POLYMER CUTTING APPARATUS AND METHOD

Abstract: A polymer cutter with a rotary cutting head (17) in which the cutting edges (20) are on the circumferential periphery of the head is described. The knife blades (18) on the rotary head have a knife angle (22) (as defined) of about 40 DEG to about 60 DEG . The apparatus is especially useful for cutting hard and/or brittle thermoplastics and may be used for preparing polymer granules or pellets.

Description: TITLE POLYMER CUTTING APPARATUS AND METHOD FIELD OF THE INVENTION This invention concerns a rotary cutting head for cutting polymer, especially polymer strands and the like, and a process for cutting polymer using this head. TECHNICAL BACKGROUND Polymers are abundant and important items of commerce, being useful in a myriad of applications. During handling, processing or reclamation of polymers it is often necessary to cut the solid (as opposed to molten) polymers into smaller pieces of various sizes and/or configurations. For instance, when thermoplastics are produced they are often cut into (uniform) pellets or gran- ules of relatively small size so they can be easily fed to a forming machine such as an injection molding machine or an extruder. In this type of an operation it is important that the pellets produced be of reasonably uniform dimensions, and that relatively little or no other sizes such as dust or other off-size particles be produced Cutters for polymers are available in many forms. In one common form a rotary head containing knives approximately parallel to the axis of rotation is used to cut polymer against a bed knife as the polymer is being fed into the cutter head. In these cutters the knives are such that they cut by combination of a slicing and a shearing action, with a narrow leading cutting edge cutting through the polymer. In such cutters the knife an- gle (see below) is typically 15-20°. This design allows for a large number of blades on a cutter head of a particular diameter, therefore increasing the cutting capacity (in weight of polymer cut per unit time) of the cutter. While cutters of this type have been popular for many years, they have certain drawbacks. Among these is cutter knife breakage and/or wear, especially when hard and/or abrasive polymers are being cut. Also particularly when hard and/or brittle polymers are being cut, cut quality is often not good, with large amounts of shattered pellets/fines, and/or long pellets and/or pellets with tails, being produced. Also when conventional cutter are being used the small knife angle makes the relatively thin knife edge prone to breakage and/or rela- tively fast wear. When breakage or excessive wear occurs, the cut quality is adversely affected, and the cutter must be shut down to resharpen or replace the worn or broken blades. This downtime is expensive in both actual maintenance costs and lost production time, and a polymer cutting apparatus which can cut at high speed with good cut quality, while at the same time requiring less downtime, would be advantageous. SUMMARY OF THE INVENTION This invention involves a rotary cutter head having an axis of rotation, comprising, one or more knives, each knife having a cutting edge on a circumferential periphery of said rotary cutter head, each knife having a knife angle of about 40° to about 60°, and provided that no point on a cutting face of said knife is further from said axis of rotation of said rotary cutter head than said cutting edge. Also described herein is a rotary cutter for cutting polymer, comprising, a bed knife, a rotary cutter head, and a means for advancing polymer into said rotary cutter head, and wherein said rotary cutter head has an axis of rotation and one or more knives, each knife having a cutting edge on a circumferential periphery of said rotary cutter head, each knife having a knife angle of about 40° to about 60°, and provided that no point on a cutting face of said knife is further from said axis of rotation of said rotary cutter head than said cutting edge. This invention also concerns a process for cutting polymer with a rotary cutter, wherein the improvement comprises, using a rotary cutter head which has an axis of rotation and one or more knives, each knife having a cutting edge on a circumferential periphery of said rotary head, each knife having a knife angle of about 40° to about 60°, and provided that no point on a cutting face of said knife is further from said axis of rotation of said rotary head than said cutting edge. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a cross section perpendicular to the axis of rotation of part of a typical rotary cutting head of this invention, together with a bed knife, a polymer strand, and a cut polymer particle. Figure 2 shows a cross section perpendicular to the axis of rotation of a typical rotary cutter head of this invention, and in particular the cutter head used in the Example. The diameter in mm of the cutter head used in the Example is given. Figure 3 shows schematically important elements of a rotary cutter head of this invention, as defined herein. Figure 4 shows the body (25) of the rotary cutter head used in the Example, with dimensions shown in mm. Figure 5 shows one of the multiplicity of cap bars (27) used in the rotary cutter head of the Examples. Dimensions are in mm. Figure 6 shows one of the multiplicity of knives (26) used in the Example with dimensions in mm. These knives were made from tungsten carbide containing 12% nickel binder. DETAILS OF THE INVENTION By a polymer herein is meant a polymer (or polymer blend) itself containing no additives, as well as polymers containing any additive or any combination of addi- tives normally found in polymers. Such additives include pigments such as Ti0 2 , antioxidants, antiozonants, toughening agents, flame retardants, lubricants, dyes, antistatic agents, antistaining agents, and fillers and reinforcing agents such as talc, clay, carbon black, milled glass, glass fiber, carbon fiber, and aramid fiber. Preferred polymers are plastics (as opposed to elastomers) , and thermoplastics are especially preferred. A more preferred polymer is a so-called thermotropic liquid crystalline polymer (LCP) , or partially aromatic polyamide, and especially preferably the thermotropic LCP. These polymers tend to be hard and brittle, and shatter relatively easily. A thermotropic liquid crystalline polymer herein is given it conventional meaning, is an LCP by the TOT test described in U.S. Patent 4,075,262, which is hereby included by reference . The polymers being cut are preferably solid polymers. By that is meant that if crystalline, they are below their crystalline melting point, and if noncrystal- line (i.e., glassy) they are below their glass transition temperatures . Figure 1 is a general view of a cross section of part of a rotary cutter head, and other parts of a cutting apparatus, according to this invention. The parts of the rotary cutter head shown are the rotary cutter head body 1 which rotates in the direction shown, several knives 2, and several cap bars 6 which hold the knives in place on 1 (with bolts) . Also shown is a stationary bed knife 3 whose mounting is not shown, a polymer strand 4 which is being fed in the direction shown, and a polymer granule 5 which has just been cut. In the cutters described herein it is preferred that clearance between the cutting edge of each knife and the bed knife be as small as practicable. This tends to give the cleanest cut, and is usually about 0.025 to about 0.25 mm, preferably about 0.050 to about 0.12 mm. Generally speaking, in such cutters the polymer is advancing into the rotor knives continuously, so after the cutting edge of each knife passes the bed knife, each cutter knife is raked away from the edge of the bed knife. In other words, the point on the knife furthest away from the axis of rotation of the rotary cutter head is normally the cutting edge of the knife, and all points on the knife cutting face are closer to the axis of rotation of the rotary cutter head than the cutting edge. Furthermore, if applicable, the rotary cutter head or its parts other than the knives are also preferably designed to allow the polymer to advance. Other designs will be obvious to the artisan to accomplish this. The knives may be separate parts which can be removed from the rotary cutter head for sharpening or replacement, or other configurations are possible, which can be held in the rotary cutter head by bolts, caps, or wedges. Or the rotary cutter head may be a single piece of metal, with the knife edges hardened. This eliminates much machining of wedges, holes, etc., which are shown in Figure 1. This is particularly useful where the knife edges don't chip or need sharpening very often. Figure 2 shows a cross section perpendicular to the axis of rotation of a full rotary cutter head according to this invention. This head has a center of rotation 24, a body 25, a multiplicity of knives 26, held to the body with a multiplicity of cap bars 27, and a multiplicity of bolts, 28. Figure 3 is a schematic diagram showing important elements of the rotary cutter head of this invention and their relationship to each other. Also included for completeness is a bed knife. The rotary head cutter of Figure 3 has an axis of rotation 13. For convenience in explanation a dashed line 14 (which is in fact a radius) has been drawn from 13 to the cutting edge 20 of the knife 18 which is mounted on body 17. Also included in Figure 3 is a bed knife 16 having cutting edge 15. Protruding from the body 17 of the rotary cutter head is knife 18 (of course more than one knife may be present) , which has knife cutting face 19, and cutting edge 20. In addition the knife also has cutting edge angle 21, a knife forward surface 23, and knife angle 22. 22 is measured from a radius such as 14 extended through cutting edge 20 to knife forward surface 23. Normally the position of 16 and 17 will be such that 15 will be approximately parallel to 13, and this position also will preferably minimize the clearance between 15 and 20 when the rotary cutter head is in operation. The cutting edge angle of the knife is 21. This angle is the angle between the knife cutting face 19 and the knife forward surface 23. If one or both of 19 and 23 is (are) curved, then 21 is taken as the angle between the tangent and the other arm of the angle or between the two tangents, (on one or both of 19 and 23) at 20. The maximum value of 21 is determined by 22 and the require- ment that no part of 19 be further from 13 than 20. 22 is the knife angle, the angle between a radius (14) from 13, and 23, and is about 40° to about 60°, preferably about 45° to about 55°, and especially preferably about 47° to about 53°. In conventional cutters this angle is believed to be typically 15-20°. No point on 19 should be further away from 13 than 20, except when 20 may be worn (see below) . This follows simply from the fact that one normally prefers to have 20 as close to 15 as is practicably while polymer is being cut. If any part of 19 is further from 13 than 20, one simply will not be able to place 20 as close to 15 as is preferred, without having 19 strike 15 when 17 is rotating. Preferably 19 should be raked back sufficiently to allow the polymer to advance after 20 has passed 15, so that the next pellet may be cut by the succeeding knife. Thus the total of 21 and 22 in degrees will normally be less than 90°. Of course as 20 suffers some wear from cutting polymer, a small portion of 19 immediately adjacent to 20 may be further from 13 than the actual edge of 20. This is permissible, but of course when 20 becomes badly worn it will preferably be sharpened to maintain a good polymer cut quality. It is preferred, although not necessary, that some, and more preferably all, of the knives 18 run the full length of 17. Provision for this is shown in Figure 4. Across the length of 17 (perpendicular to the cross section shown in Figure 2), 20 may be parallel to 13, or may be helically disposed to 13. Therefore usually essentially the entire length of 20 will be at a constant distance from 13. A preferred helical angle is about 0° to about 3° from 13. When 17 is quite long, and for instance may be cutting many strands of polymer, the overall strain on any knife is lessened if that knife con- tacts the polymer strands in sequence, as it would do if the blade was helically disposed about 13. As mentioned above, the more blades on the cutter head, generally the larger the amount of polymer that can be cut (from strands to pellets for instance) per unit time. Thus at a given cutter head speed (rpm) , the strand feed rate may be varied to obtain a given length of cut. If the number of blades on the cutter head is decreased, the feed rate of the polymer strand must be decreased and/or the speed of the cutter head increased to maintain the cut pellet size. Decreasing pellet production rate usually increases cost, so is not desirable, while increasing cutter head speed may be done, but there is usually a practical limit on this because of mechani- cal considerations. From geometrical considerations, at larger knife angles fewer and fewer cutting blades may be on a cutter head of a given diameter. While it may be desirable from a purely cut quality and downtime (less blade wear) perspective to have as large a knife angle as possible, the optimum knife angle will be a compromise between cut quality and/or downtime, and the productivity of the cutter. This invention also includes an apparatus for cutting polymers which includes the rotary cutter head de- scribed above, a bed knife, and a means for advancing polymer into the rotary cutter head. The bed knife is a usually stationary item that is placed so that clearance between the knives of the rotary cutter head 17 pass as close to the bed knife as reasonably possible while the rotary cutter head is rotating. The "knife edge" of the bed knife will usually have an angle of about 90°, and serves to keep the polymer from bending or otherwise moving as the knives 18 strike the polymer. Most commonly, the polymer is fed over a surface of the bed knife into the rotary cutter head, as shown in Figure 1. The polymer 4 is usually advanced continuously as shown in Figure 1, respectively, into the path of the knives 2 of the rotary cutter head. This requires a means for advancing the polymer. If the cutter apparatus is so disposed the polymer may "fall" into the rotary cutter head by gravity. More commonly though the polymer is fed by means of one or more pairs of feed rolls or a pair of feed conveyors. By feed rolls is meant a pair of rolls having a nip between them. Polymer is fed into the nip and the rolls are driven to move the polymer through the nip and into the rotary cutter head. A feed conveyor is similar, being a conveyor-like apparatus with two con- veyor belts with the polymer being driven between the belts. The polymer may simply be fed by being extruded from a die, the movement through and out of the die being the feed mechanism to the rotary cutter head. Other methods of feeding are known. Inherent in the above discussion is a description of a process for cutting polymer using the rotary cutter head described herein. Many different shapes of polymer may be cut, such as sheets, strands, ribbons and tubes, especially thick-walled tubes. If the polymer to be cut is too thin, such as a small diameter fiber or a thin film the polymer may bend and may or may not be cut, but even if cut the cut may not be of good quality, i.e., be ragged, generate fines (smaller particles than desired) or cut only partially or unevenly. It is preferred that the smallest cross sectional dimension of the polymer to be cut is about 1 mm or more, preferably about 2 mm or more. The maximum dimension will depend on the polymer being cut as well as the power of the cutter apparatus and the mechanical stress the cutting apparatus can endure . One preferred form to be cut is one or more polymer strands. By a strand is meant a rod-like essentially continuous length of polymer whose largest cross sectional dimension is no more than 6 times, preferably no more than 3 times, and more preferably no more than 2 times greater than its smallest cross sectional dimension. A preferred cross sectional for a strand is ap- proximately circular or square, with circular being especially preferred. It is preferred that the largest cross sectional dimension of the strand be about 1 to about 8 mm, preferably about 2 mm to about 4 mm. Cutting of strands into relatively short pieces, about 1 to about 8 mm long, or expressed another way the length to diameter ratio of the pellet is about 1, gives an especially useful form of polymer usually called pellets or granules. This is the most common form of solid polymer which is fed to injection molding machines, extruders, and the like. Preferably the cutting edge of the blade is not ground as thin as possible, since this results in a weak cutting edge (because of the thin metal) which is prone to chip. Rather it is preferred that the blade edge is cylindrically ground. By this meant that the cutting edge of the blade is ground in a generally circular (arc) configuration (cylindrical land) with a given radius. Preferably this radius is about 0.10 mm to about 1.3 mm, more preferably about 0.25 mm to about 1.0 mm, and espe- cially preferably about 0.35 mm to about 0.80 mm. The cutter described herein gives cut polymer, especially pellets, and particularly with hard/brittle polymers which are often of a superior quality to conventional cutter, especially in regard to improving pellet uniformity and reducing dust and fines. In addition, the blades often last longer, i.e., tend to chip and/or wear less than in a conventional cutter. EXAMPLE A cutter in accordance with our invention was used to cut circular cross section polymer strands using the cutter head of our invention. The cutter and various parts with their dimensions that was used is shown in Figures 2, and 4-6 (the cutter head was affixed to the rest of the cutter and rotated by a shaft passing though the head) . The diameter of the strands entering the cutter was about 2.3 mm, and the length produced was also about 2.3 mm. The strands were melt extruded through die holes 4.0 mm in diameter and the strands while molten were drawn down to about 2.3 mm in diameter by the feed mechanism of the cutter. The strands were solid by the time they actually reached the cutter feed mechanism. Over a period of slightly over 3 months a total of about 301,000 kg of various liquid crystalline polymers (LCPs) were passed through this cutter (in campaigns, not continuously), yielding about 277,000 kg of good quality pellets. Most of the LCPs has 30-40% by weight of glass fiber filler in them, as well as other materials such as carbon black or Ti0 2 . During this time period the bed knife was flipped or changed several times because of poor cut quality, and the rotor blades exhibited relatively little wear. However about 90% of the way (by weight) through this test, the left edge of the rotor (blades) apparently contacted the bed knife, and some chipping occurred there . Compared to a cutter of the prior art, the knives were not dulled as quickly on this cutter, and the quality of the pellets cut was improved, especially in re- spect to the shape of the pellets themselves and elimination of long pellets. While this invention has been described with respect to what is at present considered to be the preferred em- bodiments, it is to be understood that the inventiion is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent formulations and functions.

Claims: CLAIMS What is claimed is: 1. A rotary cutter head having an axis of rotation, comprising, one or more knives, each knife having a cut- ting edge on a circumferential periphery of said rotary cutter head, each knife having a knife angle of about 40° to about 60°, and provided that no point on a cutting face of said knife is further from said axis of rotation of said rotary cutter head than said cutting edge. 2. A rotary cutter for cutting polymer, comprising, a bed knife, a rotary cutter head, and a means for advancing polymer into said rotary cutter head, and wherein said rotary cutter head has an axis of rotation and one or more knives, each knife having a cutting edge on a circumferential periphery of said rotary cutter head, each knife having a knife angle of about 40° to about 60°, and provided that no point on a cutting face of said knife is further from said axis of rotation of said rotary cutter head than said cutting edge. 3. A process for cutting polymer with a rotary cutter, wherein the improvement comprises, using a rotary cutter head which has an axis of rotation and one or more knives, each knife having a cutting edge on a circumferential periphery of said rotary head, each knife having a knife angle of about 40° to about 60°, and provided that no point on a cutting face of said knife is further from said axis of rotation of said rotary head than said cutting edge.

Inventors: FISHER GEORGE F, WAGGONER MARION G, FISHER, GEORGE, F., WAGGONER, MARION, G.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 2000
Main classification: B27G13
IPC codes: B02C18, B26D1, B27G13, B29B9