Abstract:
A method and a device for spreading a fiber strand to provide a strip-type fiber strand. In particular, the initial fiber strand is provided with an initial width and thickness, and is then spread to form the strip-type fiber strand having a greater final width and a smaller final thickness as compared to the initial width and thickness. The fiber strand consists of continuous multifilament fibers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national phase application pursuant to 35 U.S.C. 371 of International Application No. PCT/EP2015/055689, filed Mar. 18, 2015, which claims priority to German Application No. 10 2014 105 464.4, filed Apr. 16, 2014. These applications are hereby incorporated by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method and a device for spreading of a fiber strand, which features an initial width and an initial thickness, to a strip-type fiber strand with greater final width and with smaller final thickness, wherein the fiber strand consists of continuous multifilament fibers. 
       BACKGROUND OF THE INVENTION 
       [0003]    The spreading of fiber strands has long been known and is employed on fiber strands made of polymer fibers to improve physical properties, since due to the spreading, twisting of the filaments in the fiber strand, for example, can be eliminated and a fiber strand with filaments positioned in the same direction are obtained. In the case of fiber strands made of carbon fibers, the spreading causes in particular a lower surface weight of the fabric or textile produced from these fiber strands. Fabric or textile produced in this manner can be used, in particular for composites. Usually a composite material is formed, by pressing a fiber fabric or fiber textile or another textile structure made of reinforcing filaments, such as carbon fibers, with a thermoplastic matrix and compressing it into a pre-product (prepreg). To keep the weight of the resulting composite as low as possible, fiber strands with the smallest possible thickness and consistent, good mechanical properties, such as tensile strength, for example, are used. A reduction in thickness and a broadening of the fiber strand is obtained by spreading of the fiber strand. 
         [0004]    In known devices, the fiber strand is spread apart in different ways. One possibility is to charge the fiber strand in an electric field, such that the filaments mutually repel each other and separate in this way. This kind of spreading method is energy-intensive and can only be used on fiber strands that are electrically conductive and can thus be electrically charged. Fiberglas or textile fibers, for instance, have to be impregnated before application of this kind of spreading method. 
         [0005]    In another spreading method, air is blown into a fiber strand in the longitudinal direction in order to open up the strand. The filaments in a fiber strand are surrounded by a release agent which promotes the handling of the fiber strand in various processing steps, such as in weaving. This release agent adheres to the fibers and in this blown method, impedes a uniform spreading of the fiber strand. Moreover, a removal of the release agent is difficult, since firstly, release agents with different properties are used, and secondly, these release agents prevent the damaging [sic], e.g. of fragile carbon fibers during subsequent processing. 
         [0006]    In another variant of the method, vibrations are introduced into the fiber strand for spreading. For example, the two documents U.S. Pat. No. 5,042,111 and U.S. Pat. No. 3,704,485 describe a method wherein sound waves are produced by a loudspeaker and a vibrating air cushion spreads a fiber strand. A method of this kind is very difficult to control and results in irregular final widths of the fiber strand. A better transfer of the vibrations to a fiber strand is obtained when the sound is introduced into a liquid, like water for example. This is described in documents U.S. Pat. No. 5,511,395 and EP 1,652,978 B1. But the disadvantage of this method is that the release agent surrounding the filaments changes in the water bath. Depending on the nature of the release agent, this can also result in a chemical change. But in every case, the quantity ratio of fiber to release agent in the fiber strand is affected, which is undesirable. Another disadvantage of the method consists in that the spread fiber strip emerging from the water has to be dewatered and dried. This drying process is energy-intensive, so that a mechanical dewatering step has to be provided before the drying. For dewatering of the strip, in the apparatus according to EP 1 652 978 B1, the spread strips are sent to a squeeze device. It turns out that besides the dewatering, also an additional spreading occurs in this squeeze process. Thus more recent developments relate to the spreading of a dry fiber strand with similar apparatus, in particular a zig-zag-control of the fiber strand across different rollers and possibly over vibration rods. In this method as well, the fiber strand is heated before, during and after the spreading by the use of a heating apparatus in order to break up thermally or chemically the release agent present on the fibers, which firstly affects the release agent, and secondly is associated with energy costs. A change to the release agent, however, is undesirable, since the release agent in the fiber strand is needed for subsequent processing steps. 
       SUMMARY OF THE INVENTION 
       [0007]    Therefore the object of the present invention is to provide a lower-cost method for spreading of fiber strands, which can be used on fiber strands of different character, and in particular which does not affect the ratio of release agent to fiber in the fiber strand. 
         [0008]    This object is attained with a method having the features of claim  1 . To implement a method of this kind, a device with the features of claim  8  can be employed. 
         [0009]    The dependent claims describe favorable features of the method and/or device. 
         [0010]    The method according to the invention is used for spreading of a fiber strand which features an initial width (extension in the y-direction) and an initial thickness (extension in the z-direction) into a strip-type fiber strand with a greater final width and with a smaller final thickness, that is, a broadening of the fiber strand transverse to its longitudinal direction (x-direction). The method can be used on all fiber strands made of a continuous multifilament fiber, that is, on fibers made of ceramic, such as, for example, silicate, basalt, glass, silicon carbide, metals like steel, aluminum, titanium, aramid, such as Kevlar, but also on polymer fibers. The purpose of the spreading can be merely to improve the physical properties, or in particular to obtain a reduced weight per surface area. Multifilament fibers with a different number of filaments can be used, such as 1K-fibers which contain 1000 filaments, but also for example, 50K fibers with 50,000 filaments. 
         [0011]    In the present method, the fiber strand to be spread is moved, proceeding from an unwinding roll, in the fiber longitudinal direction, through a spreader station, where a spreading takes place, and then the spread, strip-type fiber strand is wound up or is immediately passed on to a subsequent manufacturing process. In the manner according to the invention, the fiber strand in the spreader station is exposed to vibrations without the use of a fluid, such as for example, an air cushion or a liquid. In this case, ultrasound waves are used which are transmitted from a sonotrode. In this respect, the sonotrode contacts the fiber strand from above or from below. The mechanical vibrations are introduced in the z-direction, that is, perpendicular to the longitudinal direction (x-direction) of the fiber strand, so that the strand widens in the transverse direction (y-direction). 
         [0012]    The used vibrations here, have a frequency of 15 to 80 kHz, preferably between 20 and 40 kHz. At frequencies of less than 20 kHz, very large sonotrodes should be used which cause the overall apparatus to be much larger and more expensive. When using frequencies greater than 40 kHz, the sonotrode does indeed become smaller, but the process tolerance is reduced to the same extent. 
         [0013]    The ultrasound vibrators are equipped with replaceable sonotrodes which introduce the high-frequency mechanical vibrations (ultrasound) from their front surfaces into the fiber strand. One or a plurality of sonotrodes can be used in the spreader station. When the fiber strands pass through the spreader station, the fiber strands can loop around the sonotrodes so that the working angle to the contact surface of the sonotrode is variable. 
         [0014]    The individual filaments in a fiber strand are surrounded by a release agent. The composition of the release agent will differ, according to the manufacturer of the fibers. Release agents based on epoxy are known. This kind of release agent promotes the processing of the fiber strands. For example, carbon fibers display a high tensile strength in the longitudinal direction, but can break very easily transverse to the fiber longitudinal direction. The release agent acts in an adverse manner in that the fibers adhere together, which impedes a spreading of the fiber strand. In a favorable manner, the release agent on the filaments of the fiber strand does not change in the spreader station. Firstly, no chemical change occurs, since the release agent does not come into contact with any medium. Secondly, the quantity ratio of fiber to release agent does not change in the overall process. The vibrations emitted by the sonotrode cause friction in the fiber strand, which generates heat and causes a softening of the release agent and promotes spreading of the fiber strand. Conductive fibers, such as carbon fibers for example, additionally contribute to the heat conduction. Since this softening of the release agent occurs only in the region of the contact surface of the sonotrode, and immediately thereafter a cooling of the fiber strand begins again, the release agent/fiber quantity ratio does not change. 
         [0015]    Upon passage through the spreader station, the fiber strand is maintained in a tensioned state. This tensioned state is adjustable and is preferably the same throughout the entire method in order to obtain the best possible, uniform spreading. The attainable spread width here is dependent on this tensioned state. The greater the tension, that is, the more tightly the fiber strand is held, the smaller is the spread width. When the tensioned state is held constant and at a constant vibration frequency, the spread width can be changed via a change in the vibration amplitude. 
         [0016]    By means of the method described above, fiber strands can be moved and uniformly spread at a high speed, preferably of at least 20 m/min. Due to this method, a fiber strand can be spread out in a process-reliable manner into a strip-type fiber strand with at least twice the final width, and preferably the final width will change by at least 5-fold. For example, a 12K carbon fiber strand with a width of 2 mm is spread at a frequency of 30 kHz to obtain a uniformly consistent, strip-type 12 mm-wide fiber strand. By means of an appropriate bandwidth limiter, the final width can be adjusted to a specified, final width value. 
         [0017]    In an advantageous manner, the method according to the invention represents a low-cost method since the fiber strand is processed when dry, no energy is required for dewatering, heating or drying of the fiber strand. Moreover, the nature of the fiber strand does not change with respect to its composition, namely the quantity ratio of multifilament fibers and release agent in the method. A strip-type fiber strand with consistent and larger final width, and with a lesser final thickness is obtained according to the desired specification, which leads to the desired, low surface weight and, when using the fiber strand for fabric or textile, results in lighter composites with consistently good mechanical properties, such as tensile strength, for instance. 
         [0018]    A device is used for this method which comprises an unwinding unit for the fiber strand to be spread, a spreader station for spreading of the used fiber strand into a strip-type fiber strand, a controllable tensioning device for consistent tensioning of the fiber strand during its movement through the spreader station, and a winding device for the spread, strip-type fiber strand. In the manner according to the invention, the spreader station contains at least one sonotrode for contacting and spreading of the fiber strand, wherein the sonotrode introduces vibrations at a frequency of between 15 kHz and 80 kHz from its contact surface from above and/or from below (z-direction) into the fiber strand, which causes a spreading of the fiber strand in the y-direction. In this respect, two or three sonotrodes in sequence will have a favorable effect on the fiber strand. The more sonotrodes contained in the spreader station, the better is the spreading result, but the apparatus will also become more expensive. For this reason, spreader stations with two to three sonotrodes are preferred. The ultrasound vibrators are equipped preferably with replaceable sonotrodes which feature on their front side, contact surfaces for contacting of the fiber strand. These contact surfaces can be planar, concave or arched in the direction of motion of the fiber strand, that is, in the fiber longitudinal direction. The width of the contact surfaces, that is, the extension of the contact surface transverse to the motion of the fiber strand, is chosen so that in every case it is greater than the final width of the strip-type fiber strand to be obtained by the spreading. 
         [0019]    For example, if at least two sonotrodes are provided in the spreader station, then two neighboring sonotrodes are arranged at a predetermined spacing. This spacing is determined according to the nature of the fiber strand material. Furthermore, neighboring sonotrodes can be arranged with respect to each other, preferably at a different alignment, so that for example, a first sonotrode contacts the fiber strand from above, and the second sonotrode contacts the fiber strand from below. This results in more consistent process results. The contact surfaces of the sonotrodes in this case are located preferably in a plane. But in the case of high tensile strength fiber strands, such as carbon fiber strands, for example, a looping around the sonotrodes is desirable. For spreading of this kind of fiber strands, the contact surfaces are arranged preferably at different heights with respect to the transiting fiber strand, so that this fiber strand is guided preferably in a zig-zag-line through the spreader station. In order to obtain a desired working angle of the fiber strand to the contact surface of the sonotrode, diverter rollers can be arranged in front of and behind the spreader station. In addition, an additional bandwidth limiter can be installed behind the spreader station in order to obtain a consistent final width of the spread, strip-type fiber strand. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principals of the invention. 
           [0021]    The invention will be explained below with reference to exemplary embodiments depicted in the drawings. They show: 
           [0022]      FIG. 1  Basic sketch of an apparatus according to the invention, 
           [0023]      FIGS. 2 a , 2 b , 2 c    Different designs of the sonotrodes 
           [0024]      FIGS. 3 a , 3 b , 3 c    Different arrangements of sonotrodes, 
           [0025]      FIG. 4  Basic sketch of another apparatus according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The invention is not restricted to these exemplary embodiments. The basic sketch in  FIG. 1  depicts the fundamental passage of a fiber strand  2  to be spread, through the spreader station  5 , proceeding from the unwinding unit  1 , out to the winding unit  8 . The fiber strand  2  in this case pertains to a 12K fiber strand, that is, the fiber strand consists of 12,000 filaments, which are arranged continuously in the fiber strands  2 , side by side, and each one surrounded by a release agent. This release agent prevents the fiber strand  2  from being damaged during its movement. The employed fiber strand  2  is delivered on a spool of the unwinding unit  1  and is unwound from this spool. Due to the unwinding of the fiber strand  2  from the spool, different unwinding positions would result with each revolution. In order that this fiber strand  2  is always supplied to the same position of the following dancing roller  3 , which conducts it to the spreader station  5 , so that the fiber strand  2  is moved in an invariant plane proceeding from the unwinding unit  1  out to the spreader station  5 , the coil in the unwinding unit  1  is rotatable in the direction of motion and is displaceable transverse to the direction of motion of the fiber strand  2 , that is, in the y-direction. Thus for example, the unwinding position of the fiber strand  2  can be determined by a sensor and the unwinding spool can be displaced in accordance with the desired unwinding position. 
         [0027]    The fiber strand  2  then arrives in the front dancing unit  3 , which operates together with the rear dancing unit  7  as tensioning devices, wherein the front dancing unit  3  is disposed in the direction of motion of the fiber strand  2 ,  2 ′ in front of the spreader station  5 , and the rear dancing unit  7  is disposed in the direction of motion of the fiber strand  2 ,  2 ′ behind the spreader station  5 . The effect is that the fiber strand  2 ,  2 ′ is consistently tensioned in the spreader station  5  during the entire process. Corresponding to the unwinding process of the fiber strand  2  from the unwinding unit  1  and the winding process of the spread fiber strand  2 ′ onto the winding unit  8 , the dancing units  3 ,  7  can counteract changing conditions which affect the tension on the fiber strand  2 ,  2 ′. In the depicted apparatus, the dancing units  3 ,  7  each comprise three rollers. But two rollers would also suffice for a uniform tensioning of the fiber strand  2 ,  2 ′. Depending on the desired fiber strand control to the spreader station  5 , a third roller of the dancing unit  3  can act as additional diverter roller for the fiber strand  2 . Additional diverter rollers  4  in front of the spreader unit  5  and/or additional diverter rollers  6  behind the spreader station  5  can be supplied, in particular for adjusting a particular working angle of the fiber strand  2  upon its entry into the spreader station. 
         [0028]    In the example of  FIG. 1 , the spreader station  5  comprises three ultrasound vibrators  51 . Each ultrasound vibrator  51  features a replaceable sonotrode  52  with a front-side contact surface  53 . The vibrations are generated in one ultrasound generator (not depicted) and the vibrations are directed by the sonotrodes  52  via their contact surface  53  into the fiber strand  2  from above and/or from below, that is, in the z-direction. In this example, the three sonotrodes  52  are arranged in sequence in the direction of motion of the fiber strand  2 ,  2 ′, wherein neighboring sonotrodes  52  are provided at different alignments in the spreader station  5 , and specifically so that the contact surfaces  53  of the first and third sonotrode  52  direct their vibrations from top to bottom into the fiber strand  2 , and the second sonotrode  52  arranged therebetween, directs the vibrations from the contact surface  53  from bottom to top into the fiber strand  2 . In this example, mechanical vibrations are introduced from the sonotrodes  52  at a frequency of 30 kHz into the fiber strand  2 ,  2 ′. A spreading of the employed fiber strand  2  occurs right at the first sonotrode  52 , that is, a spreading of the fiber strand in a lateral direction (y-direction) occurs. This spreading is enhanced in transit of the fiber strand  2  upon contact with the following sonotrodes  52 . The passage of the fiber strand  2 ,  2 ′ through the spreader station  5  takes place horizontally in the illustrated example, that is, with no upward or downward deflection. A progression of this kind is selected, in particular, for sensitive or elastic fiber strands. 
         [0029]    The spread, strip-type fiber strand  2 ′ emerging from the spreader station  5  is guided over the rear dancing unit  7  of the winding unit  8 , where the spread fiber strand  2 ′ is wound up onto a spool with a corresponding winding tension. In this regard the spool in the winding unit  8  can be designed as a torque-controlled winding spool. 
         [0030]      FIGS. 2 a , 2 b , 2 c    depict different sonotrodes  52 ′,  52 ″,  52 ′″. In order to prevent damage to a fiber strand  2  upon its contact with the sonotrodes  52 ′,  52 ″,  52 ′″, the particular contact surfaces  53 ′,  53 ″,  53 ′″ can have a radius in the direction of motion of the fiber strand  2 , for example, like the arched contact surface  53 ″ in  FIG. 2 b   . This makes possible an easier looping around this sonotrode  52 ″ during the spreading process, without the fiber strand  2  being damaged during such looping; see  FIG. 3 b   . Also, the sonotrode  52 ′″ according to  FIG. 2 a    features an arched contact surface  53 ′″, which has the added advantage that such sonotrodes  52 ′″ can be arranged inside each other in one spreader device  5 , as depicted in  FIG. 3 a   .  FIG. 2 c    additionally shows a sonotrode  52 ′ which likewise features a radius at the outer edge  54  of the contact surface  53 ′ and in the middle, a back-set plane  55 . When using this kind of sonotrode  52 ′ in a spreader apparatus, the fiber strand  2  is tensioned across the edge  54  and when the vibrations are applied, will have a free space for vibrating due to the back-set plane  55 .  FIG. 3 c    shows one possible arrangement of several such sonotrodes  52 ′. 
         [0031]    A looping around the sonotrodes  52 , as depicted in  FIGS. 3 a , 3 b , 3 c   , is preferred for carbon fiber strands. The fiber strand  2  is supplied at a steep working angle to the contact surface  53 ′,  53 ″,  53 ′″ of the sonotrodes  52 ′,  52 ″,  52 ′″. This is also possible when the first and third sonotrodes  52  are lowered with respect to the second sonotrode  52 , as in the example of  FIG. 1 , so that the contact surfaces  53  are no longer arranged at the same height, but rather the first and third contact surfaces  53  are positioned lower in comparison to the second contact surface  53 . 
         [0032]      FIG. 4  depicts another exemplary embodiment. In this example, several fiber strands  2  are spread simultaneously, that is, several fiber strands  2  are guided side by side through the spreader device  5 , and several spread fiber strands  2 ′, that is, fiber strands widened in the y-direction, leave the spreader unit  5 . In order to obtain a single, wide fiber strand  2 ′″ from these several, spread fiber strands  2 ′, the individually spread fiber strands  2 ′ are sent to a supply unit  9  which collects the fiber strands  2 ′ into a common fiber strand  2 ″. In addition, for standardizing and/or for additional widening of the combined fiber strand  2 ″, it may pass through an additional spreader device  5 ′. Wide fiber strands  2 ′″ produced in this way can be used advantageously for the production of knitted fabrics or textiles. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           1  Unwinding unit 
           2  Fiber strand, not spread 
           2 ′ Fiber strand, spread 
           2 ″ Fiber strand composed of several spread, combined single strands 
           2 ′″ Fiber strand, combined from single strands 
           3  Front dancing roller unit 
           4 ,  4 ′ Diverter roller 
           5 ,  5 ′ Spreader station 
           51  Ultrasound vibrator 
           52 ,  52 ′,  52 ″,  52 ′″ Sonotrode 
           53 ,  53 ′,  53 ″,  53 ′″ Contact surface 
           54  Edge 
           55  Plane 
           6 ,  6 ′ Diverter roller 
           7  Rear dancing roller 
           8  Winding unit 
           9  Supply unit