Resin-Level, Hydrostatic-Pressure and Free-Surface Control in Vat Photopolymerisation: Geometry, Refill and Drift

Vat photopolymerisation consumes and redistributes liquid resin while the optical and mechanical process assumes a stable reference level. Global liquid height sets hydrostatic pressure, immersion depth, refill head and free-surface geometry. Local menisci, waves and constrained gaps can depart from that global state. A level sensor can therefore be accurate yet insufficient, and a fixed refill volume can be repeatable yet wrong after density, temperature or geometry changes. This review defines qualification of resin-level, hydrostatic-pressure and free-surface control. It separates inventory, bulk level, local interface, meniscus, pressure head and dynamic disturbance; establishes calibrated level-volume geometry; accounts for platform displacement and cured-volume removal; and couples make-up dosing to settling and bubble control. Sensors are challenged for wetting, fouling, optical interference and loss of signal. No universal level tolerance, refill increment, pressure head, dead band or settling delay is supported. The deliverable is a bounded level-control window that preserves geometry and refill capability throughout a declared build.