U.S. patent application number 09/840513 was filed with the patent office on 2001-08-16 for fibrous supported polymer encapsulated electrical component.
This patent application is currently assigned to Watlow Polymer Technologies. Invention is credited to Rutherford, James M..
Application Number | 20010014212 09/840513 |
Document ID | / |
Family ID | 23198202 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010014212 |
Kind Code |
A1 |
Rutherford, James M. |
August 16, 2001 |
Fibrous supported polymer encapsulated electrical component
Abstract
Heating elements, electrical devices and processes for
manufacturing these components are provided. The heating elements
and electrical components employ a fibrous support layer, such as a
non-woven glass mat, which provides structural support to the
relatively thin cross-section of the electrical device or
resistance wire when a polymeric layer is applied.
Inventors: |
Rutherford, James M.;
(Dresbach, MN) |
Correspondence
Address: |
DUANE, MORRIS & HECKSCHER LLP
One Liberty Place
Philadelphia
PA
19103-7396
US
|
Assignee: |
Watlow Polymer Technologies
|
Family ID: |
23198202 |
Appl. No.: |
09/840513 |
Filed: |
April 23, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09840513 |
Apr 23, 2001 |
|
|
|
09309429 |
May 11, 1999 |
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Current U.S.
Class: |
392/503 ;
219/544; 219/549 |
Current CPC
Class: |
H05B 3/54 20130101; H05B
2203/014 20130101; H05B 2203/017 20130101; H05K 3/28 20130101; H05B
3/36 20130101; H05B 3/56 20130101; H05B 2203/013 20130101; H05B
2203/003 20130101 |
Class at
Publication: |
392/503 ;
219/544; 219/549 |
International
Class: |
H05B 003/34 |
Claims
We claim:
1. An insulated electrical component comprising: a. an electrical
device having a portion having a relatively thin cross-section,
said portion being susceptible to distortion or dislocation by a
viscous flow of a liquid or semi-liquid polymeric composition
during an encapsulation of said electrical device by said polymeric
composition; b. a porous, fibrous support layer disposed with said
electrical device; and c. a polymeric layer comprising said
polymeric composition substantially encapsulating said fibrous
support layer and said electrical device whereby said fibrous
support layer provides structural support to said relatively thin
cross-section of said electrical device when said polymeric
composition is caused to flow through said porous, fibrous support
layer and around said electrical device during said
encapsulation.
2. The heating element of claim 1 wherein said portion of said
electrical device comprises an electrical resistance heating wire
disposed in a looped circuit path.
3. The electrical component of claim 2 wherein said fibrous support
layer comprises an open pore glass mat joined to said electrical
resistance heating wire.
4. The electrical component of claim 3 wherein said open pore glass
mat comprises an air permeability rating of at least about 500
ft.sup.3/min.
5. The electrical component of claim 1 wherein said fibrous support
layer is joined to said electrical device prior to
encapsulation.
6. The electrical component of claim I wherein said electrical
device comprises one or more of: an integrated circuit component,
an electrical conductor path, a power source or resistance
wire.
7. The electrical component of claim I wherein said polymeric layer
is disposed through said fibrous support layer and in contact with
said electrical device, without substantially displacing said
electrical device relative to said fibrous support layer.
8. The electrical component of claim 6 wherein said electrical
device and said polymeric layer are molded into a unified, hermetic
structure.
9. The electrical component of claim 1 wherein said fibrous support
layer comprises a glass mat having cut glass fibers joined together
with a resinous adhesive, and having an air permeability rating of
at least about 1000 ft.sup.3/min.
10. The electrical component of claim I wherein said electrical
device comprises an electrical resistance heating wire, said
electrical component further comprising a thermostat or thermistor
adapted to regulate current through said electrical resistance
heating wire upon reaching a preselected temperature limit.
11. A resistance heating element comprising: a. an electrical
resistance heating wire disposed in a circuit path, said wire being
susceptible to distortion or dislocation by a viscous flow of a
liquid or semi-liquid polymeric composition, said wire having a
pair of terminal end portions; b. a pair of electrical conductors
fixed to said terminal end portions of said wire; c. a fibrous
layer disposed with said wire; and d. a high temperature polymeric
layer substantially encapsulating said circuit path and said
fibrous layer, whereby said fibrous layer assists in supporting
said circuit path to minimize distortion of said wire during said
encapsulation.
12. The resistance heating element of claim 11 having a power
rating of about 5-10,000 W.
13. The heating element of claim 12 wherein said polymeric layer
comprises a non-electrically conductive, thermally-conductive
additive.
14. The heating element of claim 11 wherein said fibrous layer is
joined to said electrical resistance heating wire before
encapsulation.
15. The heating element of claim 14 wherein said electrical
resistance heating member comprises a Ni--Cr wire having a thin
cross-section which is capable of being dislocated by the
encapsulation with said polymeric layer, when said fibrous layer is
not used.
16. The heating element of claim 11 wherein said polymeric layer
comprises an epoxy resin bonded to said fibrous layer and
electrical resistance heating member.
17. The heating element of claim 16 wherein said fibrous layer
comprises a chopped glass mat joined to said electrical resistance
heating member.
18. A method of making an insulated, electrical component,
comprising: providing an electrical device having a relatively thin
cross-section, which is susceptible to distortion during
encapsulation by a liquid or semi-liquid polymeric composition;
adhering said electrical device onto a fibrous support layer; and
encapsulating said fibrous support layer and said electrical device
within a polymer layer comprising said polymeric composition,
whereby said fibrous support layer provides structural support to
said relatively thin cross-section of said electrical device to
minimize distortion or dislocation during said encapsulation.
19. The method of claim 18, wherein said encapsulation step
comprises molding said polymeric composition over said fibrous
support layer and said electrical device to form a hermetic
coating.
20. The method of claim 18, wherein said fibrous support layer
comprises a non-woven or woven glass mat, having an air
permeability rating of at least about 1,000 cubic feet per
minute.
21. The method of claim 19, wherein said electrical device
comprises a Ni--Cr resistance heating wire, and said polymer layer
comprises a thermosetting resin.
22. The method of claim 21, wherein said thermosetting resin
comprises an non-electrical conducting, thermally conducting
additive.
23. The method of claim 19, wherein said electrical device and said
fibrous support layer are subject to heat or pressure or both
during said encapsulation, whereby said polymeric composition wets
the surfaces of said fibrous support layer and said electrical
device.
24. The method of claim 18, in which said insulated electrical
component has a flexural modulus at least 50% greater than the
flexural modulus of said polymer layer, without said fibrous
support layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S.
application Ser. No. 09/309,429, filed May 11, 1999.
FIELD OF THE INVENTION
[0002] This invention relates to polymer encapsulated electrical
components, and more particularly, to fibrous support layer
reinforced electrical components, such as electrical resistance
wires and integrated circuits, encapsulated in a polymeric
layer.
BACKGROUND OF THE INVENTION
[0003] Electrical resistance heating elements are used in many
industrial, commercial and residential applications. They have been
used to heat electroplating baths in the plating industry and can
be found in the baseboard heaters of many homes. A typical
construction for an electrical resistance heating element includes
a pair of terminal pins brazed to the ends of a Ni--Cr coil, which
is then axially disposed through a U-shaped tubular metal sheath.
The resistance coil is insulated from the metal sheath by a
powdered ceramic material, usually magnesium oxide.
[0004] More modern heating elements have been developed using
polymeric insulating materials around a resistance heating wire,
such as disclosed in U.S. Pat. No. 5,586,214. These more recent
devices employ resinous coatings which are often injection molded
over the resistance wire. Since resistance wire is often extremely
pliable, injection molding pressures are known to distort the
circuit pattern for the wire in unacceptable ways. One solution
described in the '214 patent is to provide a polymer inner mold
having a series of grooves for receiving the wire and holding it in
place while a thermoplastic coating is injection molded over the
assembly. This technique has been difficult to implement when
thermoplastic materials are loaded with significant amounts of
ceramic additives. Such mixtures are viscous and require great
pressures of 10,000-25,000 psi to fill the mold properly. Even high
mold pressures are sometimes insufficient to fill the details of
the mold properly and the greater the mold pressure, the more
stress is applied to the circuit pattern.
[0005] In still a further method described in U.S. Pat. No.
3,968,348, a resistance heating wire is disposed between a pair of
fiberglass mats. A third fiberglass mat carries a heat dissipating
metal foil. The three mats are separately impregnated with a
thermosetting polyester resin and cured together to form a rigid,
fluid impervious laminated structure. While laminating techniques
have occasionally produced acceptable products, they often leave
air gaps in the cross-section which make uniform heating difficult.
Additionally, insufficient bonding to the glass mats or resistance
wire can cause delamination, especially due to the difference in
thermal expansion rates during heating and cooling cycles.
[0006] While such methods for creating resistance heating elements
with thermoplastic or thermosetting polymers are known, there
remains a need for better manufacturing processes which can further
reduce the cost and improve the quality of polymer heaters. There
also remains a need for more structural integrity during heating
cycles and more effective thermal dissipation of heat from the
resistance wire.
SUMMARY OF THE INVENTION
[0007] The present invention provides polymer coated electrical
components and methods of fabricating such components, and is
particularly useful in the manufacturing of electrical resistance
heating elements. In one preferred embodiment of the invention, an
electrical device is provided having a portion having a relatively
thin cross-section, which is normally susceptible to distortion or
dislocation by a viscous flow of a liquid or semi-liquid polymeric
composition during an encapsulation of said electrical device by
said polymeric composition. The electrical device is supported by a
fibrous support layer disposed with the device. A polymeric layer
is substantially encapsulated around the fibrous support layer and
the electrical device.
[0008] It is known from experience that the application of
polymeric material under pressure to delicate electrical devices
can distort or move such devices in a mold. This invention employs
a fibrous support layer to support electrical devices, such as thin
gauge resistance heating wire, to provide structural support to the
relatively thin cross-section of the electrical device so that a
viscous polymeric composition can be used to encapsulate the device
without distortion, displacement or loss of quality control.
[0009] The devices and methods of this invention enable relatively
thin cross-sections as small as 0.01-0.3 cm to be encapsulated
without distortion. The fibrous support layers of this invention
help to maintain stability of viscous flows of thermoplastic or
thermosetting material and act as a ballast or dampening factor.
They also can be porous enough to allow the flow of polymeric
material to penetrate through their pores so as to thoroughly coat
the fibers and the electronic device. The flowing of polymeric
materials through the fibrous support layers of this invention help
to "wet" the fibers of these support layers to avoid a fracture
initiation site along the boundary between the support layers and
polymer, which has been known to cause part failure, especially
when a part is heated.
[0010] The use of polymeric preforms in the manufacturing
techniques of certain embodiments of this invention helps to more
evenly distribute polymeric material over a greater amount of the
device's surface. The use of preforms and compression molding
together, when compared to older injection molding techniques,
distributes the molding forces more uniformly along the surface of
the fibrous support layer to minimize movement of the electronic
device during molding, and assist in the creation of a unified
hermetic structure. Additionally, when thermosetting polymers are
used, the use of the preform and fibrous support layers of this
invention permits a lower compressive force to be used to liquefy
thermosetting resins to permit them to flow properly. The preferred
glass mats of this invention also improve the flexural modulus of
the component by at least 50% over the unreinforced polymer used
for encapsulation, and, preferably, have an air permeability rating
of at least about 500 ft.sup.3/min.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings illustrate preferred embodiments
of the invention as well as other information pertinent to the
disclosure, in which:
[0012] FIG. 1: is a front perspective view of a insulated
electronic electrical component of this invention;
[0013] FIG. 2: is a top, cross-sectional view of the electrical
component of FIG. 1, taken through line 2-2;
[0014] FIG. 3: is a right side, cross-sectional view of the
electrical component of FIG. 1, taken through line 3-3, and
illustrating the component parts in diagrammatic illustration;
[0015] FIG. 4: is an exploded front perspective view of an
alternative encapsulated semiconductor chip component of this
invention illustrating conductor paths located on the fibrous
reinforcement; and
[0016] FIG. 5: is a flow diagram of a preferred method of
manufacturing the insulated electrical components of this
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] As used herein, the following terms are defined:
[0018] "Substantially Encapsulating" means that at least 85% of the
surface area of the designated member is provided with polymeric
material, but does not necessarily mean that the coating is
hermetic;
[0019] "Serpentine Path" means a path which has one or more curves
for increasing the amount of electrical resistance material in a
given volume of polymeric matrix, for example, for controlling the
thermal expansion of the element;
[0020] "Melting Temperature" means the point at which a polymeric
substance begins to liquefy because of heat;
[0021] "Degradation Temperature" means the temperature at which a
thermoplastic or thermosetting resin begins to lose its mechanical
or physical properties because of thermal damage;
[0022] "Melt Bond" means the bond between two members integrally
joined, whereby the atoms or molecules of one member mixes with the
atoms or molecules of the other.
[0023] "Low Volatile Content" means polymeric mixtures, suitable
for adhesives, which contain less than 10 wt % volatile components
after full set, for example, during use in the final component.
[0024] The present invention provides insulated electrical
components, including an electrical device having a relatively thin
cross-section, a fibrous support system disposed over the
electrical device and a polymeric layer substantially encapsulating
the fibrous support and the electrical device whereby the fibrous
support provides structural support to the relatively thin
cross-section of the electrical device when the polymeric layer is
applied in a mold under pressure.
[0025] The electrical components and heating elements of this
invention are designed to operate at power ratings of as low as 5
watts and as great as 10,000 watts with watt densities of 5
watts/inch.sup.2 to about 25 watts/inch.sup.2. For example, heating
elements can be produced for heating paper in an ink jet printer at
watt densities of only about 5-8 watts/inch.sup.2 using a polymeric
layer of polyphenylene sulfide, epoxy or silicone without thermally
conductive additives. On the other hand, immersion heaters for
heating hot water can be produced with watt densities of about
20-25 watts/inch.sup.2 by using an epoxy resin loaded with about 50
weight percent Al.sub.2O.sub.3 or MgO ceramic powder with about
5-10 weight percent glass fiber reinforcement.
[0026] Although the invention contemplates using a glass mat layer
adhered to an electrical resistance heating wire, teachings are
also provided for encapsulating integrated circuit components, such
microchips or small circuit boards, power sources, transformers or
magnetic devices. Such devices can optionally include a thermostat
or thermistor control for regulating current through the device
upon reaching a pre-selected temperature limit.
Heating Element Construction
[0027] In a first embodiment of the invention, shown in FIG. 1, a
heating element 100 is provided having an end cap portion 18, a
pair of terminal electrical conductors 16 and 14 and a temperature
control device 10, such as a thermistor or thermocouple, for
regulating the current through the element 100 to protect against
overheating.
[0028] The preferred circuit 22 of the heating element 100 of this
invention is illustrated in the cross-sectional view of FIG. 2. The
circuit 22 includes a resistance heating material, which is ideally
a resistance heating wire 26 wound into a serpentine path
containing about 20-51 windings, or, a resistance heating material,
such as a foil or printed circuit, or powdered conducting or
semi-conducting metals, polymers, graphite, or carbon.
Alternatively, a laser trimmed or photoetched Al or Cu foil could
be employed. More preferably the resistance heating wire 26
includes a Ni--Cr alloy, although certain copper, steel, and
stainless-steel alloy wires could be suitable. Whatever material is
selected, it should be electrically conductive, and heat resistant.
The resistance heating wire is preferably terminated with a
grommets 21 and 27, and cold pins 23 and 28, or alternatively a
conductor of another, or further construction, such as flat
electrical conductors 16 and 14, for connection to a power
source.
[0029] With reference to FIG. 3, there is shown in magnified cross
section, a preferred detailed construction of a heating element 100
of this invention. The element 100 of this embodiment includes the
polymer layer 113 or matrix, which is preferably of a
high-temperature variety including a melting or degradation
temperature of greater than 93.degree. C. (200.degree. F.). High
temperature polymers known to resist deformation and melting at
operating temperatures of about 75-85.degree. C. are particularly
useful for this purpose. Both thermoplastics and thermosetting
polymers can be used. Preferred thermoplastic materials include,
for example, for higher temperature applications: fluorocarbons,
polyaryl-sulphones, polyimides, and polyetheretherkeytones,
polyphenylene sulfides, polyether sulphones, and mixtures and
co-polymers of these thermoplastics. Preferred thermosetting
polymers include epoxies, phenolics, and silicones. Liquid-crystal
polymers can also be employed for improving high-temperature use.
The most preferred materials for the purposes of the current
embodiment of this invention are compression or sheet molding
compounds of epoxy reinforced with about 50-60 wt % glass fiber. A
variety of commercial epoxies are available which are based on
phenol, bisphenol, aromatic diacids, aromatic polyamines and
others, for example, Litex 930, available from Quantum Composites,
Midland, Mich.
[0030] For lower temperature use, less expensive polymers such as
polyethylene, polypropylene, or polyvinylchloride could be
used.
[0031] As stated above, the polymeric layers of this invention
preferably also include about 1-60 wt. %, more preferably about
5-10 wt. %, reinforcing fibers, such as glass, carbon, aramid,
steel, boron, silicon carbide, polyethylene, polyimide, or graphite
fibers. The fibers 112 of FIG. 3 can be disposed throughout the
polymeric material prior to molding or forming the element 100, in
single filament, multifilament thread, yarn, roving, non-woven or
woven fabric.
[0032] In addition to reinforcing fibers, this invention
contemplates the use of thermally conducting, preferably
non-electrically conducting, additives 110 shown in FIG. 3. The
thermally-conducting additives 110 desirably include about 30-70
wt. % ceramic powder, flake or fiber, such as, for example,
Al.sub.2O.sub.3, MgO, ZrO.sub.2, Boron nitride, silicon nitride,
Y.sub.2O.sub.3, SiC, SiO.sub.2, TiO.sub.2, etc., or a thermoplastic
or thermosetting polymer which is more thermally conductive than
the polymer matrix suggested to be used with the polymer layer 113.
For example, small amounts of liquid-crystal polymer or
polyphenylene sulfide particles can be added to a less expensive
base polymer such as epoxy or polyvinyl chloride, to improve
thermal conductivity.
[0033] In order to support the preferred resistance heating element
100 of this invention, a fibrous support layer 114 is desirably
employed to hold it in place while the polymer layer 113 is applied
under pressure. The fibrous support layer 114 should allow the
polymeric resin of the heating element 100 to flow through its
structure so as to encapsulate the preferred resistance heating
wire 26 or material. However, it should be resilient enough to
allow viscous polymer materials, which contain large amounts of
glass fibers and ceramic powder, to pass through its course
openings without substantially deforming the circuit 22. It will
become apparent to one of ordinary skill in the art that the
circuit 22 will employ resistance wire having a very fine diameter
or cross-sectional thickness, preferably, about 0.01-0.3 cm or
less, and that compression molding or injection molding, for
example, could cause a dislocation force, pushing the resistance
heating wire 26 or other electrical device into the mold wall
surface. This could cause unintentional shorts or rejected
components.
[0034] In the preferred embodiment of this invention, the fibrous
support layers 114 comprise resilient nonconducting fibers, such as
those made from glass, boron, rayon, Kevlar aramid, or other
polymers, for example, fibers made from the same polymer as the
matrix of the polymer layer 113. More preferably, the supporting
layers include a pair of non-woven cut glass mats, such as
Dura-Glass glass mat from Johns Manville having an air permeability
rating of at least 1000 ft.sup.3/min. for thermosetting resins and
at least 500 ft.sup.3/min. for low viscosity thermoplastic resins.
Dura-Glass mats are relatively thin, non-woven, fiberglass mats
composed of cut glass filaments oriented in a random pattern,
bonded together with a resinous binder, such as modified
acrylic.
[0035] The fibrous support layers 114 of this invention can be
bonded with adhesive 116 to the resistance heating wire 26 or other
electrical device. Good examples of adhesives useful for this
purpose include 3M 77 and 4550 spray adhesives. More preferably,
the adhesive 116 is a low volatile content adhesive such as
acrylic, epoxy, silicon, phenolic, or ester-based adhesives, which
can be extruded or molded around a resistance heating wire 26, or
other electrical device, or applied as a paste between the
electrical device and the fibrous support layers 114. Such adhesive
substances can be thermally conductive, or include thermally
conductive additives, such as Al.sub.2O.sub.3, MgO, etc. in amounts
of about 30-70% wt. to increase thermal dissipation from the
electrical device and permit greater watt densities, such as those
greater than 5 watts/in.sup.2 without burning or melting the
polymer layer 113. Such thermally conductive, low volatile
containing adhesives should effectively wet the fibrous support
layers and electronic device, helping to join them together. They
also should minimize air gaps between the fibrous support layers
114 and the polymer layers 113, to create more homogenous heat
dissipation. They also can increase the tensile strength of the
element 100 in cross-sectional direction marked by line 3-3 of FIG.
1, by discouraging delamination along the fibrous support layers
114 during heating and cooling cycles.
Integrated Circuit Encapsulation
[0036] With reference to FIG. 4, there is shown an integrated
circuit encapsulation 300, including an integrated circuit or
"silicon chip" 305 having a plurality of leads 312. The chip 305 of
this embodiment can be a microprocessor or memory chip, for
example, or a small circuit board having a small cross-sectional
dimension, e.g., less than about 0.01-0.3 cm, and designed to be
encapsulated in a thermoplastic or thermosetting material. As shown
in FIG. 4, the integrated circuit 305 can be disposed on a first
fibrous support layer 324, which can have a pattern of conductor
paths 308 disposed thereon. Such paths can be copper traces
electroplated, photo etched, or applied with a foil and then laser
trimmed to produce an electrically conductive path to the edge of
the fibrous support layer 324. The support layer 324, together with
fibrous support layer 322, helped to provide structural support and
a more uniform heat dissipation from the silicon chip 305. The
fibrous support layers 324 and 322 can be made of the same
materials as the fibrous support layers 114 disclosed previously.
Ideally they are made of a non-conductive material, such as a
non-woven glass mat. After the fibrous support layers 324 and 322
are joined together with a suitable adhesive substance to entrap
the silicon chip 305, they are placed with the silicon chip 305
between a pair of polymeric layer preforms 310 and 320 in a
compression molding device. The polymeric layer preforms 310 and
320 can optionally include thermally conductive additives 314 and
fibrous reinforcement 316, as discussed previously for the heating
element embodiment 100.
Preferred Molding Operation
[0037] As shown in FIG. 5, a preferred flow diagram for
manufacturing heating elements and other electrical components is
provided. The operation begins by preparing a drawing of the device
on an appropriate computer program 201. A pin layout step 202 is
provided to locate an electrical resistance wire, for example, in a
circuit path. A two dimensional winding operation 204 is then used
to dispose the electrical resistance wire along the pin layout over
a bottom mat which is fed from a bottom mat feeder 203.
Simultaneously with the arrangement of the resistance wire along
the pin layout, a top sheet feeder 205 disposes a top fibrous
support layer into an adhesive operation 206 where the adhesive is
applied to at least the top fibrous support layer. Alternatively, a
foil, instead of a resistance wire, can be disposed on the bottom
mat and then laser etched to create a circuit path. Or, in a
further embodiment, a thermally conductive adhesive can be applied
to an electrical resistance wire which is then disposed and adhered
between the top and bottom fibrous support layers. Optionally,
stitching can be used instead of, or in addition to, adhesive.
[0038] The two fibrous support layers 114 are then joined with the
wire, foil or device in the capture step 210. The fibrous support
lay is then trimmed by a cutter at trim step 211 followed by a
termination step 212, in which the ends of the conductive
resistance heating wire are terminated by crimping a grommet and
cold pin to each end of the wire. Following termination, the
element precursor is inserted between a pair of polymer layers or
preforms. These polymer preforms can include an optional cavity
shape or curvature for receiving the wire or chip, or they can be
flat.
[0039] The polymer preforms are disposed on both sides of the
element precursor and then subject to a preferred compression
molding operation.
[0040] When epoxy is used as the polymeric resin, the platen
temperature is set at about the cure temperature for the resin, or
about 350-450.degree. F. The platens are adjusted to a compressive
load of about 500-4,000 PSI, which allows the epoxy to flow slowly
through the porous glass mat layers and permit the epoxy resin to
wet and bond to the glass and electrical resistance wire surfaces.
At this point, the resin has a viscous yet fluid nature for flowing
through the porous fibrous support layers.
[0041] It has been observed that when the polymer preforms are in
place in the mold, they tend to distribute the viscous epoxy
material uniformly throughout the mold, requiring less force to
liquefy. Usually less than 5,000 psi is required to liquefy and
permit the resin to flow, which is substantially less than most
injection molding operations which require over 10,000 psi
generally to fill the mold with a molten polymer.
[0042] It has also been observed that the preferred fibrous support
layers 114 of this invention, glass mats having an air permeability
rating of at least about 1,000 ft.sup.3/min., help to maintain the
stability of the flow of resinous material and create a ballast or
dampening factor as the liquefied epoxy flows through the glass
material. This permits the electrical component or resistance wire
to substantially hold its circuit path placement and location in
the mold during resin infiltration. It also substantially
eliminates a fracture initiation site along the glass mat during
use, since the resin wets and bonds to the glass fibers and
resistance wire and forms a substantially continuous phase through
the cross-section of the product.
[0043] From the foregoing, it can be realized that this invention
provides improved heating elements and other electrical devices
which contain fibrous support layers and improved processing
features for helping to develop improved properties. These
properties include greater wetting of the fibrous reinforcement and
more resistance to fracture along the fibrous reinforcement polymer
boundary, especially during heating cycles. Conductive polymer
materials are also disclosed for improved thermal dissipation from
electrical resistance wire or other devices to improve watt density
without burning the disclosed polymer coatings. Compression molding
techniques are also provided which include improved ballasting
techniques for easier molding and fibrous mat reinforcement for
increasing the strength of polymer heaters. Although various
embodiments have been illustrated, this is for the purpose of
describing and not limiting the invention. Various modifications,
which will become apparent to one skilled in the art, are within
the scope of this invention described in the attached claims.
* * * * *