U.S. patent application number 09/782352 was filed with the patent office on 2002-04-11 for heated food service shelf for warming cookies and the like.
Invention is credited to Arx, Theodore Von, Laken, Keith, Schlesselman, John W..
Application Number | 20020040901 09/782352 |
Document ID | / |
Family ID | 24575681 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020040901 |
Kind Code |
A1 |
Laken, Keith ; et
al. |
April 11, 2002 |
Heated food service shelf for warming cookies and the like
Abstract
A heating element assembly in the form of a heating shelf and a
method of manufacturing heating shelf assemblies. The heating shelf
may be used in display cabinets to heat ready made foods such as
cookies, muffins, donuts, pizza, sandwiches and the like. The
preferred heating shelf includes thermochromic materials, or an LED
indicator, which provide a visual indica of shelf temperature. The
preferred heating shelf provides intimate contact with the heated
food products, thus optimizing heat transfer between the heating
shelf and the food products. Optionally provided, varied surface
watt density in the heating shelf allows for accurate heat
placement such that the food products can be evenly warmed while
avoiding over warming. In another embodiment, the heating shelf
includes two resistance heating elements. The first heating element
is a temperature booster used for defrosting and heating, while the
second heating element is a maintenance heater to maintain heated
food at a serving temperature.
Inventors: |
Laken, Keith; (Winona,
MN) ; Schlesselman, John W.; (Fountain City, WI)
; Arx, Theodore Von; (La Crescent, MN) |
Correspondence
Address: |
WILLIAM H. MURRAY
DUANE MORRIS & HECKSCHER LLP
ONE LIBERTY PLACE
PHILADELPHIA
PA
19103-7396
US
|
Family ID: |
24575681 |
Appl. No.: |
09/782352 |
Filed: |
February 12, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09782352 |
Feb 12, 2001 |
|
|
|
09642215 |
Aug 18, 2000 |
|
|
|
Current U.S.
Class: |
219/544 ;
219/466.1; 219/468.1; 219/549 |
Current CPC
Class: |
B29K 2027/18 20130101;
B29K 2069/00 20130101; B29K 2071/00 20130101; B29K 2023/10
20130101; B29K 2079/08 20130101; B29K 2081/06 20130101; B29K
2081/04 20130101; B29C 66/82661 20130101; B29C 66/91221 20130101;
B32B 2305/34 20130101; B29C 65/348 20130101; B29C 66/729 20130101;
B29C 66/91411 20130101; H05B 2203/004 20130101; B29C 51/02
20130101; B29C 66/71 20130101; B29C 66/71 20130101; B29C 2793/0081
20130101; B29C 65/1445 20130101; B29C 66/71 20130101; B32B 27/00
20130101; B32B 37/206 20130101; B29C 65/3444 20130101; B29C 66/432
20130101; Y10T 29/49082 20150115; B29C 51/00 20130101; B29K
2023/0683 20130101; B29K 2079/085 20130101; B29K 2023/12 20130101;
B29C 66/4326 20130101; B29C 66/71 20130101; B29C 66/91214 20130101;
B29C 65/1458 20130101; B29C 66/71 20130101; B29C 66/81831 20130101;
H05B 2203/003 20130101; B29C 65/1464 20130101; B29C 66/133
20130101; B65D 81/3476 20130101; B65D 2581/3428 20130101; H05B
2203/014 20130101; B29C 65/3492 20130101; B29C 66/81871 20130101;
B29K 2023/12 20130101; H05B 2203/037 20130101; B29C 65/3448
20130101; B29C 65/3488 20130101; B29C 66/91421 20130101; Y10T
29/49083 20150115; B32B 1/02 20130101; B29C 65/3428 20130101; H05B
2203/013 20130101; B29C 51/12 20130101; B29C 65/342 20130101; B29C
66/73921 20130101; B29C 65/3468 20130101; B29C 70/82 20130101; B29C
66/71 20130101; B29C 65/3412 20130101; B32B 3/08 20130101; B32B
37/06 20130101; H05B 2203/017 20130101; B29C 66/4312 20130101; B29C
66/433 20130101; A47J 36/2483 20130101; B29C 70/885 20130101; B29C
65/1406 20130101; B29C 66/45 20130101; B29C 66/71 20130101; B29C
66/83413 20130101; B32B 2310/022 20130101; B29C 53/04 20130101;
B29C 66/91218 20130101; H05B 3/36 20130101; B29C 66/71 20130101;
B29C 66/71 20130101; B29C 66/71 20130101; B32B 3/06 20130101; B32B
2310/0806 20130101; Y10T 29/49085 20150115; B29C 66/431 20130101;
B29C 66/91645 20130101; B29C 66/91655 20130101; B29L 2031/779
20130101; B29C 65/346 20130101; B29C 66/71 20130101; B29C 65/02
20130101; H05B 2203/002 20130101 |
Class at
Publication: |
219/544 ;
219/466.1; 219/468.1; 219/549 |
International
Class: |
H05B 003/68; H05B
003/36 |
Claims
We claim:
1. A method of manufacturing a heating shelf, comprising the steps
of: (a) disposing at least one resistance heating element between
first and second thermoplastic sheets, at least of the
thermoplastic sheets having a visible feature that changes with
temperature, each of the at least one resistance heating elements
comprising: (i) a supporting substrate; and (ii) an electrical
resistance heating material, wherein the electrical resistance
heating material is one of attached to and supported in the
substrate, the electrical resistance heating material forming a
circuit path; (b) laminating the first and second thermoplastic
sheets such that each of the at least one resistance heating
element is secured between the first and second thermoplastic
sheets to form a reformable structure; and (c) forming the
structure into a heating shelf.
2. The method of claim 1 wherein the at least one thermoplastic
sheet, having a visible feature that changes with temperature, is
one of a thermochromic material and an LED indicator.
3. The method of claim 1 wherein each heating element further
comprises: at least one flap portion, capable of rotation about a
first axis of rotation, at least one of the circuit paths
continuing onto the flap portion, wherein the step of forming
includes rotating the flap portion about the first axis to provide
resistance heating in at least two planes.
4. The method of claim 1, wherein said step of laminating includes
the steps of heating said thermoplastic sheets and compressing said
thermoplastic sheets to laminate the resistance heating elements
between the thermoplastic sheets.
5. The method of claim 1, wherein said step of forming includes the
step of thermoforming the reformable structure into the heating
shelf, whereby said supporting substrate and electrical resistance
material resist forces which are capable of breaking or shorting
said circuit path.
6. The method of claim 1, further comprising the step of cutting
the continuous element structure into a foldable profile before
forming the continuous reformable structure into the heating
shelf.
7. The method of claim 1, further comprising the steps of: (d)
energizing at least one of the resistance heating elements to
soften the thermoplastic sheets; and (e) overmolding the heating
shelf with a thermoplastic, the steps of energizing and overmolding
timed such that the thermoplastic sheets and over molded
thermoplastic form a substantially homogenous structure.
8. A method of manufacturing a heating shelf, comprising the steps
of: (a) disposing at least one resistance heating element between
first and second thermoplastic sheets, the at least one resistance
heating element comprising: (i) a supporting substrate; and (ii) at
least one circuit path, each of the circuit paths comprising an
electrical resistance heating material attached to the supporting
substrate, at least one of the circuit paths having terminal end
portions, at least one of the circuit paths continuing onto a first
flap portion of the substrate capable of rotation about a first
axis of rotation; and (b) laminating the first and second
thermoplastic sheets such that the at least one resistance heating
element is secured between the first and second thermoplastic
sheets; (c) attaching a material having a visible feature that
changes with temperature to the heating shelf.
9. The method of claim 8 wherein the material having a visible
feature that changes with temperature is one of a thermochromic
material and LED indicator.
10. The method of claim 8 wherein the step of attaching comprises
laminating the material having a visible feature that changes with
temperature to the heating shelf.
11. The method of claim 9, wherein the thermochromic material is
disposed between the first and second thermoplastic sheets.
12. A method of manufacturing a sheet of heating element
assemblies, comprising the steps of: (a) disposing at least one
sheet of resistance heating elements between first and second
thermoplastic sheets, at least one of the thermoplastic sheets
having a visible feature that changes with temperature, each of the
resistance heating elements attached to a supporting substrate and
forming a circuit path, at least one of the circuit paths having
terminal end portions, at least one of the circuit paths continuing
onto a first flap portion of the substrate capable of rotation
about a first axis of rotation; and (b) laminating the first and
second thermoplastic sheets such that the at least one sheet of
resistance heating elements is secured between the first and second
thermoplastic sheets to form a reformable structure.
13. The method of claim 12 wherein the at least one thermoplastic
sheet having a visible feature that changes with temperature, is
one of a thermochromic material and LED indicator.
14. The method of claim 12, further comprising the steps of
removing at least one heating element assembly from the sheet of
heating element assemblies, the removed heating element assembly
being a reformable structure, and forming the reformable structure
into a final element assembly configuration wherein at least the
first flap portion of the substrate is rotated about the first axis
to provide resistance heating in at least two planes.
15. The method of claim 12, further comprising the steps of cutting
at least one of the heating element assemblies into a foldable
profile before forming the reformable structure into the final
element assembly configuration.
16. The method of claim 15, wherein said step of cutting includes
the step of stamping or die cutting at least one of the heating
element assemblies into the profile.
17. A heating element assembly, comprising: (a) a first
thermoplastic sheet; (b) a second thermoplastic sheet, at least one
of the thermoplastic sheets having a visible feature that changes
with temperature; and (c) a resistance heating element secured
between the first and second thermoplastic sheets, the resistance
heating element being attached to a supporting substrate and
forming a at least one circuit path having terminal end portions,
at least one of the circuit paths continuing onto a first flap
portion of the substrate capable of rotation about a first axis of
rotation, wherein the thermoplastic sheets and resistance heating
element are laminated together to form a reformable structure, the
reformable structure formed into a final element assembly
configuration wherein at least the flap portions is rotated about
the first axis to provide resistance heating in at least two
planes.
18. The heating element assembly of claim 17 wherein the at least
one thermoplastic sheet having a visible feature that changes with
temperature, is one of a thermochromic material and LED
indicator.
19. The heating element assembly of claim 18, wherein the
thermoplastic sheets are affixed with an adhesive.
20. The heating element assembly of claim 18, wherein the
thermoplastic sheets are attached by one of fusing and
laminating.
21. The heating element assembly of claim 17, wherein the
reformable structure is thermoformed into said final element
assembly configuration.
22. The heating element assembly of claim 17, wherein the
reformable structure is cut into a foldable profile.
23. The heating element assembly of claim 17, wherein the
electrical resistance heating material is at least one of glued,
sewn and fused to the supporting substrate.
24. The heating element assembly of claim 17, wherein the
electrical resistance heating material is sewn to said supporting
substrate with a thread.
25. The heating element assembly of claim 17, wherein the
supporting substrate comprises at least one of a woven and
non-woven fibrous layer.
26. The heating element assembly of claim 17, wherein the
supporting substrate is a thermoplastic sheet.
27. The heating element assembly of claim 17, wherein the
supporting substrate includes thermally conductive additives.
28. The heating element assembly of claim 17, wherein at least one
of the thermoplastic sheets includes a thermally conductive
coating.
29. The heating element assembly of claim 17, further comprising a
secondary device secured between the first and second thermoplastic
sheets.
30. The heated element assembly of claim 17, wherein one of the
thermoplastic sheets is thicker than the other thermoplastic
sheet.
31. The heating element assembly of claim 17, wherein the heating
element assembly is over molded with a thermoplastic such that the
over molded thermoplastic and thermoplastic sheets form a
substantially homogenous structure.
32. The heating assembly of claim 17, wherein at least one circuit
path is a continuous loop, which is capable of being energized by
at least one of high frequency radiation and magnetic
induction.
33. The heating assembly of claim 28, wherein the secondary device
is one of, a thermistor, a sensor and a thermocouple.
34. The heating assembly of claim 17, wherein at least one of the
thermoplastic sheets is Polyetherimide.
35. The heating assembly of claim 17 wherein the final element
assembly is hermetically sealed.
36. The heating assembly of claim 17, wherein the heating element
assembly has a bottom and the circuit path density in the bottom of
the heating element assembly is greater than the circuit path
density in the flap portions.
37. The heating assembly of claim 17, wherein the flap portions are
outwardly flared to provide for nested engagement with a second
identical heating element assembly.
38. A method of manufacturing a sheet of heating element
assemblies, comprising the steps of: (a) disposing at least one
sheet of resistance heating elements between first and second
thermoplastic sheets, the at least one sheet of resistance heating
elements being attached to a supporting substrate and forming a
plurality of spaced apart circuit paths each of the circuit paths
having terminal end portions and each of the circuit paths
continuing onto a first flap portion of the substrate capable of
rotation about a first axis of rotation, wherein the thermoplastic
sheets and resistance heating element are laminated together to
form a reformable structure, the reformable structure formed into a
final element assembly configuration where the flap portion is
rotated about the first axis to provide resistance heating in at
least two planes. (i) a supporting substrate; and (ii) at least one
circuit path, each of the circuit paths comprising an electrical
resistance heating material attached to the supporting substrate,
at least one of the circuit paths having terminal end portions, at
least one of the circuit paths continuing onto a first flap portion
onto a first flap portion of a resistance heating element capable
of rotation about a first axis of rotation; and (b) disposing a
sheet of material having a visible feature that changes with
temperature between the first and second thermoplastic sheets. (c)
attaching the first and second thermoplastic sheets such that the
at least one sheet of resistance heating elements is secured
between the first and second thermoplastic sheets to form a
continuous element structure, wherein the first and second
thermoplastic sheets and resistance heating elements are laminated
such that the sheet of resistance heating elements is secured
between the first and second thermoplastic sheets to form a
reformable structure.
39. The method of claim 38 wherein the sheet of material having a
visible feature that changes with temperature, is
thermochromic.
40. The method of claim 38 wherein the sheet of heating element
asemblies further compries an adhesive attaching said first and
second thermoplastic sheets. changes with temperature, is
thermochromic.
41. The method of claim 38 wherein the first and second
thermoplastic sheets are attached by one of fusing and
laminating.
42. The method of claim 38 wherein the electrical resistance
heating material is at least one of glued, sewn and fused to the
supporting substrate.
43. The method of claim 38 wherein said electrical resistance
heating material is sewn to said supporting substrate with a
thread.
42. The method of claim 38 wherein the supporting substrate
comprises at least one of a woven and non-woven fibrous layer.
43. The method of claim 38, wherein the supporting substrate is an
extruded thermoplastic sheet.
44. The method of claim 38 wherein the heating element assembly
further comprises a plurality of secondary devices, each of said
secondary devices disposed between said first and second
thermoplastic sheets and associated with one of said circuit
paths.
45. The method of claim 38 wherein at least one of the
thermoplastic sheets includes a thermally conductive coating.
46. A heating shelf, comprising: (a) a first thermoplastic sheet;
(b) a second thermoplastic sheet; (c) a resistance heating element
disposed between the first and second thermoplastic sheets, the
resistance heating element comprising: (i) a supporting substrate
including at least one circuit path, comprising an electrical
resistance heating material attached to, or disposed within, the
supporting substrate, said circuit path having terminal end
portions and continuing onto the flap portion of the substrate; and
(d) a material having a visible feature that changes with
temperature attached to the heating shelf, wherein the
thermoplastic sheets and said electrical resistance heating
material are laminated together to form a reformable structure, the
reformable structure formed into a final element assembly wherein
the flap portion is rotated about the first axis to provide
resistance heating in at least two planes.
Description
FIELD OF THE INVENTION
[0001] This invention relates to electrical resistance heating
elements, and more particularly to formable thermoplastic laminate
heating element assemblies.
BACKGROUND OF THE INVENTION
[0002] Methods for providing reformable heating element assemblies
are described in Applicant's co-pending application Ser. No.
09/642,215, herein incorporated in its entirety by reference.
[0003] In the food service industry, display cabinets are commonly
used to display food products for retail sale. As an example, many
convenience stores have display cabinets that may feature varied
food products such as donuts, muffins, cookies and the like. Heated
food service cabinets are also used in nursing homes and hospitals
and in food service applications on board airliners and cruise
ships. Often times, these cabinets are fitted with heated shelves,
which keep the foods warmed to desired serving temperatures. The
present method for applying heat to shelving is to attach tubular
elements to a sheet metal framework that is attached to the bottom
side of a shelf. The sheet metal framework provides a means of
electrical enclosure, preventing exposure to live electrical parts.
However, the resulting heatable shelf assemblies average
approximately 2 inches in thickness, thus providing an inefficient
use of limited cabinet space. Further, such assemblies are
expensive to manufacture, distribute and maintain.
[0004] Electrically heated steel shelves may also pose significant
safety risks to food service workers and consumers. Because heated
steel shelves typically lack visible features to indicate the
presence of heat, workers and consumers are susceptible to burn
injuries as they remove foods from the heated steel shelves.
Moreover, humans may be exposed to significant electrical hazards
through contact with the electrically charged metal shelves.
[0005] Therefore, improved apparatus and methods for heated cabinet
shelving are desirable. The ideal heating shelf would eliminate the
risk of electrical hazard by insulating the user from direct
contact with resistance heating elements. The preferred shelf
design would also include one or more visible features that change
with heat, to provide a readily perceptible heat indicia. In
addition, the preferred heating shelf would include multiple
resistance heating elements to provide both temperature boosting
for initial heating, and maintenance heating for maintaining heated
foods at a serving temperature. The preferred design would also be
adaptable to for use with existing cabinet designs, while providing
for improved utilization of existing cabinet space. Finally, the
improved heatable shelf design would be cost effective to produce
and operate.
SUMMARY OF THE INVENTION
[0006] The present invention provides a heating element assembly in
the form of a heating shelf and a method of manufacturing heating
shelf assemblies. The heating shelf may be used in existing food
service transport and display cabinets and shelves for controlled
heating of ready made food products such as cookies, muffins,
donuts, pizza, sandwiches and the like. The preferred heating shelf
optionally includes thermochromic materials (i.e, the materials
change color with temperature), or lighted displays, such as an LED
warning light, thus providing a visual indication of heating shelf
temperature. Other features may include varied surface watt density
for accurate heat placement and multiple resistance elements for
initial temperature boosting and temperature maintenance.
[0007] The present invention as described above provides several
benefits. One or more intricate resistance circuit paths of one or
more resistance heating materials, such as NiCr wire, graphite
scrim, conductive polymers etc., may be laminated between
thermoplastic sheets, wherein the planar resistance heating element
may then be reformed, as by thermoforming, drawing, or moldings,
with the laminated structure to provide heat on one or more heat
planes.
[0008] These heating structures provide intimate contact between
the contents of the heating structures and the heat source, thereby
providing inherent energy consumption advantages as well as the
ability to intimately locate secondary devices such as thermistors,
sensors, thermocouples, RTDs, etcetera, in proximity to the
contents being heated or conditions being observed or recorded.
[0009] The heating element assembly also allows for an infinite
number of circuit path shapes, and designs, allowing the circuit
path to correspond to the general shape of a desired end product
utilizing the heating element assembly. The heating element
assembly may be folded to occupy a predefined space in an end
product and to provide heat in more than one plane, thermoformed
into a desired three dimensional heated plane, or stamped or die
cut into a predetermined flat shape which may, then, be folded or
thermoformed into a desired three dimensional heated shape. The
heating element assembly thereby emulates well known sheet metal
processing or known plastic forming processes and techniques.
[0010] The heating element assembly according to the present
invention may also be over molded in a molding process whereby the
resistance heating element is energized to soften the thermoplastic
sheets and the heating element assembly is over molded with a
thermoplastic to form a detailed molded structure. The energizing
and overmolding steps may be timed such that the thermoplastic
sheets and over molded thermoplastic form a substantially
homogenous structure accurately capturing and positioning the
resistance heating element within the structure. Alternatively, the
heating element assembly may soften during mold flow without
additional energizing.
[0011] In addition, thermochromic materials, or lighted displays,
such as colored LEDs and thermometers, may be integrally formed
with the heating shelf to provide a visual indicia of shelf
temperatures.
[0012] In another embodiment of the present invention, a sheet of
heating element assemblies comprises a first thermoplastic sheet, a
second thermoplastic sheet affixed to the first thermoplastic
sheet, and a sheet of resistance heating elements secured between
and to the first and second thermoplastic sheets. The sheet of
resistance heating elements includes a supporting substrate having
a first surface thereon and a plurality spaced circuit paths, each
of the circuit paths comprising at least one electrical resistance
heating material attached to the supporting substrate wherein at
least one of the circuit paths has terminal end portions.
[0013] The sheet of heating element assemblies of this embodiment
provides several benefits. The sheet may be inexpensively and
efficiently produced using mass production techniques. The sheet
may be collected into a roll, allowing the later separation and use
of individual heating element assemblies or group of heated element
assemblies as described above. The sheet, may be further, or
alternatively, processed using various secondary fabrication
techniques, such as stamping, die cutting, or overmolding.
[0014] The above and other features of the present invention will
be better understood from the following detailed description of the
preferred embodiments of the invention which is provided in
connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The accompanying drawings illustrate preferred embodiments
of the invention, as well as other information pertinent to the
disclosure, in which:
[0016] FIG. 1 is a top plan view of a pair of resistance wires
disposed in predetermined circuit paths according to an exemplary
embodiment of the invention;
[0017] FIG. 2 is a front perspective view of a preferred
programmable sewing machine and computer for manufacturing
resistance heating elements;
[0018] FIG. 3 is an isometric view of a first embodiment of the
heating element assembly according to the invention, with a portion
of a top laminate surface removed to reveal a portion of the
resistance heating element;
[0019] FIG. 4 is a partial cross-sectional elevation view of the
heating element assembly shown in FIG. 3, taken along line 4-4;
[0020] FIG. 5 is a partial cross-sectional view of a multi-layered
heating element assembly according to the invention;
[0021] FIG. 6 is a diagram of an exemplary method of manufacturing
a sheet of heated element assemblies according to the
invention;
[0022] FIG. 7 is a diagram of a sheet of resistance heating
elements shown in partial view according to the invention;
[0023] FIG. 8 is a top plan view of a resistance heating element
assembly wherein the laminated structure has been cut to form a
profile for a heating container which may be folded to form a three
dimensional heater assembly;
[0024] FIG. 9 is a top plan view of a heating element assembly
including the resistance heating element of FIG. 8 wherein a
portion the top laminated surface has been removed to show the
resistance heating element, before being formed into a final
configuration; and
[0025] FIG. 10 is as a performance graph of a heating assembly
according to the invention, in which the heating assembly is used
to heat prepackaged, baked cookies.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0026] The present invention provides a thermoplastic laminate
heating element assembly including resistance heating elements, in
the form of a heating shelf. As used herein, the following terms
are defined:
[0027] "Laminate" means to unite, for example, layers of laminate
via bonding them together, usually with heat, pressure and/or
adhesive. It normally is used to refer to flat sheets, but also can
include rods and tubes. The term refers to a product made by such
bonding;
[0028] "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;
[0029] "Melting Temperature" means the point at which a fusible
substance begins to melt;
[0030] "Melting Temperature Range" means the temperature range over
which a fusible substance starts to melt and then becomes a liquid
or semi-liquid;
[0031] "Degradation Temperature" means the temperature at which a
thermoplastic begins to permanently lose its mechanical or physical
properties because of thermal damage to the polymer's molecular
chains;
[0032] "Evacuating" means reducing air or trapped air bubbles by,
for example, vacuum or pressurized inert gas, such as argon, or by
bubbling the gas through a liquid polymer.
[0033] "Fusion Bond" means the bond between two fusible members
integrally joined, whereby the polymer molecules of one member mix
with the molecules of the other. A Fusion Bond can occur, even in
the absence of any direct or chemical bond between individual
polymer chains contained within said members;
[0034] "Fused" means the physical flowing of a material, such as
ceramic, glass, metal or polymer, hot or cold, caused by heat,
pressure or both;
[0035] "Electrofused" means to cause a portion of a fusible
material to flow and fuse by resistance heating;
[0036] "Stress Relief" means reducing internal stresses in a
fusible material by raising the temperature of the material or
material portion above its stress relief temperature, but
preferably below its Heat Deflection Temperature; and
[0037] "Flap" or "Flap Portion" means a portion of an element which
can be folded without damaging the element structure.
RESISTANCE HEATING ELEMENT
[0038] With reference to FIGS. 1-9, there is shown a first
embodiment of a resistance heating element 10, preferably having
about 50-95% of the surface area of the heated shelf. The preferred
resistance heating element 10 may include a regulating device for
controlling electric current. Such a device can include, for
example, a thermistor, a thermocouple, or a RTD, for preventing
overheating of the polymeric materials disclosed in this invention.
The resistance heating elements 10 of this invention can take on
any number of shapes and sizes, including squares, ovals, irregular
circumference shapes, tubes, cup shapes and container shapes. Sizes
can range from less than one inch square to 21 in..times.26 in.
with a single sewing operation, and greater sizes can be available
if multiple elements are joined together. Greater sizes are also
available with roll or continuous element forms.
[0039] As shown in FIG. 1, a first embodiment of a resistance
heating element 10 includes a resistance wire 12 disposed in spiral
circuit path. The ends of the resistance wire 12 are coupled to a
pair of electrical connectors 15 and 16 using known techniques such
as, riveting, grommeting, brazing, clinching, compression fitting
or welding. The circuit includes a resistance heating material,
which may be a resistance heating wire 12 wound into a serpentine
path containing, for example, about 3-200 windings, or, a
resistance heating material, such as ribbon, a foil or printed
circuit, or a conductive coating or ink. Preferably the resistance
heating wire 12 includes a Ni--Cr alloy, although certain copper,
steel, and stainless-steel alloys could be suitable. A positive
temperature coefficient wire may also be suitable. The resistance
heating material can be provided in separate parallel paths, or in
separate layers. Whatever material is selected, it should be
electrically conductive, and heat resistant. It should also be
resilient to subsequent forming operations, either on its owns, as
in the case of a wire or scrim, or encapsulated with a polymer. A
tensile strength of at least about 10,000 psi, and preferably at
least about 50,000 psi, for the fiber or resulting composite is
helpful. (See ASTM D 3379, D3039).
[0040] Alternatively, continuous or closed loop heating wires may
be provided, in which case current is induced into the heating
element by means such as high frequency radiation or magnetic
induction.
SUBSTRATES
[0041] As used herein, the term "supporting substrate" refers to
the base material on which the resistance material, such as wires,
are applied, or impregnated within, as is the case with graphite
powder, for example. The supporting substrate 11 of this invention
should be capable of being pierced, penetrated, or surrounded, by a
sewing needle for permitting the sewing operation. Other than this
mechanical limitation, the substrates of this invention can take on
many shapes and sizes. Flat flexible substrates are preferably used
for attaching an electrical resistance wire with a thread.
Non-plastic materials, such as glasses, semiconductive materials,
and metals, can be employed so long as they have a pierceable
cross-sectional thickness, e.g., less than a 0.010 inch-0.020 inch,
or a high degree of porosity or openings therethrough, such as a
grid, scrim, woven or non-woven fabric, for permitting the sewing
needle of this invention to form an adequate stitch. The supporting
substrate 11 of this invention need not necessarily contribute to
the mechanical properties of the final heating element, but may
contain high strength fibers. Such fibers could contain glass,
aramid fibers melt-bonded or joined with an adhesive to form a
scrim, woven or non-woven mat.
[0042] Alternatively, the supporting substrate 11 of this invention
may contain ordinary, natural, or synthetic fibers, such as cotton,
wool, silk, rayon, nylon, polyester, polypropylene, polyethylene,
etc. The supporting substrate may also comprise a synthetic fiber
such as Kevlar that has good thermal uniformity and strength. The
advantage of using ordinary textile fibers, is that they are
available in many thicknesses and textures and can provide an
infinite variety of chemistry, porosity and melt-bonding ability.
The fibers of this invention, whether they be plastic, natural,
ceramic or metal, can be woven, or spun-bonded to produce non-woven
textile fabrics.
[0043] Specific examples of supporting substrates 11 useful in this
invention include polymer, ceramic, glass, or metallic films, such
as non-woven fiberglass mats bonded with an adhesive or sizing
material such as model 8440 glass mat available from Johns
Manville, Inc. Additional substrates can include polymer
impregnated fabric organic fabric weaves, such as those containing
nylon, rayon, or hemp etc., porous mica-filled plate or sheet, and
thermoplastic sheet film material. In one embodiment, the
supporting substrate 11 contains a polymeric resin which is also
used in either the first thermoplastic sheet 110 or second
thermoplastic sheet 105, or both of a heated element assembly 100
described below. Such a resin can be provided in woven or non-woven
fibrous form, or in thin sheet material having a thickness of 0.020
inch or less. Thermoplastic materials can be used for the
supporting substrate 11 which will melt-bond or liquefy with the
thermoplastic sheets 110, 105, so as to blend into a substantially
uniform structure.
SEWING OPERATION
[0044] With reference to FIG. 2, the preferred programmable sewing
machine 20 will now be described. The preferred programmable sewing
machine is one of a number of powerful embroidery design systems
that use advanced technology to guide an element designer through
design creation, set-up and manufacturing. The preferred
programmable sewing machine 20 is linked with a computer 22, such
as a personal computer or server, adapted to activate the sewing
operations. The computer 22 preferably contains or has access to,
embroidery or CAD software for creating thread paths, borders,
stitch effects, etc.
[0045] The programmable sewing machine 20 includes a series of
bobbins 24 for loading thread and resistance heating wire or fine
resistance heating ribbon. Preferably, the bobbins 24 are pre-wound
to control tension since tension, without excessive slack, in both
the top and bottom bobbins 24 is very important to the successful
capturing of resistance heating wire on a substrate. The thread
used should be of a size recommended for the preferred programmable
sewing machine. It must have consistent thickness since thread
breakage is a common mode of failure in using programmable sewing
machines. An industrial quality nylon, polyester or rayon thread is
highly desirable. Also, a high heat resistant thread may be used,
such as a Kevlar thread or Nomex thread known to be stable up to
500.degree. F. and available from Saunders Thread Co. of Gastonia,
N.C.
[0046] The programmable sewing machine preferably has 1-20 heads
and can measure 6 ft in width by 19 feet long. The sewing range of
each head is about 10.6 inches by 26 inches, and with every other
head shut off, the sewing range is about 21 inches by 26 inches. An
acceptable programmable sewing machine is the Tajima Model No.
TMLG116-627W (LT Version) from Tajima, Inc., Japan.
[0047] The preferred method of capturing a resistance heating wire
12 onto a supporting substrate 11 in this invention will now be
described. First, an operator selects a proper resistive element
material, for example, Ni-Cr wire, in its proper form. Next, a
proper supporting substrate 11, such as 8440 glass mat, is provided
in a form suitable for sewing. The design for the element is
preprogrammed into the computer 22 prior to initiating operation of
the programmable sewing machine 20. As with any ordinary sewing
machine, the programmable sewing machine 20 of this invention
contains at least two threads, one thread is directed through the
top surface of the supporting substrate, and the other is directed
from below. The two threads are intertwined or knotted, ideally
somewhere in the thickness of the supporting substrate 11, so that
one cannot view the knot when looking at the stitch and the
resulting resistance heating element 10. As a top needle penetrates
the substrate 11 and picks up a loop of thread mechanically with
the aid of the mechanical device underneath, it then pulls it
upward toward the center of the substrate 11 and if the substrate
is consistent and the thread tension is consistent, the knots will
be relatively hidden. In a preferred embodiment of this invention,
the resistance heating wire 12 is provided from a bobbin in
tension. The preferred programmable sewing machine 20 of this
invention provides a third thread bobbin for the electrical
resistance wire 12, so that the programmable sewing machine 20 can
lay the resistance wire 12, down just in front of the top needle.
The preferred operation of this invention provides a zig zag or
cross stitch pattern, whereby the top needle criss-crosses back and
forth as the supporting substrate 11 is moved, similar to the way
an ornamental rope is joined to a fabric in an embroidery
operation. A simple looping stitch with a thread 14 is also shown.
By guiding the top needle over either side of the resistance
heating wire 12, the heating wire 12, is captured in a very
effective manner, the process being computer controlled so that the
pattern can be electronically downloaded into the computer 22 and
automatically sewn onto a substrate of choice.
[0048] The programmable sewing machine 20 can sew an electrical
resistance heating wire 12 having a diameter or thickness of 0.005
inch -0.25 inch, onto a supporting substrate 11 at a rate of about
10-500 stitches per minute, saving valuable time and associated
cost in making resistance heating elements.
[0049] The ability to mechanically attach resistive elements, such
as wires, films and ribbons, to substrates provides a multitude of
design possibilities in both shape and material selection.
Designers may mix and match substrate materials by selecting their
porosity, thickness, density and contoured shape with selected
resistance heating materials ranging in cross-section from very
small diameters of about 0.005 inch to rectangular and irregular
shapes, to thin films. Also, secondary devices such as circuits,
including microprocessors, fiberoptic fibers or optoelectronic
devices, (LEDs, lasers) microwave devices (power amplifiers, radar)
and antenna, high temperature sensors, power supply devices (power
transmission, motor controls) and memory chips, could be added for
controlling temperature, visual inspection of environments,
communications, and recording temperature cycles, for example. The
overall thickness of the resistance heating element is merely
limited by the vertical maximum position of the needle end, less
the wire feed, which is presently about 0.5 inch, but may be
designed in the future to be as great as 1 inch or more. Resistive
element width is not nearly so limited, since the transverse motion
of the needle can range up to one foot or more.
[0050] The use of known embroidery machinery in the fabrication of
resistance heating elements allows for a wide variety of raw
materials and substrates to be combined with various resistance
heating materials. The above construction techniques and sewing
operation also provide the ability to manufacture multi-layered
substrates, including embedded metallic and thermally conductive
layers with resistance wires wrapped in an electrically insulating
coating, so as to avoid shorting of electric current. This permits
the application of a resistance heating wire to both sides of the
thermally conductive metallic layer, such as aluminum foil, for
more homogeneously distributing resistance heat.
THERMOPLASTIC LAMINATE HEATING ELEMENT ASSEMBLY AND HEATING SHELF
CONSTRUCTION
[0051] FIG. 3 shows an exemplary heating element assembly 100, in
the form of a heating shelf, according to the invention. The
heating element assembly 100 includes a resistance heating element
10 disposed between laminated first and second thermoplastic sheets
105, 110. For illustrative purposes, the first thermoplastic sheet
105 is shown partially removed from the second thermoplastic sheet
110. The resistance heating element 10, described above, at least
substantially encompasses the circuit path, defined by resistance
wire 12.
[0052] The supporting substrate of the resistance heating element
10 has a thickness between 0.005 inch and 0.25 inch, and is
preferably 0.025 inch thick. The supporting substrate should be
flexible, either under ambient conditions or under heat or
mechanical stress, or both. A thin semi-rigid heating element
assembly 100 allows for closer proximity of the resistance heating
wire 12 to an object to be heated when the heating element assembly
is formed into a final element assembly, such as a heating shelf.
Because less heat needs to be generated by the resistance heating
element 10 to provide heat to the outer surfaces of a thin heating
element assembly 100, materials having lower RTI (Relative Thermal
Index) ratings can be successfully used in thin heating element
assemblies.
[0053] The thermoplastic sheets 105, 110 are laminated to each
other to secure resistance heating element 10 and to form a
reformable continuous element structure. The thermoplastic sheets
105, 110 may be heated and compressed under sufficient pressure to
effectively fuse the thermoplastic sheets together. A portion of
this heat may come from energizing the resistance heating element
10. Alternatively, thermosetting polymer layers could be employed,
such as B-stage epoxy sheet or pre-preg material.
[0054] Preferred thermoplastic materials include, for example:
fluorocarbons, polypropylene, polycarbonate, polyetherimide,
polyether sulfone, polyaryl-sulfones, polyimides, and
polyetherkeytones, polyphenylene sulfides, polyether sulfones, and
mixtures and co-polymers of these thermoplastics. An acceptable
thermoplastic polyetherimide is available from the General Electric
Company under the trademark ULTEM.
[0055] It is further understood that, although thermoplastic
materials are preferable for forming fusible layers because they
are generally heat-flowable, some thermoplastics, notably
polytetraflouroethylene (PTFE) and ultra high-molecular-weight
polyethylene (UHMWPE) do not flow under heat alone. Also, many
thermoplastics are capable of flowing without heat, under
mechanical pressure only.
[0056] Acceptable results were achieved when forming a heating
element assembly under the conditions indicated in TABLE 1 as
follows:
1TABLE THICKNESS OF SHEET PRESSURE TIME TEMP. MATERIAL (inch) (PSI)
(minutes) (.degree. F.) Polypropylene 0.009 22 10 350 Polycarbonate
0.009 22 10 380 Polysulfone 0.019 22 15 420 Polyetherimide 0.009 44
10 430 Polyethersulfone 0.009 44 10 460
[0057] Where no vacuum was applied, "thickness" is the thickness of
the thermoplastic sheets in inches, "pressure" represents the
amount of pressure (in psi) applied to the assembly during
lamination, "temperature" is the temperature applied during
lamination, and "time" is the length of time that the pressure and
heat were applied. It will be understood the above-identified
material thicknesses used in forming exemplary embodiments of the
assembly described herein are merely provided by way of example.
Materials of differing thicknesses may also be used to achieve
acceptable results without departing from the scope of the
invention.
[0058] The first and second thermoplastic sheets 105, 110 and
resistance heating element 10 of the heating element assembly 100
may also be laminated to each other using an adhesive. In one
embodiment of the present invention, an adhesive to hold the
materials together, which may be an ultraviolet curable adhesive,
may be disposed between the resistance heating element 10 and the
first thermoplastic sheet 105 and between the resistance heating
element 10 and the second thermoplastic sheet 110, as well as
between areas of the thermoplastic sheets 105, 110 which are
aligned to be in direct contact. An ultraviolet curable adhesive
may be used that is activated by ultraviolet light and then begins
to gradually cure. In this embodiment of the present invention, the
adhesive may be activated by exposing it to ultraviolet light
before providing the second of the thermoplastic sheets 105, 110.
The thermoplastic sheets 105, 110 may then be compressed to
substantially remove any air from between the sheets 105, 110 and
to secure resistance heating element 10 therebetween.
[0059] FIG. 5 illustrates that a heating element assembly 100a may
include a plurality of heated layers. A second resistance heating
element 10a may be laminated between one of thermoplastic sheets
105,110 and a third thermoplastic sheet 115.
[0060] The thicknesses of thermoplastic sheets 105, 110 and the
thickness of supporting substrate 11 and resistance heating wire 12
are preferably selected to form a reformable continuous element
structure that maintains its integrity when the element is formed
into a final element structure. The preferred heating element
assembly 100 according to the invention, then, is a semi-rigid
structure in that it may be reformed, such as by simply molding,
folding or unfolding under heat, pressure, or a combination thereof
as required by the chosen thermoplastics, into a desired shape
without sacrificing structural integrity.
[0061] Heating shelves 100 according to the present invention
provide several advantages over non-rigid and rigid shelves or
containers, which do not include a heat source. The heat source,
i.e., the resistance heating element 10, intimately surrounds the
contents of a shelf 100, which may be, for example, a food product
such as cookies, muffins, donuts, pizza, sandwiches, or other
contents, whether they be solid, semi-solid or liquid. Also,
secondary devices as described above, such as temperature gauges,
sensors, thermocouples, and RTD's may be disposed more intimately
with the contents or conditions that are being monitored.
[0062] A heating shelf 100 may also be positioned in a mold, over
molded, or both, to form a selected molded heated structure. Some
plastics may be energized prior to and/or during over molding for
improved bonding with the over molding material. A heating shelf
100 may optionally be thermoformed to conform to at least a part of
the mold structure and to preferentially align the resistance
heating element within the mold. Once the heating shelf is
positioned within a mold, the resistance heating element 10 of the
heating shelf 100 may be energized to soften the thermoplastic
sheets, and the heating shelf may be over molded with a
thermoplastic. The energizing and overmolding may be timed such
that the thermoplastic sheets and over molded thermoplastic form a
substantially homogenous structure when solidified. Alternatively,
the thermoplastic sheets may be allowed to soften as a result of
mold flow alone. The thermoplastic materials of the sheets and over
molded thermoplastic are preferably matched to further facilitate
the creation of a homogenous structure. The supporting substrate 11
may also be selected to be a thermoplastic to better promote the
formation of a homogenous structure. The energizing may be timed to
soften the thermoplastic sheets before, after, or during the
overmolding process, depending upon the standard molding
parameters, such as the flow characteristic of the selected
thermoplastics, the injection molding fill time, the fill velocity,
and mold cycle. The assembly is also amenable to other molding
processes, such as injection molding, compression molding,
thermoforming, and injection-compression molding.
[0063] FIGS. 8 and 9 illustrate an exemplary heating element
assembly, which may be formed into a heating shelf 100 final
element assembly. FIG. 8 is a top plan view of an exemplary
resistance heating element 400. The resistance heating element 400
includes a supporting substrate 405 shaped in the profile of a
flattened container. The profile may either be initially shaped in
this profile shape or cut to the profile shape from a larger
supporting substrate. Resistance heating material is affixed to the
supporting substrate 405 and is preferably resistance wire 410 sewn
to supporting substrate 405.
[0064] The resistance heating element 400 shown in FIG. 8 includes
a plurality of flap portions 420 capable of rotation about a first
axis of rotation indicated generally at fold lines 425. The circuit
path 415 formed by resistance wire 410 terminates at terminal end
portions 412.
[0065] FIG. 9 is a top plan view of a heating element assembly 500.
The resistance heating element 400 is laminated between two
thermoplastic sheets, only the top sheet 505 of which is shown, to
form a reformable continuous element structure. A portion of the
thermoplastic sheet 505 is shown removed in order to show the
resistance heating element 400.
[0066] The dashed lines 530 indicate portions of the laminated
structure that may be removed, such as by stamping or die cutting,
from the laminated structure to leave a foldable profile which may
be formed into a non-planar shelf. Alternatively, the heating shelf
may be formed without foldable flap portions. The remaining dashed
lines of FIG. 9 indicate fold lines. Other alternatives may include
integrally forming geometry features, which facilitate the assembly
of the heating shelf with existing display cabinet
configurations.
[0067] A heating shelf 100 may be formed by folding the heating
element 500 along the dashed lines of FIG. 9 and in the direction
of the arrows shown in FIG. 3. The flaps 420 of the resistance
heating element 400 are laminated between thermoplastic layers and
are folded into the shelf shape shown in FIG. 3. The folding step
may include rethermalizing the thermoplastic structure while
folding in order to thermoform the structure into the desired heat
planes, or, alternatively, folding the thermoplastic structure into
the desired heat planes and then rethermalizing the structure,
although it is recognized that the latter method introduces
residual stresses in the bend areas. The heating shelf 100 may
optionally be formed with outwardly flared sides. This feature
permits multiple shelves to be stacked in nested engagement, which
reduces spatial requirements for both storage and shipping.
[0068] It should be apparent that the heating shelf 100 can
optionally provide heat on five different interior planes may, but
is formed from an easily manufactured planar heating element 500.
It should further be apparent that the present invention is not
limited in any way to the heating shelf configuration 100 or
heating element 500 described above. Rather, the above describe
method of manufacturing and heating element structure may be used
to forms cups, enclosed containers, boxes, or any other structure
which may be formed from a planar profile. The heating shelves and
other configurations can include planar elements made from
resistance heating wires, scrim, woven and nonwoven fabric and
conductive filing such as conductive polymers, inks and foils. Such
planar forms should have sufficient tensile strength to resist
mechanical distortion of the circuit path, or heater distribution
profile, during forming of the final product.
[0069] A sheet of heating element assemblies and a method of
manufacturing the same is described hereafter. In another exemplary
embodiment of the present invention, a sheet of heating element
assemblies 225 is provided, as shown in FIG. 6. The sheet of
heating element assemblies 225 includes first and second affixed
thermoplastic sheets, as described above, and a sheet of resistance
heating elements 200 (FIG. 7) secured between and to the first and
second thermoplastic sheets. Essentially, the sheet of resistance
heating elements 200 comprises a plurality of connected resistance
heating elements 10. The sheet of resistance heating elements 200
comprises a supporting substrate 205 and a plurality of spaced
circuit paths 207, each of the circuit paths 207 comprising an
electrical resistance heating material attached to the supporting
substrate 205 to define a circuit path, which includes a pair of
terminal end portions 208, 209. The shape of the circuit paths 207
is merely illustrative of circuit path shapes, and any circuit path
shape may be chosen to support the particular end use for a heating
element assembly included in the sheet of heated element assemblies
225. Alternatively, conductive polymers or fabrics made from
resistance heating material could be employed. The dashed lines of
FIG. 7 indicate where an individual resistance heating element may
be removed from the sheets of resistance heating elements 225.
[0070] A sheet 225 of heating element assemblies may be
manufactured using conventional mass production and continuous flow
techniques, such as are described in U.S. Pat. No. 5,184,969 to
Sharpless et al., the entirety of which is incorporated herein by
reference. For example, as illustrated in FIG. 6, first and second
thermoplastic sheets 210, 212 may be provided from a source, such
as rolls 214, 216 of thermoplastic sheets, or extruded using known
extrusion techniques as a part of the manufacturing process. One
manufacturer of such thermoplastic sheet extruders is Killion
Extruders Inc. of Cedar Grove, N.J. Likewise, a sheet of resistance
heating elements 200 may be provided from a source, such as roll
218. Sheet 200 may be manufactured as described above in the
"Sewing Operation" section. The sheets 200, 212, 214 may be made to
converge, such as by rollers 224, between a heat source, such as
radiant heating panels 220, to soften the thermoplastic sheets 210,
212. A series of rollers 222 compresses the three sheets 200, 212,
214 into a sheet of heated element assemblies 225, thereby also
removing air from between the sheets 200, 212, 214. The rollers 222
may also provide heat to help fuse the sheets 200, 212, 214 and/or
may be used to cool freshly laminated sheets 200, 212, 214 to help
solidify the heated sheets into the sheet of heated element
assemblies 225 after compression.
[0071] It should be apparent that a sheet of a plurality of
multiple-layered heating element assemblies, such as a sheet
including a plurality of heating element assemblies 100a of FIG. 5,
may also be manufactured simply by including a third thermoplastic
sheet and a second sheet of resistance heating elements to the
process described above.
[0072] Regardless of the specific manufacturing technique, the
sheet of heating element assemblies 225 may be collected into a
roll 230. The roll 230 may then be used by an original equipment
manufacture (OEM) for any desired manufacturing purpose. For
example, the OEM may separate or cut individual heating element
assemblies from the roll and include the heating element assembly
in a desired product, by molding, adhesive or ultrasonic bonding,
for example, into a container or molded product. An individually
manufactured heating element assembly as mentioned above or a
heating element assembly removed from a sheet of heating element
assemblies 225, because of its resiliency and good mechanical
properties, is amenable to secondary manufacturing techniques, such
as die cutting, stamping, or thermoforming to a desired shape or
combination thereof as described above. Each heating element
assembly may be cut or stamped into a preselected shape for use in
a particular end product even while still a part of sheet 225 and
then collected into a roll 230. The circuit paths of the resistance
heating element of the heating element assembly may be
appropriately shaped to conform to the desired shape of a selected
product and heat planes in which the heating element assembly is to
be included or formed.
[0073] The formable semi-rigid feature of the heating element
assemblies of the present invention provides a designer the
opportunity to include the assembly in complex heat planes. The
assembly may be cut to a desired formable shape, and the circuit
path is preferably designed to substantially conform to this shape
or the desired heat planes. The assembly may then be rethermalized
and folded to conform to the heat planes designed for the assembly
to occupy.
[0074] A preferred thermoplastic sheet may range from approximately
0.004 inch to 0.100 inch. Thus, the thickness of the thermoplastic
sheets of a heating element assembly may be chosen to effectively
bias heat generated by a resistance heating element in a selected
direction. The supporting substrate itself also may provide an
insulation barrier when the circuit path is oriented towards, for
example, contents to be heated and the supporting substrate is
oriented toward an outer or gripping surface.
[0075] Similarly, one or both of the thermoplastic sheets of a
heating element assembly 100 or heating element assembly 500 may be
coated with a thermally conductive coating that promotes a uniform
heat plane on the heated element assembly. An example of such a
coating may be found on anti-static bags or Electrostatic
Interference (ESI) resistive bags used to package and protect
semiconductor chips. Also, thermally conductive, but preferably not
electrically conductive, additive may be added to the thermoplastic
sheets to promote heat distribution. Examples of such additive may
be ceramic powders, 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, etcetera. A thermally conductive layer and/or
additive is useful because a resistance wire typically does not
cover all of the surface area of a resistance heating element
10.
[0076] Advantageously, a heating assembly, formed in accordance
with the invention, may be provided having varying surface watt
densities, to provide for accurate heat placement. Other
alternatives include providing a heating shelf having a plurality
of resistance heating elements, in which case one element could be
used for initial temperature boosting, while a second resistance
element could be used for maintenance heating.
EXPERIMENTAL RESULTS
[0077] A heating shelf was formed having a resistance heating
circuit path sandwiched between laminated layers of thermoplastic.
The thermoplastic material used for both the top and bottom of the
heating shelf assembly was ULTEM 1000. The top of the heating shelf
was formed with two sheets of ULTEM 1000 having a total thickness
of 0.02 inch. The bottom of the heating shelf was formed from
laminated sheets having a total thickness of 0.095 inch. It will be
understood that materials used in forming the heating shelf are not
limited to the precise thicknesses defined herein, which are merely
provided by way of example. A resistance heating circuit path was
formed using resistance heating wire having a total impedance of
approximately 289 ohms. The resistance heating wire may comprise a
plurality of twisted, braided or parallel individual wires having a
collective diameter of between about 0.010 inch to 0.050 inch. The
resistance heating wire was sewn to a fiberglass scrim substrate
having an uncompressed thickness of approximately 0.030 inch. It
will be understood that materials used in forming the heating shelf
are not limited to the precise thicknesses defined herein, which
are merely provided by way of example. The resistance heating wire
was patterned in a spiral design starting in the center of the
shelf with 1/2 inch spacing, which is progressively reduced to 1/4
inch.
[0078] The substrate, having a resistance heating wire sewn
thereto, was placed between the top and bottom thermoplastic sheets
to form a heating element assembly. Next, the heating element
assembly was sandwiched in a manufacturing assembly. To this end, a
Teflon sheet was placed adjacent to the exposed surface of each
thermoplastic sheet, a layer of silicon rubber was placed adjacent
each Teflon sheet, and a stainless steel plate was placed adjacent
each silicon rubber sheet. The Teflon prevents the thermoplastic
sheets from adhering to the manufacturing assembly, while the
silicon rubber sheets provide a cushion which allows for even
distribution of the hydraulic pressure applied by the heat press.
The stainless steel sheets act as stiffening agents to facilitate
handling of the otherwise pliable assembly.
[0079] The resulting manufacturing assembly was then placed in a
conventional heated press, with temperature platens preheated to
450 degrees Fahrenheit. The assembly was heated for 20 minutes at a
pressure of 20,000 lbs. The assembly was then air cooled for 20
minutes, followed by a 2 minute water cooling period. The heater
was then trimmed to final dimensions using a belt sander.
[0080] After forming and cooling the heating element assembly, the
assembly was reheated along bend lines, about which the two flap
portions were folded to reform the assembly into a heating
shelf.
[0081] A performance graph for the above-described heating shelf is
shown in FIG. 10. The heating shelf was placed on two laterally
spaced wood strips, each having a width 0.75 inch. The baked
cookies for testing, packaged in pairs in polyethylene bags, were
placed on the heating shelf and warmed to a desired serving
temperature. The cookies were then removed from the shelf.
[0082] The performance graph shows that the cookie temperature at
the center of the shelf stabilized at 133 degrees Fahrenheit, and
the cookie temperature at the edge stabilized at 128 degrees
Fahrenheit. The loaded heater temperature was 155 degrees
Fahrenheit. After the cookies were removed, the heater stabilized
at 124 degrees Fahrenheit.
ADVANTAGES OF THE INVENTION
[0083] A heating shelf in accordance with the invention provides
more efficient heating of food products. Indeed, experimental
results have shown that the present invention consumes 1/3 less
wattage than traditional heating methods. This significant power
savings is attributed in part to the intimate contact achievable
between the heating shelf and the food product as compared to
conventional heating methods. Another factor attributing to
improved heating efficiency is the ability to design and
manufacture the product with a varied watt density, thereby
allowing the accurate placement of heat such that the food product
can evenly warmed throughout, while preventing over warming of food
product.
[0084] Also, the heating shelf is hermetically sealed, making the
shelf suitable for direct contact with food products, and allowing
for the utilization of conventional cleaning techniques such as
dishwashers etcetera, without compromising the integrity of the
shelf.
[0085] Yet another advantage of the invention is the thin yet rigid
shelf geometry for more efficient utilization of existing cabinet
space.
[0086] The preferred heating shelf has an operating voltage of 120
Vac, thereby making the heating shelf mobile as compared to other
comparable devices requiring 240 Vac supply source.
[0087] Further, as described above, the heating shelf of the
present invention lends itself to many automated and secondary
manufacturing techniques, such as stamping, die cutting, and
overmolding, to name a few. Designers can easily choose
thermoplastics and other materials for their designs that meet
required RTI (relative thermal index) requirements for specific
applications by following standard design techniques and parameters
set by materials manufacturers Also, heating shelves such as
described above allow the food industry to efficiently and
effectively reheat prepared foods, as is often required of
businesses that operate large or small food service venues or that
purchase from distributors of prepared foods. Also, among the many
advantages of the present invention is the ability to intimately
locate a secondary device captured between the thermoplastic
sheets, such as a memory device or other data collector within
close proximity to a food product, thereby allowing more accurate
data collection, such as disclosed in commonly owned U.S. Pat. No.
6,417,335, herein incorporated in its entirety by reference. This
data, as an example, may be used to prove that a food was prepared
at a temperature and for a time period sufficient to kill the E.
coli bacteria.
[0088] Although various embodiments have been illustrated, this is
for the purpose of describing, but not limiting the invention. The
assembly line described above is merely illustrative of one means
of forming a sheet of heated element assemblies. Further, the
supporting substrate shapes and circuit paths described above and
shown in the drawings are merely illustrative of possible circuit
paths, and one of ordinary skill should appreciate that these
shapes and circuit patterns may be designed in other manners to
accommodate the great flexibility in uses and number of uses for
the heating element assembly of the present invention. Therefore,
various modifications which will become apparent to one skilled in
the art, are within the scope of this invention described in the
attached claims.
* * * * *