Before welding, polyethylene has to be put into the so-called thermoplastic state. In this condition, the macromolecules are free to move without any noteworthy intermolecular bonds, i.e. even the semi-crystalline structures that only begin to melt at higher temperatures (melting temperature of crystallite) have dissolved.

Note:

In thermoplastic state, the material is not liquid but plastic (hard-flowing). The reason for this are the long molecular chains that randomly intertwine when superficially melted.