Such forming tools, particularly high-pressure forming tools, have already been used for some time, particularly for the synthesis of diamonds or cubical boron nitride or the like. A forming tool of this kind is known from DE 38 34 996 C2. During operation, not only pressures of 50 to 80 kbar are exerted in the hollow working space by pressure pistons, but an electrical heating also provides an operat- ing temperature of more than 1200OC in the working space loaded by high pressure. The heating current may have an intensity of 500 to 2500 Ampere, and is produced by means of an alternating voltage of 4 to 10 volts.

To extend the life of the die, it is surrounded closely by rings of steel and a prestressed bandage of steel. However, the heavy current flowing through the material to be pressed does not only provide the heating of the material to be pressed, which is desired additionally to the heating caused by the pressing process itself, it also causes an additional heating of the armouring, so that an intensive cooling of the forming tool is required. Surprisingly, it has turned out in practice that an armouring with a wound bandage is more heavily heated by the additional electrical heating than an armouring having only wound steel rings.

The power dissipation may amount to more than 3 kW. Accord- ingly, a higher cooling performance is also required.

The invention is based on the task of providing a forming tool as mentioned in the introduction, in which a reduced power dissipation occurs.

According to the invention, this task is solved in that the windings of the bandage are mutually electrically isolated and the magnetic field of the heating current is linked with at least one secondary current circuit linked with the bandage, the electrical conductivity of the major part of the length of this circuit being at least equal to that of aluminium.

Surprisingly, this solution has turned out to result in electrical conditions, which are comparable to those of a toroidal core transformer. In this connection the heating current is the primary current and the armouring is the magnet core. The magnetic flow produced in the armouring by the heating current induces an electrical current in the secondary current circuit, whose magnetic field also pro- duces a magnetic flow in the armouring, which is directed against the magnetic flow produced by the heating current.

As the secondary current circuit forms almost a short- circuiting current circuit, the secondary current intensity and thus also the magnetic flow produced by the secondary current are correspondingly high, and accordingly, the to- tal magnetic induction in the armouring is relatively low.

The reduction of the induction then causes a correspond- ingly heavy reduction not only of the eddy current losses, which are created without the secondary induction, but also of the hysteresis losses, which are proportional to the

third exponent of the induction or the magnetic field in- tensity, respectively, and which are normally much higher than the eddy current losses, however, now being heavily reduced. Due to the mutual isolation of the windings of the bandage, it practically acts as a metal package, so that the eddy current losses in the bandage are even further re- duced. Even though parts of the armouring are placed in the secondary current circuit, the additional ohmic heat losses caused in these parts are substantially lower than the re- duction of the hysteresis and eddy current losses in the armouring. Particularly the heat development in the sensi- tive bandage is heavily reduced. It therefore has a sub- stantially longer life.

In detail it can be provided that each secondary current circuit has on each axial front side of the armouring at least one electrical conductor extending across the ban- dage, and radially outside the circumference of the bandage at least one axial conductor connecting the radial conduc- tors, the electrical conductivity of at least the radial conductors being at least equal to that of aluminium.

In this connection, parts of the secondary current circuit can be made up of the armouring, so that correspondingly less additional material is required for the secondary cur- rent circuit.

So, each radial conductor on the front sides of the armour- ing can form an annular disc and the axial conductors can be made up of axial rods, or the axial conductor on the ra- dial outside of the armouring can be made up of an sur- rounding ring. In this embodiment, the conductors have an

accordingly large cross section, so that the ohmic power dissipation converted in it is very low.

The annular discs can limit at least one cooling channel on the front sides of the armouring. Thus, they have a double function, namely being a secondary current circuit and a cooling channel limitation. Heat from both the armouring and the secondary current circuit can be dissipated through the cooling channel.

When the radial conductors are electrically isolated to- wards the axial front sides of the bandage, the secondary current does not flow through the bandage either.

Preferably, it is ensured that the radial conductors, each together with an axial conductor, form a bow, having on the front sides and on the radial outside of the armouring a distance from the armouring, each of its radial inner leg ends being connected via an electrical conductor with at least one armouring ring surrounding the die radially in- side the bandage, the conductivity of said conductor being at least equal to that of aluminium. Making the conductor as a bow will make an isolation between the radial conduc- tors and the bandage superfluous. The whole bow can be made of the same material, and with a sufficiently large cross section it provides a more efficient reduction of the in- duction in the armouring and of all electrical losses.

Preferably, one ingredient in the conductors is copper.

Thus, they themselves cause only insignificant ohmic losses.

However, it is also possible that the windings of the ban- dage are mutually electrically isolated and the magnetic field of the heating current is linked with a secondary current circuit, which is linked with the bandage, which circuit is, over the most of its length, made as a U-shaped bow. Thus, the electrical conductivity of the bow can also be lower than that of aluminium.

In the following, the invention and its different embodi- ments are explained on the basis of preferred embodiments shown in the enclosed drawings : Fig. 1 an axial section through a first embodiment of a forming tool according to the invention, Fig. 2 a part of an axial section through a second em- bodiment of a forming tool according to the inven- tion Fig. 3 a part of an axial section through a third embodi- ment of a forming tool according to the invention The forming tools shown are high-pressure forming tools.

According to Fig. 1 the forming tool comprises a die 1 of a high-pressure resistant material, here sintered hardmetal, alternatively of ceramic or steel. In the die 1, a tool in the shape of a block 2 consisting of, among other things, graphite is arranged, upon which a pressure P of about 50 to 80 kbar is exerted by means of counteracting pressure pistons 3 for the purpose of producing synthetic diamonds.

At the same time, with the purpose of heating the block 2, a pulsating heating current Il, for example in the shape of

square pulses, most frequently, however, in the shape of a sine-shaped alternating current with the usual mains fre- quency of 50 to 60 Hz and an intensity of 500 to 2500 A with an operating voltage from 4 to 10 V, is led through the pressure pistons 3 and the block 2, to provide an heat- ing of the block 2, additionally to the heating provided by the compression pressure, up to a total of approximately 1200° to 1400°. Between the die 1 and the block 2 is ar- ranged a bush 4 of a thermally isolating material, prefera- bly pyrophyllite, to protect the die 1, that is, the sin- tered hardmetal, from plasticizing, which might occur in connection with the high pressure and surface temperatures of more than 250°C. In case of plasticizing, the inner di- ameter of the die 1 would increase, causing micro or macro cracks in the die, or causing the die to fall completely apart. To protect the die further, it is surrounded by an armouring, which consists of a sleeve 5 surrounding the die 1, a first prestressing ring 6 surrounding the sleeve 5, a second prestressing ring 7 surrounding the prestressing ring 6, a bandage 8 surrounding the prestressing ring 7 un- der prestress, the bandage 8 consisting of a plurality of windings made of a steel band, and an outer ring 9 sur- rounding the bandage 8. The contact faces between the sleeve 5 and the prestressing ring 6 are conical. The sleeve 5 is shrunk onto the die 1, the prestressing ring 6 is shrunk onto the sleeve 5, and the prestressing ring 7 is shrunk onto the prestressing ring 6. The bandage 8 wound firmly around the prestressing ring 7 causes an extension of the life of the prestressing rings 6 and 7. Additionally to the prestressing rings 6 and 7 shown, further prestress- ing rings may be provided to ensure that the die 1 can stand the heavy radial pressure forces. The sleeve 5, the

prestressing rings 6 and 7 and the outer ring 9 are also made of steel.

Because of the bandage 8 of steel band, the permissible pressure P of the pressure pistons 3 can be increased by 25 to 40%, compared with an embodiment in which the bandage 8 is replaced by an additional prestressing ring. Together with the clamping ring 10, the outer ring 9 forms a hous- ing. The clamping ring ensures that the tool can be mounted.

The windings of the bandage 8 are mutually electrically isolated by a thin isolating layer, for example by means of an oxidation of their surfaces or by means of a separately applied isolating means.

The magnetic field of the heating current I1 is linked with at least one secondary current circuit 11, which is linked with the bandage 8 (that is, linked in the way of chain links), the electrical conductivity of this circuit 11 be- ing, over the major part of its length, at least equal to that of aluminium, preferably, however, to that of copper.

In stead of one single secondary current circuit 11, as shown, several secondary current circuits 11 can be dis- tributed on the circumference of the die 1.

On each axial front side of the armouring 5 to 10, each secondary current circuit has an electrical conductor 12 extending radially across the bandage 8, and radially out- side the circumference of the bandage 8 is an axial conduc- tor 13 connecting the radial conductors 12. The electrical conductivity of the conductors 12 and 13 is at least equal to that of aluminium, preferably, however, equal to that of

copper. Together with the axial conductor 13, the radial conductors 12 form a U-shaped bow, which has, on the front sides and the radial outside of the armouring (5 to 10), a distance from the armouring (5 to 10), each inner radial leg end of the bow being connected with at least one ar- mouring ring, here the prestressing ring 6 and/or the sleeve 5, surrounding the die 1 inside the bandage 8 by means of an electrical conductor 14 having at least the same conductivity than aluminium, preferably, however, than copper. The conductor 14 can, however, also be made as a thin layer of silver or soldering material.

Each of the pressure pistons 3 is also surrounded by an ar- mouring, comprising, in the same way as the armouring of the die 1, rings 15,16 and 17 and surrounding each other under prestressing. The axial height of this armouring (15 to 17) decreases on its axial inside over the free end of the bow in the direction of the centre axis 18 of the die 1.

Without the conductors 12,13,14 and the isolation between the windings of the bandage 8, when assuming that the wind- ings are not oxidised, even though this is often the case in a natural manner, the heating current Il would, during operation, cause ohmic losses in the armouring 5 to 10 be- cause of eddy currents, and additionally high hysteresis losses through a resetting of the existing steel armouring.

True enough, the eddy currents are relatively low, as they produce a magnetic field, which is directed opposite to the primary magnetic induction, on the other hand, the hystere- sis losses are very high. These losses may cause an over- heating of the die 1 and also of the armouring, so that the life of both die 1 and armouring is limited. Adding the

electrical conductors 12,13, which form the bow, and the electrical conductor 14 connecting the bow with the sleeve 5 and the prestressing ring 6, creates a secondary current circuit 11 across the bow 12,13, the conductor 14, the sleeve 5 and the prestressing ring 6. In this secondary current circuit 11, which practically forms a short- circuiting circuit, the primary induction, which is pro- duced by the heating current Il, induces a very heavy sec- ondary current I2, whose magnetic field is linked with the armouring (5 to 10) and is directed against the primary in- duction, so that the total induction of the armouring is substantially lower than the primary induction. Conse- quently, and because of the mutual isolation of the wind- ings of the bandage 8, which acts as a package of mutually isolated, very thin metal sheets, not only the eddy cur- rents induced in the armouring, but to an even higher de- gree also the hysteresis losses, are substantially reduced.

The reduction of the eddy current and hysteresis losses is much larger than the increase of the ohmic losses in the sleeve and the prestressing ring 6 caused by the secondary current 12, so that the thermal load of the armouring and thus also the die 1 is substantially lower. The heat pro- duced in the bow (12,13) and the conductors 14 by the oh- mic loads can also be kept very low by a correspondingly large dimensioning of the cross section of these conduc- tors, which preferably have a square cross section, so that no significant additional thermal load occurs on the ar- mouring parts 5,6 through the heat dissipation from the bow via the conductor 14 to the armouring parts 5,6.

In the embodiment according to Fig. 2, each of the radial conductors 19 of the secondary current circuit 11 forms an annular disc on each axial front side of the armouring, and

the outer ring 9 of the armouring also forms an axial con- ductor of the secondary current circuit 11 on the radial outside of the armouring. Accordingly, each of the prestressing rings 6 and 7 forms an axial conductor on the radial inside of the bandage 8, as the conductors 19 made as annular discs bear on the axial front sides of the prestressing rings 6 and 7 and of the outer ring 9, the conductors 19 being fixed on the front sides of the outer ring 9 by means of screws 20 distributed on the circumfer- ence of the annular discs 19 and the outer ring 9. The an- nular discs 19 are preferably made of copper, whereas the prestressing rings 6,7 and the outer ring 9 are still of steel. Additionally, the annular discs 19 limit at least one, here two cooling channels 21 and 22 on the front sides of the armouring, the cooling channels 21,22 extending concentrically around the centre axis 18 of the die 1, the cooling channel 21 being arranged near the die 1. Also the annular discs 19 are arranged concentrically to the centre axis 18 of the die 1. In this connection, both annular discs bear firmly, to be well conductive, for example by means of soldering, with the axial inner surface of a ra- dially inside annular bulge 23 on one end face of the prestressing ring 6, with an axial inner surface of a sec- ond annular bulge 24 on each end face of the prestressing ring 7 and with an axial inner surface of a third annular bulge 25 on each one axial outer end face of the outer ring 9 or the clamping ring 10, respectively. In this connec- tion, each of the annular bulges 23 and 24 limits the sides of one of the cooling channels 21, whereas each of the an- nular bulges 24 and 25 limits the sides of one of the cool- ing channels 22. Further, the cooling channels 21 are addi- tionally sealed against the armouring by means of sealing rings 26 and the cooling channels 22 by means of sealing

rings 27. One of the annular discs 19 is provided with inlet and outlet bores 28 (of which only one is shown) and the other annular discs 19 is provided with inlet and out- let bores 29 (of which only one is shown) for a cooling fluid, here water. Further, the cooling channels 21 can be connected through at least one cooling channel 30, which penetrates the armouring, here the prestressing ring 6 ly- ing next to the die 1, so that the cooling fluid also flows through the armouring near the die 1.

The cooling channels 22 also have inlet and outlet bores for the cooling fluid (not shown). Further, the cooling channels 22 are provided with an electrically isolating layer on the insides facing the bandage 8, the cooling channels bridging the front faces of the armouring 8, not being electrically connected with these front faces through the cooling medium.

Otherwise, this embodiment is equal to the embodiment ac- cording to Fig. 1.

Also in the embodiment according to Fig. 2, a secondary current I2 is induced in the secondary current circuit 11, the secondary current flowing via the annular discs 19, the outer ring 9 and the prestressing rings 6 and 7, its mag- netic field reducing the primary induction produced by the heating current I1 in the armouring, so that the eddy cur- rents induced in the armouring by the resulting induction, and the eddy current losses caused by the eddy currents and the hysteresis losses are substantially reduced in relation to an embodiment without the secondary current circuit 11.

The sleeve 5, the prestressing rings 6,7 and the bandage 8 are thus generally less heated, in spite of the secondary

current 12 flowing through the prestressing rings 6,7 and the outer ring 9. The cooling by the cooling liquid in the cooling channels 21,22 and 30 provides an additional re- duction of the temperature of the armouring and thus also of the die 1. Compared with the embodiment in Fig. 1, the cross-section of the secondary current circuit 11 carrying the secondary current 12 is larger, so that also the same current intensity of the secondary current 12 will cause less heating of the secondary current circuit 11 than in the first embodiment.

Additionally, after an increase of their outer diameter, the radial outer edges of the annular discs 19 can be con- nected by means of an additional conductor in the shape of a ring of copper or in the shape of axial rods of copper.

The secondary current 12 would then substantially flow through these additional conductors and not via the outer ring 9. Accordingly, the heating of the outer ring 9 and thus also of the bandage 8 through the outer ring 9 would be reduced.

The embodiment according to Fig. 3 in principle only dif- fers from the one according to Fig. 2 in that the annular discs 19 limit no cooling channels, however again being electrically isolated towards the front faces of the ban- dage 8 and possibly also of the outer ring 9 and the clamp- ing ring 10 by means of an isolating layer (not shown), have a larger outer diameter than the annular discs 19 ac- cording to Fig. 2 and are connected by means of screws 32 between their radial outer circumferential edges through conductors 31 in the shape of rods of copper. In this case, the secondary current 12 flows not only through the prestressing rings 6 and 7, but also through the sleeve 5,

so that the ohmic resistance in the secondary current cir- cuit 11 is correspondingly lower.

Otherwise, the mode of operation is principally the same as in the first embodiment.