Method For Isostatic Compression, Such As The Manufacture Of Powder Bodies

Abstract

A body to be compressed is inserted within a flexible container containing a compression medium compatible with the body and the container is then placed within an outer compression chamber containing another compression medium compatible with the container and chamber.

Description

BACKGROUND OF THE INVENTION

The present invention relates, among other things, to a method of subjecting bodies or liquid to high pressure, such as in the manufacture of powder bodies, for example hard metal products, electrical heating elements comprising an electrical conductor surrounded by a body of compressed powder material and possibly a surrounding, tubular casing, welding or furnace electrodes, etc.

In a known method of manufacturing such bodies, for example, heating elements for washing machines, etc., a central conductor is packed into a surrounding powder, in this case of insulating and heat-conducting type, possibly surrounded by a tube or other casing and the body is pressed in a suitably shaped tool or between rollers having a certain groove shape so that the powder (and casing) is compressed around the central conductor. This method has certain disadvantages. A tool of a certain shape is required and it is difficult to avoid cracks, irregularities and air pockets in the finished product especially if it is afterwards bent or shaped to a nonlinear shape such as zigzag or spiral which is usual for heating elements. The difficulty connected with the requirement of a special shape is also apparent with hard metal bodies (usually powder of hard material particles such as tungsten carbide and a binding metal such as nickel or cobalt), welding electrodes, etc. and the possibility has therefore been considered of manufacturing such bodies by other methods.

One such known method consists of placing one or more powder bodies, for example, of the above mentioned type, in a high pressure chamber for gaseous or hydraulic pressure medium and producing a high pressure in the chamber so that the bodies are compressed on all sides and the powder becomes even and relatively free from pockets of air, etc. By means of such isostatic compression it is also possible to manufacture bodies having a more complicated shape, such as curved hard metal blades, heating elements in spiral shape or having some other nonlinear shape, etc., and the curing is preferably carried out before compacting. Sintering may take place either before and/or afterwards. This method provides high quality products, high production speed and good accuracy of dimensions even without subsequent calibration. The cost of manufacturing the bodies is low.

One disadvantage which sometimes arises with this isostatic compression is that for the pressure medium in the pressure chamber (pressure container) it is necessary to use a pressure medium which is suitable for the pressure chamber, for example, noncorroding, but which may cause trouble in the compressed body. A pressure medium suitable for the compressed body, on the other hand, may be unsuitable for the pressure container. As an example may be mentioned that hydraulic oil in a finished hard metal body may decrease its usefulness. Hydraulic oil in a heating element, due to the nonconducting characteristic of the oil, makes the element difficult to test for insulation and remaining oil may cause an unpleasant odor when the heating element is used, and the oil may be difficult to remove. Disadvantages with similarly manufactured electrodes could also be shown.

With, for example, isostatic compression of hard metal bodies it is preferred to use a pressure medium of water-soluble type, such as glycerine and ethylene glycol, and in the manufacture of heating elements, water. Such pressure media, however, may cause corrosion and the like in the walls of a pressure chamber which in turn may cause rupture or other damages in the high pressure apparatus. It can thus be said that it is often difficult to find a pressure medium which is suitable both for the high pressure container and for the compressed object.

STATEMENT OF THE INVENTION

The invention provides a solution of these and other similar problems and is characterized in that objects to be compressed are placed in a container having at least one elastomeric or displaceable wall or bottom, which container is also filled with pressure medium, sealed and placed in a pressure chamber for hydraulic or pneumatic pressure medium, after which the pressure in the chamber is increased and the container compressed so that the object within is also compressed. The advantages of isostatic compression are thus utilized to the full and at the same time large scale production is effected. The pressure medium operating on the object can be selected taking the object into consideration and that in the out pressure chamber can be selected with a view to the technical conditions required in the chamber. Thus, expensive molding tools are avoided and the shape or length of the object is irrelevant.

The invention also relates to a means for performing this method. The means is characterized in that it comprises a container having at least one elastomeric or displaceable wall or bottom for powder bodies or the like and for pressure medium, which container can be sealed and immersed in a pressure chamber to compress the container and bodies placed in this, by means of a pressure medium in the chamber. Such a means operates effectively as an inner pressure vessel. The elastic wall, etc. rapidly alters shape when pressure is effected in the outer chamber and the inner container is compressed until pressure equality is reached in container and chamber. The bodies in the container will be isostatically compressed with the same pressure as occurs in the chamber without coming into contact with the outer pressure medium. Suitably, the container is provided with a bottom in the form of a perforated wall, a grid or net which is surrounded by the elastic wall. Instead of an elastomeric wall (bottom) in the form of an elastomeric membrane, the wall may be displaceable in the form of a piston, movable in a cylindrical space. Regarding the minimum volume for the compressible parts of the container, see below.

DESCRIPTION OF THE DRAWINGS

The invention is further exemplified in the accompanying drawings where FIG. 1 shows a container in a high pressure chamber, which container is provided with a wall (bottom) in the form of an elastomeric membrane. FIGS. 2 and 3 show alternative embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a container 12, which by means of a cross member or other lifting device, can be lowered into a pressure chamber 11, said container having thin metal walls, such as thin steel. The container 12 may of course be placed in the chamber before being filled. In certain cases several containers may be placed in the same pressure chamber. A powder body, or several powder bodies, here for example a spiral heating element billet 13, is placed in the container 12 and then the container filled with suitable pressure medium, such as water and closed with the lid 14.

The powder body or bodies may consist of hard metal bodies, consisting of hard material grains (tungsten carbide, etc.) and binder (Co or Ni), possibly enclosed in a thin casing of plastic or the like which is removed after compression and before subsequent treatment such as turning. The powder in the hard metal bodies must be compressed to become sufficiently dense and strong so that it can be subsequently machined, for example in a lathe, before being sintered after which further machining is extremely difficult. It is desirable to prevent substances penetrating into the powder which might deteriorate the quality of the bodies, and this is done by using a pressure medium which is harmless when in direct contact with the body and by enclosing the body in a casing during the compression. There is a risk, however, that a small amount of pressure medium will penetrate into the powder body when the casing is removed and for this reason the composition of the pressure medium is also important. A suitable pressure medium is glycerine, possibly with the addition of ethylene glycol.

The powder body 13 may also comprise heating elements or thermoelements having straight or curved shape, consisting of one or more central electrical conductors surrounded by a powder of electrically insulating, but heat-conducting type, such as aluminum oxide or quartz. A thin plastic casing, later removable, or a sheathing tube to hold the powder together, sealed at the ends before the compression, may be placed around the body.

In order to obtain satisfactory strength, heat-conductivity and other desired properties, the powder in these bodies must be strongly compressed and this should be done after the body has been bent to its final shape. As can be seen, the heating element 13 in the case shown is meander formed before the compression. The method can also be used for welding and furnace electrodes, machine components in powder form, hotplates, powder bodies consisting of several layers, etc. A suitable pressure medium of the manufacture of heating elements (with or without casing) is water which is electrically conducting and thus permits the usual insulation test in the finished product, but which evaporates and disappears when the element is used.

At the bottom of the container 12 a perforated plate 15, net or grid is arranged to permit the passage of pressure medium and below this an elastomeric or flexible membrane or sack 16 is sealingly arranged. The membrane encloses a compression space which must be at least as large as the decrease in total space which takes place in the container 12 when full pressure has been reached in the outer chamber, that is the space at 16 (sack space) must at least correspond to the compression of the total space in the container 12. The wall may consist of a relatively thin-walled, cylindrical tube. Around the membrane or sack 16 is arranged a supporting space 17 axially limited by a wall or support to carry the container and protect the sack 16. The support space 17 may be open at the bottom or limited by a perforated plate 18, net or grid to permit the passage of pressure medium from the outer pressure chamber 19 to the supporting space 17.

The container 12 with supporting space 17 and sack 16 is lowered into an outer pressure chamber for gaseous or hydraulic pressure medium, such as hydraulic oil which is water repellent and noncorrosive. The container 12 may be positioned or suspended in the chamber 11 and the latter pressurized to 500--40,000 atm., for example, 6000 atm. by means of a conventional pump and/or press means 20.

Instead of a sack or membrane at the bottom, other walls of the container, or all of these, may be flexible so that they can be compressed until pressure equilibrium is achieved between the outer and inner pressure spaces. It is also possible, as shown in FIG. 2, to replace the membrane by a piston movable in a cylindrical space 21 sealing the bottom of the container 12. The shape of the container is in principle immaterial.

When the container 12 has been filled with the object and pressure medium, in this case heating element billet and water, and sealed at 14 after air has been removed, it is lowered into the pressure chamber 11 after which the pressure in the outer chamber is raised to, for example, 6000 atm., and this pressure also operates on the container 12 and its sack 16. The container 12 is compressed and the sack space decreases so that pressure equilibrium is achieved between the outer and inner pressure spaces. The container may possibly also be heated to a certain extent. The bodies 13 are compressed isostatically by the inner pressure and when the pressure ceases in the chamber 11 the sack space expands again. As an example of the degree of compression, it may be mentioned that at a pressure of 6000 atm. the container (space and sack space) is compressed to 84 percent of its original volume. The sack space must therefore be at least 16 percent of the inner total volume.

After the pressure medium has been poured into the container 12 the total volume assumes its greatest value and during the isostatic compressure it assumes its minimum volume. The object in the container is isostatically compressed and becomes uniformly and homogeneously compressed.

FIG. 3 shows a container 12 with a completely closed bottom 22 through which at least one tube 23 communicates with the sack space and the container space. As can be seen, the sack 16 may also be attached to some other part of the support space 17, for example, around the perforated plate 18. The bottom 22 may also be provided with channels at its periphery.

The method according to the above may also be used for the manufacture of plastic bodies in which case a billet of base material and curing agent is subjected to isostatic compression, possibly also heat and is thus cured. Liquids may also be subjected to isostatic pressure in order to undergo certain alterations. In conclusion, the above method relates to the separation of two different pressure media for the reasons given above.

The method and means according to the above can be varied in many ways within the scope of the following claims.

Claims

We claim:

1. Method of subjecting bodies to high pressure, preferably the manufacture of powder bodies, such as hard metal products, electrical heating elements comprising an electric conductor surrounded by a body of compressed power material and possibly a surrounding, tubular casing, and welding and furnace electrodes, said method comprising isostatic compression, characterized in that objects to be compressed are placed in a hollow container with at least one elastomeric or displaceable wall or bottom, which container is also filled with liquid pressure medium and is sealed and placed in a pressure chamber for hydraulic or pneumatic pressure medium, after which the pressure in the chamber is increased and the container compressed so that the container including liquid pressure medium and objects is also compressed and thus the objects are isostatically compressed.

2. Method of manufacturing powder bodies according to claim 1, characterized in that before compression the powder body is placed in a thin casing of plastic, metal foil or the like, substantially only to keep the powder material together before the compression.

3. Method of manufacturing electrical heating elements according to claim 1, characterized in that a skeleton tube sealed at the ends and surrounding the powder body is used as casing.

4. Method of manufacturing electrical heating elements according to claim 1, characterized in that the container is filled with water and the chamber, outside the container, is filled with hydraulic oil before compressing.