Laser inversion enables multi-materials 3D printing

Date:
July 27, 2020
Source:
Columbia University School of Engineering and Applied Science
Summary:
Selective laser sintering is one of the most widely used processes
in additive manufacturing, but it is limited to printing with a
single material at a time. Robotics engineers have now developed a
new approach to overcome this limitation: By inverting the laser
so that it points upwards, they've invented a way to enable SLS
to use -- at the same time -- multiple materials.



FULL STORY ========================================================================== Additive manufacturing -- or 3D printing -- uses digital manufacturing processes to fabricate components that are light, strong, and require
no special tooling to produce. Over the past decade, the field has
experienced staggering growth, at a rate of more than 20% per year,
printing pieces that range from aircraft components and car parts to
medical and dental implants out of metals and engineering polymers. One of
the most widely used manufacturing processes, selective laser sintering
(SLS), prints parts out of micron-scale material powders using a laser:
the laser heats the particles to the point where they fuse together to
form a solid mass.


========================================================================== "Additive manufacturing is key to economic resilience," say Hod
Lipson, James and Sally Scapa Professor of Innovation (Mechanical
Engineering). "All of us care about this technology -- it's going to
save us. But there's a catch." The catch is that SLS technologies have
been limited to printing with a single material at a time: the entire
part has to be made of just that one powder.

"Now, let me ask you," Lipson continues, "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." Wondering how to solve this challenge, Lipson and his PhD student John Whitehead used their expertise in robotics
to develop a new approach to overcome these SLS limitations. By inverting
the laser so that it points upwards, they invented a way to enable SLS to
use -- at the same time - - multiple materials. Their working prototype,
along with a print sample that contained two different materials in the
same layer, was recently published online by Additive Manufacturing as
part of its December 2020 issue.

"Our initial results are exciting," says Whitehead, the study's lead
author, "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." Selective laser sintering traditionally
has involved fusing together material particles using a laser pointing
downward into a heated print bed. A solid object is built from the
bottom up, with the printer placing down a uniform layer of powder and
using the laser to selectively fuse some material in the layer. The
printer then deposits a second layer of powder onto the first layer,
the laser fuses new material to the material in the previous layer,
and the process is repeated over and over until the part is completed.

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

"Also," adds Whitehead, "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. 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 researchers decided to find a way to eliminate the need for a powder
bed entirely. They set up multiple transparent glass plates, each coated
with a thin layer of a different plastic powder. They lowered a print
platform onto the upper surface of one of the powders, and directed a
laser beam 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 the paper, the team demonstrated their working prototype by generating
a 50 layer thick, 2.18mm sample out of thermoplastic polyurethane (TPU)
powder with an average layer height of 43.6 microns and a multi-material
nylon and TPU print with an average layer height of 71 microns. 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.

"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,"
Lipson notes. "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.


========================================================================== Story Source: Materials provided by Columbia_University_School_of_Engineering_and_Applied Science. Original
written by Holly Evarts. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. John Whitehead, Hod Lipson. Inverted multi-material laser sintering.

Additive Manufacturing, 2020; 36: 101440 DOI:
10.1016/j.addma.2020.101440 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200727145805.htm

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