Medical heat-shrink tubing from Tyco Electronics shrinks without processing aids such as an overtube or fusion sleeve.

Heat-shrink tubing provides a way to apply tight, protective coverings to components subjected to the extremes of heat, corrosion, shock, moisture, and other harmful environmental conditions. It is used in medical applications for electrical insulation, bundling components, tube joining, catheter-tip forming, adding stiffness to catheters, and as a cover for needles. It can be an alternative to coatings, eliminating solvents and chemicals for a uniform and repeatable surface.

Advanced Polymers, Salem, N.H., (www.advpoly.com) makes shrink tubing from polyethylene terephtalate (PET) in diameters from 0.008 to 1.250 in. Wall thicknesses range from 0.0001 to 0.004 in. and standard length is 50 in. The company says shrinkage is a function of temperature: the higher the temperature, the higher the shrinkage. Shrinking temperature ranges from about 185 to 374F, with a typical shrinking temperature of 302F recommended for most applications.

Unrestricted, tubing will shrink radially and axially. In most cases, the best overall performance comes from minimal shrinkage (less than 15 to 20%) except when a drawing process is used or unless axial shrinkage is restricted. To restrict axial shrinkage, hold the ends of the shrink tube while it's heated to prevent the tube from shrinking in length. This results in radial shrinkage up to 50% or more depending on temperature. A sizing chart at www.advpoly.com/Products/ShrinkTubing/Catalog/Default.aspx shows how diameters change after heat shrinking.

Heat-shrink tubing from Parker TexLoc, Ft Worth, Tex., (www.texloc.com) has two layers for electrical insulation. Texflour fluoropolymer double-shrink heat shrinkable tubing consists of an outer layer of Texfluor PTFE heat shrink with an inner layer of Texfluor FEP tubing. When using double shrink tubing, the part is encapsulated by the FEP tubing as the PTFE shrinks, melting the FEP and creating a waterproof protective covering.

Double shrink tubing comes in sizes as small as 0.036-in. expanded, up to 1-in. expanded with an operating temperature of -100 to 450F. Custom sizes can be manufactured and secondary operations such as custom shrinking, slitting, scoring, hole punching, cutting, and kitting are available.

Two-layered heat shrink tubing from Parker TexLoc keeps out moisture, dust, heat, and corrosion.

Another line of medical heat-shrink tubing speeds and simplifies installations by eliminating expensive processing aids such as an overtube or fusion sleeve. The Altera MT-PBX tubing from Tyco Electronics, Harrisburg, Pa., (www.tycoelectronics.com) uses a single-wall construction of polyether block amide copolymer (PEBA) for shrink ratios of either 2:1 or 4:1. Custom ratios are available. Using PEBA materials that meet U.S. Pharmacopeia Class VI standards, the tubing withstands repeated sterilization by autoclave, gamma radiation, ethylene oxide, steam, and dry heat.

Applications include strain relief, mechanical protection, insulation, and color coding. Altera MT-PBX is available in application-specific formulations and an array of colors. Options include an interior adhesive layer and radiopaque fillers.

Heat-shrink tubing also comes from manufacturer Zeus Inc., Orangeburg, S.C. (www.zeusinc.com). The most commonly-used materials in Zeus' heat-shrink tubing are PTFE and FEP. The company offers some guidelines for good results:

• Ensure good ventilation in the work area because fumes may cause nausea and dizziness.

• Heat-shrink PTPE tubing requires 650F ±25F and FEP tubing requires 420F ±50F to fully shrink, or recover. Actual temps may be higher or lower depending on design and dimensions of the heat shrink, application technique, and other factors. The mandrel, or part to be covered, must withstand this temperature range.

• The mandrel being covered may act as a heat sink, literally freezing the tubing before it has a chance to form completely and smoothly. Zeus recommends preheating large diameter mandrels.

• Even heating and cooling of all sides provides best results. Uneven heating or cooling tends to split the side still in the “gel” state while the other side is in the hard or crystalline state — especially upon recovery. Shrink starts when the tubing is brought to the “gel” state but completes its recovery during the cooling cycle. In its gel state, the tubing becomes crystal clear and glassy in appearance.

• Ovens are the most reliable way to recover heat shrink products because they ensure even heating and reduce the risk of overheating the material, which can lead to brittleness and cracking. Heat guns can also be used to recover heat-shrink material.