Since it is known that crosslinked polyethylene can be welded, it is clear that the mechanisms acting in the interior of a welded joint are not adequately explained yet. There are several criteria, hypotheses and models that attempt to describe what happens in a welded feint. In this paper, two new promising hypotheses, the Mixed Crystal Hypothesis and the Nexus Hypothesis, are introduced. The Mixed Crystal Hypothesis can be proven by the help of several different examination methods. Other examinations support the Nexus Hypothesis.