Pressure forming uses compressed air above the sheet while air is evacuated from the mold side. An imbalance in either circuit can produce one-sided draw, delayed detail, local thinning, or cavity-to-cavity variation.
Leaks at the pressure box, inadequate supply volume, slow fill valves, restrictive piping, unequal venting, and pressure regulator droop are common causes. The same pattern can also be created by a thermal gradient, so pressure should not be blamed until sheet temperature is mapped.
Diagnostic method
Pressure above the sheet and pressure below it should be recorded through the forming event. The useful variable is the differential across the sheet, not either gauge in isolation. A stable compressor header does not prove that the pressure box reaches the required condition quickly.
Seals, valve flow, receiver capacity, and distribution passages should be corrected before increasing set pressure. Tooling and machine ratings must never be exceeded in an attempt to overcome a restriction. Once airflow is stable, local heater zoning and vent balance can be tuned to equalize draw.