Abstract:
The invention relates to a method for the production of a cutting filament for a device used to cut vegetation such as a trimmer or border cutter. The filament is made of a synthetic material having extended molecular chains. According to the invention, (a) the filament is brought to a controlled viscous state, (b) the filament is stretched according to the length thereof in order to perform a first longitudinal molecular orientation, (c) a cross-sectional change, resulting in a partial reorientation of molecular chains in a transversal direction, is imposed upon the filament, whereupon filaments having multiple molecular orientations are produced, having improved mechanical properties (e.g. a better resistance to tearing).

Description:
BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates in general to plant cutting apparatus such as grass trimmers, edge trimmers and the like, and aims more particularly at new cutting filaments for such apparatus. 
     2) Description of Related Art 
     A cutting filament is usually made by extrusion/drawing of polyamide, and significant advances in these filaments have been known for some years: shapes adapted to reduce noise, improve cutting efficiency, etc., strands made of several materials to improve cutting efficiency, improving biodegradability, reducing cost, etc. 
     However, all the known filaments are still basically made by the same extrusion/drawing technique in which, at a given stage of the process, or on several occasions, the filament at a temperature at which it has a controlled viscosity is subjected to a longitudinal stretching. 
     The consequence of this is to give the molecular chains an average orientation extending in the longitudinal direction of the filament, in order to give it a high tensile strength and limit breakages during work. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to offer the cutting filaments new possibilities of mechanical improvement based on a selective and controlled orientation of the molecular chains. 
     having elongated molecular chains, characterized in that it comprises the following steps:
     (a) bringing the filament to a state of controlled viscosity,   (b) drawing the filament lengthwise to produce a first longitudinal molecular orientation,   (c) imposing on the filament a change of cross section capable of partially reorienting the molecular chains in a transverse direction.   

     Certain preferred, but nonlimiting, aspects of this method are as follows:
         the method also comprises the step consisting in:       (d) imposing on the filament a second change of cross section capable of causing a second partial reorientation of the molecular chains in a transverse direction.
       the second change of cross section is made in a general direction substantially identical to that of the first change of cross section.   the second change of cross section is made in a general direction substantially orthogonal to that of the first change of cross section.   the second change of cross section is made partially in a general direction substantially identical to that of the first change of cross section and partially in a general direction substantially orthogonal to that of the first change of cross section.   the filament has, before the implementation of step (c), a uniform cross section whose dimensions in two orthogonal directions are similar.   the step (c) comprises a flattening of the filament.   the step (d) comprises an at least local flattening of the filament.   the step (c) comprises a localized flattening and a localized thickening of the filament.   the change of cross section, or at least the last change of cross section, of the filament is capable of forming a filament comprising a body and at least one wing protruding from the body.   the change of cross section of the filament, or at least one of the changes of cross section, comprises forcing the filament through a series of dies of progressively different sections.   the change of cross section of the filament, or at least one of the changes of cross section, comprises forcing the filament through a single die of variable section.   the method also comprises a step of cutting the filament whose section has been changed into a plurality of individual subfilaments in the longitudinal direction of the filament.   
       

     According to a second aspect, the invention proposes a cutting filament for a plant cutting apparatus such as a grass trimmer or edge trimmer, the filament being made of a synthetic material with elongated molecular chains such as a polyamide, characterized in that, in at least one zone of the cross section of the filament, the orientation of the molecular chains diverges from a longitudinal orientation. 
     Certain preferred, but nonlimiting, aspects of this filament are as follows:
         the filament comprises a body and at least one wing protruding from the body, and said wing forms a zone in which the orientation of the molecular chains diverges from a longitudinal orientation.   the wing has a generally triangular cross section.   in the body of the filament, the molecular chains are oriented essentially in the longitudinal direction of the filament.   over most of its cross section, there are molecular chains oriented longitudinally and molecular chains oriented generally in a given transverse direction.   over most of its cross section, there are molecular chains oriented longitudinally, molecular chains oriented generally in a first given transverse direction and molecular chains oriented generally in a second given transverse direction.   the first and second transverse directions are essentially orthogonal to one another.       

    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other aspects, objects and advantages of the present invention will better appear on reading the following detailed description of preferred embodiments of the latter, given as a nonlimiting example and made with reference to the appended drawings in which: 
         FIG. 1  is a view in perspective of a cutting filament according to the prior art, 
         FIG. 2  is a view in perspective of a cutting filament according to an exemplary embodiment of the invention, 
         FIG. 3  illustrates the evolution of a filament in terms of cross section and molecular orientation during the implementation of a manufacturing method according to the present invention, and 
         FIGS. 4 to 6  illustrate three examples of the implementation of a manufacturing method according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first of all to  FIG. 1 , it shows a cutting filament  10  for a grass trimmer, edge trimmer or similar element, which has in this instance a cross section that is square or somewhat like a flattened rhombus. Here it is made of polyamide 6 (PA6). 
     This filament has been manufactured by a conventional extrusion/drawing method, in which the filament, in a given state of viscosity (determined by controlling its temperature), has undergone a longitudinal drawing in a determined drawing ratio, so that the molecular chains of polyamide have mostly adopted an orientation  01  in the longitudinal direction of the filament. 
     This conventional method makes it possible to obtain cutting filaments which, due to this molecular orientation, have an excellent tensile strength, and therefore a resistance to shearing during the work of cutting plants. 
       FIG. 2  illustrates a cutting filament made according to the invention. In this filament, there are a body part  11  and two wings  12 ,  13  situated laterally on either side of the body. 
     Via a method such as will be detailed hereinafter, the filament has:
         in its central region, mainly at the body  11 , a longitudinal molecular orientation  01 , in the same manner as in the filament  10  of the prior art shown in  FIG. 1 ;   in the region of at least one of its wings (here the wing  12  that is most prominent), a molecular orientation  02  that is mainly oriented transversely to the longitudinal direction of the filament.       

     Thus, thanks to this double orientation, the cutting filament has enhanced mechanical properties, with a plant cutting region furnished with a mainly transverse molecular orientation, hence a better resistance to wear by transverse wrenching of the filament material, and a body region retaining a mainly longitudinal molecular orientation, to retain overall a good tensile strength. 
     It will be observed here that, in the opposite wing  13 , there is an intermediate molecular orientation between the longitudinal and transverse orientations. 
     It will also be observed that the working part  12  of the filament may be furnished with arrangements (teeth, specific anti-wear coating, etc.) designed to enhance the properties of the filament particularly with respect to cutting efficiency. 
       FIG. 3  illustrates the behavior of the filament during the implementation of a method according to the present invention. 
     A conventional extrusion/drawing method with circular die has made it possible to culminate in a cutting filament as illustrated in its phase  101 , with a circular cross section and a mainly longitudinal molecular orientation  01 . 
     Thanks to a set of dies producing a sort of mixing of the filament still in the viscous state, phase  102  culminates in a filament with a generally square cross section with rounded corners, in which the molecular orientation, although remaining mainly longitudinal, has begun to diverge from this direction. A new set of dies transforms the cross section of the filament from its generally square phase to a phase  103  that is, for example, that corresponding to the end product of  FIG. 2 . 
     During this operation, and owing in particular to the disorientation begun in the preceding step and the limited thickness of the lateral wing  12  of the filament, the molecular chains in this region have taken on a mainly transverse orientation  02  resulting from the progressive creep of the filament material toward this region. 
     This produces a filament corresponding to that of  FIG. 2 . 
     It will be observed here that, instead of a set of dies of constant section gradually bringing the transverse section of the filament from one shape to another, it is possible to provide dies of changing section to culminate essentially in the same type of result. 
       FIG. 4  illustrates a first concrete application of a method according to the present invention. 
     The point of departure is a filament  10  of square cross section obtained by a conventional extrusion/drawing method, resulting in a molecular orientation  01  that is mainly longitudinal. 
     This filament has, for example, a section of 4.5×4.5 mm. 
     In step  401 , this strand enters a die  20  in two parts  21 ,  21  made identically and one of which is symmetrical with the other relative to a horizontal mid-plane. 
     Thus, each die part has a main working surface  211  that is substantially flat, in the middle of which a slight circular indentation  212  is formed. 
     The spacing between the two parts  21 ,  21  is initially such that the distance between the facing working surfaces is substantially equal to the height of the filament  10 . 
     These two parts of the die are placed on the production line to come progressively closer to one another until there results the situation corresponding to step  402 . Between steps  401  and  402 , the filament is in a controlled state of viscosity, has been compressed vertically, causing a lateral creep of its material and hence a partial transverse (or oblique) reorientation of the molecular chains of the polyamide 6. In the present example, the general dimensions of the transition defined by the die in step  402  are approximately 1.75×8 mm. 
     The section of the filament is then again worked to pass from the die illustrated in step  402  to a die  22  as illustrated in connection with step  403 , either in steps, or progressively in a variable geometry die. 
     Here again the die  22  comprises two identical parts  23 ,  23  that are symmetrical relative to a horizontal mid-plane, with a flattened V-shaped indentation  231  over the whole width and, at the base of this flattened V, a narrower channel  232  with a semicircular base. 
     This shape causes, on the one hand, a thinning of the lateral regions of the filament to a triangular shape, and, on the other hand, a thickening of its central region (whose dimensions are approximately 4×1.75 mm in the present example). 
     This has the effect of further accentuating the transverse orientation  02  given to the molecular chains of polyamide in the lateral regions of the filament. At the same time, it is observed that the central region of the filament has undergone, on the one hand, from step  401  to step  402 , a horizontal flattening then, from step  402  to step  403 , a flattening in the vertical direction, which has the effect of creating, in this region, a molecular multi-orientation favorable to the mechanical strength of the filament not only in its longitudinal direction but also in all the transverse directions. 
     This makes it possible in particular to reduce, or even eliminate, the phenomena of fraying of the cutting filament at its free end. 
       FIG. 5  illustrates a second exemplary embodiment of the method according to the invention, which aims to produce a cutting filament having the same contour as in the preceding example, but with further enhanced properties. 
     In this example, steps  501  and  502  are identical to steps  401  and  402  of the preceding example. After step  502 , the filament is rotated through 90° before being engaged in a die  22  identical to that described with reference to  FIG. 4 , but whose two elements have been spaced further apart to be able to receive the flattened filament, then oriented vertically (step  503 ). 
     As a variant, the filament  10  retains its orientation but the die  22  is turned 90° relative to the illustration of  FIG. 5 . 
     The two parts of the die  22  are then brought progressively closer together (steps  504  and  505 ) in order therefore for the filament to be worked in a direction perpendicular to that in which it was worked between steps  501  and  502 . As an example, the total height of the filament changes from approximately 8 mm (step  503 ) to approximately 4 mm (step  505 ). 
     The filament obtained at the end of step  505  has, in terms of contours, the same shape as that obtained at the end of step  403  of  FIG. 4 ; the essential difference was the fact that it undergoes a double thinning, in two orthogonal directions (knowing that, as a variant, two directions that are oblique relative to one another can be provided). 
     The multi-orientation of the polyamide molecules is this accentuated. 
       FIG. 6  illustrates another embodiment of the invention, which uses a die  24  with two elements  25  having flat facing surfaces  251 . 
     In step  601 , a filament  10  of generally square section is received between the two parts  25  of the die  24 . Between steps  601  and  602 , the two parts of the die are brought closer together, to culminate in a generally flat filament, whose thickness is, for example, between 2 and 4 mm, and whose width is typically equal to several times this thickness. During this operation, the orientation of the molecular chains has been partly transformed into a lateral orientation  02 , as illustrated in step  602 . 
     In step  603 , the flattened filament, after being taken out of the die  24 , is cut up with the aid of appropriate blades (not shown) into a plurality of filaments having the same height, but a reduced width. In the present example, this step produces four individual filaments  10   a  to  10   d , with a generally square cross section. The small terminal parts  15  or  16  are scrapped or recycled. 
     In each filament, the molecular chains have an orientation that is no longer solely longitudinal. This provides better mechanical properties, particularly less tendency to fray, without undesirably degrading the tensile strength in the longitudinal direction. 
     According to a variant (not shown) of this embodiment, a first flattening of the filament can be achieved as in step  602 , and a second flattening in an orthogonal direction (in the same spirit as in the embodiment of  FIG. 5 ), in order to further accentuate the disorientation of the molecular chains of polyamide by carrying out this disorientation in an additional direction. 
     It will be noted here that the square section filaments obtained in step  603  may be either packaged for marketing as is, or undergo any other treatments, and particularly shaping treatments (formation of cutting teeth, etc.), coating, etc. 
     Naturally, the present invention is in no way limited to the embodiments described hereinabove and shown in the drawings, but those skilled in the art will be able to apply many variants and modifications thereto. 
     In particular, it is understood that the invention may be combined by those skilled in the art with many other enhancements generally known in the field of cutting filaments (filaments of a particular shape and/or poly-materials to enhance cutting efficiency, reduce operating noise, improve biodegradability, prevent sticking phenomena, etc.).