New printing technique for strong materials

Materials World magazine
1 Mar 2018

Rotational additive manufacturing using polymer fibres could make prosthetics stronger. Ellis Davies reports.

A novel circular 3D-printing method could give better control over the arrangement of short fibres in polymer matrices, which enables the creation of structural materials optimised for strength, stiffness, and damage tolerance. Developed by researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences, USA, the method, known as rotational 3D printing, could have a wide range of applications such as the manufacture of prosthetics, sporting equipment, and low cost mould for composites.

Rotational 3D printing gives a greater level of control over fibre architecture in printed composites, and has the ability to change orientation without changing the tool path used to deposit the material. This means that near optimal fibre arrangements can be achieved at every location in the printed part, resulting in higher strength and stiffness using less material. A particular printing head is required, which is unique because it uses the viscosity of the ink itself to reorient fibres in a desirable way, rather than relying on magnetic or electric fields. The rotational depositional nozzle manipulates the flow of the viscous ink to arrange fibres in a spiral.

The method changes how the carrier resin flows in order to move fibres. Because of this, the head concept could be used on any material extrusion printing method, from fused filament fabrication and direct ink writing, to large-scale thermoplastic additive manufacturing – and with any filler material, from carbon and glass fibres to metallic or ceramic whiskers and platelets.

‘This technique allows a new level of control over fibre orientation in 3D printed composites that has not previously been possible,’ said Brett Compton, co-author of the study, and now Assistant Professor in Mechanical Engineering at the University of Tennessee, USA.

‘We have demonstrated that the unique fibre arrangements that result from our printing process lead to significant improvements in damage tolerance over existing 3D printed composite materials,’ he said.

A wider reach

Jordan Raney, co-author of the study and now Assistant Professor of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania, USA, spoke to Materials World about the broader context of this development. ‘Natural composites like wood, bone, and shells have received a lot of attention in the mechanics and materials communities because of how well they reconcile competing properties,’ he said. ‘For example, they are both tough and strong, but also lightweight. Natural materials achieve this not by using exotic, rare elements, but by using highly optimised arrangements of simple and abundant elements.’ The team’s system is notable in enabling users to control fibre arrangements inside their 3D printed materials, providing a new way to optimise the microstructure for improved mechanical properties.

The team believes the new method and attachment could have broad commercial use. ‘Potential commercial applications range from fabricating stronger, lighter prosthetics and sporting equipment tailored to specific individuals, to low cost moulds for traditional composites manufacturing with tailored heat flow properties,’ Compton said.

The greater level of control, given by rotational 3D printing, could also expand the design space that can be exploited to optimise materials’ properties, the researchers say.