Molded-in threads can be designed into parts made of engineering thermoplastic resins. Threads always should have radiused roots and should not have feather edges ¡ª to avoid stress concentrations. The Recommended Design for Molded-in Thread illustration shows examples of good design for molded-in external and internal threads. For additional information, see molded-in threads in Fasteners. Threads also form undercuts and should be treated as such when the part is being removed from the mold i.e., by provision of unscrewing mechanisms, collapsible cores, etc. Every effort should be made to locate external threads on the parting line of the mold where economics and mold reliability are most favorable.