Abstract:
This method consists essentially in manufacturing dripper units ( 4 ) and attaching them longitudinally to a thread ( 16 ) by positioning them at intervals along the latter. For this purpose, the thread ( 16 ) is introduced into at least one notch ( 9 ) of the dripper unit ( 4 ) so as to clamp it. The width of the notch is less than the diameter of the thread. The thread fitted with dripper units is introduced into the tube ( 1 ) while the latter is being formed in an extruder so as to draw the dripper units ( 4 ) therein one after the other. The dripper units are then heat welded to the inner wall of the tube during its formation. The insertion of the thread ( 16 ) into the notch ( 9 ) is achieved via a wheel ( 18 ) onto whose the edge it is guided. In this manner, the whole method can be implemented continuously without any intermittent operations.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns a method for manufacturing a drip irrigation tube. 
     As is well known in the art, tubes of this type are perforated at regular distances with small holes through which the water can flow into the earth at a low flow rate. This flow rate is fixed by flow limiters called “dripper units” mounted within the tube facing the holes and having the shape of a small block attached to the inner surface of the tube. Limitation of the flow rate is assured by a labyrinth formed in the block. A description of such a “dripper unit” can be found in European Patent Application No 0 715 926 filed by the Applicant of the present invention. 
     A method for manufacturing drip irrigation tubes is known from U.S. Pat. No. 5,324,379. It consists of drawing the drips inside the tube while the latter is being formed in an extruder or an extruding station, the dripper units being attached to the inner wall of the tube via heat welding, when the latter is still warm at the extruder output. In order to do this, the dripper units are introduced into the tube in formation by being drawn by a thread to which they are attached longitudinally at points separated from each other by a distance equal to the distance which will separate them in the completed tube. The aforecited U.S. Patent provides several alternatives for fixing the dripper units to the thread. One of these alternatives, shown in FIGS. 9 to 14 of the Patent, consists in making knots in the thread and fitting each dripper unit with a slit driving lug extending at right angles to the dripper unit&#39;s direction of movement in the installation. The knots in the thread can be locked behind these lugs, in order for a given dripper unit to be able to be driven by the preceding one which is already attached to the tube in the extruder die. 
     The thread is knotted in a knotting station which takes an end of the thread to form each knot, this station being situated upstream of a station distributing the dripper units. 
     This method has a serious drawback which lies in the fact that it can only be intermittent, i.e. the thread must be stopped each time in order to form the knots. This has a direct effect on the global manufacturing speed of the tube. The other alternatives disclosed in this U.S. Patent allowing the dripper units to be attached to the thread all have the same drawback, the author of the Patent even envisaging preparing threads fitted with dripper units in advance and winding them onto reserve spools. In addition to the risk of seeing the thread and the dripper units becoming entangled when the spool is unwound, the manufacturing process has to be stopped periodically to replace an empty spool with a full spool loaded with a thread and dripper units, an operation which is no more satisfactory than those involved in the other alternatives disclosed. 
     SUMMARY OF THE INVENTION 
     An object of the invention is thus to provide a drip irrigation tube manufacturing method overcoming the aforecited drawbacks and allowing any intermittent operation during the irrigation tube manufacturing process to be voided. 
     The invention thus concerns a method as defined in claim  1 . 
     As a result of the features of this method, the dripper units are attached to the thread during an operation which can be performed while the dripper units continue to move towards the extruder. This operation can be performed at the same speed as that at which the tube is formed, the thread also being drawn at this same speed. 
     Other features and advantages of the invention will appear during the following description, which is given solely by way of example with reference to the annexed drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a section of drip irrigation tube according to the invention; 
     FIG. 2 is also a perspective view, on a larger scale, of a dripper unit used in the irrigation tube according to the invention; 
     FIGS. 3A and 3B show via partial schematic lateral elevation views, an installation allowing the drip irrigation tube manufacturing method according to the invention to be implemented; 
     FIG. 4 is a large scale transverse cross-sectional view along the line IV—IV of FIG. 3A; 
     FIG. 5 is a transverse cross-sectional view along the line V—V of FIG.  3 B and on the same scale as the latter, 
     FIG. 6 shows an alternative embodiment of a waiting station allowing the method according to the invention to be implemented, and 
     FIG. 7 shows another alternative embodiment of the installation allowing the method according to the invention to be implemented. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a sprinkling or irrigation tube  1 , made of plastic material, for example polyethylene, which has, at a given pitch, holes  2  through which water can flow at a low flow rate to water vegetables  3 . Dripper units  4  are attached within tube  1  facing each hole  2 , their structure being shown more clearly in FIG.  2 . 
     Each dripper unit  4  takes the general form of a small oblong hollow block made of plastic material of a general parallelepiped shape whose surface  5 , intended to be adjacent to the wall of tube  1 , is curved to mould itself to said wall once dripper unit  4  is put in place. A collecting chamber  6 , which communicates with a hole  2  of tube  1  and with passages  7  opening into the cavity  8  (visible in FIG. 4) delimited by the block of dripper unit  4 , is formed in surface  5 . Channels  7   a  and  7   b  forming a labyrinth connecting collecting chamber  6  to passages  7 , are also made in surface  5 . 
     In the completed tube, dripper units  4  are attached, preferably by heat welding, via their entire surface  5  to the inner face of tube  1 . Collecting chamber  6  then opens into hole  2  through which water can flow at a predetermined rate through the labyrinth formed by channels  7   a  and  7   b.    
     According to the invention, each dripper unit  4  has two notches  9  provided in the lugs formed, in the example, by the respective end walls delimiting cavity  8  in dripper unit  4 . Each notch  9  opens towards the interior of tube  1  when dripper unit  4  is mounted in place. It will be seen hereinafter that the width of this notch  9  is carefully defined to fulfill one of the essential functions of the invention. It is to be noted that, according to an alternative, the body of dripper unit  4  could have only one notch  9  or, possibly, more than two notches distributed over its length in transverse walls or partitions passing through cavity  8 . 
     It will be noted in FIG. 1 that the length of each dripper unit  4  is designated  1 , while the pitch with which dripper units  4  are repeated in the longitudinal direction of tube  1  is designated d. 
     The method and the installation allowing a tube such as that shown in FIGS. 1 and 2 to be manufactured will now be described. 
     Dripper units  4  in the form shown in FIG.  2  and manufactured in advance, are brought to point  10  (FIG.  3 A), for example from a vibrating hopper feeder well known in the art (not shown in the drawings). Longitudinally positioned, they follow a rectilinear trajectory succeeding each other and being in contact with each other. They are thus introduced into a first conveyor or caterpillar type take off  11 , hereinafter the “caterpillar”. This caterpillar  11  includes an endless upper conveyor belt  12   a  and a lower endless conveyor belt  12   b  whose respective lower and upper sides are parallel and held apart from each other at a slightly smaller distance than the thickness of a dripper unit  4 . As conveyor belts  12   a  and  12   b  are consequently driven in such a way that their respective lower and upper sides move in the direction of arrows Fa and Fb, dripper units  4  are driven longitudinally (towards the left in FIG.  3 A). Caterpillar  11  is arranged for imposing on dripper units  4  a predetermined progression speed which is preferably fixed by the following ratio: 
     
       
           V   ch11   =V   1 *1 /d   
       
     
     in which: 
     V ch11  caterpillar  11  advancing speed, 
     V 1  manufacturing speed of tube  1   
     l, d distance values indicated hereinbefore 
     First caterpillar  11  is followed by a transfer guide  13  formed by a block in which a passage  14  is arranged having a transverse rectangular cross-section substantially equal to the transverse cross-section of a dripper unit  4 . 
     Guide  13  is followed by a station  15  for attaching dripper units  4  onto a thread  16 . This station  15 , hereinafter the “second caterpillar”, includes three endless conveyor belts  15   a,    15   b  and  15   c  (see also FIG.  4 ), namely an upper belt  15   a  and two lower belts  15   b  and  15   c,  these latter being placed side by side below upper belt  15   a  and each having a slightly smaller width than the width of belt  15   a.  Upper belt  15   a  is driven in the direction of arrow Fc and lower belts  15   b  and  15   c  are driven in the direction of arrow Fd. The lower side of belt  15   a  is situated at a distance from the upper sides of lower belts  15   b  and  15   c,  so as to arrange a gap between them whose height is slightly less than the thickness of a dripper unit  4 . 
     The active length of second caterpillar  15  is preferably equal to approximately the sum of length l and distance d as defined hereinbefore. 
     Thread  16  is unwound from a spool  17  in the direction of arrow Fe. It may have a diameter of 0.5 mm for example, and be made of plastic material such as polyethylene or polypropylene. 
     Attaching station or second caterpillar  15  also includes an insertion wheel  18  rotatably mounted in the direction of arrow Ff about an axis  19  at right angles to the direction of progression of dripper units  4 . This wheel  18  is formed of a disc  20  (FIG. 4) having a thin rim  21  whose edge is formed so as to have a circular concave groove  22  of a radius substantially corresponding to the radius of the cross-section of thread  16 . The thickness of rim  21  is preferably 0.4 mm. 
     Insertion wheel  18  is placed relative to second caterpillar  15  in such a way that its peripheral portion rotates between lower belts  15   b  and  15   c,  and groove  22  is situated above the plane formed by the upper sides of said belts. Moreover, notches  9  formed in the end walls of dripper units  4  have a width which is less than the diameter of thread  16 . Thus, according to an essential aspect of the invention, thread  16 , by passing over insertion wheel  18 , is inserted into notches  9  of each dripper unit  4  which passes between belts  15   a,    15   b  and  15   c,  which is seen clearly in FIG.  4 . This insertion causes thread  16  to be caught in notches  9  and thus thread  16  and dripper unit  4  to be attached. It will be noted that in order to facilitate insertion, notch  9  widens towards wheel  18 . 
     Second caterpillar  15  is followed by a waiting station  23 , a first portion of which appears in FIG. 3A, and a second portion of which appears in FIG.  3 B. Waiting station  23  is essentially formed of a guide-bar  24  disposed longitudinally in the advancing direction of dripper units  4 . It has over its entire length a longitudinal groove  25  of reverse T-shaped cross-section, the transverse bar having substantially the same cross-section as a dripper unit  4 . Below guide-bar  24 , groove  25  opens into a volume delimited by a caisson  26  in which a partial vacuum is maintained by suction (not shown). Above guide-bar  24 , groove  25  communicates with the atmosphere through orifices  27  provided in steps in order to assure a draught through caisson  26 . At the downstream end of guide-bar  24 , groove  25  opens upwards towards an opening  28  of larger dimension in order to allow a brush  29  to pass, said brush being suspended above guide-bar  24  so as to obstruct groove  25 . Thus, brush  29  act as a brake, the passage downstream of dripper units  4  being prevented by brush  29  which only yields if sufficient traction is exerted on said dripper units. 
     According to another essential aspect of the invention, waiting station  23  allows a waiting line  30  to form, consisting of a predetermined number of dripper units  4 , which, when stopped by the brake or brush  29 , accumulate behind each other in guide-bar  24 , while thread  16  forms loops  16   a  as shown in FIGS. 3A,  3 B and  5 . 
     Waiting station  23  also includes a sensor  31  which is capable, through another opening  32  which enables groove  25  to communicate with the open air, of sensing the presence of a dripper unit  4  at this location and of counting the number of dripper units  4  in waiting station  23 . Sensor  31  is connected to a control device  33  responsible for regulating, as a function of the number of dripper units  4  situated in waiting station  23 , the speed of a driving motor  34  for belts  15   a,    15   b  and l 5   c  and for insertion wheel  18  in attaching station  15 . The mechanical torque between motor  34  and belts  15   a,    15   b  and  15   c  is represented by dot and dash lines in FIG.  3 A. 
     The advancing speed of dripper units  4  into attaching station  15  is adjusted to a value VD equal to extrusion speed V 2  of tube  1  increased or decreased by a correction value C which can vary as a function of the signal provided by sensor  31 . Moreover, the length of belts  15   a,    15   b  and  15   c  is chosen to be substantially equal to the value d+l (see FIG.  1 ). These arrangements allow a determined number of dripper units  4  always to be held in waiting line  30 . 
     The installation according to the invention also includes an extrusion station  35 . This station includes an extrusion head  36  receiving molten plastic material in a melting chamber (not shown), and supplying a semi-formed tube to a calibration cylinder or die  38 . From there, formed tube  1  passes into a cooling station  39 , then into a regulated drawing station and into a perforation station (not shown). In the latter, cooled tube  1  is perforated at intervals at right angles to collecting chambers  6  for dripper units  4 . As these three stations of the installation are well known to those skilled in the art, they are not described in detail here. 
     A passage  38 , through which extends a support table for dripper units  4 , passes through extrusion head  35 . Likewise, die  38  has a central passage  40  into which extends the downstream part of table  41 . The latter is intended first to allow transfer of dripper units  4  to die  37  and, in said die, to assure application pressure of dripper units  4  against the inner wall of semi-formed tube  1 , said pressure guaranteeing good heat welding of dripper units  4  to the wall of tube  1 . 
     According to another important aspect of the invention, panel  41  is fitted with a third caterpillar  43  formed of an endless belt  44  whose upper side passes into a longitudinal groove  45  of panel  41 , so that its upper surface is flush with the upper surface thereof. Endless belt  44  also passes over a motor device  46  and over return rollers  47 ,  48  and  49 , while the lower side passes into a groove  50  made in the inner surface of panel  41 . Motor device  46  is arranged to drive third caterpillar  43  at extrusion speed V 2  of tube  1 . 
     Third caterpillar  43  adds a particular advantage, especially when the wall of tube  1  has a relatively small thickness. Third caterpillar  43  is able to considerably reduce the friction undergone by dripper units  4  caused by their sliding over table  41 , when they pass into die  38  where they are attached to tube  1 . This is important to the extent that tube  1  has to transmit the traction force to dripper units  4  and to thread  16  allowing them to continue on from waiting line  30 , the pressure with which dripper units  4  are made to be heat welded to the wall of tube  1  being able to be relatively significant. 
     In short, it has been established that, as a result of the method according to the invention, manufacturing of the irrigation tube can occur without any discontinuity, in particular without inopportune acceleration or deceleration of the moving elements in the installation and the tube being formed. 
     It will be noted that according to an alternative embodiment of the installation, waiting station  23  can be replaced by a dancer or take-up device  60  such as shown in FIG.  6 . This dancer  60  includes conventionally two fixed pulleys  62  and  63  between which is inserted a moving pulley  64  connected to a first end of an arm  65  hinged by its second end onto a frame B. 
     Arm  65  is spring-biased to move the fixed pulleys away from each other as a result of return means  66  formed by a spring. These return means  66  may also be formed by a counterweight which is fixed in relation to the hinge axis of arm  65  on an opposite part to that carrying moving part  64 . Thread  16  carrying dripper units  4  coming from second caterpillar  15  is thus deviated by fixed pulley  62  wound onto moving pulley  64 , and again deviated towards extrusion station  36  by second fixed pulley  63 . Dancer device  60  is associated with first guide means  68  fixed onto frame B, and to second moving guide means  69 , attached to arm  65 . Fixed guide means  68  are arranged between first fixed pulley  62  and moving pulley  64 , while moving guide means  69  are arranged between moving pulley  64  and second fixed pulley  63 . These guide means  68 ,  69  are essential for preventing any rotation of dripper units  4  about themselves before entry into the extrusion station. As a result of this dancer device  60 , the advancing speed of dripper units  4  can thus be adjusted as a function of the extrusion speed. 
     FIG. 7 shows another alternative embodiment of the installation wherein the same elements as those described in conjunction with the preceding Figures are designated by the same numerical references. 
     According to this variant, waiting station  23  has been omitted and replaced by a longitudinal groove similar to groove  25  described in conjunction with FIGS. 3A and 3B. Caterpillar  15  has also been shortened so that no more than one dripper unit  4  is driven by belts  15   a  and  15   b  at one time. Typically, the active length of caterpillar  15  is equal to three times the length of a dripper unit  4 . In this variant, thread  16  is unreeled from spool  17  without tension, for example by flyer pay off. Thread  16  which carries dripper units  4  is thus only driven by the tube which has just been formed to which the dripper units are attached. This is made possible to the extent that the thread is unreeled without tension from spool  17 , where the dripper units have a very low weight and where the friction coefficient of the dripper units in groove  25  is very low. Thus, during locking of the thread onto the dripper unit, the latter is driven by caterpillar  15  at a speed V substantially higher than V 1  or V 2 . Since V is higher than V 1  or V 2 , a little slack is created in the thread between caterpillar  15  and the extrusion station. As soon as the dripper unit which is attached onto the thread leaves caterpillar  15 , the thread carrying dripper units  4  is driven only by the tube at speed V 2 . Caterpillar  15  thus only drives dripper unit  4  before and after attachment thereof to the thread, but never drives the thread alone.