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Heat shrink tubing (or, commonly, heat shrink or heatshrink) is a mechanically expanded extruded plastic tube ordinarily made of nylon or polyolefin, which shrinks in one plane only (its diameter) 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. Heat shrink is used to insulate wires, providing abrasion resistance and environmental protection for stranded and solid wire conductors, connections, joints and terminals in electrical engineering. 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.
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 other 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 heatshrink 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 heat shrink material used, providing a snug fit over irregularly shaped joints. There is also longitudinal shrinking, usually unwanted and to a lesser extent than narrowing, of 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.
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.
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.
Depending on the material used, there are two ways that heat shrink may work. If the material contains many monomers, the monomers polymerise when the tubing is heated. This increases the density of the material as the monomers become bonded together, therefore taking up less space. Accordingly, the volume of the material shrinks.
Heat shrink can also be expansion-based. This process involves producing normal tubing, then heating it to just above the polymer's crystalline melting point and mechanically stretching the tubing (often by inflating it with a gas); finally, it is rapidly cooled. Later, when heated, the tubing relaxes back to its un-expanded size.
For external use, heat shrink tubing often has a UV stabiliser added.
Different applications require different materials:
- PTFE (fluoropolymer) tubes have a wide operating temperature range (-55 to 175 °C), a low coefficient of friction, and high resistance to chemicals and punctures.
- Viton, another fluoropolymer with high chemical resistance, is widely used in hydraulic equipment. It is highly flexible, with a very wide operating temperature range of -55 to 220 °C, making it suitable for protecting sensitive devices against heat.
- Polyvinylidene fluoride (PVDF) tubes are intended for high temperature applications.
- Fluorinated ethylene propylene (FEP) is a lower-cost alternative to PTFE.
- Elastomeric tubes maintain high flexibility even at low temperatures and meet stringent international specifications. Their operating temperature range is -75 to 150°C. The material is resistant to many chemicals (including diesel and petrol) and has good resistance to abrasion, even in severe environmental conditions. A common shrink ratio is 2:1.
- Silicone rubber offers excellent resistance to scrape abrasion and high flexibility. Its operating temperature range is -50 to 200°C
- Polyolefin tubes, the most common kind, have maximum continuous-use temperatures from -55 to 135 °C, and are used by the military, aerospace and railway industries. They are flexible and fast-shrinking, and manufactured in a wide range of colors (including clear), which can be used for color-coding wires. With the exception of black, they tend to have lower resistance to ultraviolet light; accordingly, only black is recommended for outdoor applications. A common shrink ratio is 2:1, but high-grade polyolefin heat shrink is also available with a 3:1 ratio.
- PVC tubes are available in several colors and can be used outdoors.
Other special materials exist, offering qualities such as resistance to diesel and aviation fuels, and there is also woven fabric, providing increased abrasion resistance in harsh environments.
Heat shrink types
Heat shrink tubing is available in a variety of colors for color-coding of wires and connections. In the early twenty-first century heat shrink tubing started to be used for PC modding to tidy up the interior of computers and provide an appearance considered pleasing. In response to this opening market, manufacturers have started producing heat shrink tubing in luminous and UV reactive varieties.
Although heat shrink is usually used for insulation, heat shrink tubing with a conductive lining is available, for use particularly on joints which are not soldered. This may be considered poor engineering practice.
Heat shrink end caps, closed at one end, are used to insulate the exposed cut ends of insulated wires.
Main Standards and certificates
UL224-2010 ASTM D 2671
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