  Vacuum-forming is a common manufacturing technique for constructing thin
plastic shell products by pressing heated plastic sheets onto a mold using
atmospheric pressure. Vacuum-forming is ubiquitous in packaging and casing
products in industry spanning fast moving consumer goods to connected devices.
Integrating advanced functionality, which may include sensing, computation and
communication, within thin structures is desirable for various next-generation
interactive devices. Hybrid additive manufacturing techniques like
thermoforming are becoming popular for prototyping freeform electronics given
its design flexibility, speed and cost-effectiveness. In this paper, we present
a new hybrid method for constructing thin, rigid and free-form interconnected
surfaces via fused deposition modelling (FDM) 3D printing and vacuum-forming.
While 3D printing a mold for vacuum-forming has been explored by many,
utilising 3D printing to construct sheet materials has remains unexplored. 3D
printing the sheet material allows embedding conductive traces within thin
layers of the substrate, which can be vacuum-formed but remain conductive and
insulated. We characterise the behaviour of the vacuum-formed 3D printed sheet,
analyse the electrical performance of 3D printed traces after vacuum-forming,
and showcase a range of examples constructed using the technique. We
demonstrate a new design interface specifically for designing conformal
interconnects, which allows designers to draw conductive patterns in 3D and
export pre-distorted sheet models ready to be 3D printed.
