CA1245065A - Method and plant for applying a metal heat-conducting layer and a metal coating layer to the bottoms of stainless steel cooking vessels - Google Patents
Method and plant for applying a metal heat-conducting layer and a metal coating layer to the bottoms of stainless steel cooking vesselsInfo
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- CA1245065A CA1245065A CA000499082A CA499082A CA1245065A CA 1245065 A CA1245065 A CA 1245065A CA 000499082 A CA000499082 A CA 000499082A CA 499082 A CA499082 A CA 499082A CA 1245065 A CA1245065 A CA 1245065A
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Abstract
METHOD AND PLANT FOR APPLYING
A METAL HEAT-CONDUCTING LAYER
AND A METAL COATING LAYER
TO THE BOTTOMS OF STAINLESS
STEEL COOKING VESSELS
ABSTRACT OF THE DISCLOSURE
A method and a plant for applying a heat-conducting metal layer and a coating layer of stainless steel to the bottoms of cooking vessels.
After the bottom of the overturned vessel has been cleaned and roughened, a first aluminium disc and a second stainless steel disc are overlapped and se-cured to the bottom of the vessel by means of a spot of welding. The overturned vessel, together with the superimposed discs, is transferred onto an induc-tion heating device which heats it from the inside to a suitable temperature, to bond the discs to the bottom of the vessel by means of a high pressure, under a fly press. The vessel with the compacted discs is then cooled by means of an air and water cooling system.
A METAL HEAT-CONDUCTING LAYER
AND A METAL COATING LAYER
TO THE BOTTOMS OF STAINLESS
STEEL COOKING VESSELS
ABSTRACT OF THE DISCLOSURE
A method and a plant for applying a heat-conducting metal layer and a coating layer of stainless steel to the bottoms of cooking vessels.
After the bottom of the overturned vessel has been cleaned and roughened, a first aluminium disc and a second stainless steel disc are overlapped and se-cured to the bottom of the vessel by means of a spot of welding. The overturned vessel, together with the superimposed discs, is transferred onto an induc-tion heating device which heats it from the inside to a suitable temperature, to bond the discs to the bottom of the vessel by means of a high pressure, under a fly press. The vessel with the compacted discs is then cooled by means of an air and water cooling system.
Description
- 2 ~ 5~6S
This -invention refers to the application of a layer of heat-conducting metal, such as aluminium or its alloys, and a stainless steel coating sheet, to the bottoms of stainless steel cooking vessels, and par-ti-cularly refers to a method and to a plant for high-pressure hot welding the aforesaid materials to -the outer surface of the bottoms of the vessels, using a particular inductive heating system which reduces heat losses to a minimum and whereby the bottom and its rounded periphera1 port-ion connecting it ~o the cyllndrical wall of the cooking vessel are prevalently heated. This invention also concerns the cooking vessels obtained according to the claimed method and with the claimed plant.
Several methods for bonding layers of a heat-conduct-ing metal, such as aluminium or its alloys, to the bottoms of cooking vessels are known, in order to improve heat diffusion, such as for example, the chill or shell casting method which consists of po'uring molten aluminium into the space between the bot-torn of the vessel and a shell fitted onto the bottom it-self; -this system presents a11 the dif-ficul-ties deriv-ing frorn chill casting and from -the work environment
This -invention refers to the application of a layer of heat-conducting metal, such as aluminium or its alloys, and a stainless steel coating sheet, to the bottoms of stainless steel cooking vessels, and par-ti-cularly refers to a method and to a plant for high-pressure hot welding the aforesaid materials to -the outer surface of the bottoms of the vessels, using a particular inductive heating system which reduces heat losses to a minimum and whereby the bottom and its rounded periphera1 port-ion connecting it ~o the cyllndrical wall of the cooking vessel are prevalently heated. This invention also concerns the cooking vessels obtained according to the claimed method and with the claimed plant.
Several methods for bonding layers of a heat-conduct-ing metal, such as aluminium or its alloys, to the bottoms of cooking vessels are known, in order to improve heat diffusion, such as for example, the chill or shell casting method which consists of po'uring molten aluminium into the space between the bot-torn of the vessel and a shell fitted onto the bottom it-self; -this system presents a11 the dif-ficul-ties deriv-ing frorn chill casting and from -the work environment
3 1~ 65 typical of foundries. A second method, namely that of metalization, consists of repeatedly spraying molt-en aluminium onto the bottom of the vessel until the desired thickness is obtained; this method involves operating in an iner-t atmosphere and with extremely complex equipment. A further method, referred to as braze welding, consis-ts of applying an aluminium disc to the bottom of the vessel, which is suitably heated and prepared with a brazing alloy in order to facilitate the bondin~ of the two parts to be con-nected; this method is also extremely complex and, moreover, requires a great deal of labor. A further method consists of suitably cleaning surfaces to be joined, heating them and then compacting or applying a high pressure sufficient to cause the plastic flow of the aluminium on the bottom of the cooking vessel, in the die of a press; this procedure also suffers from most of the drawbacks of the previously mentioned methods, in that it is extremely complex and difficult to automatize.
It is also known to use inductive heating devices in applying heat-conducting material to the bottoms of cooking vessels, which offer the advantage of a 5~6~
lccalized heating of -the bottom of the vessel; these systems involve applying~ from the outside and on the bottom of the cooking vessel, a medium or high-frequency electromagnetic generator, and interposing between the bottom of the vessel and the generator, an iron disc also referred to as "susceptor disc", for closing the magnetic circuit. This method of inductior, heating from the outside, is subject how-ever to a number of drawbacks which have limited tech-nological developments in this specific sector. In particular, the need to apply the susceptor disc each time to the bottom of the vessel, and to bring it all into contact with the inductive generator, limits the possibilities of automating the production cycle whenever it is required to use a heat-diffusing disc in aluminium or its alloys and a protective disc in stainless steel; moreover, the external disposi-tion both of the inductive generator and of the metal disc for closing the magnetic circuit, subjects the latter to repeated sharp changes in temperature dur-ing the entire production cycle and prevents close contact with the surfaces to be heated, thereby giving rise to considerable dissipation of heat towards the -- 5 - ~ 2 4S~16 5 environment; it also makes it impossible to heat the outer curved portion of the bottom connecting it to the cylindrical wall of the cooking vessel.
A scope o-f this invention is to provide a method for the high-pressure or impact application of a heat-diffusing disc, in aluminium or its alloys, to the bottom of a cooking vessel, and a protective disc in stainless steel, without the use of additional welding material and heating the bottom of the vessel and the discs to be applied, in an extremely short period of time, thus reducing heat losses towards the environment to a minimum~ so as to shorten the production cycle and to permit a high degree of auto-mation of the latter.
A further scope of this invention is to provide an inductive heating device, for applying heat-conducting material to the bottoms of cooking vessels according to the claimed method, which permits a high degree of heat transmission on the bottom and on the outer rounded portion of the vessel, thus reducing heat losses to the lo~lest possible degree, due to the fact that the heat generator is brought into close contact with all the surfaces to be heated.
- 6 - 1245~65 A still further scope is to provide a plant for im-plementation of the claimed method~ whereby it is possible to achieve full automation of the entire production cycle, and which comprises a step-cooling stage after the compacting operation in order to pre-vent internal stress from occurring in the vessel.
A still further scope is to provide a cooking vessell obtained by means of the claimed method, provided with a heat-conducting layer, in aluminium or its a110ys and an external layer in stainless stee1 which covers the bottom and its outer rounded portion con-necting it to the wall of the vessel, in which the edge of the stainless steel layer penetrates into the shaped edge of the layer of aluminium, thus ob-taining a protection in stainless steel whilst eli-minating the presence of any ledges or external pro-trusions wha-tsoever which are unattractive in appear-ance and could trap dirt or cause it to form on the outside of the vessel itself. For this reason, the cooking vessel thus obtained can also be very advan-tageously used as a lid member superimposed on another vessel for cooking food, thanks to the stainless steel coating of the entire bottom of the vessel.
7 ~Z4~65 This inYention basical'ly CGnSiStS of a method for applying a layer of heat-conducting metal material, and a protective layer in stainless steel to the bottom of a cooking vessel, said bottom being joined to a 'lateral wa'll by means of an intermediate rounded portion, said method comprising the steps of :
-- cleaning the external surface of the bottom of the vessel and the intermediate rounded portion;
- roughening said surfaces, for example, by means of a sandblasting operation;
- applying, to the sandblasted bottom of the over-turned vessel a first or intermediate disc in heat-conducting metal material and a second stainless steel disc placed over the first one, and securing the two discs to the bottom of the vessel by spot welding so as to enable them to expand freely;
- heating said discs, the bottom of the vessel and its outer rounded portion, by means of an induction heating device pressed against the bottom and inside the vessel;
- subse~uently compacting the aforesaid discs against the bottom of the vessel by exerting a sufficiently high pressure or impact force as to cause the inter-- 8 ~ 6 5 mediate heat-conducting disc to expand, causing the discs to adhere to each other and to the bottom of the vessel and its peripheral rounded portion.
According to a further feature of the method accord-ing to the invention, after the compacting phase of the discs on the bottom of the vessel, the latter is cooled by effecting a cooling in air and water so as to quickly eliminate all the accumulated heat in order to carry out the subsequent smoothing of the bottom.
According to a further feature of the method and of the product claimed herein, the impaction must be carried out in such a way as to cause the outer edge of the stainless steel disc to penetrate into the shaped edge of the intermediate disc in heat-conduct-ing material so that, during the subsequent process-ing stage, any ledges or protrusions around the peri-pheral rounded portion of the coated bottom are tot-ally eliminated.
This and further feature of this invention will be apparent from the following description, with refer-ence to the examples in the accompanying drawings, in which:
9 1~45~5 Fig. l shows a general layout of an automatic plant for applying heat-conducting material to the bottom of cooking vessels, according to this invention;
Figs. 2, 3, 4, 5 and 6 show the basic steps in the method according to this invention and the features of the vessel thus obtained;
Fig. 7 is a graph illustrating the double-cooling step;
Fig. 8 shows a longitudinal cross-section of a par-ticular embodiment of the induction heating device constituting part of the plant of figure l~
Figure l shows an automatic plant for applying a layer of heat-conducting material to the outer surface of the bottom of stainless steel vessels lO which, as shown in figure 2, have a flat bottom ll which joins with a cylindrical wall portion 12 by means of a peri-pheral rounded portion 13. The plant extends along a path designed to limit the overall dimensions and the time required for carrying out the entire work cycle, to a minimum. In general, the vessels lO on a trolley 14 and, if necessary, already cleaned, are transferred onto a first conveyor 15 at the end of which a first mechanical arm 17 transfers a vessel lO
- - o - ~2~5~S
onto the turntable 18 of a sandblasting unit 1~ where--in the outer surface of the bottom 11 is roughened in order to facilitate the bonding of the aforemen-tioned layer of heat-conducting material to the ves-sel. A second mechanical arm 20 transfers the indi-vidual vessels 10 from the turn-table 18 onto a second conveyor 21 which carries them towards a unit 22 where the bottom 11 of the overturned vessel is provi-ded with a first disc 23 of heat-conducting metal material, for example, aluminium or its alloys, and a second disc 24 made of stainless steel, superim-posed on the first one.
The discs 23 and 24 are stacked on a rotary loading table 25 where a mechanical arm 26 picks them up and transfers them onto the bottom of an overturned vessel that a mechanical arm 27 had previously trans-ferred from the conveyor 21 beneath a spot welding unit 29 which carries out a central spot welding in order to secure the discs 23, 24 to the bottbm of the vessel, leaving them freedom to expand radially when heated.
Once the discs have been secured to the bottom of the vessel, in a perfec-tly centered position, a mecha-5~6~
nical arm 30 transfers the vessel, now indicated by10', together with the relative discs 23, 24, onto a conveyor 31, which carries them forward towards a heating unit 32, described further on~ onto which they are transferred by means of the mechanical arm 34. In the unit32, the vessel 10' is heated from the inside by placing it over an induction heater 32a (Fig. 3) having -the same shape as the inside of the vessel, on which it is held by means of a presser element 32b in order to heat, by contact, the bottom 11 and the peripheral rounded portion 13, and the superimposed discs 23 and 24, bringing them to a tem-perature ranging from 400 to approximately 600C, in order to achieve the plastic deformation of the aluminium disc 23 and the subsequent high-pressure welding phase.
After it has been heated to the appropriate tempera-ture, the vessel 10' is picked up by a mechanical arm 35 and transferred onto the anvil 36a (Fig. 4) of a fly press 36 (Fig. 1) where the discs 23 and 24 are compacted tightly together and to the bottom 11 of the cooking vessel, by means of a high impact prssure of over 600 Kg/sq.m, for example, ranging - 12 ~ 4~rV~5 between 600 and 3000 Kg/sq.m, which gives rise to a solid hot bonding of the parts. In addition to compacting the discs 23, 24 and the bottom 11 of the vessel, the press 36 simultaneously carries out the shaping of the edges of the discs against the bottom and the peripheral rounded portion of the vessel, by means of a special moulding die 36b ~Fig. 4) hav-ing an impression 36c which determines the final shape of the discs on the bottom of the cooking vessel 10 .
After the discs 23 and 24 have been firmly compacted on the bottom 11 of the cooking vessel 10', the latter is transferred onto a conveyor 37 by means of another mechanical arm 38 and is conveyed through a quick cooling tunnel 39 having a first cooling section 39a with jets of air and a second cooling section 39b with jets of water, in order to cool the vessels 10' to room temperature extremely rapidly and at the same time avoiding the occurrence of severe internal stress which could cause excessive bulging of the bo-t-tom of the vessel or loosening of the cohesive forces between the discs and the bottom of the vessel itself.
This cooling is also necessary for the subsequent -3 12~65 opera-t-ion for smoothing the bottoms of the vessels.
Upon completion of the welding and smoothing opera-tions, the cooking vessels 10' are transferred by means of a trolley 40 to the subsequent processing step7 during which the protruding edge of the heat-conducting disc 23 is made tangent to the rounded portion 13 of the bottom of the vessel (Figs. 5 and 6).
The figures from 2 to 7 show in greater detail the main stages of the described method. In general, the metals to be bonded are aluminium or its alloys with regard to the heat-conducting disc 23, and stain-less s-teel with regard to the cooking vessel 10 and the external disc 24 covering the bottom. The thick-ness of the various parts may vary from time to time according to specific requirements; for non-limitative informational purposes, it is pointed out that the thickness of the aluminium disc 23 may range between approximately 2 and 8 mm, whereas the thickness of the stainless steel disc 24 may range between approxi-mately 4 and 7 tenths of a millimetre. Moreover, it can be seen from figure 2 that the aluminium disc 23 must initially have a smaller diameter than the - 14 - ~2~5~65 external diameter of the vessel, so that it remains perfectly in contact with the flat portion 11 of the bottom of the vessel, during the heating, whereas the stainless steel disc 24 has such a diameter as to enable it to be bent around its outer edge 24' (Fig. 5) during the compacting, so that such edge 24' penetrates comple-tely and covers most of the shaped edge 23' of the aluminium disc23', thereby almost totally coveriny the bottom of the vessel.
As mentioned previously, on leaving the compacting press 36, the cooking vessels 10' are cooled rapidly by means of a double air-water system, running along a path defined by the cooling tunnel 39 in order -to cool them to room temperature avoiding sharp changes in iemperature which could give rise to undesirable permanent internal stress between the discs and the bottom of the vessel. Consequently, in the first part 39a of the tunnel, a preliminary cooling phase is carried out with jets of air so as to reduce the temperature Tl of the vessel bottom, and of the discs, which is roughly around 420 - ~8CC, to a T2 tempera-ture equal to or less than half, roughly around 180 - 200C, which is then followed by a cooling phase - 15 ~ 65 with jets of water in the second half of the tunnel to eliminate, due to their greater intensity, the residual heat, which is now at a lower temperature, and cool the vessel to room temperature within a limi-ted space L.
The straight lines A and B in the graph of figure 7 show, by way of example, the succession of the two cooling phases along a tunnel 39; in the same graph~
the straight line Al shows the progress of the cooling in the second half of the tunnel, if the cooling had continued with air: in this case, the vessel 10' would reach room temperature over a much longer path and, therefore, would take a much longer time, due to the lower degree of efficacy of air conling, to low tem-peratures.
On the other hand, if the cooling were to be carried out with water alone, from the beginning of the tun-nel, according to the straight line Bl, the vessel 10' would reach room temperature Ta within an ex-tremely short space of time, thereby subjecting the vessel to a violent thermal shock, which would give rise to harmful uncontrollable internal stress. Con-sequently, the mixed air/water cooling system is that - 16 - ~ 2 ~5~6 5 which permits the vessels to be cooled within a short space and time, without negatively influencing them, due to the reasons explained previously.
Lastly, figure 8 of the drawings shows a pa~ticular embodiment o-f -the induction heating device 32 which is characterized by the factthat it incorporates a susce,otor disc for closing the magnetic circuit, and by the fact that it carries out a localized heating both of the bottom and of the rounded portion connect-ing the latter to the wall of the vessel, from the inside of the vessel itself, thus reducing heat losses to a minimum. Moreover, due to the internal heating of the vessel and to the particular induction heat-ing device with built-in susceptor disc, as shown in figure 8, it is possible to improve the operating cycle of the plant, in that it can be completely auto-mated.
The induction heating device of figure 8 comprises a heating head 42, made of soft metal, for example bronze, so as not to damage the inside of the stain-less steel vessel, and in order to constitute a good heat conductor; the heating head 42 adapts perfectly to the internal surface of the flat bottom 11 of the s~
vessel 1~, the rounded portion 13 and a short portion of the lateral wall 12. The heating head 42 is ther-mically in contac-t with a susceptor disc 42b, for example made of iron, interposed between the heating head 42 and a disc made o-f insulating material 43.
The insulatiny disc 43 is, in turn, supported by the annular edge 44 of a second disc 45, made of thermally insulating materia1, beneath the first one and suppor-ted by the cylinclrical casing 46 of the heating de-vice; the insulating discs 43 and 45 may be made with any thickness and in any suitable material, for ex-ample, material comprising asbestos.
Between the upper insulating disc 43 and the lower insulating disc 45, a circular chamber 47 is formed, which houses an electromagnetic inductor consisting of a tubular spirally-wound conductor 48, which is cooled constantly by circulation of water fed at the end 48a and made to flow out from the end 48b which cross the base plate 49. Consequently, by circulating an electric current of suitable intensity through the concentric turns of the inductor 48, a strong variable electromagne-tic field is generated having flux lines which are closed by means of the suscep-tor - -8 - ~24~65 disc 42, causing the circulation -in the latter of intense electric currents which heat it rapidly. The generated heat is immediately transmit-ted to the head 42 which heats the bottom 11 of the cooking vessel, -the edge 13 and the discs 23, 24 to bring them to the required temperature for carrying out the hot compacting phase, by applying a high impac-t pressure by means of a fly press, as explained previously.
As the vessel 10' is heated from the inside, all the generated heat is immediately given up and used for heating; moreover, due to the disposition of the su-sceptor disc 42 inside the housing 42a provided in the heating head 42, it is possible to automatize all the phases of the operative cycle of the plant of figure 13 reducing the down times which would otherwise occur with conventional induction heating systems.
It is also known to use inductive heating devices in applying heat-conducting material to the bottoms of cooking vessels, which offer the advantage of a 5~6~
lccalized heating of -the bottom of the vessel; these systems involve applying~ from the outside and on the bottom of the cooking vessel, a medium or high-frequency electromagnetic generator, and interposing between the bottom of the vessel and the generator, an iron disc also referred to as "susceptor disc", for closing the magnetic circuit. This method of inductior, heating from the outside, is subject how-ever to a number of drawbacks which have limited tech-nological developments in this specific sector. In particular, the need to apply the susceptor disc each time to the bottom of the vessel, and to bring it all into contact with the inductive generator, limits the possibilities of automating the production cycle whenever it is required to use a heat-diffusing disc in aluminium or its alloys and a protective disc in stainless steel; moreover, the external disposi-tion both of the inductive generator and of the metal disc for closing the magnetic circuit, subjects the latter to repeated sharp changes in temperature dur-ing the entire production cycle and prevents close contact with the surfaces to be heated, thereby giving rise to considerable dissipation of heat towards the -- 5 - ~ 2 4S~16 5 environment; it also makes it impossible to heat the outer curved portion of the bottom connecting it to the cylindrical wall of the cooking vessel.
A scope o-f this invention is to provide a method for the high-pressure or impact application of a heat-diffusing disc, in aluminium or its alloys, to the bottom of a cooking vessel, and a protective disc in stainless steel, without the use of additional welding material and heating the bottom of the vessel and the discs to be applied, in an extremely short period of time, thus reducing heat losses towards the environment to a minimum~ so as to shorten the production cycle and to permit a high degree of auto-mation of the latter.
A further scope of this invention is to provide an inductive heating device, for applying heat-conducting material to the bottoms of cooking vessels according to the claimed method, which permits a high degree of heat transmission on the bottom and on the outer rounded portion of the vessel, thus reducing heat losses to the lo~lest possible degree, due to the fact that the heat generator is brought into close contact with all the surfaces to be heated.
- 6 - 1245~65 A still further scope is to provide a plant for im-plementation of the claimed method~ whereby it is possible to achieve full automation of the entire production cycle, and which comprises a step-cooling stage after the compacting operation in order to pre-vent internal stress from occurring in the vessel.
A still further scope is to provide a cooking vessell obtained by means of the claimed method, provided with a heat-conducting layer, in aluminium or its a110ys and an external layer in stainless stee1 which covers the bottom and its outer rounded portion con-necting it to the wall of the vessel, in which the edge of the stainless steel layer penetrates into the shaped edge of the layer of aluminium, thus ob-taining a protection in stainless steel whilst eli-minating the presence of any ledges or external pro-trusions wha-tsoever which are unattractive in appear-ance and could trap dirt or cause it to form on the outside of the vessel itself. For this reason, the cooking vessel thus obtained can also be very advan-tageously used as a lid member superimposed on another vessel for cooking food, thanks to the stainless steel coating of the entire bottom of the vessel.
7 ~Z4~65 This inYention basical'ly CGnSiStS of a method for applying a layer of heat-conducting metal material, and a protective layer in stainless steel to the bottom of a cooking vessel, said bottom being joined to a 'lateral wa'll by means of an intermediate rounded portion, said method comprising the steps of :
-- cleaning the external surface of the bottom of the vessel and the intermediate rounded portion;
- roughening said surfaces, for example, by means of a sandblasting operation;
- applying, to the sandblasted bottom of the over-turned vessel a first or intermediate disc in heat-conducting metal material and a second stainless steel disc placed over the first one, and securing the two discs to the bottom of the vessel by spot welding so as to enable them to expand freely;
- heating said discs, the bottom of the vessel and its outer rounded portion, by means of an induction heating device pressed against the bottom and inside the vessel;
- subse~uently compacting the aforesaid discs against the bottom of the vessel by exerting a sufficiently high pressure or impact force as to cause the inter-- 8 ~ 6 5 mediate heat-conducting disc to expand, causing the discs to adhere to each other and to the bottom of the vessel and its peripheral rounded portion.
According to a further feature of the method accord-ing to the invention, after the compacting phase of the discs on the bottom of the vessel, the latter is cooled by effecting a cooling in air and water so as to quickly eliminate all the accumulated heat in order to carry out the subsequent smoothing of the bottom.
According to a further feature of the method and of the product claimed herein, the impaction must be carried out in such a way as to cause the outer edge of the stainless steel disc to penetrate into the shaped edge of the intermediate disc in heat-conduct-ing material so that, during the subsequent process-ing stage, any ledges or protrusions around the peri-pheral rounded portion of the coated bottom are tot-ally eliminated.
This and further feature of this invention will be apparent from the following description, with refer-ence to the examples in the accompanying drawings, in which:
9 1~45~5 Fig. l shows a general layout of an automatic plant for applying heat-conducting material to the bottom of cooking vessels, according to this invention;
Figs. 2, 3, 4, 5 and 6 show the basic steps in the method according to this invention and the features of the vessel thus obtained;
Fig. 7 is a graph illustrating the double-cooling step;
Fig. 8 shows a longitudinal cross-section of a par-ticular embodiment of the induction heating device constituting part of the plant of figure l~
Figure l shows an automatic plant for applying a layer of heat-conducting material to the outer surface of the bottom of stainless steel vessels lO which, as shown in figure 2, have a flat bottom ll which joins with a cylindrical wall portion 12 by means of a peri-pheral rounded portion 13. The plant extends along a path designed to limit the overall dimensions and the time required for carrying out the entire work cycle, to a minimum. In general, the vessels lO on a trolley 14 and, if necessary, already cleaned, are transferred onto a first conveyor 15 at the end of which a first mechanical arm 17 transfers a vessel lO
- - o - ~2~5~S
onto the turntable 18 of a sandblasting unit 1~ where--in the outer surface of the bottom 11 is roughened in order to facilitate the bonding of the aforemen-tioned layer of heat-conducting material to the ves-sel. A second mechanical arm 20 transfers the indi-vidual vessels 10 from the turn-table 18 onto a second conveyor 21 which carries them towards a unit 22 where the bottom 11 of the overturned vessel is provi-ded with a first disc 23 of heat-conducting metal material, for example, aluminium or its alloys, and a second disc 24 made of stainless steel, superim-posed on the first one.
The discs 23 and 24 are stacked on a rotary loading table 25 where a mechanical arm 26 picks them up and transfers them onto the bottom of an overturned vessel that a mechanical arm 27 had previously trans-ferred from the conveyor 21 beneath a spot welding unit 29 which carries out a central spot welding in order to secure the discs 23, 24 to the bottbm of the vessel, leaving them freedom to expand radially when heated.
Once the discs have been secured to the bottom of the vessel, in a perfec-tly centered position, a mecha-5~6~
nical arm 30 transfers the vessel, now indicated by10', together with the relative discs 23, 24, onto a conveyor 31, which carries them forward towards a heating unit 32, described further on~ onto which they are transferred by means of the mechanical arm 34. In the unit32, the vessel 10' is heated from the inside by placing it over an induction heater 32a (Fig. 3) having -the same shape as the inside of the vessel, on which it is held by means of a presser element 32b in order to heat, by contact, the bottom 11 and the peripheral rounded portion 13, and the superimposed discs 23 and 24, bringing them to a tem-perature ranging from 400 to approximately 600C, in order to achieve the plastic deformation of the aluminium disc 23 and the subsequent high-pressure welding phase.
After it has been heated to the appropriate tempera-ture, the vessel 10' is picked up by a mechanical arm 35 and transferred onto the anvil 36a (Fig. 4) of a fly press 36 (Fig. 1) where the discs 23 and 24 are compacted tightly together and to the bottom 11 of the cooking vessel, by means of a high impact prssure of over 600 Kg/sq.m, for example, ranging - 12 ~ 4~rV~5 between 600 and 3000 Kg/sq.m, which gives rise to a solid hot bonding of the parts. In addition to compacting the discs 23, 24 and the bottom 11 of the vessel, the press 36 simultaneously carries out the shaping of the edges of the discs against the bottom and the peripheral rounded portion of the vessel, by means of a special moulding die 36b ~Fig. 4) hav-ing an impression 36c which determines the final shape of the discs on the bottom of the cooking vessel 10 .
After the discs 23 and 24 have been firmly compacted on the bottom 11 of the cooking vessel 10', the latter is transferred onto a conveyor 37 by means of another mechanical arm 38 and is conveyed through a quick cooling tunnel 39 having a first cooling section 39a with jets of air and a second cooling section 39b with jets of water, in order to cool the vessels 10' to room temperature extremely rapidly and at the same time avoiding the occurrence of severe internal stress which could cause excessive bulging of the bo-t-tom of the vessel or loosening of the cohesive forces between the discs and the bottom of the vessel itself.
This cooling is also necessary for the subsequent -3 12~65 opera-t-ion for smoothing the bottoms of the vessels.
Upon completion of the welding and smoothing opera-tions, the cooking vessels 10' are transferred by means of a trolley 40 to the subsequent processing step7 during which the protruding edge of the heat-conducting disc 23 is made tangent to the rounded portion 13 of the bottom of the vessel (Figs. 5 and 6).
The figures from 2 to 7 show in greater detail the main stages of the described method. In general, the metals to be bonded are aluminium or its alloys with regard to the heat-conducting disc 23, and stain-less s-teel with regard to the cooking vessel 10 and the external disc 24 covering the bottom. The thick-ness of the various parts may vary from time to time according to specific requirements; for non-limitative informational purposes, it is pointed out that the thickness of the aluminium disc 23 may range between approximately 2 and 8 mm, whereas the thickness of the stainless steel disc 24 may range between approxi-mately 4 and 7 tenths of a millimetre. Moreover, it can be seen from figure 2 that the aluminium disc 23 must initially have a smaller diameter than the - 14 - ~2~5~65 external diameter of the vessel, so that it remains perfectly in contact with the flat portion 11 of the bottom of the vessel, during the heating, whereas the stainless steel disc 24 has such a diameter as to enable it to be bent around its outer edge 24' (Fig. 5) during the compacting, so that such edge 24' penetrates comple-tely and covers most of the shaped edge 23' of the aluminium disc23', thereby almost totally coveriny the bottom of the vessel.
As mentioned previously, on leaving the compacting press 36, the cooking vessels 10' are cooled rapidly by means of a double air-water system, running along a path defined by the cooling tunnel 39 in order -to cool them to room temperature avoiding sharp changes in iemperature which could give rise to undesirable permanent internal stress between the discs and the bottom of the vessel. Consequently, in the first part 39a of the tunnel, a preliminary cooling phase is carried out with jets of air so as to reduce the temperature Tl of the vessel bottom, and of the discs, which is roughly around 420 - ~8CC, to a T2 tempera-ture equal to or less than half, roughly around 180 - 200C, which is then followed by a cooling phase - 15 ~ 65 with jets of water in the second half of the tunnel to eliminate, due to their greater intensity, the residual heat, which is now at a lower temperature, and cool the vessel to room temperature within a limi-ted space L.
The straight lines A and B in the graph of figure 7 show, by way of example, the succession of the two cooling phases along a tunnel 39; in the same graph~
the straight line Al shows the progress of the cooling in the second half of the tunnel, if the cooling had continued with air: in this case, the vessel 10' would reach room temperature over a much longer path and, therefore, would take a much longer time, due to the lower degree of efficacy of air conling, to low tem-peratures.
On the other hand, if the cooling were to be carried out with water alone, from the beginning of the tun-nel, according to the straight line Bl, the vessel 10' would reach room temperature Ta within an ex-tremely short space of time, thereby subjecting the vessel to a violent thermal shock, which would give rise to harmful uncontrollable internal stress. Con-sequently, the mixed air/water cooling system is that - 16 - ~ 2 ~5~6 5 which permits the vessels to be cooled within a short space and time, without negatively influencing them, due to the reasons explained previously.
Lastly, figure 8 of the drawings shows a pa~ticular embodiment o-f -the induction heating device 32 which is characterized by the factthat it incorporates a susce,otor disc for closing the magnetic circuit, and by the fact that it carries out a localized heating both of the bottom and of the rounded portion connect-ing the latter to the wall of the vessel, from the inside of the vessel itself, thus reducing heat losses to a minimum. Moreover, due to the internal heating of the vessel and to the particular induction heat-ing device with built-in susceptor disc, as shown in figure 8, it is possible to improve the operating cycle of the plant, in that it can be completely auto-mated.
The induction heating device of figure 8 comprises a heating head 42, made of soft metal, for example bronze, so as not to damage the inside of the stain-less steel vessel, and in order to constitute a good heat conductor; the heating head 42 adapts perfectly to the internal surface of the flat bottom 11 of the s~
vessel 1~, the rounded portion 13 and a short portion of the lateral wall 12. The heating head 42 is ther-mically in contac-t with a susceptor disc 42b, for example made of iron, interposed between the heating head 42 and a disc made o-f insulating material 43.
The insulatiny disc 43 is, in turn, supported by the annular edge 44 of a second disc 45, made of thermally insulating materia1, beneath the first one and suppor-ted by the cylinclrical casing 46 of the heating de-vice; the insulating discs 43 and 45 may be made with any thickness and in any suitable material, for ex-ample, material comprising asbestos.
Between the upper insulating disc 43 and the lower insulating disc 45, a circular chamber 47 is formed, which houses an electromagnetic inductor consisting of a tubular spirally-wound conductor 48, which is cooled constantly by circulation of water fed at the end 48a and made to flow out from the end 48b which cross the base plate 49. Consequently, by circulating an electric current of suitable intensity through the concentric turns of the inductor 48, a strong variable electromagne-tic field is generated having flux lines which are closed by means of the suscep-tor - -8 - ~24~65 disc 42, causing the circulation -in the latter of intense electric currents which heat it rapidly. The generated heat is immediately transmit-ted to the head 42 which heats the bottom 11 of the cooking vessel, -the edge 13 and the discs 23, 24 to bring them to the required temperature for carrying out the hot compacting phase, by applying a high impac-t pressure by means of a fly press, as explained previously.
As the vessel 10' is heated from the inside, all the generated heat is immediately given up and used for heating; moreover, due to the disposition of the su-sceptor disc 42 inside the housing 42a provided in the heating head 42, it is possible to automatize all the phases of the operative cycle of the plant of figure 13 reducing the down times which would otherwise occur with conventional induction heating systems.
Claims (12)
1. A method for applying a layer of heat-conducting metal and a layer of stainless steel on the bottom of stainless steel cooking vessels comprising a flat bottom which is joined to a cylindrical wall portion by means of a peripheral rounded portion, said method comprising the steps of:
- cleaning the external surface of the bottom of the vessel;
- roughening said surface;
- applying a first heat-conducting metal disc, and a second stainless steel coating disc onto the roughened bottom of the overturned vessel, securing said discs to the bottom of the vessel so as to enable them to expand freely;
- heating the flat portion and the peripheral rounded portion of the bottom of the vessel and the super-imposed discs, by providing them with heat by contact and from the inside of the vessel;
- and exerting a sufficiently high impact pressure to compact the discs together and to the bottom of the vessel.
- cleaning the external surface of the bottom of the vessel;
- roughening said surface;
- applying a first heat-conducting metal disc, and a second stainless steel coating disc onto the roughened bottom of the overturned vessel, securing said discs to the bottom of the vessel so as to enable them to expand freely;
- heating the flat portion and the peripheral rounded portion of the bottom of the vessel and the super-imposed discs, by providing them with heat by contact and from the inside of the vessel;
- and exerting a sufficiently high impact pressure to compact the discs together and to the bottom of the vessel.
2. A method as claimed in claim 1, in which the compacting of the discs to the bottom of the vessel causes the disc of heat-conducting material to expand and the outer edge of the coating disc to fold against the rounded portion connecting the bottom to the cylindrical wall portion of the vessel, making the folded edge of the coating disc penetrate into the shaped edge of the aforesaid heat-conducting disc.
3. A method as claimed in claim 1, in which the roughening of the bottom of the vessel is carried out by sandblasting.
4. A method as claimed in claim 1, in which, during the heating phase, the vessel is placed in an overturned position, on an inductive heating element which adapts to the internal shape of the vessel, and in which the heating is carried out on the flat part and on the peripheral rounded part of the bottom of the vessel.
5. A method as claimed in claim 1, in which the layer of heat-conducting material consists of a disc member of aluminium or its alloy.
6. A method as claimed in claims 1 and 5, in which the bottom of the vessel and the aforesaid discs are heated to a temperature ranging from 400 to 600°C.
7. A method as claimed in claim 1, in which the discs are compacted to the bottom of the vessel with an impact pressure force ranging from 600 to 3000 Kg/sq.cm.
8. A method as claimed in claim 1, further comprising a quick cooling phase, following the compacting phase.
9. A method as claimed in claim 8, in which the vessel is cooled by means of a preliminary cooling phase with jets of air, followed by a second colling phase with jets of water.
10. A method as claimed in claim 9, in which the temperature of the vessel, during the air-cooling phase, is lowered to a value equal to or lower than half the temperature of the vessel at the end of the compacting phase.
11. A plant for applying a heat-conducting disc and a coating disc to the bottom of a cooking vessel, according to the method described in the previous claims, the plant comprising:
- conveyor means for moving each overturned vessel along a cleaning line and towards a unit for sandblasting the bottom of the vessel;
- means for conveying the sandblasted vessels towards a unit for positioning a heat-conducting disc and a coating disc on the bottom of the over-turned vessel, and means for transferring the over-turned vessel with the superimposed discs, onto an induction heating device which adapts to the internal shape of the vessel;
- means operating in sequence to the previous ones, to transfer each individual vessel together with the discs, from the heating device to a high-pressure impacting press, and for subsequently transferring each vessel from the press onto a conveyor to move them along a cooling path.
- conveyor means for moving each overturned vessel along a cleaning line and towards a unit for sandblasting the bottom of the vessel;
- means for conveying the sandblasted vessels towards a unit for positioning a heat-conducting disc and a coating disc on the bottom of the over-turned vessel, and means for transferring the over-turned vessel with the superimposed discs, onto an induction heating device which adapts to the internal shape of the vessel;
- means operating in sequence to the previous ones, to transfer each individual vessel together with the discs, from the heating device to a high-pressure impacting press, and for subsequently transferring each vessel from the press onto a conveyor to move them along a cooling path.
12. A plant as claimed in claim 11, in which the cooling path is delimited by a tunnel comprising a first cooling portion with jets of air, and a second cooling portion with jets of water.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000499082A CA1245065A (en) | 1986-01-07 | 1986-01-07 | Method and plant for applying a metal heat-conducting layer and a metal coating layer to the bottoms of stainless steel cooking vessels |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000499082A CA1245065A (en) | 1986-01-07 | 1986-01-07 | Method and plant for applying a metal heat-conducting layer and a metal coating layer to the bottoms of stainless steel cooking vessels |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1245065A true CA1245065A (en) | 1988-11-22 |
Family
ID=4132233
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000499082A Expired CA1245065A (en) | 1986-01-07 | 1986-01-07 | Method and plant for applying a metal heat-conducting layer and a metal coating layer to the bottoms of stainless steel cooking vessels |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1245065A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU2010285441B2 (en) * | 2009-08-17 | 2014-12-04 | Zhejiang Aishida Electric Co., Ltd. | Manufacturing method for thermally sprayed sandwich bottom cooking pot |
| CN106821004A (en) * | 2016-12-07 | 2017-06-13 | 杭州点厨科技有限公司 | Intelligent electric cooker |
-
1986
- 1986-01-07 CA CA000499082A patent/CA1245065A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU2010285441B2 (en) * | 2009-08-17 | 2014-12-04 | Zhejiang Aishida Electric Co., Ltd. | Manufacturing method for thermally sprayed sandwich bottom cooking pot |
| CN106821004A (en) * | 2016-12-07 | 2017-06-13 | 杭州点厨科技有限公司 | Intelligent electric cooker |
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