Why it is necessary to heat metal surfaces before shrinking of tubing?

It is better to heat up metal surfaces first to avoid the creation of so-called “cold scars”. Due to the thermal conductivity of the metal, the tube gets cold in some spots too fast in the process of shrinking. This non-even cooling causes cold scars that are in fact areas with different thickness of the wall. Preliminary heating can also help to avoid condensed water vapor provoked by differences in temperatures.