The racetrack effect occurs when flow races through thick sections of the cavity before the thin sections have filled.
Racetrack Effect
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 How thickness affects flow), the plastic could be forced to reach all parts of the thick rim at the same time. This would result in balanced flows.