Induced Birefringence in 3D Printing: Concealing Information Optically within Printed Objects

Abstract

This paper presents a novel method to spatially vary the intra-layer birefringence of Fused Filament Fabricated (FFF) 3D printed parts by controlling polymer chain alignment during extrusion along individual rasters. The role of print speed, extrusion factor and layer separation on the birefringence of single PLLA layer is explored, at layer thicknesses ranging from 50 to 125 µm and print speeds between 1000 and 6000 mm/min. We explore the cumulative and subtractive effect of multiple PLLA layers to elicit transmitted colours corresponding to a range of retardations, achieve complete extinction, and 3D print a physical Michel-Levy chart. It was found that, by increasing print speed and reducing layer separation and extrusion factor, a birefringence of up to Δn = 9 x 10-4 could be achieved in printed single layers. In multi-layer structures, retardations in the range of 0 to 800 nm were demonstrated. These results suggest that spatially varied birefringence can be used to store data, text or form colour images, which can be resolved when parts are illuminated between crossed polarizing films. We utilise this effect to present a new steganographic printing technique to embed information within bulk printed parts. These techniques might find application in a wide range of photonic devices and 3D printed optics, where spatial control over molecular alignment and associated influence on the propagation of light is desirable including the ability to encode information within a 3D print.