Field

The invention pertains to a method of producing long fibre thermoplastic (LFT) extruded material, a long fibre thermoplastic (LFT) material, and a single screw extruder.

Introduction

The present invention pertains in an aspect to a method of making long fibre thermoplastic (LFT) material, in particular for making LFT extruded material (extrudate). The LFT material can also be referred to as long fibre reinforced thermoplastic material. The LFT extruded material can for instance be used for producing a wide array of products comprising LFT materials, such as parts in automotive as door panels, instrument carrier or front-end of vehicles, and also parts in other fields such as electronics. The LFT extruded material is for instance used as a feedstock for injection moulding to form LFT products. The LFT extruded material is for instance cut to pellets and the pellets are used for injection moulding Injection moulding of LFT materials is known as such, and uses as feed material often LFT pellets. Such LFT pellets are for instance 5 to 15 mm long, preferably about 11 mm long, with a diameter of for instance 1-5 mm. The individual LFT pellets comprise fibres (e.g. carbon fibres, glass fibres or natural fibres) as well as thermoplastic polymer. The inventive method can be used for making such pellets with improved properties and/or in a more economical way.

Current exemplary methods of making LFT pellets are based on wire coating or thermoplastic pultrusion using dry continuous fibres. Herein, ‘dry’ indicates that the fibres are not yet embedded in, for example, a thermoplastic material. For instance, in some prior art methods for making LFT pellets, dry fibres are wire-coated with a thermoplastic polymer. Wire coating and pultrusion each give strands of the coated or impregnated fibres which are dimensioned (e.g. cut) into pellets of e.g. 11 mm in length. Longer pellets are difficult to process with injection moulding. The fibres are continuous and are aligned in the strands obtained by wire coating and pultrusion. In the individual pellets made by cutting such strands, the fibres are aligned in parallel and extend in parallel arrangement over the length of the pellet. With the currently available typical production methods for making LFT pellets, the fibre length is equal to the pellet length. Long fibres are desirable because this contributes to the mechanical properties of the injection moulded LFT parts. Extrusion, as distinct from pultrusion, is generally not used for making LFT strands or pellets. It is desired to provide a method of making LFT material, e.g. LFT pellets, wherein the produced LFT material is suitable for further manufacturing steps for making LFT articles, such as LFT injection moulding or a further extrusion process, and wherein the method is more economical and/or gives LFT material with improved properties. US 2002/0089082 describes a method for the production of fibre-reinforced plastic compositions wherein an apparatus with a double screw plasticizing extruder is used, wherein melted plastic and a fibre material are combined in an entrance opening of the plasticizing extruder. EP 1008435B1 describes a method for producing fibre-reinforced plastic masses by using a plastifying extruder, in which fibres and thermoplastic granulate and/or long-fibre-reinforced recyclate chips are plastified and are withdrawn as plastified material which can be further processed. Large-sized long- fibre-reinforced recyclate chips with a circular area of approx. 50 mm diameter and with a length of up to 200 mm are weighed in and introduced in a dosed manner into a heated tube worm conveyor device and are dried by pre-heating up to the point shortly before they become sticky. Thereafter the chips are introduced into the plastifying extruder. Summary The invention pertains in a first aspect to a method of producing long fibre thermoplastic (LFT) extruded material, wherein the method comprises: A) providing a first feed stream comprising thermoplastic material; B) providing a second feed stream comprising thermoplastic composite (TPC) feed material, for example flakes or chips, wherein the TPC feed material comprises pieces, wherein preferably the pieces individually comprise both fibres and thermoplastic material; wherein the method comprises an extrusion step carried out in an extrusion system, wherein the extrusion system comprises an extruder, wherein the extruder comprises a barrel, a single screw, and an outlet opening and has an extrusion direction, wherein the screw provides a channel in the extruder, wherein the barrel is provided with a first inlet opening and a second inlet opening, wherein the first inlet opening and the second inlet opening are separate and spaced apart from each other, wherein the second inlet opening is arranged downstream of the first inlet opening in the extrusion direction, wherein the extrusion step involves: C) supplying said first feed stream through said first inlet opening into said channel in the extruder, and conveying said first feed stream with said screw from said first inlet opening to said second inlet opening; D) supplying said second feed stream through said second inlet opening into said channel in the extruder thereby combining and mixing said first and second feed stream inside said channel in a first mixing zone of said extruder to give a first mixed material; and F) conveying a stream comprising said first mixed material through said outlet opening to give LFT extruded material. The invention pertains in a further aspect to a long fibre thermoplastic (LFT) material obtainable by this method. The invention pertains in a further aspect to a pellet comprising fibres and thermoplastic material, wherein the pellet has a length and a diameter, wherein the fibres are preferably oriented in the length direction of the pellets, and are preferably at least partially curved, and wherein preferably at least some of the fibres in the pellet have a length which is greater than the length of the pellet and at least some of the fibres in an individual pellet have a length which is smaller than the length of the pellet. The invention pertains in a further aspect to a single screw extrusion system for producing long-fibre reinforced thermoplastic material, comprising an extruder barrel, an axially rotatable extruder screw within the extruder barrel, and a die comprising an outlet opening, wherein the screw comprises a shaft and one or more helical flights, wherein the barrel comprises a first inlet and a second inlet, wherein the first inlet, second inlet and the outlet are spaced apart from each other, and wherein the first inlet is provided with a first feeder and wherein the second inlet is provided with a second feeder. The extrusion system is preferably suitable for the method. The method is preferably performed with the extrusion system. Brief description of the drawings Figure 1 schematically illustrates an example extrusion system according to the invention. Figure 2 schematically illustrates an example method according to the invention. In particular, Fig. 2A illustrates an example extrusion step and some other steps and Fig.2B illustrates some example steps of providing the second feed stream. Figure 3 schematically illustrates a comparative method. Figure 4 schematically illustrates an example extrusion system according to the invention. Figure 5 schematically illustrates a preferred die head of an extrusion system according to the invention. In Fig. 5, Panel A illustrates a preferred tapered die head; Panel B illustrates a straight die head. Figure 6 schematically illustrates an example extruder setup. Figure 7 illustrates a histogram The figures are illustrative only and do not limit the invention. Detailed description The present invention is broadly based on the judicious insight that LFT materials, for instance LFT pellets, that are suitable for manufacturing LFT articles, for example with LFT injection moulding and/or extrusion moulding, can be made in an advantageous way by using an extrusion step performed in an extruder comprising a screw, preferably in a single screw extruder, wherein thermoplastic material is supplied as first feed stream into the extruder at a first inlet opening and wherein TPC feed material, for example TPC chips and/or flakes, is supplied as second feed stream into the extruder at a downstream second opening, such that the TPC feed material is compounded with the thermoplastic material. This was found to surprisingly allow for low fibre attrition during the extrusion process. The first and the second feed stream are different from each other. The abbreviation “TPC” as used herein stands for “thermoplastic composite”. The second feed stream comprises TPC feed material. The TPC feed material comprises for instance TPC chips and/or TPC flakes and/or for example TPC pellets. Other TPC feed materials are also possible. The TPC feed material comprises for instance discrete solid pieces of TPC material. The TPC feed material is for instance provided as a stream of solid pieces of TPC material. The pieces include, for example, TPC chips and/or TPC flakes. The abbreviation “LFT” stands for “long fiber thermoplastic”. A long fiber thermoplastic material, as used herein, indicates a material comprising, or substantially consisting of, or even essentially consisting of, a thermoplastic polymer and long fibres; wherein the polymer can be for instance a raw polymer or a polymer compounded with additives. Long fibres have a typical length of 0.5 to 50 mm, more preferable 1 to 20 mm, and most preferable 2 to 10 mm and optionally have a typical diameter of 1.0-100 µm, 2-50 µm and most preferable 5- 30 µm. In an LFT material the long fibres are for instance embedded in the thermoplastic polymer, as is the case, for example, for fibres impregnated with a thermoplastic polymer. In an example embodiment, the TPC feed material comprises LFT material wherein the fibres are embedded in the thermoplastic polymer. Preferably, the TPC feed material, in particular the solid pieces, in particular the preferred chips or flakes, comprise fibers, wherein these fibres have for example a length of 0.5 to 50 mm, more preferable 1 to 20 mm, and most preferable 2 to 10 mm and optionally have a typical diameter of 1.0-100 µm, 2- 50 µm and most preferable 5-30 µm. A thermoplastic composite (TPC) material, as used herein, indicates a material comprising, or substantially consisting of, or even essentially consisting of, a thermoplastic polymer and (dry, impregnated short, long and continuous or mixtures thereof) fibres; wherein the polymer can be for instance a raw polymer or a polymer compounded with additives and wherein the fibres are, for instance, dry or impregnated, and are for instance short or long, or continuous, or mixtures of any of such fibres. The TPC feed material as used herein is for example LFT feed material. In some embodiments of the TPC feed material, the fibres are embedded in the thermoplastic polymer, for instance in case of fibres impregnated with a thermoplastic polymer. The TPC feed material is for instance provided as a stream of solid pieces of TPC material, i.e. discrete solid pieces. The discrete solid pieces typically have at least one dimension of at least 1.0 mm and typically have at least two perpendicular dimensions of at least 1.0 mm. The discrete solid pieces preferably individually comprise fibres and thermoplastic material. These fibres have a fibre length, preferably of at least 1.0 mm. The second feed stream may comprise additional components such as dry fibre fragments or particles of pure thermoplastic resin. In an example embodiment, the TPC feed material may comprise flakes of TPC material with a thickness of at least 100 µm, typically with a thickness of less than 50 mm and a smallest size in a direction perpendicular to the thickness direction of at least 1 mm, at least 10 mm, or at least 100 mm. The discrete pieces of the TPC feed material typically have a size of less than 10 cm in at least one dimension. The TPC feed material comprises for instance TPC flakes and/or TPC chips. Such TPC flakes have more particularly for instance a length of 2 mm to 40 mm, a width of 2 mm to 40 mm and a thickness less than the length and/or less than 5.0 mm; for instance with a thickness of 0.1 to 4.0 mm. TPC chips have for instance a length (L) of 2 to 50 mm, a width (W) of 2 to 50 mm and a thickness (H) of, for instance, 0.1 L < H < L. The TPC flakes have for instance an aspect ratio L/H of at least 10, for instance of at least 20. The TPC flakes have an edge and two sides, and for instance a surface area, per each of such side, of at least 5 mm² or at least 10 mm². The TPC flakes with such dimensions are for instance LFT flakes. The TPC flakes are for instance obtainable by cutting or chopping material from a tape. The TPC chips are for instance obtainable by shredding CFRT (Continuous Fibre Reinforced Thermoplastic) material, such as shredding a laminate, or shredding an end of life part of TPC material. The TPC chips can also be obtained e.g. as production waste from a process of making CFRT sheets. The TPC feed material, such as flakes and chips, are typically abrasive because of the fibre content. In some embodiments, the TPC feed material comprises for instance size- reduced (such as shredded) waste material, which waste material comprises or consists of TPC material. The TPC feed material is not restricted to any specific thermoplastic polymer and is also not restricted to specific fibrous materials. Figure 1 schematically illustrates an example of a preferred method and apparatus according to the invention. The apparatus comprises an extrusion system (1) which comprises an extruder (2). The extruder (2) comprises a barrel (3), and a single screw (4). The screw (4) is axially rotatable. The barrel is a hollow vessel, typically a cylindrical vessel having a length and a diameter. The screw is arranged in the barrel, typically in the length of the barrel, and comprises a cylindrical shaft (11) and a flight (12), typically one or more helical flights. The flight pitch is shown merely schematically. The screw provides for a channel (8) in the extruder adapted for receiving material to be extruded. In operation, material to be extruded is received in the channel through an inlet and conveyed by rotating the screw in the extrusion direction (ED) to the outlet opening (5). The material melts at least in part in the channel due to the shear action of the screw and barrel to form a melt, and the molten material is extruded into extruded material at the outlet opening which is typically provided in a die. In the method of the invention, the first feed thermoplastic material (A) is supplied into the channel through a first inlet opening (6) and is received in the channel (8). The first feed material is conveyed over a distance (S1) through the extruder to the downstream second inlet opening (7). The thermoplastic material is already at least partly molten when it arrives at the second inlet opening (7) where TPC feed material (B) is introduced into the extruder and combined and mixed with the at least partly molten first feed thermoplastic material in a mixing zone (9) of said extruder (2) to give a first mixed material (C). As an independent preferred feature, the extruder comprises a heating element (10) that is arranged in the screw, for instance in the shaft. The heating element is for instance a resistive heater. The heating element is adapted for selectively heating the first compounded material (C) in the mixing zone (9) of the channel (8). This local heating reduces the friction in the extruder locally and thereby provides for good mixing of the TPC feed material of the second feed stream (B) with the molten first feed material (A) with advantageously lower fibre breakage. The resulting compounded material (C) is conveyed to the die (15) and the extruded material (D) is received from the outlet opening (5). As an independent preferred feature, the die (15) is tapered in the extrusion direction as shown in Fig. 1. The extrusion system (1) further comprises a first feeder (16) for first inlet opening (6) and a second feeder (17) for the second inlet opening (7). The second feeder is for instance a crammer feeder. The second feeder is for instance provided with heating elements (not shown) for heating the TPC feed material prior to entry into the extruder. Particularly advantageous, the TPC feed material as used in the present invention can be waste material. This provides the advantage that the method is an environmentally friendly method of processing the material and that the TPC feedstock is economical. The present invention provides a way of recycling certain materials which are hitherto considered as waste materials. Moreover, the inventive process provides for a very elegant way to convert a waste material into a high quality recycled LFT material, for instance into high quality LFT pellets that can be used for LFT injection moulding. In a highly preferred embodiment, which however does not limit the invention, the TPC feed material comprises the separated side edges (e.g. trims) of impregnated fibre layer, such as impregnated fibre tape or sheet. Such tapes are an intermediate stage in certain production methods for continuous fibre reinforced material. The material can also be e.g. chips issued from shredding of CFRT materials such as CFRT sheets or CFRT articles, such as CFRT car parts. In some embodiments, the method of making LFT extruded material according to the present invention provides for the recycling of continuous fibre reinforced thermoplastic (CFRT) waste material. In some embodiments, the present invention pertains to a method of producing LFT extruded material, wherein the method comprises a providing such separated side edges as at least part of the second feed stream. Furthermore, the second feed stream is in some embodiments at least in part provided by recycle streams of process of making CFRT materials and CFRT parts, components and articles and/or recycle streams from end of life CFRT articles. In a preferred embodiment, which does not limit the invention in any way, the step B of providing the second feed material comprises: B1) providing dry continuous fibres, B2) impregnating said fibres with a thermoplastic resin to give an impregnated fibre continuous sheet having edges, B3) separating at least part of said edges from said impregnated fibre continuous sheet to give flakes, which flakes are separated edge pieces and which flakes individually contain fibre and thermoplastic material, and a trimmed continuous sheet, B4) using said flakes as at least part of said second feed. B5) optionally sizing the trimmed continuous sheet into sheets or tapes, and optionally making laminates from the sheets or tapes. The sheets, tapes or laminates are for instance placed in a mould and subjected to moulding (e.g. with heat) so as to a shaped CFRT article. Figure 2 schematically illustrates an example a preferred method according to the invention. In this preferred embodiment, which does not limit the invention, the method of making LFT extruded material comprises, as illustrated in Fig. 2A, the extrusion step (205) using thermoplastic resin (TP) and TPC material, for instance LFT material (for example streams A1, A2, A3, A4 as illustrated in Fig. 2B, and combinations of these with each other and/or with other streams), to give LFT extruded material strands, which are furthermore sized (206) (i.e. dimensioned, pelletized) into LFT pellets. Furthermore, the LFT pellets are optionally injection moulded or extruded (207) to give LFT parts; these parts are for instance LFT articles or LFT components. In this preferred embodiment, the step of providing the second feed material comprises a process for making CFRT material (Fig. 2B). The process comprises impregnating (201) a continuous dry fibres (e.g. glass fibres, natural fibre or carbon fibres) with thermoplastic polymer resin to give an impregnated continuous sheet or tape (single layer sheet). The fibres are for instance provided as woven or nonwoven or as unidirectional tape. The sheet, having edges of lower quality, is typically treated (202), e.g., trimmed, to remove these edges. The removed edges are cut or chopped and reduced in size (202A) and collected as LFT flakes (A1). In an example embodiment, the TPC feed material is provided by supplying such LFT flakes to the extrusion step. The sheet with edges removed is still a continuous sheet and is laminated (203) to CFRT material, such as CFRT laminate. The CFRT laminate is for instance trimmed (204) to give trimmed laminate (a “blank”) and trimming waste. The trimming waste is for instance sized (e.g. shredded) into LFT chips (A2) which are for instance used as at least part of the TPC feed material. The CFRT material can be used for processing (208) to give CFRT articles. For instance, the processing 208) is moulding the material by placing the solid CFRT material, such as CFRT blanks, in a mould, closing the mould and moulding the material using heat and/or pressure to give moulded CFRT article. The moulding stage is typically done separately from the manufacture of the CFRT sheets, e.g. after storage and transport of the CFRT material. Off-spec articles from the processing (208) can be sized (e.g. shred) to chips (A3) which are optionally used as at least part of the TPC feed material. Furthermore, end-of-life CFRT articles (e.g. post-consumer articles) can be sized (209) (such as shred) into chips (A4) which are optionally used as at least part of the TPC feed material. The illustrated TPC feed materials (A1, A2, A3 and A4) are only examples and do not limit the invention in any way. LFT flakes resulting from the edge removal step (202), e.g., from the edge trimming step, are considered as a waste product in the prior art, and are, for instance, burned in an incinerator, or ground to dust in a grinding step in prior art methods. In the present invention, the LFT flakes, but also other types of TPC material, can be used as TPC feed material for the extrusion step (205) according to the invention, as second feed stream together with a separate first feed stream comprising thermoplastic material, to produce LFT extruded material (extrudate). The inventive method in general further preferably comprises step of pelletizing (206) the LFT extruded material from the extrusion step (205) to give LFT pellets. These pellets are optionally stored and/or transported and can optionally be further used, for instance for LFT injection moulding. The individual LFT pellets comprise the fibres and the thermoplastic material. The LFT pellets can be used for instance for LFT injection moulding (207). Comparative preparation methods for making LFT pellets are, as illustrated, based on combining (208) continuous fibres and thermoplastic resin to give continuous strands, wherein the combining step is for instance wire coating or pultrusion, and a further step of sizing (209) of the strands into LFT pellets. The LFT pellets according to the present invention are very suitable for LFT injection moulding. In such LFT injection moulding process (207), which is an optional further step of the inventive method, the LFT pellets are heated such that the thermoplastic fraction melts and the melt containing fibres is injected into the mould. The mould is closed and the material in the mould solidifies in the mould to give the injection moulded LFT part which is a shaped article and which comprises thermoplastic polymer and the fibres. The fibres desirably have a relatively long length within the shaped article. The injection moulded LFT part is for instance a car part. In this preferred embodiment of the inventive method, the TPC flakes and TPC chips obtained essentially as waste material in a normal CFRT manufacturing process are surprisingly and very advantageously recycled into a high value product. This embodiment of the inventive method compares very favourable to currently used methods such as incinerating the flakes or chips or grinding the flakes or chips into dust, which dust can be used as low-cost fillers. Figure 3 schematically illustrates a comparative method of making LFT articles, not according to the invention. Herein, thermoplastic resin and continuous dry fibres are combined in a process (305) such as pultrusion or wire coating to give LFT strands which are sized (e.g. cut) (306) into pellets. The pellets can be used for LFT injection moulding and/or LFT extrusion (307). In a preferred embodiment, the TPC feed material of the second and/or optional third feed stream comprises flakes, chips, shredded material from a CFRT waste stream, or a combination thereof; more preferably in an amount of at least 90 wt.% or at least 95 wt.% of the second respectively third feed stream. The flakes or chips preferably consist of TPC material and can be obtained from any suitable source. The present invention accordingly generally pertains in an aspect to a method of making long fibre thermoplastic (LFT) extruded material, such as extrudate strands or pellets. The method comprises providing a first feed stream comprising thermoplastic material and a separate second feed stream comprising TPC feed material. The first and the second feed stream are different from each other; in particular the feed streams have a different chemical composition. The first feed stream generally preferably does not contain fibres, specifically preferably does not contain thermosetting polymeric or non-polymeric fibres, or preferably less than 5.0 wt.% or less than 1.0 wt.% of such fibres in total relative to the first feed stream in total. Preferably, the first feed stream comprises less than 5.0 wt.% carbon fibres, less than 5.0 wt.% glass fibres, and/or less than 5.0 wt.% natural fibres. The first feed stream is preferably a solid material and is introduced into the extruder in the form of pellets. The first feed stream comprises preferably at least 80 wt.% or at least 90 wt.% of thermoplastic polymers in total, relative to total weight of the first feed stream. The first feed stream may comprise one or more thermoplastic polymers. The first feed stream preferably comprises at least 80 wt.% unreinforced thermoplastic polymer, which can be for instance virgin polymer, recycle polymer, or a combination of these. The TPC feed material of the second feed stream and preferably used third feed stream generally comprises for instance flakes or chips of TPC material, preferably of LFT material. The TPC feed material comprises pieces of TPC material, such as flakes or chips, wherein preferably individual pieces comprise fibres and thermoplastic material. In a preferred embodiment, at least the second or third feed stream, or both, comprises LFT material so as to supply long fibres for the LFT product. As discussed herein, the TPC feed material may be obtained from a CFRT material or CFRT article or part production process or from an end of life CFRT article or part. The CFRT article is for example a woven, a non-woven or a unidirectional CFRT article. The TPC feed material can for instance also be obtained from LFT parts and articles in general, such as from LFT injection moulded parts and articles and/or LFT extruded parts and articles, such as by shredding end of life LFT parts and articles. The TPC feed material can also be obtained as production waste from methods of making LFT parts and articles. The TPC feed material, preferably as LFT feed material, comprises pieces, which pieces for example individually consist of at least partly impregnated fibres, wherein the fibres are partly or entirely impregnated with a thermoplastic polymer. The TPC feed material may also comprise other components such as dry fibres. The TPC feed material comprises for instance substantially only one thermoplastic polymer, e.g. at least 90 wt.% of the thermoplastic fraction of the TPC feed material is a single polymer. Preferably, the polymer or polymer blend of the TPC material is the same polymer as the polymer or polymer blend used for the first feed material. The thermoplastic polymer is for instance a polymer selected from the group consisting of polyolefine, polyamides, polycarbonate, polyphenylene sulphide, polyaryletherketone, and polyethylenimine. The term “thermoplastic polymer” as used herein includes at least plastic polymers that become pliable or mouldable at a certain elevated temperature and solidifies upon cooling. In some embodiments, which do not limit the invention, the thermoplastic polymer is not cross-linked and is not cured. Preferably the first feed material comprises at least 80 wt.% based on total first feed material, of the same polymer or polymer blend as used for at least 80 wt.% of the TPC feed material. The TPC feed material of the second feed stream comprises fibres, wherein the fibres are for example selected from the group consisting of glass fibre, carbon fibre and natural fibre. Preferably the natural fibre is for instance selected from the group consisting of hemp, flax, jute and cellulose fibres. The TPC feed material may comprise additional fibres. The TPC feed material comprises for instance at least 20 wt.% fibres or at least 30 wt.% fibres, and typically less than 80 wt.% fibres. The TPC feed material comprises for instance 30 to 70 wt.% fibres or e.g. 50 to 70 wt.% fibres. The TPC feed material of the second and third feed may have a different fibre content, such as differing by at least 5 absolute percent point or at least 10 absolute percent point. In this way, fluctuations in the fibre content of the first, second and/or third feed stream can be off-set by adjusting the feed rate of the first, second and/or third feed. The method comprises an extrusion step which is carried out in an extrusion system. The extrusion system comprises an extruder which comprises a barrel with therein a screw and an outlet opening. The outlet opening is provided in a die at the downstream end of the extruder. The extruder has an extrusion direction which is in the length direction of the screw towards the outlet opening. The extruder is a single screw extruder, as opposed to a twin screw extruder. Hence, the barrel contains not more than one screw at each position in the length of the barrel. The screw provides a channel in the extruder. The channel is an open space provided between the shaft of the screw, the flights, and the inner wall of the barrel. The barrel is for instance a cylindrical vessel. In operation, material is conveyed through the channel in the extrusion direction by the rotating action of the screw. The extruder comprises a first inlet for the first feed stream and a separate second inlet for the second feed stream. The first inlet and the second inlet are spaced apart from each other in the extrusion direction. The second inlet opening is arranged downstream of the first inlet opening in the extrusion direction. The second inlet opening is for instance spaced apart from the first inlet opening by at least 10% or at least 20% of the screw length, and typically less than 90% or 80% of the screw length. This provides for good mixing of the second feed stream, in particular the TPC feed material, with the first feed stream as well as for good extrusion at the die. The first inlet comprises a first opening in the barrel for supplying the first feed material to the screw. The second inlet comprises a second opening in the barrel for supplying the second feed material to the screw. The method of the invention involves supplying the first feed material into the extruder through the first inlet, and supplying the second feed material into the extruder through the second inlet. In the method, the first feed stream is introduced into the extruder through an inlet at a position upstream of the inlet for the second feed stream. Hence, the first feed material is conveyed by the screw for at least some distance from the first inlet to the second inlet. Furthermore, the screw channel is already partially but not entirely filled with material at the section of the extruder where the second feed stream is provided into the extruder. Hence, the second inlet provides for a mixing zone in the extruder where the initially separate first and the second feed material are combined and compounded with each other, by the action of the screw which is present in said zone. The compounded material is then further conveyed by the screw in the direction of the die of the extruder, through the channel, preferably with a third feed stream being added through a third inlet opening which is arranged downstream from and spaced apart from the second inlet opening. At the die, the compounded material passes through a die or nozzle to give extruded LFT material, in particular extruded strands. The conveying steps as discussed involve rotating the screw, preferably with low shear so as to reduce fibre attrition. This provides the advantage that desirably high fibre fractions can be achieved in the LFT extruded material, as well as large fibre lengths. In the invention, the first, second and the optional third inlet opening are each provided as openings in the barrel at a position in the extrusion direction (and at a position in the barrel length) where also the screw is provided. The barrel is typically a cylindrical vessel having a length and a diameter. The first, second and optional third inlet are each individually provided at such a position on the barrel length that a cross-section of the barrel perpendicular to the barrel length and through said inlet also goes through said screw. In the present invention, the extruder comprising the first and second and the optional third inlet opening also contains the die. Hence, the die is typically arranged parallel with the screw and in the projection of the screw in the extrusion direction. The die is typically mounted or attached to the barrel containing these inlets. In a preferred embodiment, the method comprises selectively heating a part of the screw with at least one heating element, which heating element is arranged in the screw at a position in the extrusion direction at of downstream of the second inlet opening. The heating element is for instance tracing. The localized heat supply advantageously provides for a relatively lower friction of the material in the channel located at the heating element (compared to omitting such heating) which provides for reduced shear and therefore more gentle mixing of the introduced material with the material that is already present in the channel (e.g. the material from the first inlet). In this way fibre wear and breakage is reduced and long fibre length is maintained. In a preferred embodiment, the method further involves providing a third feed stream comprising TPC feed material and the extruder comprises a third inlet opening for said third feed stream. The third inlet opening is spaced apart and downstream from the second inlet opening. The extrusion step further preferably involves conveying the first compounded material from the first mixing zone, with the screw, to the third inlet opening; and supplying the third feed stream through the third inlet opening into the channel inside the extruder. In this way the method preferably involves combining and mixing the first compounded material with the third feed stream inside the channel in a second mixing zone of the extruder (2) to give a second compounded material. The method further preferably involves conveying a stream comprising the second compounded material to the die, and in particular through the outlet opening (e.g. through the nozzle) to give LFT extruded material. Fig. 4 schematically illustrates a preferred method and apparatus according to the invention. The reference numerals are the same as in Figure 1 unless otherwise specified. The pitch of the flight (12) is schematically illustrated. The extruder comprises a third inlet (13) for a third feed stream (E) which comprises TPC feed material; this TPC feed material can have the same or a different composition than the second feed stream (B). The third inlet opening (13) is configured for supplying the third feed stream (E) into the channel (8) in the extruder (2). In this way, the first compounded material (C) is combined and compounded with the third feed stream (E) inside the channel (8) in a second mixing zone (14) of the extruder (2). This gives a second compounded material (F) which is in turn conveyed to the die (15) and, optionally after addition of further components through optional further inlet openings, through the outlet opening (5) of the die to give LFT extruded material (D). The third inlet opening (13) is provided with a feeder (18). Preferably, the first, second and third feeder (16,17,18) are each provided with a driver for providing feed into the inlet opening at variable feed rates. The second and third feeder (17,18) are each preferably provided with a gravimetric sensor for measuring the gravimetric density of the feed material. This density can be used for determining the fibre content of the feed material. The extrusion system (1) further comprises a controller (19) for adjusting the feed rates of the first, second and third feeder (16,17,18) using the information from the gravimetric sensors in order to compensate for variations in the fibre content of the feed streams so as to obtain a controlled fibre content in the extruded material. This advantageously provides for homogenizing the fibre weight fraction of the output material, in particular in case the second and/or third feed are at least in part waste material that is recycled by the inventive method. The third inlet opening (13) is spaced apart from the second inlet opening (7) by a distance (ΔS2) which allows for effective mixing. The third inlet opening (13) is spaced apart from the die (15) by a distance (ΔS3) which allows for good extrusion. In a preferred embodiment of the method and of the inventive pellet, the long-fibre reinforced thermoplastic material obtained with the method and/or the pellet comprises 20-75 wt.% of fibres by total weight of the long-fibre reinforced thermoplastic material, preferably 30-60 wt.%. Such high fibre fractions contribute to good mechanical properties of articles and parts prepared from the material. In a preferred embodiment, which does not limit the invention, the thermoplastic material comprised in the second feed stream comprises a thermoplastic polymer which has a glass transition temperature T _g and a melting temperature T _m , and step B for example involves heating said second feed stream material to a temperature that is i) above said temperature T _g and ii) equal to or lower than said temperature T _m , preferably less than 10.0ºC lower, or most preferably less than 5.0ºC lower than T _m . The heating is preferably done in the feeder of the second inlet opening. The preferred method further involves supplying the heated second feed stream through said second inlet opening. The advantage of this pre-heating is that less heating is needed in the extruder which thus compensates for the reduced heating due to less frictional heating. In a further preferred embodiment wherein a third feed stream is used, the third feed stream preferably comprises a thermoplastic polymer which has a glass transition temperature T _g and a melting temperature T _m , and step E preferably involves heating said third feed stream material to a temperature that is i) above said temperature T _g and ii) equal to or lower than said temperature T _m , preferably less than 10.0ºC lower, or most preferably less than 5.0ºC lower than T _m . The heating is preferably done in the feeder of the third inlet opening. The advantage of this pre-heating is that less heating is needed in the extruder, such as in the mixing zone of the third inlet. The pre-heating of the third feed stream is preferably combined with the preferred pre-heating of the second feed stream. The method optionally further comprises pelletizing the LFT extruded material, in particular the strands, into LFT pellets. Preferably the LFT pellets are suitable for LFT injection moulding and/or for LFT extrusion, and/or for compression moulding. Preferably, the pellets have a length in the range of 2-25 mm, preferably 6- 15 mm. Pellets with such a length are particularly suitable for LFT injection moulding. In a further aspect, the invention pertains to an extrusion system. The extrusion system is suitable for producing long-fibre reinforced thermoplastic material. Preferred features for the extrusion system as discussed in connection with the method apply equally to the extrusion system. The extrusion step of the inventive method of producing LFT extruded material is preferably conducted in an extrusion system as described herein. However, the extrusion system is not restricted to such a use. The extrusion system comprises an extruder, which comprises an extruder barrel, an axially rotatable extruder screw within the extruder barrel, and a die comprising an outlet opening, wherein the screw comprising a shaft and one or more helical flights; the flights are for instance mounted on the shaft or are unitary with the shaft. The flights provide for a channel in the extruder for receiving and conveying material to be extruded. The barrel comprises a first inlet and a second inlet, wherein the first inlet, second inlet and the outlet are spaced apart from each other. The first inlet is preferably provided with a first feeder. The second inlet is preferably provided with a second feeder. The second feeder is for instance a crammer feeder or a side feeder, or for example a further extruder. The second feeder preferably comprises a heating element for heating the second feed material. Such heating element for pre-heating of the feed is advantageous to avoid the need for high shear in the extruder. The system is configured for supplying a first feed stream into the extruder, in particular into the channel, through the first opening and supplying a second feed stream into the extruder through the second inlet, wherein the first and second feed stream are different from each other; the feed streams are preferably as discussed in connection with the method. The system for instance comprises separate supply lines for the first and second feed to the respective inlets. An example extrusion system according to the invention is schematically illustrated in Fig. 1. Preferably, the second feeder comprises a gravimetric sensor for measuring the density of the second feed stream, and the extrusion system preferably comprises a controller coupled to the first feeder and/or the second feeder so as to control the first feeder and/or the second feeder using signals from said gravimetric sensor. In an embodiment, the controller is coupled to both the first and second feeder. In this embodiment, the first and second inlet are hence in communication with each other. In this embodiment the system provides for controlled dosing which enables stable and precise fibre content of the extruded material even in case of a second and/or third feed stream with fluctuating fibre content. The measured density is indicative of fibre content because typically the fibres have a different, e.g. higher, density than the thermoplastic component. The method preferably comprises (e.g. continuously) measuring the density while carrying out the extrusion step. This preferred embodiment is illustrated in Fig. 4. Preferably, the extrusion system further comprises a third inlet opening for a third feed stream, as discussed. Preferably the third inlet opening is provided with a third feeder. The third feeder is for instance a crammer feeder or side feeder. Preferably, the third feeder comprises a further gravimetric sensor for measuring the density of the third feed stream. This sensor is also coupled to the preferred controller of the extrusion system. The controller preferably allows for controlling the first feeder, second feeder and third feeder using signals from said gravimetric sensors of the second and third feeder. A gravimetric sensor may also be provided to the first feeder. Other sensors can also be provided in the extrusion system and coupled to the controller. Such a preferred embodiment is illustrated in Fig. 4. The extrusion system may further comprise a cooler downstream of the outlet, for cooling continuous extruded strands received from the outlet, and optionally also comprising a pelletizer downstream of the cooler to transform the strands into pellets. In a preferred embodiment, the screw comprises a heating element. The heating element is for instance an electric heater and/or tracing. The heating element is for instance located in the shaft. The heating element is adapted for selectively heating only a part of the screw, in particular for heating only a section of the screw in the length direction. This can advantageously be used to reduce or avoid any temperature drop, or at least avoiding or preventing significant temperature drops, caused by the introduction of material into the extruder with the second and optional third inlet opening. One or more heating elements may be provided in the screw. In some embodiments, the screw comprises multiple heating elements which are located in the shaft and which are spaced apart along the length axis of the screw. The production method of the invention may involve heating of the barrel. The preferred localized heating by the preferred heating element located in the screw is for instance applied in addition to the heating of the barrel. The heating element provides for selective heating of only a part in the length direction of the screw, for instance for heating a part of the screw in length direction that is less than 80% or less than 50% or even less than 20% of the total screw length. In an aspect, the invention also pertains to such a screw with a heating element as such. The screw, preferably with such a heating element, preferably has a pitch length of 10-40 mm, preferably 15-30 mm, and/or a channel depth of 5-30 mm, preferably 15-25 mm. The screw preferably has a length to diameter ratio of 10-30, preferably 15-25. The screw preferably has a compression ratio of 4 : 1 to 1 : 4, preferably 3 : 1 to 1 : 1. Such a screw is preferably comprised in the extrusion system of the invention as single screw and is preferably used in the method of the invention. Preferably, the die comprises a tapered die head which is optionally configured to be heated by heating means. The advantage of the tapered die head is that fibre breakage is reduced, in particular fibre breakage in the transition of the mixed material from the barrel (and channel) into the die. The die head is for instance tapered with the wide opening at the upstream side next to the barrel and the narrow opening at the downstream side with the nozzle. The die head is for instance a conically tapered vessel. Advantageously, discontinuities between the barrel and the die are avoided. The tapered die head is also used in a preferred embodiment of the inventive method. Figure 5 schematically illustrates die heads for the extrusion system according to the invention and for the method according to the invention. Panel A illustrates the preferred tapered die head. Panel B illustrates a less preferred straight die head. Reference numerals are the same as in Fig.1. The extruder (2) comprises a die head (20) between the barrel (3) and the nozzle or outlet opening (5); the die head is tapered in panel A and a straight in panel B. The fibres (21) are schematically shown as well as their movements during the extrusion step (arrows). In panel A the fibres are gradually fed through the tapered die head into the nozzle with less fibre attrition than in the die head of panel B. Figure 6 schematically illustrates an example extruder configuration with granules inlet (1), flakes/chips inlet (2), die (4), cooling section (4), and pelletizer (5). Figure 7 illustrates a fiber length distribution histogram highlighting the length at 50% (D50) and the length at 90% (D90), as these values are used in for instance Example 1. The invention pertains, in a further aspect, to the LFT material obtainable with the method. The method allows for producing LFT material with relatively long fibres and a relatively high fibre fraction using discontinuous long fibres. For instance the material is in the form of pellets and for example the material has a ratio of D90 fibre length to initial fibre length (I) of at least 50%, at least 60%, at least 70% or at least 80%, for example with glass or carbon fibre, and/or a ratio of D50 fibre length to initial fibre length (I) of at least 20%. Herein, D90 indicates the point in the fibre length distribution (histogram), by frequency, up to and including which, 90% of the total fibres is ‘contained’. For example, if the D90 is 11 mm, this means that 90% of the fibres has a size of 11 mm or smaller, and 10% of the fibres has a length larger than 11 mm, as illustrated in Fig. 7. The D50 fibre length is calculated correspondingly, wherein 50% of the fibres has a smaller length than the indicated value and 50% a larger length. D90 (D50) indicates the fibre length up to and including which, 90% (50%) of the total fibres counted is contained. The D90 value of the material is important for the (mechanical) properties of LFT articles manufactured from the LFT material, for example with LFT injection moulding. The fibre length of individual fibres may be determined using e.g. optical image analysis. The initial fibre length is the fibre length of the TCP material fed to the extruder (input fibre length), preferably the D50 fibre length value of the TCP material fed to the extruder. In a preferred embodiment, the initial fibre length is taken as equal to the length (largest dimension) of the TPC pieces (e.g. chips or flakes, or TPC pellets) fed to the extruder, i.e. the input length, in particular for flakes. TPC pellets include thermoplastic pellets with fibers which have a maximum length within the size of the pellet. In an example embodiment, the initial fibre length is taken as equal to the length of the produced LFT pellets. Accordingly, in some embodiments, the LFT pellet has a ratio of D90 to pellet length of at least 50%, at least 60%, at least 70% or at least 80%. In a further aspect the invention pertains to pellets comprising fibres and thermoplastic material, wherein the pellets have a length and a diameter, for example obtainable with the method of the invention. The pellets are for instance (substantially) cylindrical bodies. The invention also provides LFT pellets having a length, for example obtainable with the method of the invention, wherein the LFT pellet has a ratio of D90 to pellet length of at least 50%, at least 60%, at least 70% or at least 80%, wherein the pellet length is e.g. in the range 10 – 20 mm, e.g.12 – 16 mm. The pellet preferably comprises a thermoplastic material and fibers as described in connection with the method. Preferably, the pellet has a fibre content of at least 20 wt.%. In an embodiment, the pellets comprise fibres that are are oriented in the length direction of the pellets, and are preferably at least partially curved. For instance, in a cross-section of the pellet perpendicular to the diameter of the pellets, the visible fibre fragments are preferentially oriented in the length direction of the pellet rather than exhibiting an anisotropic distribution. For instance, in a cross-section of the pellet perpendicular to the diameter of the pellets, fibre fragments are visible rather than entire fibres due to the curvature (bending) of the fibres. This curvature provides a difference with pellets made with pultrusion or wire coating of continuous dry fibre. In a preferred embodiment, the fibres in the pellets are discontinuous. Preferably, the pellets are a LFT material obtainable with the method, more preferably with a ratio of D50/I of at least 20% and/or D90/I of at least 50% or at least 80% as discussed hereinabove. In an embodiment, at least some of the fibres in an individual pellet have a length which is greater than the length of the pellet, for instance wherein at least 1 % of the fibres, by number of fibres, has a length that is at least 110% or at least 120% of the pellet length. More specifically, preferably at least some of the fibres, such as at least 1% of the fibres (by number of fibres), have a length of at least 50% of the length of the pellet and/or preferably at least some of the fibres, such as at least 1% of the fibres (by number of fibres), are not oriented in the length direction of the pellet. In a preferred embodiment, at least some of the fibres in the pellet are not oriented in the length direction of the pellet and have a length which is at least 50% of the length of the pellet. Optionally also in these embodiments, some of the fibres are oriented in the length direction of the pellets, and wherein at least some of the fibres are curved. In an embodiment, at least some of the fibres in an individual pellet have a length which is smaller than the length of the pellet, such as less than 90% of the pellet length. For instance, in the pellet at least 1 % of the fibres, by number of fibres, can have a length that is less than 90% of the pellet length. Both the smaller and the larger fibres provide a difference with pellets made using pultrusion or wire coating of continuous dry fibre. The fibre length distribution can be determined for instance after removal of the matrix, such as by ashing the pellets and/or by melting and/or dissolving the thermoplastic component of the pellets, and optical measurement of the fibres, and/or for instance with X-ray imaging without removal of the matrix. Preferably the average fibre length (by number average) is at least 1.0 mm or at least 2.0 mm or at least 5.0 mm and/or is at least 30% or at least 50% of the pellet length. The fibres and the thermoplastic polymer are preferably as discussed in connection with the method. The pellets are preferably obtainable with the method according to the invention. In a preferred embodiment, the fibres in the pellet are selected from the group of glass fibres, natural fibres and carbon fibres. The dimensions of the pellets are preferably as discussed for the method. In a preferred embodiment, the inventive pellets have a homogenous fibre fraction. In an example embodiment, the inventive pellets have a standard deviation in fibre content that is less than 20%, or even less than 10% of the mean fibre content, with the standard deviation as fraction (percentage) of the mean fibre content value, based on samples of typically 100 mm³, more typical 50 mm ³ and most typical 25 mm ³ of the extruded material (pellets) and based on wt.% fibres as fibre content. As an example, the mean fibre content is e.g. 50 wt.% and the standard deviation is less than 10 wt.% (20% of 50wt.%). Preferably, the pellets are an LFT material obtainable with the method, more preferably with a ratio of D50/I of at least 20% and/or D90/I of at least 50% or at least 80% as discussed hereinabove. In conclusion the invention pertains to a method of producing long fibre thermoplastic (LFT) extruded material using an extruder with a first inlet for thermoplastic material and a second separate downstream inlet opening for TPC feed material. Preferences for the extrusion system apply also for the extrusion system used in the method. Preferences for the materials used in the method apply also to the inventive pellets. Preferences for the inventive pellets apply also for the product obtained with the method. The verb “comprising” as used herein indicate that other elements (components, steps, features) other than those recited may additionally be present. Examples The invention will now be illustrated by the following Example(s) which are illustrative only and do not limit the invention or the claims. Example 1 The extrusion method was tested for various samples to produce pellets as indicated in Table 1, using glass fiber (GF) or carbon fiber containing tape and other thermoplastic composite (TPC) feed material, and an extruder setup MF as illustrated in Fig. 6. In sample #1, heated material was used. The feed thermoplastic granules were of the same type of thermoplastic polymer as used in the tape / TPC material (PP or PA6). The fiber length in the product is expressed as the max. length that 50% (D50) respectively 90% (D90) of the fibers have, percentage by frequency (fiber shape length distribution histogram; see e.g. Fig.7 for a histogram). I.e, 90% respectively 50% of the fibers have a smaller length, percentage by total number of counted fibers. For D90, 10% of fibers have a fibre length longer than the D90 value. These long fibers are important for the (mechanical) properties of the product, e.g. pellet, and the articles manufactured from the pellet. Table 1A shows the fibre length retention by the ratio of the product fibre length (D50 respectively D90) to the input length I. Fiber length was determined by burning pellets and optical image analysis using Fibreshape™ software and a FibreShape M scanner. The tape starting material contains continuous fibers aligned in the tape direction; cutting the tape into flakes yield an initial fiber length equal to the flake length; the flake length is the input length. In contradistinction, cutting TPC starting material such as fiber mats or laminates or woven parts/production waste lead to a broad range of input fiber length. The results demonstrate that good fiber length retention was achieved. In Table 1, nozzle diameter refers to the die. Input length indicates initial fibre length, in the example the length of the chips / flakes fed to the extruder. For a reference extruder process with various starting materials, the D90/I ratio was typically 17% to 32%.