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1 Copyright © 2020 by ASME
Proceedings of the ASME 2020
Internal International Symposium on Flexible Automation
ISFA2020
July 5-9, 2020, Chicago, IL, USA
ISFA2020-9654
ACTIVE INTERLAYER HEATING FOR SUSTAINABLE SELECTIVE LASER SINTERING
WITH RECLAIMED POLYAMIDE 12 POWDERS
Feifei Yang, Tianyu Jiang, Xu Chen1
Department of Mechanical Engineering
University of Washington
Seattle, Washington, 98195
Greg Lalier, John Bartolone
Unilever Research & Development
Trumbull, Connecticut, 06611
ABSTRACT
Selective laser sintering (SLS) technology produces a
substantial amount of un-sintered polyamide 12 powders after
the manufacturing process. Failure to recycle and reuse these
aged powders not only leads to economic losses but also is
environmentally unfriendly. This is particularly problematic for
powder particles close to the heat-affected zones that go through
severe thermal degradations during the laser sintering
processes. Limited procedures exist for systematically reusing
such extremely aged powders. This work proposes a systematic
method to maximize reusability of aged and extremely aged
polyamide 12 powders. Building on a previously untapped
interlayer heating, pre-processing, and a systematic mixing of
powder materials, we show how reclaimed polyamide 12
powders can be consistently reprinted into functional samples,
with mechanical properties even superior to current industrial
norms. In particular, the proposed method can yield printed
samples with 18.04% higher tensile strength and 55.29% larger
elongation at break using as much as 30% of extremely aged
powders compared to the benchmark sample.
Keywords: Selective laser sintering; Powder reuse; Powder
aging and degradation; Interlayer heating; Sustainability
INTRODUCTION
Selective laser sintering (SLS) is one of the most well
established and commonly used additive manufacturing (AM)
techniques to rapidly manufacture three-dimensional
components [1-4]. Polymeric powders, semicrystalline or
amorphous, are the first and still the most widely applied
materials in SLS [1,5,6]. Parts printed using amorphous polymer
powders are partially consolidated, and consequently can be
1
Contact author: chx@uw.edu
useful for applications when the strength and durability of parts
are not dominant [7]. On the contrary, parts printed using
semicrystalline polymer powders are fully consolidated with
high mechanical strength and effectively weakened warpage.
Among the semicrystalline polymers, polyamide families are
most popular for SLS, and polyamide 12 dominates the market
because of the capability to generate strong parts for common
applications [3,4,8].
Despite the popular applications of the polyamide 12
powders in SLS, the volume ratio of powders that translate to
parts is small (5% - 15%). The 85% to 95% residual powders
went through deteriorate physical and chemical degradations in
the intricate fabricating processes [3,9], but have the potential to
be recycled and reused for further applications [3,4,10,11].
However, the deteriorated powders have reduced surface
morphologies, larger and more complex molecular chains,
decreased flowability, and deteriorated mechanical and thermal
properties, which make it challenging to reuse them directly
[3,4,10-12]. Also, polyamide 12 powder is relatively expensive,
priced around $150/kg in 2019 [9]. Abandonment of the residual
polyamide 12 powders can cause not only economic losses but
also environmental pollution. Thus, reclaim and reuse are
difficult yet necessary for a sustainable SLS AM.
Relevant works on the reuse of the reclaimed polyamide 12
powders have been reported in recent years. L. Feng et al. [9]
reclaimed polyamide 12 from SLS and made the powders into
filaments for fused deposition modeling (FDM). Clarifying the
aging mechanisms on thermal behavior, coalescence behavior
and the resulting crystallinity, microstructure and mechanical
properties, and investigating systematic aging mechanisms and
microstructural evolution, P. Chen et al. [3] and S. Dadbakhsh et
al. [4] characterized the aging process of polyamide 12 powders
2 Copyright © 2020 by ASME
in SLS. Dotchev et al. [10], Wegner et al. [13] and Josupeit et al.
[14] verified the decreased flowability of the reclaimed
polyamide 12 powders compared to new ones through a melt
volume rate (MVR) index. The effect of powder reuse on
mechanical properties has been studied by R.D. Goodridge et al.
[15] and K. Wudy et al. [16], who pointed out the changes of
tensile strength as well as elongation at break in parts built from
aged polyamide 12 powders. In addition, the relationships
between preheating temperature [13], energy density [13,17],
combined dwelling time between layers, energy density [18], and
part quality were studied.
Although existing efforts have sought to understand the
aging mechanisms and reuse of the degraded polyamide 12
powders, the possibility and feasibility of reusing the residual
polyamide 12 powders remain not fully exploited. In particular,
the reuse of the extremely aged polyamide 12 powders close to
the heat-affected zones (HAZs)
2
has not been reported. This
paper seeks to bridge the missing link and proposes a new
method, hereby referred to as active interlayer heating, to build
parts with the reclaimed and extremely aged polyamide 12
powders. We discuss systematic approaches to reuse the
polyamide 12 powders of different degradation levels, different
mixing percentages, and different combinations with multiple
layer printing and superior mechanical properties. The results
show that reclaimed polyamide 12 powders can be consistently
reprinted into functional samples, with mechanical properties
comparable or even superior to current industrial norms.
1 Proposed active interlayer heating for reclaimed
polyamide 12 powders
The main procedures of the proposed method include
powder collection, powder preprocessing, powder mixing,
powder characterizations, parameter control, SLS with interlayer
heating, and part characterizations, of which the flowchart is
presented in Figure 1.
1.1 Materials sample preparation
Materials sample preparation includes powder collection,
powder preprocessing and powder mixing. We collected
polyamide 12 powders purchased from EOS Corp. and reclaimed
from standard SLS processes on an EOS P 390 machine. The
powders cover three levels of degradation: (i) new powders
without heat treatment, (ii) aged powders located far away from
the HAZs during SLS and currently reused in the industry, and
(iii) extremely aged powders located close to the HAZs and not
being actively reused in SLS.
The extremely aged powders could not be coated smoothly
in the SLS chamber because of the existence of the aggregated
large particles, as a result of striking drop in flowability. In this
work, a sieving process was applied to the extremely aged
powders prior to printing. This process was done in a fume hood
using a sieve with the mesh size of 200 µm to grind the powders.
The extremely aged powders after the sieving process were
coated smoothly in the SLS chamber.
2
Areas close to the laser-material interaction during sintering.
Powder samples
Powder mixing
Powder preprocess
Powder characterizations
Parameter control
Part characterizations
Interlayer heating based SLS
New polyamide 12 powder
Aged polyamide 12 powder
Extremely aged polyamide 12 powder
Sieving process
Pure powders
New-aged mixed powders
New-extremely aged mixed powders
New-aged-extremely aged mixed powders
Particle size/shape analysis
Scanning electron microscope (SEM)
Differential scanning calorimetry (DSC)
Fourier-transform infrared spectroscopy (FTIR)
X-Ray Diffraction (XRD)
Preheating temperature
Laser power
Laser speed
Scan spacing
Layer thickness
Layer numbers of samples
Tensile strength
Printing of pure powders
Printing of benchmark sample
Printing of mixed powders without interlayer heating
Printing of mixed powders with interlayer heating
Proposed method for recycling of aged and extremely aged polyamide 12 powders
Elongation at break
Young's modulus
Figure 1: PROPOSED METHOD FOR REUSING AGED AND
EXTREMELY AGED POLYAMIDE 12 POWDERS
Four different groups of powders were used in experiment
(as shown in Figure 1): pure powders, mix of new and aged
powders, mix of new and extremely aged powders, and mix of
new, aged and extremely aged powders, to be specific.
1.2 Powder characterizations
The particle size/shape distribution of new, aged, and
extremely aged polyamide 12 powder samples were examined
using a PartAn 3-D particle size and shape analyzer. To get
reasonable test results, over 500,000 particles were measured in
each experiment. Scanning electron microscope (SEM) was
carried out to evaluate the variations in micro surface
morphologies of the collected powders. We used a Teneo
LVSEM and coated the samples with a thickness of 3 nanometers
of Pt/Pd to increase material conductivity. To measure the
thermal properties, we conducted differential scanning
calorimetry (DSC) on the powder samples of different
degradation degrees using a TA Instruments DSC Q20. To
3 Copyright © 2020 by ASME
measure the differences of molecular components and the
chemical microstructures between new and reclaimed materials,
we applied Fourier-transform infrared spectroscopy (FTIR) to
different powder samples. We also use X-Ray Diffraction (XRD)
to check crystalline properties of different polyamide 12
materials.
1.3 Parameter control
As shown in Figure 1, six parameters that have significant
influences on part qualities were controlled in this study:
preheating temperature, laser power, laser speed, scan spacing,
layer thickness, and layer numbers. These parameters were
accurately controlled by our in-house developed SLS testbed
(Figure 2) in experiment. The selected optimal parameter
settings suitable for powders of different degradation levels were
160 °C preheating temperature; 3000 mm/s scan speed; 18 W
laser power; 0.3 mm scan spacing; and 150 µm layer thickness.
In our testing, we printed 10-layer tensile bar samples for each
powder combination.
Figure 2: IN-HOUSE BUILT AND CUSTOMIZED SLS
TESTBEDS
1.4 SLS with interlayer heating
The proposed SLS with interlayer heating applies carefully
controlled thermal energy to the specimens being printed in-
between each printed layer. In this process, powder materials
were controlled to maintain at a preheating temperature (160 °C
for polyamide 12) for a certain heating time. In this work, we
tested 0 second (no interlayer heating) and 60 seconds interlayer
heating to powders and powder mixtures with different
degradation levels to explore the influences of interlayer heating
on part mechanical properties. Through the experiment results, it
was verified that 60 seconds interlayer heating can provide the
10-layer samples with enough heat and energy.
We carried out a series of continuous experiments to explore
the reusability of aged and extremely aged powders and the
influences of interlayer heating on part properties, as shown in
Figure 3. The experiments were separated into six stages, with a
distinct objective for each stage. In stage 1, pure powders were
used to verify the feasibility of multi-layer printing with
reclaimed powders. In stage 2, the currently available industrial
reuse combination (50%-50% new-aged mixed powders) were
used to print the benchmark samples of which the mechanical
properties were regarded as the baselines in this research. The
aim of stage 3 was to check the part mechanical properties when
using new-extremely aged mixed powders with the percentages
of extremely aged powders increasing. In stage 4, to examine the
influences of interlayer heating on part mechanical properties, all
the mixed powder combinations in stages 2 and 3 were reapplied
and were printed with 60 seconds interlayer heating. By
comparing the mechanical properties of samples in stages 2, 3,
and 4, influences of interlayer heating on part properties were
obtained. In stage 5, different from the previous stages using the
combinations of two kinds of powders, parts were printed with
mixed powders composed of three types of powders to verify the
feasibility and potential benefits of this practice. To ensure the
part quality, the combinations of 30% new - 30% aged - 40%
extremely aged powders and 30% new - 40% aged - 30%
extremely aged powders were selected to print. To examine the
influences of interlayer heating, the experiments in stage 5 were
repeated with 60 seconds interlayer heating in stage 6.
The SLS with interlayer heating method can provide enough
heat and energy to promote the coalescence behaviors of
reclaimed powders and improve part densification, and the
mixed new-reclaimed powders can improve the diverse grain
sizes in reclaimed materials, which are both helpful to improve
part tensile strengths when using reclaimed polyamide 12
powders. The method can also explore the influences of
interlayer heating and powder qualities on part microstructures,
like crystalline ratios and crystal sizes, and part elongation at
break.
1.5 Part characterizations
Tensile tests were carried out on all the SLS fabricated
samples. The samples were designed based on ASTM standards
and were stretched for each tensile measurement using an Instron
5869 Electromechanical testing system with a maximum load
frame capacity of 50 kN equipped with Blue Hill control
software. A polishing treatment was done on both the top and
bottom surfaces of part to remove any skirmish un-sintered
particles prior to the tensile testing for accurate measurement.
2 Results and discussions
2.1 Powder test results
The particle size distributions show that the percentages of
the particles with the diameter greater than 100 µm are 21%,
22%, and 24% for new powder, aged powder, and extremely
aged powder, respectively, showing a mild increase after the
physical/chemical degradations. The obtained particle shape
distributions show that the percentages of particles with good
sphericity (0.9~1.0) for new, aged, and extremely aged powders
are 21%, 16%, and 14%, respectively. The sphericity of the
reclaimed powder decreases mildly, which eventually increases
part surface roughness. In other words, the percentage of
particles with non-spherical and irregular shapes or rough edges
increases.
The SEM results show that with a 500x magnification, few
aggregated large size particles were found in new powders, while
there were more large size particles found in aged powders and
extremely aged powders as aggregation exacerbated. At a 2000x
4 Copyright © 2020 by ASME
magnification, there were no cracks in new powders, obvious
cracks in aged powders, and more cracks in extremely aged
powder in the SEM scope. These cracks were attributed to the
evaporation of moisture or alcohol during the sintering cycles or
repeated expansions/shrinkages during multiple times recycling.
Stage 1
Stage 2
Stage 3
100% new powder
100% aged powder
To verify the feasibility of
multi-layer printing with
reclaimed powders
No
No
50% new + 50% aged
To print the benchmark
sample No
No
100% extremely aged
powder No
To evaluate part properties
when using new-
extremely aged mixed
powders
70% new + 30%
extremely aged
60% new + 40%
extremely aged
50% new + 50%
extremely aged
40% new + 60%
extremely aged
30% new + 70%
extremely aged
20% new + 80%
extremely aged
10% new + 90%
extremely aged
No
No
No
No
No
No
Stage 4
60 s
To examine the influences
of interlayer heating when
using two kinds of
powders
70% new + 30%
extremely aged
60% new + 40%
extremely aged
50% new + 50%
extremely aged
40% new + 60%
extremely aged
30% new + 70%
extremely aged
20% new + 80%
extremely aged
10% new + 90%
extremely aged
60 s
60 s
60 s
60 s
60 s
60 s
Stage 5 30% new + 30% aged +
40% extremely aged
30% new + 40% aged +
30% extremely aged
To verify the feasibility of
printing with new-aged-
extremely aged powders
No
No
Stage 6 30% new + 30% aged +
40% extremely aged
30% new + 40% aged +
30% extremely aged
To examine the influences
of interlayer heating using
three kinds of powders
60 s
60 s
Objectives Combinations Interlayer
heating
50% new + 50% aged 60 s
Figure 3: EXPERIMENT PLANS ABOUT REUSABILITY OF
AGED AND EXTREMELY AGED POWDERS AND INFLUENCES
OF INTERLAYER HEATING (THE MIXED POWDERS ARE IN
VOLUME PERCENTAGES)
The DSC test results exhibit that the onset melting
temperatures decrease for reclaimed powders (in the heating
cycle), which is important knowledge for parameter settings of
SLS printing when the aged or extremely aged powders are used.
The setting principle of the preheating temperature is that the
preheating temperature should be set close to the material onset
melting temperature or melting point [12,16,17]. The decrease of
onset melting temperature of aged and extremely aged powders
gave us significant instructions when printing aged or extremely
aged powder because the appropriate setting of preheating
temperature is dominant to keep the sintering processes going
and to ensure the part quality.
FTIR test results present that peaks at the vibrational
frequencies of 1369.23, 1159.03, 1062.60 and 948.82 cm−1
slightly decreased in aged powders and extremely aged powders
compared to new powders, which was a sign of oxidation
reactions. Therefore, like the material aging after the sintering
process, the aging mechanisms of the recycled powders impacted
by the existence of laser and high temperature are thermal
oxidations.
The XRD test results of new, aged, and extremely aged
polyamide 12 powders presented similar behaviors and are in
different colors. The XRD peak at 10.98° was selected as a
reference of crystallization. These curves all exhibited α-
structures with very high crystallinities, unstable structures and
oriented in an anti-parallel manner.
2.2 Part test results
Based on the proposed method, we printed successfully
tensile bars with 10 layers of powders. Using differently
degraded materials, the same series of optimized parameters
were applied to all the designed experiments to ensure the
equivalent processing conditions. Tensile test was done, and the
results are analyzed in this section.
Known from the stress-strain curves of samples printed
using pure powders, the average tensile strengths of samples
printed using pure new, aged and extremely aged powders are
respectively 22.96 Mpa, 18.12 Mpa, and 11.08 Mpa, and that of
samples using aged and extremely aged powders decrease by
21.09% and 51.75% compared to new ones. The average
Young’s modulus of samples printed using pure new, aged, and
extremely aged powders are respectively 503.67 Mpa, 358.50
Mpa, and 177.00 Mpa, and that of samples using aged and
extremely aged powders decrease by 28.82% and 64.86%
compared to new ones. The average elongations at break of
samples using pure new, aged, and extremely aged powders are
respectively 5.31%, 12.12%, and 9.56%, and that of samples
using aged/extremely aged powders increase by 56.20% and
44.46% compared to new SLS parts. The reason for this is
attributed to that the reclaimed powders have smaller crystal size,
increased flexibility and decreased brittleness. Inversely, parts
using new powders with larger crystal size are easier to break
down before the separation of crystals, with decreased flexibility.
The samples printed using the 50%-50% new-aged mixed
powders are taken as the benchmark samples, of which the
mechanical properties are baselines in the proposed method.
Calculating the average mechanical properties of the benchmark
samples, the baselines of tensile strength, Young’s modulus and
5 Copyright © 2020 by ASME
elongation at break are respectively 25.80 Mpa, 568 Mpa, and
11.36%.
Samples using new-extremely aged mixed powders (from
70%-30% new-extremely aged mixed to 10%-90% new-
extremely aged mixed) without interlayer heating were printed
in stage 3. Known from the stress-strain curves of these samples
that with the percentages of extremely aged powders increasing
from 30% to 90%, the average tensile strengths of samples are
24.69 Mpa, 29.18 Mpa, 25.32 Mpa, 24.44 Mpa, 20.93 Mpa,
22.65 Mpa and 29.97 Mpa, and the average elongations at break
are 7.10%, 7.76%, 8.60%, 8.43%, 12.33%, 13.38% and 15.36%.
From an overall perspective, the average tensile strength of parts
from the combination of 60%-40% new-extremely aged mixed
powders to the combination of 30%-70% new-extremely aged
mixed powders show decreases. But this didn’t apply to the
remaining mixing percentages, which can be ascribed to that the
mechanical properties of samples are not only related to the
properties of powders but also relevant to the thermal or laser
conditions in the sintering chamber. However, the elongations at
break increase with the increasing of extremely aged powders
because the microstructure changes.
The comparisons of stress-strain curves of benchmark
samples and samples using new-extremely aged mixed powders
with and without interlayer heating are shown in Figure 4. As
shown, the tensile strengths of some samples remain no
significant changes after interlayer heating, while that of the
other samples increase. In Figure 4 (a), (b), (c), (e), (f), the tensile
strengths of samples with 60 seconds interlayer heating
increased by 25.19%, 36.10%, 30.13%, 5.46% and 22.51%.
Thus, the tensile strengths of samples using mixed powders can
be improved to some extent after interlayer heating because of
the better melting and coalescence behaviors of particles on each
layer.
0 2 4 6 8 10 12 14
0
5
10
15
20
25
30
35
25.80 Mpa
23.62 Mpa
25.58 Mpa
28.19 Mpa 31.22 Mpa
31.93 Mpa
Stress/Mpa
Strain/%
60 s interlayer heating + sample 1
60 s interlayer heating + sample 2
60 s interlayer heating + sample 3
no interlayer heating + sample 1
no interlayer heating + sample 2
no interlayer heating + sample 3
50 % new + 50 % aged 33.76 Mpa 32.30 Mpa
25.19%
(a) 50% new + 50% aged
0246810
0
5
10
15
20
25
30
35
40
33.61 Mpa
37.20 Mpa
30.02 Mpa
25.99 Mpa
70 % new + 30 % extremely aged
Stress/%
Strain/%
no interlayer heating + sample 1
no interlayer heating + sample 2
60 s interlayer heating + sample 1
60 s interlayer heating + sample 2
23.40 Mpa 24.70 Mpa
36.10%
(b) 70% new + 30% extremely aged
0 2 4 6 8 10
0
5
10
15
20
25
30
35
40
45
30.13%
29.18 Mpa
37.97 Mpa
42.26 Mpa
38.56 Mpa
33.10 Mpa
27.67 Mpa
Stress/Mpa
Strain/%
no interlayer heating + sample 1
no interlayer heating + sample 2
60 s interlayer heating + sample 1
60 s interlayer heating + sample 2
60 s interlayer heating + sample 3
60% new + 40% extremely aged
30.69 Mpa
(c) 60% new + 40% extremely aged
0 2 4 6 8 10
0
5
10
15
20
25
30 50% new + 50% extremely aged
Stress/Mpa
Strain/%
no interlayer heating + sample 1
no interlayer heating + sample 2
no interlayer heating + sample 3
60 s interlayer heating + sample 1
60 s interlayer heating + sample 2
60 s interlayer heating + sample 3
(d) 50% new + 50% extremely aged
6 Copyright © 2020 by ASME
0246810
0
5
10
15
20
25
30
5.46%
26.39 Mpa
25.02 Mpa
26.90 Mpa
25.87 Mpa
23.85 Mpa
25.59 Mpa
40% new + 60% extremely aged
Stress/Mpa
Strain/%
no interlayer heating + sample 1
no interlayer heating + sample 2
no interlayer heating + sample 3
60 s interlayer heating + sample 1
60 s interlayer heating + sample 2
25.62 Mpa
(e) 40% new + 60% extremely aged
0 2 4 6 8 10 12 14
0
5
10
15
20
25
30
20.93 Mpa
20.16 Mpa
21.71 Mpa 21.17 Mpa
27.10 Mpa
Stress/Mpa
Strain/%
60 s interlayer heating + sample 1
60 s interlayer heating + sample 2
60 s interlayer heating + sample 3
no interlayer heating + sample 1
no interlayer heating + sample 2
30 % new + 70 % extremely aged 28.64 Mpa 25.64 Mpa
22.51%
(f) 30% new + 70% extremely aged
0 2 4 6 8 10 12 14 16
0
5
10
15
20
25 23.61 Mpa
21.66 Mpa 22.19 Mpa
Stress/Mpa
Strain/%
60 s interlayer heating + sample 1
60 s interlayer heating + sample 2
no interlayer heating + sample 1
no interlayer heating + sample 2
20% new + 80% extremely aged
22.71 Mpa
(g) 20% new + 80% extremely aged
0 5 10 15 20
0
5
10
15
20
25
30
35
Stress/Mpa
Strain/%
60 s interlayer heating + sample 1
60 s interlayer heating + sample 2
60 s interlayer heating + sample 3
no interlayer heating + sample 1
no interlayer heating + sample 2
no interlayer heating + sample 3
10% new + 90% extremely aged
(h) 10% new + 90% extremely aged
Figure 4: COMPARISONS OF STRESS-STRAIN CURVES OF
BENCHMARK SAMPLES AND SAMPLES USING NEW-
EXTREMELY AGED MIXED POWDERS WITH AND WITHOUT
INTERLAYER HEATING
Comparisons of stress-strain curves of samples using new-
aged-extremely aged mixed powders with and without interlayer
heating show that after 60 seconds interlayer heating, the tensile
strengths of samples using 30%-30%-40% new-aged-extremely
aged mixed powders increase by 1.19%, and that of samples
using 30%-40%-30% new-aged-extremely aged mixed powders
increase by 18.04%. It is concluded that the tensile strengths of
samples using new-aged-extremely aged mixed powders can be
improved after interlayer heating.
2.3 Discussions
We compare here sample tensile strengths without interlayer
heating and with 60 seconds interlayer heating. The baseline
tensile strength comes from the samples printed from the 50%-
50% new-aged blend without interlayer heating, i.e., 25.80 Mpa.
For samples without interlayer heating, the blends of which the
tensile strengths are larger than the baseline are 40%-60% new-
extremely aged, 30%-30%-40% new-aged-extremely aged, and
10%-90% new-extremely aged. When the percentages of
extremely aged powders increased from 50% to 80%, the
averaged tensile strengths decrease. The reasons are that it is
more difficult to move dislocations through and between the
grains when there are more extremely aged powders. For
samples with interlayer heating, it is concluded that there are
three blends with 60 seconds interlayer heating of which the
tensile strengths are larger than the baseline (25. 80 Mpa), 50%-
50% new-aged, 40%-60% new-extremely aged and 30%-30%-
40% new-aged-extremely aged (56.78%, 25.19% and 24.69%
better than baseline). From an overall perspective, samples using
more new powders have larger tensile strengths.
The comparisons of sample elongations at break are shown
in Figure 5, (a) without interlayer heating, (b) with 60 seconds
interlayer heating. The baseline of elongation at break is also
from the samples printed using 50%-50% new-aged blend
without interlayer heating, 11.36%. Known from Figure 5 (a) and
7 Copyright © 2020 by ASME
(b), there are several blends that have similar or larger values of
elongations at break compared to the baseline. For the samples
without interlayer heating, the samples using more extremely
aged powders have larger elongation. The largest elongation is
from the 10% new - 90% extremely aged blend (35.30% better
than baseline), for increased extremely aged powders yields the
smaller crystal size, and flexibility increases. The other mixing
percentages of samples without interlayer heating of which the
elongations at break are better than baselines are 30%-30%-40%
new-aged-extremely aged and 30%-40%-30% new-aged-
extremely aged mixed blends. For the samples with interlayer
heating, the largest elongation is from the 30%-40%-30% new-
aged-extremely aged mixed blend (64.63% better than baseline),
of which the elongation at break increases by 55.29% after
interlayer heating (from 11.63% to 18.06%) for interlayer
heating enhances bonding and microstructures.
Sample 1 Sample 2 Sample 3
0
6
8
10
12
14
16
18
20
Average:
11.63%
Average:
11.81%
Average:
15.37%Average:
13.71%
Average:
12.43%
Elongation at break/%
50% aged + 50% new 30% extremely aged + 70% new
40% extremely aged + 60% new 50% extremely aged + 50% new
60% extremely aged + 40% new 70% extremely aged + 30% new
80% extremely aged + 20% new 90% extremely aged + 10% new
30% new + 30% aged + 40% extremely aged
30% new + 40% aged + 30% extremely aged
Average:
11.36%
No interlayer heating
(a) Without interlayer heating
Sample 1 Sample 2 Sample 3
0
8
10
12
14
16
18
20
22
24
26
Average:
18.06%
Average:
16.59%
Average:
13.04%
Average:
11.06%
Elongation at break/%
50 % aged + 50 % new 30 % extremely aged + 70 % new
40 % extremely aged + 60 % new 50 % extremely aged + 50 % new
60 % extremely aged + 40 % new 70 % extremely aged + 30 % new
80 % extremely aged + 20 % new 90 % extremely aged + 10 % new
30 % new + 30 % aged + 40 % extremely aged
30 % new + 40 % aged + 30 % extremely aged
60 s interlayer heating
Average:
10.97%
(b) With 60 seconds interlayer heating
Figure 5: COMPARISONS OF SAMPLE ELONGATIONS AT
BREAK
In addition, parts with high percentages of extremely aged
powders have lower Young’s modulus. Also, the Young’s
modulus of samples can be controlled consistently at the same
level as the standard benchmark samples (50%-50% new-aged
mixed blend) in the proposed method.
3. Conclusions
A comprehensive method was proposed in this paper to
explore the possibility and feasibility of reusing the differently
degraded polyamide 12 powders in different combinations,
especially the extremely aged polyamide 12 powders close to the
heat-affected zones. The proposed method was successfully
applied to the practices of reusing reclaimed polyamide 12
powders into functional samples, with mechanical properties
even superior to current industrial norms.
Tensile test results show that parts with higher tensile
strength (e.g. 56.78% better than the baseline) and larger
elongation (e.g. 35.30% better than the baseline) are obtained. In
particular, the proposed method yields printed samples with
18.04% higher tensile strength and 55.29% larger elongation at
break using as much as 30% of extremely aged powders
compared to the benchmark sample. Besides, Young’s modulus
of samples is controlled consistently at the same level as the
standard benchmark samples.
ACKNOWLEDGEMENTS
The work was supported in part by a research grant from
Unilever and by NSF award 1953155.
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