In the mold design process of plastic injection molding, it is necessary to know the molding shrinkage rate. It reflects the molding shrinkage, cavity surface temperature, and injection molding pressure of several major thermoplastics.
The main runner is the first place where the plastic melt passes to the injection mold cavity. It introduces the plastic melt from the nozzle of the injection molding machine into the runner or injection mold cavity. Its shape is conical, which facilitates the smooth flow of the melt forward.
When the mold is opened, the sprue aggregate can be pulled out smoothly. The size of the sprue directly affects the flow rate of the plastic melt and the filling time. Because the sprue has to repeatedly contact and collide with the high-temperature plastic and the injection molding machine nozzle. It is usually not directly opened on the fixed mold. Instead, it is separately designed as a main runner sleeve and inserted into the fixed mold.
The sprue sleeve is usually made of high carbon tool steel.
The main points of the main runner design of the injection mold are as follows:
- To facilitate pulling out the aggregate of the pouring system from the main runner and considering the expansion of the plastic melt, the main runner is designed into a conical shape.
- The large end of the main runner is rounded. And its radius is r=1～3mm to reduce the resistance of the flow velocity to the transition.
- Under the condition that the plastic parts are formed well, the length of the main runnershould be as short as possible.Otherwise it will increase the aggregate of the runner and the pressure loss. What’s more, the plastic melt will cool down too much and affect the injection molding.
- Tomake the molten plastic completely enter the main flow runner without overflowing, we should connect the main flow runner closely with the nozzle of the injection machine. And we should design the joint of the main flow runner as a hemispherical pit. The main runner part is often designed as a detachable runner bushing. So that high-quality steel can be separately processed and heat treated. Therefore, electric heating is used at the runner to increase the material temperature.