Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR MOUNTING CERAMIC COOKTOPS IN APPLIANCE
The present invention is directed to the manufacture of
stoves,cooktops, and ranges (hereinafter, collectively,
"stoves") which carry a ceramic cooktop surface within a
metal frame.
Stoves intended for home use frequently have a glass-ceramic
("ceramic) cooking surface. Below the surface may be mounted
ceramic hobs which are heated with resistance wire such as
nickel-chromium wire, or quartz halogen lamps. The cooktop
surface is often colored and/or textured, and is in general
considered an aesthetic improvement over conventional electric
cooktops with exposed heating coils. The cooktops are commonly
termed "ceramic" although they are sometimes also termed
"glass ceramicas well.
In freestanding stoves, in ranges, and in drop-in cooktop
units, the "stove" 1 (Figure 1) has a top surface 5 which is a
steel stamping, and contains an opening into which the ceramic
cooktop 2 is positioned, the opening further characterized by
edges or side walls 8 (Figure 3) and ceramic cooktop support
surface 5a. For aesthetic reasons, it is often desirable to
position the ceramic cooktop in its recess without the use of
external clamps or rings. As shown in Figure 2, a sealing
compound 6, generally a silicone, fills the gap between the
metal frame and the ceramic cooktop.
Conventionally, the method employed to mount the cooktop is
both labor intensive and messy. First, (Figures 3, 4) double
sided adhesive spacer pads 7 are hand applied from a roll
supplying the adhesive pads 7 an release paper. The ceramic
cooktop is then centered in the opening and pressed onto the
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pads. The pads keep the cooktop from contacting the metal
frame 5 and provide a space between the ceramic cooktop 2 and
the support surface 5a which can be filled with sealant 6. It
is desirable not only to fill the gap between the edge of the
ceramic cooktop and its receiving hole, but also to force
sealant below the cooktop along 5a. When the sealant cures,
its adhesive properties are largely responsible for holding
the cooktop in place.
Applying the sealant is a hand operation, and the sealant must
be difficultly flowable and preferably thixotropic, since
otherwise it will flow past the adhesive pads 7 and drip into
the space below the cooktop, as shown at 6c in Figure 3.
Applying too much sealant can independently cause exuding of
sealant as well. Due to the stiff nature of the sealant, the
sealant must be forced into the space between the cooktop and
frame with some difficulty, and hand tooling must be used
immediately following its application in order to provide a
smooth, uniform surface. Often, several "passes" must be made
in order to provide the desired aesthetics, with final
cleaning up often involving removal of cured sealant with a
razor blade. It is not uncommon for a portion of the stoves to
be reworked by removing the ceramic top, cleaning it and the
frame of silicone, and remounting. As the silicone is, in
general, not water soluble, and is also thick and tacky, the
overall operation can be quite messy.
It would be desirable to be able to seal and adhesively bond
ceramic cooktops within metal frames in a rapid and uniform
manner, while avoiding at least some, and preferably most or
all of the manual steps of assembly and finishing. It would be
further desirable to form a uniform width of adhesive sealant
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below the cooktop and present an aesthetic sealant surface
with little or no hand tooling or cleanup.
Ceramic cooktops are mounted within metal frames having a
support surface by supplying a dam around the inner periphery
of the support surface, positioning a ceramic cooktop onto the
dam, and injecting a measured quantity of low viscosity, self-
leveling, curable silicone between the cooktop and the metal
frame, the curable silicone being prevented from leaving the
inner periphery of the support surface by the dam, and
extending upwards to uniformly fill the gap between the
ceramic cooktop edge and the metal frame.
Therefore, the subject of the invention is a process for
mounting a ceramic cooktop in a stove,comprising:
a) supplying a frame having an opening therein
dimensioned to receive a ceramic cooktop, the opening
having a periphery, and having a support surface
extending inwardly from the periphery of the opening, the
support surface and opening periphery connected by side
walls extending therebetween, the ceramic cooktop having
a bottom surface, a top surface, and an edge connecting
the top surface and bottom surface;
b) providing a dam an the support surface extending
continuously around the support surface and having a
height such that a ceramic cooktop, when positioned
within the opening in the frame and resting on the dam,
will have a spaced relationship between the bottom
surface of the ceramic cooktop and at least a portion of
support surface;
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c) positioning a ceramic cooktop within the opening in
the frame, the ceramic cooktop bottom surface resting on
the dam, and the edge of the ceramic cooktop spaced apart
from the side walls of the frame;
d) applying a flowable, curable silicone between the
side walls of the frame and the edges of the ceramic
cooktop and allowing the silicone to level to provide a
visible seal between the frame and the ceramic cooktop.
FIGURE 1 illustrates a free standing range having a
ceramic cooktop.
FIGURE 2 illustrates a close-up of the top of a range,
showing the sealant between the cooktop and the frame.
FIGURES 3 and 4 illustrate prior art processes of
mounting ceramic cooktops.
FIGURES 5, 6, and 7 illustrate embodiments of the present
invention process of mounting ceramic cooktops.
FIGURE 8 illustrates a further embodiment of a support
surface and dam in accordance with the present
invention.
The metal frame per se of the range may be made in any
conventional marnner. In general, as shown in Figures 3 - 8,
the range has an opening to receive the ceramic cooktop, a
depressed ledge or "support surface" 5a of a depth such that
the thickness of the ceramic cooktop and a dam (as later
described) may be accommodated, while presenting a
substantially flush surface with respect to the top 5 of the
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metal frame and the top of the ceramic cooktop 2. In some
designs, it may be desirable that the surfaces not be flush.
The key to the present invention is the ability to use a low
viscosity silicone sealant with self leveling capability, in
particular, at a known volume. In the prior art process, such
a silicone could not be used, since it would merely flow past
the adhesive pads and trickle down into the space below the
cooktop.
Moreover, the volume of sealant used would vary with the
depth of penetration into the space between the cooktop 2 and
the support surface 5a, and would vary also with the shape,
size, and positioning of the adhesive pads.
By forming a"dam" which is preferably proximate the inner
edge or "periphery" of the support surface, or alternatively,
spaced apart from and preferably parallel to the periphery of
the ceramic cooktop, liquid silicone is prevented from flowing
into the range interior. Such a dam is shown at 10 in Figures
5, 6, and 7. Moreover, because the dam is of uniform height
and placement, the required volume of silicone may be measured
or calculated with reasonable accuracy, such that robotic
dispensing tools may be employed. By injecting just the right
amount of silicone, which is also self-leveling, the cooktop
may simply be left alone following injection, and the
silicone will form a smooth, uniform, and aesthetic seal
between the frame and the ceramic cooktop. In most cases, no
hand tooling or clean-up will be required.
The dam may be supplied by three distinct methods, each of
which are embodiments of the present invention. In the first,
and most preferred embodiment, the dam is supplied as a
rapidly solidifying bead of elastomer and/or adhesive,
preferably applied by robotic means in order to provide
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uniform application. In this embodiment, the rapidly
solidifying adhesive may be a hot melt adhesive of any
chemical type, preferably one which is heat and oxidation
resistant, such as a silicone hot melt adhesive. Most
preferably, however, the dam is composed of a very viscous
and/or thixotropic, curable adhesive which rapidly cures to a
rubbery state. In either case, the bead thus produced should
preferably have the characteristics of a tacky, rubbery
elastomer.
The bead is preferably tacky in order that the cooktop, once
positioned, will not move, even if the stove is jostled or
inadvertently (or purposefully) tilted during manufacture, or
during the cure period of the later applied self-leveling
silicone. However, rather than being tacky, the bead may have
characteristics known in the elastomer industry as "froggy
hand", which provides the elastomer with a tendency to grip
smooth surfaces without itself being tacky. However, if
tilting and jostling of the stove following positioning of the
cooktop is prevented, the bead must only provide a modest
locating capability together with the ability to prevent flow
of liquid silicone past the bead (its "dam" function). In such
cases, a non-tacky and only modestly deformable elastomer or
polymer may be used.
The rapidly solidifying adhesive is preferably a silicone
adhesive, since organopolysiloxanes (silicones) have very
advantageous characteristics. In particular, silicones are
thermally resistant to relatively high temperatures, and can
thus readily survive the temperatures expected near the
cooktop periphery, even under conditions where all burners
are used for extended periods. Second, silicones
are oxidatively stable, and thus long life and continued
elasticity can be expected,even after long periods of use.
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When a silicone bead is used for the dam, it may be of any
rapidly curing type, if not a hot-melt type. Suitable
silicones are, for example, peroxide curing, condensation
curing, and addition curing. Addition curing silicones are
preferable. Since it is most desirable that the curing rate
be quite rapid, two-component addition curing systems are
preferred. Such systems are commercially available and also
can be easily formulated. The organopolysiloxanes employed
may contain any organic groups in addition to the reactive
groups, for example but not by limitation, methyl, ethyl,
propyl, phenyl, chlorophenyl, etc. Halogenated and in
particular, fluorinated alkyl groups such as hexafluoropropyl
and heptafluoropropyl groups may be present as well. For
reasons of economy, in particular, methyl and phenyl groups
are preferred, most preferably methyl groups. Thus, the
preferred elastomers are substantially polydimethylsiloxanes.
The reactive groups in addition-curable silicone elastomers
which cure by hydrosilylation are Si-bonded hydrogen and Si-C
bonded ethylenic or ethylynic unsaturation. Both, a-, w-
terminated silicones as well as silicones containing pendent
reactive groups may be used. The cured hardness is generally
influenced by the crosslink density, with higher crosslink
densities providing harder elastomers. The uncured viscosity
is largely determined, in the absence of fillers and
thixotropes, by the chain length and hence molecular weight of
the individual organopolysiloxanes. Addition of fillers, in
particular fine particle size fillers, increases the viscosity
and may render the composition thixotropic, as may also
viscosity and/or flow regulators.
Further examples of materials which may be used for a rapidly
setting dam are epoxy resin-based elastomers, particularly
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those which incorporate fluorinated compounds for heat
resistance, and polyurethanes. Other curable polymer systems
such as cyanate resins, bismaleimide resins, and the like can
also be used, often at higher cost. Hot melt adhesives based
on polyesters, polyamides, polyimides, mixtures thereof, and
the like may also be used. However, it is preferred that the
raised polymer bead or dam be constructed of a silicone
elastomer. When hot melt adhesives are employed, it is
preferable that these be the type which crosslink to a
thermoset polymer following application, or be thermally
stable at elevated temperatures, for example the aliphatic
polyamides such as polyamide 6, polyamide 6,6, polyamide 6,12,
and the like.
For minimum usage of silicone or other dam material, and hence
the highest economy, it is desirable that the bead of material
laid down assume a roughly half-circular shape in cross-section
following application, with the flattened side of the half-
circle abutting the surface of the frame. In practice,
however, a somewhat vertically flattened or elliptical shape
is obtained, and in principle, a quite broad (quite flattened)
cross-section may be used, so long as the height is
satisfactory for maintaining the ceramic cooktop at the design
height. A suitable silicone elastomer formulation is
ELASTOSIL RT 722, a two component, RTV-2 elastomer available
from Wacker Chemical Corporation, Adrian, Michigan, and Wacker
Chemie AG, Muenchen, Germany. This elastomer composition
exhibits a curing time of about 10 minutes at 150 C. The
curing time is only critical to the extent that longer curing
compositions may slow down the production cycle, or may, in
some compositions allow for such flow that only a broad bead
can be created. An initial cure which provides a dimensionally
stable bead followed by a more complete cure is perfectly
acceptable also. The curing time is preferably between 5 and
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20 minutes, short curing times being limited by the
application speed, i.e. the elastomer should not cure while
still in the "gun".
The rapidly solidifying polymer bead may be laid down by hand,
particularly if pneumatic guns are employed to exude a
constant amount of material. However, the use of manual
techniques should be avoided, and it is highly preferable that
a conventional robot be employed. Such robots generally
contain an articulated arm which directs the nozzle or "gun"
through which rapidly settable polymer flows. In addition to
eliminating manual steps during manufacture, the robotic
application has the distinct advantage of far greater
consistency. The bead should be applied so as to produce at
least one substantially continuous dam which will prevent the
flow of later applied flowable silicone past the dam. Small
"vacancies" in the continuous dam can be tolerated so long as
these do not allow for substantial leakage past the dam. The
end of the continuous bead may abut the beginning of the bead,
may overlap its flanks, or have any other termination such
that substantial leakage of low viscosity silicone past the
dam and into the stovetop interior is prevented.
In a second, less preferred embodiment, a continuous dam of
already solid elastomer is applied, either by hand or by
machine (i.e. a robot). In one such embodiment, a continuous
roll of double sided adhesive tape of suitable thickness is
hand applied, or preferably applied by robotic means. For hand
application, for example, continuous linear strips which abut
at their corners to provide a continuous
dam may be used, or a lengthy strip which is bent around the
corners may be used. In such applications, it is preferable
that the width of the strip not be excessive, in order that
bending around corners may be facilitated. The same modes of
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applications may be made robotically, which is to be preferred
over manual methods. For manual methods in particular, the
support surface from which the ceramic cooktop will be spaced
apart may be manufactured with a groove, step, or other
locating device to assist with proper location and positioning
of the dam. As with the rapidly settable dam, any "vacancies"
in the continuous dam should be small enough so as to avoid
leakage of low viscosity silicone past the dam.
In a third embodiment, the support surface itself is provided
with a raised dam as shown in Figure 8 at 5b, for example by
stamping the dam at the same time as the stamping of the
frame. In this embodiment, the dam will have no positioning
restraining ability an its own. For example, the ceramic
cooktop, once positioned, may slide if the stove is tilted, or
may be easily jostled out of position, However, it is possible
to spray the dam with an adhesive or tacky substance to
provide this restraining feature. In this case, manual
application of spray adhesive or tacky substance may be used,
since the dam is already positioned, and application of the
adhesive spray can be done very quickly.
Thus, by the term "dam" is broadly meant a raised area above
the level of the support surface located between the inner
periphery of the support surface and tbe sidewalls of the
opening in the frame, which is capable of eliminating or
substantially eliminating the flow of flowable silicone past
the dam after the ceramic cooktop has been positioned in the
opening, resting on the dam. By " substantially eliminatingis meant preventing
the flow of flowable silicone into the
interior of the stovetop to the extent that the commercial
viability of the process is not defeated. Small amounts of
leakage may be tolerated, however the amount should be small
enough so as not to require any substantial cleanup,
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preferably not more than 10 mL over the entire periphery
between the dam and the bottom surface of the ceramic cooktop
or through small vacancies in the dam, more preferably less
than 5 mL and most preferably less than 1 mL. Most preferably,
only isolated droplets or no flowable silicone at all passes
by the dam. By "dam" is also meant a continuous structure,
i.e. in the form of a loop, such that such leakage is
prevented. A discontinuous structure with discontinuities of
sufficient size such that considerable flowable silicone can
flow through or past the discontinuities is not a dam within
the scope of the invention. It should be noted, however, that
a dam is not necessarily a single, unitary, closed structure.
For example, a raised spiral, where the pathway between the
entrance, through which flowable silicone can flow, to the
exit is so long that either the viscosity and surface tension
of the flowable silicone or its cure rate prevent flow out of
the spiral into the stove interior is within the scope of the
invention, as are a plurality of concentric rings having
openings in adjoining rings, such that a labyrinthal path is
presented to the flowable silicone. The principle
consideration is the function of the dam, prevention of
substantial leakage of flowable silicone, and maintenance of
proper spacing between the support surface and the ceramic
cooktop, and not its exact configuration.
Regardless of which method of dam construction (or combination
thereof) is used, the cooktop is then placed atop the support
surface in the proper spatial relationships with the edges 8
of the frame. This operation is also preferably robotically
performed.
Finally, a low viscosity, curable silicone (11) is applied
between the edge 9 of the ceramic cooktop and the edge 8 of
the frame. While hand application is possible, and is far less
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time consuming than the application of conventional, high
viscosity silicone, robotic application is far to be
preferred. Hand application, while saving some time, still in
general would require hand tooling for finishing, since even
the most skilled worker will be unable to attain the precision
required such that the silicone self levels to a uniform,
aesthetic seal between edges 8 and 9. On the other hand, the
volumetric output of adhesive, lineal speed, etc., of a robot
is easily adjusted within fine limits, such that hand tooling
should not be required. Robotic application is thus preferred.
The amount of silicone dispersed can be calculated from the
volume encased between the dam and the surface height of the
silicone between edges 8 and 9. The delivery rate can be
adjusted to supply more silicone at corners if the "dam" does
not follow the cooktop periphery at these locations.
The silicone composition is preferably an addition-curable
composition with a relatively low viscosity and a moderate gel
time. The gel time and viscosity have, essentially, an inverse
relationship, since it is desired that the silicone be self
leveling, i.e. that it flows to fill any voids, and also forms
a smooth and level seal between the ceramic cooktop edge and
the stovetop frame. The more viscous the composition, the more
flow time prior to gelation will be required. For
substantially the same reasons, the amount of particulates in
the silicone will, in general, be much less than in a caulk or
traditional sealant. The composition will offen be pigmented,
i.e. white, gray, or black, by incorporation of pigments such
as ground chalk, titania, black transparent iron oxide, or
carbon black. As with most pigments, a relatively small
amount is generally all that is required. lt may be desirable
to leave the composition colorless and translucent or
transparent. For such compositions, either no filler of any
kind is used, or fillers such as very small particle size
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silica or titania, preferably the former, i.e. fumed silica,
are used. The cure time is preferably between 5 and 30
minutes, more preferably between 5 and 20 minutes. The cure
time may also be much longer, and as indicated earlier, full
cure may even occur only upon use of the stove, although this
is not desired.
The addition-curable silicone composition, like that of the
polymer dam previously described, contains both Si-bound
ethylenic or ethylynic unsaturation, and Si-bound hydrogen,
and a hydrosilylation catalyst. The amount of catalyst is
generally lower than that of the rapid-curing bead silicone,
since a slower curing composition is desired. The amount can
be easily adjusted by one skilled in the art to vary the gel
time and cure rate, or even to adjust these rates for changing
climatic conditions, i.e. the temperature in the manufacturing
facility.
lt is, of course, possible to employ other types of curable
liquid silicones, for example those whose cure is facilitated
with peroxides, or condensation curing systems, for example
those employing silanol-functional organopolysiloxanes and
alkoxysilanes or alkoxysiloxanes as crosslinking agents, or
those employing silanol-functional organopolysiloxanes and
silanes or siloxanes (including polysiloxanes) bearing Si-
bonded hydrogen. Such curable RTV-2 and LTV-2 silicone
elastomers are well known, and are available from numerous
sources. RTV-1 compositions may in principle be used,
although they are not preferred, and RTV-1 compositions
containing catalysts or initiators which are activated at
only modestly elevated temperature may be used, if the stove
can be heated, for example in a heat tunnel, etc. However,
elastomers which require elevated temperature cure are not
desirable.
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The viscosity of the curable, liquid, self leveling silicone
composition, as applied, is preferably between 10 mPa=s and
100,000 mPa=s, more preferably between 5,000 and 15,000 and
most preferably between 8,000 mPa=s and 10,000 mPa=s. A
composition with a viscosity (25 C) of 9,000 mPa=s has been
found very advantageous. As the viscosity which is suitable
is dependent an many factors, including the distance of
separation between the ceramic cooktop edge and the metal
frame, and between the ceramic cooktop bottom surface and the
support surface, higher or lower viscosity silicones may also
be useful for certain applications. The viscosity should
increase at a moderate pace following application, to give
the composition time enough to flow and level. Cure should
reach a degree of completion after, for example, but not by
limitation, 10 - 15 minutes, whereby the stove or stovetop
can be manipulated without damage to the sealant, but
complete cure may be prolonged, even over days or weeks, or
may be effectuated during use of the appliance, when the
temperature of the appliance is elevated. The self-leveling
silicone may cure at ambient temperature, generally in the
range of 15 C to 30 C, or at higher temperatures, for example
but not by limitation, up to 150 C.
Preferred flowable, low viscosity is silicones are two
component addition curable silicones with viscosities in the
range of 6,000 to 9,000 mPa=s, most preferred is ELASTOSIL
RT749, available from Wacker Chemical Corporation and from
Wacker Chemie AG. These silicones have a cure time of 10
minutes at 150 C.
Thus, in a preferred process according to the invention, and
as illustrated in Figure 6, a stovetop having a frame 5
dimensioned to receive a ceramic cooktop 2, the frame having a
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support surface 5a, is positioned within reach of a robot
which dispenses a rapidly curing, very viscous and/or
thixotropic RTV-2 silicone through nozzle or "gun" 12. The
robot supplies the RTV-2 silicone 11 through a nozzle to form
a raised, substantially semi-circular bead or "dam" 10 of
tacky silicone elastomer onto the support surface and at a
pre-defined location, such as a bead parallel to the edge of
the support surface or parallel to the edge position of the
later-applied ceramic cooktop, the positioning of the frame
and placement of the dam preferably being such that the bead
always occupies the same position with respect to the frame
for each stove of the same type. The dam is substantially
continuous so as to prevent leakage of later applied flowable
silicone past the dam.
A second robot then positions a ceramic cooktop within the
opening in the frame, the lower surface of the ceramic cooktop
resting on the raised bead (dam) of silicone, defining a
"seal" volume extending outwards from the outer periphery of
the dam towards the side or edge of the frame, and upwards to
the level of the top of the cooktop and frame or the lower of
these, should they not be at the same height.
Finally, a further robot (or the same robot with a further
silicone supply and/or nozzle) dispenses a flowable silicone
11 around the periphery of the cooktop, between the frame and
cooktop, in an amount which is substantially the same as the
defined seal volume. The flowable silicone is allowed to fill
the defined seal volume and to level itself, forming, as it
hardens (cures), a non-tooled seal 13 between the edge of the
frame and the edge of the cooktop.
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In the preferred embodiment described above, it is of course
possible to have but one or two robots perform all the
functions described. Moreover, it is possible to transport the
stovetop to the robot, or to transport the robot to the
stovetop. It is envisioned that the most preferable and
economic production method will have the stove or stovetop
transported on a conveyor past three robots, pausing at each
long enough for the respective robotic functions to be
performed. Many variations are clearly possible.
In less preferred embodiments, the preferred embodiment is
substantially followed, but with certain deviations. For
example, in one embodiment, the dam is applied robotically as
is also application of the flowable silicone. However, the
cooktop is manually positioned. In another embodiment, the
application of a polymer dam is not practiced; rather, a
support surface with a raised section which provides the
function of a dam is used instead, as illustrated at 5b in
Figure 8.
While it is preferable, for the sake of simplicity, to apply
the polymer dam in the same manner from stovetop to stovetop of
the same model, it is of course possible to vary the position,
width, height, etc. of the dam applied by the robot and
maintain the data in hardware or software to allow the seal
volume to be calculated for each respective unit. However,
this embodiment, while clearly within the scope of the
invention, is not preferred.
By terms such as "providing a damis meant that a dam is
present, regardless of whether it is supplied as a raised
portion of metal produced while stamping the frame from sheet
metal, or applied as a continuous strip of solid
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adhesive or a continuous bead of rapidly solidifying adhesive.
By the term "support surface" is meant an inward extension and
usually horizontal extension of the frame onto which the
ceramic cooktop rests, through the intermediary of the dam.
The support surface is preferably integral with the frame, but
may also be separately manufactured and attached to the frame.
Due to the heightened positioning accuracy inherent in the
inventive process, the width of the support surface may be
made narrower than in prior support shelves, onto which
adhesive pads were placed. As indicated previously "stove" is
inclusive of stoves, ranges, drop-in cooktops, etc. By
"flowable", when not defined by a viscosity limitation, is
meant a composition which flows by itself under the influence
of gravity.
While embodiments of the invention have been illustrated and
described, it is not intended that these embodiments
illustrate and describe all possible forms of the invention.
Rather, the words used in the specification are words of
description rather than limitation, and it is understood that
various changes may be made without departing from the spirit
and scope of the invention.
