FR2659250A1 - Cutter for equipping a cutting head of a granulating machine, and granulating machine - Google Patents

Abstract

The subject of the present invention relates to cutters and to granulating machines equipped with cutters which provide good cutting quality for the entire wear life of the cutters, and increased longevity of the cutter blades. The cutters include a heel (2) which is fixed to the said cutting head and which is extended by a blade (13), which extends along an axis (XX1) and which includes a free end (11), which blade includes a cutting edge (7) and a wear zone (14) and is delimited by an inner face (4), an outer face (6), a front face (5) and a rear face (12), the said blade (13) including a minimum cross-section (10), from which the cross-sections increase when moving towards the said free end. The technical field of the invention is the field of manufacturing plastics.

Description

Knife to equip a cutting head with a granulator and
granulator.

DESCRIPTION
The object of the present invention relates to knives intended to equip the cutting heads with granulators, that is to say machines for manufacturing plastic granules, such as polyethylene, polypropylene, etc.

 Numerous documents describe the general technical framework of the invention; the FR patent. 2,318,719 describes the general structure of these machines which include an extruder 30 comprising a horizontal cylinder 31 and a screw 32 which kneads the plastic in the molten state; the screw 32 feeds a pelleting machine called hereafter granulator via a die which is generally planar.

 The granulator essentially comprises a hub called the cutting head which rotates on an axis perpendicular to the plane of the die, which cutting head carries a number of knives intended to cut the extruded strands of plastic material which come out of the holes in the die.

 FR patents. 2,256,812 and U.S. 4,529,370 describe specific devices and methods for these granulators.

 The FR patent. 2,256,812 relates to methods and devices for adjusting the position of the knives with respect to the die, by monitoring the progress of the cutting head towards the die and by detecting the contact between the knives and the die.

 U.S. Patent 4,529,370 describes an automatic control device for the position of the cutting head and the knives produced according to FIGS. 10 to 12.

 According to this document, the knives include a blade support 220, a mounting interface 218 and a blade 222 bounded by faces 241, 243 and 250, which are designed to facilitate the evacuation of the granules and to sharpen the blade as and when measurement of its wear, in order to maintain a good quality of cut.

 Modern granulators generally operate in a cavity traversed by a liquid, which is generally water, and which is used to cool the product and to evacuate it outside the cavity.

 It nevertheless appears that most of the existing industrial cutting systems do not generally guarantee prolonged operation without defect in the quality of cutting; However, these machines are intended to operate continuously and have increasingly large dimensions and production capacities, which can reach several tens of tonnes per hour.

 The quality defects are generally caused by an unsuitable distance between the knife and the die and cause the appearance of fines or tow, which will disturb the cutting and the recovery of the granules; this distance adjustment defect frequently appears on only part of the knife blade.

This distance is a function of the substantially axial thrust of the product on the knives, of the axial prestress in the opposite direction which tends to press the cutting head against the die and which is produced by means described in particular in the patents raised; this distance depends of course on the mechanical resistance of the knife and its support, as well as on the other forces to which the knife is subjected. like the hydrodynamic force which is exerted by the coolant and which opposes the rotational movement of the knife.

 Furthermore, replacing worn knives requires stopping the machine and thus causing substantial production losses; indeed, on some machines, the knives must be replaced every fortnight.

 The problem to be solved is therefore to provide knives and granulators equipped with knives which ensure good quality of cut, throughout the period of wear of the knife, and an increased life of the knife blades.

 Another object of the invention is to provide knives and granulators equipped with these knives which prevent parasitic friction between the knives and the die, which friction is likely to cause vibrations detrimental to the quality of cut as well as a unnecessary energy consumption.

 A knife according to the invention equips a cutting head of a machine for manufacturing plastic granules, which cutting head is cooled by a liquid, is rotated about an axis and is placed opposite a flat die which knives have a heel which is fixed to said cutting head and which is extended by a blade, which is elongated along an axis and which has a free end, which blade has a cutting edge and a wear zone which is bounded at its end near said heel by a foot force, which blade is bounded by an internal face, an external face, a front face and a rear face, said blade having a minimum cross section, from which the cross sections increase when moving towards said free end.

 Advantageously, a knife according to the invention further comprises a substantially planar front face, forming an acute angle with said internal face, so that the hydrodynamic thrust exerted on said front face, induces a bending of said blade towards said die, the surface of said front face gradually increasing from said minimum section towards said free end.

 Preferably, a knife according to the invention is such that the center of gravity of the blade portion between said minimum section and said free end is located between a plane perpendicular to said axis of rotation, passing through the geometric center of said section minimum, and said outer face, so that the action of the centrifugal force applied to said blade portion tends to bring said blade portion closer to said die.

 The objectives of the invention are achieved by a granulator whose cutting head is equipped with knives according to the invention.

 An advantage provided by the knife according to the invention is that the blade and its wear zone are perfectly pressed over their entire length against the die and consequently, the quality of cut and the duration of use of the knife are markedly increased.

 Another advantage provided by the invention is that the parasitic friction between the knives and the die is practically impossible and therefore does not come to produce vibrations and deteriorate the operation of the granulator.

 The following description refers to the appended drawings which represent, without any limiting character, examples of embodiment of knives and granulators equipped with knives according to the invention.

 Figure 1 is a perspective view of an embodiment of a knife according to the invention.

 Figure 2 is a view along II-II of Figure 1, which shows a partial section of the die and the cutting head.

 Figure 3 is a view along III-III of Figure 1.

 Figure 4 is a view along IV-IV of Figure 1.

 Figure 5 is a view along V-V of Figure 1.

 Figure 6 is a view along line VI-VI of Figure 1.

 Figure 7 is a view along VII-VII of Figure 1.

 FIG. 8 is a section along VIII-VIII of FIG. 6.

 FIG. 9 is a section along IX-IX of FIG. 6.

 Figure 10 is a section along X-X of Figure 6.

 In Figure 1 is shown in perspective an embodiment of a knife according to the invention.

 This knife is intended to equip the cutting head with a granulator whose axis of rotation is YY1; the head rotates along this axis in the direction of arrow W.

 The knife is mounted on said cutting head by means of screws or bolts which pass through the holes 3 in the heel 2 of the knife. These holes have a drilling axis 1 parallel to said axis of rotation YY1.

 Advantageously, the holes 3 include a countersink which makes it possible to drown the head of the fixing screw so that the latter is not prominent and cannot touch said die.

 Said heel 2 is of substantially parallelepiped shape and is used to embed said knife on said cutting head.

 Said heel 2 is extended by a blade 13 which is elongated along an axis XX1; this axis XX1 is perpendicular to said axis of rotation YY1, so that during rotation, said axis XX1 describes a plane, which is the general case where the granulators are placed opposite a plane die.

 This axis can be radial and therefore meet said axis of rotation YY1, or not be radial, which is the case in the embodiment illustrated in FIGS. 1 to 8.

 Said blade 13 comprises over at least part of its length, a wear area 14 which is delimited by a front face 5, a cutting face 8 and a rear face of the wear area 9; the intersection of said front face 5 and cutting face 8 constitutes the cutting edge 7, which generates a plane during the rotation of said knife around said axis YY1.

 In order to ensure good cutting conditions, said plane generated by said cutting edge 7 must be maintained, parallel to the plane of the die, at a constant distance therefrom, of the order of a few tenths of a millimeter.

 Said blade is itself limited by said front face 5, an internal face 4a, 4b, an external face 6 and a rear face 12; it extends along the axis XX1, from said heel 2 to its free end 11.

 In a preferred embodiment shown in Figure 1, said faces 4a, 4b, 5, 6, 8, 9, 12 are planar; the manufacture of the knife is indeed facilitated in the case of flat faces.

 Generally, said knife is manufactured by mechanical machining such as milling, turning, grinding, from a blank which may be a cylinder of circular or rectangular section; the material usually used is a steel specially designed for cutting tools.

 Advantageously, said planar faces are interconnected by connection fillets.

 Figure 1 highlights the minimum section 10 of said blade 13; in fact, in the embodiment according to the invention shown in this figure, it can be seen that the cross section of said blade 13 decreases when one moves from said heel 2 towards said minimum section 10, then increases from said minimum section when moving towards said free end 11.

 Said cross sections are the sections of said blade 13 in planes perpendicular to said axis XX1.

 It can also be seen in FIG. 1 that said minimum section 10 corresponds to a minimum width of said front face 5; in fact, in a preferred embodiment according to the invention, said width of said front face 5 decreases from said heel to said minimum section, then gradually increases from said minimum section 10 to said end free 11, where it has a value substantially double that corresponding to said minimum section 10.

 FIG. 1 shows the geometric center C of said minimum section 10.

 For the remainder of the description and the other figures, in order to simplify the description, we choose to pass said axis XX1 through said geometric center C of said minimum section 10.

 Also shown in this figure is the plane P1 perpendicular to said axis of rotation YY1 and passing through said geometric center C; said plane P1 therefore contains said axis XX1.

 In FIG. 1, it can be seen that said wear zone 14 is limited at its ends by a foot face 16 situated on the side of said heel 2 and a head face 17 situated in the vicinity of said free end 11, said foot face and head face are such that as said blade 13 wears, said cutting face 8 keeps a substantially constant surface, so that the friction forces of said knife on said die are substantially independent of the wear of said blade 13. Thus, such a knife provides an improved and constant quality of cut over time.

 In the embodiments of knives according to the invention, said minimum section 10 is located at a distance from said base face (16) of between 0 and 0.75 times the total length of said cutting edge and, preferably between 0.3 and 0.5 times said total length.

 In the preferred embodiment shown in Figures 1 and following, said minimum section 10 is located at a distance from said foot face 16, substantially equal to one third of the total length of said cutting edge.

 It is thus understood that, thanks to the particular characteristics of a knife according to the invention, set out above, the increase in the surface of said front face 5 from said minimum section increases the effects of the hydrodynamic force exerted by the water, which tend to bring said blade 13 closer to said die by bending effect of the knife in a direction parallel to YY1, and to improve the quality of the cut.

 It can be seen in FIG. 1 that, in a preferred embodiment according to the invention, the internal face 4 has two flat faces 4a, 4b making an angle between them of the order of 1700; the face 4a is perpendicular to said axis YY1, while said face 4b is slightly inclined relative to said face 4a and causes a thinning of the rear face 12 and consequently reduces the drag due to the hydrodynamic forces exerted by the coolant.

 It can also be seen that said blade 13 has a connection face 18 which connects said external face 6 to said rear face 12, so that the drag due to the hydrodynamic thrust is reduced.

 It is also noted that said wear zone 14 of said blade 13 is prominent with respect to the rectangular parallelepiped P2, in which said heel 2 and the other part of said blade 13 are not subject to wear, so that up to the complete wear of said wear zone 14, no other part of said knife can rub against said die.

 Figures 2 to 10 explain certain details of embodiment of knives according to the invention.

 Figure 2 highlights the prominence of said wear zone 14 relative to said blade 13; it can also be seen in this figure that said wear area 14 is delimited by said rear face of wear area 9, said cutting face 8 and said foot faces 16 and head face 17.

 A partial section of said die 20 and of said cutting head 19 has been added to this figure, in order to better explain the operation of said granulators.

 Said die 20 is a planar die, the planar face 22 of which is perpendicular to said axis of rotation YY1 and to the plane of FIG. 2.

 It can be seen that said die is provided with a large number of holes 21, through which said plastic material arrives in the molten state, in the direction of the arrows P.

 Advantageously, it can be seen that the length and the position of said wear zone 14 are chosen to correspond exactly to said planar face 22, which is prominent in the part of said die 20 which comprises said holes 21; thus, thanks to the prominence of said wear zone 14 and to the particular shape of said foot faces 16 and head face 17, said cutting face 8 keeps a substantially constant surface throughout the period of use of the knife; moreover, no other part of said knife rubs against said die 20 and therefore does not cause vibrations liable to affect the quality of cut.

 It can be seen in FIG. 2 that said fixing holes 3 include a counterbore; these holes are intended to receive screws or any equivalent means for fixing said knives to said cutting head 19.

 It can also be seen that according to this embodiment of the invention, said blade 13 has a minimum cross section 10; it can also be seen that, advantageously, said rear face 12 of said knife has a width which decreases from said heel 2 to said minimum section 10, then which increases from said minimum section to said free end 11.

 Preferably, the edges of said heel 2 are equipped with chamfers 23. It can be seen that the axis XX1 passes through said geometric center C of said minimum section 10; this axis constitutes the trace of said plane P1 in the plane of FIG. 2; we see that in the embodiment according to the invention, the center of gravity G of the portion of said blade 13 between said minimum section 10 and said free end 11, is between said axis XX1 and said external face 6, this which means that said center of gravity G is between said plane P1 and said external face 6; in this way, the action of one of the components of said centrifugal force applied to said portion of said blade 13, tends to bring said blade portion closer to said die 20 and to improve the quality of cut.

 It can also be seen that the ratio between the average thicknesses of said knife measured along axes of the plane of FIG. 2 parallel to said axis of rotation YY1 in said minimum section 10, on the one hand, and on said heel 2, on the other hand, is between 0.2 and 0.8 and is advantageously equal to 0.4.

 FIG. 3 shows a knife according to the invention on the side of its internal face 4.

 Said heel 2 has chamfers 23 on its edges. Said wear zone 14 is limited by said cutting face 8 terminated by the cutting edge 7, said rear wear zone face 9 and said foot face 16 and head face 17; we see that in a preferred embodiment according to the invention, said internal face 4 comprises two flat portions 4a, 4b which extend over the entire length of said knife which is for example of the order of 170 millimeters and that said face of the foot of said wear zone 16 and internal face 4a are connected by a fillet 25.

 Figures 4 and 5 are views of two ends of the knife according to the invention.

 It can be seen in these figures that said heel 2 and said blade 13 are limited by common faces, said internal face 4b and said rear face 12; said internal face 4b forms an obtuse angle A with said internal face 4a, which is substantially equal to 170 degrees, so that the width of said rear face 12 is reduced and the drag forces and the rotational driving power of said cutting heads are decreased.

 FIGS. 4 and 5 clearly show that said wear zone delimited by said front face 5, said cutting face 8 and said rear face of cutting zone 9 is prominent relative to the rest of said blade 13.

 Advantageously, said wear zone exceeds said internal face 4 by a distance D which is the available wear depth which is for example between 1 and 10 millimeters and, preferably substantially equal to 4.5 millimeters, which allows an increase in the life of the knife which can reach 50%.

 Preferably, said front face 5 and rear face of wear zone 9 are parallel, so that said wear zone keeps a thickness E constant throughout the duration of use of the knife, which thickness is for example equal to 2 millimeters; this has the effect of improving the quality of the cut over time.

 Advantageously, said rear face of wear zone 9 and internal face 4b are connected by a fillet 24.

 We also see in these figures that said front face 5 makes an acute angle B with said internal face 4, so that during the displacement of the knife in the coolant, the hydrodynamic thrust exerted on said front face induces a bending of said blade towards said die.

 It can be seen in FIG. 6 that, in accordance with the characteristics of the invention, said blade 13 has a minimum cross section 10; it can also be seen that, advantageously, said front face 5 of said knife has a width which decreases from said heel 2 to said minimum section 10, then which increases from said minimum section to said free end 11.

 Preferably, the edges of said heel 2 are equipped with chamfers 23. It can be seen that the axis XX1 passes through said geometric center C of said minimum section 10; this axis constitutes the trace of said plane P1 in the plane of FIG. 6; we see that in the embodiment according to the invention, the center of gravity G of the portion of said blade 13 between said minimum section 10 and said free end 11, is between said axis XX1 and said external face 6, this which means that said center of gravity G is between said plane P1 and said external face 6; in this way, the action of one of the components of said centrifugal force applied to said portion of said blade 13, tends to bring said blade portion closer to said die 20 and to improve the quality of cut.

 In FIG. 7, we see said front face 5, external face 6 and connection face 18; we also see that the axis ZZ1 is substantially tangent to a cylinder whose axis is said axis of rotation YY1; in the embodiment illustrated in FIGS. 2 to 10, said axis ZZ1 makes an angle F close to 600; this is due to the fact that the knife as illustrated is not mounted radially on said cutting head; if this were the case, said angle F would be worth substantially 900. This known characteristic of mounting the knife illustrated in these figures does not in any way affect the characteristics of the invention as described in this document.

 Advantageously, in the embodiment according to the invention, said foot faces 16 and head face 17 of said wear zone as well as said free end 11 and the edge 26 of the external face 27 of said heel 2 are substantially parallel to the ZZ1 axis.

 Thus, said foot faces 16 and head faces 17 are contained in planes substantially tangential to cylinders the axis of which coincides with said axis of rotation YY1, so that, as and when wear of said blade 13, said cutting face 8 keeps a substantially constant surface and so that the friction forces of said knife on said die are independent of the wear of said blade 13.

 In a preferred embodiment of knives according to the invention, said blade 13 has a notch 28 which connects said foot face 16 of said wear zone to said heel 2, via said fillet 25, so that when the wear of said wear zone of said blade 13, no contact occurs between said knife and said die except those possible between said cutting face 8 and said die. This notch avoids parasitic vibrations of the knife which can affect the quality of the cut.

 Figures 8 to 10 show cross sections of said blade of a knife according to the invention.

 Figure 8 shows said minimum section 10, Figure 10 shows a cross section close to said free end 11, and Figure 9 shows a cross section intermediate between them.

 We see on these sections that said blade is delimited by said faces 8, 9, 4b, 12, 18, 6, 5.

 It can be seen that on the one hand, the surface and the inertia of said cross sections increase when we pass from FIG. 8 to FIG. 9 and then to FIG. 10, which means that said cross sections increase when we move from said minimum section towards said free end, and that on the other hand, the surface of said front face 5 increases in the same way, correlatively to the increase in the transverse width L of said front face.

 The technical field of the invention is the field of manufacturing plastics.

Claims (10)

 1. Knife for equipping a cutting head with a machine for manufacturing plastic granules, which cutting head is cooled by a liquid and is rotated about an axis (YY1) and is placed opposite a flat die, which knives have a heel (2) which is fixed to said cutting head and which is extended by a blade (13), which is elongated along an axis (XX1) and which has a free end (11), which blade has a cutting edge (7) and a wear zone (14) which is delimited at its end close to said heel by a foot face (16), which blade is delimited by an internal face (4), a face external (6), a front face (5) and a rear face (12), characterized in that said blade (13) has a minimum cross section (10), from which the cross sections increase as one moves toward said free end.  2. Knife according to claim 1, characterized in that said minimum section (10) is located at a distance from said foot face (16) between 0 and 0.75 times the total length of said cutting edge and, preferably , between 0.3 and 0.5 times said total length.  3. Knife according to any one of claims 1 and 2, of the type comprising a front face (5), substantially planar, forming an acute angle with said internal face (4), so that the hydrodynamic thrust exerted on said front face, induces bending of said blade (13) towards said die, characterized in that the surface of said front face gradually increases from said minimum section (10) towards said free end (11).  4. Knife according to claim 3, characterized in that the transverse width of said front face (5) is maximum at said free end (11) and is substantially equal to twice the minimum width corresponding to said minimum section (10).  5. Knife according to any one of claims 1 to 4, characterized in that the center of gravity (G) of the blade portion between said minimum section and said free end is located between a plane (P1) perpendicular to said axis of rotation (YY1), passing through the geometric center (C) of said minimum section (10), and said external face (6), so that the action of one of the components of the centrifugal force applied to said portion of blade tends to bring said blade portion closer to said die.  6. Knife according to any one of claims 1 to 5, characterized in that said blade (13) comprises at least one connection face (18), which connects said external face (6) to said rear face (12), so that the drag due to the hydrodynamic thrust is reduced.  7. Knife according to any one of claims 1 to 6, characterized in that said wear zone (14) of said blade is prominent relative to the rectangular parallelepiped in which are contained said heel (2) and the other part of said blade (13) not subject to wear, so that until the complete wear of said wear zone (14), no other part of said knife rubs against said die.  8. Knife according to any one of claims 1 to 7, characterized in that said wear zone (14) is limited by a rear face (9) of wear zone, substantially planar and parallel to said front face ( 5), and by a foot face (16) and a head face (17) contained in planes substantially tangential to cylinders whose axis coincides with said axis of rotation (YY1), so that, as and as the wear of said blade (13), said cutting face (8) keeps a substantially constant surface and so that the frictional forces of said knife on said die are substantially independent of the wear of said blade ( 13).  9. Knife according to any one of claims 1 to 8, characterized in that the ratio between the average thicknesses of said knife measured along axes parallel to said axis of rotation (YY1) in said minimum section (10) on the one hand and on said heel (2) on the other hand, is between 0.2 and 0.8.  10. Granulator intended for manufacturing granules of plastic material, equipped with a cutting head, which cutting head is cooled by a liquid and is driven in rotation around an axis (YY1) and is placed opposite a flat die, characterized in that said cutting head is equipped with knives according to any one of claims 1 to 9.