Vacuum dead volume is air space that must be evacuated but does not contribute to forming the part. It may exist in an oversized vacuum box, unused manifold branches, deep baseboard cavities, long hoses, or abandoned ports. The final gauge reading can look acceptable even though the sheet reaches the mold too slowly.
Confirm the delay
Pressure should be recorded at the receiver, machine manifold, and tool if test points are available. A large delay between valve opening and pressure change at the mold indicates volume or flow restriction downstream. Testing the machine with the tool isolated helps separate pump and receiver performance from tool-side dead volume.
Unused spaces should be reduced with rigid, temperature-compatible inserts or redesigned manifolds that do not obstruct service access or cooling. Hose length and internal volume should be minimized without creating sharp bends. The correction must be verified from the pressure-time curve at the tool; extending heating time only masks the delayed response and increases thermal risk.