Device and method for the in-situ production of hybrid, continuously fiber-reinforced and highly oriented thermoplastic fiber composite structures with high fiber volume content and strong geometric curvatures

Abstract

Device designed for the additive manufacturing of components from fiber-reinforced semi-finished products with a rectangular cross-section, where this cross-section is either rectangular with a longer side of the cross-section >1000µm to <3600µm and a shorter side >100µm to <300µm, or is square with one side of the square cross-section of >100µm to <3600µm, having: • At least one feeding unit for feeding strip-shaped or flat semi-finished products with a rectangular cross-section, • At least one conveying unit, • At least one heating element or heater, • At least one conversion unit as well as • At least one outlet, characterized in that the forming device is designed to transform a square cross-section of the drawn-in strip-shaped or flat semi-finished products into an annular cross-section, and that the forming device has an inner radius of more than 150 µm and less than 5000 µm, and that the device has at least one cutting device designed for cutting fiber-reinforced materials

Description

[0001] The invention relates to a method and a device for the in-situ production of hybrid, continuously fiber-reinforced and highly oriented thermoplastic fiber composite structures with high fiber volume content and strong geometric curvatures.

[0002] Additive manufacturing processes are of great importance in the industrial and scientific sectors. Their ease of application, in particular, makes prototypes, small series, and individual, even customer-specific, production of the most complex components and assemblies possible with machine precision at any time. To expand the application area to include the use of fiber-reinforced materials, the state of the art teaches the following:

[0003] US 2022 / 0001600 A1 describes various embodiments related to 3D printers, fiber-reinforced tapes, and their methods of use. In one embodiment, a void-free reinforced strand-like bundle of individual fibers is introduced into a conduit nozzle. The reinforced filament comprises a core, which may be continuous or semi-continuous, and a matrix material surrounding the core. The reinforced filament is heated to a temperature above the melting temperature of the matrix material and below the melting temperature of the core before the strand material is extruded from the nozzle.

[0004] WO 2020 / 094829 A1 describes a printhead for the additive manufacturing of a fiber composite material. The fiber composite material comprises fiber reinforcement in a polymer matrix, and the printhead comprises • an infiltration unit for mixing and / or infiltrating a fiber roving with a molten polymer; • at least one feed for a polymer and / or a fiber roving to the infiltration unit; • a heating element for at least partially melting the polymer within the infiltration unit; • at least one deflection element within the infiltration unit and • a discharge for the resulting fiber composite material from the infiltration unit.

[0005] The molten polymer is conveyed within the infiltration unit. The polymer flow direction runs from the inlet to the outlet along a channel.

[0006] The fiber roving can be guided within the channel by deflection around the deflection element in sections transverse to the polymer flow direction from the feed to the discharge.

[0007] Furthermore, the use of a print head for additive manufacturing as well as a method for additive manufacturing and a fiber composite material produced by the print head are also the subject of the disclosure.

[0008] EP 3 909 699 A1 describes a method for producing a nozzle for additive manufacturing. The method comprises the following steps: • providing a starting body with a hollow section, • Moving the starting body and a tool relative to each other and • Pressing a tool against an outer contour of the hollow portion of the starting body while moving the starting body and the tool relative to each other, in such a way that the outer contour of the hollow portion of the starting body is at least partially tapered by forming. Furthermore, the invention relates to a manufacturing device for producing a nozzle for additive manufacturing using the disclosed method.

[0009] WO 2018 / 165103 A describes a device and a method for improving the material properties of fiber-reinforced components.

[0010] The described device is designed for additive manufacturing and includes • a conveyor device, • a positioning system to which the conveyor device is attached; • a conveying channel communicating with an outlet opening of the conveying device, and • with an adjusting device assigned to the conveying channel for the controlled change of an orientation of a discharge opening of the conveying channel, wherein the conveying device is designed as an extruder or as a pump which is adapted to convey a flowable suspension and wherein the extruder or the pump is movable in translation by the positioning system. Furthermore, the invention relates to a method for producing a body from an anisotropic material using a described device, wherein the flowable suspension contains fibers which are aligned when conveyed out of the conveying channel.

[0011] US 2021 / 0362406 A1 describes a continuous fiber-reinforced thermoplastic composite material in which a plurality of yarns or ribbon-like intermediate materials are combined to form a rod shape, as well as a method for producing the same. The composite material is suitable for producing a reinforcing material for various thermoplastic injection-molded products and for producing a fiber-reinforced thermoplastic part using 3D printing. In the manufacturing process, a plurality of yarns or ribbon-like intermediate materials are continuously inserted into a heating unit and heated to at least the melting temperature of a thermoplastic resin in the intermediate material to melt a portion of the thermoplastic resin on the surface of the intermediate material. At least two intermediate materials are then passed through a nozzle to produce a rod shape.

[0012] US 2020 / 0324462 A1 relates to a starting material for use in an extruder in an additive manufacturing system, which is configured as a braided, intermingled tow filament, and a method for producing the same. The method comprises providing a bundle of intermingled tow material having a fiber count in the range of about 1,000 fibers to about 25,000 fibers, with thermoplastic fibers intermingled therewith, wherein the tow material in the filament is in the range of about 50 to 75 volume percent and the volume percentage of the thermoplastic material is in the range of about 25 volume percent to about 50 volume percent. The method comprises dividing the length of the intermingled tow material into sections, twisting each section into a strand to form a plurality of strands of twisted tow material, and braiding the strands together.

[0013] US 2022 / 0001600 A1 describes three-dimensional printing of reinforced filaments and methods of using them. The method includes impregnating a continuous core element with a matrix material at a first coextrusion die of a printhead to form a continuous core reinforced filament, coating an outer surface of the continuous core reinforced filament at a second coextrusion die of the printhead, attaching the coated continuous core reinforced filament to a surface at a point, and pulling the coated continuous core reinforced filament from the attachment point as the printhead is moved relative to the part. In one embodiment, a void-free reinforced filament is fed into a conduit die.The reinforced filament comprises a core, which can be continuous or semi-continuous, and a matrix material surrounding the core. The reinforced filament is heated to a temperature above the melting temperature of the matrix material and below the melting temperature of the core before the filament is drawn from the conduit die.

[0014] None of the available documents describes a process or device that makes highly ordered fiber-reinforced semi-finished products accessible to additive processes.

[0015] The object of the invention is to overcome the obvious disadvantages of the prior art and to propose a device and a method which can convert existing ribbon-shaped filaments into a suitable form for 3D printing or for an additive manufacturing process.

[0016] According to the invention, the object is achieved by the device according to claim 1 and the method according to claim 7.

[0017] A device according to the invention is designed for the additive manufacturing of components from fiber-reinforced semi-finished products with a square cross-section, where this cross-section is either rectangular with a longer side of the cross-section >1000µm to <3600µm and a shorter side >100µm to <300µm, or is square with one side of the square cross-section of >100µm to <3600µm, and has at least: • a feed unit for feeding strip-shaped or flat semi-finished products with a square cross-section, • a conveyor unit, • a heating element or heater, • a forming device and • an outlet.

[0018] The device according to the invention is characterized in that the forming device according to the invention is designed to form a square cross-section of the drawn-in strip-shaped or flat semi-finished products into a circular cross-section, and that the forming device has an inner radius of more than 150 µm and less than 5000 µm, and that the device has at least one cutting device which is designed for cutting fibre-reinforced materials.

[0019] For the purposes of this document, a 3D printing head is the part of a 3D printer capable of preparing a semi-finished product in such a way that it can be combined to form a new component. This type of combination of semi-finished products into components is a step-by-step process and is more generally referred to as additive manufacturing or 3D printing—also for the purposes of this document.

[0020] For a device suitable for additive manufacturing or 3D printing, the 3D print head is moved using a positioning device to precisely dispense the previously prepared semi-finished product. For example, and without limitation, the 3D print head is moved in two orthogonal spatial directions (x-direction and y-direction). To create the third spatial dimension (z-direction), a positioning device can also be provided on the print head. Furthermore, as a non-exhaustive example, it is also conceivable that the print head itself is immobile and the print bed, i.e. the location on which the component is manufactured, is positioned beneath the print head, or that the print head is positioned translationally only along one spatial axis and the print bed is moved along the remaining spatial axes to manufacture the component.

[0021] For the purposes of this document, fiber-reinforced semi-finished products are understood to be a type of semi-finished product that is fundamentally suitable for additive manufacturing processes.

[0022] The pre-impregnated fiber material - in other words fiber and matrix - is also understood by definition as a semi-finished product.

[0023] Such suitable semi-finished products are often made of a thermoplastic polymer. This polymer exhibits characteristic temperatures at which the desired properties for 3D printing are achieved. Above the so-called glass transition temperature, the material becomes sufficiently fluid that it can be extruded from the outlet of the 3D printing head and is moldable. Above the so-called decomposition temperature, the chemical structure of the thermoplastic polymer can change to such an extent that it loses all fluidity and is therefore no longer accessible to additive manufacturing.

[0024] The temperature at which the semi-finished product has good processing properties is a material-specific and therefore characteristic temperature.

[0025] In the case of fiber-reinforced materials, this thermoplastic material is also referred to as the matrix material or as the polymer matrix system of the semi-finished product in which fibers are embedded to create the fiber reinforcement.

[0026] The combination of matrix material and reinforcing fiber material is referred to as fiber-reinforced semi-finished product or, in the sense of this document, as filament.

[0027] Such filaments are often offered commercially with different cross-sectional geometries. Round or flat geometries are common in the cross-sections of the filaments. Flat filaments are characterized by a mostly rectangular cross-section, with a ribbon-like appearance. Thus, these flat filaments are also referred to as ribbon-shaped semi-finished products or ribbon-shaped or flat filaments.

[0028] To move the filaments through the print head, a mechanical feed device is required. This is referred to below as the feed unit. Furthermore, a heating element or heater is used to generate the temperature characteristic for processing the filament.

[0029] The term "forming device" refers to a mechanical device for changing the cross-section of the inserted filaments. To perform the forming process, the filament should first be heated to its characteristic processing temperature. It is possible that this temperature may differ from the temperature the filament has upon or after leaving the print head.

[0030] In embodiments of the invention, an additional forming device is arranged in front of the print head according to the invention in such a way that the forming of any filament cross-section - for example and without being limited to a round cross-section or a square cross-section - into a ribbon-shaped filament takes place.

[0031] When a ribbon-shaped filament is formed by the forming device, the resulting cross-section is circular. In this document, a circular cross-section is understood to include not only a mathematically defined circular ring, but also a deformed circular ring, a circular ring segment, or a deformed circular ring segment. The deformation is a deviation from an ideal circular ring geometry. For example, and without limitation, the elliptical equivalent of a circular ring is also considered a deformed circular ring.

[0032] In embodiments of the invention, the fiber reinforcement of the fiber-reinforced semi-finished product is a continuous fiber reinforcement or a bundle of individual filaments. The fiber reinforcement is selected from plastic fibers, basalt fibers, carbon fibers, glass fibers, natural fibers, or a mixture thereof.

[0033] The plastic fibers are selected such that their glass transition temperature is above the melting temperature of the respective matrix material. For example, and without limitation, this can be achieved using aramid fibers or polyetheretherketone (PEEK) fibers.

[0034] Continuous fiber reinforcement or a bundle of individual filaments is advantageous because it allows a variety of commercially available semi-finished products to be processed in the print head according to the invention, thus increasing the range of such material combinations. This further expands the range of applications and the economic advantage of cost reduction or savings.

[0035] In embodiments of the invention, the feed device and the conveyor device are identical. This is advantageous because it enables compact designs of the print head according to the invention. This also ensures use in smaller machines for additive manufacturing, which advantageously increases economic applicability.

[0036] In embodiments of the invention, the heating element has a thermal power of more than 45 W and less than 200 W.

[0037] In embodiments, the heating element has a thermal power of more than 50 W and less than 175 W, and particularly preferably more than 55 W and less than 150 W. This is advantageous because it provides a sufficiently large amount of thermal power to heat the matrix material of the fiber-reinforced semi-finished product. Due to the potential thermal conduction of the fibers embedded in the matrix material, the desired heating must occur quickly; otherwise, the desired formability and flowability of the matrix material cannot be maintained.

[0038] In embodiments of the invention, the forming device is designed at least in one piece. This is advantageous because, on the one hand, a compact design of a print head according to the invention can be achieved, and, on the other hand, the manufacturing effort for producing the forming element is kept to a minimum. Both of these factors have a direct economic impact, since a compact design makes the nozzle accessible even for smaller machines, and simpler manufacturing reduces production costs.

[0039] In embodiments of the invention, the forming device has an inner radius of more than 150 µm and less than 5000 µm, preferably of more than 300 µm and less than 3500 µm and particularly preferably of more than 500 µm and less than 3000 µm.

[0040] This is advantageous because it ensures optimal forming of the intended flat strip semi-finished product. The optimized contouring of the forming device enables geometric cross-sectional changes with very low forming forces. Especially in conjunction with the thermal device, this enables low-damage forming of the sometimes very brittle materials.

[0041] In embodiments of the invention, the forming device is the outlet of the print head according to the invention, advantageously due to its design. The outlet geometry advantageously enables the precise deposition and consolidation of the fiber strands. The final contour of the consolidated component can only be ensured through highly accurate and repeatable, precisely locally variable deposition. The consolidation of the composite material must be carried out in such a way that the reinforcing fibers are not damaged during deflection and forming. Fiber damage would otherwise have a very negative impact on the component properties, and this is advantageously avoided in this embodiment.

[0042] In further embodiments of the invention, a very short contact zone between the heating element and the semi-finished product is formed in the forming element. This results in a short residence time of the semi-finished product in the hot area, and the necessary melting temperatures must be reached very quickly. Overriding the conventional processing temperature and the circular cross-section of the formed semi-finished product advantageously promote rapid heat transfer from the contact zone of the forming nozzle to the semi-finished product. This results in advantageously lower porosity and good layer bonding, i.e., good adhesion of the different printing layers.

[0043] In embodiments of the invention, the device comprises at least one cutting device designed for cutting fiber-reinforced materials. This advantageously improves the handling of the device, as any desired length of filament can be cut as needed. This eliminates the need for complex pre-cutting, resulting in an economic advantage, as commercially available continuous material can be used.

[0044] A further aspect of the invention relates to a method for producing a fiber-reinforced component by means of additive manufacturing and a square, fiber-reinforced semi-finished product, wherein the method according to the invention comprises the following steps: • drawing in a fiber-reinforced semi-finished product with a square cross-section, • first heating of the drawn-in semi-finished fiber product within a nozzle according to the invention to a temperature above the glass transition temperature of the polymer matrix system of the semi-finished product and below the melting temperature of the fiber material of the semi-finished product, • Transformation of the square cross-section into the cross-section of a circular ring, • Discharge of the semi-finished product with a cross-section of a circular ring and • Flatten and join to form an additively manufactured component using positioning.

[0045] In embodiments of the invention, the quadrangular cross-section of the fiber-reinforced semi-finished product is a rectangular cross-section. A rectangular cross-section is characterized by a longer side and a shorter side. In a rectangular embodiment, the longer side of the rectangular cross-section measures more than 1000 µm and less than 3600 µm, and the shorter side of the cross-section measures more than 100 µm and less than 300 µm. In a square embodiment, one side of the square cross-section measures more than 100 µm and less than 3600 µm.

[0046] The cross-sections are measured at a material temperature of 20°C and 50% relative humidity in an air atmosphere. The use of these filaments is advantageous because they are commercially readily available, thus eliminating the need for costly fabrication.

[0047] In embodiments of the invention, the fiber-reinforced semi-finished product is drawn in at a speed of more than 3 mm / s and less than 10 mm / s. This is advantageous because it ensures sufficient heating of the semi-finished product—more precisely, the matrix material. Thus, the forming process according to the invention is controlled and promotes targeted discharge of the formed semi-finished product from the outlet of the print head according to the invention.

[0048] In embodiments of the invention, the nozzle temperature is set to more than 250°C and less than 450°C. Temperatures are measured using a suitable contact thermometer, for example, but not exclusively, under standard atmospheric conditions (usually in an air environment at 1013 hPa air pressure and 50% relative humidity). This is advantageous because the melting temperatures of conventional thermoplastics, and often the melting temperatures of the matrix materials of semi-finished fiber products, lie within this temperature range. This is therefore extremely advantageous for engineering plastics.

[0049] Furthermore, the use of a device according to the invention for producing fiber-reinforced components is also part of the invention. The fiber-reinforced components are produced in particular using the method according to the invention.

[0050] For the realization of the invention, it is also expedient to combine the above-described inventive embodiments, embodiments and features of the claims.

[0051] The subject matter of the invention is described in more detail below with reference to non-limiting figures and an embodiment. Fig. Figure 1 schematically shows the structure of a fiber-reinforced semi-finished product prior to introduction into a forming device according to the invention. In particular, a semi-finished product with a rectangular cross-section is shown. The edge lengths are designated 001 and 002. Furthermore, a section plane through the semi-finished product is drawn at points AA. The fiber reinforcement in the rectangular cross-section is also indicated by the black dots shown. Fig. Figure 2 shows the introduction of a fiber-reinforced semi-finished product into a nozzle according to the invention. Two sectional planes are drawn into the nozzle. The first sectional plane is shown in the upper half of the image and is drawn above the forming device according to the invention. On the left side of the image, the cross-section of the sectional plane is shown in more detail; the introduced semi-finished product can be seen with a still square cross-section. The lower half of the image shows the lower sectional plane in the nozzle according to the invention. Here, too, the cross-section can be seen in the left half of the image, which shows the forming of the fiber-reinforced semi-finished product with a square cross-section into a circular cross-section. The recessed circular ring segment is closed by applying the thus formed semi-finished product to the print bed - or already placed material. Fig.Figure 3 schematically shows the discharge of a previously formed strip-shaped semi-finished product. Due to the blind seam's migration across the web width and a certain oversize diameter, the individual filaments can be deposited with minimal tension (due to displacement) during cornering. Purely due to geometrically determined length ratios, within a tight radius on a printed web with a certain web width, compression on the inside and stretching on the outside of the continuous fibers will occur (this effect becomes even more significant with larger web widths). The optimized forming unit counteracts this with the measures described above.

[0052] In one embodiment, the 3D printing head according to the invention is designed for a semi-finished product that uses polycarbonate as the matrix material and carbon fibers as fiber reinforcement. At the same time, the method according to the invention for producing a fiber-reinforced component using additive manufacturing and a fiber-reinforced semi-finished product is described in more detail.

[0053] The feed unit according to the invention, which is designed for feeding strip-shaped semi-finished products, has a rectangular opening with the dimensions (I × b) mm 2 The corresponding filaments have a rectangular cross-section with the dimensions (I × b) mm 2 and are characterized by a flat or ribbon-shaped appearance.

[0054] The conveyor unit according to the invention is implemented by an electric motor with a power of 40 W. A rotational movement and a clamping mechanism establish mechanical contact between the semi-finished product and the motor shaft to ensure the desired conveying speed of 6 mm / s, even when heated. This represents the first process step of drawing in a fiber-reinforced semi-finished product with a square cross-section.

[0055] The heated state of the print head according to the invention is generated by a heating element with a thermal output of 60 W. The heating element enables the print head to reach temperatures from room temperature (usually 22 °C) up to 450 °C. In the heated state referred to here, the print head has a temperature of 360 °C. This reflects the process step of first heating the drawn-in semi-finished fiber product within a nozzle according to the invention to a temperature above the glass transition temperature of the polymer matrix system of the semi-finished product and below the decomposition temperature of the polymer matrix system of the semi-finished product.

[0056] The thus heated print head then heats the conveyed semi-finished product in a contact zone, which then changes its cross-section from a flat, band-like appearance to a tubular appearance in the forming device according to the invention. At this point, the process step of forming the square cross-section into the cross-section of a circular ring takes place.

[0057] Immediately after the forming unit, an outlet is placed which already enables the discharge of the formed semi-finished product, which corresponds to the process step of discharging the semi-finished product with the cross-section of a circular ring.

[0058] Subsequently, the process step of flattening and joining to form an additively manufactured component by means of positioning is carried out.

[0059] For better handling, in this embodiment, a cutting device is arranged upstream of the outlet in order to be able to cut the carbon fibers of the semi-finished product used in a controlled manner, thus achieving good control over the amount of semi-finished product used. Reference symbol 001 Length 002 Width 003 Seam AA intersection axis AA CC cutting axis CC DD Cutting axis DD R bending radius

Claims

[1] Device designed for the additive manufacturing of components from fiber-reinforced semi-finished products with a square cross-section, where this cross-section is either rectangular with a longer side of the cross-section >1000µm to <3600µm and a shorter side >100µm to <300µm, or is square with one side of the square cross-section of >100µm to <3600µm, comprising: • At least one feeding unit for feeding strip-shaped or flat semi-finished products with a square cross-section, • At least one conveyor unit, • At least one heating element or heater, • At least one forming device and • At least one outlet, characterized by that the forming device is designed to form a square cross-section of the drawn-in strip-shaped or flat semi-finished products into a circular cross-section, and that the forming device has an inner radius of more than 150 µm and less than 5000 µm, and that the device has at least one cutting device which is designed for cutting fibre-reinforced materials [2] Device according to claim 1, characterized by that the fiber reinforcement of the fiber-reinforced semi-finished product is a continuous fiber reinforcement and is selected from plastic fibers, basalt fibers, carbon fibers, glass fibers, natural fibers or a mixture thereof. [3] Device according to one of the preceding claims, characterized by that the feed device and the conveyor device are identical. [4] Device according to one of the preceding claims, characterized by that the heating element has a thermal power of more than 45 W and less than 200 W. [5] Device according to one of the preceding claims, characterized bythat the forming device is designed as at least one piece. [6] Device according to one of the preceding claims, characterized by that the forming device is the outlet. [7] Method for producing a fiber-reinforced component by means of additive manufacturing and a square, fiber-reinforced semi-finished product, the method comprising the following steps: • drawing in a fiber-reinforced semi-finished product with a square cross-section, • first heating of the drawn-in semi-finished fiber product within a nozzle according to the invention to a temperature above the glass transition temperature of the polymer matrix system of the semi-finished product and below the melting temperature of the fiber material of the semi-finished product, • Transformation of the square cross-section into the cross-section of a circular ring, • Discharge of the semi-finished product with a cross-section of a circular ring and • Flatten and join to form an additively manufactured component using positioning. [8] Method according to claim 7, characterized by that the quadrangular cross-section of the fiber-reinforced semi-finished product is a rectangular cross-section, wherein the longer side of the rectangular cross-section measures more than 1000 µm and less than 3600 µm and the shorter side of the cross-section measures more than 100 µm and less than 300 µm or is a square cross-section, wherein one side of the square cross-section measures more than 100 µm and less than 3600 µm. [9] Method according to one of claims 7 or 8, characterized by that the fiber-reinforced semi-finished product is drawn in at a speed of more than 3 mm / s and less than 10 mm / s. [10] Method according to one of claims 7 to 9, characterized by that the nozzle temperature is more than 250°C and less than 450°C. [11] Use of a device according to one of claims 1 to 6 for producing fiber-reinforced components.

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