IE67548B1

IE67548B1 - A pigment mixing method and apparatus

Info

Publication number: IE67548B1
Application number: IE417390A
Authority: IE
Inventors: Tomothy Agustus O'brien; Richard Packenham Lynch; Bryan Featherstone Lynch; David Seward Lynch; Matthew Gerard Collins
Original assignee: Wildes Holding Limited
Priority date: 1990-11-20
Filing date: 1990-11-20
Publication date: 1996-04-17
Also published as: GB2249987B; GB9025650D0; GB2249987A; BE1002627A7; IE904173A1

Abstract

A method for mixing pigment material in powder or granular form with a plastics material in similar form for extrusion or injection moulding comprises delivering the plastics material downwardly through a vertical throat (34) formed by a duct (28) into a barrel (1) of extrusion or injection moulding apparatus. The pigment material is delivered downwardly into the flow of plastics material through a nozzle (38) located axially centrally in the throat (34). The nozzle (38) is positioned just above a pair of mixing screws (10) located in the barrel (1) and the plastics material and pigment material are delivered into the barrel (1) for subsequent mixing, melting and compression for delivery through a nozzle (4) in flowable particulate form.

Description

A pigment mixing method and apparatus The present invention relates to a method and apparatus for mixing a pigment material with a plastics material prior to delivery of the mixed pigment and plastics material from a mixing and heating barrel of plastics processing apparatus, wherein the pigment material and plastics material are in a non-liquid form having fluid-like flow characteristics. The invention also relates to delivery apparatus for delivering the pigment material and plastics material into the mixing and heating barrel. Further, the invention relates to a plastics processing apparatus comprising the delivery apparatus for carrying out the method. The invention also relates to an injection moulded article and an extruded article formed from a mixture of pigment material and plastics material having been mixed according to the invention.

In the production of articles of coloured plastics material whether produced by injection moulding or extrusion, it is normal to mix a pigment material of the appropriate colour with an unpigmented plastics material, and in certain cases with a partly coloured plastics material. Generally, where the pigment material and plastics material are in non-liquid form, they are in particulate or granular form. Such plastics material may be, for example, polypropylene, polyethylene, polystyrene, PVC and the like. In general, the pigment material is mixed with the Λ plastics material prior to delivery to a mixing and heating barrel of the extrusion or injection moulding apparatus. Cyclonic mixers are typically used.

Cyclonic mixers may be mounted in a remote location from the extrusion or injection moulding apparatus, and the mixed pigment and plastics materials may be transported or conveyed by a suitable conveyor means to the extrusion or injection moulding apparatus.

Alternately, a cyclonic mixer may be mounted above a hopper feeding the plastic material into the mixing and heating barrel of the extrusion or injection moulding apparatus and the mixed plastics material and pigment material is delivered from the cyclonic mixer directly into the hopper from where it is in turn delivered into the mixing and heating barrel of the extrusion apparatus or injection moulding apparatus.

Such mixing methods and apparatus for mixing pigment material with a plastics material tend to have a number of disadvantages. Firstly, they are relatively > inflexible in that it is not easy to change from one colour to another. This is a particular disadvantage where one wishes to run off short runs of products each of a different colour. To change from one colour to another, it is essential to clean out the entire mixing apparatus and conveying apparatus for conveying the mixed plastics and pigment material from the mixing apparatus to the extrusion or injection moulding apparatus. The cleaning is essential to ensure that the mixing and conveying apparatus is free of material of the previous colour. Further, where a drift in colour is discovered in a product being produced, a considerable number of products which, in general, are rejects have to be run through the apparatus before the colour drift can be corrected. A further disadvantage of these known mixing methods and apparatus is that the apparatus tends to be relatively costly, and where the mixing apparatus is mounted in a remote location, additional costs are incurred in either transporting or conveying the mixed plastics and pigment materials to the extrusion or injection moulding apparatus. Additionally, where mixing takes place in a remote location, the problem with colour drift is even greater in that a greater quantity of product of the incorrect colour must be run off to waste before the colour drift can be corrected.

There is therefore a need for a method and apparatus for mixing pigment material with plastics material which overcomes these problems. There is also a need for plastics processing apparatus, for example, an extrusion apparatus or an injection moulding apparatus comprising the mixing apparatus which likewise overcomes the problems of known apparatus. Further, Λ there is a need for an article of plastics material to be produced using the method and apparatus.

The invention is directed towards providing such a method and apparatus, and a plastic processing apparatus and a plastics article produced according to the method and apparatus.

According to the invention, there is provided a method for mixing pigment material with a plastics material prior to delivery of the mixed pigment and plastics material from a mixing and heating barrel of plastics processing apparatus, wherein the mixing and heating barrel defines a hollow interior region within which mixing of the pigment and plastics material takes place, and wherein the pigment material and plastics material are in a non-liquid form and have fluid-like flow characteristics, the method comprising the steps of delivering the plastics material in a generally downwardly directed flow through a throat into the > hollow interior region of the barrel, delivering the pigment material in a generally downward direction into the generally downwardly directed flow of plastics material at a position substantially centrally located relative to the transverse cross sectional area of the downwardly directed flow of plastics material, and mixing the pigment material and plastics material in the hollow interior region of the barrel.

Preferably, the pigment material is delivered into the downward flow of plastics material just prior to the plastics material entering the hollow interior region of the barrel.

In one embodiment of the invention, a mixing screw is rotatably mounted in the hollow interior region of the barrel and the pigment material is delivered into the downward flow of plastics material vertically above the mixing screw.

In another embodiment of the invention, a pair of parallel mixing screws are rotatably mounted about respective rotational axes in the hollow interior region of the barrel, the pigment material being delivered into the downward flow of plastics material intermediate vertical plane containing the rotational axes of the mixing screws.

Advantageously, the plastics material flows through the throat with full flow characteristics.

Additionally, the invention provides delivery apparatus for delivering pigment material into a downward flow of plastics material for carrying out the method according to the invention, the delivery apparatus being for mounting on a mixing and heating barrel of plastics processing apparatus, wherein the mixing and heating barrel defines a hollow interior region forming a mixing compartment for mixing the pigment material and plastics material, the hollow interior region being formed by a pair of co-operating bores, and a pair of co-operating mixing screws being rotatable about respective parallel axes in the respective bores for mixing plastics material and pigment material and for pressurising the mixed pigmented material in the hollow interior region, and a barrel inlet being provided in the barrel to the hollow interior region, the delivery apparatus comprising a throat for mounting on the mixing and heating barrel, the throat terminating in a barrel engaging end for engaging the barrel and communicating with the hollow interior region through the barrel inlet, the throat extending in a generally upwardly direction from the barrel engaging end for delivering the plastics material into the hollow > interior region in a generally downwardly direction, and a nozzle means for delivering the pigment material into the downward flow of plastics material, the nozzle means having a nozzle outlet located substantially centrally relative to the transverse cross sectional area of the throat, and being directed in a generally downward direction for directing the pigment material downwardly into the downward flow of plastics material, and the nozzle outlet, in use, being disposed intermediate a pair of vertical planes containing the rotational axes of the mixing screws, and being disposed at a vertical height not greater than 25 mm to 30 mm above the upper surface of the bores forming the hollow interior region.

Preferably, the nozzle outlet terminates adjacent the barrel engaging end of the throat.

The invention also provides a plastics processing apparatus comprising a mixing and heating barrel defining a hollow interior region for mixing pigment material and plastics material, the hollow interior region being formed by a pair of co-operating bores, and a pair of co-operating mixing screws being rotatable about respective parallel axes in the respective bores for mixing plastics material and pigment material and for pressurising the mixed pigmented material in the hollow interior region, and a barrel inlet communicating with the hollow interior region, and delivery apparatus according to the invention wherein the throat is mounted on the barrel and communicates with the hollow interior region through the barrel inlet, the nozzle outlet being located intermediate a pair of vertical planes containing the rotational axes of the mixing screws and being disposed at a vertical height not greater than 25 mm to 30 mm above the upper surface of the bores forming the hollow interior region.

Further, the invention provides an extruded article of coloured plastics material, the extruded article being extruded from a mixture of plastics material and pigment material having been mixed according to the method according to the invention.

Additionally, the invention provides an extruded article of coloured plastics material extruded from a mixture of plastics material and pigment material and having been extruded from the apparatus according to the invention.

Additionally, the invention provides an injection moulded article of coloured plastics material, the injection moulded article being moulded from a mixture v of plastics material and pigment material having been mixed according to the method according to the invention.

Furthermore, the invention provides an injection moulded article of coloured plastics material moulded in the plastics processing apparatus according to the invention.

The invention will be more clearly understood from the following description of a preferred embodiment thereof given by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of an extrusion apparatus according to the invention comprising delivery apparatus also according to the invention for mixing pigment material and plastics material, Fig. 2 is a partly cut away perspective view of the extrusion apparatus of Fig. 1, Fig. 3 is a sectional elevational view of the portion of the extrusion apparatus of Fig. 1, Fig. 5 is a sectional end view of the extrusion apparatus of Fig. 1 on the line V-V of Fig. 3.

Fig. 4 is a sectional plan view of the portion of the extrusion apparatus of Fig. 1 on the line IVIV of Fig. 3, and Referring to the drawings there is illustrated a mixing and heating barrel according to the invention indicated generally by the reference numeral 1 for mixing pigment material and plastics material and for subsequently melting the plastics material. The mixing and heating barrel is of the type suitable for mounting in plastics processing apparatus, typically an extrusion or injection moulding apparatus (neither of which are shown) for respectively extruding or injection moulding components of plastics material. Extrusion and injection moulding apparatus will be well known to those skilled in the art, and it is assumed that a description of such apparatus is not necessary. The barrel 1 defines a hollow interior region 2 within which the pigment material and plastics material are mixed as will be described in detail below. The barrel terminates in a nozzle 4 having an outlet bore 5 from the interior region 2 through which the melted pigmented material is delivered. Where the barrel 1 is mounted in an extrusion apparatus, the outlet bore 5 of the nozzle 4 forms the cross section of the article (not shown) to be extruded, and the extruded article of plastics material is extruded from the nozzle 4. Where » the barrel 1 is mounted in an injection moulding apparatus, the nozzle 4 delivers the melted mixed pigmented plastics material into an injection mould within which the component or components of plastics material are formed. Such extrusion and injection moulding processes will be well known to those skilled in the art. Plastics material and pigment material is delivered into the interior region 2 of the barrel 1 through an inlet 6 which communicates through the barrel 1 with the interior region 2. Delivery apparatus also according to the invention indicated generally by the reference numeral 8 mounted on the barrel 1 delivers plastics material and pigment material through the inlet 6 into the interior region 2 and facilitates mixing of the plastics material and pigment material in the barrel 1. This is described in more detail below.

In this embodiment of the invention, the barrel 1 and delivery apparatus 8 are particularly suitable for use with plastics materials and pigment materials which are in non-liquid form having fluid like flow characteristics. Typically, such plastics materials and pigment materials may be in particulate form, for example, in powder form, or they may be in granular form. Indeed, in certain cases, the plastics material and pigment material may be formed in relatively large lumps, provided they both have fluid like flow characteristics. Further, the plastics material may be in one form while the pigment material may be in an alternative form.

Returning to the barrel 1 the hollow interior region 2 is formed by a pair of co-operating bores 9 which extend longitudinally of the barrel 1. A pair of cooperating mixing screws 10 are rotatable about parallel axes 12 in the bores 9 for mixing plastics material and pigment material and for pressurizing the mixed pigmented material for delivery through the nozzle 4. Shafts 14 extending from the mixing screws 10 are rotatably carried in bearings (not shown) in an end housing 16 extending from the barrel 1. An electric motor 18 drives the shafts 14 through a gear box 19 which is mounted on the end housing 16. The shafts 14 and mixing screws 10 are driven to contra-rotate relative to each other for mixing and urge the plastics material and pigment material in the direction of the arrow A from an upstream end 20 to a downstream end 21 of the interior region 2. The interior region 2 of the barrel 1 forms three zones, namely, an upstream feeding zone 24 towards the upstream end 20, a downstream compression zone 25 at the downstream end 21 and a mixing zone 26 intermediate the feeding and compression zones 24 and 25 respectively.

Plastics material and pigment material is delivered into the feeding zone 24 through the inlet 6 and the plastic and pigment material is delivered from the feeding zone 24 through the mixing sone 25 by the screws 10 and in turn into and through the compression zone 25 where the mixed pigmented plastics material is pressurized for delivery through the nozzle 4.

Although not illustrated, the pitch and depth of the flights of the screws 10 in the compression zone 25 reduces in a downstream direction for pressurizing the mixed pigmented plastics material. This will be well known to those skilled in the art.

Heater bands (not shown) are provided circumferentially around the barrel 1 for heating and melting the mixed pigmented plastics material in the interior region 2. The heater bands are arranged on the barrel 1 so that the temperature profile of the plastics material in the interior region 2 increases in a downstream direction from the upstream end 20 to the downstream end 21. The temperature profile is set so that melting of the plastics material and pigment material takes place towards the end of the mixing zone. Such heater bands and their arrangement on the barrel 1 will be well known to those skilled in the art.

The delivery apparatus 8 comprises a duct 28 which extends from a hopper 29 within which the plastics material is stored. The duct 28 terminates in a barrel engaging end 30 which engages a recess 31 in the barrel 1 extending around the inlet 6. The duct 28 forms a throat 34 of circular transverse cross section through which the plastics material is delivered from the hopper 29 through the inlet 6 into the feeding zone 24 of the hollow interior region 2. The duct 28 extends in a generally upward direction from the barrel 1 so that the plastics material flowing through the throat 34 of the duct 28 flows in a generally downward direction towards the inlet 6. In practice, the plastics material flows through the throat 34 with full flow characteristics.

A hopper 36 stores pigment material for delivery through a nozzle means, namely, a nozzle 38 into the throat 34 of the duct 28 for mixing with the plastics material flowing through the throat 34. Metering means comprising an auger screw conveyor 39 meters the pigment material from the hopper 36, and in turn delivers the pigment material through a pipe 40 to the nozzle 38. The auger conveyor 39 is powered by a variable speed electric motor 41, the speed of which is controlled for controlling the rate of delivery of pigment material through the nozzle 38. An opening 42 in the duct 28 accommodates the pipe 40 into the throat » 34. The periphery of the pipe 40 adjacent the opening is seam welded to the duct 28 to seal the opening The nozzle 38 defines a nozzle outlet 43 of circular cross section through which the pigment material is delivered into the flow of plastics material through the throat 34. The nozzle 38 is centrally located in the throat 34 relative to the transverse cross section of the throat 34 so that the nozzle outlet 43 is coaxial with the axis of the throat 34. The nozzle outlet 43 is downwardly directed, and accordingly, pigment material delivered into the throat 34 of the duct 28 through the nozzle outlet 43 is delivered axially downwardly in the direction of the downward flow of the plastics material and centrally of the flow of plastics material relative to the cross sectional area of the throat 34. In this embodiment of the invention, the inlet 6 is of circular cross sectional area and the throat 34 of the duct 28 is co-axial with the inlet 6. The inlet 6 and throat 34 are centrally located intermediate a pair of planes 44 which contain the axes 12 of the mixing screws 10. Accordingly, the nozzle outlet 43 being co-axial with the throat 34 is also centrally located between the planes 44. Thus, the pigment material is delivered into the plastics material at a position substantially halfway between the vertical planes 44 containing the axes 12 of the mixing screws 10.

The nozzle outlet 43 terminates in the throat 34 of the duct 28 at a position just above the barrel engaging end 30, see Figs. 3 and 5, and accordingly, above an upper portion 45 of the surface which defines the interior region 2 adjacent the inlet 6. In this embodiment of the invention, the vertical height a of the nozzle outlet 43 above the upper surface 45 is approximately 25 mm to 30 mm. The diameter of the throat 34 adjacent the nozzle outlet 43 is 120 mm, while the diameter of the nozzle outlet 43 is 25 mm.

Suitable support frameworks for supporting the hoppers 29 and 36 above the barrel 1 are provided. However, for clarity, these support frameworks are not illustrated.

In use, plastics material in powder form is delivered into the hopper 29. The plastics material may be unpigmented plastics material or pigmented plastics material but pigmented to a different colour than that required, or to a paler shade of the colour than that required. The pigment material in powder form is delivered into the hopper 36. In practice, where unpigmented plastics material in powder form is used, the particle size of the powder, in general, is of the order of 250 microns. The particle size for the pigment material powder is 1,200 microns. With the barrel 1 raised to its working temperature by the heater bands and the mixing screws 10 rotating in contra-rotation, the plastics material is allowed to flow downwardly through the throat 34 of the duct 28 with full flow characteristics. The auger screw conveyor 39 is set to deliver the pigment material at the desired metered rate through the nozzle outlet 43 into the centre of the flow of plastics material. On the plastics material and pigment material entering the feeding zone 24 through the inlet 6, the pigment material is dispersed throughout the plastics material by the mixing screws 10. As the plastics material and pigment material are delivered by the mixing screw 10 through the feeding zone 24 into the mixing zone 25, further mixing and dispersion of the pigment material through the plastics material is achieved. In fact, it has been found that as the plastics material with the pigment material dispersed therethrough approaches the end of the mixing zone 24, relatively thorough mixing and dispersion of the pigment material throughout the plastics material is achieved. As the plastics material and pigment material commence to melt towards the end of the mixing zone 24, further mixing of the pigment material and plastic material takes place in liquid form. The mixed pigmented plastics material is then subjected to the extrusion pressure in the compression zone 25 for extrusion through the outlet bore 5 of the nozzle 4. At this stage, the plastics material may be extruded if the nozzle 4 is an extrusion nozzle, or alternatively, delivered into an injection mould mounted in injection moulding apparatus .

It has been found that by virtue of the fact that the pigment material is delivered substantially centrally into the flow of plastics material, and furthermore, because the pigment material is delivered into the plastics material in the direction of flow of the plastics material that a considerably more even dispersion of the pigment material through the plastics material is achieved in the feeding and mixing zones 24 and 26 than has been achieved heretofore. Precisely why this is achieved is not fully understood. It is believed that feeding the pigment material centrally into the flow of plastics material in the direction of flow of the plastics material considerably contributes to the improved mixing results. By virtue of the fact that the pigment material is centrally delivered into the flow of plastics material, relatively even dispersion of the pigment material through the plastics material in the feeding and mixing zones 24 and 26 is achieved.

Accordingly, the method and apparatus for mixing the pigment material and plastics material has considerable advantages over mixing methods and apparatus known heretofore. By virtue of the fact that initial mixing takes place as the pigment material is being delivered into the flow of plastics material sufficient to ensure relatively even distribution of the pigment material through the plastics material during the subsequent mixing in the feeding and mixing zones 24 and 26 of the barrel 1, there is no need for premixing of the pigment material and plastics material in, for example, a cyclonic mixer or the like, which has been the practice known heretofore. This considerably reduces the associated costs of mixing pigment and plastics material. Furthermore, a relatively flexible apparatus which can readily easily be changed over from one colour to another or from one colour shade to another colour shade is provided. Additionally, should a colour drift be detected in the product being produced, this can readily easily be corrected with a minimum of product being lost. In general, such corrections are achieved by altering the flow rate of the pigment material by adjusting the feed rate of the auger screw conveyor 39.

It has been found that the barrel 1 and delivery apparatus 8 are particularly suitable for use with extrusion apparatus for extruding pipes, ducts and the like. For example, the extrusion apparatus may be used for producing corrugated or smooth wall land drainage pipes. Further, the extrusion apparatus may be used for extruding waste pipes, feed water pipes and ducting and conduit of the type which would normally be used in buildings for carrying cables and the like. Such pipes, ducting and conduit will be well known to those skilled in the art.

Needless to say, the barrel and delivery apparatus may also be used in an injection moulding apparatus for injection moulding components or the like.

While the delivery apparatus has been described as comprising a duct forming a throat for delivering plastics material with full flow characteristics of diameter of 120 mm, it is believed that good results would be achieved provided the throat is of diameter within the range 50 mm to 300 mm. Better results would be achieved with a throat diameter in the range of 90 mm to 150 mm. Furthermore, while the diameter of the nozzle outlet for delivering the pigment material into the throat has been described as being of 25 mm, it is believed that good results would be achieved provided the nozzle outlet is of diameter in the range of 12.5 mm to 150 mm, and better results would be achieved with a nozzle outlet of diameter in the range of 20 mm to 30 mm. Needless to say, it will of course be appreciated that while the nozzle outlet and throat have been described as being of circular transverse cross sectional area the transverse cross sectional area of the throat and nozzle outlet may be of any other suitable or desired cross sectional area.

While the plastics material has been described as being in powder form of 250 microns particle size and the pigment material has been described as being in powder form of 1,200 particle size, plastics material and pigment material of any other particle sizes may be used without departing from the scope of the invention. Needless to say, as mentioned above, the plastics material and pigment material may be in any other form besides powder form, for example, the plastics and pigment materials may be in granular form, lump form or the like provided they retain fluid-like flow characteristics .

Where the barrel and delivery apparatus is mounted in extruding apparatus for extruding pipes, ducts and the like, it is envisaged that the plastics material will be PVC, and in certain cases may be polypropylene, polyethylene, ABS material or any other plastics material.

While the delivery apparatus has been described with the nozzle outlet opening being at a vertical height a of approximately 25 mm to 30 mm above the upper surface 45 of the hollow interior region 2, it is believed that optimum results are achieved when the nozzle outlet opening is substantially adjacent the barrel engaging end of the throat.

While a particular construction of nozzle has been described, any other suitable construction of nozzle means may be provided without departing from the scope of the invention.

While the metering means for metering the flow rate of pigment material into the plastics material has been described as being an auger screw conveyor, any other suitable metering means may be provided without departing from the scope of the invention. Indeed, in certain cases, it is envisaged that a butterfly valve may be used.

Claims

1. A method for mixing pigment material with a plastics material prior to delivery of the mixed pigment and plastics material from a mixing and heating 5 barrel of plastics processing apparatus, wherein the mixing and heating barrel defines a hollow interior region within which mixing of the pigment and plastics material takes place, and wherein the pigment material and plastics material are in a non-liquid form and have 10 fluid-like flow characteristics, the method comprising the steps of: delivering the plastics material in a generally downwardly directed flow through a throat into the hollow interior region of the barrel, 15 delivering the pigment material in a generally downward direction into the generally downwardly directed flow of plastics material at a position substantially centrally located relative to the transverse cross sectional area of the downwardly 20 directed flow of plastics material, and mixing the pigment material and plastics material in the hollow interior region of the barrel.

2. A method as claimed in Claim 1 in which the pigment material is delivered into the downward flow of 25 plastics material just prior to the plastics material entering the hollow interior region of the barrel.

3. A method as claimed in Claim 1 or 2 in which a mixing screw is rotatably mounted in the hollow interior region of the barrel and the pigment material is delivered into the downward flow of plastics material vertically above the mixing screw.

4. A method as claimed in any preceding claim in which a pair of parallel mixing screws are rotatably mounted about respective rotational axes in the hollow interior region of the barrel, the pigment material being delivered into the downward flow of plastics material intermediate vertical plane containing the rotational axes of the mixing screws.

5. A method as claimed in any preceding claim in which the plastics material flows through the throat with full flow characteristics.

6. A method as claimed in any preceding claim in which the throat is of circular inner transverse cross section.

7. A method as claimed in any preceding claim in which the pigment material is delivered axially into the downward flow of plastics material at a location coaxial with the downward flow of the plastics material.

8. A method as claimed in any preceding claim in which the pigment material is delivered into the downward flow of plastics material through a nozzle means, the nozzle means defining an outlet axially directed relative to the throat and co-axially therewith.

9. A method as claimed in any preceding claim in which the rate of delivery of the pigment material into the plastics material is metered.

10. A method as claimed in Claim 9 in which the pigment material is metered by an auger screws.

11. A method as claimed in any preceding claim in which the hollow interior region of the mixing and heating barrel defines an upstream feeding zone and a downstream compression zone, and a mixing zone intermediate the feeding zone and compression zone, and the method includes the step of delivering the plastics material into the hollow interior region at the feeding zone.

12. A method as claimed in any preceding claim in which the transverse cross sectional diameter the downward flow of plastics material at the position where the pigment material is delivered is in the range of 50 mm to 300 mm.

13. A method as claimed in any preceding claim in which the transverse cross sectional diameter of the downward flow of plastics material at the position where the pigment material is delivered is in the range 5 of 90 mm to 150 mm.

14. A method as claimed in any preceding claim in which the transverse cross sectional diameter of the downward flow of plastics material at the position where the pigment material is approximately 120 mm. 10

15. A method as claimed in any preceding claim in which the transverse cross sectional diameter of the flow of pigment material at the position where the pigment material is delivered is in the range of 12.5 mm to 50 mm.

16. A method as claimed in any preceding claim in which the transverse cross sectional diameter of the flow of pigment material at the position where the pigment material is delivered is in the range of 20 mm to 30 mm. 20

17. A method as claimed in any preceding claim in which the transverse cross sectional diameter of the flow of pigment material at the position where the pigment material is delivered is approximately 25 mm.

18. A method as claimed in any of Claims 8 to 17 when dependent on Claims 2 and 8 in which the hollow interior region defines an elongated bore for accommodating the mixing screw, and the nozzle outlet is positioned at a height of not more than 25 mm to 30 mm above the inner surface of the screw bore defining an inlet to the bore.

19. A method as claimed in any preceding claim in which the pigment material is in powder form of particle size of approximately 1,200 microns.

20. A method as claimed in any preceding claim in which the plastics material is in powder form of particle size of approximately 250 microns.

21. A method as claimed in any preceding claim in which the pigment material is in granular form.

22. A method as claimed in any preceding claim in which the plastics material is in granular form.

23. A method substantially as described with reference to, and as illustrated in the accompanying drawings.

24. Delivery apparatus for delivering pigment material into a downward flow of plastics material for carrying out the method of any of Claims 1 to 23, the delivery apparatus being for mounting on a mixing and heating barrel of plastics processing apparatus, wherein the mixing and heating barrel defines a hollow interior region forming a mixing compartment for mixing the pigment material and plastics material, the hollow interior region being formed by a pair of co-operating bores, and a pair of co-operating mixing screws being rotatable about respective parallel axes in the respective bores for mixing plastics material and pigment material and for pressurising the mixed pigmented material in the hollow interior region, and a barrel inlet being provided in the barrel to the hollow interior region, the delivery apparatus comprising a throat for mounting on the mixing and heating barrel, the throat terminating in a barrel engaging end for engaging the barrel and communicating with the hollow interior region through the barrel inlet, the throat extending in a generally upwardly direction from the barrel engaging end for delivering the plastics material into the hollow interior region in a generally downwardly direction, and a nozzle means for delivering the pigment material into the downward flow of plastics material, the nozzle means having a nozzle outlet located substantially centrally relative to the transverse cross sectional area of the throat, and being directed in a generally downward direction for directing the pigment material downwardly into the downward flow of plastics material, and the nozzle outlet, in use, being disposed intermediate a pair of vertical planes containing the rotational axes of the 5 mixing screws, and being disposed at a vertical height not greater than 25 mm to 30 mm above the upper surface of the bores forming the hollow interior region.

25. Delivery apparatus as claimed in Claim 24 in which the nozzle outlet terminates adjacent the barrel 10 engaging end of the throat.

26. Delivery apparatus as claimed in Claim 24 or 25 in which the nozzle outlet, in use, is disposed substantially halfway between the said vertical planes.

27. Delivery apparatus as claimed in any of Claims 24 15 to 26 in which the throat is of circular inner transverse cross section.

28. Delivery apparatus as claimed in any of Claims 24 to 27 in which the nozzle outlet is axially disposed relative to the throat and co-axial therewith. 20

29. Delivery apparatus as claimed in any of Claims 24 to 28 in which a feed pipe extends from the nozzle through a side wall of the throat for delivering pigment material to the nozzle outlet.

30. Delivery apparatus as claimed in any of Claims 24 to 29 in which the inner transverse cross section of the throat adjacent the nozzle outlet is of diameter in 5 the range of 50 mm to 300 mm.

31. Delivery apparatus as claimed in any of Claims 24 to 30 in which the inner transverse cross section of the throat adjacent the nozzle outlet is of diameter in the range of 90 mm to 150 mm. 10

32. Delivery apparatus as claimed in any of Claims 24 to 31 in which the inner transverse cross section of the throat adjacent the nozzle outlet is of diameter of approximately 120 mm.

33. Delivery apparatus as claimed in any of Claims 24 15 to 32 in which the transverse cross section of the nozzle outlet is of diameter in the range of 12.5 mm to 50 mm.

34. Delivery apparatus as claimed in any of Claims 24 to 33 in which the transverse cross section of the 20 nozzle outlet is of diameter in the range of 20 mm to 30 mm.

35. Delivery apparatus as claimed in any of Claims 24 to 34 in which the transverse cross section of the nozzle outlet is of diameter of approximately 25 mm.

36. Delivery apparatus as claimed in any of Claims 24 to 35 in which a metering means is provided for metering the flow of pigment material to the nozzle outlet.

37. Delivery apparatus as claimed in Claim 36 in which the metering apparatus comprises an auger screw.

38. Delivery apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings .

39. A plastics processing apparatus comprising a mixing and heating barrel defining a hollow interior region for mixing pigment material and plastics material, the hollow interior region being formed by a pair of co-operating bores, and a pair of co-operating mixing screws being rotatable about respective parallel axes in the respective bores for mixing plastics material and pigment material and for pressurising the mixed pigmented material in the hollow interior region, and a barrel inlet communicating with the hollow interior region, and delivery apparatus as claimed in any of Claims 24 to 38 in which the throat is mounted on the barrel and communicates with the hollow interior region through the barrel inlet, the nozzle outlet being located intermediate a pair of vertical planes containing the rotational axes of the mixing screws and being disposed at a vertical height not greater than 25 mm to 30 mm above the upper surface of the bores forming the hollow interior region.

40. A plastics processing apparatus as claimed in Claim 39 in which the hollow interior region of the mixing and heating barrel comprises an upstream feeding zone and a downstream compression zone and a mixing zone intermediate the feeding zone and compression zone and the barrel inlet is provided to the feeding zone.

41. A plastics processing apparatus as claimed in Claim 39 or 40 in which the plastics processing apparatus is an injection moulding apparatus.

42. A plastics processing apparatus as claimed in Claim 39 or 40 in which the plastics processing apparatus is a plastics extrusion apparatus.

43. A plastics processing apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings.

44. An extruded article of coloured plastics material, the extruded article being extruded from a mixture of plastics material and pigment material having been mixed according to the method of any of Claims 1 to 23. 5

45. An extruded article of coloured plastics material extruded from a mixture of plastics material and pigment material and having been extruded from the apparatus of any of Claims 39 to 43.

46. An extruded pipe of coloured plastics material, 10 the extruded pipe being extruded from a mixture of plastics material and pigment material having been mixed according to the method of any of Claims 1 to 23.

47. An extruded pipe of coloured plastics material extruded from a mixture of plastics material and 15 pigment material and having been extruded from the apparatus of any of Claims 39 to 43.

48. An injection moulded article of coloured plastics material, the injection moulded article being moulded from a mixture of plastics material and pigment 20 material having been mixed according to the method of any of Claims 1 to 23.

49. An injection moulded article of coloured plastics material moulded in the plastics processing apparatus of any of Claims 39 to 43.