The expanding role of Shrink Tubing
Most engineers are familiar with heatshrinkable tubing and its application areas ranging from aerospace and transportation to medical and commercial electronics. It can be an alternative way of insulating connections as well as sealing electrical harnesses and many other components from harsh environments.
In recent years the technology of heat-shrink tubing has been refined to allow its use in tiny medical products that have the width of a human hair, and in high-temperature aerospace equipment where it must stand up to ovenlike environments. Expertise in polymer science lets suppliers devise special blends of tubing material that can handle these and other specialized applications.
Other developments in polymer formulation have yielded heat-shrinkable tubing with low-smoke emissions and versions with zero halogen content, a requirement for use in mass-transit systems.-Certain high-performance tubing formulations have been devised to stand up in low temperatures as well. For example, some polymers based on silicone and elastomers can flex even when they see temperatures down to 73°C.
It is useful to review how heat-shrinkable tubing is manufactured as a way of understanding its possibilities. Radiation cross-linking is what gives heatshrinktubing its special properties. Manufacturing typically starts by extruding organic polymer material as a tube. The tube then gets exposed to a high-energy electron beam. The resulting crosslinking creates additional bonds in the molecular chains making up the polymer. These extra links enhance some of the polymer properties such as temperature and solvent resistance. But mostimportantly, they give the polymer a memory for its shape, size, and the thickness of its walls.
The extruded tube gets heated and expanded to a larger diameter. It is cooled and delivered to customers in this expanded state. The tubing shrinks back to its original state with the application of the right amount of heat energy. Interestingly, shrink tubing becomes stable once it has shrunk back to its original dimensions. Additional applications of heat even exceeding the shrink temperature have no effect on it.
The radiation cross-linking process also helps determine the tubing shrink ratio, one of the most important properties of a heat-shrinkable product. The shrink ratio is the ratio between the tubing dimensions in its expanded and fully recovered (unexpanded) states. About 70% of all heat-shrink tubing has a shrink ratio of 2:1. It is possible to obtain tubing with shrink ratios of 3:1, 4:1, and even 6:1. The bigger ratios tend to find use on repair points or bulky connectors, where a large knot resides on a slender wire or tube.
Recently, researchers have developed tubing with a 10:1 shrink ratio. This material is commercially available though only in short cut lengths.
Heat-shrinkable tubing comes in styles that are designed to handle specific kinds of applications. The most common types of heat-shrinkable products include single-wall, dual-wall, heavy-duty, heat-shrinkable fabric, and molded shapes.
Single-wall tubing is a candidate for insulation, strain relief, protection from abrasion, and mechanical strain. One widely used single-wall tubing is called Versafit. It is a UL VW-1-rated low-shrink temperature polyolefin. (Shrink temperatures considered "low" are those in the 90°C range, not quite as hot as boiling water.) Attributes of this tubing family include a high degree of flame retardance. Mechanical properties such as flexibility and a 2:1 shrink ratio make it a candidate for general-purpose insulation of in-line electrical components. For example, it often can be found insulating wire splices or serving as post insulation for uninsulated terminals or connectors.
Other, more specialized single-wall heat-shrinkable tubing are also available. An example is RNF-100 tubing, which exhibits high abrasion and solvent resistance. Another is RW-175, which is made from semirigid polyvinylidene fluoride.-It is designed to handle high temperatures. Its shrink temperature is in the same range as its operational temperature, 175°C. It is a candidate for demanding applications that involve protecting delicate electronic components. RW-175 is translucent, so it simplifies the inspection of the protected component. Another possibility for demanding environments is DR-25, flexible, chemically resistant elastomeric tubing. It especially suits military ground vehicles as well as off-road equipment and race cars because it performs well in high temperatures and in the presence of moisture and fluids.
There are a variety of other options for single-wall tubing. Some types boast bright colors or have a shiny appearance that blends in with consumer electronics. Others have high shrink ratios to accommodate irregular shapes. Some are even hot stampable. Additionally, single-wall tubing can also be considered for decorative applications or other uses that have nothing to do with electronics. Manufacturers of hand tools sometimes shrink tubing around the grip of their products for ergonomics or wear protection.
Dual-wall tubing excels at corrosion protection and sealing. It consists of exterior tubing, much like single wall, combined with an inner layer typically composed of either an encapsulant or adhesive. Encapsulants protect connections and components from splashes and corrosion. Adhesives go a step further by helping to seal the tube to plastic, metal, rubber, or other substrates. Applying heat to this specialized tubing shrinks the outer layer while the inner portion melts and flows to protect against environmental factors.
In addition to its sealing attributes, dual-wall tubing generally has higher shrink ratios. So it tends to find use in repairs as well as in OEM equipment. ATUM tubing, for example, lets users repair damaged cable jackets without removing the connectors.
Likewise, FL2500 tubing has a 4:1 shrink ratio, which lets a few sizes of the material cover a wide range of needs. It is also flame retardant and serves as a mechanically tough barrier against fluids and moisture over an extended temperature range. In fact, this type of dual-wall tubing is specially designed for insulation, strain relief, and sealing of automotive wire splices and components.
SCL tubing can be helpful in situations that demand encapsulation. SCL is semirigid polyolefin tubing that shields substrates from splashes and moisture and provides a high degree of mechanical protection against vibration and flexing.
Particularly harsh environments can call for special heavy-wall tubing. The thicker walls in this material give more impact resistance. The trade-off is that more polymer goes into the thicker walls, so this type of tubing costs more than styles with thinner walls. An example is RHW tubing, which can have an outside wall up to about 4-mm thick. With RHW, engineers can literally shrink an insulating conduit around a bundle of wire or other substrate. RHW is lined with adhesive, so it provides a moisture-resistant and impact-resistant shell. Intermediate versions like RMW (rugged medium wall) have walls up to 3-mm thick that provide moderate impact resistance.
Highly specialized versions of single and dual-wall tubing have been developed for demanding and intricate applications. For instance, the Altera family is a medical-grade tubing line especially formulated for USP Class VI requirements of delicate electrosurgical equipment. (The key trait of medical-grade shrink tubing is that it can be sterilized using various methods favored by medical-device manufacturers.) Similarly, RT555 heat-shrinkable fluoropolymer stands up to high temperatures, solvents, corrosive chemicals, and radiation. It is highly flame retardant and lightweight. These traits make it suitable for use near jet or automotive engines.
HANDLING SPECIAL PROBLEMS
Heat-shrinkable shapes can be molded as well as extruded. This permits the insulation of special shapes and sizes. To fabricate such pieces the base material is first molded, then cross-linked and finally expanded. A typical application for molded shapes is as a boot used for sealing the back of connectors and sensors. Molded shapes are usually characterized by having multiple openings, features at different angles, or both. Molded shapes tend to be more expensive than extruded tubing, however. The cost of tubing lengths is typically measured in cents; the cost of molded shapes gets into the range of dollars. Some common molded shapes include transitions, breakouts, boots, caps and connector backshells for circulars and D-subminiatures.
There are even heat-shrinkable fabrics. They are designed primarily to prevent abrasion, rather than to just act as insulators. They are often applied to components such as rubber hoses, plastic pipes, and harness wiring bundles. This fabric incorporates heat-shrinkable polyolefin "threads" in the circumferential direction and nonshrinkable polyester strands in the longitudinal direction that are woven together.
There are several ways of installing heat-shrinkable products. Of course, heat guns remain the primary means of handling repair work and small-scale jobs. Even assemblies in substantial volumes such as automotive harnesses may be handled off-line with heat guns, usually in the same area as wire splicing. Higher-volume needs may require more specialized heating equipment such as belt heaters or tunnel ovens. The latter option is a frequent choice for bulky objects such as hoses done in mass production. The shrink-material supplier generally gets involved in designing such equipment to optimize factors such as consistency, coverage, and energy efficiency.