WO2005087993A1

WO2005087993A1 - Method and device for cutting fibres into pieces of a short length

Info

Publication number: WO2005087993A1
Application number: PCT/NL2005/000146
Authority: WO
Inventors: Willem Frans Van Der Mast
Original assignee: Willem Frans Van Der Mast
Priority date: 2004-03-01
Filing date: 2005-02-28
Publication date: 2005-09-22
Also published as: DE602005007334D1; EP1723272B1; PL1723272T3; EP1723272A1; SI1723272T1; NL1025607C2; ES2308444T3; ATE397685T1

Abstract

A method and a device for cutting glass fibres or other fibres, such as carbon fibres, into pieces, wherein a fibre is carried into the pinch between two mating knife rollers whose knives alternately press against an alternating side of the fibre, and wherein the fibre is held in place by clamping ribs provided beside the knives on the two rollers.

Description

METHOD AND DEVICE FOR CUTTING FIBRES INTO PIECES OF A SHORT LENGTH

The invention relates to a method for cutting glass fibres or other fibres into pieces, wherein the fibre is divided into pieces by pressing a knife against the fibre. In the Netherlands a patent has been granted to such a method and device under number 1012286.

According to the current state of the art, the fibre is carried between two rollers, viz. a knife roller comprising several substantially radially outwardly oriented knives distributed over the circumference, which knives have a cutting edge that extends substantially parallel to the axis of the roller, and a pressure roller, which rollers are driven in opposite directions, with the fibre being clamped down between pressure ribs present between the knives on the knife roller, and a pressure roller, wherein the knife cuts the fibre substantially without any counter pressure from the pressure roller. During the cutting operation, the knife can extend into a groove in the pressure roller to a position some distance above the bottom of said groove.

In this way the fibre can be engaged at two positions by the two rollers, whilst the knife is pressed against the fibre between said positions. The piece of fibre is longer in that case than the distance between said two positions, so that the fibre can be strongly bent over the cutting edge of the knife without exerting much force, and subsequently break.

The fibre is also bent over the edges of the groove in the counter pressure roller upon being clamped down by the counter pressure ribs, albeit to a much smaller extent than over the knife. Usually a concave surface is present between the grooves in the pressure roller, as described in Dutch patent No. 1012286, into which the counter pressure ribs fall. This concave portion causes the fibres to be bent more strongly over the edge of the grooves. Said bending of the fibre results in a reduced cohesion of the fibre bundle, so that the pieces will fall apart after cutting. In combination with the limited amount of grip that the pressure ribs are capable of exerting on the fibre, this may result in the pieces of fibre exhibiting frayed cut surf ces. Furthermore, the known device is less suitable for cutting pieces of relatively -short length. Short pieces have a length of less than 5 mm_? for example. Furthermore, in existing devices for cutting short pieces the space between the knives is so small that the pieces being cut frequently get wedged between the knives, which interferes with the cutting operation.

The object of the invention is to provide a method and device for dividing fibres into pieces, which device is suitable for cutting pieces of relatively short length, in which a better cohesion is maintained within the fibre pieces and in which the cut pieces will fall out of the spaces between the knives more easily.

In order to accomplish that object, a fibre is divided into pieces by pressing a first knife against the fibre and likewise pressing a knife oriented substantially in the opposite direction against the fibre from the opposite side at a location other than directly opposite the first knife, wherein the fibre is held in place by first clamping ribs arranged beside the first knives and second clamping ribs arranged beside the second knives, wherein a first clamping rib mates with a second clamping rib. The fibre is divided into pieces as a result of the fibre being bent over the cutting edges of the knives in this manner and breaking at the location where said bending takes place. The knives do not come into contact with each other during this operation, and the mutual distance between the cutting edges of the first knives and the cutting edges of the second knives is substantially equal to the desired length of the pieces of fibre being cut.

An advantage of the method is the fact that a fibre is hardly bent between the cutting edges, so that the cohesion within the fibre remains and the length of the pieces is defined more precisely.

Another advantage of the method is the fact that the mutual spacing between the first knives .and between the second knives is substantially slightly less than twice the length of the pieces into which a fibre, is cut, as a result of which the pieces of fibre can fall out of the opening between the knives more easily after the fibre has been cut and the knives have been retracted.

An advantage of the method is furthermore the fact that there is nearly twice as much space between the knives for mounting clamping ribs and or supports for the knives than is the case with existing methods, whilst furthermore this makes it possible to construct a device in such a manner that the clamping ribs and the knives can be placed into the device and be removed therefrom independently of each other.

The fibre is held in place by clamping ribs. The clamping ribs are mounted beside the two side faces of each knife, they extend in the longitudinal direction of the knife, parallel thereto. The clamping ribs can be depressed against spring force in a direction opposite to the direction in which the knife beside which the rib is mounted pushes against the fibre. A first clamping rib of each first knife mates with a second clamping rib of each second knife, with a first clamping rib and a second clamping rib pushing against both sides of the fibre in mutually opposite directions, substantially directly opposite each other at a location between the two knives pushing in opposite directions, the first clamping fib pushing in the direction into which the first knife pushes and the second clamping rib pushing in the direction into which the second knives pushes. A first clamping rib is preferably of identical shape to a second clamping rib. In the depressed condition of the clamping ribs, the contact surface between the mating first and second clamping rib preferably coincides substantially with the plane formed by the cutting edge of the nearest first knife and the cutting edge of the nearest second knife beside said two mating clamping ribs. Generally, the first clamping ribs cannot touch the second knives and the second clamping ribs cannot touch the first knives, either. Preferably, the cutting edges of the knives do not touch the clamping ribs .

The clamping ribs are made of an elastic material, or they are of compound construction, comprising an elastic contact surface .

The clamping ribs maintain the tension in the fibre and ensure that breaking of the fibre will take place in the order in which the knives alternately load the fibre from alternating sides.

In a preferred embodiment, a fibre is carried between two knife rollers, a first knife roller and a second knife roller, each being provided with several substantially radially outwardly oriented knives on the cylindrical surface- thereof, whose cutting edges extend substantially parallel to the roller axis, which rollers are driven at the same peripheral velocity in mutually opposite directions, wherein the fibre is clamped down between clamping ribs present on the surface of the two rollers, beside the knives.

In the text the term "first" ("second") is used, which is understood to mean "associated with the first (second) roller". Furthermore the term "opposing" is used, which is understood to mean "associated with the other roller".

In one preferred embodiment a clamping rib does not exert a mechanical load on a knife upon being depressed.

In one preferred embodiment, the knives are supported by the roller construction up to a position as close to their cutting edge as possible so as to prevent knife fracture caused by flexural strain. This makes it possible to use hard knives made of usually brittle materials.

In one preferred embodiment, two clamping ribs present between two knives on one of the two rollers are combined into one clamping section, in the outwardly facing surface of which a groove is present between the two clamping ribs, parallel to the adjacent cutting edges, which groove provides space for a knife of the opposing roller, so that the cutting edges of the knives cannot touch the clamping ribs and/or the clamping section.

In a preferred embodiment, the clamping sections are provided with a fixing profile on their radially inwardly facing side, which fixing profile fits in a recess of the roller. Said fixing profile is shaped in such a manner that it can be moved in the direction of roller axis into the recess in the roller, thereby fixing the clamping section to the roller. Said fixing profile may be configured to slightly diverge or taper in the direction of the roller axis so as to make it easier to slide the sections into a recess in a roller.

The first knives of the first roller extend between the second knives of the second roller and vice versa, with this understanding that the imaginary cylindrical surfaces of the two rollers in which the cutting edges lie intersect each other. The sum of the radii of the two aforesaid imaginary cylinders minus the centre-to-centre distance between the two knife rollers is referred to as the "penetration depth". A fibre is thus placed in zigzag shape over the cutting edges. The "pitch circle" of a knife roller is the circle which has the axis of the roller as its centre and which has a radius having a length equal to the distance from the axis to the cutting edges of said roller minus substantially half the penetration depth.

A fibre is bent over the cutting edges of the knives as a^' result of the tension that is generated by combining the tensile force in the fibre being introduced and the tension generated by the movement into one another of the first and the second knives, whilst the mating clamping ribs hold the fibre in place, preventing it from sliding or slipping over the cutting edges in the fibre direction.

The movement into one another of the opposing knives during rotation of the two rollers causes the tension in a fibre to increase during said movement of the opposing knives with respect to each other, and at some stage of said movement of the opposing knives the fibre will break. The aim is for a fibre to break on respective successive knives. This is effected by means of the clamping ribs. The clamping ribs of the two rollers work together and clamp down the fibre, so that the fibre, for example after having been broken over a first knife, will remain under tension over the following second knife. As a knife movement progresses during rotation, also the pressure force of the mating clamping ribs against each other increases before . the penetration depth is reached, enabling the clamping ribs to hold a fibre in place prior to and after the moment of breaking of the fibre.

Generally it can be stated that the larger the penetration depth of the two rollers into each other, the greater the extent to which a fibre will bend over the cutting edges and the easier the fibre will break. The risk of the fibre sliding over the cutting edges in the fibre direction as a result of the tension in the fibre with increased bending decreases as the fibre is bent more acutely over the cutting edge .

With regard to processes in which the invention is implemented it generally applies that a certain amount of tension prevails in a fibre and in that the device must exert a pulling force on the fibre for introducing the fibre. Frequently said a pulling force increases along with the velocity of the fibre. If said tension would cause a fibre to break prematurely over a cutting edge, without being clamped down by the clamping ribs yet, introduction of the fibre would not take place. Consequently, the fibre must not be bent too strongly over the knives at the start of the pinch - between the rollers, before the clamping ribs have an adequate grip on the fibre. To that end, the clamping ribs may be made to extend so far outwards that they prevent the fibre from being bent to strongly over a cutting edge or from touching the cutting edge upon being introduced between the rollers. Subsequently, upon depression by the opposing clamping rib, the knife is pressed sufficiently hard against the fibre yet upon continued rotation for cutting or breaking the fibre.

Preferably, the two _.rollers are identical in shape and have substantially the same diameter. The diameters may differ from each other, however. To ensure that the amount of wear is evenly distributed over the circumference, for example, the two rollers preferably each comprise a different number of knives.

The mutual spacing between the knives, measured on the pitch circle of the two rollers, is preferably the same for both rollers. It is possible to cut pieces of a different length or pieces of varying length by leaving out some of the knives .

One preferred embodiment comprises clamping sections, which are symmetric with respect to the radial that extends through the heart of the groove. In the preferred embodiment, the clamping ribs are preferably made of an elastic material, which progressively offers resistance upon compression. It is possible to use other shapes comprising cavities, recesses or possibly compound mechanical constructions in the clamping sections in order t - effect a desired spring characteristic. Furthermore, combinations of materials having different properties are possible.

In one preferred embodiment, the space between the first (second) tensioning ribs and the adjacent first (second) knives diverges in outward direction from a point located some distance away from the cutting edge of the knife, so that dirt can readily come loose.

Preferably, the knives are fixedly mounted to the knife rollers, and the first (second) clamping ribs between the first (second) knives extend substantially slightly less far, just as far or further radially outwards as (than) the first (second) knives, and during cutting the clamping ribs of the opposing roller push the knives into a position in which the contact surface between the mating ribs substantially coincides with the plane that is defined by the cutting edges of the nearest first knife and the nearest second knife adjacent to the mating clamping ribs. As a result, the flexural strain placed on the fibre by the clamping ribs is minimized.

In one preferred embodiment, the contact surface perpendicular to the longitudinal direction of the clamping rib is convex rather than flat, seen in sectional view, so that the fibre is engaged from the centre of the contact surface, with said area of contact further extending in either direction to the adjacent cutting edges through further depression of the clamping ribs. If a wet fibre is processed, this leads to a better removal of moisture from the contact surface. Furthermore, the pieces of fibre will become detached from the contact surface more easily after passing the pinch.

The invention furthermore relates to a device for cutting fibres into pieces, wherein a fibre is divided into pieces by pressing a first knife against the fibre and pressing a second knife, which extends in a direction substantially opposite the first knife, against the fibre from the opposite side at a different location than directly opposite the first knife, wherein the fibre is held in position by mating first and second clamping ribs disposed beside the first and second knives. In the preferred device, the fibre is carried into the pinch between two knife rollers, a first knife roller and a second knife roller, each comprising several substantially radially outwardly oriented knives having cutting edges which extend substantially parallel to the roller axis, which rollers are driven at the same peripheral velocity in mutually opposite directions, with the fibre being clamped down between the clamping ribs present beside the knives on the surface of the two rollers. The knives of one roller extend between the knives of the opposing roller, and vice versa, with this understanding that the imaginary cylindrical surfaces of the two rollers in which the cutting edges lie intersect each other. As a result, the fibre is laid in a zigzag shape over the cutting edges.

To explain the invention more fully, a few embodiments of a device for cutting fibres will now be described with reference to the drawing.

Fig. 1 is a sectional view of the device for cutting fibres; Fig. 2.1 is a sectional view of a first clamping section;

Fig. 2.2 is a sectional view of a second clamping section;

Fig. 3 is a sectional view of an embodiment of the circumference of the core of the first roller;

Fig. 4 is a sectional view of the pinch between the rollers; Fig. 5.1 is a sectional view of an example of a clamping section;

Fig. 5.2 is. a sectional view of the example of Fig. 5.1 in depressed. condition;

Fig. 6.1 is a sectional view of an example of a clamping section 9;

Fig. 6.2 is a sectional view of the example of Fig. 6.1 in depressed condition;

Fig. 7.1 is a sectional view of an example of the clamping section 9 in which one of the ribs 11 is not elastic; and Fig. 7.2 is a sectional view of Fig. 7.1 in depressed condition .

The figures are merely schematic representations, in which like parts are indicated by the same numerals.

Fig. 1 is a sectional view of the device perpendicular to the roller axes 3 and 4 of the first roller 1 and the second roller 2, which are rotated in the direction indicated by the arrows and which are driven at the same peripheral velocity.

To ensure that the rollers 1 and 2 will rotate at the same peripheral velocity, said rollers 1 by 2 may be provided at one of their ends with mating gears, whose axes coincide with the roller axes 3 and 4 and whose pitch circle corresponds with that of the roller 1 or 2 with which they have the axis 3 or 4 in common, in such a manner that it is ensured in this way that the knives 7 and 8 of the rollers 1 and 2 will at all times fall substantially centrally between each other in the pinch between the rollers 1 and 2. A fibre 5 is introduced into the pinch from above and exits it in the form of identical pieces 6. The fibre 5 may be a glass fibre, a carbon fibre, a quartz fibre, a basalt fibre or another fibre, preferably of a brittle material. The introduction of the fibre 5 from above is merely an example of a manner of introducing the fibre, the roller axes may also be positioned one above the other or otherwise, so that in principle a fibre can be introduced from any other direction in the plane of drawing. Furthermore it is possible to have a fibre run along with one of the rollers, at a specific angle with respect to the circumference thereof, before the pinch.

A fibre 5 is generally composed of a bundle of 50 to 5000 or more solid filaments of identical diameter, which diameter may range between 5 and 30 μm. In dry condition, the filaments are held together, whether or not in separate, smaller bundles, by means of a binder, in wet condition, the cohesion is less strong and more vulnerable. The fibre can be processed both in wet form and in dry form. Usually, a cutting machine draws a fibre 5 from a bobbin with a velocity of 50 - 700 m/min, or out of a moulding plate or mould with a velocity of 600 - 2500 m/min. The first roller 1 and the second roller 2 are circumferentially provided with fixed first knives 7 and second knives 8, respectively, having cutting edges 13. The knives 7 and 8 are generally identical to each other, or each other's mirror image. Furthermore, clamping sections 9 and 10 are provided along the circumference of the rollers 1 and 2. The spacing between the cutting edges 13 on one roller is substantially slightly less than twice the desired length of the pieces 6 that are to be cut from the fibre 5. A usual diameter of the rollers 1 and 2 is about 150 mm to 250 mm in the case of a spacing of about 6-10 mm between the first knives or the second knives and a desired length of the pieces 6 of 3-5 mm. The shorter the desired length of the pieces 6, the smaller the diameter of the rollers may be.

Figs. 2.1. and 2.2 are sectional views of a clamping section 9 and a clamping section 10, respectively, the plane of drawing being substantially perpendicular to the direction of the roller axes. The figures only differ from each other in that some of the reference numerals are different. The clamping sections 9 and 10 are shown in relation to their respective first knives 7 and second knives 8 and in relation to a portion of the first roller 1 and the second roller 2 that is bounded by the curve as shown. The first clamping section 9 is substantially identical to the second clamping section 10. The clamping sections 9 and 10 are composed of two clamping ribs 11 and 12, respectively, which have outwardly oriented contact surfaces 14 and 15. The contact surfaces 14 and 15 are convex. On the side that faces outwards, the clamping sections 9 and 10 are provided with a recess 16, which is provided to prevent the cutting edges 13 of the opposing knives 8 and 7 from touching the clamping ribs 11 and 12. On their inwardly facing sides, the clamping sections 9 and 10 are provided with a profile 18 that exhibits a constriction 19, which functions to make it possible to mount the clamping sections 9 and 10 to the rollers 1 and 2. The rollers 1 and 2 are provided with a recess (21 in Fig. 3), into which the profile 18 exhibiting the constriction 19 fits. The profile 18 may be configured to taper off in its longitudinal direction so as to facilitate the mounting operation. The length of the clamping ribs 11 and 12, perpendicular to the plane of drawing, is substantially equal to the length of the knives 7 and 8, likewise perpendicular to the plane of drawing. Outwardly diverging recesses 17 are present between the knives 7 and 8 and the clamping ribs 11 and 12. The depth of said recesses must be sufficient so as not to interfere with the springing action of the clamping ribs 11 and 12 and to allow the contact surfaces 14 and 15 to move to a sufficient extent without loading the knives 7 and 8 mechanically. The divergence of the recess 17 also functions to prevent dirt being retained. The surfaces of the contact surfaces 14 and 15 are preferably elastic so as to obtain an adequate grip on the fibre. The clamping ribs are placed into abutment with the supporting surfaces 22 of the rollers 1 and 2 with their inwardly facing sides 23.

Fig. 3 is a sectional view of the circumference of the first roller 1, in which the roller axis extends perpendicularly to the plane of drawing again. The first roller 1 is provided with recesses 20, in which the knives 7 can be mounted, and recesses 21, in which the fixing profiles 18 exhibiting the constriction 19 of the clamping sections fit. Like the fixing profiles 18, the recesses 21 may taper off in the direction of their longitudinal axis. The recesses 20 for the knives 7 extend as far outwards round the knives 17 as possible so as to minimise the extent to which the knives 7 are loaded by the pressure that is present in the clamping ribs while clamping down a fibre. The surfaces 22 of the rollers support the inwardly facing sides of the ribs when said ribs are being depressed. The circumference of the second roller 2 is usually identical thereto; the diameter and the number of second knives 8 and second clamping sections 10 may differ from the diameter and the number of first knives 7 and first clamping sections 9 of the first roller 1, however.

The clamping sections 9 and 10 are separately mounted on the rollers 1 and 2. It is possible to construct the clamping sections 9 and/or 10 for a roller 1 and/or 2 in such a manner that they can be fitted as one whole, for example by making them in one piece on a ring.

Fig. 4 is a sectional view of the pinch between the rollers 1 and 2; in this case, too, the plane of drawing is perpendicular to the roller axes. A fibre 5 arrives from above and just touches the cutting edge 25. The surfaces 14 and 15 being in contact with each other of the first clamping ribs 11 and the second clamping ribs 12 are specifically shown in pairs 31 and 32, 33 and 34, 35 and 36, 37 and 38, 39 and 40, 41 and 42, clamping down the fibre 5. The cutting edges 13 that are relevant to the explanation in Fig. 4 are indicated at 24, 25, 26, 27, 28, 29 and 30. The fibre 5 is kept clear of the cutting edge 24 by the contact surface 31. The contact surfaces 31 and 32 clamp round the fibre and carry it along in downward direction, slightly aided by the cutting edge 25. The fibre 5 already experiences an increasing tension between the cutting edges 25 and 26, because the spacing between the cutting edges 25 and 26 is slightly larger than the spacing between the cutting edges 24 and 25 at the moment when the cutting edge 24 just makes contact with the fibre 5. The surfaces 33 and 34 hold down the fibre 5 more strongly than the surfaces 31 and 32, because of the greater depression. The extent to which the fibre is bent over the cutting edges increases as the fibre moves further towards the centre of the pinch. The resistance^" against slip of the fibre 5 in the fibre direction increases over the cutting edge 25 towards the cutting edge 27. If the spring force and the shape of the clamping ribs 8 and 9, the penetration depth of the knives 7 and 8 and the knife spacing 50 are correctly geared to each other, the fibre 5 will break when the cutting edge is positioned between 26 and 27 or between 27 ^'and 28. For the sake of simplicity of the explanation, it is assumed that the cutting edge is positioned between 26 and 27 in this example. The fibre has already been cut on the cutting edge 27, and the contact surfaces 35 and 36 must have sufficient grip on the fibre 5 for holding it in place and bending the fibre over the cutting edge 26 upon further rotation of the rollers 1 and 2 until it breaks. The pieces of the fibre 6 that have been cut exit the pinch at the bottom side.

Fig. 5.1 is a sectional view of an example of a first clamping section 9 comprising clamping ribs 11, knives 7 and an encircled portion of the first roller 1, the plane of drawing again being substantially perpendicular to the roller axes. The contact surfaces 14 in non-depressed condition coincide with the imaginary cylindrical circumference of the roller, in which the cutting edges 13 are present. The recess 16 for the knife of the opposing roller is configured such that the contact surfaces 14 will be depressed further beside the recess 16 than beside the cutting edges 13. This is done in order to minimise the extent to which the contact surface 14 differs from the plane defined by the nearest cutting edges upon depression.

Fig. 5.2 is a sectional view of the example of a Fig. 5.1, together with the opposing clamping section 10, second clamping ribs 12, second knives 8 and second contact surfaces

15 and an encircled portion of the second roller 2 and 1 at the location of the pinch, which functions to provide an image of the extent of the depression.

Fig. 6.1 is a sectional view, substantially perpendicular to the roller ^"axes, of an embodiment of a first clamping section 9 comprising first knives 7, wherein the first contact surfaces 14, after a first contact with the knives 7 and 8, are more preformed in the direction in which the fibre 5 will extend than in the preceding examples. Penetration is deeper here, and the fibre 5 is bent more strongly over the cutting edges 13 here.

Fig. 6.2 is a sectional view of the example of Fig. 6.1, together with the opposing second clamping sections 10, second knives 8 and second contact surfaces 15, and a portion 15 of the second knife roller 2 at the location of the pinch, which functions to provide an image of the extent of depression. The extent to which the clamping ribs 11 and 12 are depressed is small in this example, the rib must have a high spring constant.

Fig. 7.1 is a sectional view, substantially perpendicularly to the roller axes, of an embodiment of 1 first clamping section 9 comprising a first knives 7 and first ribs 11, in which the clamping section 9 consists of one rib. The other rib 46 comprising and a contact surface 47 is not elastic in this case, it is fixed to the core of the roller 1.

Fig. 7.2 is a sectional view of the example or Fig. 7.1 together with the opposing second clamping sections 10 comprising ribs 12 and contact surfaces 15 and ribs 48 having contact surfaces 49, second knives 8 and a portion of the second knife roller 2 drawn at the location of the pinch, which functions to provide an image of the extent of depression. The clamping ribs 11 and 12 are depressed more strongly in this example, because the opposing clamping ribs 46 and 48 are not depressible. The aim of the embodiment of Figs. 7.1. and 7.2 is to have the increase of the degree of bending of the fibre over the cutting edges 13 and the increase of the spacing between the cutting edges 13, and thus the tension in the fibre 5 upon entry of the pinch, take place in a more controlled manner, so that it is ensured that the fibre will break on the desired cutting edge at all times .

Claims

1. A method for cutting glass fibres into pieces, wherein a fibre (5) is carried into the pinch between a first knife roller (1) provided with substantially axially oriented first knives (7) and a second knife roller (2) provided with substantially axially oriented second knives (8), wherein the first knife roller (1) and the second knife roller (2) rotate in mutually opposite directions at the same peripheral velocity.

2. A method according to claim 1, wherein first clamping ribs- (11) are provided on the surface of the first knife roller (1) for clamping down and/or holding the fibre (5) in place.

3. A method according to claim 1 or 2, wherein second clamping ribs (12) are provided on the surface of the second knife roller (2) for clamping down and/or holding the fibre (5) in place.

4. A method according to any one of the preceding claims, wherein the surface of the first and/or the second knife roller (2) is provided with substantially axially oriented grooves (16), into which the second and/or the first knives can extend during rotation.

5. A method according to any one of the preceding claims, wherein the first (11) and/or the second (12) clamping ribs are made of an elastic material.

6. A method according to any one of the preceding claims, wherein the first knife roller (1) has a first longitudinal axis (3) and a second knife roller (2) has a second longitudinal axis (4), which first longitudinal axis (3) and second longitudinal axis (4) extend substantially parallel to each other, wherein the surfaces (14) of the first knife roller (1) and the surfaces (15) of the second knife roller (2) touch each other and wherein the first knives (7) fall between the second knives (8).

7. A method according to any one of the preceding claims, wherein the first clamping ribs (11) and the second clamping ribs (12) elastically deform each other for the purpose of holding the fibre (5) in place on the contact surface (14) between the first (1) and the second (2) knife roller.

8. A method according to any one of the preceding claims, wherein the contact surfaces (14) of the first knife roller (1) and/or the contact surfaces (15) of the second knife roller (2) have an outwardly facing convex surface .

9. A method according to any one of the preceding claims, wherein the contact surface (14) of the first clamping rib (11) and the contact surface (15) of the second clamping rib (12) that mate with each other substantially coincide after deformation caused by depression with the plane defined by the cutting edge (13) of the nearest first knife (7) and the cutting edge (13) of the nearest second knife (8) .

10. A method according to any one of the preceding claims, wherein said first clamping ribs (11) are combined in pairs into a first clamping section (9) .

11. A method according to any one of the preceding claims, wherein said second clamping ribs (12) are combined in pairs into a second clamping section (10).

12. A method according to any one of the preceding claims, wherein said first clamping section (9) is made of an elastic material .

13. A method according to any one of the preceding claims, wherein said second clamping section (10) is made of an elastic material.

I

14. A method according to any one of the preceding claims, wherein a clamping section (9 or 10) is provided with a fixing profile (18) on the side facing towards the axis (3 or 4) of the knife roller (1 or 2), which fixing profile extends substantially parallel to the roller axis (3 or 4) and which fits in a recess (21) in the first roller (1) or the second roller (2).

15. A method according to any one of the preceding claims, wherein said fixing profile (18) is tapered in the direction substantially parallel to the axis (3 or 4) of the knife roller (1 or 2) .

16. A method according to any one of the preceding claims, wherein the first clamping ribs (11) and/or first clamping sections (9) are interconnected.

17. A method according to any one of the preceding claims, wherein the second clamping ribs (12) and/or second clamping sections (10) are interconnected.

18. A method according to any one of the preceding claims, wherein the first (1) and the second (2) knife roller are substantially identical in shape.

19. A method according to any one of the preceding claims, wherein the number of the first knives (7) is different from the number of second knives (8) .

20. A method according to any one of the preceding claims, wherein the mutual spacing between the first knives (7) and the mutual spacing between the second knives (8) are substantially identical.

21. A method according to any one of the preceding claims, wherein the clamping sections (9, 10) are substantially . symmetrical with respect to the radial.

22. A method according to any one of the preceding claims, wherein the elastic material of the clamping ribs (11, 12) progressively offers resistance upon compression.

23. A method according to any one of the preceding claims, wherein an interspace (17) diverging in radial direction is formed between the knives (7, 8) and the clamping ribs (11, 12) for the purpose of discharging dirt.

24. A method according to any one of the preceding claims, wherein one clamping rib (11, 12) of each pair of clamping ribs (11, 12) of the two knife rollers (1, 2) that are in contact with each other is at least partially made of a more elastic material than the other clamping rib (11, 12) .

25. A device for dividing fibres into pieces, comprising a first knife roller (1) provided with substantially axially oriented first knives (7) and a second knife roller (2) provided with its substantially axially second knives (8), which knife rollers (1, 2) touch each other -and which rotate in mutually opposite directions and the same peripheral velocity.