Although we take it for granted, versatile and widely used heat shrink tubing is the result of advanced material science, and is available in a broad range of varieties and options.
Part 1 of this FAQ looked at the basics of HST and its history. This part looks at the many variations and standard options that are widely available. As with many innovations, when there’s a good idea that easily and neatly solves many problems with little or no “headache,” it is soon extended to meet additional needs in similar or even unrelated applications. This is true for HST.
Q: Is 2:1 the only expanded/recovered ratio available?
A: Not at all. Other standard ratios are 3:1, 4:1, and even 6:1. In general, HST vendors recommend using the smallest shrink ratio that fits the application; use 6:1 if 2:1 or 3:1 will fit.
Q: What about material thickness?
In addition to standard thin-wall HST, variations are available with double, triple, and even thicker walls. These are typically used for heavier industrial cables as they have more strength, but the final diameter of the splice is also thicker, of course
Q: Is HST like the early Ford Model T, where the quip was “you can have any color you want, as long as it’s black”?
A: Absolutely not. HST is available in just about every color (Figure 1). Sometimes the colors are for color coding to help identify cables such as red for power, black for ground, or green for low voltage. In other cases, it is used to enhance the appearance and appeal of the product, such as in “open-sided”, performance-boosted PCs used for gaming. There is even clear HST which allows the splice or whatever is being covered to be visible.
Q: Is HST available with special linings?
A: Two special linings are widely used. One is a dual plastic lining where the inner lining melts as the outer one shrinks, thus encapsulating the wire splice. The other has a meltable adhesive very similar to hot-melt glue; when the tubing is heating and shrinks, the resin adheres to the spice and also forms a watertight, corrosion-resistant covering.
Q: What about high-temperature applications?
A: Standard HST shrinks at about 90⁰C to 100⁰C. However, there are versions where the tubing itself can withstand higher temperatures, so the shrink temperatures are also higher with ratings of 150⁰C and 200⁰C. There’s even Teflon-based HST, which combines extremely high shrink temperature of 250⁰C with extremely thin walls, so it becomes an almost invisible “skin” which does not interfere with sliding of the cable and minimizes added bulk.
Q: Cables often need to be labeled and marked; does HST interfere with this?
A: Yes, and no. Some HST materials are difficult to write on even with a special marker, while others are not. There are also standard, pre-printed HST cables (for example, marked with basic 1, 2, 3,…), and most HST vendors will custom-mark tubing, so the user only has to cut, slide it on, and shrink it. Of course, many users will choose separate label-makers, such as those from Brother, and can then print and label whatever they want, as needed.
Q: What are some other specialty versions?
A: Vendors offer HST with various enhanced attributes, in addition to high temperature:
On the mechanical areas, HST with extra abrasion resistance is used where the cable the HST is covering will be pulled through conduits and plenums, or be physically abused.
Other HST features additional resistance to oil and chemicals.
For applications where the HST is used not only to provide basic protection and insulation but also mechanical support, version are available with much-higher tensile strength, as much as three times.
Although the polymer of HST is a dielectric, not all-dielectric have the same voltage rating. Some HSTs feature higher-voltage dielectrics for applications which require that performance.
Q: What about regulatory codes and compliance?
A: In the temperature area, HST versions are offered which meet the UL VW-1 flame test, which defines and limits a material’s support of ongoing combustion (Figure 2). This “vertical flame” test assesses the material’s ability to resist fire along a vertically suspended wire, as the goal is to prevent the spread of fire, especially in plenums and conduits.
Q: How do you shrink the tubing?
A: A direct flame can be used from a match for small, thin-wall tubing, or a butane-fueled torch for larger pieces. However, open flames bring obvious dangers and risks, so it’s highly preferred to use electric or closed-flame, fueled hot-air guns. These can be direct “pistol” style similar to a hairdryer (which can be used, if it is hot enough and the HST is a lower-temperature version) or a hot-air source with a curved nozzle and airflow path for better coverage (Figure 3). Note that unlike the soldering of pipes, which requires both temperature and heat energy and thus a flame, HST does not need much heat energy, and thus a much-safer hot-air gun will work.
Q: How is HST sold?
A: Thin-wall, mass-market HST is widely available at hardware and home-improvement stores for the general public. It requires no special skills, training, or ability to use — that’s one of the many virtues of HST. In addition to these stores, electronic-specialty stores (Fry’s, You-Do-It Electronics) offer it in many variations, and every electronic distributor carries it. Typical retail-outlet diameters range from 1/8 inch to 1 inch and even 2”, often in lengths of six or 12 inches.
Q: How much does HST cost?
A: Obviously, there is no single answer, given the wide variety of options among which to choose. As examples, a package of six 6-inch pieces of ¼-inch black polyolefin-based tubing retails for about $3, while a four-foot length of ¾ inch, dual-wall, adhesive-lined HST is about $10.
Q: How is it packaged for sale?
A: In many ways — on rolls, as cut tubes ranging from a few inches to several feet and more, and even in handy “variety-pack” kits for field use (Figure 4).
Heat shrink tubing is one of those very handy, “how did we live without it?” products, similar to cellophane tape, zipper-lock bags, and adhesive bandages (to cite just a few others) which looks simple and are easy to use, solves many problems (some are the original ones, and many are not). Yet each requires substantial material science expertise, experience, and production know-how to make into a low-cost, widely available reality.
EE World References
- Wikipedia, “Heat-shrink tubing”
- Science Direct, “Cross-Linked Polymers”
- Cable Organizer, “Heat Shrink Tubing: How It Was Invented, and Why We Use It”
- Epec Engineered Technologies, “What is Heat Shrink Tubing and How is it Used?”
- Allied Wire and Cable, “Frequently Asked Questions: Heat Shrink Tubing”
- Alpha Wire, “The Perfect FIT® for Any Need”
- HellermannTyton, “Transparent heat shrink tubing allows for easy visual control of splices”
- Techflex, “What is the VW-1 test?”