Air can bypass the tool face through the interface between the mold and baseboard, around inserts, through mounting holes, or across warped vacuum-box surfaces. The machine circuit may pass a leak test while the assembled tool performs poorly.
The tool should be tested in stages: machine only, baseboard and box, then complete mold. Sealing unused holes and isolating sections can identify the interface. A local defect near an insert or fastener often indicates an internal bypass rather than blocked face vents.
Mating surfaces should be flat, clean, and supported. Sealants and gaskets must tolerate tool temperature and service without migrating into vacuum passages. Fasteners should provide even compression; tightening one area excessively can warp the baseboard and create a new leak elsewhere.