DE1558159B2 - PROCESS AND DEVICE FOR VACUUM PASTING OF PRECISION PARTS MADE OF METAL WITH THE HIGHEST PURITY - Google Patents

Description

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The invention relates to a vacuum casting process or it may be necessary to zerfür the molds Precision metal parts with the highest purity, break to remove the cast parts, In the case of the form and the metal to be cast there is a common but processed spectrographically pure graphite Are exposed to high vacuums and molds are very expensive, exists for commercial others the molten metal in at least a 5 turns an urgent need to reuse heated Mold cavity is poured. The hereditary forms.

The invention also relates to a device for From the Austrian patent specification 225 860 is

Implementation of the method according to the invention. it is basically known to metal under high vacuum

The requirements for material purity are not only to be melted and cleaned, but

known from the vacuum switch industry, can also be potted with the io.

Contacts are required, the content of which is gaseous. A major disadvantage of this known

gen impurities below 10 ³ parts per driving is that the cleaning process with

Million must be. One reason for this high level of potting has ended.

requirements for the purity is that every furthermore are from the French patent specification

Contamination in contact, a method and a device can be used at the grain boundaries 15 1 420 195

zen to concentrate seeks. Gaseous impurities are known for the directional solidification of castings

Cures are particularly undesirable as they cause during.

the switch actuation by heat from the Kon- The invention is based on the object a

clocks are released. This will create the printing method and apparatus using

increase in the switch to a value at which the 20 which

Switch the electric current no longer enables lower cleaning of cast parts,

breaks. The invention seeks to solve this problem

Perhaps the best known method of demonstrating that the solidification of the metal within the len of such pure metals is to control the zone melting mold cavity in such a way that the advance. In this process, a narrow, intrusive solid-liquid boundary layer is slowly moved with no molten cross-section through a solid block obstruction from the lower and outer portions of the metal to be cleaned as the block progresses through the mold cavity and is fully exposed to a high vacuum. This procedure ends within a blind-ending riser,
is based on the differences in the solubility of the base metal. If thus the will be used.

narrow zone of molten metal by the A preferred embodiment of the invention Block moves, the impurities concentrate - is characterized by the fact that the solidification dagung of the block in the liquid before it is controlled by that immediately urgent solid interface, so that the impurities 35 provided above each mold cavity After pushing one end of the block, so much molten metal was poured into the funnel the. At the same time, the high vacuum is used to ensure that every mold cavity and the pouring melt to degas. This means that the funnel will be filled.

vacuum removes those impurities that another preferred embodiment sees have a relatively high vapor pressure, 40 before that the solidification is controlled in that as well as those that form gaseous reaction products concentrically around the mold cavity, which induction coil seen pumped out by the pumping device at a speed will. moved up to a maximum of about 50 mm / h

Most uses of such metals will be.

highest purity usually require that the 45 one to carry out the invention

Cleaned metal is still being cast and machined. Method particularly suitable device draws

must, without a renewed contamination of the fact that a melting device with

Metal occurs. In particular, the casting becomes that of an outlet means for the molten metal

Metals difficult because the metal, as soon as it is molten, has at least one mold cavity

zen is easily present through the atmosphere or the shape 50, which is an upwardly protruding

can be contaminated. To such impurities blindly ending riser has that further a

conditions to avoid, the casting is usually provided in the molding heater and that

Spectrographically pure graphite forms made within the high vacuum above the molded part

men that are under high vacuum. If a pouring funnel is also arranged, which one by means of such a casting technique, the problem of the unimportant volume fraction of the metal can be absorbed

cleaning is solved, so the casting process can.

Due to the high vacuum very complicated and cause- For the simultaneous production of several cast parts

mildly significant difficulties and costs. The is preferably provided that a plurality of

The greatest disadvantage is that, due to the room boundaries, they are arranged vertically on top of one another and are encircled together 60 of the connected mold cavities are usually provided in the case of vacuum casting,

is limited to one casting per casting process. each ending with a blind ending riser

There have already been numerous attempts to equip them.

several cast parts in a vacuum casting process. The arrangement of the molded parts is preferred

at the same time. These attempts have each made that a plurality of molded parts but led to processes in which the graphite 65 made of graphite are present, which are vertically aligned

molds were not reusable. The thermi- other are arranged in a high vacuum between

see contractions between a plurality of which mold cavities are formed.

Cast parts can destroy the molds, another advantageous embodiment of the

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inventive device is characterized according to FIG. 1 the device has a closed, that the mold cavities with each other by a separate vessel or a vacuum chamber 10, the Sprue channel are connected, the between the circumference by an elongated, vertically arranged, pipe catch and the molded parts and a receiving yoke-shaped part 11 is formed with removable is formed, in which the molded parts are arranged 5 covers 12 and 13 are provided which are attached thereto are and that the sprue are cursed with each other. A high vacuum pump device 16 is used There is a tendency towards vertical slots in all for evacuating the chamber 10 via a suction tube Moldings except the lowest and 14, which is in connection with the upper part of the Kammit the outer circumferences of the mold cavities in terms of value 10. The pumping device should be within the commitment. io chamber 10 maintains a suitably high vacuum

According to a further embodiment of the invention. The size of the vacuum depends on

The device according to the invention is provided that, generally speaking, depends on the type of metal being cast.

half of the melting facility an alloy container When casting copper and copper alloys for

is provided, which is within a closed vacuum switch contact, the pressure in the chamber

The container is in which the forms receiver 10 below 510 "" ⁵ mmHg. The required variable

Mende yoke is arranged and that the alloy vacuum for the casting of other metals is in the

container has a Charching device with which vacuum casting technology is well known and needs

rather the content of the container in the melting facility - therefore not to be explained in detail here,

tion is transferred. The tubular part 11 is made of a non-conductive,

To change the geometrical dimensions of the 20 high temperature resistant material manufactured such as

Castings can be provided that the geome- high temperature glass, silica or ceramic materials,

Irish dimensions of castings that are impermeable to gases. There should be another

Mold insert is provided, which in a device such as an ion measuring device 15

several mold cavities can be used. preferably be provided on the tube 14 to the

This insert can advantageously be used to measure the vacuum generated within the chamber 10,

forms that a fusing with the casting within the vacuum chamber 10 lies on one

metallic insert is provided, which in each suitable, insulating support part 21 is made of spectro-

Mold cavity can be used. graphically pure graphite mounting yoke

It is advantageous to preheat the pouring funnel. This yoke consists primarily of one

Often provided that the molding-Heizeinrich- 3 ° circular-cylindrical, open-top container, which is in the

device consists of an induction coil, the concen-bottom with an ejection hole 19 is provided. That

trisch is wrapped around the molded parts. insulating support part 21 can be made of any high-

Finally, to heat the molded parts, there is a pre-temperature-resistant material that forms the yoke

In some cases it is provided that a pouring funnel 20 is held above the cover 12. Preferably will

Heating device is provided, which consists of a concentrated 35 the support part 21 by a short, cylindrical quartz

Trically around the pouring funnel around the induction tube shown, which stands upright on the cover 12,

tion coil consists. as it F i g. 1 shows.

The achievable according to the invention technical within the receiving yoke 20 are in progress consists essentially in the fact that the right direction over one another is spectrographically pure Metal to be processed before casting from none 40 graphite molds 22, 23 and 24 stacked in such a way that needs to be particularly pure, as the melt between every two adjacent molded parts requires molding, holding the melt and the casting itself cavity 25 is created. The mold parts 22, 23 and 24 the desired high degree of purity in the cast part is guaranteed in close contact with one another by means of a Afford. In addition, according to the invention, a number of different cast parts can be held at the same time, which are composed of spectrographically pure graphite and are connected to the receiving yoke 20 will be presented. by some means such as

F i g. 1 is a section through a high vacuum and a removable pin 27, releasably connected

casting device for precision casting of the highest purity. In this way, through the sleeve 26 and

according to one embodiment of the invention; the surface of the uppermost molded part 24 is a heat

Fig. 2 shows an enlarged section through two 50 receiving containers 28 made of spectrographically pure

spectrographically pure graphite forms, such as those formed with graphite.

Devices similar to Figure 1 are used; All molded parts 22, 23 and 24 as well as the sleeve

F i g. 3, 4 and 5 show different mold inserts, 26 are machined with a circular cylindrical

how to make a multitude of under- see shapes so that they fit snugly into the

Different configurations of the castings used 55 receiving yoke 20 fit into it. The mold cavities that

can be. can take any shape you want are in

The device shown in the drawing, especially the abutting flat surfaces of the

special the shapes, is or are primarily molded parts incorporated.

for casting vacuum switch contacts of the highest quality designed. The following description relates to approximately 60 example arranged so that a plurality hence primarily this particular application of castings made in a single cast inventive method and. that becomes invention. The lowermost molded part 22 is in its above device. Even if the casting of surface is provided with part of the mold cavity, pressure switch contacts the most prominent appearance while the uppermost molded part 24 in its lower part represents the invention, it is clear that 65 surface has part of a mold cavity, the invention is not limited to this, but between the parts 22 and 24 are shown in FIG. 1 is also provided for a plurality of intermediate mold parts 23 in every type of casting process. In Fig. 2 can use the highest purity. only one intermediate molded part 23 is shown.

When producing a single cast part, only the molded part 22 and the molded part 24 are required.

Each mold part 23 and 24 must be provided with a sprue, which the mold cavities 25 with the Heat absorbing container 28 connects. This is preferably done for each molded part 23 and 24 with one reached single vertical channel or slot 29 provided on the outside. The slots 29 lie in the vertical direction in alignment within the receiving yoke 20 and form a sprue between the molded parts and the yoke 20. Of course, the lowermost molded part 22 does not need to be provided with such a slot. If necessary, the sleeve 26 should also have a slot or Have incision 31, which forms the access to the sprue, which is represented by the slots 29 will.

The top of each mold cavity 25 is approximately in the middle of the cavity with a blind ending Riser 30 provided (Fig. 2). Because the device is exposed to a high vacuum during casting the molten metal will substantially fill the blind riser 30. That means that at In high vacuum there are practically no air pockets in the mold cavity 25 to prevent it could that the molten metal substantially fills the winch Steiger. It is essential that the blind riser 30 takes up a considerable proportion of the volume of the total cavity, at least 3% and preferably 5 to 20%. It is also important that the surfaces of the mold cavities 25 merge smoothly into the blind riser 30, so that there are no high spots on the casting except the riser itself are available.

Inside the vacuum chamber 10 lies above the receiving yoke 20 and the yoke located therein Forms a crucible 35 made of spectrographically pure graphite. According to FIG. 1 is on that Holding yoke 20 placed a quartz tube 37 on which the crucible rests. The Crucible 35 should have a receiving volume for the melt which is greater than the volume of the mold cavities 25 and the volume of the container 28. The outlet plug 36 of the crucible 35 must of course can be actuated from outside the vacuum chamber 10.

As mentioned above, those are parts that are in direct contact with the melted Metal come, preferably made from spectrographically pure graphite. However, this only applies to Metals that do not wet graphite. Those metals that wet graphite need device parts, which are made of another fire-resistant material, such as zirconium. If such a If other material is used, it is desirable to use a deoxidizer on the melt before casting add such as carbon, sulfur or the like.

If desired, an alloy container 40 can also be included within the vacuum chamber 10 a charging plug 41 can be provided, which above the crucible 35 by some means, such as a quartz tube 42 is held. As will be explained further below, such a Alloy container 40 may be advantageous in those applications in which the alloy constituents cannot be cleaned together in the crucible 35. When such an alloy container 40 is provided, the charging plug 41 must be actuatable from outside the vacuum chamber 10. This can be done in a simple manner by means of a rod 43 which extends through the upper cover 13. A vacuum seal 44 must be provided between the rod 43 and the cover 13, see above that no air can enter the chamber 10 when it is under a vacuum. If so desired the outlet plug 36 of the crucible

ίο 35 through the charging plug 41 of the container 40 are also operated. For this purpose, a molybdenum or tantalum wire is used, which is inserted between the plug 41 and the plug 36 sits, and has so much play that the charging plug 41 without lifting the Plug 36 can be raised.

In order to be able to melt the metal charge in the crucible 35, it is concentric around the crucible 35 a water-cooled high frequency induction coil 45 is placed. Known as the principle of induction heating

ao does not need to be discussed in detail here. In the same way is concentric a high-frequency induction coil 46 is placed around the receiving yoke 20 and provides preheating, to degas the molds and control the solidification of the castings, and which continues to be low Guaranteed freezing speed, as will be explained below. Also required the narrow gate preheats so that the metal in the gate does not solidify before it has not filled the mold cavities. Since the crucible 35 and the receiving yoke 20 are evacuated Chamber are located, little or no thermal insulation is required. Like in the Drawing shown, the two induction coils 45 and 46 are concentric around the tubular Part 11 as well as around the crucible and the yoke 20. The induction coils can of course also lie within the part 11 if suitable passages through the pipe 11 are provided.

When the induction coils inside the chamber

10 are arranged, the part 11 can be made of stainless steel or a similar material.

Since the yoke 20 and the shapes are near the

Lid 12 are, it may be necessary to provide the lid with a water cooling system, so that the seal between the lid 12 and the tube

11 does not overheat. This can be accomplished with a pipe loop 48 that is in contact with the Lid 12 is standing and water flows through it.

In operation, the device must be assembled as explained above, with that to be potted Metal is within crucible 35. There should be enough metal to make the warming tank 28 as well as the mold cavities 25 can fill. Then the vacuum pumping device 16 set in action, which creates a suitable vacuum within the chamber 10 and this vacuum Maintains throughout the work process. At the same time by the induction coils 45 and 46 a high frequency current as well as water is sent to the crucible 35 and the molds as well as the container 28 to heat up. When the metal is melted in the crucible 35 and in the chamber 10 a high vacuum has been generated, the molten metal is degassed by the high vacuum and partially cleaned as well as deoxidized by the crucible made of spectrographically pure graphite. This cleaning by vacuum is at least partially for that

Cleaning of many melts, in particular melts of highly heat-resistant metals, is known. In particular partial cleaning is carried out by the

•: Evaporation of certain pollutant gases, elements and oxydulene, especially the elements and oxydulene compared to the base metal have a relatively high vapor pressure. The combination of high temperature and high vacuum causes the impurities to boil out as gases with a relatively high vapor pressure and through the Pump device 16 are withdrawn. In addition, the oxygen in the melt is mixed with the carbon of the graphite crucible 35 react and form carbon monoxide, which deoxidizes the melt. In addition, since the system has a significant amount of graphite, the small amount of The existing atmosphere consists primarily of carbon monoxide and carbon dioxide produced by the The usual reduction continues to clean the melt. As soon as the metal melts in the crucible 35 is it is necessary to keep the melt in the crucible for a longer period of time in order to partially achieve the above Allow cleaning to run.

When the molten metal in crucible 35 is vacuum cleaned, it is ready for casting. It is required that the container 28 and the molds are heated to a temperature that is at least the Molten metal temperature prior to casting. The outlet plug 37 raised so that the molten metal flows from the crucible 35 into the preheated container 28, where some of the melt enters the mold cavities 25 through the sprue, which passes through the slots 29 and the yoke 20 is formed. When all of the metal has leaked out, the cavities 25 and the Container 28 can be substantially filled with molten metal. The induction coils 45 and 46 are then switched off and the system can slowly cool down.

It is important that a substantial proportion of the molten metal remain within the container 28, while the metal in the cavities 25

\ solidified to control the solidification of the castings in the mold cavities. The volume of metal in container 28 should be at least 20% of the total mold volume and preferably between 30 and 50%. By maintaining the heat sink above the molds, the castings solidify from below and the outsides upwards and inwards, with the castings still being cleaned by zone cleaning. As the liquid-solid boundary layer progresses slowly from below and outside, the impurities are pressed upwards and inwards and finally solidify within the winches risers 30. Then, after the castings have been removed from the molds, the risers are separated, with the impurities also being removed. This shows the importance of the cavity formation and the arrangement of the blind risers. As mentioned above, the riser winches 30 should be arranged relative to the mold cavity 25 such that the liquid-solid boundary layer can advance without obstruction from the lower and outer parts of the cavity 25 and terminate entirely within the riser 30. This advancing boundary layer must not terminate within any portion of the mold cavity that is retained as part of the finished casting.

In this way, each casting becomes individual as it solidifies within the mold cavity zone-cleaned.

It is also important that the castings cool slowly so that large-grain castings result. That should end. happened because some constituents of the impurities are partially concentrated at the grain boundaries, if adjacent Bodies freeze and merge. At the

ίο As the individual ·., grain boundaries progress into the liquid, the impurities are advanced further, as explained above. Thus, when adjacent grains Can. meet a common interface, some constituents of the impurities will concentrate at this interface. So when the cast parts cool slowly, larger grains form, which lead to a smaller number of grain boundaries. With a smaller number of grain boundaries, a larger proportion of the impurities is driven into the riser to be removed later. In addition, slower cooling brings the shrink holes into the upper surfaces of the metal in the blind riser.
After the castings and the container 28 have solidified and have cooled completely, the pumping device 16 can be switched off and air can be admitted into the chamber 10. Then the whole. Molding device can be removed from the chamber 10 by removing either the lid 12 or the lid 13. In order to get the castings out of the molds, the pin 27 and the sleeve 26 within the yoke 20 are first removed. A rod or narrow cylinder, not shown, is then used to eject the moldings 22, 23 and 24, the castings, the container 28 and the sleeve 26 from the yoke 20 via the ejection hole 19. Since only a single sprue 29 is provided on the periphery of the mold parts 23 and 24, any thermal contraction in the cast will not damage the molds. Furthermore, the metal of the sprue can easily be bent in such a way that all of the molded parts 23 and 24 between the cast parts and the heat sink can be removed for subsequent reuse. The heat sink and the metal of the sprue can then be separated or broken off from the castings and reused in the next casting process. The blind risers are processed from the cast parts themselves, so that finished cast parts with the highest purity result. In some applications of the highest purity castings, it is sometimes desirable to add a different metal to the base metal being cast. If both or all of the alloy components are highly heat-resistant, they can be melted and cleaned together in the crucible 35. However, if the one or more constituents of the alloy have a relatively high vapor pressure, it should not be melted directly with the base metal in crucible 35, as the high vacuum would vaporize the additional metal, cleaning the base metal and not achieving its purpose . In these cases, the relatively easily evaporable alloy constituent should not be added to the melt until immediately before casting. For this purpose, the base metal, which is relatively difficult to evaporate, can be melted in the crucible 35, as explained above

will. The easily vaporizable alloy component is introduced into the alloy container 40, where it remains relatively cool and solid while the base metal is melted and vacuum cleaned. Shortly before the casting, the charging plug 41 is raised so that the alloy metal is in the melt of the base metal in the crucible 35 can fall. Then once the alloy metal is melted and turned up has mixed with the base metal, the casting can be carried out as explained above.

In vacuum casting, it is desirable to have a certain flexibility in the design of the cast parts to enable yourself. This is especially true for the casting of vacuum switch contacts, where for different Applications for different compositions and geometrical designs the contact tip are desired. The contacts all have a similarly shaped outer circumference provided, which is referred to as "swirl" and clearly emerges from the shape of the mold cavities 25. The desired contact area, however, can change significantly.

The device explained and the method described enable different ones in the simplest possible way Arrangements for changing the shape of the contact tip. For example, the molded parts 22, 23 and 24 of FIG. 2 designed in such a way that a single pair of contacts is formed which are connected to a flat, protruding contact tip are provided, which protrudes over the contact body and through the Metal is formed in cavity 50.

The geometry and design of this contact tip can now easily be changed without doing so the original moldings 22, 23 and 24 would have to be changed. This happens because different Tip inserts are provided which are introduced into the cavities 50 prior to casting. In some applications of vacuum switches, for example, it is desirable to provide contacts, which consist of two different metals, for example the main body of the Contacts made of copper and the contact tip made of tungsten or molybdenum or alloys of these substances consists. By means of a graphite insert 51 (FIG. 3) which produces a recess and which is inserted into the cavity 50 is used, a casting can be made with a recess or depression in the one preformed contact tip made of tungsten, molybdenum or the like can be soldered in.

If so desired, such a casting can also be passed through directly over a preformed metal tip can be made that such a tip 53 (Fig. 5) is inserted into the cavity 50 prior to casting will. Then, when the metal solidifies in the mold cavity, it becomes naturally with that of the other Metal existing, preformed metal tip welded. In this case it is no longer necessary then solder the contact tip into the casting. It is clear that other forms of contact tips can easily be provided by appropriate inserts. For example leads the use of F i g. 4 into a one-piece casting with an annular contact surface is provided.

It has already been mentioned above that the essence of the present invention is that of the shape cavity 25 has a blind riser 30 and that the heat sink is provided to a controlled To achieve solidification of the cast parts in such a way that they clean themselves through the zone cleaning. In addition to this special cleaning, the invention has other advantages. For example, allowed the stacking of the molds in connection with a single pouring of one side of the Molding The casting of multiple castings without destroying or damaging the molds would have to. The only pouring on the side of the molds

ίο also gives a certain flexibility insofar as without Changing the shapes through different inserts, as explained above, obtained different castings can be without the metal supply would be disturbed. It still matters that the metal to be processed does not need to be of any particular purity before casting, because that Melting, holding the melt and the casting itself bring about the desired high degree of purity in the cast part. It is clear that numerous modifications and changes can be made without to depart from the scope of the invention. For example, other methods of controlling the Apply solidification of the castings. In such a method, the induction coil 46 can be axially Direction relative to the molds to be movably attached to the container with the molten metal to avoid. With such an arrangement, the induction coil 46 remains after the potting of the Metal turned on and is then slowly moved upwards relative to the shapes, making the desired Solidification sets in. Satisfactory results have been obtained when the induction coil 46 is moved upward at a rate of less than about 50 mm / h.

Claims (14)

Patent claims: 1. Vacuum casting process for precision metal parts with the highest purity, in the form and metal to be cast are exposed to a common high vacuum and in which the molten metal is poured into at least one heated mold cavity, thereby characterized in that the solidification of the metal within the mold cavity (25) such is controlled that the advancing solid-liquid boundary layer slowly without hindrance from the lower and outer parts of the mold cavity progresses and completely within a blind ending riser (30) ends. 2. Vacuum casting method according to claim 1, characterized in that a plurality of mold cavities (25) can be used. 3. Vacuum casting method according to claim 1 or 2, characterized in that the solidification is controlled in that by a directly above each mold cavity (25) provided pouring funnel (28) so much molten metal is poured that each Mold cavity (25) and the pouring funnel (28) are filled. 4. Vacuum casting method according to claim 1 or 2, characterized in that the solidification is controlled by having a concentrically provided around the mold cavity (25) Induction coil (46) with one speed speed up to a maximum of about 50 mm / h. 5. Device for performing the method according to claim 1, characterized in that that a melting device (35) having an outlet device (36) for the molten metal it is provided that at least one mold cavity (25) is present, which one upwards protruding blind-ended riser (30) that furthermore a molding heating device (46) is provided and that within the high vacuum above the molded part (23) a pouring funnel (28) is arranged, which take up a substantial volume fraction of the metal can. 6. Apparatus according to claim 5, characterized in that a plurality of perpendicular one above the other and interconnected mold cavities (25) are provided, which are each equipped with a blind-ending riser (30). 7. Apparatus according to claim 5 or 6, characterized in that a plurality of Molded parts (22, 23, 24) made of graphite are present, which are perpendicular to each other in a high vacuum are arranged, between which mold cavities (25) are formed. 8. Device according to one of claims 5 to 7, characterized in that the mold cavities (25) are connected to one another by a sprue (29) between the circumference and the molded parts (23, 24) and a receiving yoke (20) is formed in which the molded parts (23, 24) are arranged and that the sprue (29) consists of mutually aligned vertical There are slots formed in all mold parts (23, 24) except for the lowermost (22) and are in communication with the outer peripheries of the mold cavities. 9. Device according to one of claims 5 to 8, characterized in that above the Melting device (35) an alloy container (40) is provided, which inside a closed Container (11,12,13) is in which the yoke receiving the forms (20) is arranged and that the alloy container has a charring device (41) with which the contents of the container are transferred to the melting device. 10. Device according to one of claims 5 to 9, characterized in that a geometric Dimensions of the cast parts changing mold insert (51, 52, 53) is provided, which can be inserted into one or more mold cavities (25). 11. Device according to one of claims 5 to 10, characterized in that one with the Cast part fusing metallic insert is provided, which in each mold cavity (25) can be used. 12. Device according to one of claims 5 to 11, characterized in that the molded part heating device (46) consists of an induction coil which is placed concentrically around the molded parts (22, 23, 24). 13. Device according to one of claims 5 to 12, characterized in that a pouring funnel heating device is provided, which consists of an induction coil (45) placed concentrically around the pouring funnel (28). 14. Device according to one of claims 5 to 13, characterized in that the heating devices (45,46) can be moved in the vertical direction at a controllable speed. 1 sheet of drawings