Abstract:
A calibration apparatus and method for making a grooved plastic spacer for a slotted core telecommunications cable. The method includes attaching a calibration apparatus having helically shaped calibration members to an extrusion die. The calibration members are inserted into respective grooves of the spacer so that, as the spacer exits from the die, the calibration members maintain the shape of the grooves until the spacer is sufficiently cooled.

Description:
This application is a Continuation of application Ser. No. 08/476,389 Jun. 7, 1995, now U.S. Pat. No. 6,066,798. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the invention is telecommunication cables having cylindrical spacers having open exterior grooves for the insertion therein of longitudinally extending telecommunications members such as insulated copper wires or coated light waveguides or ribbons containing light waveguides. Such cables are sometimes referred to as slotted core cables. The invention concerns apparatus and methods for the manufacture of the grooved spacer, sometimes called the slotted core. 
     It is well known that such grooved spacers may comprise a layer of plastic extruded over a core strength member. One or more grooves are formed in the external surface of the plastic, and a telecommunications member is inserted into a groove. The grooves may follow paths which are helical in shape or which have a direction of lay which reverses at periodic intervals. 
     With the more recent trend toward inserting a stack of light waveguide ribbons in a groove, especially in cables having a high fiber count and high fiber density, irregularities in the exact shape of the floor and walls of U-shaped grooves become more significant in the performance of the cable. The portions of the plastic material between adjacent grooves and forming the groove walls are referred to as the ribs of the spacer. Even if the grooves are initially extruded having the desired U-shape, a wall of a groove may slump inward into the groove due to the force of gravity, the angular momentum on the legs caused by rotating the spacer during manufacturing, or flow in the extruded melt due to cooling effects. Spacers of high density cables become more susceptible to slumping as the width of the grooves increases to accommodate wider ribbons having more light waveguides therein, because the width of the plastic at the base of a rib between the corners at the bottom of adjacent grooves becomes smaller. The spacers also become more susceptible to slumping as the height of the grooves increases. Such wall slumping can cause unacceptable attenuation in light waveguide ribbons inserted into the groove, especially when the cable is bent. 
     Hulin, U.S. Pat. No. 4,272,472, describes a prior art technique for making a cylindrical member having near its exterior surface a plurality of rounded ducts, each of which is almost closed at the outer surface of the cylindrical member by a pair of oppositely facing lips directed towards each other. Longitudinally extending pins mounted to a crosshead extruder tip and distributed along a circle of diameter smaller than the die opening extend downstream of the die to completely fill and delimit the ducts in the extruded member. A flexible wire having a smaller diameter than the pins is mounted at the distal end of each pin. To obtain the ducts following helical paths, the pins and the tip are rotated by a motor. A knife or wire at the distal end of each flexible wire is used to cut a narrow passage to the exterior surface of the cylindrical member between the lips of the duct. Specialized extrusion equipment is required to rotate the tip. 
     Yataki, U.S. Pat. No. 4,474,426, addresses the problem of exterior groove dimensional stability in a grooved spacer for telecommunications cable by grinding the grooves following extrusion of the cylindrical member. However, such grinding results in wasted plastic, introduces an extra processing step, and requires specialized equipment. 
     Matsuno et al., U.S. Pat. No. 4,814,133 addresses the problem of exterior groove stability in a grooved spacer for telecommunications cable by using a two-step extrusion of the plastic, to provide a central strength member, an intermediate annular plastic layer over the central strength member, and an exterior plastic layer having grooves in its exterior surface. The examples given include grooves having a depth of up to 2.4 mm in high density polyethylene (HDPE); however, groove depths of up to 1.4 mm are shown when linear low density polyethylene is used. The method reduces the volume of the plastic in the grooved exterior layer, but does not address postextrusion groove dimensional stability. 
     To address the problem of groove distortion in a grooved spacer for telecommunications cable when the extruded plastic spacer contacts cooling water in a cooling vat, Schneider, U.S. Pat. No. 5,380,472, provides a plurality of shaping disks housed in the cooling vat. The shaping disks have web-like projections extending radially inward toward the axis of the grooved spacer. Each projection is inserted into a groove. The shaping disks may be used in making a grooved spacer formed of HDPE having six grooves, each having a groove width of 1.5 mm and a height of 2.7 mm made using the shaping disks. 
     In practice, the risk of the grooved spacer becoming lodged in the shaping disks during processing is significant. The extrusion of a spacer formed of a plastic material having a melt flow index higher than that of HDPE and having grooves of a depth greater than 3.0 mm is difficult using the shaping disks. The disks must be mounted to each other by rods or the like to maintain exact spacing and helical alignment of the projections. The resulting apparatus is expensive to build and can be difficult to properly clean during a back-up of the plastic material. Other disadvantages are that the distance between the extruder die and the cooling vat cannot be adjusted after the apparatus is mounted in the cooling vat, and the apparatus is relatively intolerant to variations in the longitudinal or angular velocities of the spacer. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide for telecommunication cables a spacer whose grooves have more stable dimensions. 
     A further object of the invention is to provide grooved spacers having grooves of any desired depth in plastic having a melt flow index higher than that of HDPE. 
     Another object of the present invention is to provide an improved calibration apparatus which does not require specialized extrusion equipment. 
     Yet another object of the invention is to provide such an apparatus that is less susceptible to back-up of the plastic material than prior art shaping disks. 
     Still another object of the invention is to provide such an apparatus that can maintain groove stability both between the extruder die and the cooling vat and also within the cooling vat. 
     These and other objects are provided, according to the present invention, by providing a calibration apparatus for making a longitudinally extending spacer having at least one groove following a helical path in the exterior surface thereof into which a longitudinally extending telecommunications member may be inserted. The calibration apparatus includes a calibration device including a mounting member mounted to an extrusion die which shapes the exterior surface of the grooved spacer. The calibration device also includes a substantially rigid, longitudinally extending, helically shaped calibration member mounted to the mounting member. At least a portion of the calibration member is inserted into said groove to maintain its dimensional stability during cooling of the plastic material subsequent to its extrusion. The calibration device preferably extends into a cooling vat provided to cool the hot extruded plastic material; however, it may stop short of the cooling vat, particularly if other calibration means, such as shaping disks, are provided in the cooling vat. 
     Because the force of the extruded plastic on the calibration members is significant, thickened members are provided on each end of the calibration device. These thickened members may serve as mounting members, such as the mounting member which is mounted to the die. 
     The portion of the calibration member inserted into the groove may have a cross-sectional area in a plane perpendicular to the thread of the groove which increases, over at least part of the longitudinal extent of the inserted portion, with increasing distance from the die. 
     The calibration member may be inserted into the groove to a depth of at least 3.0 mm over at least part of the longitudinal extent of the calibration member. 
     The die for shaping extruded plastic melt may have a helically shaped passage for imparting an initial momentum to the melt in the direction of the spaces between the calibration members. 
     The inserted portion of the first calibration member has a cross-sectional area perpendicular to the thread of the groove which is smaller than the cross-sectional area of the groove. In particular, the depth of insertion of the first calibration member into said groove is less than the depth of the groove. In spacers having generally U-shaped grooves each having a flat bottom and two sides perpendicular thereto, the calibration member may touch both of the groove sides, but does not touch the groove bottom, 
     The calibration apparatus may also include a second calibration device mounted by a coupling to the downstream end of the first calibration device. The second calibration device may be provided to accommodate processing lines in which the desired distance between the extruder die and the cooling vat is greater than the maximum length of an available first calibration device. Like the first calibration device, the second calibration device has thickened mounting members on each end thereof and at least one substantially rigid, longitudinally extending, helically shaped calibration member, at least a portion of the calibration member being inserted into the groove during manufacturing. A rigid cylindrical coupler having at least one wall perforation therein may be fastened at each of its ends over a thickened mounting member provided at an end of a calibration device to join the two calibration devices into a single calibration apparatus. 
     The depth of insertion of the second calibration member into the groove increases with increasing distance from the die for a predetermined distance from the point of the second calibration member which is nearest to the die. 
     Letting the number of grooves in the spacer be n, each calibration device preferably has n flights of calibration members. 
     If the calibration apparatus extends into the cooling vat, a vacuum apparatus may be used to create a low pressure atmosphere condition in the cooling vat to withstand through a difference in air pressure the outward flow of liquid coolant from the cooling vat along the grooved spacer. A vacuum, blast pipe, or other means preventing the liquid from reaching the extruder crosshead should be used. 
     In order to align one calibration member with the die or to align two calibration members located at longitudinally spaced apart positions along the processing line, a rod having one or more helically shaped external grooves therein having the same pitch as the groove of the spacer may be inserted over each of the calibration members and to the entrance of the die. 
     The apparatus and methods described herein may advantageously be used in providing telecommunications cable spacers having grooves of a desired depth made of plastic material having a relatively high melt flow index. For instance, the plastic material may have a melt flow index of equal to or greater than about 0.65 g/10 min. Such a telecommunications cable comprises a longitudinally extending spacer having at least an outer layer thereof formed of the plastic material, which plastic material may be formed over an axial strength member. Formed in at least the outer layer of the spacer are a plurality of longitudinally extending grooves each following helical paths, each groove for supporting a longitudinally extending telecommunications member therein. Each of said grooves may have a cross-sectional shape transverse to its longitudinal extent generally in the form of a “U” with a flat bottom, two corners, and two sides, each side substantially perpendicular to said bottom and joined to said bottom at a corner, and a plurality of longitudinally extending ribs, each rib between two adjacent grooves and each rib delimiting two of said groove sides. Each rib may have a longitudinally extending base between adjacent groove bottom corners whose thickness does not exceed about 1.3 mm. An outer jacket surrounds the spacer, and a water blocking material may be disposed between said spacer and said outer jacket. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of the invention are described in the several drawings, in which: 
     FIG. 1 is a perspective view of a prior art grooved spacer; 
     FIG. 2 is a perspective view of a calibration device according to the invention; 
     FIG. 3 is a sectional view of the calibration device and grooved spacer taken along line  3 — 3  of FIG. 1; 
     FIG. 4 is a sectional view of the calibration device and grooved spacer taken along line  4 — 4  of FIG. 1; 
     FIG. 5 is a side elevation view of the process for registration of the calibration device and the extruder die; 
     FIG. 6 is a schematic view of the calibration device as used in a manufacturing line; 
     FIG. 7 is a schematic view of an alternate embodiment of the calibration device in use in a manufacturing line; 
     FIG. 8 is a longitudinal sectional view of the mounting member and upstream end of a second calibration device; and, 
     FIG. 9 is a cross-sectional view of a cable which may be made using the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which one or more preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the disclosure will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. The drawings are not necessarily drawn to scale but are configured to clearly illustrate the invention. 
     A typical prior art grooved spacer  43 , sometimes called a slotted core rod, is shown in FIG.  1 . Grooved spacer  43  is a component of a slotted core type telecommunications cable. One or more telecommunications elements, such as light waveguides or light waveguide ribbons, commonly known as optical fibers or optical fiber ribbons, will be inserted in the grooves of spacer  43 , and spacer  43  is combined with other cable elements, such as waterblocking materials, strength members, and an outer jacket to form the cable. Spacer  43  typically consists of a central strength member  10  and a layer of plastic material  11  extruded over strength member  10 . The longitudinal axis of the spacer is the same as the longitudinal axis of strength member  10 . Strength member  10  may be formed of any of several materials, such as steel or glass reinforced plastic. 
     Plastic material  11  may be a single layer as shown, or alternatively may be a laminate of two or more layers of plastic material. Grooves  12  in plastic material  11  may have any of a variety of shapes bounded by spacer ribs  44 , the most common being a U-shape to best accommodate one or more light waveguide ribbons to be inserted therein. The grooves commonly have either a reverse alternating lay or a simple helical lay as shown by grooves  12  of FIG.  1 . The calibration device according to this invention is adapted to produce grooves having a simple helical lay. Grooves  12  each have a thread  41 , and a binder is typically wrapped on surface  42  forming the outer diameter of spacer  43 . 
     With reference to FIG. 6, a grooved spacer is commonly manufactured by drawing the strength member  10  from a payoff reel  30  and passing strength member  10  through the crosshead of extruder  18 . Extruder  18  extrudes a layer of plastic material  1   1  over strength member  10 . The grooves  12  are formed in plastic material  11  through radially inwardly extending projections in extruder die  19 . The formed grooved spacer is then passed through a cooling vat  33  holding a cooling material such as water  34 , and the grooved spacer is then taken up on takeup reel  31 . The direction from payoff reel  30  to cooling vat  33  will be called herein the throughput or downstream direction. A pulling device  40  may be used to draw the spacer from payoff reel  30 . Pulling device  40  may be located between the cooling vat  33  and takeup reel  31 . Other features in FIG. 6 will be discussed herein below as part of the invention. 
     Strength member  10  is normally rotated about its longitudinal axis by a gripping device either before or after it reaches extruder  18  to produce the helical shape of grooves  12 , in which case payoff reel  30  and takeup reel  31  will be rotated to accommodate such motion. However, the helical shape may be produced by other means, such as rotation of components of the crosshead of extruder  18 ; the calibration apparatus according to the invention may be used regardless of the means of producing the helical shape of grooves  12 . If the crosshead is rotated, then the calibration apparatus must also be rotated. 
     The calibration apparatus as shown in FIG. 2 may include a first calibration device  51  including calibration members  14 , a second calibration device  52  including second calibration members  16 , and a coupling  15  joining them. The calibration apparatus may be made as a single unit without a coupler as shown in FIG. 7, but the use of shorter calibration devices and a coupling may be necessary to produce a calibration apparatus of the desired length. The exterior shape of the calibration devices is produced by a lathe; a four axis milling machine may be used to produce the helical spaces between the calibration members; and a wire EDM device may be used to produce the central bore of a calibration device. 
     First calibration device  51  comprises calibration members  14  joining mounting members  13  and  53 , while second calibration device  52  comprises calibration member  16  joining mounting members  59  and  17 . Curved indentations  54  allow convenient removal of waste plastic material. Mounting member  13  is mounted to die  19  by screws inserted through holes  55 , 56  in mounting member  13 . Coupling  15  is joined to calibration devices  51  and  52  through screws in holes  60  and  61 . Coupling member  15  may alternatively be a C-shaped member clamped to the mounting members. 
     The length of the calibration apparatus depends upon the speed of the processing line, the type of plastic material forming at least the exterior layer of the spacer, and the temperature to which the plastic material is heated during extrusion. Examples will be found below. 
     Before insertion of the mounting screws, the calibration device is aligned as shown in FIG. 5. A calibration rod  20 , which has the same shape as the spacer to be manufactured, is inserted through the first and second calibration devices and to the exit passages of die  19 . Rod  20  and its handle  21  are rotated as rod  20  is inserted. The angular positions of the calibration devices are adjusted as necessary to permit passage of rod  20 , and the mounting screws are tightened to couple the second calibration device to coupler  15  and the first calibration unit to coupling  15  and die  19  when all components are in alignment. If helical passages in die  19  are provided, then rod  20  may be inserted a short distance within die  19 . Rod  20  is then removed, undergoing rotation during removal. 
     In order to minimize occurrences in which the extruded plastic material is dragged at the exit of die  19 , more clearance is provided for the hot plastic melt forming material  11  at its exit from die  19 , while less clearance is necessary thereafter. Reference is made to FIGS. 2,  3 , and  4 , with a section of the grooved spacer included in FIGS. 3 and 4 for clarification. Distal portions  57  of calibration members  14  are initially inserted into the respective grooves  12  to a depth D 1 . As shown by a comparison of FIGS. 3 and 4, the depth of insertion in grooves  12  may gradually become larger over the interval between line  3 — 3  to line  4 — 4 , as D 2  is greater than distance D 1 ; such is due to a gradual increase in the radial length of calibration members  14 . The width of the respective calibration members  14  also may increase over the interval between line  3 — 3  to line  4 — 4 . Thus, the cross-sectional area of the respective distal portions  57  decreases over the interval from line  4 — 4  to line  3 — 3 ; that is, in the direction opposite the throughput direction of the spacer. The cross-sectional area of the distal portions  57  may remain constant over the interval from line  4 — 4  to coupler  15 . 
     The portion of a calibration member which is inserted into a groove  12  has a cross-sectional area less than the cross-sectional area of groove  12 . An intended clearance is left between the calibration members and the floor  45  or  47  of a groove  12 . However, the calibration members may contact sides  46  of grooves  12 . The length of calibration members  14  is 10 cm or longer along the longitudinal axis of the spacer, and may be inserted into a spacer groove to a depth of 3.0 mm or more. 
     Mounting member  50  of second calibration device  52  may include calibration members  16  which may be inclined at an angle of about sixty degrees from the horizontal axis over a distance D 4  as shown in FIG. 8 to decrease the risk of the spacer becoming lodged as it traverses between the two calibration devices. 
     A calibration apparatus is shown in FIG. 6 as it is used in a manufacturing line. An alternate embodiment with only a single calibration device supported by post  66  is shown in FIG. 7. A device  32 , similar to that described in the Schneider reference, blows air as shown by arrows toward the entrance to cooling vat  33  to ensure that water  34  does not flow out along the calibration device or the grooved spacer to reach die  19 . A vacuum pump device  35  may also be used to create a partial vacuum within cooling vat  33  to create an atmospheric pressure differential between the interior and the exterior of cooling vat  33 , in order to inhibit the escape of water  34  along the spacer in the direction of die  19 . Coupling  15  fits through a cylindrical slip ring  63  having a raised shoulder  64 . Shoulder  64  abuts wall adapter  90  which is mounted to vat  33 . 
     A cross-section of a cable  70  made using the calibration device is shown in FIG.  9 . Spacer ribs  75  have a base width  80  lying between adjacent groove corners  81 ,  82 . The root diameter of the spacer is the length of a line passing through the center of strength member  74  and reaching a groove floor  77 . The outer diameter of the spacer reaches the outer extent of ribs  75 , shown as surfaces  42  in FIG.  1 . Grooves  76  are delimited by walls  78  formed by ribs  75 , and each groove  76  holds a stack of optical fiber ribbons  71 . The spacer is surrounded by a binder  72 , a water-blocking tape  79 , and an outer sheath  73 . 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Parameters of examples of manufacturing lines making grooved spacers 
               
             
          
           
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 1 
                 2 
                 3 
                 4 
               
               
                   
                   
               
             
          
           
               
                 spacer 
                 plastic 
                 MDPE 
                 MDPE 
                 MDPE 
                 MDPE 
               
               
                   
                 material 
                 blend 
                 blend 
                 blend 
                 blend 
               
               
                   
                 melt flow 
                 0.65 
                 0.65 
                 0.65 
                 0.65 
               
               
                   
                 index, g/10 
               
               
                   
                 min. 
               
               
                   
                 root 
                 16.1 
                 6.9 
                 7.2 
                 9.6 
               
               
                   
                 diameter, 
               
               
                   
                 mm 
               
               
                   
                 outer 
                 25.4 
                 14.1 
                 16.8 
                 19.1 
               
               
                   
                 diameter, 
               
               
                   
                 mm 
               
               
                   
                 groove 
                 2.7 
                 2.5 
                 3.8 
                 2.7 
               
               
                   
                 width, mm 
               
               
                   
                 groove 
                 4.5 
                 3.5 
                 4.3 
                 4.8 
               
               
                   
                 depth, mm 
               
               
                   
                 # of grooves 
                 14 
                 6 
                 5 
                 8 
               
               
                   
                 rib base 
                 1.0 
                 1.1 
                 1.1 
                 1.2 
               
               
                   
                 width, mm 
               
               
                 process- 
                 line speed, 
                 6 
                 10 
                 10 
                 8 to 10 
               
               
                 ing 
                 m/min 
               
               
                   
                 temperature 
                 180-205 
                 180-205 
                 180-205 
                 180-205 
               
               
                   
                 of extruded 
               
               
                   
                 melt, ° C. 
               
               
                 calibra- 
                 # of  
                 two 
                 two 
                 one 
                 two 
               
               
                 tion 
                 calibration 
               
               
                 appara- 
                 devices 
               
               
                 tus 
               
               
                 calibra- 
                 distance of 
                 7 
                 7 
                 5 
                 7 
               
               
                 tion 
                 initial 
               
               
                 members 
                 taper, inches 
               
               
                   
                 initial width, 
                 1.5 
                 2.5 
                 2.0 
                 1.5 
               
               
                   
                 mm 
               
               
                   
                 initial root 
                 19.0 
                 9.5 
                 10.0 
                 12.5 
               
               
                   
                 diameter, 
               
               
                   
                 mm 
               
               
                   
                 final width, 
                 2.5 
                 2.5 
                 3.8 
                 2.5 
               
               
                   
                 mm 
               
               
                   
                 final root 
                 18.0 
                 8.5 
                 10.0 
                 12.5 
               
               
                   
                 diameter, 
               
               
                   
                 mm 
               
               
                   
               
             
          
         
       
     
     In the examples shown in Table 1 above, the distance of initial taper, initial width, and initial root diameter apply to the upstream portion of a first calibration device, such as shown between lines  3 — 3  and  4 — 4  of FIG.  2 . The remainder of the first calibration device and a second calibration device, if any, have a width and root diameter corresponding to the final width and final root diameter. As applied to a calibration device, the term “root diameter” means the diameter of the bore of the calibration device between the radially inwardmost surfaces of the calibration members. 
     In example 1 above, the calibration members were untapered for a length of 1 inch and then tapered over a distance of six inches. Tapers were smooth for examples 2, 3, and 4. 
     It is to be understood that the invention is not limited to the exact details of the construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art without departing from the scope of the invention.