Abstrict
An electric heater assembly (16) adaptable for battery operation
with hand held devices for attaching, sealing or shrinking heat
activated plastic or mylar film materials includes a heating element
(20) comprising a planar electrically insulating substrate (18)
having a heating element (20) being a thick film conductor delineated
and fired on one of its planar surfaces in a configuration that
realizes local high power density in the heating element and the
resultant production of localized high heat energy within the substrate
with minimized power consumption from an external electrical source.
Enclosing the heater assembly is a sheet metal sheath (24) to provide
mechanical protection and to effect heat transfer to a work. The
heater assembly is positioned, with the heating element facing up,
inside the sheath with its opposing surface atop of, and in intimate
contact with, a planar base (24a) of the sheath, a workface (26)
comprising an opposing side of the base.
Claims
I claim:
1. A portable hand held heating device for attaching, sealing or
shrinking heat activated plastic or mylar film materials comprising
a handle composed of an electrically insulating material and having
a distal end with a recess, a proximal end with an opening and an
internal cavity through the length of said handle and connecting
said recess and said opening;
a heater assembly projecting from said distal end of said handle
including a planar electrically insulating substrate having a heating
element comprising a thick film conductor delineated in a U-shaped
pattern and fired on one of its planar surfaces with its opposing
planar surface adapted to apply heat to an opposing side of a workface
by means of direct intimate contact to said opposing side of said
workface, the perimeter of said substrate wherein being in the shape
of an isosceles triangle with an elongated altitude and slightly
convex curved equal sides, a vertex of said isosceles triangle corresponding
to a frontal point of said substrate and an opposing base of said
isosceles triangle corresponding to a trailing edge of said substrate,
said U-shaped pattern having two enlarged conductor contact pads,
one each contact pad being located on and forming a terminus of
a U-leg, said contact pads being proximal to said trailing edge
of said substrate, said heating element wherein, when delineated
and fired, being of a suitable surface area and sheet resistivity
to be adaptable for battery operation when electric power is supplied
to said heating element from an external source of electrical energy;
means for enclosing said heater assembly to provide mechanical
protection, and to effect efficient heat transfer from said heater
assembly to said opposing side of said workface, said means being
a sheet metal sheath including:
a horizontal planar base with a trailing edge and with an upper
surface having a configuration the same as the configuration of
said substrate and with dimensions slightly larger than the dimensions
of said substrate and
three surrounding upstanding sides of equal height, two of said
sides being equal in length intersecting and mutually terminating
at one of their ends to form a leading edge of said sheath immediately
ahead of said frontal point of said substrate, a third rear side
of said sheath rising from said trailing edge of said base of said
sheath and mutually terminating with each of said two sides of equal
length at their opposing ends from said leading edge, said leading
edge and said rear side being swept back at identical angles relative
to said frontal point of said substrate and to said trailing edge
of said base, said upper surface of said base receiving placement
with proper alignment of said substrate with said heating element
face up and with said opposing planar surface of said substrate
in intimate contact with said upper surface of said base, and with
said workface comprising an opposing undersurface of said base;
means for attaching said heater assembly and said sheath to said
distal end of said handle comprising a mounting body composed of
an electrically insulating material of low thermal conductivity
shaped so as to form fit inside of said sheath thus holding said
leading edge of said sheath in proper position, said mounting body
having a recess of sufficient size in an undersurface in order to
rest atop said heater assembly while avoiding physical contact with
said heating element, an upper surface of said mounting body being
horizontal and planar, and having a mounting tongue projecting upward
from said upper surface and fitting into said recess in said distal
end of said handle, said mounting body held to said handle by means
of a mounting screw inserted through a hole bored laterally through
one side of said distal end, through said mounting tongue, and emerging
from an opposing side of said distal end, said screw being secured
by a nut on said opposing side, said means wherein further including
a spring wire latch bent in a manner which allows one end of said
latch to be inserted into a small hole located in an approximate
center of a planar face of said rear side of said sheath with an
opposing end of said latch bent so as to engage a spring wire hook
anchored on said handle near said distal end of said handle thereby
securing said sheath to said handle;
means for providing electrical continuity from said heater assembly
to said external source of electrical energy comprising two metal
leaf spring contacts, each said contact being bent near one of its
ends to allow mounting to said mounting body with each said contact
being mounted within said recess of said mounting body by means
of a screw, one said screw for each said contact being inserted
through a hole located in said end of each said contact and through
a hole bored to accommodate each said screw through said recess
in said undersurface of said mounting body and emerging at said
upper surface of said mounting body and secured by a nut, an opposing
unattached end of each said contact being bent near each said unattached
end in a manner to permit alignment and to make electrical contact
in registry with said contact pads of said thick film conductor,
each said screw and said nut being connected to said external source
of electrical energy by means of electrical leads extending through
said recessed distal end, said internal cavity and said opening
in said proximal end of said handle.
2. A portable hand held heating device for attaching, sealing or
shrinking heat activated plastic or mylar film materials comprising
a handle composed of an electrically insulating material and having
a distal end with a recess, a proximal end with an opening and an
internal cavity through the length of said handle and connecting
said recess and said opening;
a heater assembly adaptable for battery operation projecting from
said distal end including a planar electrically insulating substrate
having a heating element comprising a thick film conductor delineated
in a U-shaped pattern and fired on one of its planar surfaces with
its opposing planar surface adapted to apply heat to an opposing
side of a workface by means of direct intimate contact to said opposing
side of said workface, the perimeter of said substrate wherein being
in the shape of an isosceles triangle with an elongated altitude
and slightly convex curved equal sides, a vertex of said isosceles
triangle corresponding to a frontal point of said substrate and
an opposing base of said isosceles triangle corresponding to a trailing
edge of said substrate, said U-shaped pattern wherein having two
enlarged conductor contact pads, one each contact pad being located
on and forming a terminus of a U-leg, said contact pads being proximal
to said trailing edge of said substrate, said U-shaped pattern wherein,
excluding contact pads, having an electrical sheet resistivity and
surface area resulting in a power density in a range of 11.6-20.0
watts per square centimeter (75-130 watts per square inch) when
electric power in a range of 3.0-6.0 watts is supplied from an electric
battery or equivalent electrical source to said heating element,
said ranges wherein being suitable to cause sufficient heating of
said heating element and said workface to permit said attaching,
sealing and shrinking of said plastic and mylar films by said workface;
means for enclosing said heater assembly to provide mechanical
protection, and to effect efficient heat transfer from said heater
assembly to said opposing side of said workface, said means being
a sheet metal sheath including:
a horizontal planar base with a trailing edge and with an upper
surface having a configuration the same as the configuration of
said substrate and with dimensions slightly larger than the dimensions
of said substrate and
three surrounding upstanding sides of equal height, two of said
sides being equal in length intersecting and mutually terminating
at one of their ends to form a leading edge of said sheath immediately
ahead of said frontal point of said substrate, a third rear side
of said sheath rising from said trailing edge of said base of said
sheath and mutually terminating with each of said two sides of equal
length at their opposing ends from said leading edge, said leading
edge and said rear side being swept back at identical angles relative
to said frontal point of said substrate and to said trailing edge
of said base, said upper surface of said base receiving placement
with proper alignment of said substrate with said heating element
face up and with said opposing planar surface of said substrate
in intimate contact with said upper surface of said base, and with
said workface comprising an opposing undersurface of said base;
means for attaching said heater assembly and said sheath to said
distal end of said handle comprising a mounting body composed of
an electrically insulating material of low thermal conductivity
shaped so as to form fit inside of said sheath thus holding said
leading edge of said sheath in proper position, said mounting body
having a recess of sufficient size in an undersurface in order to
rest atop said heater assembly while avoiding physical contact with
said heating element, and upper surface of said mounting body being
horizontal and planar, and having a mounting tongue projecting upward
from said upper surface and fitting into said recess in said distal
end of said handle, said mounting body held to said handle by means
of a mounting screw inserted through a hole bored laterally through
one side of said distal end, through said mounting tongue, and emerging
from an opposing side of said distal end, said screw being secured
by a nut on said opposing side, said means wherein further including
a spring wire latch bent in a manner which allows one end of said
latch to be inserted into a small hole located in an approximate
center of a planar face of said rear side of said sheath with an
opposing end of said latch bent so as to engage a spring wire hook
anchored on said handle near said distal end of said handle thereby
securing said sheath to said handle;
means for providing electrical continuity from said heater assembly
to said electrical source comprising two metal leaf spring contacts,
each said contact being bent near one of its ends to allow mounting
to said mounting body with each said contact being mounted within
said recess of said mounting body by means of a screw, one said
screw for each said contact being inserted through a hole located
in said end of each said contact and through a hole bored to accomodate
each said screw through said recess in said undersurface of said
mounting body and emerging at said upper surface of said mounting
body and secured by a nut, an opposing unattached end of each said
contact being bent near each said unattached end in a manner to
permit alignment and to make electrical contact in registry with
said contact pads of said thick film conductor, each said screw
and said nut being connected to said electrical source by means
of electrical leads extending through said recessed distal end,
said internal cavity and said opening in said proximal end of said
handle.
Description BACKGROUND--FIELD OF INVENTION
This invention relates to heater assemblies comprising thick film
electrical conductors formed on electrically insulated substrates
to provide local high power density with minimized power consumption
from an external electrical source and it also relates to the reduction
of heat loss from the heater assemblies to the surroundings through
other than a workface.
BACKGROUND--DESCRIPTION OF PRIOR ART
Applicant is unaware of any electric heater assemblies adapted
for use in hand held heat attaching, sealing, or shrinking devices
for heat activated plastic or mylar film materials using a thick
film conductor on an electrically insulating substrate to provide
local high power density with resultant high heat energy supplied
to a work with minimized power consumption from an external electrical
source in a manner that reduces heat loss to surroundings through
other than a workface.
Heater assemblies for attaching, sealing or shrinking of heat activated
plastic or mylar films such as are used for covering model aircraft
are well known and typically consist of metal resistance ribbon
or wire wrapped around a planar piece of insulating material and
placed in a metal shoe which utilizes the sole of the shoe as a
workface. The ends of the ribbon or wire are connected to terminals
which, in turn, are connected to an external electrical source.
Due to the necessity of providing an air gap between the heater
assembly and the shoe in order to electrically isolate the heating
element there is no intimate contact between the heater assembly
and the shoe resulting in substantial reduction in heating efficiency.
In addition this construction results in a bulky device requiring
unnecessary expenditure of electrical energy, thus precluding direct
current, or, more specifically, battery operation. Alternate approaches
in which heater assemblies housing a resistance element sandwiched
between layers of insulating material all encased in a metal enclosure
are also known as shown in U.S. Pat. Nos. 3,808,573, in 4,571,482,
and 5,081,340. A heating unit comprising a thick film heating element
and sensor is described in U.S. Pat. No. 4,859,835 in which the
intrinsic temperature coefficient of resistivity of the thick film
material is utilized to limit the power input to the heating element
by means of a control circuit. None of these inventions, however,
are adapted for use as the tool portion of a hand held, low power
consumption heat attaching, sealing or shrinking device for heat
activated plastic or molar films, nor do they disclose my heater
assembly of thick film conductor on an electrically insulating substrate
formed and disposed in a configuration that results in local high
power density, highly efficient heat transfer to a workface, reduced
heat loss from within the heater assembly, and reduced power consumption
from an external electrical source thus making possible the use
of direct current or battery powered, as well as alternating current
electrical sources.
OBJECTS AND ADVANTAGES
It is therefore the primary object of this invention to provide
an electrical heater assembly adapted to be used in battery powered,
portable hand held devices for attaching, sealing or shrinking heat
activated plastic or mylar films.
It is another object of the present invention to provide a heater
assembly comprising a thick film conductor delineated and fired
on an electrically insulated substrate in a configuration that produces
local high power density and efficient heat transfer through a workface
and minimizes power consumption from an external electrical source
thus rendering possible the use of direct current and battery powered,
as well as alternating current electrical sources.
It is a further object of the present invention to provide a heater
assembly that reduces heat loss from within said heater assembly
through surroundings other than a workface.
It is yet another object of the present invention to provide a
heater assembly which combines simplicity and durability but yet
is inexpensive in construction.
These and other objects will in part be obvious and in part pointed
out in the following description and accompanying drawings which
are merely illustrative of the present invention.
The foregoing objects are obtained by providing a heater assembly
comprising a heating element being a thick film conductor pattern
delineated and fired on one planar surface of a planar electrically
insulating substrate formed in a configuration that realizes local
high power density in the conductor, with the resultant production
of localized high heat energy within the substrate, with minimized
power consumption from an external electrical source. Enclosing
the heater assembly is a sheath made of sheet metal such as stainless
steel to provide mechanical protection and efficient heat transfer
from the heater assembly through a workface which comprises a planar
undersurface of the sheath. Also partially enclosed within the sheath
is a dielectric mounting body having low thermal conductivity for:
(a) aiding in maintaining proper position of the heater assembly
and sheath; and
(b) serving as a mount for electrical contacts and external wire
connections; and
(c) reducing heat loss through surroundings other than the workface;
and
(d) providing a means of attaching the heater assembly and sheath
to a device handle.
A simple spring wire latch connecting a rear side of the sheath
to the handle is provided for securing the sheath in position.
DRAWING FIGURES
FIG. 1 is a perspective view of the enclosure of my low power sealer
connected to a handle,
FIG. 2 is an exploded view of the heater assembly, sheath, elements
and mounting body in the preferred embodiment,
FIG. 3 is a cutaway view of the heater assembly and mounting body,
FIG. 4 is a plane view of the heater assembly, and
FIG. 5 is a perspective view taken from outside the right rear
of the sheath and distal end of the handle.
DESCRIPTION OF THE PREFERRED EMBODIMENT--FIGS. 1 THROUGH 5
Referring now to the drawings where like characters of reference
indicate like elements in each figure, 10 indicates generally, as
shown in FIG. 1, a heat attaching, sealing, and shrinking device
for heat activated plastic or mylar films with an electric heater
assembly 12 of my low power sealer held by a mounting body 22 which
is attached to a handle 14, the mounting body 22 having a pair of
terminal screws 34 and 34' secured by two nuts 35 and 35' connected
to a pair of external leads 36 and 36'.
As shown in FIG. 2, the enclosure 12 includes a heater assembly
16 and a sheet metal sheath 24.
The heater assembly 16 comprises a planar dielectric clad metal
substrate 18, in the shape of an isosceles triangle with an elongated
altitude and slightly convex curved equal sides as best seen in
FIG. 4. The substrate 18 has a frontal point 18a and a trailing
edge 18b corresponding respectively to a vertex and an opposing
base of the isosceles triangle. Delineated and fired in a U-shaped
pattern on an upper planar surface of the substrate 18 is a heating
element 20 comprising a thick film conductor with two enlarged conductor
contact pads 28 and 30. The contact pads 28 and 30 extend from points
29 and 31 respectively to a terminus of each U-leg of the heating
element 20 with the termini proximal to the trailing edge 18b of
the substrate 18. The particular operating characteristics of the
heating element 20 and the functions of the contact pads 28 and
30 will be more fully described later.
As can be seen in FIG. 2 and FIG. 3 the heater assembly 16, with
the heating element 20 face up, is enclosed inside the sheet metal
sheath 24 atop of, and intimate contact with, a planar horizontal
base 24a of the sheath 24. The configuration and dimensions of the
upper surface of the base 24a are similar to and with dimensions
slightly larger than the substrate 18 to permit proper positioning
of the substrate 18. A workface 26 comprises the undersurface of
the base 24a of the sheath 24. The sheath 24 has three upstanding
sides of equal height, with two of the sides, being equal in length,
intersecting and mutually terminating at one each of their ends
to form a leading edge 24b of the sheath 24 immediately ahead of
the frontal point 18a of the substrate 18. A third, rear side 24c
of the sheath 24 rises from a trailing edge 24d of the base 24a
and mutually terminates with each of the two sides of equal length
at their opposing ends from the leading edge 24b. Both the leading
edge 24b and the rear side 24c of the sheath 24 are swept back at
identical angles relative to the frontal point 18a of the substrate
18 and the trailing edge 24d of the base 24a. Means, for attaching
the heater assembly 16 and the sheath 24 to the handle 14, and,
to aid in making external electrical connections are provided by
a mounting body 22 formed from insulating material of low thermal
conductivity. The mounting body 22, with its lower portion contoured
to form fit within the sheath 24 atop the heater assembly 16, has
a planar horizontal upper surface with a mounting tongue 23 projecting
from said upper surface above the enclosure of the sheath 24, the
tongue 23 fitting into a recessed distal end (not shown) of the
handle 14. The lower portion of the mounting body 22 serves in part
to maintain proper positioning of the heater assembly 14 with the
mounting body 22. The underside of the mounting body 22 is recessed
in order to (a) minimize physical contact with the heater assembly
16 thus reducing heat loss, and (b) accommodate a pair of sheet
metal conductor contacts 32 and 32' made of leaf spring material
such as phosphorous bronze, and (c) avoid physical contact with
the heating element 20 excepting upon the conductor contact pads
28 and 30 by the conductor contacts 32 and 32'. As can be seen in
FIG. 2 and FIG. 3 the contacts 32 and 32' are bent near both of
their ends in a manner to allow attachment to the mounting body
22 and to make electrical contact with contact pads 28 and 30. The
attachment of the conductor contacts 32 and 32' to the mounting
body 22 is accomplished by inserting a pair of terminal screws 34
and 34' through holes in one end of the conductor contacts 32 and
32' and in the recessed underside of the mounting body 22 and securing
on the opposing upper surface of the mounting body 22 by two nuts
35 and 35'. The unattached ends of the contacts 32 and 32' are bent
to align and make electrical contact in registry with conductor
contact pads 28 and 30. As shown in FIG. 1 the terminal screws 34
and 34' and nuts 35 and 35' are connected to the external leads
36 and 36' as previously mentioned, through a recessed distal end,
through an internal cavity, and, through an opening in a proximal
end of the handle 14.
The sheath 24, being held in position in part by the contours of
the mounting body 22, is secured to the handle 14, as indicated
in FIG. 5, by means of a simple spring wire latch 38 bent in such
a manner as to be inserted with one end into a small hole 24e, located
in the approximate center of the rear side 24c of the sheath 24,
shown in FIG. 2, and engaged at it's opposing end through a small
spring wire hook 39 anchored near the distal end of the handle 14.
As previously mentioned, the tongue 23 of the mounting body 22 fits
inside the recessed distal end of the handle 14 and is attached
therewith by means of a screw inserted through aligning holes on
each side of the recessed distal end of the handle 14 and through
the tongue 23 and held by a nut on the emergent end of the aforementioned
screw, thus securing the mounting body 22 and the heater assembly
12 to the handle 14.
The heating element 20 comprising a thick film conductor material
is composed in part of a metal, metal oxide or metal alloy or combinations
thereof such as Conductor Paste Number C-4800 by Hereaus Cermalloy
of West Conshohocken, Pa. The paste may be applied to the substrate
by screen printing and firing at approximately 600.degree. C. in
clean dry air. For the purpose of the present invention any thick
film conductor material having an electrical sheet resistivity that
results in a power density in the range of 11.6-20 volts per square
centimeter (75-130 watts per square inch) of delineated and fired
conductor area, when a prespecified amount of electric power is
supplied from an external electric source to the thick film conductor,
is suitable.
The mounting body 22 may be molded from an epoxy such as Stycast.RTM.
2662 by Emerson and Cuming of Woburn, Mass., or machined or molded
from any other material having similar properties that can endure
operating temperatures in the range of 15.degree. C.-250.degree.
C. without degradation. The epoxy may be cast in a mold such as
silicone and cured at 175.degree. C.-200.degree. C. for approximately
6 hours.
The electrically insulating substrate 18 is approximately 1.0-1.8
millimeter in thickness and may comprise a material such as porcelain
steel.
Theory of Operation
In defining generalized operating criteria for my low power sealer
it was empirically determined that in order to achieve effective
heating characteristics:
(a) power densities in the range of 11.6-20 watts per square centimeter
(75-130 watts per square inch) of conductor area, excluding contact
pads, are required,
(b) the maximum percentage of thick film conductor area coverage
of a substrate, excluding contact pads, is less than or equal to
30 percent of the total area of one planar face of the substrate
in order to minimize the total power consumption from an external
electrical source,
(c) the minimum ratio of the thick film conductor line width, again
excluding contact pads, to the thickness of the substrate is 0.65
in order to promote rapid, even heat transfer within the body of
the substrate and between the substrate and a work surface, and
(d) there exists intimate contact between the heater assembly and
the work surface.
In the preferred embodiment of my low power sealer the total thick
film conductor area coverage of the substrate, excluding contact
pads, is approximately 25 percent of the area on one planar face.
The ratio of the thick film conductor line width, excluding contact
pads, to the thickness of the substrate is nominally unity. The
power density is 11.6 watts per square centimeter (75 watts per
square inch) of delineated and fired thick film conductor area,
excluding contact pads, and my sealer is operable, but not limited
to such operation, on 3.0 watts of power from an external electrical
source.
The utilization of 3 watts of external power in a thick film conductor
pattern having a power density of 11.6 watts per square centimeter
(75 watts per square inch) of conductor area results in a total
conductor area of 0.25 centimeter (0.04 square inch). By extrapolation,
utilization of the previously stated higher power density of 20
watts per square centimeter (130 watts per square inch) of conductor
in the above conductor area of 0.25 centimeter (0.04 square inch)
requires an external power input of approximately 6 watts. Either
case renders the heater assembly adaptable for battery operation.
Operation--FIGS. 1,2,3
In operation, as electric current is applied across the heating
element 16 at terminal screws 34 and 34' contacts 32 and 32' and
contact pads 28 and 30, the temperature of the heating element 20
rises rapidly due to the influx of electrical energy into a relatively
small conductor area and resultant local high power density.
Effective heat distribution within the body of the substrate 18
from the heating element 20, and, heat transfer to the base 24a
and the workface 26 are achieved because of the essentially matching
dimensions of the line width of the heating element 20, excluding
contact pads 28 and 30, and the thickness of the substrate 18, and
because of the intimate contact existing between the substrate 18,
the base 24a and hence the workface 26. Power consumption from the
external electric source is minimized due to the reduced area coverage
of the heating element 20 on the substrate 18. Heat loss to the
surroundings other than through the workface 26 is reduced by the
recess in the mounting body 22 permitting only necessary physical
contact with the heater assembly 16, and, the low thermal conductivity
composition of the mounting body 22.
SUMMARY, RAMIFICATIONS AND SCOPE
Accordingly, the reader will see that my low power sealer offers
several advantages, specifically: local high power density with
effective heating characteristics within the heating element, efficient
heat transfer between the heating element and the workface, reduced
heat loss, and minimized electric power consumption. Furthermore,
the heater assembly has the additional advantages that:
Direct current or battery operation is feasible; thus making the
device portable;
Energy savings in alternating current powered devices can result;
and
Construction is simple making my low power sealer readily producible
at low cost.
While a preferred embodiment of my low power sealer has been shown
and described, it will be realized that modifications may be made
thereto without departing from the scope of the following claims.
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