increase the swirling motion imparted
to the melt as it enters the cavity.
The nozzle bushing 10 is securely mounted in the
cavity plate 12 and back plate l~ by insulation bushing portion
46. The cavity plate 12 which is cooled by cooling element 48
~ is separated from the heated nozzle bushing 10 by air gaps 50
to reduce heat losses. The outer end 52 of a pin 54 seated in
a hole 56 in the nozzle bushing lO is received in a slot 58 in
the back plate 14 in order to prevent rotation of the nozzle
bushing by the force of the melt on the spiral blade member 42.
In use, the nozzle bushing lO is located in the mold
between the molding machine and the cavity 18. Power is applied
to the heating element 32 through lead 36 and operation commences
after the nozzle bushing is heated up. Pressurized melt rom
the molding machine is injected through the hot runne:r passage
16. Melt pressure is applied from the molding machine in
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1 impulses. After the application of each pressure impulse to
fill the cavity, the melt solidifies in the area of the gate 26
and the mold is opened to eject the molded product and then
closed again before the next pressure impulseO Temperature
control is very critical to dependable operation, particularly
in the gate area. SuEficient heat must be provided by the
heating element 32 to maintain smooth, even melt flow, without
preventing the cooling element 48 from cooling the cavity and
gate sufficiently to provide for rapid solidification and
ejection. When a pressure impulse is applied, the melt flows
rapidly through the not runner passage 16 and the spiral shape
of the blade member 42 imparts a swirling motion to the melt as
it passes through the gate 26 into the cavity 18. This swirling
motion of the melt through the gate is accelerated by the
gradually decreasing pitch of the spiral blade member so that it
lS carried as far as possible into the cavity 18 to provide the
whole product with the increased strength resulting from
unidirectional moledular orientation of the melt being avoided.
In addition, this curving motion of the melt as it leaves the
gate has the effect of reducing stringing of the melt when the
mold opens to eject the product, presumably because the molecular
orientation of the melt is no longer generally perpendicular to
the parting line.
In the preferred embodiment, the inner core portion 20
is formed of a corrosion resistant material such as stainless
steel to withstand corrosive effects of some melts, as well as
to provide the necessary strength. The outer sleeve portion 40
is also formed of stainless steel to provide a durable finish
and to withstand any corrosive gases escaping from the gate area.
The spiral blade member 42 is formed of high strength steel and
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1 the conductive portion 34 is formed of copper which is cast
over the heating element 32 and ~ereby bonded to the surface of
the coils as well as to the outer surface 38 of the inner core
portion 20. The copper is highly conductive and this integral
structure provides ~or the rapid transfer of heat from the coils
of the heating element 32 and its generally uniform application
to the outer surface 38 of the inner core portion 20. This
structure provides the necessary strength to withstand the
repeated high pressure loading while allowing the thickness of
the lesser conductive inner core portion 20 to be minimized.
Thus there is uniform heat application to the melt along the
length of the nozzle bushing, while avoiding temperature build-
ups which could result in the heating element burning-out or
deterioration of the melt. The surfaces of the stainless steel
blade member 42 are smooth to reduce friction losses with tha
melt as well as to avoid any "dead spots".
Although the description of this invention has been
given with respect to a single embodiment, it is not to be
construed in a limiting sense. Many variations and modifications
will now occur to those skilled in the art. In particular,
other blade configurations could be used, for instance only
having it extend along part of the length of the hot runner
passage 16 ending at the gate 26. Additional pins 54 or other
means could be used to prevent rotation of the nozzle bushing,
and alternate materials could be used for various molding appli-
cations. For a definition of the invention, reference is made
to the appended claims.
