Many everyday surfaces, from car interiors to glasses lenses, have biaxial curvature meaning that at a given point the surface is curved in every direction, like that of a sphere or ellipsoid. Creating displays and active optical films that conform to these complex shapes has long been a challenge. Conventional liquid crystal (LC) cells are built on flat glass, and cannot be biaxially shaped, as well as being heavy and fragile.

Organic electronics, on the other hand, has made possible the manufacture of transistors at production temperatures that don’t exceed 100°C, meaning that all types of flexible substrates can be used without damage, including those with optically ideal properties such as TAC film. FlexEnable has broadened that low temperature processing capability to include not only organic thin-film transistors (OTFTs), but also liquid crystal cell fabrication, allowing LC optics and complete organic displays (ePaper and OLCD) to be manufactured on ultra-thin, flexible substrates.

While conforming these plastic films to a uniaxially curved surface (such as that of a cylinder or cone) is relatively straightforward, biaxial curvature, achieved by 3D thermoforming the film, requires the film to be stretched. 3D forming opens up exciting new possibilities for displays and optical applications, particularly in areas like automotive surfaces and AR/VR.

Andrew Russel, Principal Industrial Designer at FlexEnable, explains how 3D forming works in the context of flexible electronics.

What is 3D forming?

3D forming is the process of shaping materials into three-dimensional shapes. All LC and OTFT cells manufactured around the world are made from flat sheet materials, so at FlexEnable we adopt these established manufacturing processes, but on flexible plastic films rather than rigid glass. We then employ simple manufacturing techniques to generate three-dimensional shapes. We do this by applying heat to the cells in an oven or on a hotplate to a precise temperature for a pre-defined time to make the materials ductile. Positive or negative pressure is applied to one side of the constrained cell for a short time at temperature; it is then removed and allowed to cool.

Simple biaxially curved shapes can be formed by constraining the perimeter of the flat cell and applying air pressure; the radius of curvature (ROC) is controlled by the level of air pressure applied.

Volume production of cells can be achieved as batch vacuum thermo-forming using existing equipment.

More complex shapes with variable radii can be formed with negative/positive air pressure into or over a mould of the desired shape. Alternatively, some form factors may be better suited to a combination of drape-forming one axis of curvature while pressure forming the other ROC.

Applications for which 3D forming is needed

AR/VR optics

The growing adoption of augmented and virtual reality (AR/VR) glasses is driving demand for advanced optical components that enhance the user experience. These include tunable lenses, which allow for dynamic focus adjustment, and pixelated dimmers, which control the amount of light entering the user’s eye. Importantly, these optical elements need to conform precisely to the curved fixed optics within the AR/VR glasses, minimising volume and weight, which can impact on the user experience. The ability to thermoform the plastic LC cells enables the creation of biaxially curved pixelated dimmers and tunable lenses that fit snugly against these curved optical surfaces, improving optical performance and visual clarity.

Automotive smart roofs

Looking beyond AR/VR applications, electric Vehicles are driving demand for smart sunroofs. Car sunroofs and car windows are biaxially curved which requires the integration of dimming films that can be 3D formed. FlexEnable’s fast-switching tintable liquid crystal cells on flexible plastic substrates allow for biaxial curvature, and the cells can even be segmented or pixelated to provide customised light control in different areas as needed. Integrated directly into the roof’s glass, this solution reduces vehicle weight and eliminates the need for traditional sunroof mechanisms.

Flexible displays

3D forming enables curved and conformable displays. Flexible OTFTs are in mass production today in the form of a curved E Ink display for the Ledger Stax cryptocurrency wallet. Using OTFTs has enabled a curve radius of 3mm and created a unique form factor and interaction method within the industry.

OTFTs can be integrated with LCD to create flexible active-matrix Organic LCD (OLCD) which allows cutting-edge designs and new functionality not possible with flat panel displays. Imagine displays that wrap around a spherical speaker or wearables.

A major technical achievement

The ability to create biaxially curved electronic devices represents a major technical achievement and shows the uniqueness of low-temperature organic electronics. As research and development continue, we can expect to see further advancements in materials, processing techniques, and device architectures, pushing the boundaries of what’s possible with 3D-formed displays and optics.