Heat-shrink tubing

Animation of heat shrink tube, before and after shrinking

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 work. 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 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 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.

Use

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.[1] 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.[1] 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%.[1] 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.

Video - Adhesive-lined heat shrink tubing shrinking

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 is specified by Daimler-Benz for automotive electrical repairs.[2]

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.

Manufacture

Heat shrink tubing was invented by Raychem Corporation[3] in 1962.[4] 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,[5] 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 term used called heat shrink with a built-in [6] thermal-memory.

For external use, heat shrink tubing often has a UV stabiliser added.

Materials

Different applications require different materials:

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.

Specialty heat-shrink tubing, known as "solder sleeves", have a tube of solder inside of the heat shrink tubing, allowing the heat source to electrically join the two wires by melting the solder and simultaneously insulate the junction with the tubing. Solder sleeves also commonly contain a ring of heat-activated sealant on the inside of each end of the tubing, allowing the connection to also be made waterproof.[11]

Heat shrink end caps, closed at one end, are used to insulate the exposed cut ends of insulated wires.

See also

Main Standards and certificates

UL224-2010 ASTM D 2671

References

  1. 1 2 3 cableorganizer.com: How to Use Heat Shrink Tubing
  2. ↑ Gilles, Tim (2015). Automotive Service. Cengage Learning. p. 546. ISBN 9781305445932. Retrieved 2016-11-20.
  3. ↑ Quality Today. IPC Industrial Press. 1994.
  4. ↑ U.S. Patent 3,396,460 (PDF). U.S. Patent Office. 1968.
  5. ↑ Accelerator apps: heat-shrink tubing, Symmetry, Dimensions of Particle Physics. V. 7, Issue 2, Apr. 10
  6. ↑ url=http://www.shrinkfit.co.uk
  7. 1 2 3 4 "3M Heat Shrink catalogue". 3M. Retrieved 30 October 2014.
  8. ↑ Kucklick, Theodore R. (2012). The Medical Device R&D Handbook, Second Edition. CRC Press. p. 19.
  9. 1 2 Wang, Xuefeng; Shaikh, Kashan A. (2009). "Interfacing Microfluidic Devices with the Macro World". In Wei-Cheng Tian, Erin Finehout. Microfluidics for Biological Applications. Springer Science & Business Media. p. 102. ISBN 9780387094809. Retrieved 2016-11-20.
  10. 1 2 3 Puckett, Larry (2015). Wiring Your Model Railrod. Kalmbach Publishing, Co. p. 88.
  11. ↑ "Solder Sleeves: Solder, heat shrink and waterproof your splices in one operation". Inventables. Retrieved 2016-01-11.
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