BACKGROUND TO THE INVENTION

This invention relates to the pressing, particularly but not exclusively hot pressing, of metals and ceramic material in powder form to produce components that have high wear and/or corrosion resistance, particularly, but not exclusively, for extruder barrels for use in screw extruders, especially twin-screw extruder barrels.

In the case of twin-screw extruders, the barrels suffer high degrees of wear and/or corrosion in operation. Current practice is to form the barrels separately as replaceable components. Methods have developed from those described in specification No. GB 1498253 but do not differ significantly in principle. An insert, having the shape of the barrel profile, is located in a housing block, thus enabling different choices of material to be made for the insert and the housing block. The working surfaces of the insert can be treated prior to assembly in order to improve their wear-resistant properties, such as by forming a wear and/or corrosion-resistant coating on the working surfaces. One such coating is described in patent specification No. EP 0459637B. An example of this form of coating is known as Borcoat™ marketed by APV Baker, which comprises a nickel-based alloy, powder-metallurgy coating consolidated and bonded onto the cast steel substrate by means of hot isostatic pressing.

Although the coating of EP 0459637B performs well, the demands of the extrusion industry require a significant improvement to the achievable performance together with a reduction in costs. Typically these requirements can be seen as a need for improved wear resistance for the processing of ceramics, high operational temperatures for processing liquid

crystal polymers and high power input with high shear rates, such as with high molecular weight, high density polythene or paper processing.

The problems with conventional barrel inserts result from the constraints imposed by the presence of the steel substrate, which restricts the choice of coating materials. This is principally due to the high levels of residual thermo-mechanical stresses which result from the large differences in the thermal expansion coefficient between the coating material and the substrate. In the Borcoat™ process this thermal stress is controlled to ensure that the coating remains adherent to the steel substrate. This results in a restriction to the available performance of the coating resultant from the restricted choice of coating materials and backing materials.

Control of the thickness of such coatings is good, but the coated insert may require additional machining to ensure a correct net final shape; machining of such hard surfaces can impose severe cost penalties. The surface coating techniques in themselves are relatively expensive and additional cost penalties are incurred due to the relatively long lead times in manufacture.

SUMMARIES OF THE INVENTION

According to one aspect of the invention we provide a shell insert for a screw extruder, being one of a pair of co-operating inserts which together define the bore(s) of the extruder, said one insert having wear and/or corrosion resistance and manufactured from powder by pressing said powder to a near-net-shape, whereby only the mating faces of said insert may require to be machined, the surfaces of said insert that define the bore(s) of the extruder requiring substantially no machining.

The use of separate inserts to define one or more extruder bores allows for accommodation of the differential in thermal expansion between the materials of the insert and the material of the housing block. Additionally, the near-net-shape of the finished pressing allows for the use of materials at the product interface which would be uneconomic in conventional extruder systems, due to the need to machine to finished dimensions.

In the case of a screw extruder, co-operating pairs of such inserts are used. The inserts of a pair are mounted into respectively the top and bottom halves of a housing such that when the two halves are clamped together the two inserts form a length of the operating bore of the extruder. Iii the case of a twin-screw extruder two bores are so formed which comprise two adjacent parallel bores of equal diameter, with the centre distance between the bores being less than their diameter. In operation the bores accommodate two parallel, inter-meshing screw shafts. For single-screw machines, the inserts define a single bore.

According to a second aspect of the invention, we provide a housing assembly for a twin-screw extruder comprising a housing block formed by two parts, wherein two wear and/or corrosion-resistant shell inserts are mounted respectively in the housing block, such that when the two parts of the housing block are clamped together the two shell inserts define the twin, parallel bores of the extruder, characterised in that each insert is manufactured complete from powder to a near-net-shape by a pressing operation, the mating faces of the insert require a relatively large amount of machining prior to assembly, the surfaces of the insert that define the bore of the extruder requiring a relatively small amount of machining.

We have realised that it would be advantageous for each extruder insert to exhibit a layered and/or graded structure and composition through its thickness. The result of our work to develop extruder barrel inserts having such a layered and/or graded structure is applicable to other pressed powder components, and is discussed in more detail hereafter.

To achieve this material structure, which is termed a Functionally Graded Material or (FGM) structure hereinafter, the powders are mixed to provide powder layers with a grading of the cermet content, for example, from its highest level at the bore wall to near zero at the insert/housing surface. This provides for high wear resistance at the bore wall and high strength and toughness where contact is made with the housing.

Most preferably the powders are compressed using hot pressing techniques.

According to a third aspect of the invention, we provide a method of producing an article having a layered and/or graded structure by subjecting powders of metals and/or ceramic material to a pressing operation in a press tool cavity defined between first and second press tools, the method comprising powder loading steps comprising:

locating in the press tool cavity an intact embryo layer of the first powder mix positioned against the first press tool so as to leave a gap between that first embryo layer and the second press tool, and filling said gap by inserting a second powder mix, preferably in flowable form, into the gap to create a second embryo layer of the second powder mix.

The intact first embryo layer can be rendered intact in various ways. The first powder mix may incorporate a binder and the first powder mix could

be applied wet to the surface of the first press tool to create, when dry, a uniform layer on the first press tool.

Preferably, however, the first powder mix incorporates a binder and is moulded into a pre-form shell which is shaped to fit the surface of the first press tool so that the pre-form shell can be arranged in the press tool cavity. Suitable location means need to be provided to hold such a pre¬ form in engagement with the first press tool whilst the second powder mix is flowed into the press tool cavity. The location means may comprise removable spacers which are withdrawn as the second powder mix is fed into the press tool cavity.

The pre-form itself could be produced as a layered body, preferably utilising a pre-forming tool that is adjustable in order to accommodate the addition of a plurality of layers of powder.

By careful weighing of the first and second powders it is possible to control precisely the quantities of powder in the first and second embryo layers and thus in the pressed article.

When an elongate article is to be pressed powder it will often be desirable to fill the press tool cavity from one end whilst the press tools are located substantially vertically with said one end uppermost. Suitable presses will usually apply pressure in a vertical direction so that it will generally be necessary to rotate the charged press tools from a vertical position to a horizontal position between filling of the press tool cavity and pressing of the powder.

In order to retain any loose powder in position in the press tool cavity during turning of the tools and any transportation to the press, it may be desirable to provide a binder with the loose powder. So powder that is not loaded into the press tool cavity as pre-form, preferably incorporates a binder.

When an elongate component to be pressed from powder has one or more locating projections, such as a locating flange, the locating projections can be arranged to, have a different composition from other parts of the component by arranging for the appropriate parts of the press tool cavity to be loaded with a different powder from powder used in other parts of the press tool cavity.

Thus by arranging for different powders to be used to fill different regions of the press tool cavity it is possible to produce by pressing a component which has different regions having different compositions and accordingly different properties, such as hardness, wear resistance or machinability.

We consider that in general it is desirable where possible to choose the inner and outer shapes of the component to be pressed from powder such that the thickness of the component in the direction of the force applied by the press tools is substantially uniform across the component whereby substantially uniform compaction of the powder mix takes place without any need for lateral motion or plastic flow of the powder mix. This is particularly desirable when the shell is to have a layered and/or graded structure.

When the pressed component is an elongate shell insert for a twin-screw extruder a pair of such shell inserts being adapted to be clamped together to

define therebetween a pair of parallel screw bores which overlap with each other, the shell in transverse cross-section preferably comprises a pair of arcuate inner surfaces meeting at a ridge and a pair of arcuate outer surfaces meeting in a trough, the centres of curvature of the inner surfaces corresponding respectively to the intended axes of the intermeshed extruder screws.

The shell insert preferably comprises along each edge a marginal flange for clamping against a complementary flange on the second insert of a pair.

The use of arcuate inner and outer surface portions facilitates machining of the press tools, and also machining of the extruder barrel housing halves to receive the shell inserts.

The inner and outer surfaces of such an elongate shell insert preferably have substantially the same radii but the centres of curvature of the outer surfaces are offset from those of the inner surfaces whereby the thickness of the shell in the direction of pressing, which is perpendicular to the mating plane of a pair of shells, is relatively constant, but the radial thickness of the shells where they meet is maintained at an adequate thickness, to provide adequate strength.

Preferably the centres of curvature of the outer surfaces are more widely spaced than the centres of curvature of the inner surfaces and disposed in the direction towards the respective inner surface from the mating plane.

For shell-like articles in general it will be desirable where possible to choose the internal and external shapes to be such that the thickness of the shell in the direction of pressing is relatively constant.

Preferably, the shape in cross-section of the press tool cavity (and of the resultant, finished shell liner) deviates from a constant vertical thickness across the width of the section at any points requiring strengthening by the amounts necessary to accommodate lateral flow of the particulate material under compression.

It is normal for multiple housing blocks to be used on an extrusion machine, arranged in-line along the length of the continuous screw shafts. Certain of the inventive inserts preferably have cut-outs positioned so as to provide entry ports for the extrusion material and such inserts can be formed so as to have adequate strength at the high load points associated with the ports.

A fourth aspect of the invention relates to a hot pressing process suitable for hot-pressing of powders.

According to the fourth aspect of the invention, a method of hot pressing processing an article from a powder of metals and/or ceramic material comprises pressing the powder between press tools in a furnace whilst the furnace is subjected to an inert gas atmosphere, the inert gas pressure in the furnace being maintained slightly above atmospheric pressure by monitoring the atmospheric pressure and controlling the furnace pressure accordingly.

By ensuring that the inert gas pressure in the furnace is always above atmospheric pressure it is ensured that there can be no leakage of air into the furnace. If there is any leakage it will be a leakage of inert gas from the furnace.

The method preferably comprises furnace purging steps in which the furnace is sealed and a vacuum is applied to the furnace, the furnace is then filled with inert gas to a pressure in excess of atmospheric pressure, and a vacuum is applied again, this sequence being repeated.

During the purging steps a furnace wall temperature in excess of dewpoint of the ambient atmosphere is preferably maintained, to avoid condensation of products on the furnace walls.

When the powder mix incorporates a binder, preferably an organic binder, one of the vacuum steps can be utilised to dispose of the binder, the furnace temperature being raised sufficiently to drive off the binder from the powder mix.

BRIEF DESCRTPTTON OF THE DRAWINGS

The various aspects of the invention will now be further described, by way of example only, with reference to the accompanying drawings in which:

Figures 1 to 3 are respectively a plan view, side elevation and perspective view of a pressed shell insert in accordance with the invention for a twin- screw extruder,

Figure 4 is an enlarged end view of the shell insert of Figures 1 to 3,

Figure 5 is an enlarged transverse cross-section of the confined powder mix in the press tool cavity prior to hot-pressing of the powder mix to create the shell insert of Figures 1 to 4,

Figure 6 is a schematic perspective exploded view showing four pairs of shell inserts adapted to be arranged in series to define the twin bores of a twin-screw extruder,

Figure 7 is a graph showing the temperatures and pressures applied during a typical hot pressing process,

Figure 8 is a schematic cross-sectional view of a suitable press for performing the hot-pressing process in accordance with the invention, and

Figure 9 illustrates an insert with a layered surface.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in Figures 1 to 4, an elongate hot-pressed shell insert 1 is of generally undulate W-shape in transverse cross-section. Pairs of identical inserts are clamped together in use between corresponding extruder housing block pairs which are adapted to be secured together in-line, in series to form a housing assembly defining overlapping parallel bores 2, 3 for intermeshing twin-screws of a twin-screw extruder (not shown).

In transverse cross-section, Figure 4, each shell insert 1 has a pair of inner arcuate surfaces 2a, 3a about centres A, B which lie on the mating plane C, and a pair of outer arcuate surfaces 4, 5 about centres D, E. The radius of curvature of surfaces 2a, 3a, 4, 5 are substantially the same but the position of centres D, E are spaced from centres A, B for a reason to be explained.

In order to provide for relatively uniform compaction of the powder mix during pressing of the mix, by relative movement together of press tools in the direction normal to plane C, that is the vertical direction in Figure 4,

we consider that it is desirable that the vertical thickness y_ of the insert should be substantially constant. This is conveniently achieved by having the curvatures of surfaces 2a, 3a, 4, 5 the same, but with the centres of curvature D, E positioned downwards in Figure 4 from A, B. However, in order to avoid an excessively thin radial thickness of the insert in regions 6 adjacent to peripheral flanges 7, the centres of curvature D, E are spaced more widely than centres A and B.

It will be appreciated that the peripheral flanges 7 on one insert abut on faces 8 with the corresponding faces 8 of the other insert of a pair, whereby surfaces 2a and 3a of the pair of inserts 1 define overlapping cylindrical bores 2, 3 for the intermeshed screws of the extruder.

The housing block part 40 (Figure 4) which receives the insert 1 is machined with part cylindrical surfaces 41, 42 complementary to surfaces 4, 5 so that the insert surfaces 4, 5 make intimate contact with the co¬ operating surfaces of the housing block. The housing block part 40 is also provided with rectangular-section recesses 43 to receive the flanges 7. Two co-operating and clamped together block parts 40 form a ^* housing block.

As shown in Figure 4, the internal arcuate surfaces 2,3 meet in a ridge 9, and the external surface 4, 5 meet in a trough 10.

Figure 5 shows the cross-sectional dimensions of the filled press tool cavity 51 prior to hot-pressing of powder (and binder) contained therein.

In order to produce an insert of near-net-shape it is necessary to weigh accurately the required amount of powder to be inserted into the press tool

cavity. The powder is most easily loaded into the press tool cavity by orienting the co-operating press tools 52, 53 (Figure 8) vertically with the open end of the cavity 51 uppermost. When all of the powder is in loose form, a mixture of powder and binder can simply be poured in to fill the cavity.

When a layered and/or graded structure is required then a pre-form of powder bound together by a suitable binder (such as an organic binder), is prepared by moulding in a suitable mould to create a pre-form which fits within one of the press tools, leaving a gap between the pre-form and the other press tool for insertion of a second powder mix.

Figure 9 illustrates an insert 1 comprising a pre-form 85 with a layered surface 80, 81 (provided by additional powder mixes). Layer 80 provides wear and corrosion resistance, and layer 81 additional mechanical strength and toughness. Layers may be linear.

When loose powder, rather than just a pre-form, is loaded into the press tool cavity 51, the loose powder may be mixed with a binder which becomes active, or is activated by suitable means such as the application of heat. This holds the powder in an even distribution in the press tool cavity 51.

The press tools 52, 53 are machined from carbon and/or graphite, and the tool cavity surfaces are preferably sprayed with a thin layer of boron nitride or a similar release agent prior to filling of the press tool cavity 51. The layer may be applied by physical or chemical vapour deposition or by chemical vapour infiltration.

Figure 7 shows a typical pressure and temperature cycle used with the press 50 of Figure 8 to achieve hot pressing of a cermet powder having the following composition: a nickel-based super alloy powder, with separate ceramic particles of 15% TiC

15% TiN

The hot-pressing cycle is preceded by furnace purging steps, using lines 73, 74 to remove air and any gaseous binder products when, as is usual, a binder is used with the powder.

The powder filled tooling is placed in the press 50 furnace between the press tools 52, 53. The press is sealed against atmosphere by a casing 70.

A nominal load is then applied to the press tools 52, 53, a load of typically l/20th of the final load, in order to retain the powder in position in the press tool cavity 51.

A vacuum of about 0.4 mbar is then applied to the casing interior 72 using draw-off line 73. High purity argon is then supplied to the interior- 72 using line 74, to a pressure of about 25 mbar above atmospheric pressure.

A vacuum of about 0.4 mbar is then re-established, and this sequence of applying a vacuum and then repressurising the furnace with argon to 25 mbar above atmospheric is repeated five times in all, in order to reduce water vapour and oxygen to a low level.

During this sequence of evacuation and repressurisation with argon, the furnace wall may be brought to a temperature to exceed dewpoint of the water vapour or gases liberated from the breakdown of the binder.

The actual hot pressing of the powder is conducted in the press tool 50 of Figure 8.

The press tool 50 comprises the co-operating pair of press tools 52, 53 which together define the tool cavity 51.

The press tools 52, 53 are mounted on platens 54, 55 at the adjacent ends of stepped structures 56, 57 disposed between the platens 54, 55 and end plates 58, 59. Reference numerals 60, 61 denote thermocouple/tie rods, which clamp press tools 52, 53, structures 56, 57 and end plates 58, 59 together.

The stepped structures 56, 57 comprise tiers of carbon or carbon-based blocks that provide load trains, which apply pressing loads to the press tools 52, 53.

These components, together with press tool support yokes 65, 66, an electrical heating element 67, a block 68 which locates the press tools 52, 53, and heating element height adjustment screws 69, are housed in a furnace casing 70, part 71 of which is of corrugated form, so that the casing can expand and contract.

Block 68 actually comprises a plurality of carbon segments which, together with press tools 52, 53, are held together by a retaining ring 76.

The interior of the furnace casing 70 is lagged with slabs of thermal insulating material 78. The lagging 78 is covered by an upper plate 77.

Using line 74, the interior 72 of the furnace casing 70 is subjected to an argon atmosphere at a pressure which is a few mbar above ambient atmospheric pressure at all times, the argon pressure being controlled (by controller 75) in an automatic manner, in response to monitored measurements of the atmospheric pressure.

It will be appreciated that the purging steps ensure that all air is removed from the casing 70, and that by carefully maintaining a super-atmospheric pressure of argon throughout the pressing cycle, it does not matter if the casing should leak slightly.

When, as is usually the case, a binder is used with the powder, then removal of the gaseous binder product may be achieved in one of the vacuum steps by suitable application of heat using heating element 67 to drive off the binder.

The hot pressing operation results in production of an insert 1 of near-net- shape. Only the mating faces 8 require to be machined. The interior surfaces 2a, 2b of the insert 1 that co-operate with the corresponding surfaces of the other insert of the pair to define the bores 2, 3 of the extruder barrel require substantially no machining.

Thus a hot pressed insert may be said to have mating faces 8 that have allowance for a relatively large amount of machining and inner bore- defining surfaces 2a, 3a, that require a relatively small amount (even zero)

of machining. Faces 8 are typically provided with 0.2 mm to 0.25 mm for machining purposes comprising grinding to a fine level.

Hot pressing of an insert 1 takes place at temperatures of 500°C to 1500°C.

Pressing pressure is 10 to 100 Mpa.

Surface hardness of an insert 1 for good surface wear resistance may vary from 30 to 75 R _c .

Metal powder used may typically have between 0 and 50% (by volume) of ceramic additions.

Metallic and ceramic powders used may vary from submicron to 150 micron in particle size. These may be mechanically alloyed as required.

Typical organic binders used include waxes or cellulose based binders.

A pre-form 85 (Figure 10) may comprise a layered body, the layers comprising wear and/or corrosion resistant layers.

Any high quality inert gas may be used instead of argon.