Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Optical fiber – waveguide – or preform
Reexamination Certificate
1998-04-28
2001-01-09
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Optical fiber, waveguide, or preform
C264S001290, C264S177160, C264S177170, C264S17800F, C264S237000, C264S348000
Reexamination Certificate
active
06171526
ABSTRACT:
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 exte
Clarke MaryAnn
McAlpine Warren W.
Aberle Timothy J.
Heitbrink Jill L.
Siecor Corporation
Staicovici Stefan
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