When the melt flows from a runner with narrow section into a cavity with large section or when the runner is very narrow and the surface roughness is rather poor, turbulence flow will form within the runner (including the cavity), thereby resulting in the turbulence flow mark on the surface of plastic parts.

The treatment measures include properly reducing screw injection rate or controlling it through hierarchical injection; opening a gate on the thick walls or side walls of plastic parts, wherein the type of gate should be propitious for the melt’s steady entering the cavity, such as tab gate, fan gate and slice gate; sometimes the section dimension of runner and gate can be properly enlarged to reduce flow resistance; optimizing structure of plastic parts when necessary to avoid uneven wall thickness wherein melt in the cavity can flow from narrow section to large and thick section; in addition, the fluidity of melt can be improved through increasing temperature of mold, barrel as well as nozzle.