An ultrasonic runner cutter is a specialized industrial device primarily used in plastic injection molding to cleanly and precisely remove excess material (\"runner\") from molded plastic parts. The runner is the solidified channel through which molten plastic enters the mold cavity. The cutting blade vibrates at an ultrasonic frequency, concentrating strong mechanical energy into the narrow runner area. This localized energy generates frictional heat at the molecular level of the plastic only at the runner, causing it to melt rapidly or become highly plasticized. Subsequently, the mechanical vibrations shear the softened material away from the main part. The entire process occurs extremely quickly (typically within a few milliseconds).