What is Insulation and Jacket Shrinkback?

Shrinkback is the tendency for the insulation of a wire to pull back from a splice or termination connector. If shrinkback occurs, an excessive amount of copper (or aluminum) conductor can be exposed and increase the risk of a short circuit. Shrinkback can also occur on the jacket of a multi-conductor cable. If the jacket pulls back from a splice or termination, it exposes the cable to dirt and/or moisture. Common causes of shrinkback and industry methods used to measure and control it are discussed below.

What Causes Shrinkback?

Shrinkback can occur when built-in mechanical stresses are present in the insulation or jacket material as the result of the manufacturing process. For example, if a copper conductor moves through an insulation extruder head at a speed slightly faster than the molten insulation compound coming out of the extruder head, the molten insulation becomes stretched. When the insulation compound is cooled, the insulation can become "frozen" in this stretched condition. If the wire is later cut into short lengths, the insulation may pull back if friction with the copper conductor is too low to hold it in position. High temperatures and wide temperature excursions tend to accelerate the process. Polymers vary in their susceptibility to shrinkback as a result of their basic chemical structure. For example, the thermal coefficient of expansion of a polymer can affect shrinkback. A higher thermal expansion coefficient means a larger volumetric change during temperature changes. This could lead to larger shrinkbacks. Table 1 contains coefficients of linear thermal expansion for common wire and cable polymer resins.1



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