Inverted laser enables SLS with multiple materials and without powder bed

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Laser beam transmitting upwards through glass. (Image: John Whitehead/Columbia Engineering)

Researchers from the University of Columbia have achieved selective laser sintering with multiple materials and without the use of a powder bed through the use of an inverted laser.

The technique, published in Additive Manufacturinghas the potential to print embedded circuits, electromechanical components and robot components, according to the scientists.

Selective laser sintering traditionally involves fusing together particles using a laser pointing downward into a heated print bed in order to build an object layer by layer.

The process works well if there is just one material used in the printing process, however using multiple materials in a single print has previously been very challenging, as once the powder layer is deposited onto the bed, it cannot be unplaced, or replaced with a different powder.

In addition, according to John Whitehead, a PhD student and lead author on the paper, in a standard printer, because each of the successive layers placed down are homogeneous, the unfused material obscures the object being printed from view, until the finished part is removed at the end of the cycle. This means that a print failure won’t necessarily be found until the print is completed, wasting time and money.

By using an inverted laser and multiple transparent plates, each coated with a thin layer of a different plastic powder, the researchers have devised a new SLS technique that eliminates the need for a powder bed entirely. 

A print platform is lowered onto the upper surface of one of the powders, and a laser beam is directed up from below the plate and through the plate’s bottom. This process selectively sinters some powder onto the print platform in a pre-programmed pattern according to a virtual blueprint. The platform is then raised with the fused material, and moved to another plate, coated with a different powder, where the process is repeated. This allows multiple materials to either be incorporated into a single layer, or stacked. Meanwhile, the old, used-up plate is replenished.

In their paper, the team demonstrated a working prototype of their setup by generating a 50 layer thick, 2.18mm sample out of thermoplastic polyurethane (TPU) powder with an average layer height of 43.6μm and a multi-material nylon and TPU print with an average layer height of 71μm. These parts demonstrated both the feasibility of the process and the capability to make stronger, denser materials by pressing the plate hard against the hanging part while sintering.

Dual thermoplastic SLS print sample. (Image: John Whitehead/Columbia Engineering)

‘This technology has the potential to print embedded circuits, electromechanical components, and even robot components,’ said Hod Lipson, a professor of mechanical engineering who co-authored the paper. ‘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. 

‘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.’

The researchers are now experimenting with metallic powders and resins in order to directly generate parts with a wider range of mechanical, electrical, and chemical properties than is possible with conventional SLS systems today.

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