The molten plastic injected from the injector nozzle will go through a sprue (sprue bush), a runner, and a gate and fill up in the cavity.
As the temperature of molten plastic is lowered while going through the sprue and runner, the viscosity will rise; therefore, the viscosity is lowered by shear heat generated when going through the gate to fill the cavity.
so learn Sprue bush design is important, becouse sprue bush is part that contact first time with melted plastic, bad design of sprue bush can make plastic product broken, especially sprue broken and runner broken.
picture below shown mold basic construction with sprue.
below is 3D model of sprue bush with 2 bolts
this type is commonly use in small mold size, until 350 ton injection machine is still used.
the other construction is 4 bolts, this sprue bush type commonly use in large mold type and mass production, more than 200 ton
and below is sprue bush with ring attachment, this type use when top plate thickness is width, but some designer prefer use large chamfer in top plate so with first model sprue bush is still can be attach
Sprue Bush Design Rule
1. Ball radius of sprue must larger than ball radius nozzle, so don't forget to check diameter of nozzle of injection machine, or you can look at injection machine catalog. for example when ball radius of nozzle of injection machine is 10 mm use ball radius sprue bush 11 until 13 mm.
2. Hole diameter inner must be larger than hole diameter nozzle, diameter nozzle is variating base on machine injection type and manufacture, is about 2 mm until 5 mm. use 0.5 mm more larger than hole diameter of nozzle.
3. check your plate thickness, make sure that length of sprue bush fit with the plate thickness.