METHOD AND DEVICE FOR SPREADING FIBER STRANDS

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

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

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

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.

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.

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.

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

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.

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.

The dependent claims describe favorable features of the method and/or device.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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

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.

The invention will be explained below with reference to exemplary embodiments depicted in the drawings. They show:

FIG. 1 Basic sketch of an apparatus according to the invention,

FIGS. 2a, 2b, 2c Different designs of the sonotrodes

FIGS. 3a, 3b, 3c Different arrangements of sonotrodes,

FIG. 4 Basic sketch of another apparatus according to the invention.

DETAILED DESCRIPTION

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.

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.

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.

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.

FIGS. 2a, 2b, 2c 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. 2b. 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. 3b. Also, the sonotrode 52′″ according to FIG. 2a 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. 3a. FIG. 2c 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. 3c shows one possible arrangement of several such sonotrodes 52′.

A looping around the sonotrodes 52, as depicted in FIGS. 3a, 3b, 3c, 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.

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

• 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

Claims

1. A method for spreading a fiber strand with an initial width and with an initial thickness to a strip-type fiber strand with a greater final width and with a smaller final thickness, wherein the fiber strand consists of continuous multifilament fibers, comprising;

moving the fiber strand from an unwinding unit to a winding unit in the longitudinal direction (x-direction); and

passing the fiber strand through a spreader station between the unwinding unit and the winding unit, while holding the fiber strand under an adjustable tension

wherein the fiber strand in the spreader station is placed under uniform tension during its entire passage;

wherein in the spreader station the fiber strand contacts at least one sonotrode and by means of vibrations introduced into the fiber strand from above and/or from below (z-direction) by a front-side contact surface of the sonotrode, the fiber strand is uniformly spread transverse to the longitudinal direction; and and

wherein the vibrations have a frequency is between 15 kHz and 80 kHz,

2. The method according to claim 1, wherein the multifilament fibers of the fiber strand are surrounded by a release agent and a quantity ratio of multifilament fibers to the release agent in the strip-type fiber strand does not change with respect to the fiber strand prior to spreading.

3. The method according to claim 1, wherein when fiber strands are made of tension-resistant fibers, the fibers strands are supplied at a work angle to a contact surface of the at least one sonotrode.

4. The method according to claim 1, wherein the fiber strand, proceeding from the unwinding unit out to the winding unit, is moved in the fiber longitudinal direction at a speed of at least 20 m/min.

5. The method according to claim 1, wherein the fiber strand, proceeding from the unwinding unit out to the spreader station, is moved continuously in an invariant plane (x-z-plane) deflected upward or downward in the fiber longitudinal direction, and does not undergo any lateral deflection transverse (y-direction) to the fiber longitudinal direction other than spreading.

6. The method according to claim 1, wherein the strip-type fiber strand produced by the spreading attains an at least 2-fold greater final width in comparison to the initial width.

7. The method according to claim 1, wherein several fiber strands are spread simultaneously and are combined into a common widely spread fiber strand.

8. A device for spreading of a fiber strand with an initial width and with an initial thickness, into a strip-type fiber strand with a greater final width and with a lesser final thickness, wherein the fiber strand consists of continuous multifilament fibers, comprising:

an unwinding device for the fiber strand for unwinding in the fiber longitudinal direction (x-direction):

a spreader station for lateral (y-direction) spreading of the fiber strand into the strip-type fiber strand;

a controllable tensioning device for uniform tensioning of the fiber strand in the fiber longitudinal direction during movement of the entire fiber strand through the spreader station; and

a winding device of the strip-type fiber strand;

wherein the spreader station contains at least one sonotrode for contacting and spreading of the fiber strand; and

wherein the sonotrode is configured to introduce along a front-side contact surface, vibrations at a frequency of between 15 kHz and 80 kHz from above and/or from below (z-direction) into the fiber strand.

9. The device according to claim 8, wherein the front-side contact surface of the sonotrode has a width greater than the final width of the band strip-type fiber strand obtained by spreading.

10. The device according to claim 8, wherein at least 2 sonotrodes are disposed in the spreader station, wherein two neighboring sonotrodes are arranged at a default spacing, and When in a default, are arranged with different alignment to each other, so that fiber strands of different characteristics can contact the sonotrodes with an adaptable looping.

11. The device according to claim 8, wherein additional deflection rollers are arranged in front of and behind the spreader station to adjust an adjusted angle of the fiber strand to the contact surface of the sonotrode.

12. The device according to claim 8, wherein the controllable tensioning device for uniform tensioning of the fiber strand consists of two dancing roller units with at least two rollers, wherein a front dancing roller unit is disposed in a direction of motion of the fiber strand in front of the spreader station, and a rear dancing roller unit is disposed behind the spreader station.

13. The device according to claim 8, wherein a bandwidth limiter is provided which is disposed behind the spreader station in the direction of movement of the fiber strand for limiting and adjusting a consistent final width of the spread.

14. The device according to claim 8, wherein for a continuous movement of the fiber strand in an invariant plane (x-z-plane) proceeding from the unwinding device out to the spreader station, the unwinding device comprises a rotatable unwinding spool which can be displaced transverse (y-direction) to the direction of movement of the fiber strand.

15. The device according to claim 14, wherein the front dancing roller unit controls the rotational velocity of the unwinding spool and the winding device comprises a torque-driven winding spool.

16. The device according to claim 8, wherein the spreader device is configured such that several fiber strands can pass through the spreader device side by side, and, a supply unit is further arranged in the direction of motion of the fiber strands behind the spreader device for combining of the individually spread fiber strands into a wide fiber strand.

17. The method according to claim 1 wherein the strip-type fiber strand produced by the spreading attains an at least 5-fold greater final width in comparison, to the initial width.

18. The method according to claim 1 wherein the strip-type fiber strand produced by the spreading attains a final width that is limited to a default value.

19. The device according to claim 16, further comprising an additional spreader device for standardizing the wide fiber strand.