Heat shrink tubing (or, commonly, heat shrink) is a tube which shrinks in diameter when heated. Its diameter and thickness can vary, and there are three main categories, thin-wall, medium-wall and thick-wall tube. It is rated by its expansion ratio, a comparative of the differences in expansion and recovery rate. Heat shrink is used to insulate wires offering abrasion resistance and environmental protection for conductors, connections, joints and terminals in electrical engineering. It can also be used to repair wires or bundle them together, to protect wires or small parts from minor abrasion, and to create cable entry seals, offering environmental sealing protection between bulkheads and adding sealability to electrical junction boxes.
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Utility
The tubing is placed over the connection to be protected and then heated with an oven, hot air gun or similar tool. Convenient, but less effective, methods for shrinking the tube include a soldering iron (held in close proximity, but not touching the tube) or the heat from a lighter. These processes cause the tubing to contract as far as one sixth of its original diameter (dependent on the heat shrink, 2:1 is the most common), providing a snug fit over irregularly shaped joints. This provides good electrical insulation, protection from dust, solvents and other foreign materials, as well as strain relief. If overheated, heat shrink tubing can melt, scorch or catch fire like any other plastic.
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 from the closely conforming materials. Heating plain, 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 this product 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 with this versatile plastic shrink tube product.
Manufacture
Heat-shrink tubing is manufactured from a thermoplastic material such as polyolefin, fluoropolymer (such as FEP, PTFE or Kynar), PVC, neoprene, silicone elastomer or Viton.
According to the exact material used, there are two ways that heat shrink may work. If the material contains many monomers, then when the tubing is heated the monomers polymerise. 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 the tubing as normal, 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 will relax back to the un-expanded size.
The material is often cross-linked through the use of electron beams, peroxides, or moisture. This cross-linking helps to make the tubing maintain its shape, both before and after shrinking.
For external use, heat shrink tubing often has a UV stabiliser added.
Materials
Different applications require different materials:
- PTFE (fluoropolymer) tubes have the widest operating temperature range (−270 to 260 °C), low coefficient of friction, and high resistance to chemicals (almost everything except molten alkali metals, fluorine gas, and chlorine trifluoride). They however have very high shrink temperature (325–340 °C, or 250 °C). The tube has to be heated uniformly until it becomes clear, a sign of the polymer changing from crystalline to amorphous; on the transition back to crystalline the tube shrinks. 1.5:1, 2:1 and 4:1 shrink ratios are available on the market.[1][2]
- Viton, another fluoropolymer with high chemical resistance. Highly flexible. Shrink temperature is 120 °C.[3][4]
- Polyvinylidene fluoride (PVDF) tubes are intended for high temperature applications; −55 °C up to 150 °C (flexible PVDF) or up to 175 °C (semi-rigid Kynar). Their coefficient of friction is second only to Teflon. The material is resistant to abrasion, cut-through, ultraviolet and gamma radiation, and is up to 3 times as strong as standard polyolefin. The tubes are rated for use in plenum spaces. Common shrink ratio is 2:1, and shrink temperature ranges around 170 °C.[5][6]
- Fluorinated ethylene propylene (FEP) is a lower-cost alternative to PTFE. Its operating temperature range is up to 204 °C and the shrink temperature is as low as 190 °C. It has high chemical resistance, lower gas permeability and higher translucence than PTFE, excellent transmission of ultraviolet rays, and excellent electrical insulation. The shrink ratios are 3:1 and 6:1.[7]
- Neoprene (chlorinated elastomer) tubes maintain high flexibility even at low temperatures. The operating temperature range is −75 to 120 °C. The material is resistant to fluids and solvents and has good resistance to abrasion. The shrink temperature is 135 °C. Neoprene tubes are used as insulation, abrasion protection, and strain relief in military and aerospace. Common shrink ratio is 2:1.[8]
- Silicone rubber offers excellent resistance to scrape abrasion and high flexibility. The shrink temperature is 200 °C.[9]
- Polyolefin tubes, the most common kind, have maximum continuous use temperature from −55 to 135 °C, with shrink temperature of 90 to 130 °C. They are manufactured in a wide range of colors (including clear), and can be used for color-coding of wires in addition to other applications. With exception of black, they tend to have lower resistance to ultraviolet light; only black is suggested for outdoor applications. Common shrink ratio is 2:1; ratios 3:1, 4:1, and 6:1 are also available; higher ratios typically need higher shrink temperature.[10][11] A version exists with shrink temperature as low as 80 °C, and operating temperature range −55 to 125 °C.[12] Flexible and semirigid versions exist.
- PVC tubes have operating temperature from −55 to 105 °C and shrink temperature 100 °C. They come in a range of colors, including clear (which is more transparent than clear polyolefin). They are higher-gloss, more scratch-resistant, tougher and stiffer than polyolefins. They are more resistant to degradation by ultraviolet light, all colors can be used outdoors.[13]
Other special materials exist, offering, e.g., resistance to diesel and aviation fuels, or vowen fabric for increased abrasion resistance in harsh environments.
Heat shrink types
Heat shrink tubing is available in a variety of colours to allow easier colour coding of wires and connections. Recently heat shrink tubing has been used more in PC modding to tidy up the interior of computers and provide a more aesthetic finish. As a reaction to this new market opening up, manufacturers have started producing heat shrink tubing in luminous and UV reactive varieties.
Although most heat shrink is used to provide insulation, heat shrink tubing is also available with a conductive lining to avoid the requirement to solder a joint before covering it. This may be considered poor engineering practice.
Similar to heat shrink tubing is heat shrink end caps. Shaped like small mugs, these may be used to insulate cut ends of wires or cables.
Heat Shrink Tubing was invented by Raychem Corporation. Leading global producers include Tyco (which owns Raychem), 3M, and Sumitomo Electric Industries (through Sumitomo Fine Polymer).
References
- ^ http://www.buyheatshrink.com/heatshrinktubing/teflon.htm
- ^ http://www.e-sonic.com/whatsnew/POW/AlphaWire/HST-Bro-0901.pdf
- ^ http://www.buyheatshrink.com/heatshrinktubing/viton-heat-shrink.htm
- ^ http://www.e-sonic.com/whatsnew/POW/AlphaWire/HST-Bro-0901.pdf
- ^ http://www.buyheatshrink.com/heatshrinktubing/kynarpvdf.htm
- ^ http://www.e-sonic.com/whatsnew/POW/AlphaWire/HST-Bro-0901.pdf][http://www.electroinsulation.com/ip2.htm
- ^ http://www.buyheatshrink.com/heatshrinktubing/fep-heat-shrink.htm
- ^ http://www.buyheatshrink.com/heatshrinktubing/neoprene-heat-shrink.htm
- ^ http://www.e-sonic.com/whatsnew/POW/AlphaWire/HST-Bro-0901.pdf
- ^ http://www.buyheatshrink.com/heatshrinktubing/2to1polyolefin.htm
- ^ http://www.e-sonic.com/whatsnew/POW/AlphaWire/HST-Bro-0901.pdf
- ^ http://www.buyheatshrink.com/heatshrinktubing/2to1-low-shrink.htm
- ^ http://www.buyheatshrink.com/heatshrinktubing/pvc-heat-shrink-tubing.htm
See also
External links
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