US3797986A

US3797986A - Device for hot pressing of ceramic materials

Info

Publication number: US3797986A
Application number: US00294830A
Authority: US
Inventors: H Onder
Original assignee: Alusuisse Holdings AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1971-10-07
Filing date: 1972-10-04
Publication date: 1974-03-19
Anticipated expiration: 1991-03-19

Abstract

A device for hot pressing of ceramic materials in powder form or fine granular form, comprising a mold defining a longitudinal cavity and built up of graphite plates arranged in succession along its length each plate lying perpendicular to the wall of the mold cavity, at least one layer between each plate and the next, these layers being resistant to high temperature, electrically ensulating, and ceasing at some distance from the wall of the mold cavity, and electrically conducting carbon material filling the slots thus resulting in the wall of the mold cavity.

Description

United States Patent Onder Mar. 19, 1974 DEVICE FOR HOT PRESSING OF CERAMIC 3.336.973 8/1967 Ratcliffe 149/32 MATERIALS imith et al .r ramarow r [75] Inventor: Harald Onder, We zik n. 3.642.400 2/1972 Theodorsen 249/162 Switzerland [73] Assignee: Swiss Aluminum Ltd., Chippis. P i a Examiner-Robert D. Baldwin Switzerland Assistant ExaminerJohn McQuade [22] Filed: Oct 4 1972 Attorney, Agent, or Firm-Ernest F. Marmorek [21] App]. No.: 294,830

[ ABSTRACT 30 F'Al't'P"tDt 1 g pp y a a 7 7 A device for hot pressing of ceramic materials in powl I Swltzer an I der form or fine granular form, comprising a mold defining a longitudinal cavity and built up of graphite [52] Cl 137 0 79 plates arranged in succession along its length each Int Cl B28b plate lying perpendicular to the wall of the mold cavity a lean one layer between each plate and the next [58] Field 2: 27:23 these layers being resistant to high temperature, electrically ensulating, and ceasing at some distance from the wall of the mold cavity, and electrically conducting carbon material filling the slots thus resulting in [56] UNITE D gIFZFI ES I IZqTENTS the wall of the mold cavity. 2.133.675 10/1938 Ten Bosch 425/330 4 Claims, 16 Drawing Figures PATENTEDHAR 19 1914 SHEET 1 BF 7 FIG] PAIENIEDHARIS 1924 V 3797' 986 sum 2 or 7 PATENTEDHAR 1 9 I974 SHEET UF 7 FIGG 3 FIG] PATENTED "AR 1 9 I974 SHEET 8 [IF 7 FIGIO FIG.9

mm m/ 0 OOOOOOOOOO FIGH BACKGROUND OF THE INVENTION The invention relates to devices for hot pressing of ceramic materials in powder or fine granular form.

The hot pressing of ceramic materials in powder or fine granular form such as boron nitride, titanium diboride, titanium nitride, zirconium diboride, and titanium carbide usually takes place in graphite molds which are open at both ends, and which are mounted in a furnace. Press plungers, which likewise consist of graphite, move axially in the mold and are operated by hydraulic cylinders arranged outside the furnace.

FIGS. 1 and 2 show schematically two known hot pressing devices in vertical section.

The conventional hot device shown schematically in FIG. 1 has an electrical furnace with tubular graphite resistance 11. A graphite tube 12 serves as mold, in which lies the mass 13 of ceramic material in powder form which is to be hot pressed. From both ends of the furnace press plungers 14 of graphite are inserted into the graphite tube 12. With their help the mass 13 is pressed together under high pressure (e.g. to 200 atmospheres) and at high temperature (e.g. to 2,500 C). Two hydraulic press cylinders 15 serve for moving the press plungers 14. At 16 is indicated a jacket of asbestos plates and at 17 a heat-insulating filling (e.g. of soot). At 18 are water-cooled connections of copper for the heating current and at 19 guide bushes of graphite. A cavity 20 is filled with protective gas. This device permits the manufacture of pressings up to about 150 mm diameter.

FIG. 2 shows schematically a known device which is employable for the production of significantly larger pressings, e.g. of 500 mm diameter and 500 mm length. The furnace is arranged vertically. A mold 22 of graphite is heated by means of convection and radiation by graphite heating bars 23, which are arranged concentrically around the mold 22. The heating bars 23 are connected together alternately above and below by connecting pieces 24 of graphite, so that all the heating bars are connected in series. At 25 current connections are indicated. At 26 is indicated an asbestos housing and at 27 a heat-insulating filling, e.g. of soot. Protective gas, e.g. argon, is introduced through a tube 28. At 29 are indicated fireproof blocks which provide a support for the mold 22 and which rest on the floor 30. The floor 30 consists of cast steel and is cooled by castin water passages 31. As in the device shown in FIG. 1, the ceramic mass 32 in powder form is pressed together by means of two plungers 33 of graphite, actuated by hydraulic press cylinders 34. At 35 is indicated the machine frame.

The maximum size of ceramic body to be produced is determined by the dimensions of the graphite mold 22. For manufacturing reasons these dimensions cannot be chosen as large as one may wish. For example, a cylindrical mold with an internal diameter of 450 mm requires an external diameter of about 1,000 mm (wall thickness 275 mm) in order to be able to withstand the circumferential tension loads which arise during pressing. The wall thickness of the mold 22 is primarily determined by the tension loads to which it is subjected, so that it must be extraordinarily large because of the very low breaking limit of the graphite.

SUMMARY OF THE INVENTION According to this invention, a device for hot pressing of ceramic materials in powder form or fine granular form comprises a mold defining a longitudinal cavity and built up of graphite plates arranged in succession along its length, each plate lying perpendicular to the wall of the mold cavity, at least one layer between each plate and the next, these layers being resistant to high temperature, electrically insulating, and ceasing at a distance preferably not less than 1 mm and not more than about 5 mm from the wall of the mold cavity, and electrically conducting carbon material filling the slots thus resulting in the wall of the mold cavity.

Therefore the devices according to the invention have a mold which simultaneously serves as the tool determining the shape and as the electrical resistance element. The said mold is able to be employed for carrying out the pressing operations described initially (FIGS. 1 and 2).

The electrically insulating layer between the graphite plates should be at least so thick that it prevents a break through of the heating current, but not so thick that the conduction of current through the carbon material in the slots is impaired. The least thickness lies around 0.1 mm. The greatest thickness will suitably not exceed 1 mm. A thin insulating layer can advantageously be applied by flame spraying, for example of aluminium oxide or boron nitride. It is sufficient if only one of the graphite layers which are to come into mutual contact is provided with the flame-sprayed insulating layer, but of course both the graphite surfaces coming into contact with one another can have this insulating layer. A thicker insulating layer can be achieved by application of elastic material, for example of ceramic fibres such as Fiberfrax" (Registered Trade Mark), which, in the manufacture of the mold, can be compressed to a thickness of for example 0.7 to 0.4 mm under the clamping force exerted on the graphite plates.-

The thickness of the graphite plates should conform with the size of the molds and the electrical data. For example, with a mold of mm internal diameter and 250 mm external diameter, with 1 mm deep slots in the inner wall and 0.4 mm thick insulating layers, one can choose graphite plates 5 to 10 mm thick. In contrast, with a mold of 500 mm internal diameter and 1,000 mm external diameter, with 3 to 5 mm deep slots in the inner wall and 0.4 mm thick insulating layers, significantly thicker graphite plates are more suitable, already for manufacturing reasons, for example from 50 to 80 mm.

The graphite plates should be made parallel-sided and have surfaces as smooth as possible.

By the presence of the electrically insulating layers, which cease at a distance not less than 1 mm from the wall of the mold cavity, the electrical heating current is compelled to flow along the inner wall of the mold. This constriction of the conducting cross sections of the mold makes possible the employment of a thickwalled mold as an electrical resistance element for attainment of very high temperatures in the mold cavity in a usable time limit and with satisfactory thermal efficiency. With the employment of a thick-walled mold without restricted conducting cross section as the electrical resistance element, due to the very low electrical resistance of the graphite body, extremely high and constructively difficultly controllable current loads would arise when working with voltages which are favourable to handle; furthermore the entire graphite body of the mold would unnecessarily have to be brought to the same temperature, which would be extremely inefficient. With a graphite mold with restricted conductive cross section and a mold cavity diameter of 450 mm it is possible to apply voltages of, for example, 20 to 40 volts, without current strengths being achieved which are difficult to control. Current strengths easily controllable lies for example at 20,000 to 30,000 amps, while current strengths of about 80,000 amps and more in such an apparatus would be difficult to control.

The form of construction of mold described above also yields the following important advantages:

a) By the use of several plates, the employment is possible of graphite of greater mechanical strength than in continuous-walled graphite molds.

b) The temperature gradient in the mold wall falls steeply from inside to outside. Because of the significant temperature gradient from inside to outside there is established, in consequence of the thermal expansion in the graphite body, a state of stress, which is superimposed on the state of stress created by the radial press pressure, so that one can speak ofa pre-stressing of the mold in the direction towards the mold cavity.

c) The combination of the small conducting cross section for the electrical heating current with the thick mold wall produces a robust construction.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate the invention. In these drawings:-

FIGS. 3 to 14 illustrate schematically various devices embodying the invention.

FIG. 3 shows in perspective in vertical longitudinal section a first device with a circular mold and FIG. 4 is a detail of FIG. 3.

FIG. 5 shows in perspective a second device with a circular mold, with omission of the front clamping elements.

FIG. 6 shows a third device entirely schematically and FIG. 7 is a section along the line A-A in FIG. 6.

FIG. 8 shows entirely schematically a fourth device with a mold which can be unloaded, in the pressing po' sition (closed position).

FIG. 9 is a purely schematic showing of a fifth device, with longitudinal mold segments, in pulled apart positron.

FIG. 10 shows the same device with the longitudinal mold segments, in pushed together position.

FIG. 11 shows in a section along the line B--B of FIG. 10 and in more detailed illustration, but still only schematically, the device according to FIGS. 9 and 10.

FIG. 12 is a diagrammatic illustration of the mold according to FIG. 10, partly in broken lines.

FIG. 13 shows schematically in plan a sixth device, with a mold divided into six longitudinal segments, in pulled apart position, and

FIG. 14 shows the same device in pushed together position (in pressing position).

FIG. 15 shows the employment of a device as stationary pressing mold and FIG. 16 shows the employment of a device as a quasiisostatic press.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The example shown in FIGS. 3 and 4 has a round mold 36 about 680 mm long, which is assembled out of nine superimposed annular graphite plates 37. The graphite plates 37 have each a thickness of mm and an external diameter of 600 mm and form a mold cavity 38 of 300 mm diameter, in which the fine granular ceramic material is pressed together with the help of graphite plungers not shown. Between each graphite plate 37 and the next there is a layer 39 0.5 mm thick. of Fiberfrax" (Registered Trande Mark), which ceases at 4 mm from the wall 40 of the mold cavity 38. This layer is resistant to high temperature, and is electrically insulating. The slots 41 (FIG. 4) remaining in the wall 40 are filled with an electrically conducting carbon material 42, for example with finely ground natural graphite. This can, during manufacture of the mold, be applied as a suspension in mineral oil. The outer wall of the mold 36 is provided with a jacket 43 several mm thick of several layers of asbestos paper. The graphite plates 37 are clamped together between water-cooled clamping and contact plates 44 of copper with the help of water-cooled clamping bolts 45 formed as weldless steel tubes, with nuts 46. For the sake of simplicity the supply of water to, in, and from the clamping and contact plates 44 is not shown; the same is true of the water supply to and from the clamping bolts 45. The clamping bolts 45 are electrically insulated from the clamping plates 44, e.g. by procelain or asbestos inserts 47. At 48 are shown the heating current connections to the clamping and contact plates 44.

In the example shown in FIG. 5 the circular mold 49, which has the same dimensions as the mold 36 in FIG. 3, is divided into four equal longitudinal segments 50. As in the construction according to FIG. 3, there is arranged between each graphite plate 51 and the next a layer 52 0.5 mm thick of Fiberfrax, which ceases at 4 mm from the wall 53 of the mold cavity 54. The slots remaining in the wall 53 are filled with electrically conducting carbon material, as in FIG. 4. There is a thermally and electrically insulated jacket 55 of asbestos on the outer wall of the mold 49. This is shown in section. For the sake of clarity the watercooled clamping and contact plates are not shown, nor are the current connections and the hollow clamping bolts, which are introduced through openings 56 in the graphite plates 51 and which hold together the graphite plates 51 with the help of nuts. The hollow clamping bolts, not shown, are electrically insulated from the clamping and contact plates as well as from the graphite plates 51. They are passed through by cooling water during use of the mold. They consist of weldless steel tube and have an external diameter of about 40 mm and an inner diameter of about 15 mm. There is a casing 57, only the rear half of which is shown. This casing 57 is preferably water-cooled. It accepts during the pressing operation alle the external forces, so that the graphite is only subjected to pressure.

FIGS. 6 and 7 show a device in which the mold 58 consists of four longitudinal segments 59, each of which is built up of seventeen rectangular graphite plates 60. The mold cavity 61 is square in cross section.

The plungers for pressing the ceramic material are, for the sake of clarity. not shown. The graphite plates of each segment are held together by two hollow watercooled clamping bolts 62 of steel, as is apparent from FIG. 7. Between the individual graphite plates 60 are arranged insulating layers 63 which can withstand high temperature, which cease at a distance not less than 1 mm from the wall 64 of the mold cavity, while the slots remaining in the wall 64 are filled with electrically conducting carbon material, as in FIG. 4. Everywhere where the graphite plates 60 do not form a part of the wall 64 of the mold cavity 61, the insulating layer 63 extends up to the edge of the graphite plates 60, for only the wall 64 of the mold cavity 61 should carry heating current. In FIG. 6 the inner boundary of the insulating layers 63 is shown in broken lines.

65 indicates a thermally and electrically insulating layer of asbestos, which is arranged between the outer edges of the longitudinal segments 59 and a ribbed steel frame 66. This frame 66 withstands all external forces during the pressing operation. The longitudinal segments 59 are only subjected to pressure. The arrows 67 indicate the delivery and removal of the cooling water for the clamping bolts 62. Insulating washers 68 of asbestos are provided at each end of each hollow clamping bolt 62, which in addition is surrounded by an insulating tube 69 of porcelain. The electrical heating current is supplied to the graphite plates 60 through watercooled contact plates 70 of copper. The ribs 71 increase the mechanical strength of the frame 66 and facilitate the removal of heat. Cooling water flows through bores 72 in the frame 66. The connections for supply and removal of water are not shown for the sake of clarity.

In the device shown in FIG. 8, the mold 73 has a rectangular internal cross section and consists of two longitudinal segments 74 and two longitudinal segments 75. These

longitudinal segments

74 and 75 have all four the same length and are, in a similar manner to the longitudinal segments 50 in FIG. 5 and to the longitudinal segments 59 in FIG. 7, assembled from graphite plates with intermediate insulation and with slots the wall 76/77 of the mold cavity 80, filled with electrically conducting carbon material. The graphite segments 74 are fastened to clamping

jaws

78 and 79 respectively, which have the same length in the longitudinal direction of the mold cavity 80 as the longitudinal segments 74. The longitudinal segments 75 are fastened to clamping jaws 81, which likewise have the same length as the longitudinal segments 75. The jaw 78 is fixed to a vertically arranged steel plate 82. Two pairs of elechydraulic cylinders 83 are fastened to the steel plate 82, of which only the upper is visible. Piston rods 84 extending out of the cylinders 83 from pistons 92, are connected to arms 85 of the clamping jaws 79 at 86 with provision for pivoting. The ability to pivot facilitates the movement of the piston rods 84 and of the arms 85. At 87 and 88 are indicated water-cooled contact plates of copper, which are pressed against the

longitudinal segments

74 or 75 with the help of watercooled clamping

bolts

89 and 90 of steel tube. These clamping

bolts

89 and 90 correspond to the clamping bolts 62 in FIG. 7 and are electrically insulated in similar manner.

In the pressing condition, a chamber 91 ofeach cylinder 83, and in consequence the piston 92, is under such oil pressure that the clamping jaw 79 is pressed in the direction towards the clamping jaw 78 against the longitudinal segments 74, and, thanks to oblique surfaces 93, presses the clamping jaws 81 against the longitudinal segments 75. In this pressing condition. the rectangular mold cavity 80 can be filled with the material to be hot pressed, in powder form, and put under pressure by introduction of press plungers of graphite, not shown, from above and below.

At the end of the hot pressing, the oil pressure in the cavity 91 of each cylinder 83 is removed and a cavity 94 is put under oil pressure. A consequent small longitudinal movement of the pistons 92 is sufficient to loosen the assembly of the

longitudinal segments

74 and 75 of the mold 73 so far that the hot-pressed body can be removed easily, and without friction on the

inner walls

76 or 77 of the

longitudinal segments

74 or 75.

For the supply and removal of current and cooling water similar means are used as have been explained with reference to the examples according to FIGS. 3 and 4, FIG. 5 and FIGS. 6 and 7.

FIGS. 9 to 11 show a device with an adjustable mold 95, which has a square internal cross section. This mold 95 consists of four longitudinal segments 96, which are so movable that the mold cavity 97 maintains its cross sectional shape (here square) both upon pulling apart and also pushing together of the longitudinal segments 96. This is made possible by the arrangement of flat sliding surfaces 98, which are parallel to the corresponding surfaces of the wall 99 of the cavity, and perpendicular to the axis of the mold. On pulling apart and on pushing together of the segments 96, corner points 100 of the cavity cross section move along diagonals 101. The mold cavity 97 is defined by four inserts 102, while main parts 103 of the segments consists of steel. Between each insert 102 and the corresponding steel part 103 there is arranged a 5 mm thick layer 104 of asbestos board as electrical insulation. At 105 there is shown a clearance, which has the purpose of avoiding sliding over one another of highly heated graphite surfaces upon pulling apart and pushing together of the mold segments 96. The arrows 106 (FIG. 9) indicate the movement of the longitudinal segments 96 during pushing together and the arrows 107 (FIG. 10) during pulling apart. At 108 is indicated a frame, shown schematically; it withstands the mechanical loads, and carries guides for the movement of the longitudinal segments 96.

FIG. 11 shows primarily the construction of the inserts 102. These inserts are built up of twelve graphite plate segments 109. The graphite plates 109 are electrically insulated from one another by 0.2 mm thick layers 110 of flame-sprayed aluminum oxide. The layer 110 ceases at a distance of 5mm from the inner wall 99 of the mold 95. The slots 111, which remain upon assembly of the inserts 102, in the wall 99 of the mold cavity 97, are filled with a carbon material having good electrical conductivity, in the same way as the slots 41 in FIG. 4.

The insert 102 is supported through the insulating layer 104 of asbestos, against the steel part 103. This steel part 103 is cooled by water which flows in passages 112. It is extended at each end of the mold to a yoke 113 with an oblique surface 114. A water-cooled wedge 115 of copper cooperates with the oblique surface 114, and, through a tension screw 116 and nut 117, clamps the graphite plates 119 together in the longitudinal direction of the mold. The wedges 115 serve also as electrical current connections. For this reason they are electrically insulated from the yoke 113 by an asbestos layer 118. After passing through the graphite plates 109 at each end of the insert 102, the electrical current flows only through a small layer of the part of the graphite insert 102 forming the wall of the mold cavity 97.

FIG. 12 shows diagrammatically, partly in broken lines, the mold 95 according to FIGS. 9 and 10, with omission of the frame 108, the two front longitudinal segments 96 and the clamping yoke 113.

FIG. 13 shows schematically in plan a mold 119 of polyhedral internal cross section with six longitudinal segments 120 in pulled apart position, and FIG. 14 shows this mold in pushed together position. Constructionally, the longitudinal segments 120 are substantially the same as the longitudinal segments 96 of FIG. 12. A frame is indicated at 121.

In conventional pressing (see FIGS. 1 and 2) the principal reason for destruction of the graphite mold is that, upon cooling, the graphite shrinks more quickly than the hot-pressed ceramic workpiece, and hence tension loads arise in the graphite which may lead to fracture of the mold. In the molds shown in FIGS. 5 to 14, the graphite layers shrink away from the workpiece, so that no tension arises in the graphite and consequently no breakage arises.

FIG. illustrates in vertical section schematically the employment of mold 122 as a stationary mold, while two press plungers 123 compress a mass 124 to be deformed. The arrows 125 indicate the pressing direction.

If the device according to FIG. 15 is provided with an opening mechanism which permits pulling apart of the mold 122, the mold exhibits in addition the advantage that it can be opened for removal of the pressed body, so that violent knocking out is not necessary. A further advantage then lies also in the possibility of choosing the cross section of the mold cavity at any desired size between open and closed position. Upon changing the cross section of the mold by moving the segments of the mold, it is necessary to exchange the plungers 123 for others of appropriate cross section.

The compression of fine granular ceramic materials in the devices according to FIGS. 3 and 4, FIG. 5, FIGS. 6 and 7, and FIG. 8 can only take place in the same way as in the devices according to FIGS. 1 and 2, that is to say with the help of press plungers of graphite, which are introduced at each end of the mold cavity into the latter under pressure. Such a compression by plungers can also be carried out with the devices according to FIGS. 9 to 12 and FIGS. 13 and 14. With the devices according to FIGS. 9 to 12 and FIGS. 13 and 14 the compression can however alternatively take place by simultaneous pressing together of the longitudinal segments 96 (FIGS. 9 to 12) or 120 (FIGS. 13 and 14). This produces a quasi-isostatic pressing action.

A purely isostatic pressing is the deformation of a body under equal pressure from all sides, for example the deformation of a sphere to a smaller sphere. In practice, however, the term isostatic pressing" is also used in respect of the deformation of a cylinder to a cylinder of smaller diameter and equal length. The term quasi-isostatic pressing" is used in this specification to mean the deformation of a prism of polyhedral cross section to a prism of geometrically corresponding but smaller cross section.

FIG. 16 illustrates in vertical section, schematically the employment of a mold 126 as a quasi-isostatic pressing mold. After introduction of the mass of material 127 to be hot pressed, the mold 126 is closed at both ends by graphite parts 128. After the closing of the mold 126 the electrical current is switched on and the mold wall 129 of the mold 126 is brought to a high temperature, e.g. to 1,800 C. Before this temperature is reached, the longitudinal segments of the mold 126 are pressed towards one another by conventional means not shown in FIG. 16, under a pressure of about atmospheres. Upon attainment of a temperature at which the material 127 transforms itself into a plastic condition, the longitudinal segments of the mold move further together, until a balance occurs between pressing pressure and compressibility. The electrical heat ing current is switched off after attainment of the desired temperature. After cooling, the mold 126 is opened (that is to say its longitudinal segments are pulled apart) and the pressed body is removed.

The force required to keep the parts 128 closed during pressing is very slight, because the axial pressure component produced during the closing movement of the longitudinal segments of the mold 126 is very small, by reason of the small plasticity of the material 127.

In conventional presses according to FIG. 1 or FIG. 2, as well as in presses according to the example according to FIGS. 9 to 11, the compression of the mate rial to be worked on must take place over the length of the mold cavity, since the cross section of the mold cavity remains constant. The press plungers require to perform strokes of substantial length, and are thus themselves of substantial length. As they are graphite they are extremely subject to breakage. Moreover there are pronounced density and pressure gradients in the compressed material due to the long strokes.

On operation with the mold according to FIG. 16 on the contrary, the pressing takes place by reduction of the cross section of the mold cavity with maintenance of its length. This pressing is here called quasi-isostatic, becuase of the six walls of the mold cavity four take active and simultaneous part in the deforming operation and at the same time only travel a short stroke. If a cube a is produced from a parallelepiped 2a.a with a compression ratio of 2:l by pressing, then with the conventional method the press stroke is 2.a/2 (each plunger moves a stroke of a/2 0.5a In production of a cube a by quasi-isostatic pressing in the sense indicated above, the starting shape is a parallelepiped with a cross section \/2 .a X Via and a heigth of a; in this case the mold segments make four individual strokes of \/2 l)a/2, that is of about only 0.20 compared with 0.5a with conventional deformation.

The greatest advantages of the hot pressing device according to FIG. 16 lie on the one hand in that the pressed bodies have a nearly isotropic structure, and on the other hand in the possibility of pressing long relatively thin bodies, the physical properties of which are constant over their entire length. These cannot be produced in equally high quality by any other method.

As in all devices for hot pressing of fine granular ceramic materials, the graphite parts, where such are used, must be protected by protective gas from oxidation. For this purpose the entire hot pressing device, with the exception of the machine frame and the hydraulic cylinders fixed to it, is best brought within a steel housing through which flows a protective gas. Nitrogen is suitable as the protective gas. Argon is safer, because it is entirely inert. Helium comes less into question by reason of its high price. Hydrogen should only be used when very strict safety precautions can be met against danger of explosion.

What I claim is:

1. A device for hot pressing of ceramic materials in powder form or fine granular form, comprising a mold defining a longitudinal cavity and built up of graphite plates arranged in succession along its length, each plate lying perpendicular to the wall of said mold cavity, at least one relatively thin layer between each plate and the next, said layers being resistant to high temperature, electrically insulating, and ceasing at a distance from the wall of said mold cavity, and electrically conducting carbon material filling the slots thus resulting in the wall of said mold cavity, means for electrically heating said mold, and opposed press members adapted to enter said longitudinal cavity for hot pressing ceramic materials therein.

2. A device according to claim 1, the electrically insulating layers ceasing at a distance not less than 1 mm and not greater than 5 mm from the wall of the mold cavity.

3. A device according to claim 1, in which the mold is divided into longitudinal segments, and surrounding these longitudinal segments there is a frame which is able to withstand the forces which emanate from the mold cavity during use of said device.

4. A device according to claim 1, in which the mold cavity has a polyhedral cross section and in which device the mold is divided into longitudinal segments having sliding surfaces which are parallel to the corresponding surfaces of said cavity and perpendicular to the axis of said mold, said segments being so movable along said sliding surfaces that the said mold cavity maintains its cross sectional shape both on pulling apart and also pushing together said longitudinal segments and in which mold, surrounding the said longitudinal segments, there is a frame which is able to withstand the forces which emanate from said mold cavity during use of the device.