Top-Down Vat Photopolymerisation: Free-Surface Optics, Recoating, Buoyancy, Supports and Scale-Up

Top-down vat photopolymerisation exposes resin at or through an upper liquid interface while the part descends into the vat. The architecture avoids a transparent vat floor and its separation forces, but introduces a coupled free-surface system. Optical path length, meniscus and waves influence exposure; recoating must restore a usable layer; buoyancy and hydrodynamic forces act on the submerged build; supports must resist those loads; and scale-up changes both surface dynamics and resin inventory. This review defines system-level qualification for top-down vat photopolymerisation. It consumes the qualified global-level state from ER-246 and the qualified local surface-readiness state from ER-244, then integrates those inputs with optical and mechanical consequences without redefining either upstream control problem. Qualification is non-compensatory across optical integration, buoyancy, supports, thermal state and long-build drift, with accepted surface and level states treated as required inputs. No universal support density or scale factor is supported.