Note: Descriptions are shown in the official language in which they were submitted.
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Cooking vessel whose bottom is reinforced by a plate and
radial grooves
The present invention concerns a cooking vessel in
which deformation of the bottom due to the effects of
heat and sudden variations in temperature to which it is
subjected in use are controlled.
Cooking vessels are generally pressed from sheet
aluminum.
During heating, the surface of the bottom of the
container is exposed to a much higher temperature than
the sides.
This results in differential thermal expansion
leading to bulging of the bottom of the container.
To prevent this unwanted phenomenon, the bottom of
the container is made slightly concave so that when
heated the bottom does not become convex due to the
effect of expansion.
However, experience shows that after a number of
heating and cooling cycles the concavity of the bottom of
the container is initially reduced and then disappears
and finally the bottom bulges (becomes convex), making
the container virtually unusable.
This result is explained by the characteristics of
the materials used to manufacture the vessel.
During heating and cooling cycles the elastic limit
is exceeded and this causes irreversible deformation.
To solve this problem, European patent 0 648 459
describes a cooking vessel whose bottom incorporates a
plate which may be in several parts and which is
centered, covering only a small portion of the bottom,
and is made from a metal or alloy having mechanical,
thermal and physical characteristics different to those
of the metal or alloy of the sheet, this plate being
embedded in the bottom. It may be partially or totally
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embedded.
The plate embedded in a localized area of the
bottom of the container and .having mechanical, thermal
and physical properties different to those of the metal
or alloy of which, the remainder of the container is made,
prevents irreversible convex.deformation of tie bottom of
the container for a very large number of heating and
cooling cycles. .
The aim of the present invention is to delay
further the time at which, after a very large number of
heating and cooling cycles, deformation of the bottom
causes it. to become convex.
According to the present invention there is provided a cooking vessel
made from a pressed metal or light alloy sheet, having a bottom provided with
an outside edge and adapted to be placed on a cooker hotplate or burner, the
bottom being, slightly concave and including a plate, said plate having a
periphery and being made from a metal or alloy having mechanical, thermal and
physical properties different to those of the metal or alloy of said sheet,
the plate
being embedded in said sheet, characterized in that the plate covers only a
relatively small central portion of the bottom and in that the surface of said
sheet
between the periphery of the plate and the outside edge of the bottom
comprises a plurality of radial grooves thus having one end adjacent to the
periphery of the plate and their other end adjacent to the outside edge of the
bottom.
Trials have shown that the simultaneous presence on
the bottom of the container of an embedded plate and
radial grooves limits deformation of the bottom and
thereby delays the time at which such deformation becomes
irreversible.
European ' patent 0 474 850 describes a cooking
vessel whose bottom is provided with radial grooves that
are not associated with an embedded plate.
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2a
The presence of the radial grooves is described in
the abovementioned patent as limiting deformation of the
bottom of the container. However, neither the teaching
of the above European patent nor that of European patent
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0 648 459 gives reason to think that the simultaneous
presence on the bottom of the container of an embedded
plate and radial grooves could have a synergistic effect,
that is to say an anti-deformation effect greater than
the sum of the effects of the plate alone and the radial
grooves alone.
The container is preferably made of sheet aluminum
and the plate is preferably made of stainless steel.
The radial grooves are preferably identical and
evenly distributed over the surface.
Other features and advantages of the invention will
become more apparent in the course of the following
description.
In the accompanying drawings, which are given by
way of non-limiting example only:
- figure 1 is a plan view of a disc adapted to be
pressed into the shape of a container in accordance with
the invention, and
- figure 2 is a view of the aforementioned disc in
cross section.
Figures 1 and 2 show a disc cut from a sheet of
metal or metal alloy such as aluminum. The disc is
adapted to be pressed, in a manner that is well known to
the skilled person, into the shape of a cooking vessel,
such as a frypan or a saucepan having a bottom adapted to
be placed on a cooker hotplate or burner.
In figure 1 the dashed line C represents the
periphery of the bottom 1 of the container.
As is also well known in this art, the bottom 1 is
made slightly concave when the disc 3 is pressed.
The central part of the bottom 1 carries a plate 2
made of a metal or alloy having properties different to
those of the metal or alloy of the disc 3.
As shown in figure 2, the plate 2 is embedded in
the metal of the bottom 1 and the periphery 2a of the
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plate 2 is adjacent a groove 4 in the bottom 1.
After the disc 3 is pressed, the concavity of the
bottom 1 is such that when it is placed on a cooker
hotplate the circumference of the surface 5 of the bottom
around the plate,2 touches the hotplate but that of the
plate 2 does not touch it.
The disc 3 is preferably an aluminum disc having a
thickness in the range from 2 mm to 6 mm.
The plate 2 is preferably a stainless steel plate.
The plate 2 preferably covers a proportion of the
surface of the bottom 1 in the range from 5% to 40%.
This relatively small proportion of the bottom 1 covered
by the plate 2 gives the best results, both with regard
to resistance to and the amplitude of deformation of the
bottom 1 due to the effect of heating and cooling cycles
and with regard to heat transmission and temperature
distribution.
These results are explained in particular by the
embedding of the plate 2 in the central area of the
bottom of the container and by the different properties
of steel compared to those of aluminum, and this prevents
the convex deformation of the bottom becoming
irreversible due to the effect of heating and cooling
cycles.
In the embodiment shown in figures 1 and 2 the
plate 2 has holes 6, for example circular holes,
uniformly spaced and distributed over all of the surface
of the plate. The plate 2 is embedded in the bottom 1.
The plate 2 can be embedded in the bottom 1 by cold
pressing, as described in European patent 0 590 860.
In this way, the plate 2 is firmly attached to the
bottom 1 of the container at a large number of points and
controls deformation of the bottom 1 because of the steel
from which the plate is made.
It is sufficient for the plate 2 to have a
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thickness in the range from 3/10 mm to 6/10 mm (for an
aluminum bottom having a thickness in the range from 2 mm
to 6 mm). Embedding it in the bottom 1 to a depth in the
range from half its thickness -to its full thickness is
5 sufficient to obtain excellent results.
As shown in figure 1, the plate 2 is in the shape
of a disc. However, the plate 2 could have a polygonal
shape or an annular shape. The plate 2 could be divided
into a number of sectors or form a grid.
In accordance with the present invention, the
surface of the bottom 1 between the plate 2 and the
outside edge C of the bottom includes a plurality of
radial grooves 7 evenly distributed over the bottom.
In the example shown in figure 1, the end of the
radial grooves 7 adjacent the plate 2 is wider than their
end adjacent the outside edge C of the bottom 1.
Also, the end of the radial grooves 7 adjacent the
plate 2 opens into a circular groove 4 surrounding the
plate 2.
The diameter within which the radial grooves 7 are
formed is in the range from 40% to 60% of the diameter of
the bottom.
The depth of the radial grooves 7 is in the range
from 0.3 mm to 1 mm.
A single cold pressing operation can fix the plate
2 to the aluminum disc and form the radial grooves 7.
The results of comparative tests of two saucepans
are given below, one being a conventional saucepan and
the other a saucepan in accordance with the invention.
The two saucepans were of exactly the same size and
made from a 3.4 mm thick aluminum disc.
The saucepan of the invention had on its bottom a
0.5 mm thick stainless steel disc 75 mm in diameter.
Both saucepans were subjected to heating and fast
cooling cycles (heating to 250°C followed by cooling with
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tap water to around 20°C).
Irreversible deformation of the bottom of the
conventional saucepan was noted after 30 cycles.
Deformation of the bottom of the saucepan of the
invention was noted only after around 500 heating and
cooling cycles.
Of course, the invention is not limited to the
examples just described and many modifications can be
made to these without departing from the scope of the
invention.
Thus the plate 2 can be any shape and need not be
centered on the bottom of the container.
The plate can be star-shaped, for example, with any
number of branches. The plate can be asymmetric in
shape, for example having one branch extending in one
direction, for example that of the handle of the vessel.
The radial grooves can have different shapes,
lengths and depths.
