Solving molding racetrack effect problems
The racetrack effect occurs when flow races through thick sections of the cavity before the thin sections have filled.
Note: Thick sections offer less resistance to flow than thin sections.
The racetrack effect indicates unbalanced flow paths and can often cause unnecessary weld lines and air traps. The following diagram shows a part with a thick rim.
The flow of plastic (red arrows) races around the rim trapping a pocket of air (blue circle).
What to do
A large difference in wall thickness throughout a part can cause problems, but is sometimes necessary from a design point of view. However, in the previous example, the racetrack effect through the thick regions is not actually the problem. The problem is unbalanced flow that allows the racetrack effect to occur. If the plastic reached all parts of the thick rim at the same time, the racetrack effect would not occur.
Flow path 1 is shorter than flow path 2. However, by slightly thickening flow path 2 or thinning flow path 1 (see flow leaders and deflectors), the plastic could be forced to reach all parts of the thick rim at the same time. This would result in balanced flows.
The above example of racetrack in a symmetrical part with a thick rim is often easy to solve. In more complicated parts, thick walls may need some thinning, the polymer injection location may need to be altered, or multiple polymer injection locations may need to be used.