Note: Descriptions are shown in the official language in which they were submitted.
`V094/11894 PCT/US93/10~7
~ -- 1 --
~2 1~7786
AN IMPROVED ARC EXTINGUISHING DEVICE
AND METHOD OF ASSEMBLING SAME
Related Application
Related co-pending U.S. Application Serial No.
07/976,077 filed on even date herewith and identified by
Assignee's Docket No. CRC-l discloses one class of material
compositions which is suitable for use in the present
application. The entire teaching and disclosure of that
co-pending application is incorporated herein by reference.
Field of the Invention
The present invention relates to circuit
breakers, circuit interrupters, electrical distribution
devices and the like, and more particularly, to an arc
extinguishing device having an improved design for use
therein.
Backqround of the Invention
Circuit breakers are commonly used to protect
branch circuits in residential and commercial buildings
against overload and fault conditions. Basically, a
circuit breaker includes a separable pair of electrical
contacts, a spring-operated mechanism for effecting
separation of the contacts, and a tripping mechanism upon
the occurrence of the overload or fault condition. A
representative circuit breaker is fully set forth in U.S.
Patent No. 2,889,429, issued to Kingdon et al. and U.S.
Serial No. 722,050, issued October 26, 1992, to Cook et
al., both commonly assigned to the assignee herein and
incorporated herein by reference.
SUBSTITUTE SHEET
WO94/11894 ~ h2~277 ~6 PCT/US93/10~7
-- 2
An electric arc is produced each time the circuit
breaker contacts are opened or closed. The detrimental
effects from the arc on other internal components is most
severe during interruption of the electrical contacts. An
arc extinguishing mechanism is used to control and
extinguish the arc and protect the other components of the
circuit breaker.
For example, a common type of arc shield, which
is placed in a recess or arc chamber of a circuit breaker
is a series of spaced magnetic plates as illustrated in
U.S. Patent No. 2,811,607 issued to Dorfman et al. Another
type or arc extinguishing mechanism is set forth in U.S.
Patent No. 2,898,427 issued to Nadeau, which discloses a
one-piece u-shaped magnetic metallic member having a
plurality of parallel slots with an arc runner portion to
lead the arc to a venting passage. U.S. Patent No.
2,429,722 to Jennings discloses an arc extinguisher using
insulating side members mounted between the legs of
u-shaped magnetic members and the side walls of the breaker
casing. Another example is U.S. Patent No. 4,616,200
issued to Fixemer et al. which discloses a molded arc
barrier projecting into the arc chamber to shield the
operating mechanism of the circuit breaker.
The need arises to distribute more power through
enclosures which are the same size or smaller. This
requires increasing the electrical rating of the circuit
breaker to carry same voltage and current density while
decreasing the size of the enclosure and the components
therein like the arc extinguishing means. The design of
the arc device must be smaller, yet able to extinguish the
electrical arc created by a higher current.
SUBSTITUTE SHEE~
~V094/1~894 ~ A~ 1 27 7 ~ 6 PCT/US93/10967
-- 3
Among the problems caused by increasing the
electrical rating of a circuit breaker is the heat emitted
by the arc created when interrupting the electrical
contacts. Without dissipation of the arc and the heat
build-up the other components of the circuit breaker will
be damaged.
Summary of the Invention
According to the present invention there is
provided an arc extinguishing device for disposition along
a pre-determined path of movement between two electrical
contacts in an electrical distribution device. The device
includes a generally u-shaped cradle member having a bight
portion defined by a bottom wall with two upstanding side
walls. The cradle member is made of a thermoplastic
resin. The cradle member includes at least one triad of
slots. One of the slots is formed in each side wall
extending from the top edge of the side wall substantially
downward towards the bottom wall. The third slot is formed
in the bottom wall of the cradle member. The triad of
slots is positioned in the same plane extending
perpendicular through the cradle member.
The arc device includes at least one u-shaped
plate having a bight portion defined by a bottom bar with
two upstAn~;ng side prongs. The plate bight portion has
sufficient width to allow the movement of the electrical
contacts between the two side walls. The thickness and
width of the plate is pre-determined to fit the side prongs
within the side wall slots and the bottom edge within the
bottom wall slot. Each plate is retained in connection
with the cradle member. The retaining means is integrally
formed with the cradle member. In the preferred
embodiments, the plate is made of a magnetic metal or a
conductive plastic composition.
SUBSTITUTE SHEET
WO94/11894 ~ A2~ ~7786 PCT/US93/10967
The present invention also includes an electrical
distribution device which includes a housing and a pair of
electrical contacts positioned within the housing. At
least one contact is movable in and out of engagement with
the other along a pre-determined path. The electrical
distribution device further includes an arc extinguishing
device disposed along the pre-determined path of movement.
The arc device is of the same description discussed above.
A method of assembling an arc extinguishing
device for disposition along a pre-determined path of
movement between two electrical contacts in an electrical
distribution device is included in the present invention.
The method includes molding an arc extinguishing device
having a cradle member of the above description from a
thermoplastic resin. The method further includes the step
of providing a u-shaped plate of the above description and
nesting the plate in the triad of slots of the cradle
member.
It is an object of the present invention to
provide an arc extinguishing device which overcomes the
aforementioned problems affecting interruption of circuit
breakers in small enclosures.
Another object of the present invention is to
provide an arc extinguishing device made of two or more
compositions creating an anode-cathode fall therebetween in
order to extinguish electric arcs of greater current in
smaller spaces than the prior art.
A further object of the invention is to provide
an improved arc extinguishing device which protects the
other components of a circuit breaker from exposure to an
electric arc at high fault levels.
Yet another object of the present invention is to
provide an arc extinguishing device which allows
flexibility of design with ease of assembly and economical
manufacture.
SUBSTITUTE SHEET
~094/118g4 CA~l 277~6 Pcr~US93/-og6~
Other and further advantages, embodiments,
variations and the like will be apparent to those skilled
in the art from the present specification taken with the
accompanying drawings and appended claims.
Brief Description of the Drawings
In the drawings, which comprise a portion of this
disclosure:
Fig. 1 is a side view of a circuit breaker
according to the present invention wherein the side cover
is removed showing the position of an embodiment of the
inventive arc extinguishing device;
Fig. 2 is an enlarged, isolated perspective view
of the embodiment of the arc extinguishing device
illustrated in Fig. l;
Fig. 3 is a top plan view of the u-shaped cradle
member disassembled from the remainder of the arc
extinguishing device embodiment illustrated in Fig. 2;
Fig. 4 is a front plan view of the u-shaped plate
disassembled from the remainder of the arc extinguishing
device embodiment illustrated in Fig. 2; and
Fig. 5 is an enlarged cross-sectional view of the
arc extinguishing device embodiment along lines 5-5 in
Fig. 3.
Detailed Description
Turning now to the drawings and referring
specifically to Fig. 1, an example of an embodiment of the
present embodiment is illustrated in the form of a remotely
controlled circuit arrangement. The arrangement includes
an insulating body or housing 10 open at one face with a
detachable cover (not shown). A line terminal 12 and a
load terminal 14 completes the circuit between a source and
a load (not shown).
SUBSTITUTE SHEE~
WO94/11894 C A~l ~1786 PCT/US93/lo~7
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The circuit path beginning at the line terminal
12 carriers current through stationary and movable contacts
16 and 18. The circuit continues through a flexible copper
conductor 20 which is connected between a carrier 22 and a
bimetal member 24. The movable contact 18 may be formed as
part of the carrier 22. A rigid conductive plate 26 is
welded to the bimetal member 24 to carry current from the
bimetal 24 to the load terminal 14.
The above-described current path is controlled
remotely and locally by a number of different components.
Some of the components are similar in structure and
operation to the corresponding components in Square D
Company Model Nos. QO-PL and QOE, and in U.S. Patent No.
4,623,859 and U.S. Serial No. 722,050 issued October 26,,
1992, both entitled "Remote Control Circuit Breaker,"
assigned to the instant assignee and incorporated herein by
reference.
Local control of the circuit breaker arrangement
is provided using the external operating handle 28
pivotally mounted about an axis 30 in the housing 10 to
control the contact carrier 22. In response to the
movement of the handle 28 to the right or left, the carrier
22 is moved counterclockwise or clockwise, respectively, by
the action a biasing spring 32. The handle 28 moves the
top of the carrier 22 to either side of the equilibrium
position, so that the bottom of the carrier 22 biases the
movable contact 18 to either the open or closed position.
The trip mechanism assembly includes an armature
34, the bimetal 24 and a yoke 36. Upon occurrence of a
moderately sustained overload, from the contact-closed
position the bimetal member 24 heats up and flexes to the
right, causing the armature 34 and the yoke 36 to swing
counterclockwise releasing the stand-off pressure of the
end of the trip lever 38. The trip lever 38 swings
clockwise about pin 40 and pulls the carrier 22 away from
the stationary contact 16 to interrupter the current path.
SUBSTITUTE SHEE~
WO94/11894 C ~ 71 8 6 PCT/US93/10967
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Similarly, upon occurrence of an extensive
current overload, the yoke 36 manifests a magnetic force
that attracts and swings the armature 34 counterclockwise.
The trip lever 38 then swings clockwise and the spring 32
pulls the carrier 22 interrupting the current path.
Remote control of the circuit breaker arrangement
is provided using a motor 42 having a shaft 44 which
rotates in one direction to pull the carrier 22 and
interrupt the current path. Rotation of the shaft 44 in
the opposite direction allows the carrier to be pulled by
spring 32 to re-establish the current path.
During a short-circuit condition or interruption
of the current path, energy is shunted around the bimetal
member 24. A shunt terminal 46 extends from the load
terminal 16 to an arc extinguishing device 50 to dissipate
the arcing current. An arc yoke 52 attracts the arc and
shunts the current around the bimetal member 24. The arc
yoke 52 providing a grounding means for draining the
electric potential generated by an interruption of the
contacts 16, 18 from the arc device 50. Other types of
electrical connections for grounding the arc device 50 are
suitable for use with the present invention.
The arc extinguishing device 50 is illustrated in
isolation in Figs. 2 through 5. The arc device 50 includes
a generally u-shaped cradle member 54 having a bight
portion 56 defined by bottom wall 58 with two upstanding
side walls 60 and 62. The side walls 6p and 62 have an
inside face 64 and an outside face 66 which terminate at
the top edges 68 and 70, respectively. The width of the
bight portion 56 is sufficient to allow movement of the
electrical contacts between the two side walls 60, 62.
The cradle member 54 is formed with at least one
triad of slots 72, 74 and 76. Slots 72 and 76 are formed
in the side walls 60 and 62, respectively, extending from
the top edges 68, 70 of the side wall substantially
downward towards the bottom wall 58. The third slot 74 is
formed in the bottom wall 58 of the cradle member. The
SU~STITUTE SHEET
WO94/1l894 ~ 7 7 ~ 6 PCT/US93/10~7
triad of slots 72, 74, and 76 are positioned in the same
plane extending perpendicular through the cradle member 54.
The arc device 50 includes at least one u-shaped
plate like 72 having a bight portion 80 defined by a bottom
bar 82 with upst~n~ing side prongs 84 and 86. The bottom
bar 82 has a bottom edge 88 and a top edge 90. The width
of the plate bight portion 80 is sufficient to allow
movement of the electrical contacts between the two side
prongs 84, 86. The thickness and width of the plate 72 is
predetermined to fit the side prongs 84, 86 within the side
wall slots 72, 76 and the bottom edge 88 within the bottom
wall slot 74.
The plate 78 is retained in connection with the
cradle member 54 by a retaining wall 92 integrally formed
with the cradle member 54. The retaining wall 92 upstands
from the bottom wall 58 and extends between the two side
walls 60, 62 of the cradle member. The retaining wall 92
abuts a face 94 of the plate. Formed with the retaining
wall 92 is a flange 96 which extends perpendicularly
towards the abutting face 94 of the plate. The flange 96
is positioned at a pre-determined height on the retaining
wall 92 to abut the top edge 90 of the bottom bar of the
plate and retain the plate 78 in position. Preferably, the
flange 96 is made of a resilient material which allows the
flange 96 to be bent in and out of position and reversible
engage the top edge 90.
Other means for retaining the plate 78 in
connection with the cradle member 54 are contemplated by
the present invention. For example, the thickness of the
plate 78 can be formed slightly greater than the size of
the side wall slots 72, 76 to provide an interference fit
therebetween.
The gases that are created during the
interruption of the electrical contacts 16, 18 are vented
through the bottom wall 58 of the cradle member.
Preferably, the bottom wall slot 74 is made larger than
need to accommodate the bottom edge 88 of the plate. This
SUBSTITUTE SHEET
~094/11894 ~ A 2 1 2 7~ B 6 PCT/US93/10~7
g
positions a hole adjacent to the bottom edge 88 of the
plate. It is suitable to form separate holes in the bottom
wall 58 near the bottom edge 88 of the plate.
In the preferred embodiment of the inventive arc
device 50, there are two triads of slots like 72, 74 and 76
formed in the u-shaped cradle member 54. Corresponding to
the two triads are two u-shaped plates like 78.
Both side walls 60 and 62 have a thickness T and
a length L as shown in Fig. 2. In the preferred
embodiment, the thickness T of each side wall 60, 62 and
substantially the entire arc device 50 is in the range of
about 0.06 to 0.1 inches and preferably measuring about
0.0625 inches. The length L of the arc device 50 as
measured along the top edges 68 and 70 is in the range of
about 0.5 to 1 incudes and preferably measuring about 0.75
inches. The length L is sufficiently long to encompass a
substantial portion of the path movement of the contact
18. The width of the bight 56 or the distance between the
side wall 60 and 62 is sufficient to allow the contacts to
pass between them.
The thickness of the plate 78 is about 0.094
inches. The distance between the top edge 90 and the
bottom edge 88 of the bottom bar is about 0.25 inches. The
width of the plate 78 is preferably greater than the width
of the cradle member 54 so that the edges of the plate
protrude beyond the outside face 66.
Preferably, the bight portion 80 of the plate is
substantially smaller than the bight portion 56 of the
cradle member. Then the bight portion 80 of the plate is
closer to the movement path of the electrical contacts 16,
18 than the bight portion 56 of the cradle member. Thus,
the plate 78 is more quickly exposed to the electric arc
because of its closer spacial position.
The dimensional measurements of the arc device 50
are adjusted to accommodate the capacity of the circuit
breaker. For the measurements discussed above, the
capacity of the circuit breaker ranges from about 15 to 30
SUBSTITUTE SHEEl-
WO94/11894 ~ A ~ ~ 277 ~ 6 PCT/US93/10967
-- 10 --
Although a circuit breaker is illustrated, the
present invention is suitable for protecting all types of
electrical distribution devices such as circuit
interrupters and the like. In practical applications, it
is still desirable to have all electrostatic sensitive
components insulated from direct contact with any part of
the arc device 50.
The cradle member 54 of the arc device 50 is made
of a thermoplastic resin. The preferred thermoplastic
resins include nylon 6, nylon 6/6, nylon ll, nylon 6/12,
and high-impact nylon. The most preferred thermoplastic
base resin is a mineral-filled nylon having the trade name
Minlon available from DuPont Company, as catalog number
l0B40.
The plate 78 of the arc device 50 is made of a
material which is more conductive than the cradle member 54
material to establish an anode-cathode fall therebetween.
One class of conductive materials for the plate 78 are
magnetic metals. Preferably, the magnetic metal used is
nickel-coated steel.
Another class of conductive materials for making
the plate 78 include conductive composites formed from
adding a conductive modifier to a thermoplastic resin. The
conductive plastic composites preferably have certain
electrical properties. The US Department of Defense
Handbook 263 describes three categories of plastic
composites for use in electrostatic discharge protection.
They are anti-static, static dissipative, and conductive.
they are anti-static, static dissipative, and conductive.
Although the resistivity characteristics of these three
categories is not entirely settled, static dissipative
composites are usually defined to have surface
resistivities of greater than 105 and less than l09
ohms per square. Since static dissipative compsites are
more conductive than anti-static composites, they dissipate
electric potential more quickly. Also, because of their
make-up, they more readily conduct the potential throughout
SUBSTITUTE SHEE~
~ 094/11894 CA~1~77~6 PCT/US93/10~7
-- 11 --
the volume of the part. Thus, static dissipative
composites allow more rapid bleed-off of electric
potential. The ASTM Standard D-257 provides uniform
procedures for determining the resistance of a material.
These methods are used to determine both surface and volume
resistivities of the composite.
The conductive composites do not change their
static dissipative electrical resistivity properties over
time because they are formed by compounding or mechanically
blending a solid conductive modifier with a thermoplastic
base resin. The homogeneity and level of dispersion of one
or more conductive modifiers within the thermoplastic base
resin are the important criteria affecting the performance
of the composites. The resulting composites are permanent
in their static dissipative properties with a surface
resistivity less than 109 ohms per square and a volume
resistivity preferably less than 10 ohms per centimeter.
Other components to the conductive composites are
limited to those conventional additives like inhibitors,
etc., needed for the composites to exhibit the requisite
manufacturing or mechanical properties discussed below.
These additives are present in small amounts relative to
the thermoplastic base resin and the conductive modifier.
The conductive modifiers include electrically
conductive powders and fibers. Preferred modifiers include
carbon black powder, carbon graphite fiber, stainless steel
fiber and powder, and nickel coated graphite fiber. Other
types of metallic coatings are suitable for use on graphite
fibers. The concentration of the conductive modifier is
effective to have the composite exhibit static dissipative
electrical resistivity properties and remove electric
potential therefrom. A suitable weight percent
concentration of the conductive modifier in the
thermoplastic base resin is in the range of about 4 to
about 20 percent. A more preferred range is about 5 to
about 15 percent by weight. The metallic containing
conductive modifiers like stainless steel and nickel
SUBSTITUTE SHEET
WO94/tl894 ~ A ~1 27 7~6 PCT/US9~10~7
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graphite have concentrations in the lower area of this
range, for example, about 5 to about 10 percent by weight.
Since amorphous carbon black has a lower aspect ratio than
graphite carbon, it needs concentrations in the higher end
of the range to impart the desired electrical properties,
like about 15 to about 20 percent by weight. The graphite
fiber is preferred in the middle of the range, for example,
about 10 to about 15 percent by weight. Preferably, the
weight concentration of stainless steel fiber in nylon 6
is about 10 percent.
Composites containing carbon black powder as he
conductive modifier are presently the most cost effective.
However, a high percent loading is required and this can
result in a composite with lessened mechanical properties
in comparison to the unmodified base resin. If higher
properties are needed, such as impact strength or
stiffness, then another conductive modifier such as carbon
fiber, is used to gain optimal properties.
The present invention also provides for using a
magnetic material as the conductive modifier in a
concentration effective to have the composite exhibit
magnetic properties which increase the mobility and further
dissipate the arc current. A preferred magnetic material
as the conductive additive is stainless steel or
nickel-coated steel. A suitable weight percent
concentration of the conductive modifier in the
thermoplastic resin is in the range of about 60 to 80
percent. Depending on the magnetic material selected as
the conductive modifier, a weight percent less than this
range improves the performance of the arc device because
the composite is conductive However, the composite does
not exhibit significant magnetic properties and the
associated advantages.
the following thermoplastic base resins are
suitable for use with the present invention in compounding
composites with most types of conductive modifiers:
polypropylene, nylon 6/6, nylon 6, nylon 11, nylon 6/12,
S~)BSTITUTE SHEET
~094/11894 ~ A 2 1 27 7 ~6 PCT/US93/10~7
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high-impact nylon, polycarbonate, polystyrene,
acrylonitrile butadiene styrene, high density polyehtylene,
low density polyethylene, polysulfone, polybutylene
terephthalate, polyethylene terephthalate, polyphenylene
sulfide, polyester thermoplastic elastomer, polyetherimide,
styrenic thermoplastic elastomer, and olefinic
thermoplastic elastomer.
The following base resins are more compatible for
compounding with carbon fiber, nickel coated graphite fiber
and stainless steel as the conductive modifier: acetal,
polyurethane thermoplastic, polyphenylene oxide,
polyetheretherketone, phenylene ether co-polymer,
polycarbonate/acrylonitrile butadiene styrene,
polyarylether ketone, polyetherketoneetherketoneketone,
polyphthalamide, and polyehterketoneketone. Other suitable
base resins include perfluoroalkoxy, ethylene
tetrafluoroethylene, and polyvinylidene fluoride.
The composite suitable for use by the present
invention exhibit certain mechanical properties for
constructing an arc device capable of being mounted within
and withstanding the environment of an electrical
distribution device like a circuit breaker. The composites
exhibit structural integrity, good impact resistance and
dimensional stability. furthermore, the composites exhibit
a sufficient heat distortion temperature and achieve a UL
temperature index of 100 C. degrees or greater, as measured
by UL Subject 756B. Preferably, the composite is flame
retardant
The following examples are set forth for the
purposes of illustration and should not be construed as
limiting.
EXAMPLES
Three sample type EH circuit breakers
manufactured by the Square D Company were utilized wherein
all the components were standardized but for the exceptions
noted herein. The circuit breakers were single pole
SUBSrITUTE SHEET
WO94/118~ CA2t27786 PCT/US93
- 14 -
construction with a 30 amp, 277 volt rating. One type EH
breaker was used as a control. The other two circuit
breakers were modified by removing the tripping mechanism
and replacing it with a smaller type QO tripping mechanism
having a lower power rating than the type EH breaker.
Within each of the type QO tripping mechanisms
was substituted an inventive arc device having a
construction as illustrated in Fig. 2 herein. The cradle
member for both inventive arc devices was made of mineral
filled nylon supplied by the DuPont Company as Minlon
10B40. The cradle member was about 0.75 inches long and
0.0625 inches thick.
Each inventive arc device used two plates
positioned in the slots formed in each cradle member. In
the first inventive arc device, the plates were made of
nickel-plated, cold rolled steel. In the second inventive
arc device, the plates were made of a nylon 6 thermoplastic
having a conductive modifier of stainless steel added
thereto in the amount of about 10 percent by weight. This
material was supplied by the RTP Company of Winona, MN as
catalogue number RTP 0288-A-X-58759 Natural.
Each set of plates measured about 0.0938 inches
thick. The distance between the top edge and bottom edge
of the bottom bar was about 0.25 inches. The inventive arc
devices had a smaller width, length and shape than the
stAnAArd arc device used in the EH circuit breaker. The
size of the arc device used in the type EH tripping
mech~nism prevented it from being positioned in the type QO
tripping ~PchAnism.
Each circuit breaker was subjected to a testing
regime wherein the circuit breaker electrical contacts were
closed on a high fault level measuring 240 vac., 14 ka, 1.5
ms. after voltage zero. The testing regime demonstrated
that the performance of the inventive arc devices with the
steel and conductive plastic plates was comparable.
Furthermore, both inventive arc devices were able to
extinguish significantly higher arc currents than the
SUBSTITUTE SHEE~
`V094/~t894 ~ A ~ ~ ~77~6 PCT/US93/10967
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standard arc device sued in the tripping mechanism of the
type QO circuit breaker.
The surface area of the cradle member S4 made of
the thermoplastic resin appears to melt when attacked by
the arc created by the interruption of the electrical
contacts 16, 18. The surface area of the cradle member 54
substantially encloses the minute ablative particles which
are disintegrated from the contacts. The liquefying of the
thermoplastic surface appears to envelope the ablative
particles, cool them, and then the surface of the
thermoplastic material rehardens. Accordingly, the
ablative particles are actually absorbed by the cradle
member 54 surface.
The thermoplastic resin and other suitable
composites also preferably emit hydrogen gas upon attack by
an electric arc. The hydrogen gas emission further cools
and extinguished the arc by a process known as outgassing.
In the preferred embodiments, the present
invention provides an anode-cathode fall between the cradle
member 54 and the plates 78. The effect created improves
the ability of the arc device to extinguish arcs of higher
energy than devices of the prior art.
Preferably, the cradle member 54 is molded by
conventional techniques such as injection molding. It is
desirable for the thermoplastic resin or other suitable
composites to exhibit high flow properties for molding thin
walls of the cradle member 54.
The plate 78 made of the magnetic metal is
preferably formed by stamping or otherwise cutting the
individual pieces from a sheet of material. The plate 78
is assembled to the cradle member 54 by manually inserting
the side prongs 84, 86 into the slots 72, 76 and snapping
the flange 96 into engagement with the top edge 90 of the
plate.
The plate 78 made of a conductive composite is
preferably molded by conventional techniques such as
injection molding. The plate 78 is then nested in the
Sl:~:BSTITUTE SHEET
wo g4/.t894 ` ~ A ~1 27 7 ~ 6 PCT/US93/10967
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cradle member 54 in a similar fashion as the plate made of
magnetic metal.
Another method of making the arc device having a
plate made of a conductive composite is with a two-part
mold. One part of the mold forms the cradle member with a
first thermoplastic composition. The second part of the
mold forms the plate in the appropriate position with a
second thermoplastic composition. The two part mold
simultaneously nests the two parts, cradle member and
plate, together during the molding of the two thermoplastic
compositions. Although the cost of manufacturing an item
with a two part mold is usually more expensive than the
other methods described herein, this technique allows
greater flexibility in design than can be inexpensively
achieved with metal-forming te~n;ques.
While particular embodiments and applications of
the present invention have been illustrated and described,
it is to be understood that the invention is not limited to
the precise construction and compositions disclosed herein
and that various modifications, changes, and variations
which will be apparent to those skilled in the art may be
made in the arrangement, operation, and details of
construction of the invention disclosed herein without
departing from the spirit and scope of the invention as
defined in the appended claims.
S~BSTITUTE SHEET
