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New Laser Inversion Technique Allows 3D Printing of Multiple Materials

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Jul 28 2020

Three-dimensional (3D) printing—also called additive manufacturing—employs digital manufacturing procedures to create strong and lightweight components without requiring any unique tooling.

*Laser beam transmitting upwards through glass. Image Credit: Columbia Engineering.*

In the last 10 years, the field of additive manufacturing has grown exponentially, at a speed of over 20% per annum. This process prints pieces that span from car parts and aircraft components to dental and medical implants out of engineering polymers and metals.

Selective laser sintering (SLS), which is one of the most extensively used manufacturing procedures, uses a laser to print components out of micrometer-scale material powders: in this method, the particles are heated by the laser to the point where they combine together to develop into a solid mass.

Additive manufacturing is key to economic resilience. All of us care about this technology—it’s going to save us. But there’s a catch.

Hod Lipson, James and Sally Scapa Professor of Innovation, Department of Mechanical Engineering, Columbia University

The difficulty is that SLS technologies can print using only one material at a time: the whole component has to be made of only that one powder.

“Now, let me ask you,” continued Lipson, “how many products are made of just one material? The limitations of printing in only one material has been haunting the industry and blocking its expansion, preventing it from reaching its full potential.”

In an effort to solve this problem, Lipson and his PhD student John Whitehead applied their know-how in robotics to design a novel method to overcome these limitations in SLS technologies. They inverted the laser so that it points in an upward direction and this allowed the SLS technique to use numerous materials simultaneously.

The duo’s working prototype, together with a print sample containing a couple of different materials in the same layer, was newly published online by the Additive Manufacturing journal as part of its December 2020 issue.

Our initial results are exciting, because they hint at a future where any part can be fabricated at the press of a button, where objects ranging from simple tools to more complex systems like robots can be removed from a printer fully formed, without the need for assembly.

John Whitehead, Study Lead Author and PhD Student, Department of Mechanical Engineering, Columbia University

In the SLS process, material particles are usually combined together by using a laser that points in a downward direction into a heated print bed. Then, a solid object is created from scratch, while the printer places down a uniform powder layer and applies the laser to selectively combine certain material in the layer.

The printer subsequently deposits another layer of powder onto the first uniform layer of powder, and the laser merges the resultant new material to the material in the earlier layer. This process is performed repeatedly until the part is finished.

A process like this would work well only when a single material is employed in the printing procedure. However, it has been very difficult to use numerous materials in a single print because as soon as the layer of powder is deposited onto the bed, it cannot be substituted, or replaced, with another powder.

Also, in a standard printer, because each of the successive layers placed down are homogeneous, the unfused material obscures your view of the object being printed, until you remove the finished part at the end of the cycle.

John Whitehead, Study Lead Author and PhD Student, Department of Mechanical Engineering, Columbia University

Whitehead continued, “Think about excavation and how you can’t be sure the fossil is intact until you completely remove it from the surrounding dirt. This means that a print failure won’t necessarily be found until the print is completed, wasting time and money.”

The scientists decided to look for a method that completely prevents the necessity for a powder bed. They subsequently set up numerous transparent glass plates, which were coated with a thin layer of another type of plastic powder. A print platform was lowered onto the upper surface of one of the powders, and a laser beam was directed upwards from beneath the plate and via the bottom of the plate.

In this process, some amount of powder is selectively sintered onto the print platform in a pre-programmed pattern in accordance with a virtual blueprint. The print platform is subsequently raised with the merged material, and shifted to another plate, which is coated with another powder, in which the process is again carried out.

This makes it possible to either integrate or stack numerous materials into a single layer. In the meantime, the old and used-up plate is fully replenished.

In this study, the researchers described their working version by creating a 2.18-mm, 50-layer thick sample from thermoplastic polyurethane (TPU) powder. This sample had an average layer height of 43.6 µm. They also created a multi-material nylon and TPU print that had an average layer height of 71 µm.

Such components not only showed the viability of the process but also demonstrated the potential to produce thicker and stronger materials by simply pressing the plate strongly against the hanging part during the sintering process.

“This technology has the potential to print embedded circuits, electromechanical components, and even robot components. It could make machine parts with graded alloys, whose material composition changes gradually from end to end, such as a turbine blade with one material used for the core and different material used for the surface coatings,” noted Lipson.

“We think this will expand laser sintering towards a wider variety of industries by enabling the fabrication of complex multi-material parts without assembly. In other words, this could be key to moving the additive manufacturing industry from printing only passive uniform parts, towards printing active integrated systems,” Lipson concluded.

The team is currently working with resins and metallic powders to directly fabricate parts that have a broader range of chemical, electrical, and mechanical characteristics than is possible with present-day traditional SLS systems.