Heat-shrink tubing (or, commonly, heat shrink or heat shrink) is a shrinkable plastic tube used to insulate wires, providing abrasion resistance and environmental protection for stranded and solid wire conductors, connections, joints, and terminals in electrical work. It can also be used to repair the insulation on wires or to bundle them together, protect wires or small parts from minor abrasion, and create cable entry seals, offering environmental sealing protection. Heat-shrink tubing is ordinarily made of nylon or polyolefin, which shrinks radially (but not longitudinally) when heated, to between one-half and one-sixth of its diameter.
Heat-shrink tubing is manufactured in a multitude of varieties and chemical makeups with the exact composition of each type being dependent on the intended application. From near microscopically thin-wall tubing to rigid, heavy-wall tubing, each type has a precise design and chemical additives that make it suitable for meeting a wide variety of environmental demands. Heat-shrink tubing is rated by its expansion ratio, a comparison of the differences in expansion and recovery rate.
The unshrunk tubing is fitted on the wire before making the connection, then slid down to cover the joint after it is made. If the fit is tight, silicone lubricant can be applied without compromising the heat-shrink material. The tubing is then shrunk to wrap tightly around the joint by heating in an oven or with a hot air gun or another source of hot gas flow. Convenient but less consistent methods for shrinking the tube include a soldering iron held close to but not touching the tube, or the heat from a lighter. Uncontrolled heat can cause uneven shrinkage, physical damage, and insulation failure, and these methods are not recommended by heat-shrink suppliers. If overheated, heat-shrink tubing can melt, scorch or catch fire like any other plastic. Heating causes the tubing to contract to between half and one-sixth of its original diameter, depending on the material used, providing a snug fit over irregularly shaped joints. There is also longitudinal shrinking, usually unwanted and to a lesser extent than narrowing, typically around 6%. The tubing provides good electrical insulation, protection from dust, solvents, and other foreign materials, and mechanical strain relief, and is mechanically held in place (unless incorrectly oversized or not properly shrunk) by its tight fit.
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