Surface World March 2019 SW_March_2019_LR | Page 20

IMF: THE INSTITUTE OF MATERIALS FINISHING
Electroforming vs 3D printing –
is it a disruptive technology?
Will 3D printing become
a disruptive technology
for one of the surface
engineering’s most
established technologies
– electroforming?
3D printing is exactly what the name implies
– the formation of 3 dimensional objects by
putting down material in a defined pattern.
It was first conceived in the 1980’s and is
also known as Rapid Prototyping (RP) or
Additive Manufacturing (AM). Since then it
has developed into an alternative method
of manufacturing complex shapes and
components in metals, polymers, ceramics
and glass. It is even used in one of the UK’s
greatest passions – food, where it is used
to create complex shapes using edible
constituents!
Despite being a relatively recently developed
technology and still finding new markets,
it is already well established in many sectors
that have traditionally used electroforming,
such as aerospace, manufacturing, medical
applications, prototyping and tooling.
To create a part by 3D printing, the
component needs to be designed on a
computer and converted to a machine code
that the printer can understand. This offers
an immediate advantage over conventional
electroforming, because the electronic
design can be tested for its performance
by computer simulation and Finite Element
Analysis (FEA). This allows any design flaws
to be identified and removed prior to any
manufacturing taking place. In doing so,
it reduces both time to market for any new
designs and ensures the component if fit
for purpose, without incurring any
manufacturing costs.
Where metal fabrication is concerned, 3D
printing is carried out by a generic process
called Selective Laser Sintering (SLS), when it
is also known as Direct Metal Laser Sintering
(DMLS). The process is relatively straight
forward in that a baseplate platform is
coated with a thin layer of metal powder
which is then partially melted, or sintered,
usually by using a CO2 or infra-red laser.
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The baseplate is then lowered by a discrete
amount to allow it to be recoated with new
powder, which is then again selectively
sintered by the laser. This creates a porous
structure that has a reduced bulk density of
about 60% that of a fully solid component.
In some circumstances, this can be used
to the component’s advantage, such as in
prototyping an injection moulding die, where
porosity can be used to help improve the
mould’s thermal cycling and speed up the
mould cycle time.
An example of DMLS manufacture is shown
in Figure 1, at turbine assembly.
Fig. 1: DMLS manufactured turbine assembly
The major problems with SLS products
are porosity and surface roughness. There
are also numerous factors that affect the
preferred particle size of materials suitable
for SLS; if they are too small, the particles
may have excessive flow characteristics, not
sinter properly and melt. On the other hand,
if they are too big, the particles create
issues with surface roughness and porosity.
The preferred diameter range is between
20-80µm, but this can influence the height
by which the platform is moved, although
a movement of about 10-40µm is generally
used. One of the advantages of having
a low flowing powder is that the residual
material can support the structure as it is
being formed.
To improve the surface finish of a 3D printed
component, a technique known as Selective
Laser Melting (SLM) can be used. This is
very similar to SLS, but is designed for use
with metals. Unlike sintering, the powder is
melted, resulting in a much more dense final
product (Figure 2). Source: Sculpteo
MARCH 2019
Fig. 2: SLM manufactured valve assembly
There are an increasing number of metals
being used with 3D printing technologies,
but the most common are aluminium and
cobalt alloys, such as AlSi7Mg0.6 and
Cobalt chrome CoCrMo as well as stainless
steels (316 etc), maraging steels, titanium
alloys (eg TiAl4V), Inconel alloys and
HastalloyX. It is also possible to use brass
and sterling silver in SLM.
In SLM, the component is manufactured in
discrete layers of between 30-80µm,
although these layer thicknesses are
becoming smaller as the equipment becomes
more refined. However, unlike SLS, the
powder is fully melted and therefore the
process requires much higher temperatures
and processing times. Because SLM fully
melts the metal, it needs a higher
temperature and hence greater energy; it
also means that when compared against
DMLS, the metal takes longer to cool down.
SLM also creates other process issues, such
as those relating to metal oxidation. During
the melting process, the metal becomes
increasingly susceptible to oxidation, so SLM
is usually done under an inert atmosphere of
nitrogen or argon; any oxygen content is
normally limited to below 500ppm. SLM is
the technology for AM of titanium and
aluminium products, as these metals are very
susceptible to oxidation, which will result in a
major loss in performance.
There is also increasing interest in using SLM,
and SLS in the manufacture of jewellery, as it
can be used to produce customised articles
in gold, silver and other metals, almost “on
the spot” as shown in Figure 3. Source:
Savorsilver.com
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