In a multi-cavity tool, cavities can compete for airflow. Those with the shortest, least restrictive path may form first while distant or poorly vented cavities lag. The problem often appears as a repeating position-specific scrap pattern.
Unequal manifold branches, local leaks, blocked vents, different cavity volumes, plug misalignment, and thermal gradients all contribute. Center cavities may also receive a different heat load than edge cavities, creating an apparent vacuum problem that is actually thermal.
Compare cavities systematically
Wall thickness, detail, sheet temperature, and local pressure response should be mapped by cavity position. Swapping or isolating manifold branches can help separate airflow from tool geometry. A defect that stays with the machine position suggests heat or motion; one that stays with a cavity suggests venting or tooling.
Branch conductance and vent area should be balanced rather than compensated with a larger pump alone. The receiver must start each cycle at a repeatable condition. Downholders, local pre-stretch control, or plug corrections may be needed when equal airflow still produces unequal material demand.