SAMPE Journal - January/February 2022 - 38

FEATURE / NEW POLYARYLETHERKETONES (PAEK) ALTERNATIVES
are believed to be responsible for isotropy of
mechanical properties, structural integrity and
strength, properties that are achieved in injection
moulding PEEK parts. When Yi et al16
determined
the Avrami exponents of PEEK 151 FF printed
parts, the values were in the range of 1-2, i.e. not
showing development of spherulites. This finding
proves that when an injection moulding optimised
polymer is processed in FF, the different thermal
condition of FF such as slower cooling and overall
lower
processing temperatures
than
injection
Figure 10. Comparison of 450G PEEK and AM 200 properties15
.
of the approach used, a slow crystallising polymer
is favourable in FF as it will maximise interlayer
bonding and will minimise crystallisation during
printing. Crystalline content is desired in AM
products for its effect on part properties but if
occurring too fast during printing, it might cause
deformation, warpage and shrinkage so severe to
cause complete failure of a building job. In this
way AM 200 has been designed to exhibit a slow
crystallisation behaviour while maintaining a level
of crystallinity typical of PEEK. Figure 8 shows the
crystallisation half-time of a typical PEEK grade
and AM 200. Crystallisation half-time is a standard
parameter to quantify the velocity of crystallisation
in semi-crystalline polymers. The longer the
crystallisation half-time, the slower the polymer
crystallises.
It is noteworthy that for any temperature shown
in the graph, AM 200 exhibits greater crystallisation
time, in other words it takes longer for the polymer
to develop crystallinity. This behaviour allows
AM 200 to be printed with both amorphous and
crystalline strategies according to the thermal
settings on the machine while also minimising
warpage during printing in both cases.
A more
in-depth
investigation
into
the
crystallisation kinetics of AM 200 versus PEEK
consists of determining, via a series of experiments
in DSC, the Avrami exponents of the polymers.
Avrami exponents are responsible for the
morphology of crystallites - the structural units
of the crystalline region. When the crystallites
develop in one- or two-dimensional shape, the
Avrami exponents have values within the 1-2 range,
if instead crystallites develop in a 3-dimensional
manner in a spherical geometry, called spherulites,
the Avrami exponents will be in the 3 and above
value range17
. PEEK polymers are well-known to
develop a spherulitic structure18-20. Spherulites
38 | SAMPE JOURNAL | JANUARY/FEBRUARY 2022
moulding do not lead to formation of spherulites,
which ultimately are responsible for high strength.
As spherulites are not formed, the expected
properties are not achieved. However, when Yi et
al16
evaluated the Avrami exponents for AM 200,
the values were around 3 and above, thus proving
development of spherulites.
It is possible that the slower crystallisation
behaviour of AM 200 not only leads to formation
of isotropic crystallites, but also allows to develop
such structure intra and inter-layer, which then
accounts for a stronger interlayer strength between
consecutive layers of a part. A better interlayer
strength especially for printed part built vertically
helps to reduce significantly the well-known weak
performance of AM polymeric part along the Z axis.
A further validation of the finding found via
crystallisation kinetics study on the different
behaviours of PEEK versus AM 200 when processed
in FF is shown in Figure 9.
Figure 9 shows the mechanical properties of
ISO527-1BA specimens printed vertically within
the 3D Gence system 34021
: AM 200 samples
showed higher strength and elongation of the PEEK
counterparts.
Further results on characterisation of
mechanical performance of specimens in 450G
built in FF and AM 200 are reported in Figure 10,
along with the typical data of an injection moulded
test specimen in PEEK.
As more data is published, AM 200 based
on LMPAEK polymer continues to show higher
strengths in printed samples, especially in the Z
direction compared to PEEK 450G.
LMPAEK CARBON FIBRE REINFORCED
GRADES
Direct Melt Extrusion of Granules
Printing of large components present some
challenges: bigger parts imply longer time, longer
tool paths for the nozzle and therefore increased
chance of localized unwanted crystallisation on
cooling that triggers warpage; but large parts also
retain more heat which can activate localized
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SAMPE Journal - January/February 2022

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