Device for the induction heating of a workpiece

A device for heating a workpiece includes a coil arrangement connected to a voltage source for generating a magnetic field. The coil arrangement is positioned around a core of electric steel sheeting having two poles spaced apart to form an air gap for accommodating a central portion of a workpiece. One of the poles is movable wherein the poles engage the workpiece which bridges the air gap or engage each other by extending through the center of an annular workpiece. The coil arrangement includes first and second coils, each positioned about one of the poles, generating a magnetic field inside and outside the core. The magnetic field concentrated in the core heats the central portion of the workpiece while outer magnetic field heats peripheral portions of the workpiece to achieve a rapid and uniform heating of the workpiece. At least one of the coils can be moved with respect to the associated pole to control the heating of the workpiece. A third coil can be positioned around the air gap and connected in series with the first and second coils.

The present invention relates to methods and devices for magnetically 
heating a workpiece consisting at least partly of metal More specifically, 
the invention is concerned with the preheating of moulding or press tools, 
the separation of metal from a polymer, the production of composite 
materials, and the hardening of adhesive between two metal members. 
For the preheating of press tools, which is necessary before the tool is 
operated, use is made at present of hot-air furnaces in which the tool is 
heated to about 450.degree.. This takes about 10 hours, which is far too 
long for an efficient production. Furthermore, it is difficult in a 
furnace, to heat the press tool to a uniform temperature across the entire 
tool cross-section. 
Hitherto, a number of techniques have been utilised to separate polymeric 
materials, such as rubber and plastic from metal. One technique involved 
burning-off of the polymer, which has caused emissions hazardous to the 
environment, and structural changes and stress in the metal. DE 2,900,655 
indicates a different technique involving the use of an induction coil. An 
object, for example a vehicle tire, is positioned within the induction 
coil which is activated by a high-frequency voltage to generate a magnetic 
field heating the metal in the vehicle tire to such an extent that the 
polymer can be detached from the metal. Generating high-frequency voltage 
is expensive and cannot be utilised for all types of objects. If the 
object has projecting metal parts, a so-called point effect is obtained 
which means that the projecting metal part is heated to red heat. 
Furthermore, the high-frequency technique is restricted to relatively thin 
objects since the penetration depth in metal is slight when use is made of 
high frequency, for example 1 to 16 kHz. 
A first object of the invention is to achieve rapid and uniform heating of 
a workpiece, without having to rely on the shape and thickness of the 
object for the desired result. PG,3 
A second object of the invention is to provide an inexpensive and 
uncomplicated heating device adapted to utilise the existing electric 
mains. 
A third object is to provide a device which, upon separation, gives clean 
and faultless final products, i.e. metal parts which are clean from 
polymeric material and which have not been affected by structural changes 
and stress, as well as polymeric material which is not heated to 
temperatures at which it is destroyed, the final products being reusable. 
A fourth object of the invention is to provide an energy-saving heating 
method by which the losses are restricted to the heat developed in the 
coils, and to losses in the core. 
These objects are achieved by devices and methods having the features 
defined in the characterising clauses of the appended claims.

FIG. 1 illustrates a core 1 of transformer sheet steel. The core has a 
stationary pole 2 and a movable pole 3. The pole 3 is movable into 
engagement with the pole 2 to form a short-circuited core. In the 
embodiment shown in FIG. 1, the movable pole 3 is displaced upwardly to 
form an air gap between itself and the stationary core 2. A first coil 4 
is positioned around the movable pole and a second coil 5 is positioned 
around the stationary pole. A third coil 6 is positioned around the air 
gap between the first and the second coil. The coils 4, 5 and 6 are 
connected in series via lines 9, 10, 11 and 12 and connected to an 
alternating voltage source 7 which is a single-phase source utilizing the 
zero conductor of the electric mains and gives, for example, 220 V at 50 
Hz. Naturally, also two or three phases may be utilised. In one of its 
aspects, the invention aims at providing adequate heating by means of the 
normal mains voltage and frequency. 
The poles 2 and 3 have been designed to retain the workpiece during 
heating, and therefore there is no need of special means for holding the 
workpiece during heating 
The workpiece consists of metal coated with a polymeric material, such as 
rubber. The workpiece may also be a press or moulding tool. In FIG. 1, the 
workpiece extends beyond the core, but also smaller workpieces of a width 
below the cross-sectional area of the core can be heated in a device 
according to the invention. One example of such a workpiece is a ball 
bearing. The workpiece may have at least one metal portion bridging the 
air gap and short-circuiting the core 1. The device is operable also if 
short-circuiting is prevented by a layer of polymeric material on the 
workpiece. It should be noted, however, that the losses increase with the 
thickness of the said layer. 
Upon application of a voltage, the coils generate a magnetic field which is 
conducted in the circuit formed by the core 1 and the workpiece 8 and also 
occurs outside the core. The magnetic field conducted in the core passes 
through the central metal portion of the workpiece 8 and heats it. The 
magnetic field outside the core will heat the peripheral metal portions of 
the workpiece by induction. By using a relatively low frequency in the 
range 16-400 Hz, adequate penetration depth in the metal is obtainable, 
and so the metal is uniformly heated. 
Upon separation of polymeric material and metal, the heating of the metal 
causes a chemical degradation of the layer of the polymeric material in 
contact with the metal. 
The frequency can be varied in dependence on the thickness of the workpiece 
If a penetration depth of about 2 cm is desired, the frequency is set at 
about 25 Hz. For heating thin-walled workpieces, the frequency may be for 
example 200 Hz. 
In FIG. 1, the coils are connected in series, but can also be connected in 
parallel, provided that their magnetic fields cooperate to heat the 
workpiece. 
At least the first coil 4 can be raised and lowered to control the heating 
of the workpiece. A microprocessor is preferably used for controlling the 
movement of the coil 4 and the movable pole 3. If desired, also the 
remaining coils can be made vertically movable. In some cases, the coil 6 
may be omitted, and then the magnetic field generated by the coils 4 and 5 
is sufficient to heat both the peripheral portions and the central portion 
of the workpiece. On the other hand, the coil 6 assists in concentrating 
the magnetic field outside the core to the workpiece peripheral portions. 
Heating a workpiece to 450.degree. C. in a conventional hot-air furnace 
would take 10 hours, but with the new technique according to the present 
invention the heating takes but a fraction of this time, for example 15 
min. The new technique gives a temperature which differs by not more than 
.+-.5.degree. C. throughout the cross-sectional area of the workpiece. 
In FIG. 2, a workpiece in the form of a ring 13 is heated. In this 
embodiment, the movable pole 3 has been lowered to engage the stationary 
pole 2. Upon activation of the cores, an electrical current is produced in 
the workpiece 13 which assists in heating the workpiece. As in the 
embodiment according to FIG. 1, the magnetic field from the coils 4. 5 and 
6 induces heat within the workpiece. 
FIG. 3 shows a device for removing polymeric material from a large 
workpiece, such as a metal tank. The device comprises a U core 14, the 
legs of which are provided each with one coil 17 and 18, respectively. The 
coils are connected in series by means of lines 19, 20 and 21 and 
connected to a voltage source 7 of the type mentioned above. The device is 
applied to the tank 22, and the magnetic field generated upon activation 
of the coils is conducted through the U core 14 and the tank region 
between the legs 15 and 16 which is heated to the desired temperature by 
controlling the applied voltage, the frequency and the time during which 
voltage is applied When the region has been heated, the device is moved to 
another region of the tank and the polymeric material of the area first 
heated simply is peeled off. 
In the embodiments described above, a short-circuited core has been used. 
In some cases, it is also possible to utilise only the poles 2 and 3 of 
the core and to dispense with the remaining core part, although a higher 
current consumption and higher losses are then obtained