Cable ties: Complexity from simplicity

The plastic cable tie. It is the poster child of Catch-22s in molding. But it’s hard to know just by looking at one.

Ken Tomasetti knows. He’s the president of Advanced Cable Ties (ACT), based in Gardner, MA. His company has become expert in molding cable ties in a variety of lengths, widths, and strengths. Jim Patenaude knows as well. He’s the president and owner of Wilderness Mold in West Hatfield, MA, a member of the Brookfield Group of companies and the moldmaker who supplies ACT with its molds, molds that are highly acclaimed for their quality, precision, and durability.

Molding cable ties is a competitive, high-volume business that, to be successful, demands fast cycles, robust processes, and nearly perfect parts. So what’s the catch?

“You look at a cable tie and it looks like a relatively simple product,” explains Patenaude. “But it’s an exercise in contradiction in reality, because the cable tie product stretches the limits of injection molding.” The bottom line is simple—the cable tie has a lousy L/D ratio.

“The molder has to get the material from the head to the tail in four-tenths of a second or it starts to freeze off and build pressure,” explains Patenaude. “Then you blow the mold open and it goes downhill from there.”

Contradictions
Running counter to all this high speed and pressure is the fact that molded-in stress in a cable tie is unacceptable. Each tie must meet certain tensile strength and Mil Spec load requirements ranging from 18 to 175 lb, which would be difficult, if not impossible, to achieve if there were molded-in stress.

Also, much of the load in a cable tie is held in the head where the end of the tie is inserted and secured. The head has molded into it a small pawl that catches the teeth of the tie and keeps it from slipping backward. If the pawl is too stiff, it exceeds the industry-recommended .2 lb of insertion force required to insert the tie. Too loose, or soft, and the pawl won’t hold the strap of the tie in place. Similarly, flash is a no-no as well. Too much of it, and excessive force again is required to insert the tie into the head.

“The hinge of the pawl is the critical factor here,” says Patenaude. “It has to be robust enough to withstand the pull on the strap against the engaged pawl. But that robustness works against you on insertions. You achieve one, but violate the other. The molder really has a small window. It’s a very fine line.”

Tooling is Key
ACT has found that fine line, thanks partly to Wilderness Mold. The company has built more than a dozen molds for ACT, ranging from six to 256 cavities, cut from prehardened 4120 and 420 stainless steel. They produce cable ties that range from 4 to 48 inches long, .090 to .40 inch thick, and .090 to .390 inch wide.

Patenaude says precision is the key to his success with cable tie tools. His shop, in business since 1973, employs 56 people and specializes in Class 101 molds for medical, electrical connector, and telecommunication applications.

With heavy investment in new technology, Wilderness runs Micron high-speed CNC cutting centers, Tree machining centers for large mold bases, a 60,000-rpm Robodrill electrode cutter, and Agie (Agitron) wire and sinker EDM centers. Production is virtually 24/7 thanks to a special camera system that allows operators to run EDM machines from their home computers. Handwork is minimal and turnaround is fast. Wilderness Mold has achieved such precision that cores and cavities of like products are interchangeable from mold to mold, minimizing the cost of spare components.

Understanding how cable ties are molded, and how they behave in the mold, has made Wilderness an expert on the subject. Precision aside, Patenaude says one factor critical to cable tie molding is cooling. He uses multiple zone cooling channels to precisely control temperature along different cable tie sections. The result is a much faster cycle.

Wilderness Mold’s tools also appear to be passing the durability test. Tomasetti says he recently had Wilderness refurbish the parting line on his oldest tool (more than 100 cavities) for the first time. The mold produced 1 billion parts over four years, without interruption.

Process and Part
The other half of the equation is the molding process. Tomasetti started ACT four years ago, entering this competitive commodity market after several years with other cable tie molders. Today ACT produces ties in 21 different sizes on 11 Sandretto presses. Attention to detail and process robustness are essential.

The cable ties are molded from nylon 6/6, which is heated to maximum temperature (600F to 640F) to reduce viscosity. Injection pressures range from 11,000 to 16,000 psi to fill each cavity in less than .6 second. The conundrum here is that, depending on cavitation, the clamp must be as large as 600 tons to counter the pressure. But shot size, in general, is too small for a standard 600-ton machine. To do the job, ACT uses customized Sandrettos with larger clamps, smaller barrels and screws, and specialized injection units.

Hot runner systems are used on some molds, depending upon cavitation and cable tie size. Similarly, some molds are equipped with cavity pressure transducers. Otherwise, molds transfer on injection pressure.

So how well is ACT molding its ties? Very well, says Tomasetti:

“My biggest challenge,” says Tomasetti, “is flowing nylon a great distance. It takes acute experience to process 48-inch strips of .090-inch nylon 6/6. There aren’t many shops that can do it.” Tomasetti has such faith in Wilderness that ACT doesn’t have a tool shop. All maintenance is performed by Patenaude and Wilderness. For four years ACT ran with one set of production tools for each of its products, confident that they would run without incident. “It takes a good molder to mold cable ties,” says Patenaude, “and there aren’t many that do it well.”