Most conventional additive manufacturing (AM) processes fabricate the designed 3D model in a planar-layer-based manner, which significantly limits the printing quality with poor surface roughness, weak strength, and wasting material and time for supporting structures. This study presents a heat-guided algorithm to generate non-planar layers using isothermal surfaces to address these drawbacks of planar layers. The designed model is placed virtually on a heated freeform substrate. The substrate's heat flows through the 3D model, which mimics the material accumulation in 3D printing. The “virtual” isothermal surfaces naturally slice a 3D model into curved layers. Different curved layers with desired functionalities can be created by tailoring the boundary conditions and the thermal conductivity for the “virtual” heat transfer problem. In addition, customized three-axis and five-axis 3D printers were prototyped to evaluate the proposed slicing method. Several test cases demonstrated the benefits, leading to a 63 % decrease in surface roughness, a 14 % increase in tensile strength, and a 50 % printing time reduction in multi-material 3D printing. Further, the presented non-planar slicing algorithm was utilized for conformal printing and to smoothen the printed parts with planar layers.

Vat photopolymerization-based additive manufacturing (AM) technology has emerging success in efficiently fabricating complex and high-resolution objects. However, it is still challenging to reduce the lateral stair-stepping defects caused by the pixelated projection pattern, especially for Liquid Crystal Display (LCD) projection whose pixels are disconnected. To tackle this lateral stair-stepping issue, we propose to defocus the curing image pattern by increasing the gap between the LCD screen and the resin vat. This gap intentionally blurs the disconnected pixels to create a continuous and smooth projection pattern. Experiments verified that the smoothened LCD pattern led to an average 81.2% reduction in surface roughness, which was much more effective than grayscale pixels. The gap between the LCD screen and the resin vat also enabled blowing air to dissipate the heat from LCD and the resin polymerization, reducing the part distortion and printing failure due to the thermal stress. The present approach improved the surface roughness and paved the way for vat photopolymerization in fabricating smooth micro-optics, microfluidic channels, and functional curved surfaces. 

Though 3D printing shows potential in fabricating complex optical components rapidly, its poor surface quality and dimensional accuracy render it unqualified for industrial optics applications. The layer steps in the building direction and the pixelated steps on each layer's contour result in inevitable microscale defects on the 3D-printed surface, far away from the nanoscale roughness required for optics. This paper reports a customized vat photopolymerization-based lens printing process, integrating unfocused image projection and precision spin coating to solve lateral and vertical stair-stepping defects. A precision aspherical lens with less than 1 nm surface roughness and 1 µm profile accuracy is demonstrated. The 3D-printed convex lens achieves a maximum MTF resolution of 347.7 lp mm−1. A mathematical model is established to predict and control the spin coating process on 3D-printed surfaces precisely. Leveraging this low-cost yet highly robust and repeatable 3D printing process, the precision fabrication of multi-scale spherical, aspherical, and axicon lenses are showcased with sizes ranging from 3 to 70 mm using high clear photocuring resins. Additionally, molds are also printed to form multi-scale PDMS-based lenses.

A new manufacturing paradigm is showcased to exclude conventional mold-dependent manufacturing of pressure sensors, which typically requires a series of complex and expensive patterning processes. This mold-free manufacturing leverages high-resolution 3D-printed multiscale microstructures as the substrate and a gas-phase conformal polymer coating technique to complete the mold-free sensing platform. The array of dome and spike structures with a controlled spike density of a 3D-printed substrate ensures a large contact surface with pressures applied and extended linearity in a wider pressure range. For uniform coating of sensing elements on the microstructured surface, oxidative chemical vapor deposition is employed to deposit a highly conformal and conductive sensing element, poly(3,4-ethylenedioxythiophene) at low temperatures (<60 °C). The fabricated pressure sensor reacts sensitively to various ranges of pressures (up to 185 kPa−1) depending on the density of the multiscale features and shows an ultrafast response time (≈36 µs). The mechanism investigations through the finite element analysis identify the effect of the multiscale structure on the figure-of-merit sensing performance. These unique findings are expected to be of significant relevance to technology that requires higher sensing capability, scalability, and facile adjustment of a sensor geometry in a cost-effective manufacturing manner.