Recoating Dynamics of Low-Viscosity Photoresins: Meniscus Motion, Settling, Bubble Entrainment and Layer Uniformity

Layer exposure in vat photopolymerisation assumes that liquid resin has replenished the target gap and reached an acceptable fluid state. That assumption fails when blade motion, platform travel, cavity geometry, wetting, viscosity, surface tension or entrained gas create a nonuniform layer. “Settling time” is often used ambiguously for free-surface relaxation, squeeze-flow equilibration, bubble escape and particle sedimentation. These processes have different timescales and acceptance variables. This review defines a qualification framework for recoating and refill of low-viscosity photoresins. It separates bulk refill, meniscus and contact-line motion, residual free-surface waves, local thickness error, bubble entrainment and dispersed-phase sedimentation. Rheology is measured at process-relevant temperature and shear history. Exposure readiness is defined by a measured layer-thickness or interface-stability tolerance rather than a fixed generic delay. Coupons and sensors challenge cavities, islands, area fraction, travel direction and repeated layers. The framework treats blade, gravity, constrained-gap and vacuum-assisted methods as different boundary conditions. No universal viscosity ceiling, lift speed, wait time, bubble-rest period or Stokes correction is supported. The defensible result is a bounded refill and layer-uniformity window for a declared resin, geometry, machine and cycle.