Method of making multi-ply bottom of clad metal cookware

A method of making a multi-ply bottom of clad metal cookware is disclosed. In the method, both an aluminum plate and a stainless steel cladding plate, externally laid on a base bottom of the clad metal cookware held on a friction press, is pressed down using an instantaneous pressure of the friction press, thus being integrated with the base bottom of the cookware in a way such that the stainless steel cladding plate is fully integrated with the base bottom of the cookware within a predetermined area including a corner of the base bottom. The instantaneous pressure of the friction press ranges from 800 tons to 1,500 tons. In the multi-ply bottom of this invention, the aluminum plate has a dense structure, and increases heat conductivity, heat conservativity, and thermal efficiency of the cookware while allowing the multi-ply bottom to be free from partially heating food in the cookware. The method also reduces the production cost of the cookware.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates, in general, to a method of making a 
multi-ply bottom of clad metal cookware and, more particularly, to a 
method of making a multi-ply bottom of clad metal cookware by externally 
integrating an aluminum plate, clad with a stainless steel plate, with the 
base bottom of the cookware into a single structure through a friction 
process using an instantaneous super-high pressure, thus almost completely 
preventing an unexpected removal of the aluminum plate from the bottom, 
the multi-ply bottom being also designed to allow food in the cookware to 
be uniformly and appropriately heated without being partially scorched or 
burnt at a position around the rounded corner of the bottom, thus making 
food tasty. 
2. Description of the Prior Art 
In the prior art, most cookware is typically made of a single metal. 
However, such single metal cookware is problematic in that it may be 
reduced in heat conductivity when it is designed to have a high degree of 
corrosion resistance, or it may be reduced in corrosion resistance when it 
is designed to have a high degree of heat conductivity. The known single 
metal cookware is thus inconvenient to users. 
In an effort to overcome the above-mentioned problems experienced in the 
single metal cookware, clad metal cookware has been proposed and somewhat 
effectively used. As well known to those skilled in the art, a clad metal 
is a sheet material which is formed by integrating two or more different 
metal plates into a multi-ply metal plate. In the kitchenware art, a 
stainless steel-clad aluminum plate, or an aluminum plate clad with a 
stainless steel plate, is typically used as a material of clad metal 
cookware. 
The stainless steel preferably has a high degree of corrosion resistance, 
but regrettably has a low degree of heat conductivity, thus being 
problematic in that it causes cookware to be partially heated when the 
bottom of the cookware is made of the stainless steel. Such a problem of 
the stainless steel is almost completely overcome when the stainless steel 
is used with aluminum in a clad metal. For example, a stainless steel 
plate, or STS304 plate, regrettably has a temperature difference of at 
least 200.degree. C. between the center and the edge. However, in a clad 
metal plate consisting of an aluminum plate clad with a stainless steel 
plate, the temperature difference between the center and the edge is not 
higher than 25.degree. C. That is, such a clad metal plate has a somewhat 
uniform temperature distribution. The clad metal plate, consisting of an 
aluminum plate clad with a stainless steel, has a high induction heating 
performance when cookware, made of said clad metal plate, is used in an 
induction heating cooker which has been widely used recently due to a high 
degree of thermal efficiency and operational stability. 
However, known clad metal cookware is problematic in that the aluminum 
plate, or the main heat transferring plate, is so thin as to render the 
bottom of the cookware to be partially heated. Therefore, such clad metal 
cookware has a substantial temperature difference between the bottom, the 
middle portion of the sidewall, and the top edge portion of the sidewall, 
thus failing to uniformly heat food. That is, food, contained in such clad 
metal cookware, is more rapidly heated at the lower portion around the 
bottom of the cookware than the upper or middle portions. This makes the 
food undercooked, partially scorched or burnt, and so the food fails to 
have a good flavor. 
In order to solve the problems experienced in such clad metal cookware, a 
multi-ply bottom brazing clad metal cookware has been proposed and used. 
An example of known multi-ply bottom brazing clad metal cookware is shown 
in FIG. 4. As shown in the drawing, the multi-ply bottom brazing clad 
metal cookware 6 has a multi-ply bottom, or a five-fly bottom 5. The above 
multi-ply bottom 5 is formed by brazing both an aluminum plate 1 and a 
stainless steel cladding plate 2 to the external surface of the base 
bottom of the cookware 6 using a pressing machine, such as a press, or a 
high-frequency heating machine. In such a case, a bonding material 3 is 
used in the brazing process. Such a multi-ply bottom 5 maximizes both the 
heat conductivity and heat conservativity of the cookware 6, thus allowing 
food in the cookware 6 to be uniformly heated and to have a good flavor. 
Such cookware 6 also conserves energy while cooking. However, the above 
multi-ply bottom brazing clad metal cookware 6 has a problem in that it is 
impossible to integrate the aluminum plate 1 to the rounded bottom of the 
cookware 6 since a sensitivity heating coil of known brazing machines is 
designed to only heat a flat surface. Therefore, food is partially 
scorched or burnt at a portion around the corner of the bottom which is 
directly affected by gas flames and fails to have a good flavor. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention has been made keeping in mind the above 
problems occurring in the prior art, and an object of the present 
invention is to provide a method of making a multi-ply bottom of clad 
metal cookware, which instantaneously integrates both an aluminum plate 
and a stainless steel cladding plate with the exterior surface of the base 
bottom of clad metal cookware using an instantaneous super-high pressure 
of a friction press in a way such that the integration of the stainless 
steel cladding plate with the base bottom of the cookware completely 
occurs within a predetermined area, including the rounded corner of the 
base bottom, without using any bonding material, and which thus maximizes 
both heat conductivity and heat conservativity of the cookware, and allows 
food in the cookware to be uniformly heated without being partially 
scorched or burnt and to have a good flavor. 
In order to accomplish the above object, the present invention provides a 
method of making a multi-ply bottom of clad metal cookware, comprising the 
step of: pressing down both an aluminum plate and a stainless steel 
cladding plate, externally laid on a base bottom of the clad metal 
cookware held on a friction press, using an instantaneous pressure of the 
friction press, thus integrating both plates with the base bottom of the 
cookware in a way such that the stainless steel cladding plate is fully 
integrated with the base bottom of the cookware within a predetermined 
area including a corner of the base bottom. 
In the preferred embodiment, the instantaneous pressure of the friction 
press ranges from 800 tons to 1,500 tons.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram, showing a method of making a multi-ply bottom of 
clad metal cookware in accordance with the preferred embodiment of this 
invention. FIG. 2 is a partially sectioned view, showing a friction 
process of integrating both an aluminum plate and a stainless steel 
cladding plate with the base bottom of the cookware in accordance with 
this invention. FIG. 3 is a sectional view of clad metal cookware having a 
multi-ply bottom formed through the friction process of this invention. 
As shown in the drawing, in order to make the multi-ply bottom of clad 
metal cookware of this invention, a raw metal sheet is cut into a 
plurality of pieces, individually having an appropriate size suitable for 
being formed as one desired piece of cookware, at a cutting step 32. 
Thereafter, each metal piece from the cutting step 32 is trimmed using a 
hydraulic trimming machine prior to being drawn into a desired 
three-dimensional shape at a drawing step 30, thus forming a piece of 
cookware. The cookware is, thereafter, washed at a washing step 34. The 
washing step 34 is followed by an integration step 36, wherein the 
cookware is subjected to a friction process. In the friction process, both 
an aluminum plate and a stainless steel cladding plate are externally 
integrated with the base bottom of the cookware using a friction press, 
thus forming a desired multi-ply bottom of the clad metal cookware. 
In the friction process, both the aluminum plate 12 and the stainless steel 
cladding plate 14 are externally laid on the base bottom of the cookware 
10, supported on the friction press, prior to being instantaneously 
pressed down using a super-high pressure of the friction press, thus being 
integrated with the base bottom. In such a case, the integration of the 
stainless steel cladding plate 14 with the base bottom of the cookware 10 
completely occurs within a predetermined area including the rounded corner 
of the base bottom. 
In the present invention, it is preferable to set the super-high pressure 
of the friction press for pressing down the two plates 12 and 14 to about 
800-1,500 tons. 
In addition, it is preferable to heat the aluminum plate 12 to a 
temperature of 450.degree. C.-500.degree. C. using an induction heater 
prior to performing the friction process. This improves the integration 
force of the aluminum plate 12 and results in a uniform integration of the 
aluminum plate 12. 
That is, during such a friction process, both the aluminum plate 12 and the 
stainless steel plate 14 are externally laid on the base bottom of the 
cookware 10, supported on the friction press, as shown in FIG. 2. The 
above aluminum plate 12 is heated to a temperature of 450.degree. 
C.-500.degree. C. using an induction heater prior to being subjected to 
the friction process, and so the ductility of the aluminum plate 12 is 
increased. After setting the two plates 12 and 14, both plates 12 and 14 
are instantaneously pressed down using a super-high pressure of the 
friction press. The induction-heated and easily ductile aluminum plate 12 
is thus uniformly spread widely within an area between the base bottom and 
the stainless steel cladding plate 14. The aluminum plate 12 is firmly and 
uniformly integrated with both the base bottom and the stainless steel 
plate 14 without leaving any cavity between the base bottom and the 
stainless steel cladding plate 14. 
As best seen in FIG. 3, the integration of the stainless steel cladding 
plate 14 with the base bottom of the cookware 10 completely occurs within 
a predetermined area including the rounded corner of the base bottom. 
Therefore, the cookware of this invention effectively prevents food from 
being partially scorched or burnt at a portion around the corner of the 
bottom even when the corner is affected by gas flames. 
The integration force of the aluminum plate 12, clad with the stainless 
steel plate 14, with the base bottom of the cookware 10 maintains 
semi-permanently, and so the clad metal cookware 10 may be effectively 
used for a lengthy period of time. In addition, the multi-ply bottom of 
this invention may be formed while somewhat freely changing the thickness 
of the aluminum plate 12 in accordance with use of cookware 10 as desired. 
In the multi-ply bottom of this invention, the aluminum plate 12 is 
substantially thick, and is integrated with the total area of the base 
bottom of the cookware 10 including the rounded corners of the base 
bottom. Therefore, the multi-ply bottom maximizes heat conductivity, heat 
conservativity, and thermal efficiency of cookware 10. This effectively 
prevents food, contained in the cookware 10, from being partially scorched 
or burnt at a portion around the corner of the bottom. 
As described above, the present invention provides a method of making a 
multi-ply bottom of clad metal cookware. The method firmly integrates both 
an aluminum plate and a stainless steel cladding plate with the base 
bottom of clad metal cookware using an instantaneous super-high pressure 
of a friction press, with the integration of the stainless steel cladding 
plate with the base bottom completely occurring within a predetermined 
area including the rounded corner of the base bottom. Therefore, the 
aluminum plate of the multi-ply bottom has a dense structure. The 
multi-ply bottom of this invention also increases heat conductivity, heat 
conservativity, and thermal efficiency of the cookware while being free 
from partially heating food in the cookware. Another advantage of the 
method resides in that its reduces the production cost of the cookware. 
Although the preferred embodiments of the present invention have been 
disclosed for illustrative purposes, those skilled in the art will 
appreciate that various modifications, additions and substitutions are 
possible, without departing from the scope and spirit of the invention as 
disclosed in the accompanying claims.