Welding is used in a variety of polymer applications. Heat is applied to soften two polymer pieces, then they are encouraged to interdiffuse to form a strong bond. The composite image below illustrates one method for doing this.

A metal wire is located about 250 µm from the area where the joint is formed. The two pieces of polymer are placed together and the wire is heated by electropmagnetic induction by placing the joint in a solenoid. One problem with this method is that too much heat causes the polymer to cross-link and become brittle. To investigate this local thermal analysis was used over the area shown above. The positions at which the measurements were taken are indicated below.

The results of the local thermal analysis are shown below.

Near the metal heater there is no softening observed (1 and 2) in the temperature range studied, thus it can be concluded that the degree of cross-linking is high. Further away a low softening point is observed (3) then, closer to the weld, this softening temperature increases (4). On the weld no softening is observed (5) then past the weld a high softening point is seen (6). The interpretation of this is that high cross-linking is seen close to the heated metal, as expected, then this decreases but then, unexpectedly, increases again approaching the weld. Beyond the weld the degree of cross-linking again declines. The explanation is probably that, as part of the welding process, the two pieces of polymer are vibrated as they are pushed together. The resultant friction provides additional heat to the polymer and in an increase in cross-linking in the immediate vicinity of the weld.