[0001] The present invention relates to a method for the manufacture of appliances for
cooking foods of any nature in an insulated enclosure (kitchen ovens, individually
or in combination with a cooktop), the casing of such appliances being composed of
walls, a bottom, and a door and provided with an adequate thermal insulation.
[0002] The thermal insulation of such ovens is usually accomplished by the formation of
an insulating lining of glass wool. This insulation system requires walls of considerable
thickness, because the conductivity of the insulating material cannot be reduced below
certain values (0.028 - 0.030 Kcal/m h °C).
[0003] These values increase considerably in response to rising temperatures: for example,
at 200 °C the conductivity of glass wool is practically twice the value indicated
above.
[0004] In addition, the employ of glass wool panels requires a number of manual operations
for moving, handling, and often also assembling the applicance, so that it is practically
inavoidable that the operator comes into contact with the material which is notoriously
harmful for man.
[0005] A provision for limiting the danger of contaminating the workplace environment with
glass fibres consists in the application of a bonding agent to the fibres to thereby
achieve a higher degree of compaction.
[0006] The presence of such bonding agent becomes apparent during the initial operating
time of the applicances when the more volatile components are released by the effect
of temperature.
[0007] Described in US 2,164,143 is an alternative insulating structure, in particular for
refrigerating appliances, obtained by the introduction of fine powders, such as metal
oxides and Kieselguhr (diatomaceous earth) into a space formed between two flexible
metal sheets by hermetically welding them to one another, and from which the air
is subsequently evacuated.
[0008] The thus obtained insulation has to be enclosed, however, in an air-tight envelope
formed by an inner casing and an outer casing joined to one another with the interposition
therebetween of a wooden framework and rubber gaskets.
[0009] This provision is necessary for protecting the insulating panel, and particularly
the respective weld seams, during further processing.
[0010] The casing obtained according to the method of US 2,164,143 does not either permit
the integration of functional components of the respective appliances (in the case
considered by the present invention, resistance heaters, burners, a thermostat, a
grill, a fan and the like).
[0011] The Italian Patent Application no. 45745-A/86 filed by the present applicant on September
18, 1986, proposes a manufacturing method permitting a refrigerating appliance to
be obtained with all of the respective functional components in an automatized process.
The insulating structure of the casing is composed of evacuated anorganic powders
filling the space between the outer walls and the interior compartment of the casing.
[0012] The feasibility and technical-economical convenience of applying the same method
also to the manufacture of oven casing, particularly of the domestic type, has been
experimentally verified.
[0013] It has in fact been shown that an insulation formed of evacuated anorganic powders
is also capable of withstanding the high temperatures generated within the oven chamber,
offering better thermal insulation characteristics than glass wool and permitting
the manufacturing process to be completely automatized.
[0014] A main object of the invention is thus the provision of an applicance for cooking
foods, provided with an insulating structure on the base of evacuated anorganic powders
of high efficiency and obtainable by an automatized process.
[0015] The invention thus provides a method for the manufacture of cooking ovens, particularly
of the domestic type, comprising a casing the metal sheet walls of which contain a
thermal insulation formed of a mass of evacuated anorganic powders and are provided
with a plurality of functional components such as burners, electric resistance heaters,
a thermostat, a fan and the like.
[0016] According to the invention this method is characterized by comprising a first step
of cutting and drilling two planar sheets having the developed shapes respectively
of the complete internal and external walls of the oven, and of mounting the respective
functional components on said sheets, a second step of bending and welding said sheets
to form two component-equipped box-shaped elements having one open side, a third step
of inserting the box-shaped elements into one another and welding them along their
edges to form an empty space therebetween, and finally a fourth step of completely
filling said empty space with an anorganic powder, and evacuating and hermetically
sealing said space.
[0017] The characteristics of the invention will become more clearly evident from the following
description, given by way of example with reference to the accompanying drawings,
wherein figs. 1 to 4 show diagrammatical representations of successive steps of the
method for the manufacture of a casing for a cooking oven according to the invention.
[0018] The insulating materials employed according to the invention are preferably expanded
perlite or diatomaceous earth.
[0019] The thermal conductivity of these materials, already low at atmospheric pressure
(about 0.04 Kcal/m h °C) is known to decrease still further under vacuum conditions:
at 0.01 mm Hg the conductivity has decreased to about one tenth.
[0020] The thermal insulation obtainable with these conductivity values is conducive to
a considerable improvement of the performance of the oven when the thickness of the
insulat ing walls remains the same as at present, or to an increase of the available
free volume when the thickness of the walls is reduced to dimensions for obtaining
the same performance as in a conventional oven. In any case there results a noticable
energy saving effect.
[0021] A further advantage obtained by the employ of perlite or diatomaceous earth derives
from the fact that the insulation remains constant over long time: there is no degradation
of the insulating material by the action of high temperatures, and thus no change
in the insulating capacity.
[0022] On the other hand, the influence of temperature on the thermal conductivity is less
then in the case of glass wool: as a matter of fact, the thermal conductivity is much
less responsive to rising temperatures and is not noticeably increased thereby.
[0023] The sealing of the powders between two metallic walls also ensures impermeability
to external agents (air), so that it is not necessary to introduce absorbent substances
(getters) for the prevention of a gradual rising of the internal pressure with the
resultant degradation of the performance of the applicance.
[0024] This provision may be necessary, however, when the powders are introduced in flexible
containers (films).
[0025] The conventional process for the manufacture of an oven usually involves the formation
of the interior walls and the bottom by suitably stamping metal sheets. These two
components are formed with perforations for the introduction8 of the necessary functional
components into the oven chamber, and welded together to form a unitary body. After
this body has been enamelled, the glass wool is applied to the outer surfaces of the
assembled casing.
[0026] In contrast therewith, the method according to the invention comprises a first step
in which an integral piece 12 having the developed shape of the complete outer walls
of the overn (fig. 1) and another integral piece 13 having the developed shape of
the complete inner walls of the oven (fig. 2) are cut from a metal sheet.
[0027] The dimensions of piece 12 have to be suitably greater than those of piece 13, because
the two box-shaped elements to be formed of these pieces are to be interconnected
in a manner leaving an empty space therebetween to be subsequently filled with insulating
powders as will become evident as the description proceeds.
[0028] In an automatic processing step preceeding a bending operation each of the two pieces
is provided with necessary perforations 14-16 for mounting the functional components
(e.g. resistance heater, thermostat etc.) of the appliance (fig.2).
[0029] The perforations formed in the external and internal walls are interconnected by
connecting pipes welded thereto at their ends to thereby ensure the hermetical sealing
of the vacuum to be subsequently created in the intervening space.
[0030] The lateral walls of the inner casing are additionally formed with elongate projections
17 to act as rails for supporting grids and shelves within the oven (fig. 3).
[0031] The method according to the invention includes the further step of bending the two
metal sheet pieces and of welding them along abutting edges to thereby form two box-shaped
elements 18, 19 each having an open side (fig.4).
[0032] The element having the smaller dimensions is then inserted into the other one so
as to create an empty space between the two elements.
[0033] During the same processing phase the two elements are welded together along their
respective edges. These edges should be formed with zones of different mechanical
strength for compensating the shrinkage of the powders during the evacuation of the
intervening space between the two elements in the subsequent processing phase. The
welding step is preferably carried out in a continuous laser welding operation. The
oven may if need be also be subjected to an enamelling process. In a further step
of the method, the intervening empty space is then filled with anorganic powders and
subsequently evacuated. To this purpose the outer casing is provided with passages
which are subsequently closed by electric welding after the introduction of the
powders and the evacuation of the intervening space. The compaction of the powders
within the intervening space is ensured by vibrating the casing with the aid of a
vibrating table whereon it is placed.
[0034] The described method for making the oven is also applied to the manufacture of the
door, which also consists of an inner and an outer sheet metal wall sealingly welded
together and filled with an evacuated powder.
[0035] This method is thus substantially analogous to that of the Italian Patent Application
no. 45745-A/86 already quoted above.
[0036] In summary, the proposed method offers the following advantages:
- an improvement of the insulating performance relative to that of glass wool for
the same wall thickness, or a corresponding reduction of the wall thickness, and thus
of the overall dimensions, for the same insulating performance;
- substantially constant thermal conductivity of the insulation with rising temperatures;
- complete impermeability of the walls, whereby the employ of absorbent substances
is rendered unnecessary and the thermal insulating characteristics are maintained
over an extended period of time;
- a major degree of automatization of the manufacturing process, resulting in greater
reliability and reduced physical contact with operating personnel; the manufacturing
process is thereby rendered less noxious to the environment of the workplace.
1. A method for the manufacture of cooking ovens, particularly of the domestic type,
comprising a casing closed by at least one door, the metal sheet walls of said casing
containing a thermal insulation formed of a mass of evacuated anorganic powders, and
being provided with a plurality of functional components such as burners, electric
resistance heaters, a thermostat, a fan and the like, said method being characterized
by comprising a first step of cutting and drilling two planar sheets (12,13) having
the developed shapes respectively of the complete internal and external walls of the
oven, and of mounting the respective functional components (14-16) on said sheets,
a second step of bending and welding said sheets (12,13) to form two component-equipped
box-shaped elements (17,18) having one open side, a third step of inserting the box-shaped
elements (17,18) into one another and welding them along their edges (19) to form
an empty space therebetween, and finally a fourth step of completely filling said
empty space with anorganic powder, and evacuating and hermetically sealing said empty
space.
2. A method according to claim 1, characterized in that the edges of the sheets to
be welded are formed with zones of different mechanical strength to thereby compensate
the shrinkage of the thermally insulating powder when creating the vacuum in the space
between said box-shaped elements.
3. A method according to claim 1, characterized in that the welding of the edges of
the sheets for joining the box-shaped elements is carried out by a continuous laser
welding operation.
4. A method according to claim 1, characterized in that said casing is subjected to
vibrations when said space is filled with said insulating powder.
5. A method according to claim 1, characterized in that the same manufacturing steps
are also carried out for the separate manufacture of said door, said first step including
the step of mounting conventional elements for containing and supporting foods on
the inner wall thereof.