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INDUCTION HEATING DEVICE AND METHOD FOR MAKING PARTS USING SAME
The invention relates to a method and a device for
heating a metal surface by induction, in particular in order to
carry out a molding or transformation, especially of
thermoplastic or thermosetting matrix composite materials.
To heat a metal surface in order to carry out
especially a molding of a part made of plastic or composite
part, there is a known way of burying inductive wires in a
volume of resin or the like, the surface of this volume to be
heated comprising a plate made of magnetic material, this plate
being called a "susceptor". The heating is obtained by
electromagnetic coupling between the inductors and the magnetic
plate.
This technology has major drawbacks that make it
difficult to exploit. Indeed, the heating of the susceptor is
not homogeneous because it is the maximum at the position of
each inductive wire and diminishes between these positions.
Furthermore, since resin is a thermal insulator it is not easy
to obtain the cooling necessary between two duty cycles.

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Furthermore, the heating and cooling cycles may alter the
mechanical properties of this resin. Finally, resin has low
resistance to impact.
The invention overcomes these drawbacks.
The device of the invention comprises a body having at
least; one part made of magnetic and heat-conductive material,
with a plurality of closed cavities in the proximity of the
surface to be heated, each cavity surrounding an inductor, the
heat produced by induction on the walls of the cavity being
transferred by conduction to the heating surface, the inter-
cavity distance and the position of these cavities relative to
the heating surface being such that the heating is substantially
uniform on this surface.
The magnetic and conductive material is, for example,
steel.
Thus, the heating of the surface is uniform, and the
efficiency is high since the coupling between each inductor and
the corresponding cavity is the optimum, with the cavity
completely surrounding the inductor. Furthermore, the material
of the body of the heating surface may be less sensitive to
ageing than a resin.
Since the magnetic material constituting the body of
the device is a thermal conductor, the cooling can be done
efficiently.
In one embodiment, to minimize thermal losses by
conduction on the opposite side to the heating surface, the part
of the body that is on the opposite side to the surface to be
heated relative to the cavities is made of a non-magnetic
material.
In one embodiment, the cavity take the form of grooves

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in two parts of the body, the first part which ends in the
surface to be heated -being made of magnetic material and the
second part, opposite the surface, being made for example of
non-magnetic material.
The grooves, and therefore the cavities, may have any
unspecified section, for example a circular section or a square
or rectangular section.
In one embodiment, for the cooling between two
surface-heating cycles, there are provided channels designed to
be crossed by a cooling fluid, these channels being located
between the cavities and the heating surface. The channels have
for example a direction parallel to the cavities. As a variant,
they have a direction perpendicular to the cavities.
According to one embodiment, each inductor has a
tubular shape in which the central channel serves for the
circulation of a cooling fluid. This cooling of the inductors
can also serve for the cooling of the body of the device between
two heating cycles.
As a variant, the inductive tube is preferably lined
with an insulator on its external surface and the external
surface of the tube, possibly the external surface of the
insulator, is at a distance from the internal wall of the cavity
so as to make a ring-shaped space for the circulation of another
cooling fluid designed to cool the body between two heating
cycles. Thus, with this embodiment, the space requirement of
the cooling means is minimized. Furthermore, the positioning of
the inductors in their cavity can be done easily.
With this last-mentioned embodiment, the thermal
losses are minimized because, during the induction heating, the
air between the walls of the cavity of the inductor constitutes

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a thermal insulator since of course the fluid for cooling
between two cycles does not flow during this heating phase.
In another embodiment, the space between each inductor
of the internal wall of the cavity is entirely filled with an
electrical insulator.
In one embodiment, a heating apparatus comprises two
devices of the type defined here above, for example one forming
a die and the other forming a punch. The two devices can be
powered in such a way that their temperatures are different, for
example so as to obtain different surface states on a same part.
The surfaces to be molded may have any unspecified
surface area.
The invention also relates to a method for the
manufacture of parts by molding or transformation by means of at
least, one heating surface using the device as defined here
above. It also relates to a method for the manufacture of parts
by molding or transformation by means of an apparatus comprising
at \cast two of these devices.
Other features and advantages of the invention shall
appear from the description of some of its embodiments, this
description being made with reference to the appended drawings,
of which:
Figure 1 is a drawing of a device according to the
invention,
Figure la shows a part of the device shown in figure
1,
Figure 2 is a top view of a device shown in figure 1,
Figure 3 is a drawing showing an alternative
embodiment of the cooling means for the device shown in figure
1, and

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Figures 4, 5 and 6 are drawings of examples of molds
according to the invention.
In the example shown in figure 1, the device 10
constitutes the half portion of a mould for the shaping and/or
transformation of a part by heating. Thus, in this example, the
device 10 forms the lower part of a mould, the upper part of
which is not shown.
In this device 10, it is therefore necessary to heat
the upper face 12 in order to transform or mould a part 14.
According to the invention, to keep the surface 12,
the device 10 comprises a body 16 which, in the example, has two
parts, 18 and 2 0 respectively. These two parts are made of
steel . The part 18 is made of magnetic steel while the part 20
is made of non-magnetic material, for example also steel.
The part 18 made of magnetic material is the one
comprising the heating surface 12. The lower portion of this
part 18, which has a generally parallelepiped shape in the
example, has circular, square or rectangular sectioned grooves
with identical grooves of the part 2 0 of the body 16
corresponding to them. Thus, when the part 18 and 20 are
assembled as shown, the grooves form channels or cavities 221#
222, etc. each of which is designed to hold an electrical
conductor 24, for example made of copper, which is crossed, for
the heating, by an alternating current at high frequency, for
example a frequency ranging from 100 to 200 KHz, in order to
induce an electromagnetic field.
As can be seen in figure 2, the various conductors 24
are connected to one another by jumpers 26.
In the example shown in figure 1 and figure 2, the
magnetic part 18 of the body 16 is crossed by channels 281, 282,

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etc. having a general direction perpendicular to the channels
221, 222. These channels 28x, 282, ... are designed to receive a
cooling fluid between two heating cycles. As a variant, there
may be provided cooling channels 301, 302 having a direction
substantially parallel to the cavities 22x, 222, etc.
In another variant, which shall be described further
below with figure 3, the cooling is done in the cavities 22.
In the example shown in figures 1 and la, the
conductor 24 is tubular so as to bring about a circulation of
fluid for cooling the conductor, and it is insulated from the
internal walls of the cavity 22 by a ring-shaped and insulating
layer 32.
The working is as follows:
The high-frequency current, whose intensity is of the
order of 100 to 200 KHz, crosses the conductor 24 and produces
an electromagnetic field which, by coupling, heats the walls of
the magnetic part of the cavity. The coupling is perfect since
the cavity completely surrounds the conductor. Thus, losses are
minimized.
The heat produced on the walls of the cavity is
propagated to the surface 12 in a diffusion zone 34 having a
substantially conical shape.
The distance from the cavities to the surface 12 and
the distance between two adjacent cavities must be such that, on
the surface 12, the diffusion zones 34 form an intersection so
that the temperature of the surface 12 remains uniform.
However, in order to minimize heat losses, the
distance from the cavities to the surface 12 should not be
excessive.
The heat losses toward the rear, i.e. in the part 20

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of the body 16, are minimized because the heat produced is
produced by the magnetic part of the cavity and not by the non-
magnetic part.
As shown in figure 2, the inductive currents 36 induce
currents in opposite directions in the cavity.
In the variant shown in figure 3, to optimize the
heating, there is no provision for cooling conduits of the type
shown in figure 1 but the cooling is obtained in each cavity.
Thus, the cavities 22 may be closer to the surface 12 and there
is no obstacle to the propagation of heat towards the surface
12.
The tubular conductor 24 is lined with an insulating
layer 40 and the section of this insulated conductor has a
dimension substantially smaller then the section of the cavity
22. Thus a ring-shaped space 42 is made between the conductor
24 and the internal surface 44 of the cavity and, in this ring-
shaped space 42, a fluid, in particular a liquid, for cooling of
the body 16 is made to flow between two heating cycles.
During the heating, the ring-shaped zone 42 is filled
with air. This feature thermally insulates the cavity of
the tube 24. In other words, the heat produced in the part 18
of: the body 16 makes practically no contribution to heating the
tube 24.
In one embodiment, the part 14 to be processed has two
surfaces that have to present different aspects. To this end,
the upper part of the mould (not shown) has a device (not shown)
similar to the device 10 described here above with a power
supp1y to the inductors that is different from the power supply
to the inductors of the lower device 10.
Thus, the heating temperature of the upper and lower

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parts may be different in order to give the different surface
states.
This possibility of different temperatures is
naturally not limited to different surface states. It may also
entail, for example, the processing of parts made of materials
that are different on each face.
Figure 4 is a view in section of a mould compliant
with the invention and designed to make a tube.
This mould therefore has two devices 50 and 52, each
having a semi-cylindrical cavity, respectively 54 and 56. These
cavities are heated as described here above, in particular as
described with reference to figures 1 and 3. The material 58 to
be shaped as a tube by the heating operation is applied by
compressed air against the induction-heated walls 54, 56.
In each of the devices, the inductors are evenly
distributed in a magnetic material around the surfaces 54, 56.
Kach of these inductors and the cooling means of the mould are
of the type shown in figure 3, i.e., each copper conductor 60 is
tubular to let a cooling fluid circulate within, and between
this conductor 60 and the cavity 62 made of magnetic material, a
ring shaped space 64 is made, filled with air during the
molding. In this space 64, a cooling fluid flows between two
molding cycles.
Figure 5 is a view similar to that of figure 4 but
pertains to the molding of a part made of composite material
having, for example, the shape of an element of an automobile
body such as a hood. In this case, there is provided a device
70 forming a punch and another device 70 forming a die. The
Inductors distributed in the vicinity of the molding surfaces,
74 and 76 respectively, so that, as described already, uniform

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temperatures are obtained on these surfaces.
Finally, figure 6 represents a mould used to obtain a
flat plate. This embodiment is distinguished from the one shown
in figures 4 and 5 by the fact that the conductors 80 have, in
this case, a rectangular or square section and that similarly
the cavities have a rectangular or square section.

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CLAIMS
1. Device for heating a surface by induction, in
particular in order to carry out a molding or transformation of
a part made of thermoplastic or thermosetting composite
material, comprising a body (16) having at least one part (18)
made of magnetic and heat-conductive material, with a plurality
of closed cavities in the proximity of the surface (12) to be
heated, each cavity surrounding an inductor (24) , the heat
produced by induction on the walls of the cavity being
transferred by conduction to the heating surface, the inter-
cavity distance and the position of these cavities relative to
the heating surface being such that the heating is substantially
uniform on this surface.
2. Device according to claim 1 wherein the magnetic
and heat-conductive material comprises steel.
3. Device according to claim 1 or 2 wherein the part
of the body (20) that is on the opposite side to the surface to
be heated relative to the cavities is made of non-magnetic
material.
4. Device according to one of the claims 1 to 3
wherein each cavity is formed by the association of two grooves,
one groove being formed in a surface of the part of the body
made of magnetic material and the other groove being formed in a
surface of another part of the body.
5. Device according to one of the claims 1 to 4
comprising conduits (281, 282; 301, 302) for the circulation of a
cooling fluid between the cavities and the heating surface.
6. Device according to one of the above claims
wherein each inductor has a section smaller than that of the

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cavity so as to form a ring-shaped space (42) for the
circulation of a cooling fluid between two heating cycles of the
surface to be heated.
7. Molding or transformation apparatus comprising at
least two devices according to one of the above claims.
8. Apparatus according to claim 7 wherein the power
supplies for the inductors of the two devices are distinct.
9. Method for making parts by molding or
transformation by means of a heating surface, making use of a
device according to one of the claims 1 to 6.
10. Method for making parts by molding or
transformation by means of an apparatus according to claim 7 or
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The invention concerns a device for heating a surface by induction, in particular for
molding or transforming a part made of thermoplastic or thermosetting composite
material. The device comprises a body (16) having at least one portion (18) made of
magnetic and heat conducting material wherein is provided a plurality of closed cavities
proximate the surface (12) to be heated, each cavity surrounding a field winding (24).
The heat produced by induction on the walls of the cavity is transferred by conduction
to the heating surface. The distance between the cavities and the position of said
cavities relative to the heating surface are such that the heating is substantially uniform
on said surface.