Note: Descriptions are shown in the official language in which they were submitted.
1 This invention relates to an improved system ~or edge
gated injection molding.
In a typical edge gated system, a number o~ cavities
are located in a cooled cavity plate around a central heated
nozzle or bushing. The fact that there is no valve or other
shut-off means to assist in melt flow control causes the
temperature and heat flow characteristics in the gate area to be
of critical importance. The system must be capable of rapidly
and repeatably filling the cavities and then opening them to
eject the molded product without plugging and without excessive
drooling. This requires an insulation or partial insulation
between the heated bushing and the cooled cavity plate so that
the bushing will remain hot enough to maintain the melt in a
molten state and the cavity plate will remain cool enough to
quickly solidify the melt when it flows into the cavities.
In the past, this insulation has been provided by
providing a space adjacent the valve gate between the bushing
and the cavity plate, and allowing it to fill with melt. The
melt solidifies, at least adjacent the cool cavity plate, and
thus provides a degree of insulation between the bushing and the
cavity plate. For instance, the applicant's U.S. Patent Numbers
3,822,856 entitled "Hot Runner Heater" which issued July 9, 1974
and 4,094,447 entitled "Heater Cast ~or Multi-Cavity Hot Runner
Edge Gate" which issued ~une 13, 1978 both show systems which
carry this one step further with the radial portions of the
runner passage being in direct contact with the cavity plate.
EIowever, these nozzle seals have previously been used by the
applicant in valve gated injection molding systems as disclosed
in U.S. Patent Number 4,043,740 entitled "Injection Molding
Nozzle Seal" which issued August 23, 1977 and in Canadian Patent
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1 Application Serial Number 356,233 filed July 15, 1980 entitled
"Injection Molding Nozzle Seal". In both of these applications,
the nozzle seal acts to conduct additional heat to the area of
the gate which facilitates seating of the valve to improve the
reliability of the system and extend the operating life of the
valve pin operating mechanism. In fact, in application serial
number 356,233, the nozzle seal itself actually forms the gate
itself in which the valve pin tip seats.
The fact that these advantages are only available in
a valve gated system reduces the possibility that such a seal
would be incorporated into other types of molding systems. More
particularly, the fact that in an edge gated molding system the
cavity plate wall containing the gates is curved points away
from the use of these seals in an edge gated system because of
the problem of leakage of the pressurized melt. If there is
leakage of the melt in the gate area, it will flow into the air
gap between the bushing and the cavity plate with the attendant
problems mentioned above.
Accordingly, it is an object of this invention to at
least partially overcome these disadvantages by providing an
edge gated injection molding system which works using hollow
seals between the bushing and the gates.
To this end, in one of its aspects, the invention
provides a hot runner edge gated injection molding system
comprising: cooled cavity plate means defining at least one
openable cavity therein; a hollow electrically heated sprue
bushing securely seated in a well in the cavity plate means, the
well having an inner wall which is spaced from the sprue bushing
to form an insulative air gap therebetween, the sprue bushing
having an elongated central melt passage extending from a melt
1 inlet to at least one channel extending radially outward from
the central passage to at least one corresponding edge gate in
the cavity plate means, the edge gates leading to said cavity;
ana at least one corresponding hollow seal having a central bore
extending between an inner end and an outer end which is dome
shaped with a central opening therethrough, the seal extending
across said air gap with its central bore being in alignment
with said radially extending channel and said gate, the inner
end being seated in a recess in the sprue bushing and the outer
end abutting against the inner wall of the well in the cavity
plate, at least a portion of the wall being slightly inwardly
tapered whereby the seal is gradually slightly inwardly deformed
as the sprue bushing is inserted into the well and the system
heated to operating temperatures to prevent substantial leakage
of the pressurized melt into the air gap.
Further objects and advantages of the invention will
appear from the following description taken together with the
accompanying drawings in which:
Figure 1 is a sectional view of a portion of an edge
gated injection molding system according to a preferred embodi-
mend of the invention, and
Figure 2 is an enlarged sectional view showing the
relationship between one of the seals and a corresponding gate
in the cavity plate.
Referring to the drawings, the edge gated injection
molding system shown has at least one hollow sprue bushing 10
with a central hot runner passage 12 therethrough. The passage
12 extends from a manifold plate 14 and branches into a number
of channels 16 extending radially outward from the central
passage 12. Each o~ the chann~ls 16 leads outwardly through a
hollow seal 18 to a gate 20 leading to a cavity 22.
,~ , ; , , ~,
1 The sprue bushing 10 is electrically heated and may
be of the general types shown in the applicant's Canadian Patent
Application Serial N.umbers 317,948 filed December 14, 1978
entitled "Sprue Bushing with Cast In Heater Element" and
363,161 filed October 24, 1980 entitled "Sprue sushing and Method
o~ Manufacture". It has a corrosion resistant inner portion 24
defining the central passage 12, a helical heating element 26
encircling the inner portion 24, and a highly conductive portion
28 cast over them. In the preferred embodiment, the inner
portion 24 is formed of a beryllium nickel alloy to withstand
the corrosive effects of the melt and the conductive portion is
formed of a beryllium copper alloy to rapidly and evenly transfer
heat from the heating element 26 to the inner portion 24. The
sprue bushing 10 is seated in a well 30 in the cavity plate 32
and is securely maintained in position by insulation bushing 34.
As the sprue bushing 10 is heated by heating element 26 and the
cavity plate 32 is cooled by cooling element 36, the insulation
bushing 34 maintains an air gap 38 between them to reduce the
heat loss.
~s clearly seen in Figure 2, each of the hollow seals
18 has a central bore 40 which extends from an inner end 42 to a
central opening 44 in a dome shaped outer end 46. Each hollow
seal 18 extends across the air gap 38 with its inner end 42
seated in a recess 48 around one of the radial channels 16 in the
sprue bushing 10 and its outer end 46abutting against the inner
wall 50 of the well 30 in the cavity plate 32. As may be seen,
the dome shaped outer end 46 has a substantially flat sealing
surface 52 formed by deformation against the cavity plate 32
around a gate 20 leading to one of the cavities 22. In this
position, the opening 44 in the dome shaped outer end 46 of the
5~
1 seal 18 is in alignment with the gate 20, and the cen~ral bore
~0 connects with one of the radial channels 16. The hollow
seal 18 is formed of a metal to provide sufficient strength to
withstand the repeated high pressure loading, but it should not
be a highly conductive metal in or~er not to result in exces-
sive heat transfer from the sprue bushing 10 to the cavity
plate 32. In the preferred embodiment, the hollow seal 18 is
formed of a titanium alloy or stainless steel.
In use, during assembly of the system a seal 18 is
located in each of the recesses 48 and the sprue bushing 10 is
then forced into the well 30 until it is seated on the insulation
bushing 34. The sprue bushing 10 and the seals 18 are sized
so that their effective combined radius is about 0.005" larger
than the radius of the inner wall 50 of the well 30 at the
gates 20. Thus, in order to facilitate insertion of the sprue
bushing 10, a port,on 54 of the wall 50 is slightly inwardly
tapered leading to the gates 20. Accordingly, when the sprue
bushing 10 is forced into the well 30, the dome shaped outer
ends 46 of the seals 18 contact the tapered portion 54 of the
~n wall 50 and are slightly resiliently inwardly deformed as they
come to rest in position around the gates 20. During insertion,
an outer face o the insulation bushing 34 is received in a
cylindrical portion of the wall 50 of the cavity plate well 30
to guide the sprue bushing 10 in proper alignment. This ensures
that the sprue bushing is properly aligned as deformation occurs
so that a secure seal is provided. When the sprue bushing 10
is heated to operating temperatures, it expands resulting in
further deformation of the dome shaped outer ends 46 of the
seals 18. This shape of the seals 18 which allows for this
slight inward deformation enables a pressure tight seal to be
5~;~
1 provided against the curved inner wall 50 of the well 30. While
it would be possible to overcome this same problem by grinding
the end of each seal 18 to the shape of the curved wall 50,
this has the disadvantages that it is expensive to do and it
would require that the seals 18 always be inserted at the correct
orientation. If the repeated application of the high injection
pressure to the heated melt does result in leakage between the
seal 18 and the curved wall 50, it will escape into the air gap
38 resulting in decreased effectiveness of the insulation, as
well as causing the possible problems on colour and material
changes discussed above.
Following assembl~, the sprue bushing 10 is heated by
applying power to the heating element 26 through leads 56. A
thermocouple (not shown) is usually provided to enable the
temperature to be accurately controlled. The cavity plate 32
is also cooled by cooling element 36 and, after temperatures
have stabilized at operating conditions, hot pressurized melt
is applied from a molding machine (not shown) or other source.
The melt flows from the manifold plate 14, through the central
passage 12, branches out into the radially extending channels
16, through the seals 18, and into the cavities 22. After the
cavities are filled, the injection pressure is withdrawn from
the melt and after the melt in the cooled cavities solidifies
the mold is opened to eject the molded products. The mold is
then closed and this process is repeated. It is important that
this process be reliably repeatable without leakage into the air
gap. This is particularly so with difficult to mold engineering
and flame retardant materials such as polycarbonate, poly-
phenylene sulfide, pol~phenylene oxides and nylon 66 because
these materials will deteriorate if trapped in the air gap 38.
~t~ 6~
1 In a system such as this where the heating elemen-t 26 extends
down close to the gates 20, the proximity of the application of
heat to the air gap 38 would further cause deterioration of the
trapped melt to an unacceptable level.
Although the disclosure describes and illustrates a
preferred embodiment of the invention, it is to be understood
that the invention is not limited to this particular invention.
Variations and modifications will occur to those skilled in the
art. For instance, while a multi-cavity system has been
described and illustrated, it will be apparent that the invention
includes a similar single cavity system. Furthermore, alternate
variàtions of the shape of the outer ends of the seals may be
provided which allow for deformation to provide the pressure
seal against the curved inner wall of the cavity plate. For a
definition of the invention, reference is made to the appended
claims.
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