Heat shrink tubing is a form of plastic tubing that contracts or shrinks in size when the heat is applied to it. It's most common application is in electrical wiring, where the plastic tubes are shrunk down to fit around exposed wires. They provide insulation and abrasion resistance, and they are even used to bundle wires when necessary. Heat shrink tubing is also an economical and effective way to protect wire splices and bare wires.
Heat shrink tubing comes in a variety of colors, flexibility levels, and temperature sensitivities for different applications, ranging from military and sensitive electrical applications to more common ones.
All heat shrink tubes are manufactured and stretched to a certain diameter. Interestingly enough, the "shrinking" process actually restores the tubing to its original size, as the stretching process is the actual alteration.
Make sure the diameter you are using is the correct diameter for your wiring. It's usually stamped on the tubing or the package as a ratio, which indicates the stretched size first and the shrinking size second. For instance, a 2:1 diameter means the tube will shrink to half the size of its current diameter. Other common sizes include 2:1, 4:1, 5:1, and 6:1.
Regardless of the application or size, all heat shrink tubing needs one thing to operate properly: heat. It's important to realize that heat sources that may be used in one setting may cause a fire or explosion in another. For example, heat sources that employ an open flame, such as candles or matches, will work to shrink the tubing, but they may also char or burn through the tubing. Be sure to read thoroughly before attempting to heat your tubes.
Electrical tools that blow hot air are a safe and effective way to provide indirect heat to any tubing. Such tools include heat guns, such as the ones used for stripping paint, or even household blow dryers that are used on your hair. (Note that the blow dryers may not always shrink the tubing, as the plastic must get very hot to actually shrink. Not all blow dryers are able to achieve such high temperatures.)
While an open flame held near the plastic will certainly get hot enough to cause a reaction in the heat shrink tubing, it's also a risky approach. You have to find the balance between holding the flame close enough to provide enough heat without igniting the tubing and causing damage to your wires and much more if the fire spreads.
Flame options for heat shrinking plastic include candles and matches, or butane lighter, many of which may need to be used in succession to reach your goal. Even a torch, such as the kind used for cooking or for soldering, can work.
WARNING: Only use these items in well-ventilated areas where no flammable vapors such as gas, propane, or oil are present.
While a light bulb certainly isn't the best alternative, if you're in a pinch and don't have access to any of the other tools, holding your piece of wiring close enough to a hot light bulb can provide the sufficient heat to shrink your tubing.
Similarly, if your splice can be safely placed in an oven, that's another feasible, albeit unorthodox, way to heat your tubing.
WARNING: Be sure to monitor the oven method closely. Putting your wire in the oven at too high a temperature or leaving it in for too long will melt or burn it.