In the world of custom cable assemblies, flexible wire and overmold solutions are used in a wide range of different contexts. Not only are they commonly employed in association with various types of industrial equipment, but they also play an important role in fields like automation, robotics, and even the automotive industry.
Custom cable assemblies are nothing if not versatile - meaning that they can be designed in a plethora of options depending on your needs. Therefore, to guarantee that you're getting a solution that will meet those needs while also exceeding your expectations, there are a few key elements of the process that you need to know more about to make certain the cable jacket and overmold are flexible enough for your application.
What Is Durometer As It Relates to Cables and Wire?
Durometer is a common measure of hardness to characterize non-metallic materials. A Shore A Durometer is a device used to measure the Shore A hardness of materials used in cable assemblies, rubber keypads, elastomers, and other materials. The device itself is a compact, pocket-sized instrument that is the industry standard to check hardness.
The probe on the Shore Durometer is pushed into the material and the resultant deflection is measured. The noted displacement corresponds to a Shore A hardness value which ranges from about 50 Shore A to 95 Shore A, with 95 Shore A being a relatively hard material. Hardness probes are commonly used to make in-process measurements to make certain the material is meeting specifications. Specific to custom cable assemblies and wire harnesses, the durometer of a cable jacket and overmold is directly related to its inherent flexibility.
Minimum Bend Radius
The minimum bend radius is another critical characteristic to note as it refers to the smallest allowed radius that a cable can bend around. During the installation and routing process, cables need to be bent or flexed in various ways to deal with certain environmental conditions. Cables can be bent around curves to provide a better fit or may need to be twisted around a conduit or flange to make installation possible in the first place.
It is important to know the minimum bend radius so that cables aren't subject to an extreme amount of stress, which could potentially cause damage to the wire jacket, center conductors, or even the connector itself. Many standards, including those from NEC, ICEA, have very strict requirements when it comes to the minimum bend radius. Some raw wire technical data sheets will note the wire’s minimum bend radius, while for others it must be estimated using sizing factors and industry rules of thumb.
A tight minimum bend radius is crucial for routing wires in small spaces.
Hi-Flex Cable Applications and Potential Opportunities for Failure
High-flex cables are commonly defined as those that come in a special kind of jacket that is not only heavy-duty but one that is supple enough to support a continuous amount of flex, and usually feature high strand count multi-conductor constructions.
One popular example of a high-flex application comes by way of process automation machines - assets that are becoming increasingly common in manufacturing facilities all the time. Here, custom cable assemblies allow equipment to work much faster than previous generations were capable of - thus allowing organizations to increase productivity while also cutting costs. As throughput increases, cycle counts increase, and the number of bends and flexes that a cable must endure continuing to rise. Cable assemblies with jackets that can survive thousands, or millions of cycles are necessary to meet program objectives.
Other examples of how these high-flex cables are used include but are not limited to pick and place equipment that is often used in robotic SMT lines, with other types of high-flex applications including measurement equipment, conveyor equipment, medical devices, and more.
The key thing to understand is that these tend to be high-stress environments, meaning that if not designed properly, there remains a high degree of risk and the potential for a system failure. Accordingly, engineers need to be mindful of the potential causes of failure. These causes include the degradation of the cable itself, a break in the center conductor, and also cracks or damage to the wire jacket.
What Happens to a Cable's Flexibility When it is Subjected to Extreme Cold?
A cable's flexibility when it is subject to extremely cold temperatures will ultimately be dictated by the cable's jacket and insulation material. Solutions made of thermoplastics may begin to stiffen when the temperature drops and become noticeably more rigid down below -10°C. If a cable is made with thermoset insulation, they are typically temperature rated all the way down to -40°C. In both situations, cables will likely lose flexibility and can even become brittle if that rating is exceeded.
Multiconductor cables routed through conduit.
Flexible Wire Jacket Material Types
There are several different types of flexible wire jacket materials to choose from depending on the project requirements. These jacket types include:
- Silicone jacketed wire offers excellent flexibility across a host of temperatures.
- PTFE jacketed wire offers good flexibility across a large temperature range and low outgassing.
- SR-PVC (semi-rigid PVC) jacketed wire is softer and more flexible than traditional PVC and is known for its low cost and widespread availability.
- TXL/GXL jacketed wire offers excellent abrasion and temperature resistance. It's also flexible while still being tough enough and resilient enough for situations where rough handling is a necessity.
- TPE (thermoplastic elastomer) jacketed wire, which is both flexible and resilient in cold temperatures. It, too, is known for its low cost and is readily available. It's also compatible with standard overmold materials that are used in custom overmolding.
Flexible Overmold Material Types
One design feature that it is essential to be aware of is the flexibility of overmolded strain relief material and shape. One notable overmold hardness value is approximately 55 Shore A. This is the softest value that is typically used in these applications. Even though the material in this situation is especially supple and can deform under certain conditions, it can also shear, tear, and even fail under high loads.
On the other end of the spectrum is overmold material rated at approximately 95 Shore A. This is among the highest hardness values available and is common for PVC-based overmold materials. These materials can be too hard, however, and can create a concentration of stress on the wire jacket and center conductor - ultimately leading to failure in the form of an open circuit.
The most flexible overmold material chosen is PVC and TPE based. These options come with a low cost and are readily available - perfect for projects that need to begin quickly allowing samples to be produced fast. Plasticizers can be added to change the stiffness and therefore the durometer of the overmolded assembly to meet more specific requirements. PVC and TPE-based overmold materials also can chemically bond to cable jackets, thus creating a strong and watertight connection.
Methods to Test Wire Jacket and Overmold Flexibility
One of the most important mechanical testing requirements to consider for custom cable assemblies is the “pull test”. This test is used to determine the maximum pull strength of a design, or to determine if a design repeatedly survives multiple loads applied. This test can stress the overmold, the wire jacket, and also the connector and crimp terminals.
One method of completing this test involves the "pull and break" technique. Similar to an ultimate tensile test, a constant pull force is applied until the wire or terminal finally breaks. The "pull, hold and break" method is also regularly used. This is non-destructive, but a pulling force is still applied and held at a specific rate for a certain length of time. The "Flex/Bend Test" is also used in a number of contexts. This mimics the way that cables are routed, bent, and flexed during installation as opposed to throughout the assembly's useful life.
During these tests, a specially designed "Bend/Flex" machine will be used and the cable under test will be attached at one end. The other end can be tied into a knot or fixtured in a way to have a certain amount of weight attached to the end. The cable is then bent over at a 90-degree angle at both ends. The machine constantly flexes the cable with both ends swinging at 180-degrees until the test is completed or there is a test anomaly noted.
Some bending and flex tests may be performed at ambient conditions and then repeated at extreme hot or cold. Increased failures are expected at cold temperatures due to jacket materials becoming more brittle at cold. Excessive heat can also break down and melt wire jackets, creating additional concerns for performance at elevated temperatures. For some projects, it is physically impossible to fit the entire test setup within a thermal chamber. Mechanical testing at ambient is preferred due to its simplicity but testing at hot or cold can help verify the design will perform as intended for the life of the project.
All of this goes a long way toward underlining the importance of having the right product, the right design, and the right manufacturing partner by your side in the first place. A full-service manufacturing partner like Epec can help pick out the right custom cable and the right materials for a given solution. Having a vertically integrated supply chain with industry-leading engineering expertise enables Epec to be a high-value manufacturing partner supporting projects from the initial design phase to volume production. Reach out today to learn more about our flexible custom cable assemblies.