A system and method for producing chopped strands from for the manufacture of chopped strand mats

Abstract

The invention relates to a system and method for producing chopped strands (10) from discontinuous fibers for manufacturing chopped strand mats. The system comprises: - first guiding means for a band-like strand; - a pair of opposed fiber distributing rollers (3, 4) of parallel horizontal axis arranged against each other at a tangential zone between them and being rotatable along their axis in opposed directions. The first roller has a circumferentially grooved profile surface made of alternate circumferential peaks (31) and valleys (32), and the second roller has a smooth surface (4'). The space between the valleys and the smooth surface houses portions of the band; - a scrapper (5) placed below the tangential zone between the rollers and provided with multiple vertical prongs (51) at its upper end. Each prong is located close to the bottom of the valleys (32); - second guiding means; and - a rotatory cutter.

Description

[0001]

[0002] “A system and method for producing chopped strands from for the manufacture of chopped strand mats”

[0003] Technical field of the invention

[0004] The invention relates to the technical field of chopped strand mats, particularly to a system and method for producing chopped strands comprising discontinuous fibers, said chopped strands being suitable for the manufacture of chopped strand mats.

[0005] Background of the invention

[0006] It is well known that fiber- re info reed composites are the best choice for the production of certain products that require specific properties in specific areas. For instance, for certain applications such as a rear parcel shelf of a motor vehicle, it is sought for the final composite material to have a high flexural and tensile modulus. The final composite is as light as possible and therefore, has a low density. Advantageously, the final composite is as homogeneous as possible (symmetry of the properties), which depends directly on the homogeneity of the starting fibrous structure. Producing fiber-reinforced composites involves shaping a fibrous structure such as a mat in a mould and then injecting a polymer-based resin in order to impregnate said structure. The resin is then solidified by crosslinking or by cooling, depending on whether the resin is thermosetting or thermoplastic. The fibrous structure must therefore exhibit a set of properties. In particular, prior to impregnation, it must present good drapability, i.e., it must be easy to shape and thus easily deformable by hand, without any tendency to form wrinkles. It is also desirable for the structure to have shape memory, that is to say, to retain the shape which it is given, for example by hand, by deforming it as little as possible under the effect of its own weight. For a given mass per unit area, the fibrous structure must also be as permeable as possible to the impregnating resin and reinforce the final material as much as possible.

[0007] A chopped strand mat (CSM) is a fiber-reinforced composite material that uses chopped strands of glass fiber laid randomly and held together with a binder or stitching and are typically 2-5 cm in length. The advantages of CSM compared with other composite materials are its strength-to-weight ratio, specific stiffness, and the equal distribution of loads in all directions on its surface. CSM can be used between layers of woven fabric to help build thickness quickly and aid in all layers bonding well together.

[0008] The substitution of fiberglass in CSM for other fibers with better mechanical properties, more recyclable, cheaper or with a smaller carbon print is desirable.

[0009] Some of these new materials are discontinuous fibers like natural fibers. However, the production of random distributed CSM using discontinuous fibers brings with it several technical challenges.

[0010] One of those challenges is to reduce the weight of the yarn of the discontinuous fibers to 30-40 tex, a size convenient to produce homogeneous and nevertheless permeable mats. Usually, discontinuous fibers, like flax fibers, do not form stable threads below 300 tex.

[0011] Patent document EP1478797 relates to a composite material comprising a fibrous structure comprising a mat. Said mat comprises a homogeneous mixture of discontinuous glass fibers and discontinuous natural fibers and a matrix comprising a thermosetting resin, said material being obtained after impregnation of said fibrous structure with said thermosetting resin. The invention disclosed herein also relates to a process for manufacturing the material, comprising shaping the fibrous structure in a mold and then carrying out the impregnation of said fibrous structure with a thermosetting resin.

[0012] The composite material disclosed in EP1478797, is a mat comprising discontinuous natural fibers and discontinuous glass fibers, the natural fibers having a length of less than 150 cm, whereas glass fibers have a length of between 10 and 200 mm. The natural fibers are separated and carded, followed by needle punching, in other words, the resulting mat will have fibers aligned in the same direction. This is not desirable because the property of the equal distribution of loads in all directions is lost. Another technical challenge is distributing randomly the chopped discontinuous fibers in a given surface because discontinuous threads tend to cake up.

[0013] The patent documents JP2022149483, EP3578711, ES2870850, JP6774651 and ES2884306 teach different methods and materials related to the production of randomly orientated chopped mats using an aggregate of discontinuous reinforcing fibres and a matrix resin.

[0014] The patent documents EP3719078, EP2808433, EP2808432, EP2810979 and ES2539902 teach new composite materials and manufacturing methods using randomly distributed carbon fibre bundles.

[0015] It is desirable to find new manufacturing methods that allow the production of chopped strand mats with new materials, especially with discontinuous fibers.

[0016] Description of the invention

[0017] With the aim of finding an alternative to the known solutions, a system for producing chopped strands suitable for the manufacture of chopped strand mats is disclosed in claim 1 . The chopped strands to be produced can be made from discontinuous fibers, like flax fibers, as well as from continuous fibers, like glass fibers and carbon fibers.

[0018] The system is characterized by comprising:

[0019] - first guiding means configured for receiving and orientating in a vertical position a band-like strand made of fibers;

[0020] - a pair of opposed fiber distributing rollers of parallel horizontal axis, the rollers being arranged against each other at a tangential zone between them and being rotatable along their axis in opposed directions, wherein the first roller of the fiber distributing rollers is a grooved roller that has a circumferentially grooved profile surface made of alternate circumferential peaks and valleys, and the second roller of the fiber distributing rollers has a smooth surface in contact with the peaks of the profile surface of the first roller along the tangential zone, the spaces between the valleys and the smooth surface in the tangential zone being configured to house portions of the band-like strand made of fibers, wherein the first guiding means are placed above the tangential zone and wherein at least one of the fiber distributing rollers is connected to a motor;

[0021] - a scrapper placed at a certain distance below the tangential zone between the fiber distributing rollers, the scrapper being provided with multiple vertical prongs at its upper end, wherein each prong is located inside a corresponding valley of the grooved profile surface of the first roller of the fiber distributing rollers, the prongs being closer to the bottom of the valleys than to the peaks;

[0022] - second guiding means arranged next to the scrapper, that configures a vertical passage below the upper end of the scrapper; and

[0023] - a rotatory cutter arranged at the lower end of the vertical passage of the second guiding means.

[0024] It should be clarified that a band-like strand made of fibers is the result of pressing and flattening a strand made of fibers in order to achieve the form of a band, that is to say, with a cross section wider than its thickness.

[0025] The fiber distributing rollers distribute and divide the fiber material of the band-like strand by compacting it into the valleys of the first roller, producing corresponding a group of portions of the band-like strand. Since said group of portions, depending on the nature of the fibers, tends to remain embedded within the valleys, the scrapper disengages the group of portions, resulting a group of strands. Then, the lower ends of said group of strands are driven and / or pulled to the rotatory cutter for each strand to be cut in shorter strands called chopped strands.

[0026] According to another feature of the system of the invention, the vertical distance between the upper end of the scrapper and the tangential zone (or the closest point between the fiber distributing rollers) between the fiber distributing rollers is comprised between 2 mm and 10 mm. Preferably, said vertical distance is about 8 mm. According to a particular embodiment of the system, the vertical distance between the upper end of the scrapper and the tangential zone between the fiber distributing rollers (the closest point between the rollers) is comprised between 2 and 6 times the thickness of the chopped strands to be produced by the system.

[0027] According to a preferred embodiment, the distance between the edges of the peaks and the bottoms of the valleys of the grooved profile surface is between 0.8 and 2.0 times the distance between two consecutive peaks, preferably 1 .0 time.

[0028] Preferably, the first roller and the second roller of the fiber distributing rollers have equal diameters. The rollers are arranged against each other with contact pressure between them at the tangential zone. The first roller is connected to a motor and the second roller rotates due to the contact with the rotating first roller.

[0029] The grooved profile surface of the first roller of the fiber distributing rollers is preferably made of a hardened metallic material and the smooth surface is made of a hardened metallic material, rubber or a plastic material. When processing flax fibers, the smooth surface is made of a hardened metallic material, when working with carbon fibers, the smooth surface is made of rubber of a hardness of 80-100 Shore A, and when working with recycled glass fibers, the smooth surface is made of rubber of a hardness of 75-80 Shore A. The scrapper is preferably made of iron or steel.

[0030] According to another preferred embodiment, the first roller of the fiber distributing rollers is provided with air recirculation means and / or air-cooling means.

[0031] Regarding the rotatory cutter of the preferred embodiment of the system, it comprises two cutter surfaces, wherein the first cutter surface is a retaining surface and the second cutter surface is a cutting blade arranged in a plane perpendicular to tangential zone between the rollers. Thus, the cutting blade cuts, according to a horizontal cutting plane, the strands that are removed by the scraper from the valleys of the grooved profile surface of the first roller of the fiber distributing rollers, that have been made by the portions of the band-like strand that have been placed and pressured within said valleys.

[0032] Also preferably, the first cutter surface is a fixed serrated or grooved surface and the cutting blade of second cutter surface is able to rotate around a cutter rotation axis that is parallel to tangential zone between the fiber distributing rollers and perpendicular to the rotation axis of the rollers.

[0033] More preferably, the first cutter surface is configured as a vertical passage of a through hole with a transversal cross section with an oblong curved shape or with a bean shape, the wall of the through hole having retaining means to retain one or more portions of the band-like strand made of fibers. The retaining means provided in the wall of the through hole is a grooved surface with teeth provided along the inner perimeter of the through hole.

[0034] In addition, in the preferred embodiment of the system, the rotatory cutter has a plurality of second cutter surfaces arranged radially with regard to the cutter rotation axis.

[0035] According to the preferred embodiment of the system, it further comprises, downwards the rotatory cutter, a pulling means in the form of two opposed pulling rollers with their axis parallel to the axis of the fiber distributing rollers. The pulling rollers are arranged against each other and are rotatable along their axis in opposed directions. Said pulling means pulls the strands before being cut in the rotary cutter and one cut in chopped strands, they throw the chopped strands in a uniform way so that they are rightly randomly distributed on a surface to produce a chopped strand mat.

[0036] Each one of the two pulling rollers has a grooved profile surface. Said grooved profile surface is configured with a groove forming a knuckle thread or a round thread. In this way, the strands resulting from the distribution of the bandlike strand in the fiber distributing rollers keeps separated and do not merge again as were in the band-like strand.

[0037] The preferred embodiment of the system contemplates that the surface of the pulling rollers has a very low surface roughness, namely, it has an Ra about between 0.2 and 0.3. This avoids the strands become embedded in the pulling rollers and no scrapper is needed.

[0038] The pressure between the pulling rollers is very low, since a compaction of the fibers is not wished. The preferred distance between the surfaces of the pulling rollers is between 0.05 and 0.1 mm.

[0039] With regard to a second aspect of the invention, a method for producing chopped strands from fibers suitable for the manufacture of chopped strand mats is disclosed. The method of the invention is characterised by comprising the following steps: a) providing a band-like strand made of discontinuous fibers; b) orientating said band-like strand in a vertical position; c) introducing said orientated band-like strand between a pair of opposed fiber distributing rollers of parallel horizontal axis, with the width of the band-like strand extending parallel to the axis of the rollers, the rollers being arranged against each other at a tangential zone between them and being rotatable along their axis in opposed directions, wherein the first roller of the fiber distributing rollers is a grooved roller that has a circumferentially grooved profile surface made of alternate circumferential peaks and valleys, and the second roller of the fiber distributing rollers has a smooth surface in contact with the peaks of the profile surface of the first roller along the tangential zone, the space between the valleys and the smooth surface in the tangential zone being configured to house portions of the band-like strand as the band-like strand is driven downwards passing between the fiber distributing rollers, wherein at least one of the rollers is connected to a motor; d) removing with a scrapper the portions of the band-like strand housed at the bottoms of the valleys while the rollers rotate, separating them from the first roller, said removed portions constituting strands; e) guiding the strands through a vertical passage below the upper end of the scrapper; and f) cutting the strands with a rotatory cutter arranged at the lower end of the vertical passage, obtaining chopped strands. According to a preferred variant of the method, before cutting the strands, the lower parts of the strands pass through the rotatory cutter and are pulled downwards by a pulling means in the form of two opposed pulling rollers with their axis parallel to the axis of the fiber distributing rollers, the pulling rollers being arranged against each other and being rotatable along their axis in opposed directions, wherein each one of the two pulling rollers has a grooved profile surface. Said pulling rollers are the ones previously disclosed with regard to the system object of the invention.

[0040] Preferably, the band-like strand has a width comprised between 0.5 cm and 8.0 cm and a thickness comprised between 0.12 mm and 2.0 mm. The band-like strand can be made of natural discontinuous fibers or continuous fibers. For instance, the method object of the invention has been carried out starting from a band-like strand of flax fibers of a width of 2.5 cm and a thickness of 0.3 mm, and also from a band-like strand of carbon fibers of a width of 1 .0 cm and a thickness of 0.15 mm.

[0041] The method of the invention is carried out by means of the system previously disclosed according to the first aspect of the invention.

[0042] According to a third aspect of the invention, a chopped strand made from fibers is disclosed, being characterised in that it is obtainable by the method according to the second aspect of the invention.

[0043] The resulting chopped strand fibers are of 30 to 40 tex, flat, without torsion or twisty, and their length is preferably around 50 mm.

[0044] A fourth aspect of the invention is referred to a chopped strand mat made of fibers, characterized in that it comprises chopped strands that are those obtained by the method according the second aspect of the invention or the chopped strands are obtainable by the method according to the second aspect of the invention.

[0045] A preferred chopped strand mat made of fibers according to the fourth aspect of the invention comprises chopped strands made of flax fibers, chopped strands made of carbon fibers, and chopped strands made of glass fibers.

[0046] Brief description of the drawings

[0047] In the attached drawings, preferred embodiments of the system, method and chopped strand mats which are object of the invention are illustrated by way of non-limiting examples. In said drawings:

[0048] Fig.1 is a schematic view which illustrates a band-like strand made of discontinuous fibers being introduced between the first grooved roller and the second smooth roller of the fiber distributing rollers,

[0049] Fig. 2 is a schematic view which illustrates how the scrapper removes the portions of the band-like strand housed in the valleys of the first grooved roller of the fiber distributing rollers forming strands,

[0050] Fig. 3 is a schematic view which illustrates how the fiber strands are pulled by the pulling rollers and then cut by the rotatory cutter into chopped strands,

[0051] Fig. 4 is a perspective view which illustrates a preferred embodiment of the system according to the invention (but without showing the pulling rollers),

[0052] Fig. 5 is a cross section view according to a vertical of the system of Fig. 4,

[0053] Fig. 6 is lateral view which illustrates a preferred embodiment of the first roller of the fiber distributing rollers, the scrapper and the second guiding means,

[0054] Fig. 7 is a schematic view which illustrates a detail relative to the tangential zone between the peaks and valleys of the grooved surface of the first fiber distributing roller and the smooth surface of the second fiber distributing roller,

[0055] Fig. 8 is a perspective view which illustrates a detail relative to the peaks and valleys of the grooved roller and the prongs of the scrapper,

[0056] Fig. 9 is a partial bottom view which illustrates a preferred embodiment of the rotatory cutter of the system of Fig. 4,

[0057] Fig. 10 shows a photograph of a flax fiber band-like strand winded up in a bobbin,

[0058] Fig. 11 shows a photograph of a chopped strand mat made of a flax fiber chopped strands produced according to the invention,

[0059] Fig. 12 shows a photograph of chopped strand mat made of flax fiber and glass fiber chopped strands according to the invention,

[0060] Fig. 13 shows a photograph of a flax fiber chopped strand, a carbon fiber chopped strand and a fiber glass chopped strand produced according to the invention, and

[0061] Fig. 14 is a graph of stress-strain curves of flax fiber chopped strand mat samples.

[0062] Detailed description of the invention

[0063] Figs. 4 and 5 depicts a system 100 for producing chopped strands 10, for example from discontinuous fibers, suitable for the manufacture of chopped strand mats. Although in the following explanation reference is made to chopped strands 10 made of discontinuous fibers, the system 100 is also able to produce chopped strands 10 from continuous fibers like glass fibers and carbon fibers.

[0064] The system 100 comprises:

[0065] - first guiding means 2 configured for receiving and orientating in a vertical position a band-like strand 11 made of discontinuous fibers;

[0066] - a pair of opposed fiber distributing rollers 3 and 4 of parallel horizontal axis, being arranged against each other and rotatable along their axis in opposite directions;

[0067] - a scrapper 5 placed at a certain distance below the closest point between the fiber distributing rollers 3 and 4, that is to say, below the tangential zone between the rollers 3 and 4 but closer to the first roller 3;

[0068] - second guiding means 6 arranged next to the scrapper 5, that configures a vertical passage 61 below the upper end of the scrapper 5; and

[0069] - a rotatory cutter 7 arranged at the lower end of the vertical passage 61 of the second guiding means 6.

[0070] Regarding the fiber distributing rollers 3 and 4, they are arranged against each other at a tangential zone between them and as said before, are rotatable along their axis in opposed directions. As seen in Fig. 6, the first roller 3 is a grooved roller that has a circumferentially grooved profile surface 3’ made of alternate circumferential peaks 31 and valleys 32 (see Fig. 7). The second roller 4 has a smooth surface 4’ in opposition to the peaks 31 of the profile surface of the first roller 3. The spaces between the valleys 32 and the smooth surface 4’ in the tangential zone between the fiber distributing rollers 3 and 4 are configured to house portions of the band-like strand 11 made, for example, of discontinuous fibers (see Fig. 10). The first roller 3 is connected to a motor and the second roller 4 rotates by contacting with the first roller 3 in the tangential zone. The fiber distributing rollers 3, 4 rotate at a same speed.

[0071] The area and dimensions of the peaks 31 and valleys 32 of the first roller 3 are appropriate to split the band-like strand 11 into portions 1 T and house them within the spaces created between the valleys 32 and the smooth surface 4’. In other words, the band-like strand 11 is divided and distributed by the peaks 31 and the smooth surface 4’ into portions 1 T and the fiber material of each portion is accumulated and compacted by the smooth surface inside the valleys 32. The number, area, shape and deepness of the valleys 32 will depend on the material, thickness and fiber density of the starting band-like strand 11 used. It is possible to not use band-like strands from the beginning, but the strand should have to undergo a widening step. Conveniently, the band-like strand 11 shall not be wider than the grooved profile surface 3’ of the first roller 3.

[0072] Fig. 7 shows an example of a grooved profile surface 3’ of the first roller 3, in particular, for processing a band-like strand 11 of flax fibers. The distance between two consecutive peaks 33 is about 0.8mm, the peaks 33 have a general triangular shape with a flat top between 0.1 and 0.2 mm. The bottom of the valleys 32 has preferably a radius of 0.3 mm and its sides an angle about 60°.

[0073] For the manufacture of the first and second rollers 3, 4 the starting point was a heat-treated steel cylinder with a hardness of 65 HrC, then, the cylinder of the first roller 3 was machined to create the valleys 32 and peaks 33. Additionally, a series of holes were drilled from front to back to circulate a coolant through the interior of the first roller 3.

[0074] The first guiding means 2 are placed above the tangential zone between the rollers 3 and 4. In Figs. 4 and 5 the first guiding means 2 are made of two flat rectangular plates oriented in V, bent at their lower end configuring a vertical channel.

[0075] Regarding the scrapper 5, Fig. 8 shows how it is provided with multiple vertical prongs 51 at its upper end. Each prong 51 is located inside a corresponding valley 32 of the grooved profile surface of the first roller 3, the prongs 51 being closer to the bottom of the valleys 32 than to the peaks 31.

[0076] The vertical distance between the upper end of the scrapper 5 and the tangential zone between the rollers 3, 4 is comprised between 2 and 10 mm. Said vertical distance depends on multiple factors like the nature of the discontinuous fibers, its “stickiness” to the valleys 32, the fiber density of the band-like strand 11 or its resistance to bending. For instance, when producing chopped strands 10 from flax fibers, said vertical distance is preferably about 8 mm.

[0077] On the other hand, the vertical distance between the upper end of the scrapper 5 and the tangential zone between the fiber distributing rollers 3, 4 (that is to say, the closest point between the rollers 3, 4) may also be selected as to be comprised between 2 and 6 times the thickness of the chopped strands to be produced by the system 100.

[0078] Regarding the rotatory cutter 7, depicted in detail in Fig. 9, it comprises a first cutter surface 71 and a plurality of second cutter surfaces 72. The second cutter surfaces 72, five in the example, are cutting blades arranged in a plane perpendicular to the tangential zone between the rollers 3, 4. The blades of the second cutter surfaces 72 are arranged radially with regard to a cutter rotation axis 7’ around which they rotate.

[0079] The first cutter surface 71 is a serrated or grooved vertical surface which is fixed to the frame of the system 100. Indeed, the first cutter surface 71 is configured as a vertical passage of a through hole made in a plate below the vertical passage 61 of the second guiding means 6 of the system 100. The transversal cross section of through hole has an oblong curved shape or a bean shape. The wall of the through hole has retaining means to retain one or more portions of the band-like strand 11 made of fibers that the scraper 5 has removed from the valleys 32 of the first roller 3. The retaining means is the teeth or grooves that made the serrated or grooved surface provided along the inner perimeter of the through hole. In Fig. 3 it can be seen that downwards the rotatory cutter 7, there is a pulling means in the form of two opposed pulling rollers 9 with their axis parallel to the axis of the fiber distributing rollers 3 and 4. The pulling rollers 9 are arranged against each other and are rotatable along their axis in opposed directions. Each one of the two pulling rollers 9 has a grooved profile surface configured with a groove forming a knuckle thread or a round thread. The surface of the pulling rollers 9 has a very low surface roughness, namely, it has an Ra about between 0.2 and 0.3. The pressure between the pulling rollers 9 is very low, since a compaction of the fibers is not wished. The preferred distance between the surfaces of the pulling rollers is between 0.05 and 0.1 mm.

[0080] Before cutting the strands 12 with the rotatory cutter 7, the lower parts of the strands 12 pass through the first cutter surface 71 and are pulled downwards by the two opposed pulling rollers 9. Then, being pulled downwards, the strands 12 are cut by second cutter surfaces 72 into chopped strands 10. The round thread of the surface of the pulling rollers 9 avoids that the strands 12 that have become separated after having passed the fiber distributing rollers 3 and 4 and the scrapper 5 come joined or connected again, which would affect to the distribution of the chopped strands 10 in the mats. The low surface roughness of the surface of the pulling rollers 9 avoids the strands 10 become embedded in the pulling rollers 9 and thus, no additional scrapper is needed.

[0081] When strands 12 pass through the vertical passage 61 of the second guiding means 6, they reach the through hole first cutter surface 71 that has a bean shape or an oblong curved shape. Said through hole has a vertical wall with a serrated or grooved surface. As depicted in Fig. 9, the grooves extend vertically but the wall could have another kind of retaining means. When the lower ends of said strands 12 go through the through hole and are pulled by the pulling roller 9, one of the cutting blades of the second cutter surfaces 72 that rotate will cut the strands 12. The cutting will be carried out in a clean way because the strands 12 will not slide or will not tend to scape due to the movement of the cutter surface 72 because the grooved surface of the wall of the through hole of the first cutter surface 71 retain the strands 12 in a position until they meet the cutting blade and because the lower ends of the strands 12 are pulled by the pulling rollers 9. Second cutter surfaces 72 can be added to the body of the rotatory cutter 7. The curved length of the circular trajectory between two consecutive second cutter surfaces 72 and the rotation speed of the rotatory cutter 7 determine the length of the chopped strands 11. It is even possible to obtain chopped strands 11 of different lengths with a same rotary cutter 7 by putting two consecutive second cutter surfaces 72 separated by a short distance and putting other two consecutive second cutter surfaces 72 with a longer distance.

[0082] Once the strands 12 are cut, the pulling rollers 9 project the chopped strands 10 in random directions, creating a dispersion cone that deposits the chopped strands 10 in a zone in a Gaussian fashion.

[0083] In a particular embodiment, the length of the chopped strands 10, and therefore the distance at which the strands 12 are cut, is similar to the length of the discontinuous fibers that form the strands 12.

[0084] The system 100 disclosed above is one of the ways to put into practice a method for producing chopped strands 10 from discontinuous fibers suitable for the manufacture of chopped strands mats. The method also allows obtaining chopped strands 10 of continuous fibers. The method comprises the following steps.

[0085] Step a) consists in providing a band-like strand 11 made of discontinuous fibers. In the case of using a conventional strand, it is possible to subject the conventional strand to a widening preliminary step. The thickness of the band-like strand 11 depends on the free area available between the peaks 31 and valleys 32 of the grooved profile of the first roller 3 and the smooth surface 4’ of the second roller 4 in order to be split and housed in its entirety. Particularly, the following band-like strands 11 with the features shown in Table 1 have been tested: Table 1

[0086] At the end of the method, the resulting chopped strands 10 starting form such band-like strands had a length of about 50 mm (as shown in Fig. 13).

[0087] In step b), the band-like material, stored in the form of a bobbin, is unwound and conveyed to the first guiding means 2 that orientate the band-like strand 11 in the direction of the tangential zone formed between the grooved profile surface 3’ of the first roller 3 and the smooth surface 4’ of the second roller 4. The width of the band-like strand 11 has to be in the direction of the axis of the rollers 3 and 4.

[0088] Then, in step c) the band-like material strand 11 is introduced between the pair of opposed fiber distributing rollers 3, 4. The band-like strand 11 is moved downwards driven by the opposed rotation of the rollers 3, 4, passing between the peaks 31 and valleys 32 of the grooved profile surface 3’ of the first roller 3 and the smooth surface 4’ of the second roller 4 and being split and housed in the valleys 32 of the grooved profile of the first roller 3 forming portions 1 T of the band-like strand 11.

[0089] In step d), the portions 1 T of the band-like strand 11 are removed from the valleys 32 of the first roller 3 with the scrapper 5. Said removed portions 1 T will be referred as strands 12 (see Fig. 2).

[0090] Next, in step e) the strands 12 are conveyed by the second guiding means 6 that configures a vertical passage 61 that avoids the matting back of the strands 12 and guide them to the cutting means.

[0091] Then, in step f), strands 12, which are pulled downwards by the pulling rollers 9, are cut according to a cutting plane perpendicular to the length of the strands 12 and the resulting chopped strands 10, coming out from the pulling rollers 9 (see Fig. 3) are scattered in random directions without intersecting, creating a projection cone that deposits the chopped strands 10 in a Gaussian fashion.

[0092] As the chopped strands 10 are ejected in random directions as they are cut from the respective strands 12 by the rotatory cutter 7 and pulled by the pulling rollers 9, the chopped strands 10 fall by gravity and are homogenously distributed on an appropriate support layer, for instance a propylene layer or on a textile layer, for producing a chopped strand mat as the one shown in Fig. 11.

[0093] The reason for including the pulling rollers 9 is due to the lack of weight of the chopped strands 10 once the strands 12 are cut. Being the fiber material so light, the strands 12 once cut and processed could jam in any part of the system 100, since the sum of gravity and mass makes them too light to fall with enough aplomb and could be unable to overcome any small jam caused by a single fiber.

[0094] That is why the pulling rollers 9 are provided, to help the chopped strands 10 come out. Thus, the pulling rollers 9 have two functions: to help the strands 12 to have a better cut and to throw the chopped strands 10 in a convenient way.

[0095] Usually, a support layer advances on a conveyor or any other transportation means located below the rotatory cutter 7, and the chopped strands 10 are deposited homogenously on the support layer. Alternatively, the chopped strands 10 are deposited homogenously on a base just to allow them to deposit them on a support, the chopped strands 10 being oriented in multiple different directions. At a later stage, at the end of the conveyor, the chopped strands 10 are sewn in order to keep them in said distribution by performing several seams and an impregnation with a resin is carried out. The seams 90 go through the thickness of the deposited chopped strands 10. Seams 90 in zig zag shape can be seen in Figs. 11 and 12, fastening flax fiber chopped strands 10a with or without glass fibers chopped strands 10b. Other strands, as carbon fiber chopped strands 10c (see Fig. 13) can also be combined. The sewing is a well-known procedure when producing chopped strand mats of glass fibers. It is very convenient that the chopped strands 10 resulting from discontinuous fibers or from other types of fibers by performing the procedure disclosed above in reference to the system 100 can be treated as other chopped strands for producing chopped strand mats.

[0096] It is convenient to control the distance (height) between the rotatory cutter 7 and the base layer or support on the conveyor by a controlling means, because if the fibers are heavy, for instance carbon fibers, a greater distance has to be left for the fibers to open further the Gauss bell. In the case of flax fibers, as they are lighter, less distance (height) is needed, because after a certain distance, they cease to have a controllable behavior, their low weight causes them to slow down in their fall and they could fall like a feather instead of falling with the inertia that transmit the rotatory cutter 7 when cutting the strands 12.

[0097] Example 1 : manufacturing of flax fiber chopped strands

[0098] In the following, a particular embodiment especially suitable for the manufacture of chopped strands 10 made of flax fibers is described.

[0099] As source material, a high TEX flax fiber band-like strand 11 was used. The flax fiber band-like strand 11 of Table 1 was used in the system 100 previously described. See the band-like strand 11 in Fig. 10.

[0100] The flax fiber band-like strand 11 , in the form of a bobbin, was unwound and conveyed into the first guiding means 2 that guided the band-like strand 11 towards to the pair of opposed fiber distributing rollers 3 and 4.

[0101] When the flax fiber band-like strand 11 was inserted between the two fiber distributing rollers 3, 4, it split up by occupying spaces between the valleys 32 of the first roller 3 and the smooth surface 4’ at the height of the tangential zone. The fiber density and thickness of the flax fiber band-like strand 11 used and its dimensions were proved to be appropriate for being split and housed by the rollers 3, 4, avoiding clogs and overheating. On their side, the area and dimensions of the valleys 32 of the first roller 3 were proved to be appropriate to house the portions of the flax fiber band-like strand 11 , compacting it to form portions 1 T of the bandlike strand 11 to be converted into flax fiber strands 12 of approximately 30-40 tex. Once the flax fiber strands portions 1 T passed the tangential zone of the fiber distributing rollers 3, 4, they tended to detach from the first roller 3 and abandon the valleys 32 because of its resistance to the bending force exerted by the rotation of the first roller 3. This detachment was favoured by the scrapper 5 with prongs 51 that dislodged the remaining flax fiber strand portions 1 T from the valleys 32. The distance between the scrapper prongs 51 and the tangential zone of the fiber distributing rollers 3, 4 was about 8 mm. This distance was calculated considering the maximum point of resistance to bending of the flax fiber portions 1 T to make the detachment in the gentlest possible way.

[0102] Once the flax fiber strand portions 1 T left the fiber distributing rollers 3, 4, they were conveyed by the second guiding means 6 that configures a vertical passage 61 that avoids that the flax fiber strands 12 matting back to the cutting means in the form of the rotary cutter 7 of Fig. 9 already explained above.

[0103] Before cutting, the flax fiber strand portions protrude from the shear 50 mm to form, after their pass through the pulling rollers 9, chopped flax fiber strands 10 of approximately 50mm of length and a linear mass of 93 g / km.

[0104] The particular configuration of the pulling rollers 9 allowed the chopped flax fiber strands 10 to be projected in random directions without intersecting, creating a dispersion cone that deposited the flax fiber chopped strands 10a in a zone in a Gaussian fashion on a propylene textile of 195 g / m2, as it will be explained below.

[0105] Example 2: manufacturing of carbon fiber chopped strands

[0106] In a similar way to example 1 , carbon fiber chopped strands 10c (see Fig. 13) were manufactured, starting from the carbon fiber band-like strand 11 of Table 1 , using the same system 100 and carrying out the same steps as described in example 1 .

[0107] The resulting carbon fibers chopped strands 10c had a length of about 50 mm and a linear mass of 135 g / km.

[0108] Example 3: manufacturing of chopped strands mats Several chopped strand mats were manufactured using the flax fibers chopped strands 10a obtained in Example 1 and the carbon fibers chopped strands 10c obtained in Example 2. The flax fibers chopped strands 10a and the carbon fibers chopped strands 10c were pulled and deposited in a Gaussian fashion on a support layer of propylene textile by means of the pulling rollers 9.

[0109] Some chopped strand mats also contained glass fibers chopped strands 10b with a linear mass of 40 g / km, but in this case, the glass fibers chopped strands 10b were not produced starting from a band-like strand 11 and were not cut and spread with the components of the system 100 since the glass fibers do not have the same problems of discontinuous fibers and they can be produced by well- known methods and systems belonging to the prior art.

[0110] Independently on their material, the chopped strands 10a, 10b and 10c, once deposited randomly uniformly on the propylene textile, were impregnated with a resin. In particular, the resin that was used is commercially available under the name of DISTRITON® 429 v1 ,2 by Polynt Composites. Said resin is an unsaturated polyester resin in styrene and is orthophthalic. The properties of the liquid resin and the properties of the cured unreinforced resin are shown, respectively, in the following tables 2 and 3.

[0111] Table 2 Table 3

[0112] The following Table 4 describes the density and composition of 7 different typologies of chopped strand mats that constitute the typologies of samples that were later tested.

[0113] Table 4

[0114] The chopped strand mats were tested after 21 days after applying the impregnation with the resin. An example of a chopped strand mat made of flax fibers chopped strands according to the reference FLL of Table 4 is shown in Fig. 11.

[0115] Tensile tests The purpose of these tests is to determine the stress-strain curves of samples of the material in order to compare their structural response under a pure tensile test.

[0116] 35 specimens (flat rectangular specimens) were tested, 5 different specimens for each typology according to Table 4. The nominal width of the samples was 20 mm. The thickness of the samples was different.

[0117] The tests that are described below were carried out in July of 2023 by a laboratory called LERMA (Elasticity and Strength of Materials Laboratory) created in 1970 by the Elasticity and Strength of Materials chair of the ETSEIB (Barcelona School of Industrial Engineering) belonging to the UPC (Polytechnic University of Catalonia).

[0118] The main characteristics of these tests are the following ones:

[0119] • Speed of testing: 5 mm / min.

[0120] • A Instron 3366 (Universal Testing Machine) was used to perform all tensile tests.

[0121] • Distance between grips: 60 mm.

[0122] • The strain has been measured by means of an extensometer.

[0123] The modus operandi was:

[0124] • All relevant dimensions of each specimen were measured.

[0125] • Sensors were set to zero before each test.

[0126] • Progressive application of displacement was applied during the test. The force was measured with a force transducer of 10 kN. The displacement was measured with an extensometer located at the specimen.

[0127] • The stress-strain curve was recorded until the appearance of the initial cracks. Nevertheless, the test continued (without the strain values measured with the extensometer) until the maximum axial load was reached.

[0128] • Some of the recommendations defined in ASTM D634-14 were used to carry out the experimental tests and to calculate the material parameters. Nevertheless, these experimental tests did not fulfill the whole requirements of ASTM D634-14, such as the shape of tensile specimens. The tensile strength has been calculated using Eq. 1.

[0129] (Eq. 1)

[0130] Fmax Maximum axial load.

[0131] Ao: Original cross-section area.

[0132] The modulus of elasticity (E) was calculated as the slope of the initial linear part of the stress-strain curve of each sample.

[0133] Tensile test of FLL samples

[0134] Table 5 provides the width and thickness of the 5 tested FLL samples (flax fiber chopped strand mat samples), as well as their results of tensile strength and modulus of elasticity.

[0135] Table 5

[0136] Fig. 14 shows the stress-strain curves of each FLL sample until the first crack appears. The values of tensile strength and modulus of elasticity can be found in Table 6. The tensile strength can be obtained after the initial cracks (when the extensometer measures are not valid due to the cracking process); therefore, the tensile strength can be higher than the maximum stress of Fig. 14.

[0137] Tensile test of FC samples

[0138] Table 6 provides the width and thickness of the 5 tested FC samples (carbon fiber chopped strand mat samples), as well as their results of tensile strength and modulus of elasticity. The tensile strength can be obtained after the initial cracks (when the extensometer measures are not valid due to the cracking process). Table 6

[0139] Tensile test of FV samples

[0140] Table 7 provides the width and thickness of the 5 tested FV samples (glass fiber chopped strand mat samples), as well as their results of tensile strength and modulus of elasticity. The tensile strength can be obtained after the initial cracks (when the extensometer measures are not valid due to the cracking process).

[0141] Table 7

[0142] Tensile tests of samples of chopped strand mats made of a combination of glass fibers, carbon fibers and / or flax fibers

[0143] Table 8 provides the width and thickness of the 5 tested FLL+FV samples (samples of flax fiber (50% weight) + glass fiber (50% weight) chopped strand mats).

[0144] Table 8

[0145] Table 9 provides the width and thickness of the 5 tested FLL+FC samples (samples of flax fiber (83% weight) + carbon fiber (17% weight) chopped strand mats). Table 9

[0146] Table 10 provides the width and thickness of the 5 tested FV+FC samples (samples of glass fiber (83% weight) + carbon fiber (17% weight) chopped strand mats). The values of specimen HT2 were very anomalous and therefore, they were neglected to calculate the mean and the standard deviation values.

[0147] Table 10

[0148] Table 11 provides the width and thickness of the 5 tested FLL+FV+FC samples (samples of flax fiber (41% weight) + glass fiber (41% weight) + carbon fiber (18% weight) chopped strand mats).

[0149] Table 11

[0150] Any combination with flax seems to be better than any other combination of mineral fibers alone. The combination of natural+ mineral fibers give to the composite a superior performance, compared with the lower properties of the one- type samples; mixing produces a multiplier effect. Although the costs of the material may be higher when using flax fibers, the superior properties give a better relation between price and the mechanical properties. In addition, the combination of glass + flax fibers would mean lower emissions on transport and utilization, as well as lower foot print, making this option a really attractive option, not only for the environment, but also for the cost itself.

Claims

1. A system (100) for producing chopped strands (10) suitable for the manufacture of chopped strand mats, characterized by comprising:- first guiding means (2) configured for receiving and orientating in a vertical position a band-like strand (11) made of fibers;- a pair of opposed fiber distributing rollers (3, 4) of parallel horizontal axis, the fiber distributing rollers being arranged against each other at a tangential zone between them and being rotatable along their axis in opposed directions, wherein the first roller (3) of the fiber distributing rollers is a grooved roller that has a circumferentially grooved profile surface (3’) made of alternate circumferential peaks (31) and valleys (32), and the second roller (4) of the fiber distributing rollers has a smooth surface (4’) in contact with the peaks (31) of the grooved profile surface (3’) of the first roller (3) along the tangential zone, the spaces between the valleys (32) and the smooth surface (4’) in the tangential zone being configured to house portions (1 T) of the band-like strand (11) made of fibers, wherein the first guiding means (2) are placed above the tangential zone and wherein at least one of the fiber distributing rollers (3, 4) is connected to a motor;- a scrapper (5) placed at a certain distance below the tangential zone between the fiber distributing rollers (3, 4), the scrapper (5) being provided with multiple vertical prongs (51) at its upper end, wherein each prong (51) is located inside a corresponding valley (32) of the grooved profile surface (3’) of the first roller (3), the prongs (51) being closer to the bottom of the valleys than to the peaks (31);- second guiding means (6) arranged next to the scrapper (5), that configures a vertical passage (61) below the upper end of the scrapper (5); and- a rotatory cutter (7) arranged at the lower end of the vertical passage (61) of the second guiding means (6).

2. The system (100) according to claim 1 , characterized in that the vertical distance between the upper end of the scrapper (5) and the tangential zone between the fiber distributing rollers (3, 4) is comprised between 2 and 10 mm.

3. The system (100) according to claim 1 , characterized in that the verticaldistance between the upper end of the scrapper (5) and the tangential zone between the fiber distributing rollers (3, 4) is comprised between 2 and 6 times the thickness of the chopped strands (10) to be produced by the system (100).

4. The system (100) according to any of the previous claims, characterised in that the first roller (3) and the second roller (4) of the fiber distributing rollers have equal diameters.

5. The system (100) according to any of the previous claims, characterised in that the rotatory cutter (7) comprises two cutter surfaces (71 , 72), wherein the first cutter surface (71) is a retaining surface and the second cutter surface (72) is a cutting blade arranged in a plane perpendicular to tangential zone between the fiber distributing rollers (3, 4).

6. The system (100) according to claim 5, characterised in that the first cutter surface (71) is a fixed surface and the cutting blade of second cutter surface (72) is able to rotate around a cutter rotation axis (7’) that is parallel to tangential zone between the fiber distributing rollers (3, 4) and perpendicular to the rotation axis of the rollers (3, 4).

7. The system (100) according to claim 6, characterised in that the first cutter surface (71) is configured as a vertical passage of a through hole with a transversal cross section with an oblong curved shape or with a bean shape, the wall of the through hole having retaining means to retain one or more portions of the band-like strand (11) made of fibers.

8. The system (100) according to claim 7, characterised in that the retaining means provided in the wall of the through hole is a grooved surface with grooves or teeth provided along the inner perimeter of the through hole.

9. The system (100) according to any of claims 5 to 8, characterised in that the rotatory cutter (7) has a plurality of second cutter surfaces (72) arranged radially with regard to the cutter rotation axis (7’).

10. The system (100) according to any of the previous claims, characterised in that the fiber distributing rollers (3, 4) are arranged against each other with contact pressure between them at the tangential zone.11 . The system (100) according to any of the previous claims, characterised in that the distance between the edges of the peaks (31) and the bottoms of the valleys (32) of the grooved profile surface (3’) is between 0.8 and 2.0 times the distance between two consecutive peaks (31), preferably 1.0 time.

12. The system (100) according to any of the previous claims, characterised in that the grooved profile surface (3’) of the first roller (3) of the fiber distributing rollers (3, 4) is made of a metallic material and the smooth surface (4’) of the second roller (4) of the fiber distributing rollers (3, 4) is made of a metallic material, rubber or a plastic material.

13. The system (100) according to any of the previous claims, characterised in that the scrapper (5) is made of iron or steel.

14. The system (100) according to any of the previous claims, characterised in that the first roller (3) of the fiber distributing rollers (3, 4) is provided with air recirculation means and / or air-cooling means.

15. The system (100) according to any of the previous claims, characterized in that if further comprises, downwards the rotatory cutter (7), a pulling means in the form of two opposed pulling rollers (9) with their axis parallel to the axis of the fiber distributing rollers (3, 4), the pulling rollers (9) being arranged against each other and being rotatable along their axis in opposed directions, wherein each one of the two pulling rollers (9) has a grooved profile surface.

16. The system (100) according to claim 15, characterized in that the grooved profile surface of each of the two pulling rollers (9) is configured with a groove forming a knuckle thread or a round thread.

17. A method for producing chopped strands (10) from fibers suitable forthe manufacture of chopped strand mats, characterised by comprising the following steps: a) providing a band-like strand (11) made of discontinuous fibers; b) orientating said band-like strand (11) in a vertical position; c) introducing said orientated band-like strand (11) between a pair of opposed fiber distributing rollers (3, 4) of parallel horizontal axis, with the width of the band-like strand (11) extending parallel to the axis of the fiber distributing rollers (3, 4), the rollers (3, 4) being arranged against each other at a tangential zone between them and being rotatable along their axis in opposed directions, wherein the first roller (3) of the fiber distributing rollers is a grooved roller that has a circumferentially grooved profile surface (3’) made of alternate circumferential peaks (31) and valleys (32), and the second roller (4) of the fiber distributing rollers (3, 4) has a smooth surface (4’) in contact with the peaks (31) of the profile surface (3’) of the first roller (3) along the tangential zone, the space between the valleys (32) and the smooth surface (4’) in the tangential zone being configured to house portions (11’) of the band-like strand (11) as the band-like strand (11) is driven downwards passing between the fiber distributing rollers (3, 4), wherein at least one of the rollers (3, 4) is connected to a motor; d) removing with a scrapper (5) the portions (11’) of the band-like strand(11) housed at the bottoms of the valleys (32) while the rollers (3, 4) rotate, separating them from the first roller (3), said removed portions constituting strands(12); e) guiding the strands (12) through a vertical passage (61) below the upper end of the scrapper (5); and f) cutting the strands (12) with a rotatory cutter (7) arranged at the lower end of the vertical passage (61), obtaining chopped strands (10).

18. The method according to claim 17, characterized in that before cutting the strands (12), the lower parts of the strands (12) pass through the rotatory cutter (7) and are pulled downwards by a pulling means in the form of two opposed pulling rollers (9) with their axis parallel to the axis of the fiber distributing rollers (3, 4), the pulling rollers (9) being arranged against each other and being rotatable along their axis in opposed directions, wherein each one of the two pulling rollers (9) has a grooved profile surface.

19. The method according to claim 18, characterized in that the grooved profile surface of each of the two pulling rollers (9) is configured with a groove forming a knuckle thread or a round thread.

20. The method according to any of claims 17 to 19, characterized in that the band-like strand (11) has a width comprised between 0.5 and 8.0 cm and a thickness comprised between 0.12 and 2.0 mm.

21. The method according to any of claims 17 to 20, characterized in that the band-like strand (11) is made of natural discontinuous fibers.

22. The method according to any of claims 17 to 21 , characterized in that is carried out by means of the system (100) according to any of claims 1 to 16.

23. A chopped strand (13) made from fibers, characterised in that it is obtainable by the method according to any of claims 17 to 22.

24. A chopped strand mat made of fibers, characterized in that it comprises chopped strands (10) obtained by the method according to any of claims 17 to 22 or chopped strands (10) according to claim 23.

25. A chopped strand mat made of fibers according to claim 24, characterized in that it comprises chopped strands (10) made of flax fibers, chopped strands (10) made of carbon fibers, and chopped strands (10) made of glass fibers.

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