Ultrasound Could Transform 3D Printing for a Future of Smart Materials

By Bruce Drinkwater
Bruce Drinkwater
Bruce Drinkwater
January 19, 2016 Updated: January 19, 2016

The advent of 3D printers supposedly means we can manufacture anything in our homes. But in reality most existing home 3D printers can only make things out of certain plastics, although there are industrial systems that can print certain metals.

What has so far been out of reach is a way to 3D print high-tech composite materials such as the carbon fiber composites that are used to build lightweight but extremely strong versions of things including tennis rackets, aerodynamic bikes and even aircraft parts. But researchers from my lab at 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 fibers set in a plastic matrix. It’s the presence of the fibers, and the fact that they are all carefully arranged, that makes these materials so impressively strong yet lightweight.

Simple Solution

At present, composite products are made by forming the fibers 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, my colleagues and I 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 fibers. 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.

Anyone for laser tennis? (Tom Llewellyn-Jones, Bruce Drinkwater and Richard Trask, Author provided)
Anyone for laser tennis? (Tom Llewellyn-Jones, Bruce Drinkwater and Richard Trask, Author provided)

The big challenge was working out how to reassemble the tiny fibers 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 fibers 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 fibers move to and align with low pressure regions in the field called nodes. The fibers are then fixed in place using a tightly focused laser beam that cures (sets) the polymer.

Smart Materials

The patterned fibers 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 fibers in liquid epoxy polymer that are formed into longer lines of fibers 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 fibers as the print progresses, producing a complex 3D architecture of fibers 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 fiber 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.

The ultrasonic technology is still in its early stages, so don’t expect to be able to buy these printers next week. But 3D printing is a very fast moving field so these ideas could well hit the market in the next few years.The Conversation

Bruce Drinkwater a professor of ultrasonics at the University of Bristol in the U.K. This article was originally published on The Conversation.