GB2525421A

GB2525421A - Extrusion apparatus

Info

Publication number: GB2525421A
Application number: GB1407249.0A
Authority: GB
Inventors: John Jardine; Dave Williams
Original assignee: BRIDGEBROOKE ENERGY Ltd
Current assignee: BRIDGEBROOKE ENERGY Ltd
Priority date: 2014-04-24
Filing date: 2014-04-24
Publication date: 2015-10-28
Anticipated expiration: 2034-04-24
Also published as: GB2525421B; GB201407249D0

Abstract

A tip element 32 for attachment to an end of an extruder screw 30 comprises a substantially cylindrical screw root having first and second end faces, the second end face being configured to locate, in use, adjacent to an end of said extruder screw; a helical flight extending radially outwards from the screw root turning through an angle of 720Â° or less between first and second ends, wherein a width of the flight is greater at the first end than the second end, and wherein the tip element is made of a metal material and a hardness of the tip element is substantially constant throughout the helical flight. The tip element preferably comprises an axial bore extending between the end faces. An extruder screw assembly is further provided comprising a main screw component 30 having a first end configured for connection to a means for rotating the screw component, the tip element of the invention configured to locate adjacent to the second end of the screw component, a nose element 34 including an elongate tapered portion and a means for securing the nose element and the tip element to the main screw element. The screw extrusion apparatus is preferably for forming biomass extrudate for use as a fuel.

Description

Extrusion Apparatus

BACKGROUND

a. Field of the Invention

This invention relates to a screw extrusion apparatus for forming biomass extrudate for use as a fuel. In particular this invention relates to a tip element for attachment to an end of an extruder screw and to an extruder screw assembly including such a tip element.

b. Related Art It is known to extrude wood material in the form of saw dust and wood chips to form wood extrudate or briquettes that may be burnt as a fuel in woodburners and barbeques, for example. As the particulate wood material is extruded, the heat and pressure causes the natural resins in the wood material to bond the saw dust and wood chips together. This extrusion process can, however, be unreliable and produce an inconsistent end product.

One known problem is that the wood material causes significant wear of the tooling of the extrusion apparatus. In particular, the screw component of the extrusion apparatus typically only operates for around 30 hours before it is necessary to repair and rebuild the flight of the screw. In some circumstances the screw component may last less than two hours before requiring repair.

The screw is typically rebuilt by replacing the worn material by welding using a material comprising tungsten carbide granules in an alloy or other hard facing materials. The process will often use multiple grades of build-up electrodes, requiring highly skilled and trained staff. The weld material is then machined or ground, often by eye, to produce the correct angle of the flight. This repair process has a number of disadvantages. Machining the flight by eye is very difficult and inconsistent, and the high temperatures of the welding process cause distortions of the shaft of the screw, ultimately leading to premature failure of the screw, as well as affecting the physical properties of the metal material. There is, therefore, a high likelihood that the extrusion apparatus will not operate correctly after the repair has been made. Additionally, the weld material has a relatively rough surface finish compared to the original material of the screw such that the saw dust and wood chips have a tendency to bind on this region of the screw during subsequent extrusion operations, causing inconsistent operation.

Due to these problems screw extrusion systems are considered unreliable and the majority of the production of wood, or biomass, briquettes is carried out using mechanical or hydraulic briquetter systems that produce a much inferior product.

The moisture content of the wood material also has a significant impact on the quality and consistency of the final product. If the moisture content of the wood material is too high then steam produced during the extrusion process will cause the extrudate to break apart and not consolidate consistently. However, if the moisture content of the wood is too low then there is a fire risk and the briquette will not form correctly.

Due to the above problems with the extrusion process it is known that it is very difficult, and in some situations impossible, for this extrusion process to be made to be commercially viable, even when clean wood material, such as the saw dust by-product of the timber industry, is used.

It is an object of the present invention to provide an improved extrusion apparatus that overcomes some of the above disadvantages of known systems.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a tip element for attachment to an end of an extruder screw, the tip element comprising: -a substantially cylindrical screw root having first and second end faces, a longitudinal axis extending between the first and second end faces defining a rotational axis of the tip element, and the second end face being configured to locate, in use, adjacent to an end of said extruder screw; and -a helical flight extending radially outwards from the screw root, the helical flight turning through an angle of 720° or less between first and second ends, said first flight end being proximate the first end face of the screw root and said second flight end being proximate the second end face of the screw root, wherein a width of the flight is greater at the first end than the second end, and wherein the tip element is made of a metal material and a hardness of the tip element is substantially constant throughout the helical flight.

Preferably the hardness of the tip element is substantially constant throughout the screw root and the helical flight.

In some embodiments the tip element further comprises an axial bore extending through the screw root between said end faces.

The width of the flight at the first end is, preferably, twice the width of the flight at the second end. Preferably the width of the flight at the first end is between 13mm and 20 mm. More preferably the width of the flight at the first end is between 15mm and 17 mm. In preferred embodiments the width of the flight increases linearly from the second end to the first end.

The tip element is preferably made from a tool steel material. In particularly preferred embodiments the tip element is made from an alloy steel having a hardness of between 65 and 75 Rockwell.

Typically the helical flight turns through an angle of between 180° and 54Q0 between first and second ends. More preferably the helical flight turns through an angle of between 3600 and 4QQO between the first and second ends.

In some embodiments an outer diameter of the root is between 40 mm and 50mm.

The pitch of the helical flight may be between 25 mm and 35 mm.

Preferably the second end face of the screw root is stepped, such that a first part of the second end face is further from the first end face than a second part of the second end face.

In some embodiments a maximum axial length of the tip element, between the first and second end faces is between 45 mm and 50 mm.

According to a second aspect of the present invention there is provided an extruder screw assembly comprising: -a main screw component comprising an elongate cylindrical root and a helical flight, the cylindrical root having opposing first and second ends and a longitudinal axis extending between the first and second ends defining a rotational axis of the screw component, a first end of said cylindrical root configured for connection to a means for rotating the screw component about said axis; -a tip element according to the first aspect of the invention, the second end face of the tip element being configured to locate adjacent to the second end of the main screw component; -a nose element including an elongate tapered portion; and -means for securing the nose element and the tip element to the main screw component such that the tip element is located between a part of the nose element and the second end of the screw component and the helical flight of the tip element is aligned with the helical flight of the screw component.

In preferred embodiments the second end of the screw component includes a cavity, the tip element includes an axial bore extending through the screw root and the nose element includes an elongate stem portion, and wherein the stem portion of the nose element extends through the axial bore and an end of the stem portion is received in the cavity.

More preferably the nose element includes an axial bore extending through the tapered portion and the stem portion, the screw component includes a threaded hole extending axially from a base of the cavity, and the assembly including securing means comprising an elongate shaft sized to be received within the axial bore and having, at a first end of the shaft, a head configured to engage with a part of the nose element and, at an opposite second end of the shaft, a threaded region configured to engage with said threaded hole.

The second end of the main screw component and the second end face of the tip element may each have a stepped profile.

In particularly preferred embodiments the tip element is engaged with the main screw component such that rotation of the main screw component causes rotation of the tip element.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a system used to form extrudate products from biomass material; Figure 2 is a schematic illustration of an alternative system used to form extrudate products from biomass material; Figure 3 is an exploded view of a part of an extruder assembly according to a preferred embodiment of the present invention; Figure 4 is a cross-sectional view along a longitudinal axis of the extruder assembly of Figure 3; Figure 5 is a side view of a tip element according to a preferred embodiment of the present invention; Figure 6 is an end view from the first end of the tip element of Figure 5; Figure 7 is an end view from the second end of the tip element of Figure 5; Figure 8 is a cross-section view along a longitudinal axis of the tip element of Figure 5; Figure 9 is a side view of the tip element of Figure 5 in a direction perpendicular to the side view of Figure 5; Figure 10 is a cross-sectional view along a longitudinal axis of a screw component of an extruder assembly according to a preferred embodiment of the present invention; Figure 11 is a side view of the screw component of Figure 10; Figure 12 is an end view from a second end of the screw component of Figure 11; Figure 13 is a side view of a nose element of an extruder assembly according to a preferred embodiment of the present invention; Figure 14 is an end view from a first, distal end of the nose element of Figure 13; Figure 15 is an end view from a second, proximal end of the nose element of Figure 13; Figure 16 is a cross-sectional view along a longitudinal axis of the nose element of Figure 13; Figure 17 is an end view of a guide bush of an extruder assembly according to a preferred embodiment of the present invention; Figure 18 is a side view of the guide bush of Figure 17; Figure 19 is a cross-sectional view along a longitudinal axis of the guide bush of Figure 17; Figure 20 is a cross-sectional view along a longitudinal axis of a die element of an extruder assembly according to a preferred embodiment of the present invention; Figure 21 is an end view from a first end of the die element of Figure 20; and Figure 22 is an end view from a second end of the die element of Figure 20.

DETAILED DESCRIPTION

Figure 1 is a schematic illustration of a preferred embodiment of an extrusion system 1 and process used to form a biomass extrudate or biomass briquettes for use as fuel. The extrusion process may be used with any suitable type of biomass material, for example, wood, rice husks, bagasse, and nut shells or husks e.g. coconut shells. The following description exemplifies the use of the process with wood material, however, it will be appreciated that the same method and system 1 may be used with any other suitable biomass material.

The raw biomass material may comprise material from a variety of sources. In the case of wood material, the raw material may, for example, be in the form of wood chips and sawdust from saw mills or timber yards, or sawdust that has previously used in stables or for other animal bedding. The raw biomass material typically comprises pieces or particles of material having a wide range of sizes, and will, in general, have a varying moisture content dependent on the source of the material, the time of year and the weather, amongst other things.

The raw material is initially received and stored in a suitable receptacle or area 2.

This receptacle or area 2 is preferably open to the surroundings so that there is a supply of air to the material which helps to dry the material.

In a first stage of the process raw biomass material is fed from the initial storage area 2 into a primary separation apparatus 4. The primary separation apparatus 4 removes the larger pieces of material from the raw biomass by means of a suitable screening apparatus. In some embodiments the screening apparatus may comprise a vibrating sieve screen or a rotary trommel screen. In other embodiments the screening apparatus comprises a star screen. Typically the primary separation apparatus 4 is configured to remove pieces of material having dimensions greater than about 75 mm. The primary separation apparatus may also comprise a magnet, for example a drum magnet, to remove metal contaminants, known as tramp metal.

The biomass material is then transferred from the primary separation apparatus 4 to a secondary separation apparatus 6. The secondary separation apparatus 6 is configured to remove heavy contaminants from the material, such as glass, metal fragments and stones, and small light contaminants, such as small pieces of plastic.

In some embodiments the material may be transferred from the primary separation apparatus 4 to the secondary separation apparatus 6 through a duct by means of a flow of air. This has the advantage that the flow of air over the material through the duct helps to remove some moisture from the material.

The second separation step preferably removes contaminants from the raw material by means of a fluidized bed separator. The fluidized bed separator comprises a vibrating table or platform and a source of air. The table is arranged at an angle such that a first end of the table is higher than an opposing second end. The particulate raw material is fed onto the table and a flow of air is passed up through the table to form a fluidized bed of material. The air flow is set such that the lighter material is raised off the surface of the table and the heavier contaminants remain on the surface of the table. The table is then vibrated so that the heavy contaminants are moved up to the first end of the table and separated from the lighter raw material.

The processed raw material is then transferred from the second separation apparatus 6 to a first storage container or silo 8. In some embodiments the processed material may be transferred from the secondary separation apparatus 6 to the silo 8 through a duct by means of a flow of air. This has the advantage that the flow of air over the material through the duct helps to remove additional moisture from the material.

Following the separation steps, the particle size of the processed raw material in the silo 8 is preferably less than 10 mm. The biomass material may additionally have been pulverized by the use of suitable pulverizing apparatus after the secondary separation step. The moisture content of the material in the silo is generally in the range of 15% to 60% by wet weight. More typically the moisture content of the material in the silo is in the range of 15% to 45% by wet weight.

In a next step in the process the biomass material is dried to the required moisture content for extrusion. In preferred embodiments the biomass material is dried by means of a flow of hot air; however, it will be appreciated that other suitable means may be used to dry the material.

In this embodiment the biomass material is transferred from the silo 8 to a rotary hot air drier 10. A stream of hot air enters the drier 10 at a first end 12 and exits the drier 10 at an opposite second end 14. The stream of hot air entering the drier is maintained at a constant temperature and a constant flow rate. Typically the temperature of the hot air entering the drier 10 will be between 400 °C and 600 °C.

Biomass material is fed continuously into the drier 10 proximate the first end 12 and is carried towards the second end 14 through the flow of hot air. The biomass material exits the drier 10 proximate the second end 14 and is then transferred to a second storage container or silo 16.

In a preferred embodiment the biomass material is fed into the drier 10, from the first silo 8, by means of a conveyance system that governs the rate at which the biomass material is conveyed from the silo 8 into the drier 10. The conveyance system may comprise a moving floor of the silo 8 and/or a conveyor screw..

A temperature sensor 18 is positioned to measure the temperature of the hot air as it exits the drier 10 at the second end 14. The output from the sensor 18 is used as an input to a control system 20 that controls the feed rate of biomass material into the drier 10 based on the measured air temperature. In particular, the control system 20 is configured to control the speed of operation of the conveyance system. Typically the operation of the conveyance system will be by means of a hydraulic drive and the control system 20 will be configured to vary the speed of the hydraulic drive. Accordingly, in some embodiments, the feed rate of material into the drier 10 is determined by the speed of rotation of a conveyor screw. In other embodiments the feed rate is determined by the speed of operation of a moving floor of the container or silo 8, which draws material to a conveyor screw rotating at a constant speed.

Biomass material entering the drier 10 having a relatively higher moisture content will cause the temperature of the hot air to decrease more, between the first and second ends 12, 14 of the drier 10, than biomass material entering the drier 10 having a relatively lower moisture content, as the air removes moisture from the material during movement through the drier 10. Consequently, as the air temperature measured by the sensor 18 decreases, the control system 20 sends a signal to the hydraulic drive to cause a decrease in the speed of operation of the conveyance system and, therefore, a decrease in the feed rate of material into the drier 10. As the measured air temperature increases, the control system 20 sends a signal to the hydraulic drive to cause an increase in the speed of operation of the conveyance system and, therefore, an increase in the feed rate of material into the drier 10. In this way, if the biomass material being fed into the drier 10 has a higher moisture content the feed rate is decreased so that a smaller volume of wet material is held within the drier 10 at any one time. If the biomass material has a lower moisture content, the feed rate can be increased as less moisture needs to be removed from the material as it passes through the drier 10.

The control system 20 is, preferably, configured to maintain the measured air temperature between pre-determined upper and lower threshold temperatures.

These threshold temperatures are set so as to control the moisture content of the biomass material exiting the drier 10 between well-defined limits. The moisture content of the material exiting the drier 10 is preferably maintained at between 2% and 10% of the wet weight, and more preferably between 4% and 8% of the wet weight of the material. The upper threshold temperature is typically between about 65 °C and 75 °C, and the lower threshold temperature is typically between about 55 °C and 65 °C.

If the moisture content of the biomass material is too high, generally greater than about 12% by wet weight, when the material is extruded, steam produced during the extrusion process will cause the extrudate to break apart and not consolidate consistently. If the moisture content of the wood is too low, generally less than about 4% by wet weight, then the extrudate will not form correctly and there is a fire risk due to the high temperatures experienced by the material during the extrusion process. Typically the biomass material will experience temperatures up to about 300 °C during extrusion.

The dried biomass material from the second silo 16 is fed continuously into an extruder 22. In some embodiments biomass material from the second silo 16 may be fed into two or more extruders 22a, 22b operating in parallel. The extruder or extruders 22 are screw extruders and produce a continuous length of extrudate from the biomass material.

At a next step in the process, the extrudate is cut into discrete lengths by means of suitable cutting apparatus 24. The cut lengths of the extrudate, or briquettes, are then packed for distribution to retail outlets and the like. The briquettes may be tied in a bundle, but preferably, the briquettes are wrapped in polythene or another suitable plastics material to keep the briquettes dry during subsequent storage.

After extrusion the extrudate is at a high temperature, typically around 80 °C to °C. The briquettes, therefore, need to be cooled before they can be packed.

A number of methods may be used to cool the briquettes, including the use of forced air to increase the rate of the cooling process. In some embodiments the briquettes may be transferred on a conveyor belt between a location of the cutting apparatus 24 and a location 26 at which the briquettes are packed. If the locations at which the briquettes are cut 24 and packed 26 are in one or more buildings, then it may be desirable if the briquettes are conveyed outside the building(s) between these two locations so that the surrounding air is used to cool the briquettes without requiring any extra energy input into the system.

In some embodiments of the extrusion system and process the secondary separation step may be subsequent to the drying step. In these embodiments of the extrusion system 101, illustrated in Figure 2, the raw biomass material is first passed from the storage area 2 into the primary separation apparatus 4, before being stored in the first storage container or silo 8. The biomass material is then dried in a drier 10 as described above before being passed through the secondary separation apparatus 6. Following this second separation step the biomass material is held in the second storage container or silo 16, before being extruded to form the briquettes. In these embodiments the biomass material may, additionally, be passed through a first mill or pulverizer 5, located between the primary separation apparatus 4 and the first storage container or silo 8, and/or a second mill or pulverizer 7, located between the secondary separation apparatus 6 and the second storage container or silo 16.

The details of an extruder assembly 28, forming part of the extruder 22, will now be described with reference to Figures 3 to 22, which illustrate a preferred embodiment of components of an extruder assembly 28 used in the extrusion process described above.

The extruder assembly 28 comprises a main screw component 30, a tip element 32, a nose element 34, a guide bush 36 and a die 38, as shown most clearly in Figures 3 and 4. In use, biomass material is forced through the extruder assembly 28 by means of the rotation of the screw component 30 and tip element 32, and the high temperatures and pressures causes consolidation of the material into an extrudate, the size and shape of which is determined by the dimensions of the die 38 that is used.

Figure 4 illustrates the components of the extruder assembly 28 in an assembled state. As shown, the tip element 32 is seated within a bore 40 of the guide bush 36 and the nose element 34 protrudes through the die 38. This arrangement causes the biomass material to be compressed into a generally cylindrical briquette having a central bore.

The individual components of the extruder assembly 28 will now be described with particular reference to Figures 5 to 22.

The main screw component 30, shown in Figures 10 to 12, comprises a generally elongate cylindrical root 42 and a helical flight 44. The cylindrical root 42 has opposing first and second ends 46, 48, and a longitudinal axis 50 of the root 42, that extends between the first and second ends 46, 48, defines a rotational axis of the screw component 30.

The first, proximal end 46 of the cylindrical root 42 is configured for connection to a means for rotating the screw component 30 about the longitudinal axis 50. Any suitable means may be used to rotate the screw component 30 as is known in the art. The second, distal end 48 of the screw component 30 is shaped such that a second end surface 52 has a stepped profile. In particular, a first part 54 of the second end surface 52 is further from the first end 46 of the screw component 30 than a second part 56 of the second end surface 52.

The second end 48 of the root 42 of the screw component 30 includes a cavity 58 that is centrally positioned and extends axially towards the first end 46. A threaded hole 60 extends axially from a base 62 of the cavity 58 towards the first end 46 of the root 42. The threaded hole 60 is also centrally positioned such that the threaded hole 60 and the cavity 58 are co-axial with each other and with the root 42.

The helical flight 44 extends radially outwards from around the root 42. The helical flight 44 extends from the second end 48 of the root 42 along the length of the root 42. In this embodiment the helical flight 44 comprises between six and seven turns. The helical flight 44 may not extend along the whole length of the root 42, however, and in some embodiments, and as shown in Figures 10 and 11 in particular, a stem portion 64 of the root 42 may extend between the first end 46 and an end 66 of the helical flight 44. The helical flight 44 does not extend around the stem portion 64 of the root 42 and the stem portion 64 therefore provides a region for connection to another part of the extruder 22. Connection between the means for rotation and the screw component 30 at the stem portion 64 is important, because if the screw component 30 is incorrectly machined or wrongly engaged in the extruder 22, then whole component is liable to break during operation.

The helical flight 44 is tapered such that an outer diameter of the helical flight 44 is larger at a first end 66 of the helical flight 44 than at a second end 68. The angle of taper is preferably between 3° and 8°, and more preferably about 5°. The pitch of the helical flight 44 is preferably between 25 mm and 35 mm, and more preferably between 28 mm and 32 mm. In this embodiment the pitch of the helical flight 44 is about 30 mm.

The screw component 30 is preferably a unitary element of one-piece construction. Typically the screw component 30 will be made from a suitable machine steel.

Figures 5 to 9 illustrate a preferred embodiment of the tip element 32 of the extruder assembly 28. The tip element 32 comprises a substantially cylindrical screw root 70 having first and second end faces 72, 74. A longitudinal axis 76 extending between the first and second end faces 72, 74 defines a rotational axis of the tip element 32. A central bore 78 extends axially through the screw root 70 between the first and second end faces 72, 74. The first end face 72 of the tip element 32 is substantially planar and the second end face 74 has a stepped profile, such that a first part 80 of the second end face 74 is further from the first end face 72 than a second part 82 of the second end face 74. In this way the second end face 74 is configured to locate adjacent to the second end 48 of the screw component 30. In particular, the stepped profile of the second end face 74 of the tip element 32 is complementary to the stepped profile of the second end surface 52 of the screw component 30.

The tip element 32 further comprises a helical flight 84 extending radially outwards around the screw root 70. The helical flight 84 extends along the length of the screw root 70 between a first end 86 proximate the first end face 72 of the root 70 and a second end 88 proximate the second end face 74 of the root 70. The length of the root 70, between the first and second end faces 72, 74, is such that the helical flight 84 turns through an angle of 720° or less between the first and second ends 86, 88, i.e. the tip element 32 includes two complete turns or less of the helical flight 84. In preferred embodiments the helical flight 84 turns through an angle of between 180° and 540° between the first and second ends 86, 88, and more preferably the helical flight 84 turns through an angle of between 3600 and 400° between the first and second ends 86, 88. In this embodiment, illustrated most clearly in Figure 9, the helical flight 84 extends through an angle of about 380°.

A width of the helical flight 84 is greater at the first end 86 than the second end 88.

In some embodiments the helical flight 84 increases in width linearly along the length of the flight 84 from the second end 88 to the first end 86. Importantly, the helical flight 84 of the tip element 32 increases in width over the last turn or half turn of the flight 84 at the first end 86, i.e. the flight 84 increases in width over a length of the flight 84 corresponding to a turn through an angle of between 360° and 180° from the first end 86. It is this final turn of the flight 84 at the first end 86 of the tip element 32 that has the greatest wear rate during the extrusion process and, accordingly, the increased width of the flight 84 in this region prolongs the operating life of the tip element 32.

In the embodiment illustrated in Figures 5 to 9, the width of the flight 84 at the first end 86 is approximately twice the width of the flight 84 at the second end 88. In preferred embodiments the width of the flight 84 at the first end 86 is between 13 mm and 20 mm. More preferably the width of the flight 84 at the first end 86 is between 15 mm and 17 mm.

The pitch of the helical flight 84 is preferably between 25 mm and 35 mm, and more preferably between 28 mm and 32 mm. In this embodiment the pitch of the helical flight is about 30 mm. It is important that the pitch of the helical flight 84 of the tip element 32 is substantially the same as the pitch of the helical flight 44 of the main screw component 30 to which it will be connected in use.

Additionally, in embodiments in which the tip element 32 comprises more than one complete turn of the helical flight 84, the helical flight 84 of the tip element 32 is tapered to match the taper of the helical flight 44 of the screw component 30 to which it will be connected. An angle of taper is, therefore, preferably between 3° and 8°, and more preferably about 5°.

The tip element 32 is made of a metal material and a hardness of the tip element 32 is substantially constant throughout the helical flight 84. This means that the wear rate of the tip element 32 is substantially constant throughout the depth of the flight 84, and an operating lifetime of the tip element 32 is more reliably predicted. In preferred embodiments the hardness of the metal material is substantially constant throughout the whole of the flight 84 and the root 70. The metal material that is used is preferably an alloy steel providing good hot hardness and wear resistance. In preferred embodiments the tip element 32 is made of a tool steel, and may be made of a high-speed tool steel. The tip element 32 is preferably made of an alloy steel though-hardened to a hardness level of between and 75 Rockwell, and more preferably about 70 Rockwell. In a particularly preferred embodiment the tip element is made from ASP®2060. The tip element 32 is preferably a unitary element of one-piece construction.

Figures 13 to 16 illustrate a preferred embodiment of the nose element 34 of the screw assembly 28. The nose element 34 comprises an elongate cylindrical stem portion 90 and an elongate tapered portion 92 having a substantially circular cross-sectional shape. A first end 94 of the stem portion 90 forms a first end 94 of the nose element 34 and the tapered portion 92 extends from a second end 96 of the stem portion 90. A diameter of the tapered portion 92 at the second end 96 of the stem portion 90 is greater than the diameter of the stem portion 90 such that an outwardly extending shoulder 98 is formed around the nose element 34. A distal end 100 of the tapered portion 92, furthest from the stem portion 90 forms a second end 100 of the nose element 34. In this embodiment a diameter of the second end 100 of the tapered portion 92 is smaller than the diameter of the stem portion 90.

A longitudinal axis 102 of the nose element 34 extends between the first and second ends 94, 100. In this embodiment a central bore 104 extends axially through the nose element 34 between the first and second ends 94, 100.

Figures 17 to 19 illustrate a preferred embodiment of the guide bush 36 of the extruder assembly 28. The guide bush 36 comprises a generally annular member 106 having an outer diameter defined by an outer surface 108 and an inner diameter defined by an inner surface 110. A longitudinal axis 112 of the guide bush 36 extends between a first end 114 and an opposing, second end 116. The inner diameter is larger at the first end 114 than the second end 116, such that the bore 40 of the guide bush 36 is tapered.

A plurality of ribs 118 protrude radially inwardly from the inner surface 110 of the guide bush 36. The ribs 118 extend longitudinally between the first and second ends 114, 116 of the guide bush 36. The ribs 118 are equally spaced around the inner surface 110 of the guide bush 36 and, in this embodiment, there are eight ribs 118. In preferred embodiments a depth of the ribs 118 in a radial direction is between 5mm and 10mm.

Figures 20 to 22 illustrate a preferred embodiment of the die 38 of the extruder assembly 28. The die 38 comprises a generally annular member 120 having an outer surface 122 and an inner surface 124. A longitudinal axis 125 of the die 38 extends between a first end 126 and an opposing, second end 128.

The inner surface 124 of a first part 130 of the die 38 proximate the first end 126 is circular and the inner surface 124 of a second part 132 of the die 38 proximate the second end 128 is octagonal. There is, therefore, a transition region 134 between the first and second parts 130, 132 in which the cross-sectional shape of a bore 136 of the die 38 changes from circular to octagonal. The dimensions of the inner surface 124 of the second part 132 of the die 38 are constant along the length of the die, parallel to the longitudinal axis 125. The diameter of the inner surface 124 of the first part 130 decreases in a direction from the first end 126 towards the transition region 134.

In this embodiment the inner surface 124 of the second part 132 is not a regular octagon but is in the shape of a truncated square. The shape of the inner surface 124 of the second part 132 of the die 38 determines the shape of the outer surface of the extrudate.

The extruder assembly 28 further comprises means (not shown) for securing the nose element 34 and the tip element 32 to the main screw component 30. In an assembled state, as shown in Figure 4, the tip element 32 is located between a part of the nose element 34 and the second end 52 of the screw component 30. In this embodiment the shoulder 98 of the nose element 34 contacts the first end face 72 of the tip element 32 and the second end face 74 of the tip element 32 contacts the second end surface 52 of the screw component 30. In particular, the first part 80 of the second end face 74 of the tip element 32 is in contact with the second part 56 of the second end surface 52 of the screw component 30, and the second part 82 of the second end face 74 of the tip element 32 is in contact with the first part 54 of the second end surface 52 of the screw component 30.

Furthermore, the stepped profile of the second end 74, 48 of each of the tip element 32 and screw component 30 means that the tip element 32 engages with the screw component 30 such that rotation of the screw component 30 causes rotation of the tip element 32 in the same direction due to an engagement surface 138 of the screw component 30 pressing against an engagement surface 140 of the tip element 32, shown most clearly in Figures 3, 4, 9 and 11.

It will be appreciated that in other embodiments the second end face 74 of the tip element 32 does not contact directly the second end surface 52 of the screw component 30. There may, for example, be a spacing element (not shown) between the tip element 32 and the screw component 30. However, in all embodiments the tip element 32 engages with the screw component 30 such that rotation of the screw component 30 causes rotation of the tip element 32.

In the assembled position the bore 78 of the tip element 32 is axially aligned with the cavity 58 of the screw component 30. Preferably the diameter of the cavity 58 is the same as the diameter of the bore 78. Additionally, the helical flight 84 of the tip element 32 is aligned with the helical flight 44 of the screw component 30. In particular the second end 88 of the helical flight 84 of the tip element 32 is aligned with the second end 68 of the helical flight 44 of the screw component 30.

The stem portion 90 of the nose element 34 extends through the bore 78 of the tip element 32 and into the cavity 58. In this way, the screw component 30, the tip element 32 and the nose element 34 are all co-axial.

In this embodiment the extruder assembly 28 includes securing means (not shown) comprising an elongate shaft sized to be received through the bore 104 of the nose element 34. The shaft preferably has, at a first end, a head configured to engage with a part of the nose element and, at an opposite second end, a threaded region configured to engage with the threaded hole 60 in the screw component 30. In preferred embodiments the securing means is a bolt having a head sized to contact the second end 100 of the nose portion 34 around the bore 104.

The direction of rotation of the securing means to secure the threaded region of the shaft into the threaded hole 60 is, preferably, the same as the direction of rotation of the screw component in use. In this way, rotation of the screw component during extrusion will always tend to tighten the connection between the screw component 30 and the nose element 34.

In other embodiments the nose element 34 and the screw component 30 may include complementary connecting means such that the nose element 34 can be connected directly to the screw component 30. For example, the cavity 58 of the screw component 30 and a part of the stem portion 90 of the nose element 34 may include complementary screw threads such that the nose element 34 is screwed directly into the cavity 58 of the screw component 30 and additional securing means are not required.

In an assembled state, the first end face 72 of the tip element 32 is preferably aligned with the second end 116 of the guide bush 36. The first end 126 of the die 38 is in contact with or is position adjacent to the second end 116 of the guide bush 36. Accordingly, the diameter of the bore 136 of the die 38 at the first end 126 is substantially the same as the diameter of the bore 40 of the guide bush 36 atitssecond end 116.

The present invention, therefore, provides an improved extrusion apparatus that overcomes disadvantages of known systems leading to a commercially viable process.

Claims

CLAIM S1. A tip element for attachment to an end of an extruder screw, the tip element comprising: -a substantially cylindrical screw root having first and second end faces, a longitudinal axis extending between the first and second end faces defining a rotational axis of the tip element, and the second end face being configured to locate, in use, adjacent to an end of said extruder screw; and -a helical flight extending radially outwards from the screw root, the helical flight turning through an angle of 7200 or less between first and second ends, said first flight end being proximate the first end face of the screw root and said second flight end being proximate the second end face of the screw root, wherein a width of the flight is greater at the first end than the second end, and wherein the tip element is made of a metal material and a hardness of the tip element is substantially constant throughout the helical flight.
2. A tip element as claimed in Claim 1, wherein the hardness of the tip element is substantially constant throughout the screw root and the helical flight.
3. A tip element as claimed in Claim 1 or Claim 2, further comprising an axial bore extending through the screw root between said end faces.
4. A tip element as claimed in any preceding claim, wherein the width of the flight at the first end is twice the width of the flight at the second end.
5. A tip element as claimed in any preceding claim, wherein the width of the flight at the first end is between 13 mm and 20 mm.
6. A tip element as claimed in any preceding claim, wherein the width of the flight at the first end is between 15 mm and 17 mm.
7. A tip element as claimed in any preceding claim, wherein the width of the flight increases linearly from the second end to the first end.
8. A tip element as claimed in any preceding claim, wherein the tip element is made from a tool steel material.
9. A tip element as claimed in Claim 8, wherein the tip element is made from an alloy steel having a hardness of between 65 and 75 Rockwell.
10. A tip element as claimed in any preceding claim, wherein the helical flight turns through an angle of between 1800 and 540° between first and second ends.
11. A tip element as claimed in any preceding claim, wherein the helical flight turns through an angle of between 3600 and 400° between the first and second ends.
12. A tip element as claimed in any preceding claim, wherein an outer diameter of the root is between 40 mm and 50 mm.
13. A tip element as claimed in any preceding claim, wherein the pitch of the helical flight is between 25 mm and 35 mm.
14. A tip element as claimed in any preceding claim, wherein the second end face of the screw root is stepped, such that a first part of the second end face is further from the first end face than a second part of the second end face.
15. A tip element as claimed in any preceding claim, wherein a maximum axial length of the tip element, between the first and second end faces is between mm and 50 mm.
16. An extruder screw assembly comprising: -a main screw component comprising an elongate cylindrical root and a helical flight, the cylindrical root having opposing first and second ends and a longitudinal axis extending between the first and second ends defining a rotational axis of the screw component, a first end of said cylindrical root configured for connection to a means for rotating the screw component about said axis; -a tip element as claimed in any one of Claims 1 to 15, the second end face of the tip element being configured to locate adjacent to the second end of the main screw component; -a nose element including an elongate tapered portion; and -means for securing the nose element and the tip element to the main screw component such that the tip element is located between a part of the nose element and the second end of the screw component and the helical flight of the tip element is aligned with the helical flight of the screw component.
17. An extruder screw assembly as claimed in Claim 16, wherein the second end of the screw component includes a cavity, the tip element includes an axial bore extending through the screw root and the nose element includes an elongate stem portion, and wherein the stem portion of the nose element extends through the axial bore and an end of the stem portion is received in the cavity.
18. An extruder screw assembly as claimed in Claim 17, wherein the nose element includes an axial bore extending through the tapered portion and the stem portion, the screw component includes a threaded hole extending axially from a base of the cavity, and the assembly including securing means comprising an elongate shaft sized to be received within the axial bore and having, at a first end of the shaft, a head configured to engage with a part of the nose element and, at an opposite second end of the shaft, a threaded region configured to engage with said threaded hole.
19. An extruder screw assembly as claimed in any one of Claims 16 to 18, wherein the second end of the main screw component and the second end face of the tip element each have a stepped profile.
20. An extruder screw assembly as claimed in any one of Claims 16 to 19, wherein the tip element is engaged with the main screw component such that rotation of the main screw component causes rotation of the tip element.
21. A tip element substantially as herein described, with reference to or as shown in Figures 5 to 9.
22. An extruder screw assembly substantially as herein described, with reference to or as shown in Figures 3 and 4.