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Shrinkable plastic tube used to insulate wires

Heat shrink tubing (or, commonly, heat shrink or heatshrink) 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 wiring. It can also be used to repair the insulation on wires or to bundle them together, to protect wires or small parts from minor abrasion, and to create cable entry seals, offering environmental sealing protection. Heat shrink tubing is ordinarily made of 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 precise design and chemical additives that make it suitable for meeting any of 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 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. Some types of heat shrink contain a layer of thermoplastic adhesive on the inside to help provide a good seal and better adhesion, while others rely on friction between the closely conforming materials. Heating non adhesive shrink tube to very near the melting point may allow it to fuse to the underlying material as well. Heatshrink tubing is sometimes sold in pre cut lengths, with a solder blob at the center of the length, as this configuration was specified by Daimler Benz for automotive electrical repairs. One application that has used heatshrink in large quantities since the early 1970s is the covering of fibreglass helical antennas, used extensively for 27 MHz CB radio. Many millions of these antennas have been coated this way.

Heat shrink tubing was invented by Raychem Corporation in 1962. It is manufactured from a thermoplastic material such as polyolefin, fluoropolymer (such as FEP, PTFE or Kynar), PVC, neoprene, silicone elastomer or Viton. The process for making heat shrink tubing is as follows: First the material is chosen based on its properties. The material is often compounded with other additives (such as colorants, stabilizers, etc.) depending on the application. A starting tube is extruded from the raw material. Next, the tube is taken to a separate process where it is cross linked, usually through radiation. The cross linking creates a memory in the tube. Then the tube is heated to just above the polymer's crystalline melting point and expanded in diameter, often by placing it in a vacuum chamber. While in the expanded state it is rapidly cooled. Later, when heated (above the crystalline melting point of the material) by the end user, the tubing shrinks back to its original extruded size. The material is often cross linked through the use of electron beams, peroxides, or moisture. This cross linking creates the memory in the tubing so that it is able to shrink back to its original extruded dimensions upon heating, producing a material called heat shrink tubing. For outdoor use, heat shrink tubing often has a UV stabiliser added.