Heat shrink tube is a mechanically expanded plastic tube, which shrinks when heated. The tubing is placed over the connection to be protected and then heated with an oven, hot air gun or similar tools. This processes causes the tube to contract and it hold tight the surface or wire. This is a very good protector of wire joints, but it can catch fire.
Heat shrink tube comes in different color and diameter. typical shrink ratio is 2:1 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.