The Design of Sprue bush (Main Runner)

  • The main runner refers to the melt flow passageway from the joint between the injection molding machine nozzle and the mold sprue bushing to the starting point of the branch runner.

–The design has to minimize melt temperature drop/pressure loss.

–Adopt the cone shape design, to facilitate removal of the condensed material

  • Forms of The Sprue Bush

  • Fixing Methods of the Sprue Bushing

  • The main runner is usually located along the center line of the plastic injection mold, overlapping with the axle of the injection molding machine nozzle.
  • In horizontal and vertical injection molding machines, the runner axle should be perpendicular to the parting surface.
  • To facilitate removal of condensed materials from the main runner, the main runner should be designed into the cone shape, with a cone angle of 2 – 6º; the diameter of the smaller end D d + (0.5~1mm), d refers to the diameter of the injection molding machine nozzle.
  • The internal surface roughness of the main runner Ra < 0.4; and the length of the main runner usually < 60mm.
  • The transition between the bigger end of the main runner and the branch runner should adopt an arc-shaped design, of which the corner radius is 1 – 3mm.
  • Since the main runner is the first contact point of the high temperature molten plastic, which also frequently make contact with the injection molding machine nozzle, damages by collision tend to occur. Generally speaking, the sprue bushing needs to be fixed to the fixed clamp plate. See the following figure for the structure of the sprue bush.
  • The hardness of the sprue bush should be lower than that of the injection molding machine nozzle.