Method Of Manufacturing Electric Heating Panels

Abstract

An electric heating panel is composed of two cakes compressed into a homogeneous unitary body of a thermosetting polymerizable resin having an inert filler with an intermediate asbestos sheet having an electric heating circuit lying wholly against one surface of said sheet, the method of manufacturing said panel consisting of impaling the sheet upon a set of spaced upstanding prongs around which the wire of said circuit is next bent, temporarily securing said wire in the pattern so formed, disimpaling said sheet and circuit, and uniting said cakes with said sheet and circuit therebetween under heat and pressure, the said cakes having a moisture content of between 8 and 12 percent by weight after drying, the panel being aluminum or gold painted on one surface if it is desired only to reflect heat from the other surface.

Description

The present invention relates to a novel electric heating panel and to a method for manufacturing the same, without the need for employing a mold conforming to the contour of the interior electric heating circuit or the necessity for an expensive and time-consuming circuit winding operation.

It is well known that electric heating panels are used in many applications. The present invention however relates to an electric heating panel utilizing a thermosetting resin having desirable heat storage properties without the need for costly curing molds for the resinous body thereof.

By "chemically inert" herein is meant chemically inert so far as the resin used is concerned. By "homogeneous" herein as related to two joined plastic bodies, is meant forming a unitary body "of the same character and quality throughout" (excepting for possible variation in density or compression as will hereinafter become apparent) and further that the term is used to distinguish the state herein called "homogeneous" from the state wherein two bodies joined together are essentially an adherent lamination of two bodies.

The electric heating panel according to the present invention is characterized by utilizing a novel electric heating element fixed within a homogeneous unitary body of a water soluble or water emulsive thermosetting polymerizable resin having a chemically inert filler.

The novel homogeneous unitary body of resin according to the present invention comprises at least two cakes of which each is a homogeneous mixture of an uncured water emulsive or water soluble thermosetting polymerizable resin, a plasticizer therefor, an accelerator therefor, a chemically inert filler, and has a moisture content of less than 12% by weight of the cake, an electric heating element being disposed between the cakes which have been pressed together substantially simultaneously to cure the same and to form them into a single homogeneous body, such electric heating element being novel in that it is characterized by a relatively thin asbestos sheet having an electric heating element in the form of a wire bent or curved to the desired pattern and existing wholly upon one side of and against the surface of said asbestos sheet.

A further novel feature of the present electric heating panel consists of the provision of a multiplicity of apertures or openings each of substantial area formed in said asbestos sheet between the runs of wire forming said circuit through which the aforesaid resin may flow in forming the stated homogeneous body.

A further object of the present invention is to provide an electric heating panel characterized in that at least one main face of the panel has a pattern of indentations embodying a multiplicity of indented cavities presenting promontories therebetween providing a protecting summital surface, the troughs of the cavities immediately overlying the wire of the electric circuitry within the panel to provide greater material density and better dispersion with diminished thickness thereat all to the end that maximum heat will be radiated from the troughs of the cavities while such troughs, being sunk, are protected against being contacted directly by foreign bodies which could cause burning or discomfort, such as for example, cloth, paper, the human skin or the like. The invention contemplates that two bodies of thermosetting plastic in uncured form and having complementary areas shall be placed on either side of the electric heating element already referred to in any such way that after placement such complementary areas, under the influence of heat and pressure will tend to move toward each other substantially simultaneously to cure the plastic in the two bodies and cause them to form a single homogeneous body. In such way the electric heating element is embedded in the homogeneous body as if molded therein, but, in fact, without the expense and equipment required for molding.

In order to make the panel according to this invention, the filler, a water soluble polymerizable resin and a plasticizer therefor are mixed in water as a cold slurry to which is added an accelerator (sometimes known as a hardener) for the resin. The slurry is then at least partially dehydrated in a forming tray. Following this the cake formed from the slurry is further dried. The presence of the accelerator aids in the dehydration of the cake and in the formation of a surface therein which although it should not be termed "tough" is tougher and more resistant to damage than the interior of the cake.

With the foregoing in view, and such other or further purposes advantages or novel features as may become apparent from consideration of this disclosure and specification, the present invention consists of the inventive concept which is comprised, embodied, embraced, or included in the product, method, process, construction, composition, arrangement or combination of parts, or new use of any of the foregoing, herein exemplified in one or more specific embodiments of such concept, reference being had to the accompanying Figures in which:

FIG. 1 is a perspective representation of the slurry tray.

FIG. 2 is a cross-sectional illustration of the vacuum drying machine.

FIG. 3 is a perspective representation of a completed heating panel depicting indentations and an intervening summital area.

FIG. 4 is an enlarged representation of the cavitations and promontories aforesaid the better to depict the relationship of the wire of the electric heating circuit as closely underlying the troughs of the cavities.

FIG. 5 is a perspective representation of the set of spaced and pointed guide prongs projecting from a supporting surface therebelow depicting the manner in which a wire electric heating pattern is stretched between said prongs and bent therearound after the said sheet of asbestos has been impaled upon said prongs, this Figure also depicting means for preserving an electric circuit heating pattern against distortion or displacement while relative transfer is being effected, and showing the plurality of apertures of substantial area formed in the asbestos sheet, the better to facilitate homogeneous union between upper and lower cakes of resin.

FIG. 6 is a perspective representation showing the asbestos sheet and electrical pattern wholly upon one surface thereof and taped in place, positioned or interposed between two cakes of resin.

FIG. 7 is a cross-sectional detail in elevation depicting a terminal boxing and pair of terminal posts for the wire of said electric heating circuit.

FIG. 8 is a schematic representation in elevation showing the two cakes of FIG. 6 and the asbestos sheet thereof being curved between the platens of a heated press.

In the drawings, like characters of reference designate similar parts in the several Figures.

As indicated above, the first step in making an electric heating panel according to the present invention is the mixing of a slurry, the principal ingredients of which are a water soluble or emulsive polymerizable resin, a filler, and a plasticizer and an accelerator for the resin. The resin in its uncured state and the filler are preferably dry mixed. To the dry mixture is added water and a plasticizer and lastly the accelerator. The reason for adding the accelerator last is it has a tendency after addition to begin reaction with other elements of the mixture and hence the mixture must soon be used once the accelerator is added. The accelerator may be any one of a number well known to those skilled in the art for use with the thermosetting resin used. Examples of such accelerates are aluminum sulphate, alum, calcium chloride, potassium sulphide and sodium carbonate.

The resin is a thermosetting, water soluble or water emulsive powdered resin in its uncured state preferably of the phenolic type such as, phenol formaldehyde. The resin can be of the one stage or the two stage curing type but must be water soluble.

The filler employed will depend upon the availability of materials and the end characteristics required in the heating panel. It has been found, however, that a filler comprising 55-70 percent by weight of silica flour and/or powdered slate, on the one hand, and 45-30 percent by weight of asbestos or fiberglass fibers on the other hand, is suitable for the purpose. It will be realized that for high temperature applications, more silica flour and less powdered slate may be used while asbestos rather than fiberglass fibers may be used.

A filler within the above limits results in an end product that has very desirable heat storing properties. The silica powder is chosen for its low moisture absorbing qualities and low heat conductivity. The slate powder has low heat conductivity, provides hardness in the product, and is desirable from the point of view of storing heat in the panel in use. The asbestos has the quality of being able to bind the product together and give it a good tensile and impact strength and resistance to heat.

The panel in general has the advantage that being a poor conductor of heat, it "feels" cooler than panels of better thermal conductivity, at the same actual temperature, and thus the risk of burning, by the panels, of bodies in contact therewith, is less in the case of the herein described panel than with higher conductivity panels at the same temperature.

Other fillers which could be used for the herein panel are: fly ash, mica powder, ground kraft fibers and the fillers mentioned two paragraphs previously in various or differing proportions.

The plasticizer is added to the mix for the purpose of creating a good resin flow during the subsequent heat and pressure stage. The choice of a plasticizer for a particular water soluble polymerizable resin is a matter of choice and skill in the art. However zinc stearate and furfural have been successfully employed while barium hydroxide and barium stearate can also be used although more expensive than the first two plasticizers mentioned.

The mixture may be colored black in one of a number of ways. This increases the thermal efficiency of the product in its function as a heating panel, and this has been found to be true even where the radiating surface of the black panel has been covered with a surface of another color for aesthetic reasons. Such black coloring may be achieved for example by adding to the mixture a black lake dye or black nicrosine dye.

It will be appreciated that variation in the amount and content of the mix is possible, where desirable. For example for lower temperature use kraft fibers may be used in the filler instead of asbestos to produce a product which is not as temperature resistant but which is more easily worked. Fly ash, which is a residue from coal burned in electric power stations, can be used instead of silica flour in the filler.

The mix when completed is poured into a forming tray which has a foraminous or screened bottom through which water may be drawn off under vacuum. Some loss of materials will ensue when the vacuum is applied and hence the gauge of the bottom apertures should be somewhat less than that of the filler. For example with a filler where, for example, the slate or silica flour is 30 gauge the apertures should be about 35 gauge.

During drying, by means of the application of vacuum to the underside of the tray, after the easily extracted water has been drawn off, a cover which forms a resistance to the air is placed over the tray. Such cover is pulled downward on to the cake formed from the slurry while the vacuum is being applied, tending to maintain the cake level and maintaining the area of the tray filled, in plan view while the volume of the cake is being diminished by the removal of moisture.

In the preferred embodiment, the top cover is also foraminous and defines the lower wall of a chamber connected to a vacuum pump. In this way when the vacuum drying, (through the tray bottom) has been completed, and the vacuum turned off, a vacuum may be applied through the top cover, causing the cake to adhere to the top cover, whereby it may be lifted intact with and by means of the cover to remove the cake from the tray. This is considered a substantial advance over prior art methods of handling such a cake which tended to cause breakage and cracking.

Following initial drying, the cake formed from the slurry is removed from the tray and stacked so that air has access thereto for further drying. Drying continues until the moisture content is in the order of under 12 percent and preferably 8 percent by weight of the cake. In connection with the drying, the temperature should be maintained low enough that pre-curing of the resin content of the panels will not take place. For most phenolic resins this means drying in a temperature of below 70.degree.-75.degree. C.

The drying process is an air drying process so that drying may take place slowly whereby undue warping may be avoided.

In FIG. 1 of the drawings there is illustrated a forming tray. It comprises a frame 12 having a depth of about 1 1/2 inch seated on a larger frame 14 having a wire mesh bottom. As already stated the mesh of the bottom should be somewhat smaller than the slate or silica being used.

In use a slurry is mixed as described above, with the accelerator added last, and poured into the frame 12 of the forming tray. Excess moisture drops through the apertures of the mesh of the frame 14 and additional moisture is drawn off through frame 14 by means of a vacuum machine, with the valve 15 open to cause the application of vacuum to the bottom.

The vacuum machine illustrated in FIG. 2 comprises a body 20 having a seat 22 for the slurry tray and a cover 24 to be described hereafter for sealing the top thereof after the tray has been seated. Vacuum is applied to the chamber on the underside of the tray by means of the vacuum pump generally indicated by the numeral 26.

The cover 24 is designed to initially sit in spaced relation to and above the tray 12. On application of the vacuum by opening valve 15 a substantial amount of moisture is drawn out of the slurry. Then the lid 24 is applied to the top of the slurry with the vacuum still applied and the lid 24 is sucked down over the slurry within the tray that is mounted within the seat 22 whereby to squeeze water from the slurry within the tray and to give an even surface to the slurry within the tray and to form a cake therefrom. Water drained from the slurry within the tray is drained out of the body 20 of the vacuum machine through the outlet 28.

The cover 24 is foraminous and forms the bottom of a chamber also defined and enclosed by side and top walls, with the chamber connected by conduit 25 through a valve 35 to the vacuum pump 36.

When the vacuum drying through chamber bottom 14 has been completed, the vacuum is turned off through valve 15 and the bottom vacuum released by means not shown, or by opening valve 28. Valve 35 is then opened so that the vacuum pump may apply vacuum through conduit 25 and the cake is then caused by the upwardly applied vacuum through the apertures in cover 24 to adhere to the latter. The cover 24 is then raised to lift the cake to which it is attached by vacuum suction, out of the tray.

A series of preformed cakes are thus formed and dryed at a temperature below 70.degree. C, and at a relatively slow rate. The 70.degree. C. limit is governed by the tendency of the resins within the slurry to react. It normally takes 3 to 7 days (usually 3 to 4 days) to slowly dry the cakes to a point where their moisture content is sufficiently low for the steps which follow. It has been found that the moisture content should be below 12 percent and preferably 8 percent by weight of the cake.

Following drying, two cakes 38 and 40 are employed in the formation of a completed heating panel 42 as depicted in the accompanying FIG. 3. Between these panels is an asbestos sheet 44 wholly upon one side whereof and in contact against such side is an electric heating circuit collectively designated 46. The ends 48 and 50 of the wire forming circuit 46 are secured to a pair of terminal posts 52 secured within cake 40. The terminals are enclosed by a boxing 54 and the wires or leads 48 and 50 feed out through the eye hole 56 into the form of cable 58.

The asbestos sheet 44 may if desired be imperforate and in that case should be of substantially less area than the cakes 38 and 40 to provide a surrounding marginal area 60 whereby the two cakes may be homogeneously united by heat and pressure after the asbestos sheet and the circuitry upon it has been positioned as aforesaid. Alternately or additionally, a plurality of apertures 62 having substantial area the better to facilitate homogeneous union of said cakes may be formed in said asbestos sheet.

The novel method of positioning a circuit wholly upon one side of an asbestos sheet and maintaining it in position and shape thereafter is well exemplified in the accompanying FIGS. 5 and 6, wherein the novel product itself, vis that of an asbestos sheet having a circuit secured wholly against one side thereof as aforesaid is shown, consists of impaling an asbestos sheet 44 upon a supporting surface 64 of a board 66 by means of a set of spaced and pointed guide prongs 68 in the manner illustrated. A wire electric heating pattern (the circuit 46) is next formed upon sheet 44 by stretching said wire between said prongs and bending it therearound. When this has been effectuated the circuit or pattern is secured against movement on the asbestos sheet preferably by the step of overlaying it with strips of such as drafting tape or the like 70 although any material may be employed which has a relatively low point of thermal disintegration so that it is destroyed by heat in the subsequent application of heat and pressure to the cakes 38 and 40.

After the circuit 46 or electric heating pattern has been secured against movement, the sheet and pattern is disimpaled from the prongs 68 and a relative transfer of said sheet and pattern as between supporting surface 64 and cake 40 is effected. For consonance between this description and the accompanying claim or claims, the cake 40 may also be designated a "first cake" and the cake 38 as a "second cake". The second cake 38 is now placed upon the first cake 40 so that the asbestos sheet and electric heating pattern is sandwiched between them after which the two cakes are united into an essentially unitary heating panel by heat and the application of pressure such as is schematically designated in the accompanying FIG. 8 where such pressure is applied by means of the pair of upper and nether platens 72 and 74.

Although for the purposes of this description and by reference to the accompanying FIGS. 5 and 6 it may have appeared as if sheet 44 must be disimpaled by lifting it from off the prongs 68 and transferring it to cake 40, it should be understood that, within the scope of the present invention means may very well be designed by the use of which the asbestos sheet 44 is held stationary while the prongs move downwardly and out of piercing relationship with the sheet to be replaced by cake 40 which thereafter supports the asbestos sheet.

The platens 72 and 74 are usually heated to some 150.degree. to 160.degree. C so that, as pressure is applied, the resins of the cakes 38 and 40 polymerize substantially simultaneously into a homogeneous mass with the electrical conducting circuit embedded therein and one of the advantages of the present invention is that during the application of heat and pressure there is comparatively little horizontal flow.

In a suitable product it has been found that, prior to curing between the aforesaid platens, the cakes 38 and 40 each have a thickness of about 1/4 inch. Following curing between the platens they are reduced to about 1/8 inch each so that the total thickness of the final product including the thin sheet of asbestos with the electric circuitry against it is approximately 1/4 inch or less having regard for the fact that the wire becomes embedded in the asbestos and that possibly the asbestos also becomes to some extent resin impregnated.

With reference to the accompanying FIGS. 3 and 4 it will be seen that the face of the platen 72 is formed with a plurality of cavitations 71 and adjacent promontaries 73 which together form a heat radiating pattern. In the accompanying FIG. 6 the shown pattern is that which might be made if the platen 72 were faced with woven wire. In FIGS. 3 and 4 the cavities are of generally pyrammidal formation. In either case, or in the case of any undulating design a multiplicity of cavities is formed with intervening promontaries the latter providing a protecting summital surface. The troughs 75 of the cavities, regardless of the shape of the cavities overlie the runs 76 of the wire forming circuit 46 to provide greater density of material at such locations and better heat dispersion with diminished thickness of material thereat while the promontories or protecting summital surface formed of such promontories collectively provides an area of reduced density in material and increased thickness of material. As a result the protecting summital surface acts as a safeguard against foreign contact with the cavitations and particularly the troughs of the cavitations and the possible effects of burning as of paper, cloth or skin in contact therewith.

If it is required that heat should emanate from only one of the main faces of the heating panel it is desirable that the opposed main face of the panel be provided with a layer of heat reflecting material such as a coating of aluminum or gold paint.

Various modifications can be made within the scope of the inventive concept disclosed. Accordingly, it is intended that what is set forth herein should be regarded as illustrative of such concept and not for the purpose of limiting protection to any particular embodiment thereof, and that only such limitations should be placed upon the scope of protection to which the inventor hereof is entitled, as justice dictates.

Claims

I claim:

1. In the construction of electric heating panels and the like the method which consists of the steps of impaling a sheet of asbestos or the like upon a set of spaced and pointed guide prongs projecting from a supporting surface therebelow, forming a wire electric heating pattern upon said sheet by stretching said wire between said prongs and bending it therearound, securing said electric heating pattern against movement on said asbestos sheet, disimpaling said sheet and pattern from said prongs and effecting a relative transfer of said sheet and pattern as between said supporting surface and a first cake of a suitable material so that said sheet and pattern is supported on said first cake, placing a second cake of suitable material upon said first cake so that said asbestos sheet and electric heating pattern is sandwiched between said cakes, and said cakes being forced together under pressure to effect the union thereof, a heat radiating surface pattern being impressed upon the outer surface of at least one of said cakes during the application of said pressure, said heat radiating pattern consisting of alternating recessed and relatively projecting areas, the troughs of said recessed areas immediately overlying the wire of said electric heating pattern so as to provide greater material density and heat dispersion with diminished thickness where they so overlie, said projecting areas providing a protecting surface of reduced density and increased thickness against foreign contact with said recessed areas.

2. In the construction of electric heating panels and the like, the method which consists of the steps of placing upon a first cake of a suitable material an electric heating pattern, then placing a second cake of suitable material upon the said first cake and said pattern so that said pattern is sandwiched between said cakes and forcing said cakes together under pressure to effect the union thereof, a heat radiating surface pattern being impressed upon the outer surface of at least one of said cakes during the application of said pressure, said heat radiating pattern consisting of alternating recessed and relatively projecting areas, the troughs of said recessed areas immediately overlying the wire of said electric heating pattern so as to provide greater material density and heat dispersion with diminished thickness where they so overlie, said projecting areas providing a protecting surface of reduced density and increased thickness against foreign contact with said recessed areas.