Walking on a high wire is one of the most thrilling and spectacular acts in a circus. It takes amazing balance, innate skill, years of practice and more than an ounce of bravery. Writing on wire or cable can be equally challenging.

Unlike most parts-marking applications, which occur on flat, stable surfaces, cable and wire pose numerous problems because they are curvaceous and flexible. Cable and wire often appear similar at first glance, but they differ in diameter and thickness. They also have different insulation materials with different consistencies. Wire can be as small as 0.16 inch in diameter, while cable is thicker and can have a diameter in excess of 3 inches. Typically, the larger the cable or wire, the easier it is to mark.

The thinner shape and configuration of wire often make it more difficult to work with than cable. Cable usually has a jacket of insulation made of polyvinyl chloride (PVC), Teflon or some other protective material; wire often is covered, too, but with a thinner jacket, which makes it more susceptible to oil residue and other surface contaminants that may impede coding or marking efforts. In most cases, PVC is easier to mark than Teflon.

The jacket material on wire and cable has continued to evolve over time. Just a few years ago, PVC was virtually the only material used, but the use of polyethylene has grown dramatically. The changing jacket materials are a continuous challenge for marking. For instance, PVC and other materials vary greatly in their composition.

Today, manufacturers are under increasing pressure to validate the products on their assembly lines. In some industries, such as automotive, aerospace and appliance, many product failures and warranty problems can be traced to increasingly sophisticated electrical and electronic systems that depend on miles of wire harness and cable assemblies.

"Marking speeds up maintenance, verifies installation and enables troubleshooting," says Mike Kasun, president of K-Sun Corp. (Somerset, WI). "Also, for safety and identification compliance, marking is very valuable."

More and more manufacturing engineers are turning to part-marking equipment and identification codes to error-proof their production processes and create an accurate history of every component from the beginning of its life to the end. Standards such as ISO9000 and TS16949 are driving the quality crusade in many companies. Marking cable and wire is a good way to insure against future quality issues that might crop up.

"There is a greater demand for traceability and conformance to requirements, both internal and external, along with confirmation of conformance," says Scott Prochaska, product manager for supplies at Videojet Technologies Inc. (Wood Dale, IL). "In addition, there is an increased use of codes related to the assembly process, for post-production service and for consumer replacement. Growth in all of these areas has resulted in the increasing need for wire marking."

Printing information on cables and wire, such as time, date and serial numbers, aids assembly, simplifies troubleshooting and provides manufacturing traceability. "With sequential bar coding and serialization, it is possible to trace every wire or cable," notes Rob Boyd, crimping manager at Schleuniger Inc. (Manchester, NH). "If necessary, a wire or cable assembly can be tracked back to the plant where it was made and the date on which it was made."

Manufacturing engineers can choose several ways to mark cable and wire. They can mark directly on the material using hot-stamp technology or use noncontact processes such as ink-jet printing or laser-coding technology. Another alternative is to use adhesive labels or label sleeves that slide over cable and wire. Each technique has advantages and disadvantages.

"Figuring out what will provide the clearest, most durable mark given the insulation material and the usage environment is a challenge," says Boyd. "If multiple methods can be used, then usually the method with the fastest processing time is chosen. However, in many cases, only one method is possible."

Contact vs. Noncontact

Printing directly onto cable or wire provides the greatest control over the mark. Indeed, it becomes a permanent part of the protective jacket. Direct marking typically withstands damage and environmental exposure better than other alternatives, such as labels.

Hot stamping is a popular form of direct part marking used for cable and wire applications. It is a cost-effective way to identify cable assemblies and wire harnesses. Heated character wheels, when pressed against marking foil, imprint engraved characters on the selected material. This proven process is suitable for marking PVC, polyethylene and other insulations.

However, some experts warn that hot stamping can damage insulation. "Hot-stamp marking is the least expensive, but also the least flexible," Boyd points out. "Only one character set will be marked on a batch of wires and changing characters requires mechanical changes.

"Hot-stamping requires a fairly consistent surface," adds Boyd. "Otherwise, the marking disks might not make good contact." However, he says a wider variety of hot-stamp foils are now available for different insulations than in the past.

In the past few years, there has been a strong increase in the use of noncontact marking. It provides flexibility in dealing with various odd-shaped components through a single printer system.

According to Videojet's Prochaska, "continuous ink-jet has been the noncontact system of choice, but laser-coding is also gaining ground, specifically on PVC jackets. Laser-coding provides an engraved mark, which is good for the entire life of the product, but needs to have a contrast to be effective. Continuous ink-jet does not have the same level of permanence, but using ultraviolet cure inks is generally effective for the intended life of the mark, and can provide the required contrast."

Because there is no contact between the marking device and the product, like in hot-stamping or labeling, ink-jet printing yields a much higher throughput. "Ink-jet printers provide a means of marking at speeds up to 1,500 feet per minute," claims Jim Mueller, sales manager at Leibinger USA (Norwalk, CT).

"Ink-jet printing is good for high-volume runs of the same printed information that is printed directly on the cable," adds Todd Fries, marketing manager at HellermannTyton (Milwaukee). "The downside is that the some inks will not show up on certain colors of wire insulation."

Indeed, ink-jet marking poses particular challenges in finding an ink color that will work with a wide variety of wire insulation colors. Ink-jet markers come in two different versions: systems for pigmented inks and systems for nonpigmented inks. The systems for nonpigmented inks are typically less expensive. Black is nonpigmented, and most all other colors are pigmented.

"However, you cannot mark a black wire with black ink," warns Schleuniger's Boyd. Therefore, if a manufacturer is using a wide variety of insulation colors, finding an ink that works well with all of them can be a big challenge. "The downside to ink-jet is that typically the systems require much more maintenance," says Boyd, "and companies have to deal with the issues of working with chemicals. However, many inks have been formulated for better adhesion."

Don't Overlook Labels

Direct part marking is ideal for applications that have high volume and low mix. However, because of offshore manufacturing and other market forces, most cable and wire shops in the United States have shifted to high-mix, low-volume production in recent years.

"In these cases, direct part marking wouldn't make sense," says Michelle Laramee, identification and labeling product manager at Tyco Electronics Identification (East Providence, RI). "These companies usually have custom applications [that] require labeling during the build and integration process."

Labeling can be a time-consuming way to mark cable and wire, but it is much easier today thanks to new equipment that has simplified the process. Labels offer a good solution due to their high quality print, the amount of information they can hold and the contrast they provide for easy legibility.

Some observers argue that they do not provide as much permanence as hot-stamping or ink-jet printing. However, HellermannTyton's Fries claims that "a good label will stay on the cable for the life of the assembly, unless the application gets into very extreme environments, such as high heat, continuous water exposure or continuous UV exposure."

Labeling is a flexible process; because labels can be applied to a wide variety of insulation materials, they offer the best solution to inconsistent surfaces. Wires and cables can be labeled with numeric, alphanumeric, hexadecimal and octal serialization, as well as bar code labels and logos.

Manual labeling typically takes much longer than hot-stamping and ink-jet printing. That translates into lower production rates. However, the emergence of in-line labeling systems makes labels more appealing for some applications. "Previously, applying labels was a separate process and was not very flexible," Schleuinger's Boyd points out. "Now, the process can be tied in line with the cut-strip process, with labels containing serialized text as well as bar codes and logos. Furthermore, scrap is significantly reduced compared to older systems."

"The primary advantage of labeling is having a clear form of ID on your wire because the print contrasts well against the label," says Tim Ziegler, product manager for automated assembly solutions at Brady Corp. (Milwaukee). "Colors or patterns on the wire jacket do not affect the readability of the ID. Bar codes, which require a certain level of contrast, can be reliably read on labels.

"Teflon-coatings or dirty wire can still be identified effectively with labeling, unlike most direct marking technologies," adds Ziegler. "The resolution and quality of print is far superior in label printing since the label surface is consistent unlike the wide variety of wire jackets, wire coatings and wire cleanliness."

Information can vary from label to label. The contrast of printed text against the white background of the label provides a legible mark.

Besides clearer identification, Ziegler says logos and other graphics become a possibility with higher resolution printing. "The flexibility of the ID is also superior in labeling," he points out. "Fonts and font sizes are limitless, and information can be changed easily on the fly. We even have labels with repositionable adhesives, allowing removal and reapplication of cable and wire identification."