Method for the manufacture of a heating element

In order to form a heating element, a coil-shaped hollow path, cut into a plate (10), is filled with an exothermally reacting pulverulent mixture and is compressed by means of a correspondingly shaped pusher (13), a platen being then applied to the upper face of the plate (10). Thereafter, the coil (2) comprised of compressed reactive mixture is lifted above the plate (10) and covered with a refractory powder to which a binder has been added and the assembly is then compressed by means of a piston (36; FIG. 6). The assembly is then placed into a metal container (5) intended to diffuse the heat for heating for example foodstuffs, after the reactive mixture has been ignited through the opening (22).

Heating elements are known in the form of a flat cake or lozenge comprising 
a layer of thermally insulating refractory material, a reactive layer 
constituted by means of a finely divided metal or metalloid, mixed with at 
least one oxidant, which is likewise finely divided, and a heat conducting 
and diffusing layer, the refractory layer comprising perpendicular walls 
creating a reaction path for the reactive layer. Elements of this type are 
described for example in European Patent No. 0084532. 
These elements may be obtained by successive casting steps. The insulating 
layer may for example be constituted by plaster reinforced with a fibrous 
material such as glass fibre. The glass fibre/plaster mixture is in 
admixture with water and the liquid mass thus obtained is poured into an 
appropriate mould, such as to obtain a flat cake having a deep channel on 
one of its faces, formed by ribs shaped perpendicular to the plane of the 
cake, said channel forming a predetermined path. After solidification of 
the plaster, the cake is removed from the mold and the reactive exothermic 
mixture, provided in a fluid state by the addition thereto of a liquid 
which can contain a binder, is poured into the channel. This liquid may 
for example be water in admixture with a small percentage of silicate. A 
part of the liquid is progressively absorbed by the plaster and the 
reactive mass densifies. As soon as the reactive mass has attained an 
adequate consistency, the diffusing and conducting layer is placed on its 
surface, which layer may be either a film of metal, such as of aluminium 
or a pulverulent layer consisting of a metallic powder or other heat 
conducting element with the addition of an incombustible binder, such as 
silicate. Subsequently the assembly thus formed is heated in order to 
evaporate off the liquid elements which it contains. If this process is 
relatively easy to put into effect, it nevertheless has a certain number 
of disadvantages which render the manufacture of the elements slow and 
costly. The solidification of the plaster necessitates a minimum time of 
15 minutes, and likewise the densification of the reactive layer takes an 
appreciable time. It is know that to facilitate its pouring, plaster 
should have an equal weight of water added to it and evaporation of this 
water necessitates use of a large quantity of energy. 
The object of the present invention is to permit the rapid manufacture of 
the heating element in question in great numbers. This invention lends 
itself to the constitution of an entirely automated chain of manufacture, 
permitting the production of these elements in a very simple manner at a 
very low price. 
To this end the present invention provides a process of manufacturing a 
heating element in the form of a flat cake comprising a support of 
generally flattened form and of thermally insulating refractory material, 
this support having walls substantially perpendicular to its length, these 
walls defining a hollow path therebetween in which is disposed an 
exothermic, finely divided, reactive mixture, said process being 
characterised in that a layer of the reactive mixture is deposited on a 
temporary support and provided with the mirror image of the said hollow 
path and a layer of a liquid or pulverulent refractory material is 
deposited on the reactive mixture layer thus formed, this layer of 
refractory material being subsequently subjected to a treatment effecting 
its hardening. 
The accompanying drawings show schematically two examples of the process 
according to the present invention, and in which: 
FIG. 1 is a section through a first flat cake along its principal plane. 
FIG. 2 is a section along the line II--II of FIG. 1. 
FIG. 3-7 show successive stages of the process of manufacture of the 
heating element according to FIGS. 1 and 2. 
FIG. 8 is a section through another flat cake along its principal plane. 
FIG. 9 is a section along the lines IX--IX of FIG. 8. 
FIGS. 10-13 show successive stages of the process of the manufacture of the 
heating element according to FIGS. 8 and 9.

FIGS. 1 and 2 show a heating element in section. This heating element is 
constituted by a layer of refractory material 1 having perpendicular walls 
3 forming a reaction route 8 in which is disposed an exothermic reactive 
mixture 2, this route starting from the centre and following a curved path 
to terminate at 4 at the end of the path. This assembly is fixed to a small 
metallic cup 5, the face 6 of which constitutes a layer which diffuses the 
heat produced by the reactive mass 2. The metallic cup 5 may be obtained 
by pressing for example and it may be fastened by rolling to the wall 7 of 
a container which contains the product to be heated. This assembly is 
obtained by the process described below. 
A moulding tool is used of the type which is currently employed in the 
sintered metal industry. It consists of a plate 10 (FIG. 3) of resistant 
material, for example of steel. In this plate, a path 8 such as shown in 
FIG. 1 is cut out for example by a wire electro erosion machine. A base 
plate 11 held by screws 12 supports the walls 9 left after cutting. A 
pressing element 13, having the shape of the hollow path 8 is fitted into 
the plate 10. This element 13 may be moved in the direction perpendicular 
to the plate 10 by the intermediary of rods 14 passing into openings 
provided in the base plate 11, these rods being fixed to a disc 15. As 
shown in FIG. 3, a funnel 16 filled with pulverulent reactive mixture 17, 
sealed at its base 18 by the plate 10 is displaced parallel to the upper 
face of this plate 10 in a manner such as to pass above the cut-out part 
and to fill the cavity formed with the pulverulent mixture 17 which 
escapes from the base of the funnel 16. 
When the moulding tool is filled with pulverulent reactive mixture a plate 
20 is placed on plate 10 (see FIG. 4) and held against the plate 10 by 
clamping means such as a jack 21. Subsequently a compression force, 
symbolised by the arrow is applied to the disc 15 to compress the contents 
of the mould against the plate 20 by the intermediary of the rods 14 and 
pressing elements 13. 
FIG. 4 shows the position of the different elements at the end of 
compression. As soon as the pression force is attained, the jack 21 is 
released, the plate 20 raised, then the disc 15 displaced to eject the 
reactive element thus obtained from the moulding tool. The ejection device 
stops when the base of the object is at the level of the upper face of the 
plate 10. 
The pulverulent reactive mixture is in admixture with an adequate quantity 
of an agglomerant agent, such as a small percentage of binding liquid or a 
colloid, such as a powdered clay etc. It is well known in pyrotechnics that 
if at the time of compression, one exceeds a certain load per unit area, 
the reactive mixture may become inactive. Compression loads generally used 
for sintering metals cannot therefore be used and, for this reason, a 
moulded element is obtained which is relatively fragile and, in view of 
its configuration, practically impossible to handle. 
In the subsequent operation, the disc 15 is held in its last position. The 
reactive element thus formed has the refractory material 1 moulded onto it 
on the spot. To this end, a mould composed of two half shells 30 and 31 
held together by clamping means symbolised by the arrows 32 and 33 (FIG. 
5) is placed on the plate 10. This mould completely encircles the reactive 
mass. The mould is then filled with a pulverulent insulating material in 
admixture with a binder. This material is then compressed by a piston 36 
(FIG. 6) thus completely covering the reactive mass, with the exception of 
the face of the latter which is in contact with the ejector. After having 
withdrawn the piston 36 (FIG. 7) the two half shells 30 and 31 forming the 
mould are separated. The compression piston is provided with a punch 
detaching a plug from the insulating mass, and thus creating an access 22 
to the reactive mass permitting activation of the latter. 
The object thus formed is sufficiently solid to be manipulated delicately. 
It may be transferred into the cup 5 (FIG. 7) and the assembly thus formed 
is placed in a press which, with the aid of an adequate piston, once again 
compresses the assembly of the reactive and insulating masses in a manner 
to effect perfect marriage to the shape of the cup. 
FIGS. 8 and 9 represent a heating element of the same type as that of FIGS. 
1 and 2 but in which the metallic cup 5 is replaced by a heat diffusing 
layer constituted by a layer 25 forming part of the heating element and 
formed by an agglomerated pulverulent mixture. 
The heating element is produced as follows. A template consisting of a 
plate 41 having the desired thickness of the heat dispersion layer is 
placed on a base 40 as shown in FIG. 10, the plate having an opening with 
the contour of the said layer. A second template 42 of a similar shape is 
placed on the first. The space thus formed by the two superposed templates 
is filled with a pulverulent mixture 26 consisting of a mass of good heat 
conductivity, such as for example, a metallic powder, silicon carbide, 
etc, admixed with an appropriate inorganic binder, such as a silicate. 
This mass, which has the approximate consistency of moist sand, may be 
deposited, with the aid of a funnel 43, onto the bed formed by the 
stencil-plates 41 and 42. This mass is then compressed with the aid of a 
piston 44. During compression, this mass diminishes substantially in 
volume and the thickness of the stencil-plate 42 is chosen in such a 
manner that, after compression, the conductive mass will have the same 
thickness as the stencil-plate 41. Once compression is effected the piston 
44 is withdrawn as well as the stencil-plate 42. The heat conducting layer 
thus formed may be displaced by sliding the stencil-plate 41, with its 
contents, on the base 40. Subsequently a mould in the form of frame 47 is 
placed on the stencil-plate 41, containing the heat diffusing layer 46, 
the frame having an opening 48 in which there is deposited a layer of 
substantially equal thickness of reactive mixture 17 in the form of a 
pulverulent mixture, in admixture with an agglomerant or a binder. This 
mixture is subsequently compressed with the aid of piston 50 having an 
engraving 51 on its face, which comes into contact with the reactive 
mixture, the contour of which engraving is such as that of the reactive 
layer to be formed. 
After this compression, the piston 50 is withdrawn and the mould 47 raised 
to obtain the assembly shown in FIG. 12, that is to say, the diffusing 
layer 25 still framed by the stencil-plate 41, on which is disposed a 
serpentine of reactive mixture 2. 
It is of advantage to give a clearance to the engraving 51 on piston 50 
sufficient to enable the compressed mass to unmould itself by gravity. If 
it proves necessary, one can provide the stamp created in the piston 50 
with an ejection device for example similar to that described in the first 
embodiment. 
The assembly represented in FIG. 12 may then be transferred by sliding onto 
a table 40 and then two elements 55 and 56 are placed on the stencil-plate 
41 to form an enclosure 57 having a section with the circumference of the 
final heating element. This enclosure 57 is filled with a pulverulent 
mixture consisting of a granulated, insulating, refractory material, in 
admixture with an inorganic binder, for example a silicate. This material 
is compressed with the aid of a piston 58, the active face of which is 
formed to imprint a supporting ridge 60 in the refractive material, and a 
slot 22 the base of which is constitued by the reactive mixture. This slot 
is for enabling the firing of the reactive mixture. 
After having withdrawn the piston 58 the assembly thus formed can be 
released by separating one or the other of the elements 55 and 56 forming 
the enclosure. The assembly obained is sufficiently solid to be able to be 
manipulated with caution, to be placed on a support permitting its 
transportation into an oven for the necessary solidification treatment of 
the binders and the release of the solvents used in the binders. 
An expanded mineral commercially known under the name of "Perlite" is 
particularly advantageous for the insulating layer. It permits the 
production of a very light insulating layer, with great insulating power, 
and the spheroidal form of the grains permits free filling of the element 
which is to be moulded onto. The insulating layer could also be made of 
plaster which would subsequently be allowed to rest until hardened. 
Different binders may be used to constitute the different layers and good 
results have been obtained notably with silicate of soda, silicate of 
potassium, phosphoric acid and its salts or esters of silicic acid.