Insulation shrinkback can increase the risk of wire shorts. It can also destroy the protective seal that the insulation and jacket are designed to provide. We know these are important problems to avoid, but what is insulation shrinkback and how can we prevent it?
What is shrinkback?
Shrinkback occurs when a cable’s insulation pulls away from a termination connector or cable splice. When it pulls back, it increases the insulation clearance or the distance between the end of the insulation and the termination device.
Increased insulation clearance leaves a part of the conductor exposed. As a result, there is a higher risk for shorting and a compromised seal that can let dirt and water into the cable. These damaging factors can cost you a lot of time and money in cable replacements.
What Causes Insulation Shrinkback?
Shrinkback is a result of mechanical stress involved with the manufacturing process. If materials are extruded at even slightly different speeds, it can cause the insulation material to stretch. Once the cable is then cut to size, the insulation will automatically return to its true size if friction isn’t enough to hold it in place around the conductor. High temperatures and exposure to a wide temperature range can cause shrinkback to occur more rapidly.
Where Does Shrinkback Happen?
Shrinkback can happen in any cable, but it is important to note it is more likely in cables with PTFE insulation. Since PTFE is a non-stick material, an adhesive is applied to the conductor during the manufacturing process to keep the insulation in place. Afterward, that adhesive can remain brittle and break apart from physical stresses.
It has also been determined that smaller gauges, specifically 24 AWG and smaller, with 7 strands or less are the most susceptible to shrinkback. Due to the small size of the stranding, the insulation does not have a lot of material to grip, making the PSI requirements for the jacket adhesion very low.
How Do We Prevent Shrinkback?
Although it may seem like shrinkback is unavoidable, there are several things you can do to minimize its effects. First, you can choose materials that are less likely to experience shrinkback and use insulation support crimps when splicing or terminating your wire. You can also check out shrinkback testing and standards developed by the American National Standards Institute (ANSI) and the Insulated Cable Engineers Association (ICEA) and then include shrinkback requirements when placing your cable order.
It is very important not to over manipulate the wire during processing to prevent the breakdown of the adhesion of the insulation to the stranding. Here are some practices that Allied uses to reduce the opportunity for shrinkback due to over-handling:
- Reels should always use the largest cores possible to eliminate memory and over-bending the wire.
- Do not over-tighten payout reels. Keep them tight enough to provide proper tension but not enough to cause drag and stretch wires.
- Minimize how often the wires pass around guide pulleys, eyelets, counter wheels, etc.
- Use the largest-sized bend radius whenever possible. The smaller and longer the bend, the bigger reduction in adhesion.
- Eliminate or minimize pressure on wires as they pass through counters, drive wheels, and drive belts.
- Reduce the number of passes through capstans or de-reelers (dancers) with small guide wheels.
Following these simple steps is the best way to fight PTFE shrinkback. They will help prevent stretching the insulation and destroying its adhesion to the stranding. Use this knowledge to defend yourself against shrinkback problems in your future wire and cable jobs. Preventing problems like these are the key to finding a cable that will last for a long time in your applications.
Allied Wire and Cable follows these best practices to mitigate shrinkback as much as possible, however, it’s never a guarantee if it’s ever truly eliminated. Results can vary from reel to reel.
Find more technical info and wire and cable resources from Allied Wire and Cable here.