EP4294617A1 - Impregnation device to produce continuous fibre reinforced thermoplastic filaments for 3d printing, and impregnation method thereof - Google Patents

Abstract

The present disclosure relates to an impregnation device (1) and method for continuously obtaining a fibre-reinforced thermoplastic filament (42) from continuous fibre strands or yarns (11) to be impregnated with a thermoplastic resin, fed through an inlet (9), said impregnation device comprising: an impregnation cavity having a rectangular cross-section and being tapered towards a circular cross- section outlet (14) of the impregnated yarn; wherein the impregnation cavity is shaped to undulate sinusoidally in a downstream direction.

Description

IMPREGNATION DEVICE TO PRODUCE CONTINUOUS FIBRE REINFORCED THERMOPLASTIC FILAMENTS FOR 3D PRINTING, AND IMPREGNATION METHOD THEREOF

TECHNICAL FIELD

[0001] The present disclosure relates to a method and a device to impregnate continuous fibres and produce continuous fibre reinforced thermoplastic filaments with thermoplastic resins for three-dimensional printing applications, such as fused deposition modelling.

BACKGROUND

[0002] Fused deposition modelling (FDM) is the most widely utilised additive manufacturing (AM) technique due to its relative low maintenance cost, compact size, low material waste and operative simplicity. In this process, a continuous polymer filament is fed into an extrusion head, wherein the material is heated above its softening point and is extruded from the nozzle, before solidifying upon cooling, and finally deposited layer-by-layer on a print surface to form a finished part. Since printed parts produced using commonly available polymers as raw materials do not present high enough mechanical properties to be applied in high performance structures, there have been numerous attempts to overcome the poor mechanical performance of three-dimensional (SD) printed parts with the addition of fibre reinforcements.

[000S] A preliminary step consisting of the fabrication of fibre reinforced composite filaments is required prior to AM processing, since targeted feedstock materials first need to be formulated and developed for a specified 3D printing machine. The final product must possess adequate rheological profile, compatible thermal processing window and suitable mechanical behaviour.

[0004] One of the most important phases involved in this manufacturing process is impregnation, during which the dry fibres are wetted by the liquid polymer resin in order to create a fully impregnated semi-finished composite section. Material properties like resin viscosity and fibre tex, as well as processing parameters such as process temperature, winding speed, roving tension and fibre volume fraction can influence the impregnation quality and, therefore, have a strong impact on the mechanical properties of the final product.

[0005] There is an increasing interest in continuous carbon fibre reinforced thermoplastic (CFRTP) due to their high processability, short production cycles, thermal stability, superior mechanical properties and reduced ecological impact, despite possessing high inherent viscosity which significantly hinders reinforcement impregnation and consolidation. In recent years, several research studies have been conducted in order to develop more efficient methods of impregnating fibres with high-viscosity thermoplastic resins so as to overcome this obstacle. One of the most promising techniques consists in a direct melting process wherein continuous fibre strands are pulled through a heated cross-head extrusion die, while simultaneously being impregnated by a molten thermoplastic matrix. The resulting fully impregnated continuous fibre reinforced composite filament or tape can subsequently be wound in a spool to be used by a specified FDM 3D printer or other similar additive manufacturing machine.

[0006] Numerous devices and methods have been proposed for impregnating different types of fibre strands with polymeric resins; see for example the U.S. Patent 5,451,355, which describes a process for manufacturing a composite thread made of glass filaments combined with thermoplastic organic material. This document describes in particular a process that enables a range of products to be obtained, from superficially coated to at least partially-impregnated composite threads with thermoplastic organic material. It is claimed that the initial thread is carried along the device at a velocity of at least 5 meters per second without impairing the uniformity of the coating of said thread and the quality of its impregnation. Concerning drawbacks, one can argue that in the referred invention, the impregnation quality is surpassed by the coating uniformity, since it would be somewhat unlikely to yield an optimum impregnation throughout the cross-section of the thread, whilst said thread is progressing at high speeds across a straightforward, tension-prone, central duct. Moreover, at a high velocity, the cooling of the thread is achieved by spraying droplets of water as it passes through, which is not an efficient means to consolidate therewith, provided that the adhesive nature of the outer layer may promote moisture intake and degradation.

[0007] The separation/consolidation aspect of individual continuous fibres represents an important feature among these types of impregnation devices; see for example the U.S. Patent 4,728,387. This document describes an assembly comprising a number of alternating convex and non-convex surfaces whereon a path line along which the continuous fibres are free to travel. This change in direction forces the passing fibre strand to spread apart and recombine successively, allowing the molten polymer resin to completely wet each individual fibre, thereby eliminating voids between them for enhanced impregnation quality. A wide array of thermoplastic resins is listed as examples of possible impregnants. However, the artisan fails to refer resins with a melting point well above 250 °C or of high-level inherent viscosity (e.g. PEEK). Furthermore, there is no mention pertaining to how the cleaning of said device is performed in the event of fibre breakage and residue build-up.

[0008] As referenced by the above-described documents, one of the main challenges of the prior art is ensuring that the fibres or filaments are spread effectively, such that each element can be individually wetted and thoroughly encapsulated by the resin. This issue becomes particularly worrisome with the increase of the inherent viscosity of the thermoplastic resin used as impregnant. Another obstacle they face is being able to secure continuous filament production cycles without developing fibre breakage and time-consuming cleaning procedures. Steps have been taken by the prior art, as it is cited in various descriptions of methods and apparatus, having the goal to resolve the aforementioned difficulties. Examples of such representations include those established in EP0320650A2 wherein the artisan inserted a removable spreading member consisting of a frusto-conical body located inside the vessel means; EP0364828A2 discloses an extrusion impregnation apparatus for the production of profiles having fibre contents from 50 to 70% by volume, further comprising a separable impregnating set equipped with deflecting elements. Although the inventor made some improvements to the art regarding fibre tension reduction by

B incorporating an undulated geometry in the impregnation chamber, and upgraded production turnover, as well as cleaning procedures, by adding a closing mechanism, it still fails to include a cooled consolidation component. This shortcoming makes the apparatus only suitable for linear profiles with considerably high fibre volume content; U.S. Patent 6,251,206 B1 describes an apparatus for opening and resin-impregnation of fibre bundles that come into contact with a variable number of opening pins arranged in a staggered pattern, wherein said bundles are pulled through at an adjustable angle. According to the method described, this setting not only improves the quality of impregnation, but also permits continuous operation cycles while ensuring high productivity, even when the composite material is pulled out at high speeds. This system is also not completely satisfactory, since increasing angles of fibre bundle winding and hooking can easily form fluff, due to fibre damage. Also, cleaning of the pins can prove to be difficult.

[0009] U.S. Patent Application 2002/0125603 Al, CN105014994A and U.S. Patent Application 2019/0184653 Al describe impregnation devices with more complex and costly machined geometries, divided in two different sections whereby one of them comprises a fibre dispersion area equipped with tension rollers and another comprises an alternate impregnation chamber featuring, for instance, a number of opening pins, tooth-shaped structures or perpendicular resin melt runners. The latter patents are particularly noteworthy because they offer to solve the limitations associated with insufficient impregnation of continuous fibres or filaments with highly viscous thermoplastic resins, while also avoiding fibre breakage. However, the referred inventions have excessively "compartmentalized" designs combined with moving parts, causing the fibre path to be too lengthy and irregular to avoid fibre damage altogether. The complexity of the machining and maintenance of said devices also make them less cost-effective for small scale productions, in the case of low volume fibre-reinforced feedstock for FDM applications.

[0010] The object of the present disclosure is to provide a device for impregnation and consolidation and a method for impregnation and consolidation of a plurality of continuous fibre strands or multifilament yarn with a thermoplastic resin matrix of high or low inherent viscosity that enables the limitations of the prior art to be surpassed in terms of design simplicity, accessible interior for fibre placement and cleaning, cost-effective impregnation quality of various fibre contents with different types of thermoplastics, while reducing material waste and labour demand.

[0011] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.

GENERAL DESCRIPTION

[0012] In accordance with the present disclosure, an impregnation device is provided to produce prepreg composite filaments consisting of thermoplastic matrix impregnated fibre lengths, such as strands or yarns, which can eliminate the foregoing problems associated with the conventional techniques. Moreover, an efficient, yet straightforward, detachable feature is implemented in the impregnation and consolidation sections of the device to deliver a mechanism for easy fibre placement and for avoiding laborious or imperfect cleaning procedures.

[0013] It is an object of the present disclosure an impregnation device for continuously obtaining a reinforced composite filament from continuous fibre strands or yarns, for being impregnated with a thermoplastic resin, fed through an inlet, said impregnation device, comprising: an impregnation cavity having a rectangular width-wise cross-section [transversal to the inlet-outlet direction] and being tapered towards a circular cross-section outlet of the impregnated strands or yarns, i.e. the reinforced filament; wherein the impregnation cavity is shaped to undulate sinusoidally in a downstream direction.

[0014] In an embodiment, the impregnation device comprises two feeders for feeding thermoplastic resin into the cavity wherein one feeder is arranged upstream in respect of the other.

[0015] In an embodiment, the feeders are arranged to inject the thermoplastic resin into the cavity in an oblique downstream angle.

[0016] In an embodiment, the impregnation cavity is shaped in a tapered conical manner. [0017] In an embodiment, the fibre strand or yarn inlet comprises a plurality of comb shaped apertures followed downstream by a convex round spreading protuberance for spreading the fibres or yarn. The convex round spreading protuberance can be a tear shaped spreading protuberance in manner to promote a smooth and progressive spreading of the strands or yarns.

[0018] In an embodiment, the impregnation cavity is formed by successive concave and convex curves.

[0019] In an embodiment, the successive concave and convex curves are formed by two parallel walls of an inner surface.

[0020] In an embodiment, a thermoplastic resin inlet is arranged horizontally and/or vertically.

[0021] In an embodiment, the spreading protuberance has the fibre strand or yarn inlet upstream and the impregnation cavity downstream and said spreading protuberance comprises a length-wise cross-section having a smooth, i.e. without raised areas or indentations, curved surface line increasing in height from a first end to a midpoint and a steeper curved surface line decreasing in height from the midpoint to a second end. Preferably the cross-section has a continuously curved surface line increasing in height from a first end to a midpoint and a steeper curved surface line decreasing in height from the midpoint to a second end.

[0022] In an embodiment, the impregnation device comprises an upper body and a lower body, wherein the thermoplastic resin inlet is at a same plane of the outlet of the impregnated strands or yarns, said plan being a plan which divides said upper and lower bodies.

[0023] In an embodiment, the impregnation device comprises a fixed hinge on a rearward side for rotating vertically at an angle between 0° to 90° the upper body.

[0024] In an embodiment, the lower body comprises two parallel walls of the inner surface that protrudes upwards from the lower body and extended along the outer limits of the lower body inner surface. [0025] In an embodiment, the upper body comprises two parallel and symmetrical sinusoidal indentations extending continuously through the inner surface of the upper body.

[0026] In an embodiment, the upper body comprises two parallel and symmetrical sinusoidal inner walls with an increasing thickness towards the outlet of the impregnated strands or yarns.

[0027] In an embodiment, the two parallel walls of the inner surface of the lower body matches with two parallel and symmetrical sinusoidal indentations of the inner surface of the upper body.

[0028] In an embodiment, the device further comprises a removable frontal cylinder attached to the outlet of the impregnated strands or yarns.

[0029] In an embodiment, the impregnation device comprises a cooling consolidation die adjacent to the removable frontal cylinder.

[0030] In an embodiment, the impregnation device further comprises at least four metal brackets having adjustable lobe knobs for supporting and/or fixing the impregnation device on a flat surface base or extrusion line.

[0031] In an embodiment, the thermoplastic resin is crystalline, semi-crystalline or amorphous.

[0032] In an embodiment, the thermoplastic resin is selected from polyolefins, polyesters, vinyl aromatic polymers, halogenated vinyl polymers, polyaryletherketones, polyamides, polyimides, polyacetals, polyacrylates, polycarbonates, thermoplastic polyurethanes, polysulfones, polyethersulfones, polyetheramides, polyetherimides, polyphenylene sulfides, polyphenylene oxides, polyphenylene ethers and mixtures thereof.

[0033] In an embodiment, the continuous fibre strand or yarn are jute fibres, flax fibres, aramid fibres, glass fibres, carbon fibres or silica fibres, or combinations thereof.

[0034] The present disclosure also relates to a method to operate the impregnating device to obtain, i.e. manufacture, a continuous fibre reinforced thermoplastic filament. [0035] Furthermore, this disclosure also concerns a continuous fibre reinforced thermoplastic filament obtained by the method disclosed.

[0036] It is disclosed an impregnation device to produce prepreg composite filaments, consisting of a thermoplastic resin matrix of high or low viscosity, reinforced with a plurality of continuous fibre strands or multifilament yarn, suitable for three- dimensional printing applications, which comprises; means for receiving and guiding the continuous fibres into the fibre inlet of the impregnation device; a removable spreading module containing a plurality of parallel strips, that can be attached to the fibre inlet of the impregnation die; a main impregnation die body, consisting of an upper and lower body or casing, whereby the upper casing can be opened by rotating vertically at a fixed hinge on the rearward side, making a maximum angle of 90°, said main impregnation die body having

I. at least two usable feed ducts having horizontal and vertical inlets located in the upper die body, perpendicular to a central duct, that guide the extruded resin thereto;

II. a removable custom-built coupling cylinder for receiving the extruded resin attached, preferably, to the horizontal feed duct inlet of the upper die body, wherein the extruder is connected;

III. a central duct located in the upper die body also diverging into at least two successive oblique distribution channels, guiding the extruded resin through the channel outlets towards an impregnation chamber;

IV. an impregnation chamber generated upon secured junction of the upper and lower die body inner surfaces, said surfaces comprising a plurality of alternating successive concave and convex curves throughout the length of the device, whereupon the matching inner surfaces of both die body sections create a cavity consisting of a tapered conical shape that undulates sinusoidally in a downstream direction; V. a path line wherein the continuous fibres are free to advance between the means for receiving the aforementioned reinforcements and the impregnation die body outlet;

VI. an interior tear-shaped spreading protuberance located on the inner surface of the lower die body, said protuberance having the first end adjacent to the fibre inlet and the second end adjacent to the beginning of the first concave surface curve of the lower die body; said protuberance further defined, when viewed in a lengthwise cross-section, by having a flat curved surface line increasing in height from the first end to the midpoint and a steeper curved surface line decreasing in height from the midpoint to the second end;

VII. two parallel sinusoidal inner walls, protruding upwards from the lower die body and extended along the outer limits of the lower die body inner surface; said walls further describing a sinusoidal pattern similar to that of the impregnation cavity, wherein the top surface line of both walls consists of an offset entity of the undulated inner body surface line; said walls furthermore matching two parallel indentations on the inner surface of the upper die body, upon enclosure of the main impregnation die body; a removable frontal cylinder attached to the impregnation die body outlet for prepreg composite filament shaping; a cooling consolidation die consisting of an upper and lower part, forming a calibrated cylindrical pathway.

[0037] In an embodiment, an extruder having vertical configuration can, alternatively, be attached to the vertical feed duct inlet through a custom-built removable coupling cylinder withal.

[0038] In an embodiment, the feed ducts may converge into the central duct located in the upper die body, said central duct subsequently diverging into at least two successive oblique distribution channels, guiding the extruded resin through the channel outlets towards the impregnation chamber.

[0039] In an embodiment, the main impregnation die body may be supported by at least four metal S brackets having adjustable lobe knobs, whereby said main impregnation die body can be levelled and fixed on a surface base or extrusion line substructure. Preferably said S brackets are removable, such that said device can tilt downwards, rotating on the axis of the coupling cylinder, describing an angle between 0° and 90°.

[0040] In an embodiment, the main impregnation die body may include an interior tear-shaped spreading protuberance located on the inner surface of the lower die body, said protuberance having the first end adjacent to the fibre inlet and the second end adjacent to the beginning of the first concave surface curve of the lower die body.

[0041] In an embodiment, the main impregnation die body may include two parallel sinusoidal inner walls, protruding upwards from the lower die body and extended along the outer limits of the lower die body inner surface.

[0042] In an embodiment, the two parallel sinusoidal inner walls may be of at least 15 mm thickness.

[0043] In an embodiment, the inner walls may describe a sinusoidal pattern similar to that of the impregnation chamber, having each sinusoidal inner wall a top surface line consistent with a 20 mm offset of the undulated inner body surface line.

[0044] In an embodiment, the two parallel sinusoidal inner walls may match two parallel and symmetrical sinusoidal indentations on the inner surface of the upper die body, at least 8 mm deep when matching the highest point of each wall structure, upon enclosure of the impregnation die.

[0045] In an embodiment, the alternating convex and concave surface curves can differ in number, having various possible lengths, heights and shapes, such as undulated, corrugated or serrated configuration, that serve the purpose of the current disclosure while avoiding fibre damage.

[0046] In an embodiment, the outlet of the curved frontal conical cavity portion may match the outlet of the upper undulated inner surface cavity of the upper die body, resulting in a circular shape that directly connects with the frontal cylinder outlet of the removable frontal cylinder. [0047] In an embodiment, the consolidation die may have multiple possible pathway shapes, can consolidate prepreg composite filaments with different complex shape cross-sections.

[0048] The present disclosure is further concerned with a method of using the disclosed impregnation device to produce prepreg composite filaments, consisting of a thermoplastic resin matrix of high or low viscosity, reinforced with a plurality of continuous fibre strands or multifilament yarn, suitable for three-dimensional printing applications, comprising:

1. introducing a single or multiple continuous fibre strands or yarns through the spreading module attached to the fibre inlet of the impregnation die while the upper die body is opened, positioning each fibre bundle laterally across the surface of the interior tear-shaped spreading protuberance, passing the resultant conjoining fibre bundle through the lower die body, further passing said fibre bundle through the frontal cylinder outlet and finally through the consolidation die into a pulling system;

2. inserting and locking the extruder, after the main impregnation die body is closed, fastened, heated and the pulling system is activated, onto the coupling cylinder attached to the horizontal feed duct inlet, wherein a thermoplastic resin melt is injected in the feed duct thereto, subsequently flowing through two successive oblique distribution channels and respective outlets, making its way into the impregnation chamber;

3. continuously drawing the newly conjoined continuous fibre bundle towards the impregnation chamber as it meanders through the undulated path line, dragging against the alternating convex and concave surfaces, creating tension reduction driven by additional spreading points to facilitate thermoplastic wetting of the individual fibres, then exiting the narrowing impregnation cavity through the frontal cylinder outlet; and

4. after the heated thermoplastic-impregnated fibre bundle is extruded from the frontal cylinder outlet, drawing through the assembled consolidation die, supplied with circulating cool water, located adjacent to said outlet, thus, forming a consolidated prepreg composite filament that is wound onto an empty spool to complete the process.

[0049] In an embodiment, the maximum service temperature that can be reached to process the thermoplastic resin and impregnate the continuous fibre lengths is higher than 400 °C.

[0050] The disclosed impregnation device is useful to impregnate several types of continuous fibre lengths, such as strands or yarns, comprising a multitude of fibres having high-temperature resistance. Thus, suitable fibres include glass fibres, carbon fibres, aramid fibres, and the like. In one particular embodiment, the impregnation device is particularly useful to impregnate continuous carbon fibres, which can commonly be found in the form of rovings. Typically, carbon fibre rovings contain a number of individual filaments with a diameter ranging between 5-10 pm.

[0051] Further in accordance with the disclosed process, wide selections of either crystalline or amorphous thermoplastic resins are employed to homogeneously impregnate the individual filaments of the selected continuous fibre lengths. The impregnation die of the current disclosure is particularly advantageous in impregnating fibre lengths with molten thermoplastic polymers and blends thereof having melting points or processing temperatures greater than 300 °C, such as polyetheretherketones, polyetherimides and polyphenylene sulfides.

[0052] Additionally, since the device and the method of the present disclosure describe a cost-effective and compact, nevertheless, efficient and versatile, means of impregnating continuous fibre lengths, due to simpler structure design, uncomplicated operation and more suitable maintenance, a wide range of prepreg composite filaments can be obtained at high production rates with increased quality of impregnation, good mechanical properties and consistent cross-section that are tailor- made for three-dimensional printing applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The present invention will be further described in connection with the attached schematic drawings, with reference to the preferred embodiments of the invention, including specific components and equipment, which should in no way be intended by those skilled in the art to limit the scope of the invention.

[0054] Fig. 1 is a schematic illustration of a preferred embodiment of the impregnation device according to the present disclosure showing the relationship of various components and apparatus used during operation.

[0055] Fig. 2 is a perspective view of the impregnation device shown in Fig. 1, comprising an impregnation die and an adjacent consolidation die.

[0056] Fig. 3 is a bottom view of the upper die body of the impregnation assembly shown in Fig. 2.

[0057] Fig. 4 is a top view of the lower die body of the impregnation assembly shown in FIG. 2.

[0058] Fig. 5 is a longitudinal cross-section view of the closed impregnation die shown in Fig. 2, when both die bodies of Figs. 3 and 4 are joined together and fastened.

DETAILED DESCRIPTION

[0059] The present disclosure succeeds in dealing with several of the issues experienced with the prior art when it comes to producing composite filaments consisting of continuous fibres impregnated with thermoplastic matrices of high or low inherent viscosity, concomitantly offering a working technological solution for manufacturing composite filament feedstock suitable for the field of three- dimensional printing.

[0060] The present disclosure relates to the development and production of an impregnation device and impregnation method thereof, to produce prepreg composite filaments consisting of thermoplastic matrix impregnated fibre lengths, such as strands or yarns, for three-dimensional printing applications. A plurality of fibres are pulled through said device comprised of an impregnation die and a consolidation die, whereby the impregnation die is mounted on a conventional base support and inserted in the manner of a cross-head at the outlet of an extruder, whence the molten material is injected in the direction of the fibres perpendicularly thereto and drawn at a constant speed through the consolidation die; said consolidation die having a calibration tool therein for diameter precision.

[0061] Using a specially designed cross-head die assembly mounted coaxially at the outlet of an extruder, a plurality of continuous fibres are pulled through a compact impregnation die, possessing an efficient working geometry and less tortuous meandering path line, that allows for the maximization of thermoplastic melt contact with the individual filaments of the fibre bundle, while at the same time generating lower amounts of tension that promote fibre breakage. In a prompt second step, the prepreg composite filament is consolidated and calibrated into a desired diameter by aid of an adjacent consolidation die. The device and method are described in greater detail with reference to the drawings below.

[0062] Referring to the accompanying drawings, Fig. 1 is a diagram illustrating, for convenience, the developed impregnation device 1 as an independent unit that can be easily mounted in a conventional thermoplastic composite co-extrusion process setting and amidst related equipment.

[0063] As can be seen, a number of fibre rovings or spools containing a plurality of continuous fibre lengths are drawn from a means for receiving and guiding said continuous lengths of fibre 2 through the impregnation device 1, comprising an impregnation die 4 and a consolidation die 36, by action of a set of pull rolls 43. The impregnation die 4 can be attached in a manner of a cross-head to a single-screw extruder 3, in either a horizontal or vertical, preferably horizontal, disposition.

[0064] In the beginning of the process, as soon as the impregnation die 4 reaches the set service temperature and the consolidation die 36 is cooled by turning on the water circulation, the pull rolls 43 are activated in order to initiate the pulling stage. Subsequently, thermoplastic resin granules are fed into the hopper of the single-screw extruder 3, previously heated at the recommended processing temperature for the selected thermoplastic material, transported along the extruder barrel through a screw at a variable speed and injected into the heated impregnation die 4. Finally, the recently impregnated and consolidated, ergo, finished composite filament is pulled and wound onto a filament storage spool 44, completing the production process.

[0065] As can be seen more clearly in Fig. 2, the specific structure of the impregnation device 1 is comprised of two die assemblies: an impregnation die 4 followed by a consolidation die 36, on a horizontal plane. The main body of the impregnation die 4 is comprised of a stainless steel block, having a minimum volume size of 320x100x140 mm and weighing approximately 30 Kg, which is divided into an upper die body 5 and a lower die body 6, whereas the upper die body 5 can be opened by rotating vertically relative to the lower die body 6, making a maximum angle of 90°. When introducing the continuous fibre lengths 11 through the impregnation die 4, it is imperative that it remains opened during this point, for precise visualization and handling of each continuous fibre length 11. To perform this action, the upper die body 5 is lifted with the aid of an eye bolt 12 screwed on the top front section. Afterwards, the impregnation die 4 is closed and fastened by any number of screws or latch clamps located along its main body.

[0066] The exiting continuous fibre lengths 11 are joined and made to pass through a removable frontal cylinder 10 possessing an outlet 14 that is responsible for the cross- section shape of the prepreg composite length, which in turn is fastened with at least four screws, displayed evenly, to also ensure that both upper die body 5 and lower die body 6 are properly locked together.

[0067] For allowing the injection of the extruded thermoplastic resin into the impregnation die 4, a removable coupling cylinder 7 is attached to the horizontal feed duct inlet 8 located on the right side of the upper die body 5, connecting the single screw extruder 3 therein; said extruder 3 can be of any type of single-screw or double screw extruder required for thermoplastic processing, provided that the removable coupling cylinder 7 used is removable and custom-built to fit the extrusion die unit. Preferably, the single-screw extruder 3 is connected to the removable coupling cylinder 7 attached to the horizontal feed duct inlet 8, while a metal cap is used to seal the vertical feed duct inlet 9 and prevent resin backflow to this zone. [0068] Alternatively, if an extruder possessing a vertically-positioned die unit is used, it can be attached to the vertical feed duct inlet 9, also through a custom-built removable coupling cylinder, in a manner similar to the aforementioned arrangement, while the horizontal feed duct inlet is sealed to, in turn, prevent backflow to this zone. Advantageously, the impregnation die 4 is supported by at least four metal S brackets having adjustable lobe knobs 13, such that the device can be levelled and fixed on a specified surface base or substructure of an extrusion line. More advantageously, said adjustable lobe knobs 13, and respective metal S brackets, can be easily removed or rotated, such that the impregnation die 4 can be tilted downwards, rotating on the axis of the coupling cylinder 7, anywhere between 0° and 90°, hindering possible backflow formation, in the event that excessively viscous thermoplastic resins are processed.

[0069] Further exhibited in the present illustration (Fig. 2), after the continuous fibre lengths 11 are made to pass through the securely closed impregnation die 4, they are subsequently drawn through the consolidation die 36, which can be placed at a specified distance to the removable frontal cylinder 10. The consolidation die 36 is a metal block, having a volume size of 80x50x50 mm and weighing less than 1 Kg, which is divided into an upper die part 37 and a lower die part 38 that can be separated from each other, for easy placement of the continuous fibre lengths 11 and thorough cleaning of its interior cavity.

[0070] Each of the die parts 37 and 38 has an array of channels for circulation of cooling water that is distributed by small tubing that are connected to water inlets 39 and water outlets 40. The interior cavity is a hollow pathway with a calibrated cross- section diameter 41, formed when the die parts 37 and 38, having identical semi- cylindrical shafts centred along each section, are joined together and fastened. After the impregnated fibres pass through the cooled consolidation die 36, a consolidated prepreg composite filament 42 is formed, having the same cross-section diameter as the calibration tool diameter 41. Although the preferred final product consists of a filament with a circular cross-section for the sake of the intended field of application, it is possible, taking into consideration the shaping of the hollow pathway, to obtain any type of complex shape cross-section filament. [0071] As detailed in Fig. 3 and Fig. 4, the main body of the impregnation die 4 consists of an upper die body 5 connected to a lower die body 6 through a fixed hinge 16 located herein at the rearmost edge of each lateral side. The upper die body 5 possesses a fibre inlet 15 whereto the continuous fibre lengths 11 are introduced, which also provides a frame casing for the removable spreading module 25 attached to the lower die body 6.

[0072] Thus, first making use of the bottom view shown in Fig. 3 as a point of reference, the continuous fibre lengths 11 are drawn in a downstream manner, initially, through the fibre inlet 15, then passing through the upper primary surface cavity 20, followed by the upper undulated inner surface cavity 23 and finally exiting through the frontal cylinder outlet 14. In a preferred embodiment, distribution of the molten thermoplastic resin in the upper die body 5 is twofold (see Fig. 5) and the introduction of said impregnant herein is carried out through a first distribution channel outlet 22 and a second distribution channel outlet 24 located on the undulated inner surface cavity 23, at the beginning of a convex curve and at the end of a concave curve, respectively.

[0073] This particular disposition of each distribution channel outlet 22 and 24 ensures that as the steady flow of thermoplastic resin reaches the continuous fibre lengths 11 wherefrom, it exerts the largest amount of shear flow in contact zones herein, yielding different magnitudes of tension force, due to the friction generated when the continuous fibre lengths 11 come into direct contact with each curve transition. The upper die body 5 further comprises an (upper) outer parting surface 17, having a 15 mm width that runs horizontally across the external limits of both lateral sides of the main body. Moving towards the centre, adjacent to both outer parting surfaces 17, there are two parallel and symmetrical sinusoidal indentations 18, also having a 15 mm width, extended continuously through the inner surface of the upper die body 5, and reaching depths of at least 8 mm in the deepest zones of said depressions.

[0074] Further moving towards the centre, there are two parallel and symmetrical upper body sinusoidal inner walls 19, beginning in the middle section of the upper die body 5 and increasing in thickness towards the frontal cylinder outlet 14. The shape of the aforementioned wall 19 structures turns the upper undulated inner surface cavity 23 into a progressively narrow pathway towards the frontal cylinder outlet 14.

[0075] Referring now to the top view shown in Fig. 4, identical to previously disclosed herein, the continuous fibre lengths 11 are drawn in a downstream manner, initially through a removable spreading module 25 containing a plurality of parallel strips, preferably four, that ensure that each roving is separated upon entry, whereof it is attached to the lower die body 6, in the area adjacent to the fibre inlet 15. Following this, the continuous fibre lengths 11 reach the lower primary surface cavity 26, whereupon the fibre filaments are spread apart across a horizontally curved surface by a tear-shaped spreading protuberance 27 located therein, said protuberance having the first end adjacent to the fibre inlet 15 and the second end adjacent to the beginning of the first concave surface curve of the lower die body 6; said protuberance further defined, when viewed in a lengthwise cross-section, by having a flat curved surface line increasing in height from the first end to the midpoint and a steeper curved surface line decreasing in height from the midpoint to the second end. Subsequently, the recently separated fibre bundle travels along a lower undulated inner surface cavity 30, comprising a plurality of alternating successive concave and convex curves, ending in a tapered convex curve carved with a narrow, arced frontal conical cavity portion 31 in the centre line of said convex curve.

[0076] Similar to the upper die body 5, the lower die body 6 further comprises a (lower) outer parting surface 28 that matches the (upper) outer parting surface 17, when the impregnation die 4 is fully closed. Moving towards the centre, adjacent to both (lower) outer parting surfaces 28, there are two parallel and symmetrical sinusoidal inner walls 29 with 15 mm thickness, protruding upwards from the lower die body 6 and extended along the outer limits of the lower undulated inner surface cavity 30; said walls further describing a sinusoidal pattern similar to that of the impregnation chamber 35 (See Fig. 5), wherein the top surface line of both sinusoidal inner walls 29 is consistent with a 20 mm offset of the undulated inner body surface line; said sinusoidal inner walls 29 furthermore matching the two parallel and symmetrical sinusoidal indentations 18 on the inner surface of the upper die body 5, at least 8 mm deep when matching the highest point of each wall structure, upon enclosure of the impregnation die 4.

[0077] Additionally, the outlet of the arced frontal conical cavity portion 31 also matches the outlet of the upper undulated inner surface cavity 23 of the upper die body 5, resulting in a circular shape that directly connects with the frontal cylinder outlet 14 of the removable frontal cylinder 10. The existence of the aforementioned thick wall structures 19 and 29 on the periphery of the upper and lower undulated inner surface cavities 23 and 30, respectively, are particularly advantageous for the reason that they are effective in preventing excessive thermoplastic resin leakage perpendicular to the polymeric material flow.

[0078] Considering that the impregnation die 4 is an assembly that can be opened while being subjected to immense pressure, it makes it more prone to this type of phenomena, since it is more difficult to maintain proper surface sealing, even with secure fastening.

[0079] Thus, in a particular embodiment, the convenience of this wall configuration upon enclosure of the impregnation die 4, aided by the fastening of the removable frontal cylinder 10, further aided by two long fastening screws that go through the upper die body 5 and lower die body 6 by way of two screw drill holes 21, creates a more compact device that, not only sustains a steady processing rate, but also ensures easier and faster cleaning procedures after the device is cooled and opened, subsequently to the conclusion of the production.

[0080] Fig. 5 represents a longitudinal cross-section view of the closed impregnation die 4 shown in Fig. 2. More specifically, Fig. 5 represents a longitudinal cross-section view of the impregnation die 4, when both upper die body 5 and lower die body 6 shown, respectively, in Figs. 3 and 4, are joined together and fastened.

[0081] In a preferred embodiment described herein, one of the two usable feed ducts located in the upper die body 5, specifically, the vertical feed duct inlet 9 that is blocked with a screwed metal cap, is perpendicular to a central duct 32 whereto the extruded thermoplastic resin initially flows from the horizontal feed duct inlet (omitted in the present illustration due to the selected view). [0082] The central duct 32 further diverges into a first distribution channel 33 and a second distribution channel 34, successively positioned in a downstream direction, guiding the molten thermoplastic resin through the first distribution channel outlet 22 and second distribution channel outlet 24 (previously described in Fig. 3), respectively, towards an impregnation chamber 35.

[0083] This impregnation chamber 35 is generated upon secured junction of the upper and lower die body inner surfaces, said surfaces comprising a plurality of alternating successive concave and convex curves throughout the length of the impregnation die 4, whereupon the matching inner surfaces of the upper die body 5 and the lower die body 6 create a cavity consisting of a tapered conical shape that undulates sinusoidally in a downstream direction.

[0084] Said cavity comprises a path line wherein the continuous fibre lengths 11 (shown in Fig. 2) are free to advance between the means for receiving the aforementioned reinforcements (which is placed near the fibre inlet 15) and the frontal cylinder outlet 14, along a passage defined by a horizontal parting plane X-X. During their course, the continuous fibre lengths 11 travel within the parallel strips of the removable spreading module 25 that separate each different fibre roving.

[0085] Next, the fibres of each roving are spread across the horizontally curved surface of the tear-shaped spreading protuberance 27, followed by a vertical shift of said reinforcements that are brought into contact with the subsequent alternating concave and convex surfaces, before finally condensing in a recombined fibre bundle, which is induced by a narrowing pathway exit. This aforementioned multi-planar diversion promotes a more rigorous detachment of the individual fibre filaments, which in turn facilitates the wetting of said filaments by the molten thermoplastic resin.

[0086] The impregnation process starts with the formation of resin flux between the upper and lower undulated inner surface cavities 23 and 30 described, respectively, in Figs. 3 and 4, and the advancing continuous fibre lengths 11, after the molten thermoplastic resin and reinforcing fibres are fed into the impregnation die 4 under predefined conditions. The continuous fibre lengths 11, which are under tension, are deflected over the curved surfaces, applying hydrostatic pressure on the resin. [0087] The hydrostatic pressure areas, which are indicated by the dashed lines in Fig. 5, constitute the driving phenomena of the impregnation process. If the angles of these deflection points have a critical value, it could lead to fibre breakage. However, if the deflection angle is high enough to allow for sufficient direction change, it can lead to tension reduction without damaging the fibres. In spite of the fact that high processing speeds can be reached using the present device, lower speeds are preferable, since increased speed generates increased tension and, as the tension increases, the fibre filaments tend to come closer to each other, causing high compaction. Therefore, an increase in tension results in a lower quality impregnation.

[0088] Although the disclosure has been described with respect to certain preferred embodiments, those skilled in the art will recognize that many aspects can be modified upon the reading and understanding of the specification. For example, an increasing number of distribution channels and outlets thereof may be machined in augmented versions of the impregnation die 4, coinciding with an increasing number of alternating convex and concave surface curves; said surface curves having various possible lengths, heights and shapes, such as undulated, corrugated or serrated configuration, that serve the purpose of the current disclosure while avoiding fibre damage.

[0089] In the present disclosure, the thermoplastic resins that are used to homogeneously impregnate the individual filaments of the selected continuous fibre lengths are either crystalline or amorphous. Moreover, any extrudable thermoplastic polymer or copolymer is suitable for the referred impregnation process. Examples of which include polyolefins (such as polyethylene and polypropylene), polyesters (such as polyethylene terephthalate and polybutylene terephthalate), vinyl aromatic polymers (such as polystyrene and acrylonitrile-butadiene-styrene), halogenated vinyl polymers (such as polyvinylchloride and vinylidene polyfluoride), polyaryletherketones (such as polyetheretherketone and polyetherketoneketone), polyamides (such as polyamide 6, 11, 12, 66, 610, and 612), polyimides, polyacetals, polyacrylates, polycarbonates, thermoplastic polyurethanes, polysulfones, polyethersulfones, polyetheramides, polyetherimides, polyphenylene sulfides, polyphenylene oxides, polyphenylene ethers and blends thereof. [0090] The extrudable thermoplastic polymers, copolymers and blends thereof can be reinforced with continuous fibre lengths that are mineral, organic or inorganic. Thus, suitable examples can be found in rovings of jute fibres, flax fibres, aramid fibres, glass fibres, carbon fibres, silica fibres and other specimens capable of reinforcing the thermoplastic resin, on condition that said thermoplastic resin can be processed at a temperature lower than the decomposition temperature of the referred fibre specimens. The combined fibre rovings also need to possess sufficient strength so they can sustain enough pulling tension without easily breaking. Ideally, the present device implements carbon fibre rovings having a linear density of 67 to 3200 tex and containing a number of individual filaments ranging from 1000 to 48000 with a diameter of 5-10 miti. However, this equipment allows the use of fibres of any linear density (tex).

[0091] Provided that some of the extrudable thermoplastic resins are considered high- performance polymeric materials due to their high-temperature melting points (polyetheretherketone, for example, has a melting point around 365 °C), the impregnation die has to be made of stainless steel or similar high-temperature resistant alloy so it can yield a maximum service temperature higher than 400 °C. The whole volume of the main impregnation die body is heated to allow the thermoplastic resin to properly flow through the inner channels and to prevent this polymeric material from solidifying.

[0092] Heating is carried out by inserting, widthwise, at least twelve 800 W-230 V cartridge heaters in both the upper and lower die body, separated by a distance of at least 12 mm between them and having a more compact distribution between the middle cross section and the downstream end, due to the propensity of the thermoplastic resin to flow in the direction of a hotter mass of metal. The amount of installed cartridge heaters allows the inner channels and impregnation chamber to reach the required maximum temperature of 400 °C, making the impregnation of any type of thermoplastic resin feasible. In addition, an auxiliary band heater with the same heating capacity as the cartridge heaters is placed around the coupling cylinder in order to allow the steady flow of thermoplastic resin from the extruder to the feed duct of the impregnation die. [0093] The temperature of the device is dependent upon the melting point and rheological behaviour of the chosen thermoplastic resin. Hence, the adjustment of the temperature value is kept between the melting point and the decomposition temperature of said polymeric material. Temperature values can be adjusted from a control panel, providing automatic control of the electrical power feed by using a three-phase temperature controller with PID microprocessor connected to a type K thermocouple installed near the horizontal parting plane of the device. In an embodiment, stone wool insulation slabs are used to cover the impregnation die body during operation, in order to minimise heat loss to the exterior. By using this thermal system and electrical set-up, it is possible to reach the aforementioned maximum service temperature in 15 min to 20 min.

[0094] The finished prepreg filament is obtained after passing through the consolidation die, at a constant pull speed. This equipment is comprised by two matching parts that form a cylindrical pathway when joined together and fastened. The cross-section of this pathway has the same diameter as the final consolidated prepreg filament, and the two parts that form it are dismountable, in order to facilitate the drawing of the fibers herein prior to the initiation of the extrusion process and enable an efficient cleaning procedure thereafter.

[0095] This consolidation die is cooled by circulating water in the interior of each part so it can maintain a constant temperature that allows the thermoplastic impregnated continuous fibres to solidify, while at the same time it acts as a calibration tool that predetermines the diameter of the final filament. For three-dimensional printing applications, such as FDM, the standard filament size is limited to 1.75 and 2.85 mm diameters. Preferably, the present disclosure forms a 2.85 mm diameter filament so it can accommodate a larger amount of continuous fibres (between 15 and 20%) and, consequently, allows the final product to possess more advantageous mechanical properties.

[0096] The following examples describe the process and the method of using the disclosed impregnation device, by impregnating continuous fibre lengths with thermoplastic resins that are drawn at a constant speed using specific implementations that set forth preferred modes contemplated by the inventors. These examples provide operating parameters to produce a composite filament whereby the reinforcing continuous fibre lengths are homogeneously impregnated, albeit not serving as a limitation on the scope of the current disclosure.

EXAMPLE 1

[0097] Using the device shown in Fig. 2, polyamide 6 (Durethan B30S 000000) granules are processed using a Periplast, Ltd single-screw extruder (D= 25 mm, L/D= 25) at a melt temperature 270 °C to impregnate, at a screw speed of 1.2 rpm, a carbon fibre bundle of 10% by weight composed of Tenax^®-E HTA40 E13 grade carbon fibre rovings (two 3K 200 tex rovings and one 6K 400 tex roving) from Toho Tenax Europe GmbH, yielding a combined value of 800 tex. The impregnated fibre bundle is drawn from the impregnation die, heated at a temperature of 295 °C, at an angle a = 11°, and passed through the cooled consolidation die at a pulling speed of 1 m/min, producing a consolidated prepreg filament that is calibrated for a 2.85 mm diameter cross-section.

EXAMPLE 2

[0098] As in Example 1, polyetheretherketone (VICTREX 450G) granules are processed using a Periplast, Ltd single-screw extruder (D= 25 mm, L/D= 25) at a melt temperature of 370 °C to impregnate, at a screw speed of 2 rpm, a carbon fibre bundle of 5% by weight composed of Tenax^®-E HTA40 E13 grade carbon fibre rovings (a single IK 67 tex roving and two 3K 200 tex rovings) from Toho Tenax Europe GmbH, yielding a combined value of 467 tex. The impregnated fibre bundle is drawn horizontally from the impregnation die, heated at a temperature of 385 °C, and passed through the cooled consolidation die at a pulling speed of 40 rpm, producing a consolidated prepreg filament that is calibrated for a 2.85 mm diameter cross-section.

[0099] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. [00100] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.

[00101] The above described embodiments are combinable.

[00102] The following claims further set out particular embodiments of the disclosure. [00103] References

1. Boissonnat, P., Loubinoux, D. and Roy, L. (1995). Process for the manufacture of a composite thread and composite products obtained from said thread, U.S. Patent No. 5,451,355 A.

2. Hilakos, W. (1988). Resin impregnation of fiber structures, U.S. Patent No. 4,728,387 A.

3. Hilakos, W. (1989). Poltrusion apparatus and method for impregnating continuous lengths of multi-filament and multi-fiber structures, European Patent No. EP 0320650A2.

4. Augustin, G., Hinrichsen, G., and Traencker, H.-J. (1990). Extrusion impregnation apparatus, European Patent No. EP 0364828A2.

5. Saito, K. and Yonaiyama, R. (2001). Method for opening and resin-impregnation to produce continuous fiber-reinforced thermoplastic resin composite material, European Patent No. 6,251,20681

6. Sakai, Y. and Yonaiyama, R. (2002). Device for producing thermoplastic resin continuous length sections reinforced with long fibers, U.S. Patent No. 2002/125603 Al.

7. Likun, S. and Huilin, Y. (2015). Continuous fiber reinforced thermoplastic resin composite material melting soaking device, Chinese Patent No. CN 105014994A.

8. Chunhua, C, Dahua, C, Xianbo, H., Yonghua, L., and Wei, X. (2019). Alternating pressure melt impregnation device and melt impregnation method using the same, U.S. Patent No. 2019/184653 Al.

Claims

C L A I M S

1. An impregnation device for continuously obtaining a fibre-reinforced thermoplastic filament from continuous fibre strands or yarns fed through an inlet for being impregnated with a thermoplastic resin, said impregnation device comprising: an impregnation cavity having a rectangular width-wise cross-section and being tapered towards a circular cross-section outlet of the impregnated strands or yarns; wherein the impregnation cavity is shaped to undulate sinusoidally in a downstream direction.

2. The impregnation device according to the previous claim comprising two feeders for feeding the thermoplastic resin into the cavity wherein one feeder is arranged upstream in respect of the other.

3. The impregnation device according to the previous claim wherein the feeders are arranged to inject the thermoplastic resin into the cavity in an oblique downstream angle.

4. The impregnation device according to any of the previous claims wherein the impregnation cavity is shaped in a tapered conical manner.

5. The impregnation device according to any of the previous claims wherein the fibre strand or yarn inlet comprises a plurality of comb-shaped apertures followed downstream by a convex round spreading protuberance for spreading the strands or yarns.

6. The impregnation device according to any of the previous claims wherein the impregnation cavity is formed by successive concave and convex curves.

7. The impregnation device according to the previous claim wherein the successive concave and convex curves are formed by two parallel walls, an upper wall and a lower wall, forming an inner surface of the cavity.

8. The impregnation device according to any of the previous claims wherein a thermoplastic resin inlet is arranged horizontally and/or vertically.

9. The impregnation device according to any of the previous claims wherein the spreading protuberance is arranged downstream from the strand or yarn inlet and arranged upstream from the impregnation cavity and said spreading protuberance comprises a lengthwise cross-section, in the inlet-outlet direction, having a smooth curved surface line increasing in height from a first end to a midpoint and a steeper curved surface line decreasing in height from the midpoint to a second end.

10. The impregnation device according to any of the previous claims wherein the impregnation device comprises an upper body and a lower body, wherein the thermoplastic resin inlet is at a same plane of the outlet of the reinforced filament, said plan being a plan which divides said upper and lower bodies.

11. The impregnation device according to the previous claim further comprising a fixed hinge on a rearward side for rotating vertically the upper body to be raised at an angle between 0° to 90°.

12. The impregnation device according to any of the claims 10-11 wherein the lower body comprises two parallel walls that protrude upwards from the lower body and extend along lateral borders of the lower body. [29]

13. The impregnation device according to any of the claims 10-12 wherein the upper body comprises two parallel and symmetrical sinusoidal indentations extending continuously through the inner surface of the upper body.

14. The impregnation device according to any of the claims 10-13 wherein the upper body comprises two parallel and symmetrical sinusoidal inner walls with an increasing thickness towards the outlet of the reinforced filament.

15. The impregnation device according to the previous claim wherein the two parallel walls of the inner surface of the lower body matches with two parallel and symmetrical sinusoidal indentations of the inner surface of the upper body.

16. The impregnation device according to any of the previous claims wherein the device further comprises a removable frontal cylinder attached to the outlet of the reinforced filament.

17. The impregnation device according to any of the previous claims, wherein comprises a cooling consolidation die adjacent to the removable frontal cylinder.

18. The impregnation device according to any of the previous claims wherein further comprises at least four metal brackets having adjustable lobe knobs for supporting and/or fixing the impregnation device on a flat surface base or extrusion line.

19. Method to operate the device according to any of the claims 1-18 to manufacture a continuous fibre reinforced thermoplastic filament.

20. The method according to the previous claim wherein the thermoplastic resin is crystalline, semi-crystalline or amorphous.

21. The method according to any of the claims 19-20 wherein the thermoplastic resin is selected from polyolefins, polyesters, vinyl aromatic polymers, halogenated vinyl polymers, polyaryletherketones, polyamides, polyimides, polyacetals, polyacrylates, polycarbonates, thermoplastic polyurethanes, polysulfones, polyethersulfones, polyetheramides, polyetherimides, polyphenylene sulfides, polyphenylene oxides, polyphenylene ethers and mixtures thereof.

22. The method according to any of the claims 19-21 wherein the continuous fibre strands or yarns are made of jute fibres, flax fibres, aramid fibres, glass fibres, carbon fibres, silica fibres, or combinations thereof.

23. Continuous fibre reinforced thermoplastic filament obtained by the method according to any of the claims 19-22.