Although I have worked with cable for many years, I had never had the opportunity to see it being made until a few months ago when I visited CommScope's cable manufacturing plant in Claremont, NC. CommScope designs and manufactures cable and makes more than 3000mi of enterprise, broadband and wireless cables per day. At the Claremont plant, the company manufactures many different products, including unshielded twisted pair (UTP), coax and fiber-optic cable. This plant is one of the company's 12 manufacturing facilities located around the world.
Manufacturing single-conductor and braid wire
The first thing that struck me when I entered the plant was its size. The manufacturing plant is held within two connected buildings that total 587,500sq ft.
The basic layout of the various manufacturing lines is similar. CommScope draws its own single-conductor and braid wire. Spools of single-conductor wire or fiber are loaded onto machines at the beginning of each line. The number of conductors or fibers needed for the finished product determines the number of spools. Filler material and strength members are also loaded, if required. The cable contents are threaded through an extruder, which creates a jacket, which then encloses the wires.
The jacket starts out as plastic pellets that are loaded into a hopper above the extruder. The appropriate color is added as the pellets are melted. The molten plastic flows into the extruder head and onto the individual wires as the wires move through the head in a continuous motion. The number of variables that have to be regulated to make this work is mind-boggling. Wire feed rate, tension, plastic flow rate and plastic temperature are just some of the items that must be closely controlled.
CommScope employs computer statistical process control to identify short and long-term manufacturing problems. An example of a short-term problem is a pressure variation in the molten plastic as it flows into an extruder. An example of a long-term problem is a bearing going bad, causing periodic fluctuations in cable thickness.
In addition, operators test each roll of cable to ensure it meets specifications before the cable moves on to subsequent manufacturing steps. Operators also test and inspect the final product.
Once the jacket is extruded onto the wires, the completed cable passes through a number of water baths. The temperature of these water baths is tightly controlled so that the finished product's jacket bonds properly to the cable.
The cable is then wound onto spools or put into conveniently sized boxes. Custom-designed computers control the whole process, with proprietary software written and maintained by the company.
Manufacturing UTP cable
I had a couple questions about how UTP cable is made: How are the wires twisted together? Are the pairs in UTP manufactured separately and then woven together, or is the entire cable manufactured in one step? But before I got to these questions, I learned something else.
The raw material for the line is bare 14-gauge copper wire. As the line moves, this wire is swaged down to the appropriate gauge. This reduces the time between loading reels of raw materials.
Once the wire is swaged to proper size, it is insulated and fed into a twinning machine. Two conductors are fed through collars with holes in them. As the wire is fed through the machine, the collars twist the two wires into a single twisted pair. The wires feed into another set of collars, which twist with a similar motion, but at a faster rate.
This process continues until the fourth stage, when the collars are twisting so rapidly that you almost cannot see the individual strands. The output of this line is a reel of one twisted pair. The four individual pairs are then loaded into a similar machine, and they are bunched together to produce the final UTP wire. From here, the reels of UTP are loaded onto a final line that adds a jacket.
Manufacturing fiber-optic cable
To make fiber-optic cable, multicolored reels of single-coated glass fiber are loaded at the beginning of the line. What happens next depends on whether the cable is loose buffer or tight buffer.
Loose buffer cable, which is typically used in outdoor applications, carries the fiber inside a loose-fitting buffer tube filled with a water-blocking gel that virtually eliminates water migration into the tube. Tight buffer cable, as the name implies, has a buffer that tightly encircles the fibers. Tight buffer cable, intended for indoor applications, does not contain any gel.
A loose buffer manufacturing line injects a water-blocking gel and then extrudes the buffer tube around the fiber. Multiple fibers in their buffer tubes are combined according to the cable type being manufactured. The fibers are wrapped in Kevlar-strength members and then fed through another extruder. They can also be fed into a machine that covers the cable in an armored jacket.
Tight buffer cable follows a manufacturing process similar to standard wire described earlier in this article.
Manufacturing coaxial cable
Watching UTP being made was quite something. But it was also a real learning experience seeing coax cable being made.
The manufacturing process starts with the cable's center conductor, which is fed off of reels and around a large capstan. Two capstans, one at the beginning of the line and the other at the end, carefully control tension and feed rate throughout the process. The center conductor is fed through an extruder, where the dielectric is applied. In this case, plastic pellets are heated to the melting point and compressed nitrogen gas is injected just before the plastic exits the extruder. The center conductor moves through the extruder as the dielectric flows onto it.
As the dielectric exits the extruder head, two things occur simultaneously. First, the plastic begins to contract as it cools. Second, the plastic begins to expand as the high-pressure gas mixed with the plastic is exposed to ambient air pressure (creating the foam dielectric).
The two processes are carefully controlled to maintain the cable diameter within tight tolerances. As part of the continuous quality control process, the center conductor is connected to ground. The cable, with its dielectric bonded to it, passes through a sparker box, where bead chains energized with a high-frequency HV signal are used to create a corona. If there is a fault in the dielectric, the corona field is disrupted and sets off a detector.
The cable is wound onto spools and is then fed through another machine called a braider. The center conductor and dielectric are fed through the center of the braider, which weaves the wire braid onto the cable. The braiders have been around for more than 100 years and were originally designed to make rope. The shielded cable is then fed through an extruder head, where the appropriate jacket is added.
ACKNOWLEDGEMENTS: Thanks to Randall Stevens, Betsy Lambert and CommScope for their hospitality and contributions to this article.
Brad Gilmer is a SMPTE Fellow and is president of Gilmer & Associates. He is also executive director of the AAF Association and executive director of the Video Services Forum.
Send questions and comments to: firstname.lastname@example.org