An extruder device and method of controlling the extrusion pressure

Abstract

An inverted extruder for extrusion of materials with high viscosity, such as aluminium, aluminium alloys, or other metal matrix composites or alloys, comprising a housing (15) having a bore (16) extending between a feeding zone (ZF) and an extrusion chamber (18), and an elongate member (14) rotatably arranged in the bore (16) about a central and longitudinal axis (x). At least a portion of the bore (16) comprises a helical recess (10) arranged in the bore wall, and the rotatable member (14) comprises a smooth outer surface or a surface having structural features that promote adherence between the rotatable member and the material which is being extruded. The housing (15) comprises one or more first fluid channels (7) arranged in the wall around the bore (16) and the rotatable member (14) comprises one or more second fluid channels (8). The invented extruder allows for a method of controlling the extrusion pressure by controlling the temperature in the rotatable member (tR) and the temperature in the wall of the bore (tB).

Description

[0001] AN EXTRUDER DEVICE AND METHOD OF CONTROLLING THE EXTRUSION PRESSURE

[0002] Technical field of the invention

[0003] The invention concerns an extruder as set out by the preamble of claim 1, and methods of controlling the extrusion pressure in a rotational extruder, as set out by the preambles of claims 15 and 18. The invention is particularly useful for extrusion of materials with high viscosity, for example metals such as aluminium, aluminium alloys, or other metal matrix composites or alloys.

[0004] Background of the invention

[0005] Extrusion of a material with high viscosity, such as aluminium or aluminium alloys, involves a process in which the material is forced through a die having a specific cross- sectional profile. The processes involves high pressures, typically between 150 to 400 MPa. Presently, dominant extrusion methods are ram extrusion and screw extrusion.

[0006] Ram extrusion involves placing a preheated billet (for example of aluminium) in an extrusion chamber, and advancing a ram against the billet so as to force the material through a die. The billet must be preheated to a temperature at which it is malleable; for aluminium approximately 400 to 500 °C.

[0007] Screw extrusion involves feeding the material to be extruded into a housing having a bore in which a screw is rotating. The material may comprise granules or particles of any suitable size, such as metal chips, particles, pulverized metal, etc., and is fed into the housing at a temperature comparable to the ambient temperature; i.e., the material is not preheated as in the ram extrusion. The compaction force imposed by the revolving screw causes the temperature in the material to increase such that the granular matter coalesces into a malleable substance before it is forced through an extrusion die.

[0008] Figure l is a schematic illustration of main components of a prior art screw extruder. A screw 51 — for example an Archimedes screw — is arranged in a bore 55 inside an extruder housing (also referred to as a "barrel") 50 and is rotatable around the extruder longitudinal axis x. The material to be extruded is fed into a feeding zone ZF through an inlet 52. The material, which may comprise a mixture of solid-state substances (e.g. chips, nodules, or granules), is conveyed along the bore by the rotating screw, through a heating and mixing zone ZM, before the mixture is compressed in a compaction zone Zc and then forced into an extrusion chamber 53 and extruded through an extrusion die 54.

[0009] A critical parameter for successful operation of a screw extruder for extrusion of materials with high viscosity, is the temperature along the screw housing, or more specifically: along the bore in which the extruder screw is arranged. Efficient screw extrusion of such materials (e.g. aluminium) requires specific temperatures along the screw. High temperatures in the forward region (i.e. near the extrusion chamber 53) will promote sticking friction (i.e., friction that causes adherence — “sticking” — between the material and the bore wall or the screw) and reduce forces required for material deformation, while limited temperature further back (i.e. near the inlet 51) will prevent sticking friction and material compaction. The ability to control the temperature gradient along the extruder bore is therefore necessary in order to avoid these problems. An objective of the present invention is to improve the cooling capability to provide a robust and stable extrusion process.

[0010] The prior art includes US 2007 / 121421 Al, which describes a multiple-shaft extruder having a core with outward leading channels for a cooling liquid. At least two housing segments are each provided with a cooling circuit with interconnected cooling bore holes for a cooling liquid, distributed in the peripheral direction and in an axially parallel manner, and which are located on the section of the housing segments that faces the process chamber. The housing segment provided with the cooling bores has at the same time a heating means on the outside circumference. Each housing segment provided both with cooling bores and with a heating means preferably has a control device which controls both the heating means and the flow of cooling liquid through the cooling bores to permit adjustment of an optimal processing temperature and a material temperature as low as possible.

[0011] The prior art also includes WO 2008 / 063076 Al, which describes a screw extruder for the continuous extrusion of materials with high viscosity, in particular metals such as aluminium and its alloys. The extruder includes an Archimedes screw rotationally provided within liner of a screw housing with an inlet for the feeding of the material to be extruded, a compacting or extrusion chamber, and an extrusion die assembly with a die which forms the shape of the desired extruded product. The design of the screw and liner is such that the required compaction takes place at the downstream end of the screw towards the extrusion chamber corresponding to up to 540° of the rotation of the screw, or up to 1,5 turns of the screw flight length, and that the solid plug of metal thus formed at the end of the screw and extrusion chamber is restricted from rigid rotation to obtain the required compaction and extrusion pressure.

[0012] Publications US 3 199 147 A, CN 211 074 655 U, US 10 035 291 B2, and DE 10 356 423 B4 also describe various extruder types.

[0013] There is a need for a device and a method whereby friction along the barrel - and hence the extrusion pressure - may be controlled more precisely than what is possible with the prior art extruders, and where the quality and throughput may be further optimised.

[0014] Another aspect of prior art screw extruders — in which a rotatable screw advances and compacts material inside a substantially smooth barrel — is that the screw is exposed to large bending moments due to fluctuating loading of the screw flights when the screw is rotating. These bending moments cause stress concentrations and may deform the screw such that it damages the barrel wall. There is thus also a need for a more robust extrusion device.

[0015] Summary of the invention

[0016] The invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention.

[0017] It is thus provided an extruder for extrusion of materials with high viscosity, such as aluminium, aluminium alloys, or other metal matrix composites or alloys, comprising a housing having a bore extending between a feeding zone and an extrusion chamber, and an elongate member rotatably arranged in the bore about a central and longitudinal axis, characterized in that at least a portion of the bore comprises a helical recess arranged in the bore wall, and the rotatable member comprises a smooth outer surface or a surface having structural features that promote adherence between the rotatable member and the material which is being extruded. In one embodiment, the helical recess is formed by a bottom portion recessed into the bore wall and a wing rising out from the bottom portion. In one embodiment, the housing comprises one or more first fluid channels arranged in the wall around the bore and the rotatable member comprises one or more second fluid channels. At least one of the first fluid channels may be arranged in a wing. In one embodiment, the housing comprises an outer housing and an inner housing, or “barrel”, the latter comprises said bore.

[0018] In one embodiment, the rotatable member comprises a core member and a sleeve arranged around the core member, and the core and sleeve are rigidly and releasably interconnected so as to rotate as one element. The sleeve may comprise a smooth outer surface or a surface having structural features that promote adherence between the sleeve and the material which is being extruded. In one embodiment, said second fluid channels are arranged circumferentially and embedded in the core member outer surface.

[0019] A conveyor screw may be attached to the rotatable member in the region of the feeding zone in order to move the material into a mixing-and-compaction zone in the bore.

[0020] In one embodiment, the inner housing material has a thermal conductivity which is greater than the thermal conductivity of the outer housing material. In one embodiment, the inner housing is secured to the outer housing whereby the inner housing is prevented from rotating. The outer housing inner surface and the inner housing outer may have surface complementary shapes.

[0021] In one embodiment, the outer housing inner surface and the inner housing outer surface have complementary frustoconical shapes with a common cone angle and the inner housing comprises a frustrum with its base on the downstream side of the housing.

[0022] In one embodiment, the sleeve material has a thermal conductivity which is greater than the thermal conductivity of the core member material.

[0023] In the invented extruder, the structurally homogenous rotatable punch may be designed with a higher stiffness than that of a rotatable screw of a similar diameter, whereby the risk of damaging the barrel wall is minimized.

[0024] In the invented extruder, the material entering the extrusion chamber from the barrel (the housing bore, or “barrel”) is advanced axially but is not rotating. This non-rotating translation promotes a higher tolerance and a more homogenous finished product compared to products by a conventional screw extruder. In conventional screw extruders, the material entering the extrusion chamber is rotating, which may lead to undulating extrusion speed and an uneven structure in the finished extruded product.

[0025] It is also provided a first method of controlling the extrusion pressure in a rotational extruder for extrusion of a material or materials with high viscosity, such as aluminium, aluminium alloys, or other metal matrix composites or alloys, wherein the extruder comprises a rotatable member arranged in a bore of a housing, and the rotational extruder comprises the extruder according to the invention, characterized in that the method comprises controlling the temperature in the rotatable member and the temperature in the wall of the bore while the rotatable member is rotating and the material or materials are within a mixing-and-compaction zone in the extruder, such that the temperature in the rotatable member is lower than the temperature of the wall of the bore.

[0026] One embodiment of the invented first method comprises allowing at least the temperature in the rotatable member to rise to a level at or above the sticking friction temperature for the material or materials to be extruded, at the beginning of the mixing- and-compaction zone, while maintaining the temperature in the rotatable member below the temperature of the wall of the bore.

[0027] One embodiment of the invented first method comprises maintaining at least the temperature of the wall of the bore above the extrusion temperature for the material or materials to be extruded, in a region towards the end of the mixing-and-compaction zone, upstream of an inlet to an extrusion chamber, while maintaining the temperature in the rotatable member below the temperature of the wall of the bore.

[0028] It is also provided a second method of controlling the extrusion pressure in a prior art rotational extruder for extrusion of a material or materials with high viscosity, such as aluminium, aluminium alloys, or other metal matrix composites or alloys, wherein the prior art extruder comprises a rotatable screw arranged in a bore in an extruder housing, characterized in that the method comprises controlling the temperature in the screw and the temperature in the wall of the bore while the screw is rotating and the material or materials are within a mixing-and-compaction zone in the extruder, such that the temperature in the screw is higher than the temperature of the wall of the bore.

[0029] One embodiment of the second invented method comprises allowing at least the temperature in the wall of the bore to rise to a level at or above the sticking friction temperature for the material or materials to be extruded, at the beginning of the mixing- and-compaction zone, while maintaining the temperature in the screw above the temperature of the wall of the bore.

[0030] The improved extrusion pressure control made possible by the invention, allows for an improved extrusion efficiency, i.e., optimisation of the relationship between extruded volume and energy consumption.

[0031] Brief description of the drawings

[0032] These and other characteristics of the invention will become clear from the following description of embodiments of the invention, given as non-restrictive examples, with reference to the attached schematic drawings, wherein:

[0033] Figure l is a sectional drawing of an example of a prior art screw extruder, as described above;

[0034] Figure 2 is a sectional drawing of certain components of an embodiment of the extruder according to the invention, the section taken along a longitudinal axis of the extruder;

[0035] Figure 3 is a perspective view of the extruder illustrated in figure 2 with certain components removed or being transparent;

[0036] Figure 4 is a side view of a portion of the extruder illustrated in figures 2 and 3, illustrating i.a. an exemplary arrangement of fluid channels in the inner housing and an optional conveyor screw;

[0037] Figure 5 corresponds to figure 4, but illustrates an alternative embodiment of the inner housing in the region of a material feeding zone;

[0038] Figure 6 is a sectional view, taken along the extruder longitudinal axis, of an embodiment of a portion of the inner housing according to the invention; Figure 7 is an enlarged view of the area marked “A” in figure 6;

[0039] Figure 8 illustrates an embodiment of the rotatable punch according to the invention;

[0040] Figure 9 corresponds to figure 8, but the conveyor screw and a sleeve have been removed to illustrate an embodiment of the rotatable core member;

[0041] Figure 10 is an exemplary diagram illustrating the temperature of the stationary housing and the temperature of the rotatable punch as functions of the distance along the barrel;

[0042] Figure 11 is an exemplary diagram illustrating friction force and shear strength as functions of temperature;

[0043] Figure 12 corresponds to figure 6, but illustrates an alternative embodiment of the inner housing;

[0044] Figure 13 is a cross-sectional view of the embodiment of the alternative embodiment of the inner housing illustrated in figure 12; and

[0045] Figure 14 is an exemplary diagram illustrating the temperature of a stationary housing and the temperature of a rotatable screw as functions of the distance along the barrel, in an extruder having a conventional screw, for example an Archimedes screw.

[0046] A person skilled in the art will understand that the drawings do not show a complete extruder, but only parts and features that are necessary for elucidating the invention.

[0047] Detailed description of embodiments of the invention

[0048] The following description may use terms such as “horizontal”, “vertical”, “lateral”, “back and forth”, “up and down”, ’’upper”, “lower”, “inner”, “outer”, “forward”, “rear”, etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader’s convenience only and shall not be limiting.

[0049] Referring initially to figure 2, the extruder according to the invention comprises an elongate member 14 — in the following referred to as a rotatable punch — rotatably arranged in a bore 16 in a static housing 15, about a central and longitudinal axis x of the extruder. The punch 14 is thus rotatably arranged in a non-moving (hence “static”) housing 15. In the illustrated embodiment, the punch is a cylindrical member, but it should be understood that the member may have other shapes; for example frustoconical. Parts required to operate the extruder (power source, bearings, seals, sensors, extrusion die, control devices, etc.) have been omitted from the drawings, as they are not required to describe the inventive features. Reference number 3 designates an interface portion via which the rotatable punch 14 is connectable to a power source such as a drive motor; features that are well known in the art. When the extruder is in operation and the punch 14 is rotating, the material to be extruded may be fed into the extruder in a feeding zone ZF (the actual inlet is not shown). The material, which may comprise a mixture of substances including aluminium or aluminium alloys, or other metal alloys or metal matrix composites yielding a high-viscosity material, is mixed and compacted in a mixing-and-compaction zone ZMC inside the bore 16 before entering an extrusion chamber and extrusion die. This mixing and compaction process is described below. In figure 2, the extrusion chamber and extrusion die are indicated schematically by dotted lines and reference number 18 in the right-hand end of the bore 16, and the arrow “D” indicates the flow of material through the extruder.

[0050] If the extruder is arranged horizontally or near horizontally, for example as shown in figure 2, a conveyor screw 5 is attached to the rotatable punch in the region of the feeding zone ZF in order to move the material into the mixing-and-compaction zone ZMC. If the extruder is arranged at an angular orientation that is sufficient for the material to gravitate into the mixing-and-compaction zone, the conveyor screw 5 is not necessary and may be omitted.

[0051] As illustrated in figure 2, the static housing 15 comprises in the illustrated embodiment an outer housing 2 and an inner housing 4, the latter also referred to in the art as a “barrel” or “liner”, and comprising the above-mentioned bore 16. This two-part design is convenient from an operational perspective, as it may be desirable or necessary to replace or refurbish the barrel 4 from time to time. However, the housing 15 may be comprised of a single element. The inner housing 4 preferably comprises a material having a high wear resistance or / and a coated bore wall. As a non-limiting example, the material may be a tool-grade steel and a wall coating may comprise diamond-like carbon (DLC) or a nickel-based superalloy.

[0052] When the extruder is in operation, the inner housing 4 is exposed to severe heat generation from the deformation of the extrudate. The heat flow from the housing to the cooling channels is limited by the thermal conductivity of the housing material, and thermal runaway will cause thermal stress that may cause plastic deformation and / or formation of cracks in the extruder bore 16. This problem is mitigated or even avoided by the invention, by the inner housing 4 comprising a material with thermal conductivity sufficiently high for transporting away the generated heat. Thus, in an alternative embodiment, inner housing (liner) 4 has a thermal conductivity which is greater than the thermal conductivity of the outer housing 2. As a non-limiting example, the thermal conductivity of the inner housing 4 is in the region 40 to 80 W / (m K), while the thermal conductivity of the outer housing 2 is around 25 W / (m K). As a nonlimiting example, the material of the inner housing is a high-thermal conductive steel or beryllium copper, and the outer housing is of regular tool-grade steel.

[0053] Referring additionally to figures 6 and 7, in the mixing-and-compaction zone ZMC, the bore 16 comprises a helical recess 10 arranged in the bore wall. The helix angle a and helix lead (i.e. the length along the bore which the helix travels in one revolution; a function of the helix angle) of the recess will be dimensioned to suit operational parameters, such as type and nature of the material to be extruded. It is apparent from the figures, however, that the helix angle and lead are towards the extrusion chamber and extrusion die 18, as indicated by the arrow “D” and explained above with reference to figure 2.

[0054] The helical recess 10 is formed by a bottom portion 9d recessed into the bore wall and an element hereinafter referred to as a “wing” 9 rising out from the bottom portion. The wing 9 is comprised of a top portion 9c having a flat surface which is parallel with extruder longitudinal axis x and arranged at a distance h from the bottom portion 9d, and oppositely arranged wall portions 9a, 9b connecting bottom and top portions. The wall portion 9b which is facing the extrusion chamber and extrusion die (the direction “D” in figures 2 and 6) is referred to as the downstream wall portion 9b, and the wall portion 9a which is facing the opposite direction is referred to as the upstream wall portion 9a. As the wing 9 is instrumental in mixing, compacting, and advancing the material through the bore 16, as will be discussed in detail below, the downstream wall portion 9b will be referred to as a “passive flank” and the upstream wall portion 9a will be referred to as an “active flank”. In the illustrated embodiment, the passive flank 9b is arranged perpendicularly to the extruder longitudinal axis x, and the active flank 9a is arranged at a flank angle f> with the extruder longitudinal axis x. However, the invention shall not be limited to the illustrated wing geometry.

[0055] Referring now to additionally to figures 3 and 4, in which the outer housing 2 is not shown, the housing 15 comprises fluid channels 7 arranged in the wall around the bore 16. In the illustrated embodiment, the fluid channels 7 are formed by grooves in the inner housing 4 and the inner wall of the outer housing 2. In the embodiment where the housing 15 is comprised of a single element and not inner and outer housings, the fluid channels 7 are embedded in the body of the housing 15.

[0056] These fluid channels will in the following be referred to as “static” fluid channels 7, as they are arranged in the static housing. In the illustrated embodiment, the static fluid channels 7 are arranged in the barrel 4 and run in a helical fashion along the mixing- and-compaction zone ZMC, consistent with the helical recess 10 and wing 9 described above.

[0057] The static fluid channels 7 are connected to a fluid supply-and-circulation system (which per se is known in the art and therefore not illustrated), whereby a fluid may be circulated through the channels 7. The fluid is preferably a coolant, such as air, oil, water or another liquid having a high thermal capacity. In any case, the temperature and flowrate of the fluid entering the channels are controlled by external devices (well known in the art). During operation of the extruder, the coolant temperature is controlled in order to control the temperature of the bore wall (and hence the friction between the bore wall and the material in the mixing-and-compaction zone ZMC), and it is therefore desirable to arrange the fluid channels 7 as close to the bore wall surface as possible. To this end, each wing 9 comprises a fluid channel 7a arranged between the passive and active flanks. In the illustrated embodiment, a plurality of static fluid channels 7, 7a have been arranged in segments Si-nalong the mixing-and-compaction zone ZMC, (see figure 4), whereby the temperature in each corresponding bore wall segment may be controlled independently. In one embodiment, the fluid channels of each segment is connected to individual coolant supply-and-circulation systems. It should be understood that the housing may comprise one or more such static fluid channels 7.

[0058] Figure 5 illustrates an embodiment of the static housing in which the bore in the region of the feeding zone ZF comprises serpentine fluid channels for coolant, connected and operated similarly to the helical channels 7a, b.

[0059] An embodiment of the rotatable punch 14 will now be described in more detail. As illustrated in figure 3, the rotatable punch 14 comprises in the illustrated embodiment a core member 1 and a sleeve 6 arranged around the core member. The core and sleeve comprise devices or features whereby they are rigidly interconnected so as to rotate as one element when the extruder is in operation. This two-part design is convenient from an operational perspective, as it may be desirable or necessary to replace or refurbish the sleeve 6 from time to time. However, the rotatable punch 14 may be comprised of a single element. The core member 1 may comprise a tool-grade steel material, and the sleeve 6 preferably steel of a harder grade than that of the core member.

[0060] When the extruder is in operation, the sleeve 6 is exposed to severe heat generation from the deformation of the extrudate. The heat flow from the sleeve to the cooling channels is limited by the thermal conductivity of the sleeve material, and thermal runaway will cause thermal stress that may cause plastic deformation and / or formation of cracks in the sleeve. This problem is mitigated or even avoided by the invention, by the sleeve 6 comprising a material with thermal conductivity sufficiently high for transporting away the generated heat. Thus, in an alternative embodiment, sleeve 6 has a thermal conductivity which is greater than the thermal conductivity of the core member 1. As a non-limiting example, the thermal conductivity of the sleeve 6 is in the region 40 to 80 W / (m K), while the thermal conductivity of the core member I is around 25 W / (m K). As a non-limiting example, the material of the sleeve is a high-thermal conductive steel or beryllium copper, and the core member is of regular tool-grade steel. The sleeve surface may also comprise a nickel-based superalloy coating for improved wear resistance and temperature control.

[0061] In one embodiment, the sleeve 6 outer surface is smooth. In another embodiment, the sleeve 6 outer surface comprises structural features that promote adherence between the rotatable punch (i.e. the sleeve) and the material which is being extruded. For example, as shown in figure 8, the sleeve 6 outer surface may comprise straight and parallel grooves 13 that are aligned with the extruder longitudinal axis x. As an alternative, the sleeve outer surface may comprise a rifling pattern (i.e., helical grooves) or a knurled pattern. A thermal insulator 17 may optionally be arranged between the sleeve 6 and conveyor 5.

[0062] Referring now to figure 9, in which the sleeve 6 has been removed, fluid channels 8 are arranged circumferentially in the rotatable punch 14. These fluid channels will in the following be referred to as “rotatable” fluid channels 8, as they are arranged in the rotatable punch. In general, the rotatable fluid channels 8 are embedded in the rotatable punch at a distance from the punch outer surface that ensures optimal control of heat flow and thermal stresses. In the illustrated embodiment, the rotatable fluid channels 8 are arranged in the core 1 outer surface and run in a helical fashion along the portion of the punch that is arranged in the mixing-and-compaction zone ZMC, consistent with the helical recess 10 and wing 9, and the static fluid channels 7, 7a described above. It should be understood that the punch may comprise one or more such rotatable fluid channels 8.

[0063] The rotatable fluid channels 8 are connected to a fluid supply-and-circulation system (which per se is known in the art and therefore not illustrated), for example via a circulation channel 11 (see figure 2), whereby a fluid may be circulated through the channels 8. The fluid is preferably a coolant, as described above with reference to the static fluid channels. During operation of the extruder, the coolant temperature is controlled in order to control the temperature of the punch (i.e. sleeve) surface (and hence the friction between the bore wall and the material in the mixing-and-compaction zone ZMC). That part of the rotatable punch which corresponds to the feeding zone ZF comprises a separate channels 8a that are connected and operated similarly to the static channels 8. The skilled person will understand that the housing and the punch comprise sensors placed at appropriate positions in order to monitor temperature, and other parameters such as pressure and strain. As an example, referring to figure 10, one or more sensors 19 are placed in the punch and the housing at least in the region of the inlet to the mixing-and-compaction zone ZMC, along the mixing-and-compaction zone, and near the inlet to the extrusion chamber and extrusion die 18.

[0064] When the extruder according to the invention is operated, the punch 14 is rotated while the material to be extruded (not shown) is fed into the housing in the feeding zone ZF. The material will enter the mixing-and-compaction zone ZMC in the upstream portion of the bore 16, by natural gravitation or / and assisted by the optional conveyor screw 5. The material properties, the punch rotational speed, and the rate at which the material is fed into the feeding zone, all contribute to determining the cooling requirements in the mixing-and-compaction zone. The material input rate and the rotational speed may be optimised for obtaining certain desired characteristics in the final, extruded, product.

[0065] The rotating punch 14 will advance the material inside the bore 16, in the direction towards the extrusion chamber and extrusion die 18, as indicated by the arrow “D” in figure 6, the objective being to mix and compact the material to a temperature which is sufficient for extrusion (i.e., a temperature at which the compacted material is malleable).

[0066] A fundamental difference between polymer extrusion and metal extrusion is that in polymers, the friction force is the product of the applied pressure multiplied by the friction coefficient multiplied by the area; i.e., the frictional force in a polymer is proportional to the pressure. For metals that exhibit sticking friction, such as aluminium, the force is simply given as the shear strength multiplied by the area. The pressure build-up in any rotational extruder (i.e., a conventional screw extruder according to the prior art or the extruder according to the present invention) is thus fundamentally different for polymers and metals.

[0067] Figure 11 illustrates how friction force F and shear strength T in a metal vary with temperature in a metal during mixing and compaction. The shear strength r generally decreases with increasing temperature, and the friction force F remains constant until sticking friction occurs at a temperature ts, which is the material-dependent critical temperature for sticking friction. In the illustrated example, in which the material is aluminium, sticking friction Fs is achieved at a temperature ts of 300 °C.

[0068] An element (such as a metal) which is being forced along bore in an extruder by means of a rotating member, is subjected to a positive pressure gradient in the downstream (D) direction - a fundamental premise for compaction and subsequent extrusion. This is the case for a conventional screw extruder having a rotating screw inside a substantially smooth bore (barrel), as well as for the extruder according to the present invention as described above. Using the present invention as an example, and referring to figures 6 and 7 as discussed above, the force on the element caused by the active flank 9a must be greater than the force on the element caused by the passive flank 9b. While the affected area, i.e. geometrical and dimensional relationship between the punch and wings (and, in a conventional extruder: between the screw and the bore) is important, it can be demonstrated that an absolute requirement for a positive pressure gradient in the downstream direction is that the shear strength between the element and the rotating member (TR) is greater than the shear strength between the element and the static member (TS), i.e.: TR > TS. For example, a ratio of TR / TS between 1.5 and 3.0 seems favourable, depending on the alloy.

[0069] Therefore, to control the pressure build-up for “rotational extrusion” (as opposed to non-rotational ram extrusion) of metals, one must control the shear strength of the metal at the different surfaces in the extruder. The term “rotational extrusion” is used for indicating that this principle applies to the extruder according to the invention as well as to a conventional screw extruder. By utilizing the fact that the shear strength of a metal is temperature-dependent, it is possible to use temperature as the control mechanism for pressure generation. This is illustrated in figure 10, which illustrates a favourable temperature difference between a rotating member (such as the rotatable punch 14) and a static bore (e.g., the wings 9 and recesses 10) along the length of the bore. The temperature tR in the rotating punch is thus balanced against the temperature tn in the static bore wall (i.e., the helical wings 9) in order to achieve sticking friction between the material and the punch and reduce friction between the material and the bore wall. Figure 10 corresponds to figure 11 in that the material has a critical temperature for sticking friction, ts, of 300 °C. The figure illustrates an extrusion method according to the invention, in which:

[0070] • The temperature in the rotating punch and the temperature in the static bore both rise due to interaction between the material, the rotating punch, and the bore wall. At the beginning of the mixing-and-compaction zone ZMC, the temperatures must be allowed to rise to levels above the critical temperature for sticking friction, ts, but the temperature in the rotating punch is at all times kept below the temperature in the static bore, i.e.: tR > ts; tB > ts; tR < tB. If necessary, preheating the extrusion screw may be necessary to ensure sticking friction, i.e. that tR > ts.

[0071] • The temperature in the rotating punch and the temperature in the static bore are maintained above the critical temperature for sticking friction throughout the mixing-and-compaction zone ZMC, while tR is maintained below tB.

[0072] • Towards the end of the bore, immediately before the inlet to the extrusion chamber and extrusion die 18, the material must have a temperature, tE, at which it is malleable and hence suitable for extrusion.

[0073] The principle described above with reference to figures 10 and 11 applies to a conventional screw extruder as well, for example a prior art screw extruder as illustrated in figure 1 and figure 14. However, in this configuration, the invented method comprises controlling the temperature in the screw (4R) and the temperature in the wall of the bore (ts) while the screw is rotating such that the temperature in the screw is higher than the temperature of the wall of the bore; i.e. tR > tB. Also, in this configuration, the method comprises allowing at least the temperature in the wall of the bore (ts) to rise to a level at or above the sticking friction temperature (ts) for the material or materials to be extruded at the beginning of the mixing-and-compaction zone (ZMC), while maintaining the temperature in the screw above the temperature of the wall of the bore; i.e. tR > tB. This is illustrated in figure 14, where a rotatable screw 71 (for example an Archimedes screw) is rotatably arranged in a housing 70 upstream of an extrusion chamber 73. A feeding sone ZF and a mixing zone ZM are schematically indicated. The housing 70 comprises fluid channels 74 whereby the temperature in the housing can be controlled, and the screw 71 comprises an optional fluid channel 75.

[0074] It should be understood that the invention applies to extrusion of materials with high viscosity, for example metals such as aluminium, aluminium alloys, or metal matrix composites. The extrusion process may be continuous or by introducing batches of material or material mixtures into the extruder.

[0075] Figures 12 and 13 illustrate an alternative embodiment of the inner housing, denoted by reference number 4’ . As described above, in one embodiment the inner housing material has a thermal conductivity which is greater than the thermal conductivity of the outer housing. As inner housing is subjected to torsion by the material being extruded, caused by the torque induced by the rotating punch or screw, the inner housing is in the illustrated embodiment secured to the outer housing (not shown in figures 12 and 13) in order to prevent the inner housing from rotating inside the outer housing. In this embodiment, the outer housing inner surface and the inner housing 4’ outer surface have complementary shapes, whereby the inner housing 4’ is prevented from rotating inside the outer housing. In the illustrated embodiment, the surfaces are undulating radially, with six lobes. It should be understood that these surfaces may have other shapes.

[0076] Referring to figure 12, the inner housing 4’ outer surface and the outer housing inner surface (not shown in figure 12) have complementary frustoconical shapes with a common cone angle y. The inner housing 4’ is thus a frustrum with its base on the downstream side D of the section inner housing. This configuration ensures a press-fit between inner and outer housings and prevents inadvertent separation when the housing 15 is removed from the extruder.

[0077] Although the invention has been described with reference to a single housing 15, the invention is equally applicable to an extruder in which the mixing-and-compaction zone ZMC comprises multiple housing sections arranged and interconnected in an end-to-end relationship to form a continuous bore 16.

[0078] In the embodiments described above, various features and details are shown in combination. The fact that several features are described with respect to a particular example should not be construed as implying that those features by necessity have to be included together in all embodiments of the invention. Conversely, features that are described with reference to different embodiments should not be construed as mutually exclusive. As a person skilled in the art readily will understand, embodiments that incorporate any subset of features described herein and that are not expressly interdependent have been contemplated by the inventor and are part of the intended disclosure. However, explicit description of all such embodiments would not contribute to the understanding of the principles of the invention, and consequently some permutations of features have been omitted for the sake of simplicity or brevity. The invention is defined by the appended claims.

Claims

Claims1. An extruder for extrusion of materials with high viscosity, such as aluminium, aluminium alloys, or other metal matrix composites or alloys, comprising a housing(15) having a bore (16) extending between a feeding zone (ZF) and an extrusion chamber (18), and an elongate member (14) rotatably arranged in the bore (16) about a central and longitudinal axis (x), characterized in that at least a portion of the bore(16) comprises a helical recess (10) arranged in the bore wall, and the rotatable member (14) comprises a smooth outer surface or a surface having structural features that promote adherence between the rotatable member and the material which is being extruded.

2. The extruder of claim 1, wherein the helical recess (10) is formed by a bottom portion (9d) recessed into the bore wall and a wing (9) rising out from the bottom portion.

3. The extruder of any one of claims 1 and 2, wherein the housing (15) comprises one or more first fluid channels (7) arranged in the wall around the bore (16) and the rotatable member (14) comprises one or more second fluid channels (8).

4. The extruder of claim 3, wherein at least one of the first fluid channels (7) is arranged in a wing (9).

5. The extruder of any one of claims 1-4, wherein the housing (15) comprises an outer housing (2) and an inner housing (4), and wherein the latter comprises said bore (16).

6. The extruder of any one of claims 1-5, wherein the rotatable member (14) comprises a core member (1) and a sleeve (6) arranged around the core member, and the core and sleeve are rigidly and releasably interconnected so as to rotate as one element.

7. The extruder of claim 6, wherein the sleeve (6) comprises a smooth outer surface, or a surface having structural features that promote adherence between the sleeve and the material which is being extruded.

8. The extruder of any one of claims 6-7, wherein said second fluid channels (8) are arranged circumferentially and embedded in the core member (1) outer surface.

9. The extruder of any one of claims 1-8, wherein a conveyor screw (5) is attached to the rotatable member (14) in the region of the feeding zone (ZF) in order to move the material into a mixing-and-compaction zone (ZMC) in the bore.

10. The extruder of any one of claims 5-9, wherein the inner housing material has a thermal conductivity which is greater than the thermal conductivity of the outer housing material.

11. The extruder of any one of claims 1-10, wherein the inner housing (4’) is secured to the outer housing whereby the inner housing is prevented from rotating.

12. The extruder of claim 11, wherein the outer housing inner surface and the inner housing outer surface have complementary shapes.

13. The extruder of any one of claims 5-12, wherein the outer housing inner surface and the inner housing outer surface have complementary frustoconical shapes with a common cone angle (y) and the inner housing (4’) comprises a frustrum with its base on the downstream side (D) of the housing.

14. The extruder of any one of claims 6-13, wherein the sleeve (6) material has a thermal conductivity which is greater than the thermal conductivity of the core member (1) material.

15. A method of controlling the extrusion pressure in a rotational extruder for extrusion of a material or materials with high viscosity, such as aluminium, aluminium alloys, or other metal matrix composites or alloys, wherein the extruder comprises a rotatable member arranged in a bore of a housing, and the rotational extruder comprises an extruder as defined by any one of claims 1-14, characterized in that the method comprises controlling the temperature in the rotatable member (ta) and the temperature in the wall of the bore (te) while the rotatable member is rotating and the material or materials are within a mixing-and-compaction zone (ZMC) in the extruder, such that the temperature in the rotatable member is lower than the temperature of the wall of the bore (tR < ts).

16. The method of claim 15, wherein the method comprises allowing at least the temperature in the rotatable member (IR) to rise to a level at or above the stickingfriction temperature (ts) for the material or materials to be extruded, at the beginning of the mixing-and-compaction zone (ZMC), while maintaining the temperature in the rotatable member below the temperature of the wall of the bore (tR < ts).

17. The method of claim 15 or claim 16, wherein the method comprises maintaining at least the temperature of the wall of the bore (ts) above the extrusion temperature (IE) for the material or materials to be extruded, in a region towards the end of the mixing- and-compaction zone (ZMC), upstream of an inlet to an extrusion chamber (18), while maintaining the temperature in the rotatable member below the temperature of the wall of the bore (tR < ts).

18. A method of controlling the extrusion pressure in a rotational extruder for extrusion of a material or materials with high viscosity, such as aluminium, aluminium alloys, or other metal matrix composites or alloys, wherein the prior art extruder comprises a rotatable screw (51; 71) arranged in a bore in an extruder housing (50; 70), characterized in that the method comprises controlling the temperature in the screw (IR) and the temperature in the wall of the bore (ts) while the screw is rotating and the material or materials are within a mixing-and-compaction zone (ZMC) in the extruder, such that the temperature in the screw is higher than the temperature of the wall of the bore (tR > ts).

19. The method of claim 18, wherein the method comprises allowing at least the temperature in the wall of the bore (ts) to rise to a level at or above the sticking friction temperature (ts) for the material or materials to be extruded, at the beginning of the mixing-and-compaction zone (ZMC), while maintaining the temperature in the screw above the temperature of the wall of the bore (tR > ts).