When a part has complex geometry, it may require more than one injection location. You must ensure that the flow of the material is balanced to avoid molding problems.
There are two stages in balancing flow. The first stage is determining how many injection locations are needed and where they should be. The second stage is designing the runner system so that the part fills evenly.
Injection locations
It is helpful when you are balancing flow paths to split the part into imaginary sections that will fill simultaneously. The following diagram shows a part that requires three injection locations to fill effectively. The three sections separated by the red lines show the sections will all fill at the same time. The pale blue arrows in the diagram indicate the flow paths of material, and the yellow cones indicate the injection locations.
Runner system
The next stage is to design the runner system so that each section fills simultaneously. For a part with two equal sections, the runners should be the same diameter, length and distance from the sprue. For parts with unequal sections, the pressure in each runner and section should be the same and all the flow fronts should meet at the same time. The following diagram shows a runner system designed to finish filling all three sections at the end of fill.
