WO2024172647A1

WO2024172647A1 - Fibre-containing thermoplastic material extrusion with extension unit

Info

Publication number: WO2024172647A1
Application number: PCT/NL2024/050071
Authority: WO
Inventors: Amandine Marie Floriane Magali-Sophie Codou; Ali Rezaei; Jozef Maria Herman Linsen; Rick Adriaan Coleta LEUVEN
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno; Chemelot Scientific Participations B.V.
Priority date: 2023-02-13
Filing date: 2024-02-13
Publication date: 2024-08-22

Abstract

The disclosure pertains to a method of producing an extruded material, the method comprising extruding a mixture comprising a thermoplastic component and fibres through a die and subsequently through an extension unit.

Description

P134022PC00 Title: FIBRE-CONTAINING THERMOPLASTIC MATERIAL EXTRUSION WITH EXTENSION UNIT Field The invention pertains to making fibre-reinforced material, in particular with extrusion. Embodiments of the invention pertains to a process for the production of long fibre thermoplastic (LFT) extruded material, preferably for producing free- flowing LFT pellets. The pellets are for instance suitable for moulding, e.g. for injection moulding. The extruded material and pellets are made using thermoplastic composite (TPC) feed material, which is for instance recycle material. The invention also pertains to equipment for extrusion processes. The invention also pertains to processes with short fibers. Introduction The present invention pertains, in an embodiment, to a process for recycling thermoplastic fibre composite parts into long fibre reinforced thermoplastic parts, for example, for recycling of long fibre reinforced thermoplastic parts or continuous fibre reinforced thermoplastic parts. Long fibre reinforced thermoplastic parts are parts typically comprising long fibres embedded in a thermoplastic matrix. In some embodiments, the material to be recycled is provided by continuous fibre thermoplastic component parts, which are used for numerous applications including automotive, and construction, for example. Such parts contain, for example, glass fibres or carbon fibres as continuous fibres, and one or more thermoplastic components. It has been proposed to recycle thermoplastic composites by conversion into long fibre thermoplastics (LFT), also known as long- fibre-reinforced thermoplastic (LFRTs). These are types of easily mouldable thermoplastic used to create a variety of components used for example in the automotive industry. The invention is in particular directed to the production of pre-compounded, pelletized, LFTs. LFTs are typically used for making parts by moulding (e.g. injection moulding and compression moulding). In particular mass production is desired. Hence, the LFT product is preferably obtained in the form of discrete solid pieces, such as e.g. pellets, which can be supplied to a unit for producing parts. The LFT material should be suitable for dosing into the unit, in particular it should be possible to dose the LFT material with a hopper. Advantageously, the (recycled) LFT pellets produced with the method of the present invention can be free-flowing. The potential applications of the LFT pellets include, without limitation, the manufacture of parts such as, for example, dashboards, bumpers and seats for cars and vehicles, parts for medical equipment, computer housings, and for furniture and facade elements for buildings to ski boxes for cars. Other applications are also possible. The present invention accordingly pertains in an aspect to a process of making long fibre thermoplastic (LFT) material, in particular for making LFT extruded material (extrudate), more in particular in the form of LFT pellets. Such LFT pellets are, for instance, 2 to 25 mm long, or 5 to 15 mm long, with a diameter of for instance 1-10 mm, e.g.1-5 mm, and have for instance a cylindrical shape. The individual LFT pellets comprise fibres (e.g. carbon fibres, glass fibres or natural fibres, or other types of fibres) as well as thermoplastic polymer. The pellets contain fibres with a length of e.g. at least 0.3 mm, or e.g. at least 0.5 mm, at least 1.0 mm, or preferably at least 2.0 mm, and typically up to 50 mm. The fibres have for instance a diameter of at least 1.0 µm or at least 2 µm or at least 5 µm and/or up to, for example, 100 µm, or max. 50 µm, or max 30 µm. The diameter is for example 1.0-100 µm, or 2-50 µm and preferably 5-35 µm, even more preferably 10µm to 25µm. The LFT extruded material produced with the present invention can for instance be used for producing a wide array of products comprising LFT materials, for example, without limitation, parts useful for automotive applications, for example door panels, instrument carrier or front-end of vehicles, and also for parts useful in other fields, such as e.g. electronics. The LFT extruded material, in particular as pellets, is for instance used as a feedstock for moulding (e.g. injection moulding) to form LFT products. The LFT extruded material is for instance cut to pellets and the pellets are used for moulding (e.g. injection moulding). In other words, it is desirable that the pellets obtained from the cutting of extruded strands are free flowing and have a smooth surface. Additionally, the pellets should impart good mechanical properties on articles made by moulding (e.g. injection moulding) of the pellets. Moreover, a desire in the art is that the pellets have a uniform size, in particular in terms of diameter. It is often desirable that the LFT pellets are free-flowing, for instance for transport and for dosing into an apparatus, such as e.g. with a hopper. For instance the LFT pellets are to be dosed into moulding equipment, e.g. into injection moulding equipment. Accordingly, flowability of the pellets is important for the processability of the material. The fibre length distribution in the pellets is important for the quality of the product. Conventional manners of making LFT pellets are wire-coating and pultrusion. Some prior art documents describe extrusion processes. However, in practice, it turns out that when manufacturing LFT pellets using discontinuous fibers, frequently extrudate is obtained with a very rough surface, with the result that pellets made from it are not free-flowing. In particular, comparative pellets (PP/ carbon fibre) were obtained with a known (reference) extrusion process. The surface of the pellets was very rough and the shape is irregular; moreover, the pellets are not-free flowing. In particular, fibres were observed to extend (protrude) from the surface of these reference pellets. Hence, it is desired to provide methods to prepare LFT extrudate with better quality of the surface, regular shape, and good fibre length retention; and to provide suitable apparatuses for such methods. In an attractive example embodiment, which does not limit the invention, the inventive method uses thermoplastic composite (TPC) feed material as part of the feed for the extrusion process, wherein the TPC feed material is derived from CFRT (Continuous Fibre Reinforced Thermoplastic) material, for instance by comminution of (e.g. shredding) CFRT waste material. Hence, embodiments of the invention pertain to the recycling of CFRT waste material into LFT pellets with a smooth surface that are suitable for moulding, e.g. for injection moulding. However, other sources of the TPC material are also possible, for instance chopped fibre reinforced thermoplastic composites, or fibers recovered from wind mill industry. EP3974138A1 describes methods of producing long fibre thermoplastic (LFT) extruded material, wherein the method comprises providing thermoplastic feed material and thermoplastic composite (TPC) feed material, for example flakes or chips. WO2022066021A1 describes methods 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 thermoplastic composite feed material. US20190184619A1 describes a method of manufacturing a long fibre reinforced thermoplastic filament wherein a mixture of fibrous material and thermoplastic material is introduced into an extruder and through an extensional flow die to preserve longer fibre lengths. The extensional flow die is a gradual angle convergent die. Summary The invention in a first aspect provides a method of producing an extruded material, wherein the method comprises extruding in an extruder a mixture comprising a thermoplastic component and fibres through a die. The invention also provides a suitable extruder and extrusion apparatus. In a preferred embodiment of the invention, the extension unit is configured for cooling a strand of the extruded mixture while confining the strand. The invention pertains to an extruder comprising a barrel, a screw, and a die, wherein the die is provided at the outlet side with an extension unit providing a channel for flow of the extruded mixture. The channel has a length Lc and a mean diameter Dc over said length Lc, wherein said mean diameter Dc is preferably at least 0.5 mm and said length Lc is preferably at least 5 times said mean diameter Dc, or at least 10 times, or at least 15 times, and wherein said channel preferably has less than 5 % deviation in the diameter from said mean diameter Dc over said length Lc. Preferably the mean diameter Dc is at least 1.0 mm, e.g. between 1.0 mm and 25 mm, or preferably between 1.0 and 10 mm, more preferably between 1.0 and 5.0 mm, or between 1.0 and 4.0 mm. An aspect of the disclosure provides a method of producing an extruded material, the method comprising extruding a mixture comprising a thermoplastic component and fibres through a die and subsequently through an extension unit. Brief description of the drawings Figure 1 schematically illustrates a section view of an example inventive extruder. Figure 2 schematically illustrates a system comprising an inventive extruder and a cooling bath. Figure 3 shows photographs of a strand and a pellet produced with a comparative extrusion method without the extension unit. Figure 4 shows photographs of a strand and a pellet produced with an inventive extrusion method with the extension unit. Figure 5 shows photographs of a strand and pellet obtained with an inventive extrusion method with the extension unit. Any embodiments illustrated in the figures are examples only and do not limit the invention. Detailed description The method advantageously allows for obtaining pellets with improved surface quality and uniform size distribution, and wherein the pellets are free- flowing. Surface quality refers in particular to improved smoothness of the surface. Moreover, pellets can be obtained with uniform cross-sectional size and shape. The invention pertains to a method of producing an extruded material, for example in the form of extruded strands, filaments, or in the form of pellets. The method comprises extruding a mixture comprising a thermoplastic component and fibres in an extruder. The method hence comprises supplying a feedstock to an extruder. The extruder is for instance a single screw or double screw extruder. A single screw extruder is preferred, to maintain the fiber length. The extruder may have one or more inlets for the feedstock. The feedstock components may be combined prior to the supply to the extruder, or in the extruder, or in a combination thereof, to provide said mixture. The feedstock comprises a thermoplastic component and fibres. In some embodiments, the fibres are provided as dry fibres, e.g. as fibres not embedded in thermoplastic material. The feedstock preferably, but not exclusively, comprises thermoplastic composite (TPC) feed material. The TPC feed material is generally provided in the form of discrete solid pieces, for example in the form of flakes or chips. Without limitation, flakes for example have a thickness of at least 0.1 mm, e.g. up to 5 mm, and a length/width of e.g. 1 mm and/or to 50 mm. The shape of the feed material is not particularly restricted but the feed material should be suitable for extrusion. The TPC feed material is for example provided as a conglomeration of discrete solid, macroscopic particles. A thermoplastic composite (TPC) material, as used herein, includes 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. 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. Additives are for instance part of the family of antioxidants, UV stabilisers, flame retardants, pigments, dyes, dispersing agents, adhesion promoters, for example modified polypropylene, in particular maleated polypropylene, antistatic agents, mold release agents, nucleating agents etc. The additives can be in the plastic(s) to recycle or can be added in the extruder or both. The materials can also contain reinforcing additives and/or reinforcing additives are added to feedstock. Reinforcing additives include, for example, inorganic reinforcing agents such as talc, high aspect ratio talc, mica, short glass fibres and glass, or organic reinforcing agents such as aramid fibres, polyester fibres, and carbon fibres. The feedstock comprises fibres, in an embodiment in particular “long fibres”, for instance embedded in a thermoplastic matrix. The fibres preferably have a length of e.g. at least 0.5 mm, at least 1.0 mm, or preferably at least 2.0 mm, and for example up to 100 mm, or up to 50 mm, or e.g. up to 10 mm or e.g. up to 15 mm. The fibres have for instance a diameter of at least 1.0 µm or at least 2 µm or at least 5 µm and/or up to, for example, 100 µm, or max. 50 µm, or max. 30 µm. The fibers are e.g. glass fibers, carbon fibers, or natural fibers, and can be selected by the skilled person as appropriate or feedstock having the appropriate fibers is selected. These lengths and diameters are average (mean) values, in particular number average. Preferably, the TPC feed pieces individually comprise such fibres embedded in a thermoplastic matrix. Advantageously in the inventive method, the average fibre length can be maintained in the extrusion step above 0.5 mm or above 1.0 mm. Maintenance of sufficient fibre length can be assisted by using a low shear extruder and preferably a low compression extruder. An example method wherein a suitable composite is extruded with fibre length above 1.0 mm after processing is described in EP 3974138, in particular Example 1 of that document, and in WO 2022/066020. Without wishing to be bound by way of theory, the presence of fibres of at least 0.5 mm or at least 1.0 mm in the extruded material, e.g. strand, at the time of release of the extruded material (e.g. strand) from the die may result in poor surface quality of pellets obtained by size reduction of the material (e.g. strand) with comparative methods without the extension unit; however the presence of a sufficient amount of fibres with such length in the pellets is highly desirable for the downstream uses of the pellets. In a further embodiment, the fibers in the feedstock and/or in the extruded material are shorter fibers. Various advantages of the extension unit, such as flexibility, are also useful for embodiments wherein short fibers are used in the feedstock. A high surface quality can advantageously also be obtained with the inventive method for short fibers; as well as advantageous stable and uniform geometrical dimensions. It is also observed fibers in the extruded material will have a fiber length distribution in embodiments, with shorter fibers and longer fibers. Also the molten material in the barrel may have a fiber length distribution in embodiments with shorter and longer fibers. The inventive method is especially, but without limitation, advantageous for embodiments with long fibers in the feedstock and in the extruded material. The fibres are preferably made from a different material than the thermoplastic component. The fibres are for example not thermoplastic. The fibres, for instance, have no melting point, or a melting point above 360ºC. The feedstock may comprise two or more types of fibres, differing e.g. in one or more of material and fibre dimensions (length and/or diameter). Possible materials for the fibres include, for example, metal fibres, glass fibre, carbon fibres, boron fibres, ceramic fibres (e.g. alumina or silica), aramid fibres, synthetic organic fibres (e.g. polyamide, polyethylene, paraphenylene, terephthalamide, polyethylene terephthalate and polyphenylene sulfide). Natural or synthetic inorganic or organic fibre material can also be used. Glass and carbon fibres are for instance particularly suitable. A combination of glass and carbon fibre is also possible. The fibres in the feedstock may be twisted or straight. The fibres may be in the form of single fibres or bundles of fibres. The feedstock may comprise two or more types of granular TPC materials, wherein different TPC pieces contain different types of fibres, or wherein individual TPC pieces contain different types of fibres. The fibres have preferably a higher tensile strength than the thermoplastic component. The fibres optionally, in some example embodiments, have a tensile strength of at least 1000 MPa, or at least 2000 MPa or at least 3000 MPa. Conventional measurement methods as defined in ISO standards can be used, optionally according to ISO 23523:2021 for polymer fibre, ISO 10618:2004 for carbon fibre, or ISO 9163:2005 (short method) for glass fibre. The fibres have preferably a higher tensile modulus of elasticity than the thermoplastic component. The fibres optionally, in some example embodiments, have a tensile modulus of at least 20 GPa, or at least 50 GPa, or at least 100 GPa, e.g. with glass or carbon fibres. Conventional measurement methods as defined in ISO standards can be used, optionally according to said standards. A single type of fibre can be used, or two or more types of fibres can be combined. In case of different types of fibres, these can be pre-combined, or be combined in the extruder. The extruded material, i.e. the product, is preferably an LFT material (long fibre thermoplastic material). The material comprises, or substantially consists of, or even essentially consists of, one or more thermoplastic polymer materials as matrix and long fibres embedded in said thermoplastic matrix. Preferably, the fibres of the extruded material have a mean length of e.g. at least 0.5 mm, at least 1.0 mm, or preferably at least 2.0 mm, or at least 3.0 mm, and for example up to 50 mm or up to 25 mm or up to 10 mm, e.g. in the range 1 to 5 mm; mean length indicating the mean based on number of particles. Preferably, these long fibres are embedded in the thermoplastic material in the extruded material. A fibre length of at least 1.0 mm may contribute to good mechanical properties of components prepared from the pellets by e.g. moulding, such as e.g. good strength of such components. A higher fibre length may contribute e.g. to strength and toughness. For example, the fibres are aligned or orientated in the extruded material. Both in the extruded material and in the TPC feed material, the polymer can be, for instance, a raw polymer or a polymer compounded with additives, as well as a recycled polymer. The recycle polymer is, e.g., a polyolefin comprising PP and/or PE. The feedstock may also comprise thermoplastic granules or particles, or combinations thereof. The feedstock may also comprise dry fibres and fibre fragments (i.e. fibres not embedded in thermoplastic material). For example fibers recovered from the recycling of wind turbine blades, boats, or PCT boards. The various components of the feedstock, in particular the discrete solid pieces, may be supplied to a single inlet of the extruder, or be distributed over one or more inlets of the extruder. The feedstock comprises, for example, less than 5.0 wt.% or less than 2.0 wt.% or less than 1.0 wt.% water. This may provide a difference with methods wherein a high moisture feedstock is used, as is the case for instance when using (natural) fibres without sufficient prior drying. Suitably a drying step is used to obtain a low moisture content of the feedstock. 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, relative to total weight of TPC feed material. These amounts apply in particular to fibres with diameter of at least 1.0 µm or at least 2 µm or at least 5 µm and/or with a length of at least 2.0 mm or at least 5.0 mm. Additionally, these preferred amounts apply to synthetic fibres such as e.g. glass fibre, carbon fibre, and synthetic polymer fibre. These preferred amounts may also apply to natural fibres. The total feedstock comprises e.g. at least 1.0 wt.% fibres, or at least 5 wt.%, or at least 10 wt.%, or at least 20 wt.%, or at least 25 % fibres, for example up to 60 wt.% fibres, or for example up to 70 wt.% fibres, especially of fibre having the above-mentioned preferred length and/or diameter, again as percentage of total feedstock. Additionally, these preferred amounts apply to synthetic fibres such as e.g. glass fibre, carbon fibre, and synthetic polymer fibre; and may also apply to natural fibres. Preferably, in a non-limiting embodiment, in the extruded material, more preferably in the pellets, the fraction of fibres with a length of at least 1 mm is at least 5 wt.% of the extruded material, or at least 10 wt.%, or at least 20 wt.%. The same preferences apply for the feedstock. However, other sizes of the fibers in the extruded material are also possible, e.g. when the feedstock contains long fibers which are to some extent reduced in size, or when the feedstock contains short fibers. The preferably used TPC feed material comprises for instance discrete solid pieces of TPC material. The TPC feed material is for instance provided to the extruder as a stream of solid pieces of TPC material. Preferably in the TPC material, the fibres are embedded in the thermoplastic material. In some embodiments, the fibres are orientated, or even aligned, in the solid pieces. In a preferred embodiment, the feedstock comprises TPC pieces, preferably flakes and/or chips, with a length of 2 mm to 50 mm, a width of 2 mm to 50 mm and a thickness less than the length and/or less than 5.0 mm; for instance with a thickness of at least 0.1 mm and/or up to 5.0 mm or up to 4.0 mm. The feedstock for example comprises TPC chips having 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 feedstock for example comprises TPC flakes having 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. These TPC pieces preferably comprise fibres with a mean length of at least 1.0 mm or at least 2.0 mm or at least 5.0 mm, in an amount of preferably at least 10 wt.% or at least 20 wt.% or at least 50 wt.% relative to total weight of the TPC pieces. For instance an example laminate contains 60 wt.% fibers. The mean length is e.g. the number-based average. A broad distribution of fiber lengths may be observed in practice. These TPC pieces are preferably at least 10 wt.% of the total feedstock, e.g. at least 20 wt.% or at least 30 wt.%, or at least 50 wt.% or at least 70 wt.% or at least 90 wt.% of the feedstock. The inventive method is in particular advantageous, without limitation, in embodiments wherein the feedstock comprises, as TPC pieces, shredded, ground, or crushed CFRT (Continuous Fibre Reinforced Thermoplastic) material. The inventive method is also advantageous for mixtures of dry fibers, or coated fibers, and thermoplastic granules, again without limitation. In some embodiments, the method furthermore comprises a step of preparing TPC granules that are used to provide at least a part of the feedstock. This step may involve, for instance, comminuting (e.g. shredding, grinding) CFRT (Continuous Fibre Reinforced Thermoplastic) material. Comminuting indicates a size reduction step, such as e.g. shredding, crushing, or grinding, to give solid pieces having preferably the above-mentioned minimum dimensions. The CFRT material is, for example, an end-of-life thermoplastic composite part, or for example obtained as production waste from a process of making CFRT sheet material. Other types of CFRT material are also possible. The TPC feed material comprises, for example, the separated side edges (e.g. trims) of impregnated fibre layer, such as e.g. impregnated fibre tape or sheet. Such tapes are an intermediate stage in certain production methods for CFRT material. The TPC feed material can also comprise, e.g., chips issued from size reduction, e.g. shredding and grinding, of CFRT materials such as e.g. CFRT sheets or CFRT articles, such as e.g. CFRT car parts. Combinations of various types of TPC feed material are also possible. The CFRT material is, for example, coming from post-industrial processed semi- finished such as offcut of laminates, or finished such as parts which are overmolded, thermoformed, injection molded, compression molded. In an example embodiment, which does not limit the invention in any way, the optional step of preparing the TPC pieces involves providing a plurality of dry continuous fibres in a sheet; impregnating said sheet of fibres with a thermoplastic resin to give an impregnated fibre continuous sheet having edges; separating at least part of said edges from said impregnated fibre continuous sheet to give flakes and a trimmed continuous sheet. The flakes in this example embodiment are separated edge pieces and the flakes individually contain fibre and thermoplastic material. The flakes are used as at least part of the TPC feed material in this example embodiment. Optionally the trimmed continuous sheet is size reduced (cut) into sheets or tapes, and optionally laminates are made 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. The feedstock comprises a thermoplastic polymeric component, as a component of the TPC feed material, as separate thermoplastic feed material, or as a combination thereof; accordingly the feedstock may comprise one or more of such thermoplastic polymeric components, e.g. a combination of fibers coated with thermoplastic coating and thermoplastic granules. The thermoplastic polymer is for instance, but without restriction, one or more polymers selected from the group consisting of polyolefin, polyamides, polycarbonate, polyphenylene sulphide, polyaryletherketone, polyethylene terephthalate, polybutylene terephthalate, polyester, polyethylenimine, polyether sulfone, polyoxymethylene, polyetherimide. The thermoplastic polymer is for instance polypropylene or polyethylene. The term “thermoplastic polymer” as used herein includes at least plastic polymers that become pliable or mouldable at a certain elevated temperature and solidify upon cooling. In some embodiments, which do not limit the invention, the thermoplastic polymer is not cross-linked and is not cured. The thermoplastic polymer has for instance a glass transition temperature and/or a melting temperature below 400ºC, or below 360ºC, or below 300ºC. The TPC feed material comprises, for example, substantially only one thermoplastic polymer, e.g. at least 90 wt.% of the thermoplastic fraction of the TPC feed material is a single polymer. The TPC feed material may also comprise a blend of polymers or blends of various grades of the same polymer. Furthermore, the TPC feed material may comprise a mixture of TPC granules having different polymeric components. Optionally, 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 component, for instance has an MFI of at least 1, or at least 5, or at least 10 g/10min, or at least 20 g/10min, ideally above 50 g/10min, or even above 100 g/10 min, with MFI measured according to ISO 1133-1. The thermoplastic component for instance comprises or consists of a polyolefin, e.g. polypropylene, with MFI of at least 50. The lower viscosity may advantageously contribute to good mixing and improved mechanical properties. The feedstock and the extruded material preferably comprise, independently, at least 10 wt.% of the thermoplastic material, e.g. polyolefin, for instance at least 20% or at least 40%, and/or for example up to 95 wt.%. In the extruded material, the thermoplastic component is typically a matrix for the fibres. The feedstock and extruded material may comprise further components, for example additives, for instance friction reducing additives or flow enhancers. In some embodiments, the thermoplastic polymeric component is provided as solid pieces comprising at least 90% or at least 95 wt.% thermoplastic polymer. The extruder, as used in the method of the invention, and as provided by an aspect of the invention, comprises a barrel, a screw (e.g., single screw or double screw), and an outlet opening provided with a die. The screw provides a channel in the extruder and can be rotated in the barrel. The screw(s) typically comprises a cylindrical shaft and a flight, typically one or more helical flights. The shaft is arranged in the extrusion direction. Optionally, 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. Preferably, TPC feed material is selectively supplied to one or more inlet openings, downstream of a first inlet opening. Optionally the material is fed by a crammer feeder, optionally the material is fed solid or in the molten state. The barrel is for instance maintained at a temperature above 100ºC, or above 150ºC, or above 200ºC, and generally above the melting point of the thermoplastic component. The screw diameter is e.g. at least 10 mm, or at least 20 mm; much larger sizes are also possible, e.g. above 100 mm. The method involves extruding the feedstock in the extruder. Optionally, the feedstock or part thereof is pre-heated. The die in an embodiment is a die-plate. In a preferred embodiment, the die comprises a converging channel for the extruded mixture, i.e. for flow of the material being extruded, and preferably comprises a tapered converged channel; i.e. converging or tapered in the extrusion direction. Preferably, the die provides a curved channel, which may advantageously contribute to fibre length retention. A curved channel in particular refers to a channel that is curved in a cross-section parallel the extrusion direction, e.g. parallel to and through the axis of the screw. The channel is for instance curved, in said cross-section, with a radius of curvature larger than 2.0 mm or larger than 5.0 mm; this curved section of the channel preferably provides for a reduction of the flow area of the channel with at least 50% (flow area is the cross-sectional area of the channel perpendicular to the extrusion direction). A die optionally has a curved part that is used to provide a reduction of the flow area of the channel. The die has an inlet for the material to be extruded and an outlet for said material. The outlet has a smaller cross section than the inlet. The die provides for a passageway (channel) for the extruded mixture with a reduction of the cross sectional area of the passageway, of typically at least 50%, more preferably at least 80% or at least 90%, e.g. a cross-sectional area reduction ratio of at least 2, or at least 5, or at least 10, or at least 20, or at least 50, or even higher, between the cross-sectional area at the inlet and at the outlet of the die; for cross-sections perpendicular to the extrusion direction. Preferably, downstream of the die, there is no restriction of the cross- sectional area of the passageway for extruded material, up to the size reduction unit (e.g. pelletizer) if used, of more than 50%, or more than 20%, or more than 10%, relative to the cross-sectional area at the outlet of the die. In case of a plurality of parallel channels in the extension unit or otherwise downstream of the die, the cross-sectional area refers to the total cross-sectional area of the channels. Preferably, any restriction of the flow area for extruded material in the extension unit is less than 20% or less than 10%, relative to the flow area at the outlet of the die; i.e. the flow area is at least 80% or at least 90% over the length of the channel, relative to the flow area at the die outlet, and based on total flow area in case of parallel channels (flow area indicating the cross-sectional area perpendicular to the flow direction). In an example embodiment, any part of the channel between the die and the extension unit and between the extension unit and the size reduction unit provides for a restriction of the flow area of 0 – 20%. It is noted that constant flow area is included in said range. In some embodiments, the variation in flow area for extruded material in the flow line for extruded material from the die to the size reduction unit, or in the extension unit, is less than 10% or less than 5% (increase or decrease) relative to the flow area at the outlet of the die. A relatively constant flow area in the extension unit provides for a uniform flow velocity of extruded material, which is advantageous. In some embodiments, the die is releasably attached to the barrel, for instance using bolts and nuts. In the invention, the die is provided with an extension unit, which can also be referred to as an extension member, that is arranged at the outlet side (downstream side) of the die. The extension unit for instance is provided by one or more separate elements, i.e. elements separate from the die. The extension unit for instance comprises two or more elements that preferably are distinct from the die. An extension unit that is a separate element and that is attached to the die, e.g. mounted on the die, is preferred. In particular, the extension unit is preferably a tubular element. This may allow for easier manufacture with a relatively large length of the channel, compared to channels that are drilled in a die. The separate extension unit also allows for better control over L/D, lower costs, better control over the melt temperature in the outlet (it can be easily cooled or heated), and easier to maintain and assemble and provides better modularity. The extension unit provides for an elongation of the confined channel for extruded material, said confined channel originating in the die. Accordingly, the extension unit provides a conduit (e.g. tube) for the extruded material, said conduit comprising or consisting of a wall, e.g. a tubular wall. If a preferably extension tube is used, the tube may be provided by one or more parts in the length direction of the tube. The method involves extruding the mixture through the die and subsequently through the extension unit. Hence, the extension unit is arranged downstream of the die. Preferably, the extension unit is directly adjoined to the die. Placing the extension unit close to the die is preferred to optimally benefit from cooling of the extruded mixture in the extension unit. The extension unit is configured for cooling the mixture from the die, i.e. for cooling the extruded mixture, while confining that mixture, i.e. while confining the extruded material, e.g. while confining a strand of the extruded mixture. Preferably the extruded material at the inlet of the extension unit is at a temperature higher than the melting temperature of the thermoplastic component(s) of the extruded material. The extension unit is configured for cooling extruded material, e.g. one or more strands of the extruded mixture. The extension unit comprises a channel, e.g. one channel or two or more parallel channels, for flow of the extruded mixture (extrudate flow channel), in particular for flow of the extruded material (e.g. strand) through the channel (or more broadly, transport of the extruded material through the channel). Hence, the extension unit may be provided as one or more ducts for the extruded material. A duct comprises a channel and a duct wall, this wall can also be referred to as the wall of the channel. The extension unit may comprise a manifold for distributing the extruded mixture to multiple parallel channels, for instance if the extension unit has a single inlet for the extruded material and multiple outlets. Alternatively, the extruder may have a plurality of extension units in parallel, wherein each extension unit is e.g. a tube with a single channel. In cross-section perpendicular to the flow direction, the channel has for instance a convex shape, i.e. given any two points in the cross-section, the cross- section of the channel contains the whole line segment that joins them. For example, the cross-section is a circle, or has another shape. The term ‘flow’ of the extruded material, as used herein, includes transport of both a fluid extruded mixture and transport of a partially or completely solidified strand of extruded material. Furthermore, a strand of extruded material, as used herein as an example type of extruded material, includes both a strand of solidified extruded material and a strand of not or only partially solidified extruded material. Extruded material, as used herein, includes material that is in the process of being extruded, and material that is already extruded. The channel in the extension unit contains a wall which completely contains the extruded material (strand of extruded material) in the circumference. In embodiments, the extruded material is partially or completely solidified in the extension unit by the cooling. For example, at least the surface of the extruded material solidifies in the extension unit. For example, at least the surface of the extruded material (e.g. strand) reaches a temperature below the melting temperature of the thermoplastic component in the channel of the extension unit. For instance, at least the surface of the extruded material (e.g. strand) reaches a temperature at least 5ºC, or at least 10ºC, or at least 20ºC, or even better at least 30ºC below the melting temperature of the thermoplastic component in the channel of the extension unit; in case of blends, lower than the thermoplastic component with the lowest melt temperature. The confinement of the extruded material in the channel of the extension unit may help avoid deformations of the surface of the extruded material by protrusion of fibres. Preferably, at least at an upstream part of the channel (e.g. upstream 10% of the length of the channel, or upstream 90% of the length of the channel), the wall is in touching contact with the extruded material over the circumference of the wall; more preferably the wall is in touching contact with the extruded material over the entire length of the channel. The wall thereby provides for confinement and stabilization of the extruded material. The wall may contain coating of the surface exposed to the extruded material, e.g. a coating of the channel. Preferably, the extruded material is at a temperature above the melting point of the thermoplastic component at the outlet of the die and at the inlet of the extension unit. The method preferably involves cooling the extruded mixture while the mixture passes through the channel and hence cooling of the extruded mixture (e.g. strand) while confined in the channel. Without wishing to be bound by way of theory, this may allow for a sufficient degree of solidification of the extruded mixture while the mixture is radially constrained in the channel, by the channel wall, thereby preventing undesirable deformation of the extruded material after release from the extruder. The extension unit may comprise, for example, one or more tubular elements, said tubular elements providing the channel(s). The channels are preferably straight, to avoid deformation of the extruded material (e.g. strands). In particular, the channels preferably have no bends in the length direction (axial direction). The diameter of the channel may be constant over the length of the channel, or may vary. For instance, the channel is converging, in the flow direction, in an upstream part and has constant diameter over a downstream part. In a broad aspect of the invention, the geometry of the channels is not particularly restricted, for instance in combination with the cooling. Certain preferred geometries of the channels are as described hereinafter. Good results have been obtained for such channels as shown in the Examples. The channel typically has a length of at least 5 times, or at least 10 times, or at least 20 times, or at least 30 times, or at least 40 times, the diameter of the channel. The diameter of the channel refers to the inner tube diameter, if the channel is provided by a tube. This relatively high ratio of length to diameter may contribute to sufficient cooling of the extruded material in the channel and thereby for dimensional control of the output extruded material, e.g. strand. A higher ratio of length to diameter (L to D) can be beneficial for obtaining higher quality extrudate. A larger L/D ratio can be especially useful for e.g. carbon fiber, e.g. a L/D ratio of at least 30. For glass fiber, an L/D ratio in the range of 10-40 was found to give good results. Example 3 demonstrates excellent surface quality obtained with high L/D values in combination with high thermal conductivity of the tube used as the extension unit. The use of tubes or pipes for the extension unit permits the use of high L/D ratios, and relatively small diameters, compared to drilled or machined metal pieces. Such L/D ratios were found to give good results (see the Examples). Diameters that are not too large are advantageous for the size of pellets and strands; e.g. tubes with such inner diameter can be used. Preferably, the diameter of the channel, in particular the mean diameter Dc, is at least 0.5 mm or at least 1.0 mm, and for instance less than 50 mm, or less than 40 mm, or less than 30 mm, or less than 20 mm, or less than 10 mm. For example, the mean diameter Dc is between 1.0 mm and 25 mm, preferably between 1.0 and 10 mm, more preferably between 2.0 and 5.0 mm. The mean diameter Dc is the mean diameter over the length of the channel. Diameters in these ranges are in particular useful for making pellets that can be used e.g. for (injection) moulding, and for using the extruded material for additive manufacturing. These diameters are preferably combined with said channel having a length of at least 5 times, or at least 10 times, or at least 20 times the mean diameter Dc of the channel. The channel has for example a circular cross- section. In case the channel has a non-circular cross section, the diameter refers to the equivalent area diameter. Preferably, the channel has a length of at least 2.5 mm, or at least 3 mm, at least 5 mm, at least 10 mm, or at least 20 mm or at least 50 mm, or at least 100 mm, and, optionally, for example, less than 50 cm or for example less than 20 cm. The channel of the extension unit typically (but not exclusively) has a substantially constant diameter over the length of the channel, said length preferably being in the above-mentioned ranges. Herein, substantially constant indicates less than 20%, or less than 10%, or less than 5% deviation from the average (mean) diameter of the channel over said length, more preferably less than 1%. Preferably, the channel has an angle of convergence in the range 0º - 10º, or in the range 0º - 5º as average over the length of the channel. In some embodiments, the channel has, at any position over the downstream 50% of the channel in the length direction, or over the entire length, an angle of convergence in the range 0º - 10º, or in the range 0º - 5º. Preferably, the flow area of an individual channel is, at any position of the channel in the length direction over said length of the channel, not less than 80% of the flow area at the inlet of the channel, preferably in the range of 80 – 120%. The method preferably involves cooling the extruded mixture during the passage of the mixture through the extension unit, in particular during the passage of extruded mixture, e.g. a strand of the extruded mixture, through the channel. The cooling is preferably performed by indirect heat exchange with a coolant fluid (liquid or gas), more preferably with a forced flow of coolant fluid. The coolant fluid is optionally a gas. The coolant fluid is preferably a liquid, at least at the inlet of the extension unit for cooling fluid. Indirect heat exchange indicates that the extruded material (e.g. a strand) and the cooling fluid are in contact with different sides of a conduit wall (the channel wall). Hence, the indirect heat exchange is through the wall of the channel. In operation, the extruded mixture is in contact with the inner surface of the wall, and the cooling fluid is in contact with the outer surface of the wall. For instance, in an embodiment, the outer surface of the wall is in contact with ambient air in operation, or for example with a cooling liquid. In further embodiments, the method involves cooling the extruded mixture during the passage of the mixture through the extension unit, in particular during the passage of extruded mixture, e.g. a strand of the extruded mixture, through the channel. Preferably, the extruder further comprises a cooling unit for cooling the extruded mixture during the passage of the mixture through the extension unit, preferably having an inlet and outlet for a fluid, e.g. a cooling fluid (coolant), and a chamber for receiving the fluid (e.g. cooling fluid) in indirect heat exchanging contact, i.e. through a heat-exchanging wall, with at least a part of the channel for extruded material of the extension unit. In embodiments with heating of the channel, the fluid is a heating fluid. Generally, the fluid can be used as a heat transfer fluid. The (cooling) fluid and extrudate are preferably separated from each other to prevent mixing. The chamber for the fluid is for instance provided as an annulus around the wall of the extrudate flow channel. The extruder may hence comprise a conduit for the fluid. The extruder may be configured for forced flow of the heat transfer fluid through the cooling unit, for instance using a fan, compressor or pump. In embodiments with cooling using a cooling fluid, the cooling fluid preferably has, at least at the inlet of the cooling unit, a temperature that is lower, preferably at least 50ºC lower or at least 100ºC lower, than the melt temperature in the extruder. The heat exchanging wall, alternatively the wall of the channel, is e.g. metallic and e.g. has a thickness of less than 5 mm, or less than 2 mm, or less than 1.0 mm. A thin wall contributes to effective heat exchange, e.g. with the fluid. Therefore, as a general preference, with or without the active cooling, the wall of the channel is e.g. metallic and e.g. has a thickness of less than 5 mm, or less than 2 mm, or less than 1.0 mm. A thin wall contributes to effective heat transfer from the extruded material, e.g. by heat exchange, e.g. with ambient or with the cooling fluid. Generally, with or without the optional active cooling, the wall of the channel may contain two or more materials, e.g. a metallic part and a non-metallic part, or two types of metallic parts. Generally, a metallic channel wall comprises a metallic part, more preferably comprises a metallic tube part, e.g. a cylindrical metallic tube part. For instance, coated tubes and bimetallic tubes and pipes could be used. The extension unit preferably comprises or is a pipe or a tube, wherein the pipe or tube provides the channel for extruded material. The pipe or tube is preferably metallic, and is for example cylindrical. Preferably, both the inner and the outer surface of the tube are cylindrical. Hence, the extension unit preferably comprises a metallic pipe or tube which receives the extruded material in operation. The pipe or tube is manufactured e.g. using hot or cold rolling, or using a hot extrusion process, or using a hot hollow forging process. Tube drawing may also be used. The extension unit is preferably flexible. Flexibility of the extension unit, e.g. the pipe or tube, can be used for providing a curved extension unit, e.g. a downward curved extension unit. Curvature refers to a curvature in the length direction, e.g. to a tube that is not straight in the length direction but is curved in the length direction. The length direction is the direction of the flow of the extruded material in the channel. This may be used to guide extrudate from a horizontal extruder to a cooling bath. For example, the extrudate after exiting the extension units enters a cooling bath, in particular the extrudate enters the liquid of the cooling bath through the horizontal gas/liquid interface of the liquid. Bending the extension tube or pipe downward is advantageous for fast transfer of the extrudate into the liquid. The extrudate is submerged in the liquid of the cooling bath for effective cooling. The extruder optionally comprises a connection element connected to the flexible extension unit, preferably to the tip of the flexible extension unit, and configured to tension or bend the flexible extension unit. The connection element is e.g. also connected to a frame or to the extruder. The connection element is for example, without limitation, arranged for downwardly bending the flexible extension unit (e.g. tube). A connection element is especially useful in case of resilient tubes. It is also possible that the tubes of the extension unit are bent during manufacture. The extruder is preferably a part of an extension system, wherein the extension system further comprises a cooling bath, e.g. a water bath, with an inlet for extrudate at an upper surface of the cooling bath. The cooling bath is arranged such that it receives extruded material from the extension unit of the extruder in operation.. A flexible extension unit is also, independently, advantageous in case of an extruder having multiple extension units. Bending the flexible tubes used as extension unit can be used e.g. for stacking the plurality of extruded strands. For instance, a metal with a relatively low Young’s modulus is used (e.g. lower than carbon steel), so as to have a low flexural modulus, such as bronze, copper, titanium or aluminium. Furthermore, tubes with a thin wall can be used to provide the flexibility of the tube, e.g. a wall thickness of less than 1.0 mm or less than 0.50 mm. Generally, the extension unit can comprise a curved tube, independent of the flexibility of the tube. For example, a rigid curved tube can be used to provide the extension unit. The use of one or more tubes as the extension unit also provides possibility of forming and flexibility of the channel after manufacturing. In case of very thin- walled extension tubes, flexibility is possible and easy before and after mounting the extension-nozzle. In case of thick-walled tubes, flexibility is preferred to be provided before mounting the extension nozzle, but also is possible after. Flexibility in form of the extension unit, e.g. tubes, provides the possibility to control the orientation and speed of the material flow. This may in turn provide for good stability and control in the produced extruded material (e.g. extruded strands). The flexibility can be in shape of a single and simple curve, or double, or more curves and forms. This gives the advantage of greater control and flexibility in the production. For instance: in a single nozzle, the material’s shape, flow-speed and flow-direction can be controlled. In double, or multi-nozzle configuration (multiple extension tubes in parallel), the material’s shape, flow-speed and flow- direction of each strand can be controlled independently. Another example is, for instance, that the material can be directed to the cooling bath (cooling unit) with desired and different angle of entry. Another advantage of the flexible and/or tube- like extension tube is easier external cooling or heating of the nozzle. This advantage can also be obtained with thin-walled straight tubes as part of the extension unit. Preferably, the wall is made at least in part of a material, e.g. a metal or alloy, that has a thermal conductivity of at least 100 W m⁻¹ K⁻¹ (W/(m•K)), or at least 200 W m⁻¹ K⁻¹ ¸or at least 300 W·m⁻¹ K⁻¹ ; with thermal conductivity measured e.g. according to ISO 8302, in particular ISO 8302:1991. For instance, the wall of the channel is made at least in part of copper, or aluminium, or tungsten, or silver, or an alloy containing one or more of these metal elements, e.g. certain types of brass and aluminium alloys. Hence, preferably the extension unit is provided by one or more tubes made at least in part of such materials (metals or alloys) with high thermal conductivity. High thermal conductivity contributes to sufficient cooling in the channel. Advantages of using tubes with high thermal conductivity are demonstrated in Example 3 and include excellent surface quality of the extruded strand, even without active cooling of the wall. For instance, the extension unit is provided by one or more tubes comprising tube parts that are made of such metals or alloys and have a thickness of preferably less than 5.0 mm. Direct contact between the heat-conductive tube part and the extruded material is not necessary, e.g. in case of tubes with a coating of the inner surface or bimetallic tubes. The extruded material, at the end of the extension unit, is e.g. in the form of strands or filaments. Filaments includes continuous strands. The extruded material as received from the extension unit was observed to have a desirable uniform size, e.g. for strands/filaments a uniform diameter. The method optionally further involves size reduction of an extruded material (e.g. strand) into, preferably, pellets, in a size reduction unit. The size reduction unit is for instance a cutter. Size reduction may involve pelletizing or cutting the extruded material, e.g. strands. The length of the pellets or cut pieces, is for instance 10 – 15 mm. The diameter of produced pellets is e.g. at least 2.0 mm, or at least 3 mm, and/or up to 8 mm. Generally, the diameter of the pellets or cut pieces is 90 – 110% of the diameter of the e.g. used strands at the inlet of the size reduction apparatus, cutter, or pelletizer. The invention hence also provides an extrusion apparatus (system) comprising an extruder as described, and a size reduction apparatus for size reduction a extruded material (strand of extruded material) from the extension unit into pellets. The size reduction apparatus is for instance a pelletizer. The size reduction apparatus is for instance a cutting apparatus. The apparatus is arranged in-line with the extruder. In some embodiments, the extruded material (e.g. strand) is used as a filament, e.g. for additive manufacturing, for instance fused deposition modelling. In such embodiments, the essentially continuous extruded strand can be used as such, e.g. without a need for size reduction particularly by pelletizing, and is for instance rolled. Hence, the use of the pellets is not limited to moulding. The pellets can also be used for other uses, e.g. for additive manufacture. For example, the extruded material, e.g. pellets, can be used for additive manufacturing of polymers, fibers, particles, and fiber-reinforced materials. For instance, the extruded material can be used for e.g., fused particle fabrication (FPF) and fused granular fabrication (FGF). Fused particle fabrication involves the layer-by-layer deposition of extruded materials to create three-dimensional objects, wherein the extruder receives particles or granular material. For example, a moving extruder is used to deposit the extruded material according to a pattern. For instance, plastic granulates (also referred to as plastic pellets) are extruded, in particular are melted and fed through an extruder with a nozzle, with the extruder moving in the horizontal plane for each layer and vertically to create a next laser. An example process is described by Nieto et al, Additive Manufacturing 23, 2018, pages 79-85, https://doi.org/10.1016/j.addma.2018.07.012; and in Woern et al, Materials 2018, 11, 1413; doi:10.3390/ma11081413. Optionally, the method furthermore comprises manufacturing a shaped article from a feedstock comprising the extruded material, e.g. by moulding of the pellets, for instance by injection moulding of the pellets, optionally with further components. Other types of moulding are also possible, such as e.g. extrusion moulding and compression moulding. The invention also provides a method of making a moulded LFT part, preferably an injection moulded LFT part, the method comprising preparing pellets with the inventive extrusion method and a pelletizing step, and subjecting the obtained pellets to moulding (preferably injection moulding) to produce the LFT part. The LFT part is in particular a shaped article. In a preferred embodiment, the die is modular and comprises means for releasably attaching the die to the barrel of the extruder. In preferred embodiments, the extruder comprises means for releasably attaching the extension unit to the die, these means are e.g. screws or nuts and bolts. Fibre length, as used herein can be determined with, e.g., an optical microscope. Average or mean fibre length refers to number-weighed mean. Figure 1 illustrates a section view of an example inventive extruder (1) comprising a barrel (not shown), a screw (not shown), and a die (2). The converging channel in the die is curved, as illustrated. The die is provided at the outlet side with an extension unit (3) providing two parallel channels for flow of the extruded mixture. The extension unit is provided by two parallel tubes. The extruder is furthermore provided with a (optionally used) cooling unit (4) which has an inlet and an outlet for cooling fluid and chamber for cooling fluid in indirect heat exchanging contact with the extruded material in the channels of the extension unit (3). The cooling unit can slide over the extension unit from an upstream position close to the die to a downstream position close to the tip (illustrated). The cooling unit (3) is provided by two tubular elements that are separate from the die. Figure 2 schematically shows an embodiment with a flexible extension tube. References are the same as in Fig. 1. The extruder comprises the die (2) with a flexible tube (3) as the extension unit. The extrudate exits the tube and enters the cooling bath (5) that is filled with a cooling liquid. The extruder optionally comprises a connecting element (not shown) to bend the flexible tube, thereby downwardly curving the tube. The optional connecting element is connected to both the tube and to the extruder or, not shown, to a frame. Hence a system is shown comprising the extruder and the cooling bath. Figure 3 shows photographs of a strand (A) and a pellet (B) produced with a comparative extrusion method without the extension unit. Figure 4 shows photographs of a strand (A) and a pellet (b) produced with an inventive extrusion method with the extension unit. The surface quality is much higher (smoother surface) than obtained with the comparative method. The inventive pellets were free flowing. It was also observed that the strands and pellets prepared with the inventive method had improved geometrical and dimensional quality. Examples The invention will now be further illustrated by the following non-limiting examples. These examples do not limit the invention and do not limit the claims. Example 1 Experiments were carried out with an extension unit as illustrated in Fig. 1, with or without active cooling using the cooling unit. Details of the cooling are given in Table 1A: cooling yes/no; position of the cooling unit (downstream tip T, middle M, upstream B), and cooling temperature. The cooling temperature was measured at the outer surface of the extension unit (3). The barrel temperature was 180ºC, 230ºC or 250ºC. The extension unit (nozzle) was provided in this example by two parallel tubes with an inner diameter of 3 mm and length 100 mm. Hence, L/D was 33. The tubes were made of steel and had a wall thickness of 2 mm. The feedstock was 40 wt.% long glass fibre reinforced PP. The results as shown in Table 1B show that with the 3 mm nozzle extension, the average fibre length of the pellets remains acceptable, the pellets are free flowing, and hence the pellets are easy to doze into a moulding machine, e.g. into an injection moulding machine. The pellets produced with and without a cooling unit meet the defined targets. In Table 1B, the median fibre length is given, as measured in the pellets, the standard deviation in the mean length, and the median fibre length (the geodesic fibre length at 50% of the counted objects), as well as the fibre fraction in wt.% (determined by ashing the matrix and weighing remnant fibres) and the diameter all of the pellets, and whether the pellets are free flowing or not. Table 1A Barrel Cooling Position Cooling T [ºC] T [ºC] A 180 - B 230 - C 250 - D 250 + T 90 E 250 + M 90 F 250 + B 90 G 230 + B 70 H 230 + B 80 I 230 + B 90 J 230 + T 70 K 230 + T 80 L 230 + T 90

Table 1B Counted Mean St. Median Fibre Diameter Free flowing fibres length dev. length fraction [mm] [mm] [mm] wt. % [mm] A 29838 1.140 0.880 0.929 35.69 2.44<d<2.89 Yes B 39016 1.110 0.790 0.945 35.97 2.15<d<3.03 Yes C 13263 1.260 0.990 1.016 37.57 2.16<d<3.35 Yes D 26972 1.110 0.610 1.008 36.63 2.72<d<3.25 Yes E 29255 1.060 0.750 0.945 36.49 2.26<d<3.05 Yes F 34088 1.090 0.670 0.984 35.74 2.41<d<2.98 Yes G 43731 0.990 0.490 0.913 33.86 2.62<d<2.9 Yes H 50939 0.950 0.500 0.881 33.98 2.79<d<3.03 Yes I 31077 1.010 0.480 0.945 32.12 1.98<d<2.58 Yes J 34420 0.980 0.480 0.913 32.94 1.98<d<2.58 Yes K 25996 0.990 0.490 0.929 32.77 1.96<d<2.57 Yes L 33390 0.970 0.460 0.921 34.15 2.04<d<2.67 Yes Example 2 Further experiments were carried out using 40 wt.% long glass fibre reinforced PP. The extension unit length (nozzle) was 60 mm or 100 mm; the inner diameter was 3 mm, two parallel extension units were used (See Table 2A). This example illustrates that free-flowing LFT pellets can also be obtained with different lengths of the extension unit, in particular also with a shorter tube length than in Example 1, and with different barrel temperatures (see Table 2B). For sample P, cooling was used with the cooling unit at the upstream position B and with 95ºC cooling temperature. Table 2A Barrel Cooling Extension T [ºC] unit length [mm] M 230 - 60 N 180 - 100 O 210 - 100 P 250 + 100 Table 2B Counted Mean St. Median Fibre Diameter Free flowing fibres length dev. length fraction [mm] [mm] [mm] wt. % [mm] ^M 13262 2.060 1.920 1.405 25.530 2.15<d<2.91 ^Yes ^N 20327 1.440 1.050 1.151 24.670 2.20<d<2.84 ^Yes ^O 14149 1.420 1.080 1.111 22.700 2.31<d<2.75 ^Yes ^P 16064 1.650 1.280 1.254 22.720 1.93<d<3.02 ^Yes Example 3 Experiments were carried using an extruder with an extension unit. Copper tubes were used as the extension unit, without active cooling; using carbon fibers and polypropylene (PP) polymer. The tubes were flexible. The extruded strands were ejected from the tubes into a water bath. Details and results are given in Table 3A and Table 3B. Average fiber length is mean weighted average. The extension unit tubes were exposed to ambient air. The copper tubes were flexible and had a length of 155 mm or 260 mm, and an inner diameter of 3 mm or 4 mm, and with L/D of the tubes of 39, 52, or 65. The wall thickness was 1 mm. The temperature of the outer surface of the tubes at the tip and at halve tube length was measured for Experiments 1 and 2. The fiber fraction was about 30 wt.%. The fiber length in the product (in the produced pellets) was in an advantageous narrow range of 0.8 – 1.0 mm. The pellets had excellent surface quality (very smooth) and were free flowing (excellent flow characteristics), and had advantageously a very narrow diameter tolerance. Figure 5 shows a representative strand (A) and pellet (B) obtained in this example, which had a very smooth surface and hence excellent surface quality. Table 3A # Length Tube inner L/D Wall Melt Temp Temp Temp (mm) diameter Thickness (Barrel) ºC mid tip (mm) (mm) ºC ºC 1 155 4 39 1 230 165 160 2 260 4 65 1 230 150 136 3 155 3 52 1 230 n.a. n.a. Table 3B # Fiber Average fiber length Pellet Surface quality Free fraction (mm) Diameter of the product flowing wt.% (mm) 1 30.5 0.8<L<1.0 mm 3.6<D<3.7 ++ ++ 2 30 0.8<L<1.0 mm 3.55<D<3.6 ++ ++ 3 30 0.8<L<1.0 mm 2.72<D<2.77 ++ ++

Claims

Claims 1. A method of producing an extruded material, the method comprising extruding in an extruder a mixture comprising a thermoplastic component and fibres through a die and subsequently through an extension unit that is configured for cooling an extruded mixture while confining the extruded material, preferably wherein the extruded material is a strand, wherein the extension unit comprises a channel and a wall of the channel.

2. A method according to claim 1, wherein the extruded material at least partially solidifies while confined in said channel of the extension unit, preferably wherein at least the surface of the extruded material solidifies in the extension unit.

3. The method according to any of the preceding claims, further comprising size reduction of the extruded material into pellets.

4. The method according to any of the preceding claims, wherein the method involves supplying a feedstock comprising thermoplastic composite (TPC) feed material to said extruder, wherein the TPC feed material comprises TPC pieces, wherein the TPC pieces individually comprise fibres and thermoplastic material.

5. The method according to any of the preceding claims, wherein the extruded material comprises fibres, wherein the fraction of fibres with a length of at least 1 mm is at least 5 wt.% of the extruded material.

6. The method according to any of the preceding claims, comprising cooling the extruded mixture during the passage of the mixture through the extension unit by indirect heat exchange with a forced flow cooling fluid, preferably cooling liquid.

7. The method according to any of the preceding claims, wherein the extension unit comprises the channel, wherein the channel is for flow or transport of the extruded mixture, and wherein the channel has a length Lc and a mean diameter Dc over said length Lc, wherein said mean diameter Dc is at least 0.5 mm and said length Lc is at least 5 times said mean diameter Dc, preferably wherein said length Lc is at least 10 times, or at least 20 times, or at least 30 times said mean diameter Dc.

8. The method according to claim 7, wherein the channel has less than 5 % deviation in the diameter from Dc over said length Lc.

9. The method according to any of the preceding claims, wherein said wall has a thickness of less than 5 mm.

10. The method according to any of the preceding claims, wherein the wall is made at least in part of a material, e.g. a metal or alloy, that has a thermal conductivity of at least 100 W· m⁻¹· K⁻¹ (W/(m * K)), or at least 200 W· m⁻¹· K⁻¹¸ or at least 300 W· m⁻¹· K⁻¹; with thermal conductivity measured e.g. according to ISO 8302.

11. The method according to claim 1, wherein - said wall has a thickness of less than 5 mm, - the channel has a length Lc and a mean diameter Dc over said length Lc, wherein said mean diameter Dc is in the range of at least 0.5 mm and said length Lc is at least 10 times said mean diameter Dc, and - the wall is made at least in part of a material that has a thermal conductivity of at least 100 W· m⁻¹· K⁻¹ (W/(m * K) ).

12. The method according to any of the preceding claims, wherein the extension unit comprises a tubular element providing said channel, preferably a tube.

13. The method according to claim 12, wherein the tubular element is curved in the length direction of the tube.

14. A method of making a moulded LFT part, comprising preparing pellets with the method of claim 3, wherein the preparation of the pellets preferably has the features of any of claims 4-13, and subjecting the pellets to moulding, preferably to injection moulding.

15. An extruder (1) comprising a barrel, a screw, and a die (2), wherein the die is provided at the outlet side with an extension unit (3) providing a channel for flow of the extruded mixture, the channel having a length Lc and a mean diameter Dc over said length Lc, wherein said mean diameter Dc is at least 0.5 mm and said length Lc is at least 5 times said mean diameter Dc; and wherein preferably: - the channels of the extension unit have a wall with a thickness of less than 5 mm; and/or - the mean diameter Dc is at least 1.0 mm, e.g. between 1.0 and 10 mm, more preferably between 1.0 and 5.0 mm, and/or - said channel has less than 5 % deviation in the diameter or thickness from said mean diameter or thickness Dc over said length L, and/or - said length Lc is at least 10 times, or at least 20 times, or at least 30 times said mean diameter Dc; and/or - the extension unit comprises a tubular element providing said channel, preferably a tube.

16. An extruder according to claim 15, the extruder further comprises a cooling unit (4) for cooling the extruded mixture during the passage of the mixture through the extension unit having an inlet and outlet for cooling fluid and a chamber for receiving the cooling fluid in indirect heat exchanging contact with at least a part channel for extruded material of the extension unit.

17. An extruder according to claim 15 or 16, wherein - the channels of the extension unit have a wall with a thickness of less than 5 mm; and - the mean diameter Dc is at least 1.0 mm, e.g. between 1.0 and 10 mm, more preferably between 1.0 and 5.0 mm, and - said length Lc is at least 10 times, or at least 20 times, or at least 30 times said mean diameter Dc.

18. An extruder according to any of claims 15-17, wherein the channels of the extension unit have a wall with a thickness of less than 2 mm, or preferably less than 1.0 mm.

19. An extruder according to any of claims 15-18, wherein the wall is made at least in part of a material, e.g. a metal or alloy, that has a thermal conductivity of at least 100 W· m⁻¹· K⁻¹ (W/(m*K)), or at least 200 W· m⁻¹· K⁻¹¸or at least 300 W· m⁻¹· K⁻¹; with thermal conductivity measured e.g. according to ISO 8302.

20. An extruder according to any of claims 15-19, wherein the extension unit comprises a tube that is curved in the length direction of the tube.

21. An extrusion apparatus comprising an extruder according to any of claims 15-20, further comprising a size reduction apparatus for size reduction of a strand of extruded material from the extension unit into pellets, wherein the size reduction apparatus is preferably a cutter.

22. The method according to any of claims 1-14, carried out in the extruder or extrusion apparatus of any of claims 15-21.