Controlled resin coating and image-based transparency assessment for VPP-printed components

Additive manufacturing of transparent polymer components produced by vat photopolymerization (VPP) is often limited by surface roughness and internal optical inhomogeneities, which degrade light transmission and visual quality. In this work, a controlled resin-coating post-processing strategy and a quantitative image-based framework for transparency assessment were developed to improve the optical performance. The effects of layer height (50, 100 μm) and printing angle (0°, 15°, 30°) were investigated. The coating process reduced surface roughness from values up to ~ 10 μm to below 0.15 μm, increasing optical transmittance from as low as 3–6% to ~ 86–89%, largely independent of the initial printing conditions. However, image-based metrics revealed that transparency quality remained strongly distance-dependent: haze increased from ~ 10% to over 40% as the air-gap distance increased from 20 mm to 60 mm, while sharpness metrics showed significant degradation, indicating persistent internal scattering effects. The results are consistent with a transition from a predominantly surface-dominated optical regime in the as-printed condition toward a regime in which bulk and interfacial effects become increasingly relevant after coating. While surface smoothing effectively restores transmitted intensity, the persistent degradation of contrast and image fidelity suggests a residual contribution from internal layer interfaces and refractive-index inhomogeneities. The proposed framework links manufacturing parameters and post-processing to optical performance and supports the design of transparent VPP-printed components.