Updated: Jan 14, 2020
Interpreting the term "parison" to be limited to a plastic tube that is outside the extruder is correct. Thus, the claimed method including "cutting and opening an extruded parison" (or "extruding a parison" and later "cutting through said parison") does not capture Donghee's process in which the plastic is cut from within a second flat die tool that is part of the extrusion equipment.
Moreover, the claimed step does not read on Donghee's process under the doctrine of equivalents because the two methods do not perform substantially the same function - Donghee's process allows independent control of the wall thickness while the patentee's process emphasizes uniform wall thickness as an advantage.
Read the case here.