Cutting module and method for cutting a strand into individual pieces

Abstract

A cutting module for cutting a strand into individual pieces, the cutting module comprising a rotatable cutter wheel and a rotatable cot wheel, the cutter wheel being rotatable and having cutting blades, the cot wheel having a cylindrical outer surface and being rotatable parallel at a distance from the cutter wheel, such that cutting edges of the cutting blades contact the outer surface of the cot wheel successively in use, so that a strand which is fed between the cutter wheel and the cot wheel is cut into individual pieces, each of the cutting blades being designed such that a cutting force is directed through the cutting blade.

Claims

1. A cutting module for cutting a strand into individual pieces, the cutting module comprising: a rotatable cutter wheel and a rotatable cot wheel, the cutter wheel being rotatable about a cutter wheel central rotational axis, and having a plurality of cutting blades, spaced apart in circumferential direction of the cutter wheel and each extending in an axial direction, each of the plurality of cutting blades having a front surface, facing forward in a direction of rotation of the cutter wheel in use, and a rear surface, at an acute tip angle with the front surface, facing rearward in the direction of rotation in use, an intersection of the front and rear surfaces defining a cutting edge, the cot wheel having a cylindrical outer surface and being rotatable about a cot wheel central rotational axis extending parallel at a distance from the cutter wheel rotational axis, such that the plurality of cutting edges of the cutting blades contact the outer surface of the cot wheel successively in use, so that a strand which is fed between the cutter wheel and the cot wheel is cut into individual pieces, each of the plurality of cutting blades being designed such that a cutting force having a direction that coincides with a virtual line intersecting the cot wheel rotational axis and the cutting edge of a cutting blade in use while the cutting blade cuts through the strand is directed through the cutting blade in use, and each of the plurality of cutting blades being designed such that the cutting force is directed through the rear half of the cutting blade in use, the rear half being between an angle bisector line of the tip angle and the rear surface of the cutting blade.

2. The cutting module according to claim 1, wherein each of the plurality of cutting blades points forward, over a forward angle, in the direction of rotation of the cutter wheel in use, the forward angle being the acute angle between the angle bisector line of the tip angle of a cutting blade and a mathematical base line intersecting the rotational axis of the cutter wheel and the cutting edge.

3. The cutting module according to claim 2, wherein the forward angle of each of the plurality of cutting blades is in the range of 3 to 10 degrees.

4. The cutting module according to claim 1, wherein each of the plurality of cutting blades points forward, over a forward angle, in the direction of rotation of the cutter wheel in use, the forward angle being the acute angle between the angle bisector line of the tip angle of a cutting blade and a mathematical base line intersecting the rotational axis of the cutter wheel and the cutting edge, and wherein the forward angle of each of the plurality of cutting blades is in the range of 3 to 10 degrees.

5. The cutting module according to claim 1, wherein a cutter wheel radius, defined by a circumscribed circle of the plurality of cutting blades, is in the range of 20 to 35 percent of a cot wheel radius, defined by the cylindrical outer surface of the cot wheel.

6. The cutting module according to claim 1, wherein the tip angle of the cutting edge is in the range of 25 to 40 degrees.

7. The cutting module according to claim 1, wherein the cot wheel has a resilient layer at least at the outer surface which resilient layer made of an elastomeric material, and wherein the distance between the cutter wheel rotational axis and the cot wheel rotational axis is such that the respective cutting edges of the cutting blades of the plurality of cutting blades successively penetrate the resilient layer of the cot wheel.

8. A method of making a composite product comprising: I) preparing a sheathed, composite strand; and II) cutting the sheathed, composite strand into individual pieces using a cutting module according to claim 1.

9. Method according to claim 8, wherein step I) comprises: i) providing a plurality of continuous fibers; ii) applying a sizing composition to coat said plurality of fibers provided in step i) iii) gathering said plurality of sized glass fibers obtained in step ii) to obtain a preimpregnated continuous glass multifilament strand containing between 2 and 25% by mass of said sizing composition; iv) applying a sheath of thermoplastic polymer around the preimpregnated continuous multifilament strand to form a sheathed, composite strand; and wherein step II) comprises: A) counter-rotating the cot wheel and the cutter wheel of the cutting module; B) feeding the sheathed, composite strand between the cutter wheel and the cot wheel, and C) cutting the sheathed, composite strand into individual pieces.

10. Method according to claim 9, wherein said fibers are glass fibers and wherein said sheathing material is polypropylene so as to obtain a composite material having a core comprising a continuous glass multifilament strand and a sheath of polypropylene surrounding said core.

11. Method according to claim 8, wherein step I) comprises: a1) unwinding from a package of at least one continuous glass multifilament strand containing at most 2% by mass of a sizing composition or a2) providing a plurality of continuous fibers, applying a sizing composition to coat said plurality of fibers provided, and gathering said plurality of sized glass fibers to obtain a sized continuous glass multifilament strand containing at most 2% by mass of said sizing composition); b) applying from 0.5 to 20% by mass of an impregnating agent to said at least one continuous glass multifilament strand to form an impregnated continuous multifilament strand; c) applying a sheath of thermoplastic polymer around the impregnated continuous multifilament strand to form a composite strand, being a sheathed continuous multifilament strand; and wherein step II) comprises: A) counter-rotating the cot wheel and the cutter wheel of the cutting module; B) feeding the sheathed, composite strand between the cutter wheel and the cot wheel, and C) cutting the sheathed, composite strand into individual pieces.

12. A cutting module for cutting a strand into individual pieces, the cutting module comprising: a rotatable cutter wheel and a rotatable cot wheel, the cutter wheel being rotatable about a cutter wheel central rotational axis, and having a plurality of cutting blades, spaced apart in circumferential direction of the cutter wheel and each extending in an axial direction, each of the plurality of cutting blades having a front surface, facing forward in a direction of rotation of the cutter wheel in use, and a rear surface, at an acute tip angle with the front surface, facing rearward in the direction of rotation in use, an intersection of the front and rear surfaces defining a cutting edge, the cot wheel having a cylindrical outer surface and being rotatable about a cot wheel central rotational axis extending parallel at a distance from the cutter wheel rotational axis, such that the plurality of cutting edges of the cutting blades contact the outer surface of the cot wheel successively in use, so that a strand which is fed between the cutter wheel and the cot wheel is cut into individual pieces, wherein each of the plurality of cutting blades points forward, over a forward angle, in the direction of rotation of the cutter wheel in use, wherein the forward angle being the acute angle between the angle bisector line of the tip angle of a cutting blade and a mathematical base line intersecting the rotational axis of the cutter wheel and the cutting edge, wherein a cutter wheel radius, defined by a circumscribed circle of the plurality of cutting blades, is in the range of 20 to 35 percent of a cot wheel radius, defined by the cylindrical outer surface of the cot wheel, and wherein each of the plurality of cutting blades is designed such that a cutting force is directed through the rear half of the cutting blade in use, the rear half being between the angle bisector line of the tip angle and the rear surface of the cutting blade.

13. The cutting module according to claim 12, wherein the tip angle of the cutting edge is in the range of 25 to 40 degrees.

14. The cutting module according to claim 12, wherein the cot wheel has a resilient layer at least at the outer surface, and wherein the distance between the cutter wheel rotational axis and the cot wheel rotational axis is such that the respective cutting edges of the cutting blades of the plurality of cutting blades successively penetrate the resilient layer of the cot wheel.

15. A method of making a composite product comprising: I) preparing a sheathed, composite strand; and II) cutting the sheathed, composite strand into individual pieces using a cutting module according to claim 12.

16. A cutting module for cutting a strand into individual pieces, the cutting module comprising: a rotatable cutter wheel and a rotatable cot wheel, the cutter wheel being rotatable about a cutter wheel central rotational axis, and having a plurality of cutting blades, spaced apart in circumferential direction of the cutter wheel and each extending in an axial direction, each of the plurality of cutting blades having a front surface, facing forward in a direction of rotation of the cutter wheel in use, and a rear surface, at an acute tip angle with the front surface, facing rearward in the direction of rotation in use, an intersection of the front and rear surfaces defining a cutting edge, the cot wheel having a cylindrical outer surface and being rotatable about a cot wheel central rotational axis extending parallel at a distance from the cutter wheel rotational axis, such that the plurality of cutting edges of the cutting blades contact the outer surface of the cot wheel successively in use, so that a strand which is fed between the cutter wheel and the cot wheel is cut into individual pieces, wherein each of the plurality of cutting blades points forward, over a forward angle in the range of 3 to 10 degrees in the direction of rotation of the cutter wheel in use; wherein the tip angle is in the range of 20 to 40 degrees; wherein the distance between the cutter and cot wheel is chosen such that in use the penetration depth is in the range of 0.3 to 2.5 millimeter; and wherein each of the plurality of cutting blades is designed such that the cutting force is directed through the rear half of the cutting blade in use, the rear half being between an angle bisector line of the tip angle and the rear surface of the cutting blade.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The present teachings are described hereinafter with reference to the accompanying drawings in which embodiments of the invention are shown and in which like reference numbers indicate the same or similar elements.

(2) FIG. 1 shows an example of a cutting module according to the present teachings, in 3-dimensional view.

(3) FIG. 2-4 shows, in cross-section, a sketch of part of the module of FIG. 1, also showing a part of a strand being cut into individual pieces, for the purpose of describing a method of designing a cutting blade of the cutting module, FIGS. 2-4 showing three consecutive states during the cut of a strand: the state on entry of a cutting blade into the strand (FIG. 2); an intermediate state (FIG. 3); and the state on exit of a cutting blade at the end of the cut through the strand (FIG. 4).

(4) FIG. 5 shows part of FIG. 2, including some additional reference signs, for further clarification.

(5) FIGS. 6-8 show, in cross-section, a single cutting blade including areas thereof through which a cutting force vector extends in use, of several respective embodiments of a cutting module according to the invention.

DESCRIPTION OF EMBODIMENTS

(6) FIG. 1 illustrates a cutting module 1 having a cutter wheel 2 and a cot wheel 3. In use, the module cuts a strand 4, partly shown, of long glass fibre reinforced polypropylene having a core comprising a continuous glass multifilament strand and a sheath surrounding said core, into individual pieces 5, or, granules. The strand is continuously fed between the cutter wheel 2 and the cot wheel 3 from a source of strand, such as a from a supply roll, or from a preceding in-line processing device such as a device arranged for applying a sheath of thermoplastic polymer around an impregnated continuous multifilament strand to form the, composite, strand, being a sheathed continuous multifilament strand. In practice a plurality of such strands are fed simultaneously, distributed over the length of the module 1, in axial direction 8, between the cutter wheel 2 and the cot wheel 3.

(7) The cutter wheel 2 is rotatable about a cutter wheel central rotational axis 6 and it has a plurality of cutting blades 7, spaced apart in circumferential direction of the cutter wheel 2 and each extending in the axial direction 8. Only two blades 7 are fully shown in FIG. 1. The cutting blades 7 are provided on a base 13 of the cutter wheel 2. As shown in FIG. 1, the cutting blades 7 may extend in the axial direction under a slight helical angle with respect to the axial direction as shown in FIG. 1. Due to this, the cutting blades 7 are twisted over the length thereof, wherein the amount of twist depends on the length of the cutter wheel 2 and thus also on the length of the cutting blades 7. This way, at each point along the length of the cutter wheel 2, the cutting blades are directed exactly in the same direction such as relating to the forward angle thereof as will be explained in more detail below.

(8) Each of the plurality of cutting blades 7 has a front surface 9, facing forward in a direction of rotation 11 of the cutter wheel 2 in use, and a rear surface 10, at an acute tip angle t with the front surface 9, facing rearward in the direction of rotation 11 in use. In FIG. 2 the half tip angle (t/2) is indicated. See also FIG. 5. The tip angle is the angle of the tip, which is the acute angle between the relevant parts of the front 9 and rear surfaces 10 of the cutting blade 7, defining the cutting edge 12 at the intersection thereof. The relevant parts of the front and rear surfaces are the parts in vicinity of the cutting edge, such as in the range up to 1 mm, 2 mm or even 3 mm from the cutting edge. This distance of for example 3 mm is indicated by the reference sign th in FIG. 5. In case of a cutting blade having flat front and rear surfaces, the value of th is irrelevant. In case that the cutting blade would however have a bullet-shape for example, i.e. having curved front and rear surfaces, the tip angle may still be well-defined using the mentioned distance.

(9) An angle bisector line ab divides the tip angle t in half. An intersection of the front and rear surfaces 9, 10 defines a cutting edge 12. The cutter wheel 2 has a cutter wheel radius r_cuw which is defined by a circumscribed circle (indicated by a dashed line in FIG. 2) of the cutting edges 12. The cutting blades 7 are made of tungsten carbide but may alternatively be made of, or comprise, other suitable materials such as High Speed Steel or ceramic materials. The number of cutting blades 7 on the cutter wheel depends on the radius of the cutter wheel and on the mentioned circular arc between cutting edges of individual cutting blades.

(10) The cot wheel 3 has a cylindrical outer surface 14 and it is rotatable about a cot wheel central rotational axis 15 extending parallel at a distance d from the cutter wheel rotational axis 6. The distance d is chosen such that the plurality of cutting edges 12 of the cutting blades 7 contact the outer surface 14 of the cot wheel 3 successively in use, so that a strand 4 which is fed between the cutter wheel 2 and the cot wheel 3 is cut into individual pieces 5, or, granules. The cot wheel 3 has a cot wheel radius r_cow defined by the cylindrical outer surface 14 of the cot wheel 3. The cot wheel 3 further has a resilient layer 17 at least at the outer surface 14. The distance d between the cutter wheel rotational axis 6 and the cot wheel rotational axis 15 is such that the respective cutting edges 12 of the cutting blades 7 of the plurality of cutting blades successively deform and penetrate the resilient layer 17 of the cot wheel 3. The resilient layer 17 of the cot wheel 3 is made of an elastomeric polyurethane. As a result of the fact that the cutting blades 7 of the cutter wheel 2 engage the cot wheel 3, in use the cot wheel 3 rotates in a direction 11′ opposite to the direction of rotation 11 of the cutter wheel. The cot wheel 3, or alternatively the cutter wheel 2 or both, may be rotatably driven by any drive means such as by an electric motor. The cutter wheel 2 is then rotated via the cot wheel 3.

(11) Each of the plurality of cutting blades 7 points forward, over a forward angle f, in the direction of rotation 11 of the cutter wheel 2 in use. The forward angle f is the angle between the angle bisector line ab and a mathematical base line, coinciding with line b in FIG. 2, intersecting the rotational axis 6 and the cutting edge 12.

(12) A method of designing a cutter wheel of a cutting module having cutting blades pointing forward over a forward angle, according to the present teachings, comprises the steps of: defining the cutter wheel radius r_cuw and the cot wheel radius r_cow, the distance d between the cutter wheel and the cot wheel, and the strand thickness; for each of the cutting blades, for one or more values of a tip angle t of each of the plurality of cutting blades: calculating a minimum forward angle f based on an entry point of the cutting edge of the cutting blade, at the start of a cut through the strand, and calculating a maximum forward angle f based on an exit point of the cutting edge of the cutting blade, at the end of a cut through the strand. Reference is made to FIG. 2, in which a cutting blade 7 of the plurality of cutting blades is cutting through a strand 4, or at least is in a state wherein the cutting blade 7 is making a first contact with the strand, i.e. the entry point. The above step of calculating a minimum forward angle f of the plurality of cutting blades may be performed by applying the cosine rule (also called the law of cosines) on a triangle having as sides the distance d between the cutter wheel 2 and cot wheel 3, a line b between the cutter wheel rotational axis 6 and the cutting edge 12 of the cutting blade 7, the length of which line b thus equals the cutter wheel radius r_cuw, and a line a between the cot wheel rotational axis 15 and the cutting edge 12, the length of which equals a cutting edge radius r_ce, at the entry point. The radius r_ce is in this case equal to the cot wheel radius r_cow plus the strand thickness t_s. Here, the angles alpha and beta are calculated using the cosine rule on the mentioned triangle; the line a coincides with the cutting force Fc, illustrated as a vector; and the line b coincides with the above mentioned mathematical base line.

(13) From the entry point, the cutting blade 7 cuts through the strand 4. FIG. 3 shows an intermediate state during the cut.

(14) The above step of calculating a maximum forward angle f of the plurality of cutting blades may be performed by applying again the cosine rule on the triangle having as sides the distance d between the cutter wheel 2 and cot wheel 3, the line b, and the line a, but this time at the exit point, upon finish of the cut, or, at the exit point, as shown in FIG. 4. The length of a is in this case considered equal to the cot wheel radius r_cow.

(15) In case that the outcome of the above calculations show that the value of the minimum forward angle is larger than the value of the maximum forward angle, the tip angle needs to be increased until the value of the minimum forward angle is at most equal to the value of the maximum forward angle.

(16) In an embodiment of the method, for the purpose of designing a cutting module of which in use the cutting force vector is at an acute angle of at least a predefined value (in degrees; the value representing a safety angle sf) with the bisector line ab as well as with the rear surface 10 at all times, sf should be added to the value of the minimum forward angle and be subtracted from the value of the maximum forward angle. For example, sf may be in the range of 0.5 to 2 degrees, such as for example 1 degree. See also FIG. 8.

Example 1

(17) In the present example, each of the plurality of cutting blades has been designed such that a cutting force (Fc) is directed through the rear three quarters of the cutting blade. That means, a cutting force (Fc) is directed between an angle bisector line (ab2) of a front half angle between the front surface of the cutting blade and the angle bisector line (ab) of the tip angle of the cutting blade, and the rear surface of the cutting blade. This part of the cutting blade is indicated by the hatched area B in FIG. 7.

(18) The radius r_cuw is 80 mm in the present example. The plurality of cutting blades 7 are spaced apart in circumferential direction of the cutter wheel 2 such that a circular arc ca between two successive cutting edges 12 (see FIG. 1) is 12 mm. The tip angle t of the cutting edge is 25 degrees. The number of cutting blades 7 on the cutter wheel depends on the radius of the cutter wheel and on the mentioned circular arc between cutting edges. In the present example, about 41 cutting blades 7 may be provided. The cot wheel radius r_cow is 300 mm. A penetration depth of the respective cutting edges 12 into the resilient layer 17 of the cot wheel 3 is 0.6 mm; that means that the distance between the rotational axes 6, 15 of the cutter and cot wheel is set at 379.4 mm.

(19) The entry of the cutting blade into the strand (FIG. 2) determines the minimum value of the forward angle. The forward angle f is at least equal to the angle alpha (α) between d and b, plus the angle beta (β) between d and a, minus half the tip angle i.e. t/2. Thus, f≥alpha+beta−t/2.

(20) The exit of the cutting blade from the strand (FIG. 4) determines the maximum value of the forward angle. In the present example, the forward angle f is at most equal to the angle alpha between d and b plus the angle beta between d and a plus one fourth of the tip angle t. Thus, f≤alpha+beta+t/4.

(21) For the present example, the forward angle f is calculated in accordance with the above described designing method, for a strand having a diameter, or, thickness, of 3.5 mm, and not taking account a safety angle sf. This method results in a minimum forward angle of about 8.2 degrees and a maximum forward angle of about 14.1 degrees. If the forward angle would then be fixed at 10 degrees, for example, a safety angle of about 1.8 degrees on entry and a safety angle of about 4.1 degrees on exit would result. Also, choosing a different value of the tip angle t would result in different values for the minimum and maximum forward angle. This means that several suitable combinations of tip angle and forward angle may result from the calculations. The same holds for the further examples below.

Example 2

(22) In the present example, each of the plurality of cutting blades 7 is designed such that a cutting force Fc, generated while a cutting blade 7 cuts through the strand 4, is directed through the rear half of that cutting blade 7, that means between an angle bisector line ab of the tip angle t and its rear surface 10. This part of the cutting blade is indicated by the hatched area A in FIG. 6.

(23) The radius r_cuw is 81.25 mm in the present example. The plurality of cutting blades 7 are spaced apart in circumferential direction of the cutter wheel 2 such that about 36 cutting blades may be provided. The tip angle t of the cutting edge is 30 degrees. The cot wheel radius r_cow is 305 mm. A penetration depth of the respective cutting edges 12 into the resilient layer 17 of the cot wheel 3 is 0.5 mm; that means that the distance between the rotational axes 6, 15 of the cutter and cot wheel is set at 385.75 mm.

(24) The entry of the cutting blade into the strand (FIG. 2) determines the minimum value of the forward angle. The forward angle f is at least equal to the angle alpha (α) between d and b, plus the angle beta (β) between d and a, minus half the tip angle i.e. t/2. Thus, f≥alpha+beta−t/2.

(25) The exit of the cutting blade from the strand (FIG. 4) determines the maximum value of the forward angle. The forward angle f is at most equal to the angle alpha between d and b plus the angle beta between d and a. Thus, f≤alpha+beta.

(26) In case that a minimum value for the safety angle would be required, The above step of calculating a minimum forward angle f of the cutting blades would then be f≥sf+alpha+beta−t/2. Similarly, the above step of calculating a maximum forward angle f of the cutting blades would then be f≤alpha+beta−sf.

(27) For the present example, the forward angle f is calculated in accordance with the above described designing method, for a strand having a diameter, or, thickness, of 3 mm, and not taking into account a (minimum) safety angle sf. This method results in a minimum forward angle of about 4 degrees and a maximum forward angle of about 7.1 degrees. If the forward angle would then be fixed at 5.5 degrees, for example, a safety angle sf of about 1.5 degrees on both sides would result. This situation is shown in FIG. 8, wherein the part of the cutting blade through which the cutting force Fc is directed from entry until exit is represented by the hatched area A*, being smaller than A because on both sides a safety angle sf is present. Although FIG. 8 shows an equal safety angle on both sides, different angles on the entry side (the right side in FIG. 8) and on the exit side (left side in FIG. 8) may be chosen, in dependence of the value of the forward angle and the tip angle. Such a safety margin may be advantageous because in that case variations such as in strand diameter/thickness may be accounted for without running the risk that the force vector Fc would be directed outside the rear half of the cutting blade. If such a safety angle would however not be desired, the cutting blade might be further optimized by decreasing the tip angle. A smaller tip angle leads to a lower load on the cutting blade during the cutting. If the tip angle would be set at about 23.7 degrees, the resulting minimum and maximum forward angles become about equal at about 7.1 degrees. In case that the minimum and maximum forward angles are equal, this means that on entry the cutting force vector coincides with the rear surface 10 of the cutting blade, whereas on exit the cutting force vector coincides with the angle bisector line ab.

Example 3

(28) In the present example, each of the plurality of cutting blades 7 is, like in example 2, designed such that a cutting force Fc, generated while a cutting blade 7 cuts through the strand 4, is directed through that cutting blade 7, between an angle bisector line ab of the tip angle t and its rear surface 10.

(29) The radius r_cuw is 75 mm in the present example. The tip angle t of the cutting edge is 32 degrees. The cot wheel radius r_cow is 320 mm. A penetration depth of the respective cutting edges 12 into the resilient layer 17 of the cot wheel 3 is 0.7 mm; that means that the distance between the rotational axes 6, 15 of the cutter and cot wheel is set at 394.30 mm.

(30) The entry of the cutting blade into the strand (FIG. 2) determines the minimum value of the forward angle. The forward angle f is at least equal to the angle alpha (α) between d and b, plus the angle beta (β) between d and a, minus half the tip angle i.e. t/2. Thus, f≥alpha+beta−t/2.

(31) The exit of the cutting blade from the strand (FIG. 4) determines the maximum value of the forward angle. The forward angle f is at most equal to the angle alpha between d and b plus the angle beta between d and a. Thus, f≤alpha+beta.

(32) For the present example, the forward angle f is calculated in accordance with the above described designing method, for a strand having a diameter, or, thickness, of 4 mm, and not taking account a safety angle sf. This method results in a minimum forward angle of about 6.6 degrees and a maximum forward angle of about 8.7 degrees. If the forward angle would then be fixed at 7.65 degrees, for example, a safety angle of about 1.05 degrees on both sides would result.

(33) The foregoing description provides embodiments of the invention by way of example only. The scope of the present invention is defined by the appended claims.