The EconomistNovember 16th 2019 Science & technology 732 corporated into the ink instead of carbon
fibres. That would please Greens, because
carbon fibres are usually made from oil-
based materials. Habib Dagher, one of the
project’s leaders, says the aim is to print
with a material containing 50% wood pro-
ducts. This would create a composite as
strong and light as aluminium. And with
further work the group hope to deposit that
material at 230kg an hour. The researchers
recently used cellulose fibres and a resin
made from maize to print a mould for con-
structing the roof of a boat. To add to the
greenery, this mould could be recycled and
the material used again.
Just press “print”
Making moulds and production tools will
be an important job for large-format 3d
printing, says Craig Blue, director of energy
efficiency at Oak Ridge. Tool-making is ex-
pensive for two reasons. It requires spe-
cialist skills. And the items produced tend
to be one-offs or to be made in small num-
bers, so there are no economies of scale.
For 3dprinters, however, the cost of mak-
ing one or many items is about the same.
There are other advantages. For exam-
ple, an Oak Ridge system was used by con-
tractors to print specially shaped moulds
for concrete castings on the façade of a 45-
storey building on the site of an old sugar
refinery in Brooklyn, New York. Usually,
such moulds are made out of wood by
skilled carpenters and might last only
three or four pourings, so builders get
through a lot of them. But, Dr Blue says, the
3d-printed versions, composed of carbon-
fibre-reinforced plastic, were able to sur-
vive at least 200 pourings.
Oak Ridge is also working on ways to
print concrete structures directly. The
practicalities of erecting massive 3d-print-
ing gantries suggest printing skyscrapers
and other large structures is probably best
not done in one go, but in smaller sections.
Printing precast concrete subunits in the
controlled conditions of a factory and then
assembling them on site can be better
suited to making complex and artistic
structures. This is the approach taken by
Xu Weiguo and his colleagues at Tsinghua
University in Beijing. They used a pair of
robotic arms that extrude concrete mixed
with polyethylene fibres to print precast
sections which were then assembled into a
26-metre footbridge that spans a pond in
an industrial park in Shanghai.
This structure is styled after the Anji
Bridge, a stone arch built around 600ad
across the Xiaohe river in Hebei province.
The replica took 450 hours to print—lei-
surely by the standards of Maine’s boat-
yard, but rapid compared both with the ten
years the original took and the pace at
which even modern building sites tend to
move. The researchers reckon production
costs were two-thirds that of making a sim-ilar bridge from conventionally cast con-
crete sections.
Other forms of 3dprinting are getting
bigger and faster, too. Chad Mirkin and his
colleagues at Northwestern University, Illi-
nois, have come up with something they
call high-area rapid printing (harp). Their
prototype can make things four metres tall,
with a cross section of nearly a square me-
tre. It does so by pulling these solid objects
out of a shallow pool of liquid polymer.
The printer scales up an existing indus-
trial process which starts with the liquid
polymer being held in a container with a
transparent base. An ultraviolet image of
the layers to be built is projected through
the base. This triggers a chemical reaction
which cures a corresponding layer of poly-
mer immediately above the base, so that it
solidifies into the image of the projected
light. The first layer attaches itself to a tool
lowered into the liquid from above. As the
tool is raised it lifts the object out of the
pool to permit subsequent layers to be add-
ed from below.
The innovation which harpbrings is
having a film of oil flow across the trans-
parent base. This oil, the researchers say,
behaves like “liquid Teflon”. It stops the
polymer layers sticking to the base and also
removes heat generated during curing. The
result is that the printer can run much fast-
er than was previously possible. It can, says
Dr Mirkin, print in a couple of hours an ob-
ject the size of an adult human being. A
conventional 3dprinter using this method
would require a couple of days.
The harpprocess allows a wide range of
materials to be printed at large scale, in-
cluding hundreds of different polymers,
each one of which could be hard, soft or
rubbery. It can also print resins that con-tain materials like silicon carbide, which
can be processed into hard-wearing heat-
resistant ceramics. Components made
from all these materials might be used in
products ranging from cars to aircraft to
buildings. The system can also be scaled up
further, adds Dr Mirkin. He has co-founded
a company, Azul 3d, to commercialise the
process and expects the first harpprinter
to be on the market in about 18 months.Heavy metal
The most difficult task 3dprinting faces,
though, is printing large metal objects. The
main way of printing in metal is to melt
successive layers of a metallic powder us-
ing a laser or an electron beam. To stop the
powder oxidising and being contaminated
by impurities in the air—or worse, explod-
ing—that process needs to be carried out in
a chamber filled with an inert gas. Scaling
this procedure up is tricky and would be ex-
ceedingly expensive.
Yet metal printing is also escaping the
box. One way it is doing so is by the deploy-
ment of large robots brandishing various
types of migwelders. “mig” stands for met-
al-inert gas. migwelders work by feeding a
sacrificial electrode made of wire through
the nozzle of a welding torch. The wire is
connected at one end to a supply of elec-
tricity and at the other to an earthed work-
piece. When the torch is held close to the
workpiece an electric arc forms between its
surface and the wire. The heat from the arc
causes the wire and adjacent metal to melt
and fuse together. During the process the
torch blows an inert gas, such as argon,
over the weld to protect it.
To turn a welding torch into a 3dprinter
the robot welds continuously over the
same area, building up layer after layer of
metal. This process is used by mx3d, a
Dutch 3d-printing company, to build a vari-
ety of metal objects. Appropriately for the
Netherlands, these include a lightweight
bicycle printed in aluminium and a 12-me-
tre long stainless-steel pedestrian bridge to
cross a canal in Amsterdam.
Relativity Space, a firm in Los Angeles,
is using large continuous-welding robots
to build parts for space rockets (see picture
on previous page). Each robot has an alu-
minium-alloy wire fed along its arm to the
print head at its tip. The print head uses a
high-temperature plasma arc to melt the
wire and deposit it in layers whilst blowing
an inert shielding gas around the arc.
3d-printed rockets, Relativity Space
says, can be made faster and with fewer
parts than conventional ones. The com-
pany has big ambitions. Its first rockets
will be used to launch satellites but it hopes
eventually to use its production system,
which it calls Stargate, to print a rocket on
the surface of Mars. When it comes to won-
dering what 3dprinting is capable of, it
Cast off! seems, even the sky is not the limit. 7