During the incubation period, heat is introduced into the joint as current is passed through the coil. Although there is no joint strength at this point, the polymer expands and the joint gap is filled. During joint formation and consolidation, melting begins. Melt pressure has begun to build, and the majority of the joint's strength is developed during this stage. The strength increase is due primarily to the constraint of the increasing molten material by the cold zones in the surrounding fitting. The plateau region signals the stabilization of the joint strength. Despite this, the heat of the joint is still increasing with time during this stage. The cooling period occurs after current is no longer applied to the coils. The molten polymer material solidifies and forms the joint.