Heat shrink tubing (or, commonly, heat shrink) is a mechanically expanded extruded plastic tube ordinarily made of nylon or polyolefin, which shrinks when heated in an effort to return to the relaxed diameter it originally had when extruded. 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 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 much as one sixth of its original diameter (depending on the heat shrink, a ratio of 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 between 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.
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.
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.
The material is often cross-linked through the use of electron beams, peroxides, or moisture. This cross-linking helps the tubing to maintain its shape, both before and after shrinking.
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.
- 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 to allow easier color 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 opening market, 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.
Heat shrink end caps are imilar to heat shrink tubing. Shaped like small mugs, they may be used to insulate the cut ends of wires or cables.
Heat shrink tubing was invented by Raychem Corporation.