Ultrasound will transform 3D printing

But researchers from Bristol University have now developed a way to transform existing 3D printers so they can also print composite materials.

When designed properly, composites have just about the best strength for their weight of any common material, making them perfect for applications that need to be very strong but light, such as aeroplanes. Composites are usually made from very long glass or carbon fibres set in a plastic matrix. It’s the presence of the fibres, and the fact that they are all carefully arranged, that makes these materials so impressively strong yet lightweight.

At present, composite products are made by forming the fibres into sheets that look a bit like stiff cloth. These are then cut to shape and assembled by hand, layer-by-layer, to create the final product. As a result, composites are expensive and not easily replicated with 3D printers.

However, Bruce Drinkwater, Professor of Ultrasonics at the University of Bristol, and fellow researchers have found a way to print composite material by making a relatively simple addition to a cheap, off-the-shelf 3D printer. The breakthrough was based on the simple idea of printing using a liquid polymer mixed with millions of tiny fibres. This makes a readily printable material that can, for example, be pushed through a tiny nozzle into the desired location. The final object can then be printed layer by layer, as with many other 3D printing processes.

The big challenge was working out how to reassemble the tiny fibres into the carefully arranged patterns needed to generate the superior strength we expect from composites. The innovation we developed was to use ultrasonic waves to form the fibres into patterns within the polymer while it’s still in its liquid state.

The ultrasound effectively creates a patterned force field in the liquid plastic and the fibres move to and align with low pressure regions in the field called nodes. The fibres are then fixed in place using a tightly focused laser beam that cures (sets) the polymer.

The patterned fibres can be thought of as a reinforcement network, just like the steel reinforcing bars that are routinely placed in concrete structures such as foundations or bridges. Our study used short glass fibres in liquid epoxy polymer that are formed into longer lines of fibres and can recreate the structure of a traditional composite.

But the process has huge flexibility and can also create patterns not possible with traditional methods. By adjusting the ultrasonic wave pattern we can steer the fibres as the print progresses, producing a complex 3D architecture of fibres rather than layers of 2D structures.

One of the particularly useful features of the ultrasonic alignment process is that almost any type, size or shape of fibre can be used. This will give product designers some completely new possibilities and allow the printing of smart materials that can repair themselves or harvest electricity from the environment. For example researchers are working on embedding networks of hollow tubes filled with uncured polymer into composites. If the material is damaged and the tubes are broken open they will “bleed” polymer that will then set and “heal” the product. These tubes could be positioned in the liquid plastic with our ultrasonic printing system.