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Process control of surface quality and part microstructure in selective laser sintering involving highly degraded polyamide 12 materials

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Abstract and Figures

Polyamide 12 is one of the most extensively used semi-crystalline polymer materials to date in selective laser sintering (SLS) additive manufacturing, or SLS 3D printing. In this powder-based direct digital manufacturing process, a substantial amount of expensive materials remains un-sintered, recyclable, and reusable. Recently, understanding the mechanisms of degradation and the reusability of reclaimed polyamide 12 powders has attracted increasing industrial and research interests. However, using reclaimed polyamide 12 powder in SLS results in problems with part surface quality such as undesirable part surface finish with poor textures and numerous un-sintered particles. Limited research is available on the improvement of part surface quality. In particular, results barely exist on improving or modifying the surface quality of parts using extremely aged powders – powders that are held close to the heat-affected zones (HAZs) and suffer from severe degradations during the sintering process. To improve the surface quality and to build interrelations between process parameters and surface quality, we propose a novel approach for SLS with (extremely) aged polyamide 12 powders. By combining material preparation, powder and part characterizations, and SLS with customized post-heating, we obtain parts with improved surface quality (e.g., reduced roughness and porosities, and eliminated un-sintered particles). Particularly, parts 3D-printed using the 30%-30%-40% new-aged-extremely-aged powder mixtures exhibit the smoothest and flattest surface with no unmolten particles and nearly zero porosity.
SEM test results of polyamide 12 powders and parts using polyamide 12 powders 3.1.2. Thermal property Figure 5 presents the DSC curves of different polyamide 12 powders and parts, revealing changes in material thermal properties. For each sample, two heating cycles were conducted. In the first more dominant heating cycle (marked with blue line), the onset melting temperatures of different powders decrease with increasing powder degradations (Figure 5a), following the sequence: new powders (183 °C) > aged powders (181 °C) > extremely aged powders (180 °C). The result indicates a broaden melting trajectory for the reclaimed powders caused by the more regular chain-folded states and higher crystalline ratios. Such results are consistent with the previous studies [1, 9]. The peak melting temperatures for the aged powders and extremely aged powders are both larger than that of the new powders, suggesting a higher melting temperature for powders after aging. Such a result is also expected giving the high-melting-point pieces in the reclaimed powders. For the part samples, both the onset melting temperatures and peak melting temperatures decrease when the level of degradations increases (Figure 5b). It is worth noting that the DSC curves exhibit two melting peaks for the part samples: the first peak originated from the early melting of molten and crystallized spherulite chains, and the second attributed to aggregated polymer molecules in the core regions of spherulites that remain unmolten during the 3D printing process.
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Process control of surface quality and part microstructure in
selective laser sintering involving highly degraded polyamide 12
materials
Feifei Yang
*
, Tianyu Jiang*, Greg Lalier
, John Bartolone, Xu Chen*
Abstract: Polyamide 12 is one of the most extensively used semi-crystalline polymer materials to date in
selective laser sintering (SLS) additive manufacturing, or SLS 3D printing. In this powder-based direct
digital manufacturing process, a substantial amount of expensive materials remains un-sintered, recyclable,
and reusable. Recently, understanding the mechanisms of degradation and the reusability of reclaimed
polyamide 12 powders has attracted increasing industrial and research interests. However, using reclaimed
polyamide 12 powder in SLS results in problems with part surface quality such as undesirable part surface
finish with poor textures and numerous un-sintered particles. Limited research is available on the
improvement of part surface quality. In particular, results barely exist on improving or modifying the
surface quality of parts using extremely aged powders powders that are held close to the heat-affected
zones (HAZs) and suffer from severe degradations during the sintering process. To improve the surface
quality and to build interrelations between process parameters and surface quality, we propose a novel
approach for SLS with (extremely) aged polyamide 12 powders. By combining material preparation,
powder and part characterizations, and SLS with customized post-heating, we obtain parts with improved
surface quality (e.g., reduced roughness and porosities, and eliminated un-sintered particles). Particularly,
parts 3D-printed using the 30%-30%-40% new-aged-extremely-aged powder mixtures exhibit the
smoothest and flattest surface with no unmolten particles and nearly zero porosity.
*
Department of Mechanical Engineering, University of Washington, Seattle, WA, 98195, USA.
Unilever Research & Development, 45 Commerce Drive, Trumbull, CT 06611.
Corresponding author.
Email addresses: yangff@uw.edu (Feifei Yang), tjiang19@uw.edu (Tianyu Jiang), Greg.Lalier@unilever.com
(Greg Lalier), John.Bartolone@unilever.com (John Bartolone), chx@uw.edu (Xu Chen)
Graphical abstract
Keywords: Selective laser sintering; Aging and degradation; Surface quality improvement; Post-heating;
Roughness and unmolten particles
1. Introduction
Capable of fabricating functional parts with diverse materials and complex geometries directly from a
digital model, selective laser sintering (SLS) is one of the most widely developed techniques in
additive manufacturing (AM) to manufacture high-quality polymeric and metallic components [1-4].
Compared with other SLS materials such as metals and ceramics, polymeric powder materials offer
benefits in low processing temperatures, controllable flowability and high corrosion resistance [5-7] in trade
of strength. Particularly, as one of the most important semi-crystalline thermoplastic polymer materials,
polyamide 12 and its reinforced/filled forms generate SLS parts with superior mechanical properties over
general amorphous materials [2].
The microstructures of polyamide 12 materials consist of a series of carbon atoms and the amide groups
(-NHCO-), forming carbon-based molecular chains and showing both amorphous regions and crystalline
regions [1, 8]. Due to the open chain ends, the molecular structures of polyamide 12 materials are prone to
molecular changes at high temperatures and during laser-material interactions. In particular, post-
condensation, chain scission and chain crosslinking reactions form the essential degradation mechanisms
of polyamide 12 powders [9, 10], and induce different material properties between reclaimed and pristine
polyamide 12 powders.
Reduced flowability is a predominant property change for reclaimed polyamide 12 powders [1]. This
property change is attributed to (i) the increase of the molecular weight originated from molecular chain
crosslinking and spherulite growth [11], and (ii) the formation of large particles aggregated from small
pieces. These large particles cause deteriorated surface finish with unmolten high-melting-point pieces in
the specimens [12]. With multiple times of reuse and repeated expansion/shrinkage in the fabrication cycles,
the surface of the reclaimed polyamide 12 powders exhibits increasing cracks and fragments, lowering the
part surface quality [13]. Also, compared to 3D printed parts using new powders with fibrillar spherulites
dominating the morphologies, parts using reclaimed powders contain coarse spherulites with rough and
uneven surface finish due to post-crystallization and spherulite growth [1, 14].
More at the level of part quality, S. Dadbakhsh et al. [1] utilized the scanning electron microscopy
(SEM) to exam the surfaces of single layer parts made from new, mixed, and aged powders, aiming to
clarify the effects and the corresponding mechanisms of in-process aging on the microstructures of
polyamide 12 specimens in SLS. M. Pavan et al. [15] investigated how thermo-temporal effects on the SLS
polyamide 12 impact part quality at both micro- (e.g., porosity and crystallinity) and macro-levels (e.g.,
dimensional accuracy) by testing the samples using a mixing ratio 50/50 new/recycled powder and an
alternate x-y scanning pattern. D.T. Pham et al. [16] and W. Yusoff et al. [14] developed different
amendment strategies through optimizing the important SLS process parameters to reduce or eliminate the
“orange peel” surface texture when using reclaimed polyamide 12. J. Guo et al. [17] presented an
experimental and analytical study to improve the surface quality of parts using reclaimed polyamide 12
materials and to clarify the interrelations between surface quality and process parameters.
Despite the aforementioned literature, it remains not clear how to maximize surface quality when using
reclaimed powders of different combinations. Moreover, there is a lack of understanding in surface
characteristics of parts sintered using extremely aged polyamide 12 powders. Held close to or wrapped by
the heat-affected zones (HAZs)
§
, these powders go through severe degradations during the sintering process.
We show, however, that such expensive materials can be reused to produce parts with fine surface textures,
reduced porosities, and free from unmolten particles. The result is obtained by developing a new strategy
to control and optimize surface quality using SLS with controlled post-heating. By material preparation,
§
Areas close to the laser-material interaction during sintering.
powder and part characterizations and SLS with controlled post-heating, we obtain a series of parts using
differently degraded powders and different combinations. Then after surface cleaning, we examine the
surface morphologies of these parts and evaluate the characteristics of the surface morphologies. The result
is that the undesirable surface finish of parts printed using reclaimed polyamide 12 powders can be
optimally improved after using the proposed strategy. In particular, parts 3D-printed using the 30%-30%-
40% new-aged-extremely-aged powder mixtures exhibit the smoothest and flattest surface with no
unmolten particles and nearly zero porosity.
2. Design of the proposed SLS for reclaimed polyamide 12
The proposed method involves (Figure 1) material preparation (Step 1) and SLS with post-heating
(Step 2). More specifically, Step 1 includes powder collection and powder preprocess, and Step 2 covers
powder/part characterizations, powder mixing, parameter control, SLS with post-heating and evaluation.
Details of these procedures are explained in the following parts.
Powder collection
Powder mixing
Powder preprocess
Parameter control SLS with post-heating
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
Preheating temperature
Laser power
Laser speed
Scan spacing
Layer thickness
Layer number
New powders without post-heating
Mixed powders without post-
heating
Mixed powders with post-heating
Proposed SLS with post-heating method
New powders with post-heating
Reclaimed powders with post-
heating
Reclaimed powders without post-
heating
Powder characterizations Part characterizations
DSC XRD FTIR
SLS with post-heating process
Evaluation
DSC XRD FTIR
Coalescence behaviors
Porosity
Unmolten particles
Roughness
Step 1
Step 2
SEM SEM
Material preparation
Microstructures
Figure 1 Proposed SLS with post-heating method to improve part surface quality when using reclaimed polyamide 12 powders
2.1. Material preparation
Powder collection: Pristine or new polyamide 12 powders were purchased from EOS Corp, and the
reclaimed powders were collected from standard SLS processes on an EOS P 390 machine. Polyamide 12
powders with 3 different degradation levels were used in this work: (1) new polyamide 12 powders with no
additional heat treatment; (2) aged polyamide 12 powders located far away from the HAZs in the SLS
chamber and are reused in industries; (3) extremely aged polyamide 12 powders adjacent to or wrapped by
the HAZs that are not reused in industries currently.
Powder preprocess: Due to the large particles or part debris caused by the high temperature and laser
induced degradation/aging, when recoating with the extremely aged powders, the powder bed is uneven
and rough on surface. A sieving process was applied to preprocess the extremely aged powders for an even
recoating surface. This process was conducted in a fume hood using a sieve with a mesh size of 200 um.
The time for sieving a batch of 500 ml powders is around 2 hours including preparation and post cleaning.
After sieving, 84%~90% of the reclaimed materials can be collected and well recoated.
2.2. Powder and part characterizations
Key characterization tests, including Scanning Electron Microscope (SEM), Differential Scanning
Calorimetry (DSC), Fourier-transform Infrared Spectroscopy (FTIR) and X-Ray Diffraction (XRD), were
carried out on 100% new, 100% aged and 100% extremely aged polyamide 12 powder samples to examine
property differences. The characterizations of parts printed using 100% new, 100% aged and 100%
extremely aged powders were also conducted using SEM, DSC, FTIR, and XRD instruments. The aims of
the characterizations are to (i) quantify the differences between the new powders and reclaimed powders to
instruct the 3D printing process, (ii) verify the feasibility of 3D printing with reclaimed polyamide 12
powders, and (iii) understand the differences between the new SLS parts and SLS parts printed using
reclaimed powders.
2.3. Proposed SLS and post-heating
2.3.1. Powder mixing
Four different kinds of powder combinations were used in this work: (i) pure powders, (ii) new and
aged powder mixture, (iii) new and extremely aged powder mixture, and (iv) new, aged, and extremely
aged powder mixture. We conducted various volume mixing percentages for these powder combinations.
2.3.2. Parameter control
Before presenting the main surface improvement control proposed in this paper, we discuss the overall
parameter control that significantly influences the properties of the sintered parts. Six key parameters were
tested in the experiments: preheating temperature, laser power, laser speed, scan spacing, layer thickness
and layer numbers of samples (Figure 1). A variety of parameter settings are suitable for new powders.
However, existing parameter settings seldom apply to the case using extremely aged powders in presence
of the degraded material properties. The general principles to select the proposed parameter settings are that
(i) these parameters are equal or close to industrial norms and (ii) the pure new, aged and extremely aged
polyamide 12 powders can all be successfully printed into parts. The nominal parameter settings selected
in this work were: preheating temperature, 160 °C; scan speed, 3000 mm/s; laser power, 18 W; scan spacing:
0.3 mm; layer thickness, 150 µm. In this work, samples are all printed with 3 layers and using the optimized
parameter settings selected to explore the part surface quality improvement.
2.3.3. SLS with post-heating
Hardware: The SLS machine used in the paper is an in-house built open-configuration research testbed,
with features comparable to commercial machines (Figure 2). We designed the testbed using a Coherent
GEM100A CO2 laser (maximum laser power: 100 W) and a Scanlab intelliSCAN 14 scanner. The powder
bed dimension is 250 mm (L) x 250 mm (W) x 150 mm (H) with a 20 µm layer thickness resolution and a
0.24 µm theoretical XY positioning resolution. The typical scan speed is 3.75 m/s and the maximum scan
speed is 30 m/s. The testbed recoats the powder with a blade and positions the powder bed with a servomotor
and a lead-screw transmission. The heating method used is a combination of radiation heating with infrared
heaters above the powder bed (200 mm) and conduction heating with a mica heater underneath the powder
bed. The software used to control the machine is an in-house developed LabVIEW program integrated with
Scanlab RTC5 API, which can read G-codes and send motion commands to the galvo scanner.
Figure 2 In-house built and customized SLS AM testbeds
Process: The proposed SLS and post-heating control apply tailored heating after the core laser-material
interaction with optimized processing parameters. The post-heating here keeps parts at the preheating
temperature (160 °C) for a controlled time after the sintering process. In this work, we tested 0 second (no
post-heating), 20 seconds, 60 seconds, 120 seconds, and 300 seconds of post-heating to different specimens
to explore the influences of post-heating on part surface morphologies. Figure 3 shows details of the
proposed five-stage SLS. In Stage 1, we 3D printed the benchmark part using 100% new powders with no
additional post-heating, and the part was used as a reference to evaluate the surface qualities of other parts.
In Stage 2, parts were printed using 100% new powders with different post-heating time (20 seconds, 60
seconds, 120 seconds, and 300 seconds) to identify the influences of post-heating on part surface qualities
when using 100% new powders. In Stage 3, we printed parts with 100% extremely aged powders at different
post-heating time. After comparing the surface qualities of parts using reclaimed powders with and without
post heating, 300-second post heating appears most effective for reclaimed powders. In Stage 4, parts were
3D printed using different powder mixtures; parts were also 3D printed with and without 300-second post-
heating to study the effects of post-heating on part surface quality when using mixed powders. In Stage 5,
we used the mixtures of three differently degraded powders (new, aged, and extremely aged powder
mixtures) with and without 300-second post-heating. The results form the basis to identify the effects of
post-heating on part surface morphology.
Figure 3 Post-heating based SLS process (The mixed powders are in volume percentages)
Mechanism: It has been verified that the pieces with a high melting point in the reclaimed polyamide 12
powders require a higher temperature to form the molten phase [1]. Thus, reclaimed polyamide 12 powders
are more difficult to melt and coalesce compared to new powders under the same sintering conditions,
leading to worse surface qualities with insufficient coalescence and numerous unmolten particles. In
addition, the numerous unmolten pieces in the reclaimed powders act as the nucleation sites for the
formation of nucleation seeds. Spherulite structures grow on the nucleation seeds once the temperature is
below the melting point. In this way, numerous spherulite structures grow on the amorphous solid phase of
parts after the layer solidifies, forming coarse spherulites and a rough surface. On the other hand, when
using new powders, the generated parts have fibrillar spherulites and smooth surface subject to little
unmolten pieces and nucleation seeds. Our proposed SLS with customized post-heating provides additional
and enough energy to promote coalescence of the reclaimed powders and to accelerate the phase change of
high-melting-point pieces. The method is also helpful to avoid the formation of numerous spherulite
structures and decrease the surface roughness. In addition, the parts using pure new powders can have high
porosity immediately after printing. The new and reclaimed powder mixtures used in the proposed method
explore the possibility of reducing part porosity.
Evaluation: After surface cleaning to remove debris on the part surface, we used SEM to compare surface
morphologies between the benchmark part and the evaluated parts in Figure 3. Compared features include
particle coalescence performances, part porosity, the number of unmolten particles, surface microstructures
and roughness. The parts were found to have significantly different surface properties when using different
powders and powder mixtures. The details are explained in the next section.
3. Experimental results and discussions
3.1. Powder- and part-level characterizations of material property changes
3.1.1. Surface morphology
Figure 4 shows the SEM characterization results for the tested polyamide 12 powders and 3D printed
parts. The new powders have a good spherical morphology, whereas the aged or extremely aged powders
have irregular shapes with cracks or gaps on the surface (Figure 4a). For the printed parts, a smooth surface
is observed when using new powders. However, many partially melt particles are found on the parts 3D
printed using aged and extremely aged powders (Figure 4b), agreeing well with our mechanismic analysis.
In Figure 4b, we cannot even see a completed surface for parts built from the extremely aged powders.
(a) Polyamide 12 powder samples
(b) Polyamide 12 part samples
Figure 4 SEM test results of polyamide 12 powders and parts using polyamide 12 powders
3.1.2. Thermal property
Figure 5 presents the DSC curves of different polyamide 12 powders and parts, revealing changes in
material thermal properties. For each sample, two heating cycles were conducted. In the first more dominant
heating cycle (marked with blue line), the onset melting temperatures of different powders decrease with
increasing powder degradations (Figure 5a), following the sequence: new powders (183 °C) > aged powders
(181 °C) > extremely aged powders (180 °C). The result indicates a broaden melting trajectory for the
reclaimed powders caused by the more regular chain-folded states and higher crystalline ratios. Such results
are consistent with the previous studies [1, 9]. The peak melting temperatures for the aged powders and
extremely aged powders are both larger than that of the new powders, suggesting a higher melting
temperature for powders after aging. Such a result is also expected giving the high-melting-point pieces in
the reclaimed powders.
For the part samples, both the onset melting temperatures and peak melting temperatures decrease
when the level of degradations increases (Figure 5b). It is worth noting that the DSC curves exhibit two
melting peaks for the part samples: the first peak originated from the early melting of molten and
crystallized spherulite chains, and the second attributed to aggregated polymer molecules in the core regions
of spherulites that remain unmolten during the 3D printing process.
(i) New polyamide 12 powder
(ii) Aged polyamide 12 powder
(iii) Extremely aged polyamide 12 powder
Cooling cyc le
(a) Polyamide 12 powder samples
(i) Part using new polyamide 12 powder
(ii) Part using aged polyamide 12 powder
(iii) Part using extremely aged polyamide 12
powder
(b) Polyamide 12 part samples
Figure 5 DSC curves of polyamide 12 (a) powder samples and (b) part samples
3.1.3. Crystalline property
Figure 6 shows the XRD results for polyamide 12 powders and parts. The powder samples mainly
exhibit the unstable α phase, which has a molecular chain oriented in an anti-parallel manner with a high
crystalline ratio [18, 19]. On the other hand, the printed parts mainly exhibit the γ phase, which has a
parallel-oriented molecular chains and is more stable than the α phase [20, 21]. The SLS processing thus
significantly altered the morphologies and the hydrogen bonds between the molecular chains with a drastic
drop in crystallinities.
α phase
- New polyamide 12 powder
- Aged polyamide 12 powder
- Extremely aged polyamide 12 powder
γ phase
- Part using new polyamide 12 powder
- Part using aged polyamide 12 powder
- Part using extremely aged polyamide 12 powder
(a) Powder samples (b) Part samples
Figure 6 XRD results of polyamide 12 (a) powder samples and (b) part samples
3.1.4. Molecular components
Though the process chamber is designed to be filled with nitrogen, 2 % - 5 % residual oxygen is
inevitable commonly [1, 22]. During elevated temperatures and in a laser environment, free hydrogen
molecule radicals can emerge to initiate the oxidation process. Previous studies have revealed that oxidation
for polyamide mainly initiates on the weakest C-H bonds located on the carbon atoms close to the nitrogen
atoms [23, 24]. Then in the propagation process, oxygen residual in the chamber gets absorbed, forming
unstable middle products or structures such as peroxide radicals and peroxides. After the termination of the
oxidation reaction with stable final products, two carbon-oxygen double bonds exist in every polyamide 12
unit on the carons adjacent to nitrogen [9].
In FTIR results, the decreases at peaks with vibrational frequencies of 3080, 1369, 1159, 1062 and 948
cm1 are verified giving the fact that thermal oxidation reactions occurred on carbons of the weakest C-H
bonds [9, 25]. Figure 7 demonstrates FTIR test results of polyamide 12 powder samples and part samples.
In Figure 7a and 7c, compared with new powders, both aged and extremely aged powders exhibit decrease
at the peaks with vibrational frequencies of 3080, 1369, 1159, 1062 and 948 cm1. For part samples, FTIR
peaks of the parts using extremely aged powders at 3080, 1369, 1159, 1062 and 948 cm1 exhibit a drastic
decrease compared with the parts using new powders and using aged powders (Figure 7b and 7d),
suggesting a more completed oxidation reaction during SLS. During the oxidation process, chain cross-
linking and chain scission occur simultaneously [26, 27]. In SLS, due to the nitrogen atmosphere, cross-
linking is found to be the dominating process [27]. In summary, the FTIR results indicate that both the
aging process and SLS process can trigger the thermal oxidation reactions for polyamide 12, and thereby
affect the microstructures and properties of polyamide 12 samples.
4000 3500 3000 2500 2000 1500 1000 500
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Absorbance
Wavenumber/cm-1
New polyamide 12 powders
Aged polyamide 12 powders
Extremely aged polyamide 12 powders
(a) FTIR of polyamide 12 powder samples
4000 3500 3000 2500 2000 1500 1000 500
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Absorbance
Wavenumber/cm-1
Part of new polyamide 12 powders
Part of aged polyamide 12 powders
Part of extremely aged polyamide 12 powders
(b) FTIR of polyamide 12 part samples
3130 3120 3110 3100 3090 3080 3070 3060
0.005
0.010
0.015
0.020
0.025
Absorbance
Wavenumber/cm-1
New polyamide 12 powders
Aged polyamide 12 powders
Extremely aged polyamide 12 powders
(c) FTIR of polyamide 12 powder samples near 3080 cm1
3140 3120 3100 3080 3060 3040
0.005
0.010
0.015
0.020
0.025
Absorbance
Wavenumber/cm-1
Part of new polyamide 12 powders
Part of aged polyamide 12 powders
Part of extremely aged polyamide 12 powders
(d) FTIR of polyamide 12 part samples near 3080 cm1
Figure 7 FTIR test results of polyamide 12 (a) powder samples, (b) part samples, (c) powder samples near 3080 cm1 and (d) part
samples near 3080 cm1
3.2. Surface quality improvements of the 3D-printed parts
3.2.1. Stage 1: printing the benchmark sample
Figure 8 presents the SEM images of the 3-layer benchmark sample printed using 100% new
polyamide 12 powders without additional heat treatment. As seen from Figure 8 (a) - (c), the part exhibits
a smooth and flat surface with no unmolten particles. Meanwhile, high porosity is observed from these
images, suggesting an insufficient densification. At a high magnification ratio of 10000 (Figure 8d), some
fine lamellae or spherulitic regions in an amorphous matrix are observed. The spherulites radiate from the
center and grow in a ringed extinction pattern. These surface characteristics of the benchmark part are used
as references to evaluate the surface quality of the other parts.
Figure 8 The SEM images of benchmark part using 100% new polyamide 12 powders at different magnification ratios (a) 200,
(b) 500, (c) 2000, and (d) 10000
3.2.2. Stage 2: the influences of post-heating on part surface morphology when using new powders
Dissimilar to Figure 8, Figure 9 shows the SEM results of the parts using 100% new polyamide 12
powders with 20 seconds, 60 seconds, 120 seconds and 300 seconds post-heating at a magnification ratio
of 500. In Figure 9, no obvious differences are observed for the parts using different post-heating time.
Compared with Figure 8b (100% new samples with no post-heating), the parts obtained in Stage 2 (printed
using the same powders and different post-heating time) exhibit a very similar surface morphology of a
smooth and flat surface with high porosity and no visible unmolten particles (Figure 9). The result suggests
that the parts in Stage 2 have similar coalescence, densification and consolidation characteristics with the
parts in Stage 1, a further indicator that the post-heating barely has an effect on the surface qualities of
samples 3D printed using new polyamide 12 powders.
Figure 9 The part using 100% new polyamide 12 powders with (a) 20 seconds, (b) 60 seconds, (c) 120 seconds and (d) 300
seconds post-heating at a magnification ratio of 500
3.2.3. Stage 3: the influences of post-heating on part surface morphology when using reclaimed
powders
Figure 10 exhibits the SEM images of parts using 100% extremely aged powders with 0 second (no
post-heating), 20 seconds, 60 seconds, 120 seconds, and 300 seconds post-heating at magnification ratios
of 500 and 2000. From Figure 10a to 10e, we observe a gradual melting and coalescing process with post-
heating time increasing. In Figure 10a (no post-heating), multiple layers of insufficiently melt particles are
observed, and every two or more particles form a neck-like bonding due to the reduction of the free surface
energies of the particles triggered by high temperature and intense laser-material interaction. With 20
seconds of post-heating (Figure 10b), the particles bonded by the neck-like structures migrate together. The
migration becomes stronger with longer post-heating (Figure 10c and 10d). Finally, a large unit, a well-
consolidated surface forms with little porosity and few unmolten pieces (Figure 10e).
Comparing Figure 8 (100% new powders with no post-heating) with Figure 10a (100% extremely aged
powders with no post-heating), we observe an obvious difference in surface morphology. Numerous visible
unmolten particles arise and fuse together to form porous structures in the parts 3D printed using extremely
aged powders (Figure 10a). The results indicate the existence of high-melting-point pieces in the extremely
aged powders, matching well with the DSC results that the peak melting temperature and melting enthalpies
of deteriorated polyamide 12 powders increase. Given the above, we conclude that the post-heating process
helps to improve the surface quality of the printed parts using extremely aged powders by maximizing the
coalescence and consolidation, and a well-consolidated surface obtains with 300 seconds post-heating
(Figure 10e).
(a) Parts using 100% extremely aged polyamide 12 powders without post-heating
(b) Parts using 100% extremely aged polyamide 12 powders with 20 seconds post-heating
(c) Parts using 100% extremely aged polyamide 12 powders with 60 seconds post-heating
(d) Parts using 100% extremely aged polyamide 12 powders with 120 seconds post-heating
(e) Parts using 100% extremely aged polyamide 12 powders with 300 seconds post-heating
Figure 10 Parts printed using 100% extremely aged polyamide 12 powders with and without post-heating
3.2.4. Stage 4: the influences of post-heating on part surface morphology when using mixed
powders
Figure 11 presents the SEM images of the parts using new-aged polyamide 12 powder mixtures and
new-extremely-aged polyamide 12 powder mixtures with (i) no post heating and (ii) 300 seconds post-
heating. Figure 11 (a) - (d) shows the SEM images of parts 3D printed using 50%-50% new-aged mixed
powders, 70%-30% new-extremely-aged mixed powders, 60%-40% new-extremely-aged mixed powders,
and 50%-50% new-extremely-aged mixed powders, respectively. The left and right subfigures, indicated
by (i) and (ii), show the results of no post heating and 300 seconds post-heating, respectively. Smooth and
flat surfaces with several unmolten particles are observed with no post heating (Figure 11a (i), 11b (i), 11c
(i) and 11d (i)), suggesting that good particle coalescence behaviors are achieved when using 50% or more
new powders. Meanwhile, the unmolten particles dramatically decrease, and smooth and flat surfaces with
high porosity are obtained with 300 seconds post-heating (Figure 11a (ii), 11b (ii), 11c (ii) and 11d (ii)).
These surfaces are similar to those of parts using 100% new powders, suggesting a significant improvement
of surface qualities by our post-heating.
Figure 11 (e) - (f) show the SEM images of parts 3D printed using 40%-60% new-extremely-aged
mixed powders and 30%-70% new-extremely-aged mixed powders, respectively. The left and right
subfigures, indicated by (i) and (ii), show the results of no post-heating and 300-second post-heating,
respectively. With no post-heating, samples exhibit severe deteriorated and distorted surface morphologies
with irregular holes or porous and plenty of unmolten particles (Figures 11e (i) and 11f (i)). With 300-
second post-heating, the improved surfaces have relatively flat morphologies with several unmolten
particles (Figures 11e (ii) and 11f (ii)). Through comparisons, the parts using 30%-70% new-extremely-
aged powder mixtures (Figures 11f (ii)) exhibit a smoother surface with lower porosity. Such improvement
can be attributed to the better densification and consolidation due to weak gas adsorption in the reclaimed
powders when enough energy was supplied.
Figures 11 (g) - (h) exhibit the SEM images of parts 3D printed using 20%-80% and 10%-90% new-
extremely-aged powder mixtures, respectively. Subfigures (i) and (ii) are results of no post-heating and
300-second post-heating, respectively. With no post-heating, the SEM images exhibit little completed
surfaces with numerous unmolten particles and multi-layer porous structures (Figures 11g (i) and 11h (i)).
These surfaces are similar to the SEM images when using 100% extremely aged powders. With 300-second
post-heating, the images exhibit very smooth and flat surfaces with almost no porous and several unmolten
particles (Figures 11g (ii) and 11h (ii)). The quality of the obtained surfaces are even better than the part
printed using 100% new powders.
(a) Parts using 50% new + 50% aged mixed polyamide 12 powders
(b) Parts using 70% new + 30% extremely aged mixed polyamide 12 powders
(c) Parts using 60% new + 40% extremely aged mixed polyamide 12 powders
(d) Parts using 50% new + 50% extremely aged mixed polyamide 12 powders
(e) Parts using 40% new + 60% extremely aged mixed polyamide 12 powders
(f) Parts using 30% new + 70% extremely aged mixed polyamide 12 powders
(g) Parts using 20% new + 80% extremely aged mixed polyamide 12 powders
(h) Parts using 10% new + 90% extremely aged mixed polyamide 12 powders
Figure 11 Parts using new-aged mixed or new-extremely-aged mixed polyamide 12 powders with no post-heating and with 300
seconds post-heating
3.2.5. Stage 5: the influences of post-heating on part surface morphology when using new, aged, and
extremely aged powder mixtures
Figure 12 demonstrates the images of parts using new-aged-extremely-aged mixed polyamide 12
powders with (i) no post-heating and (ii) 300 seconds post-heating. The mixtures used are 30%-30%-40%
and 30%-40%-30% new-aged-extremely-aged mixed polyamide 12 powders. In these combinations, 30%
new powders and 70% reclaimed powders were used. All using 30% new powders with no post-heating,
the parts in Figure 12a (i) and 12b (i) have better-coalesced surfaces compared to the part in Figure 11f (i).
Because much more extremely aged powders were used in Stage 4 (Figure 11f, 70%) than in Stage 5 (Figure
12, 30% and 40%), making it more difficult to fuse the materials. With 300-second post-heating, the parts
exhibit superior surface morphologies with smoother surfaces and less unmolten particles and porosity
(Figures 12a (ii) and 12b (ii)) than the parts with no post-heating (Figures 12a (i) and 12b (i)). In particular,
the part 3D printed using 30%-30%-40% new-aged-extremely-aged mixed powders has the smoothest and
flattest surface with no unmolten particles and almost no porosities (Figure 12a (ii)).
(a) Parts using 30% new + 30% aged + 40% extremely aged mixed polyamide 12 powders
(b) Parts using 30% new + 40% aged + 30% extremely aged mixed polyamide 12 powders
Figure 12 Parts using new-aged-extremely-aged mixed polyamide 12 powders with no post-heating and with 300 seconds post-
heating
3.3. Discussions
3.3.1. Unmolten particles, coalescence, roughness and porosity
To compare the number of unmolten particles, coalescence performances and roughness of the sintered
parts with no post-heating, the 3-layer parts were printed using different polyamide 12 combinations, with
the SEM images shown in Figure 13. In general, parts using 100% new powders have the best coalescence
performance, and almost no unmolten particles (Figure 13a). However, parts using more extremely aged
powders exhibit worse coalesced surfaces with drastically increased unmolten particles (Figure 13 (b)-(l)).
Parts using more new powders have smoother and flatter surfaces, while parts using mixed powders obtain
worse surfaces as the percentages of reclaimed powders increase from 30% to 70%, as shown in Figure 13
(b)-(g) and Figure 13 (k)-(l). In particular, when using 80%~100% extremely aged powders (Figure 13 (h)-
(j)), numerous insufficiently melt particles are observed, and no consolidated surfaces form.
Figure 13 Comparisons of unmolten particles, coalescence and roughness of parts using polyamide 12 powders of different
combinations without post-heating
The 3-layer parts with 300 seconds post-heating were also printed using different polyamide 12
combinations. To compare the number of unmolten particles, coalescence performances and roughness,
Figure 14 shows the SEM images of these 3D printed parts. Except for the part using 100% new powders,
the images in Figure 14 exhibit better surfaces with enhanced coalescence, decreased unmolten particles
and improved smoothness with the proposed post-heating. In Figure 14 (a)-(e) and (k), the images of parts
printed using 100% new powders, 50%-50% new-aged mixed powders, 70%-30% new-extremely-aged
mixed powders, 60%-40% new-extremely-aged mixed powders, 50%-50% new-extremely-aged mixed
powders and 30%-30%-40% new-aged-extremely-aged mixed powders, respectively, the surfaces are
smooth and flat without any unmolten particles. However, in Figure 14 (f)-(j) and (l), the images of parts
printed using 40%-60% new-extremely-aged mixtures, 30%-70% new-extremely-aged mixtures, 20%-80%
new-extremely-aged mixtures, 10%-90% new-extremely-aged mixtures, 100% extremely aged powders
and 30%-40%-30% new-aged-extremely-aged mixtures, respectively, the parts exhibit several unmolten
pieces. These results indicate that the parts using a high percentage of new powders obtain improved smooth
and flat surfaces with no unmolten particles with post-heating. These surfaces are comparable to those of
part using new powders. The parts using a high percentage of extremely aged powders get improved
surfaces with post-heating, but there still exist several unmolten particles.
As for the SEM image in Figure 14, the parts also display different porosities. In Figure 14 (a) to (d),
the images of parts printed using 100% new powders, 50%-50% new-aged mixtures, 70%-30% new-
extremely-aged mixtures and 60%-40% new-extremely-aged mixtures, respectively, there are more large
size pores than the other parts. In Figure 14 (e), (f) and (l), the images of parts printed using 50%-50% new-
extremely-aged mixtures, 40%-60% new-extremely-aged mixtures and 30%-40%-30% new-aged-
extremely-aged mixtures, respectively, the number and size of pores decrease significantly. In the
remaining images of Figure 14 (g)-(k), almost no pores are observed. These results suggest that the parts
using more new powders tend to have more pores. The parts using more extremely aged powders with post-
heating display lower porosity due to better consolidation.
Figure 14 Comparisons of unmolten particles, coalescence and roughness of parts using polyamide 12 powders of different
combinations with 300 seconds post-heating
3.3.2. Microstructures
Microstructure is another important variable in the printed parts when using reclaimed powders. Figure
15a and 15b present the microstructures of the parts with no post-heating and with 300 seconds post-heating
at a magnification ratio of 10000, respectively. Different powders and powder mixtures yield different
microstructures. Parts using new powders exhibits fine fibrillar/lamellae spherulitic regions in amorphous
matrixes (Figure 15a), due to the aggregations of chain-folded crystallites radiating from the center and
growing to be spherical in shape. On the other hand, the parts using extremely aged powders present coarser
spherulites spreading all over the matrix (Figure 15a). Due to the slightly aging, the spherulite roughness
in the part using aged powders behaves in the middle between those observed in the parts using new and
extremely aged powders.
The parts using different powders with 300 seconds post-heating show similar characteristics (Figure
15b). Parts using new powders show fine lamellae. Extremely aged powders lead to coarse spherulites. And
parts using aged powders show intermediate morphologies. It can be concluded that the microstructures of
parts are largely impacted by the aging status of powders rather than post-heating.
(a) Polyamide 12 parts without post-heating
(b) Polyamide 12 parts with 300 seconds post-heating
Figure 15 Microstructure examinations of (a) polyamide 12 parts without post-heating, and (b) polyamide 12 parts with 300
seconds post-heating at a magnification of 10000
4. Conclusions
This work proposes an SLS with post-heating to improve surface quality of 3D printed parts using
reclaimed polyamide 12 powders. The proposed method decreases roughness and porosity of the printed
parts, and eliminates unmolten particles. The effects of post-heating on the surface quality using different
powder mixtures were studied. In particular, SEM reveals surface features, including the number of
unmolten particles, coalescence performances, roughness, porosity and microstructures.
The tested parts using 100% new powders with different post-heating time show similar surface
properties, suggesting that post-heating barely affected the surface quality of parts using 100% new powders.
The parts using 100% extremely aged powders with no post-heating exhibit multiple layers of insufficiently
melt particles. However, a 300-second post-heating yields a well-consolidated surface with little porosity
and a drastically reduced un-molten particles.
The unmolten particles disappear on the parts using 50% or more new powders with 300-second post-
heating, showing smooth and flat surfaces with high porosity. These surfaces are comparable to those of
parts using 100% new powders. When using 60% or 70% reclaimed powders with 40% or 30% new
powders, severely deteriorated part surface quality with irregular holes and numerous unmolten particles
arise in the case with no post-heating. With the proposed 300-second post-heating, we obtain smooth parts
with few unmolten particles and drastically decreased porosity thanks to the improved densification and
consolidation. When using 80% or 90% reclaimed powders with 20% or 10% new powders, numerous
visible unmolten particles and multi-layer porous structures occur in the case with no post-heating. The
surfaces are similar to those of the parts using 100% extremely aged powders. With the proposed 300-
second post-heating, we obtain smooth and flat surfaces with almost zero porosity and only few unmolten
particles. The resulting surface morphologies are even better than parts 3D printed using 100% new
powders.
These results validate the effectiveness of our proposed SLS with post-heating in improving the surface
qualities when reusing polyamide 12 powders. The improved surface qualities of different powder mixtures
may provide useful information for reusing reclaimed polyamide 12 powders in the SLS industry.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Acknowledgments
The work was supported in part by a research grant from Unilever and by NSF award 1953155.
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The biaxial stretching behavior of polyamides 6 and 11 has been investigated in relation to the organization of H bonds within their crystalline structure. Contrary to the case of mesophases, the H-bonded sheet-like structures of both PA6 and PA11 display poor ability for biaxial orientation due to their strong mechanical anisotropy. It is shown that the occurrence of a Brill transition has a dramatic impact on the mechanical behavior. A huge and abrupt improvement of the biaxial stretchability, up to a draw ratio λ × λ around 3 × 3, similar to the one achieved for the mesophases, is reached for samples initially crystallized into the less perfect α phases, as soon as the draw temperature is higher than TBrill. The biaxial stretchability of polyamides remains nevertheless limited as compared to that of polyolefins. The biaxial drawing response appears as a sensitive probe of the structural organization. It strongly supports the assumption that high temperature (HT) structures are rather characterized by a random distribution of H bonds around the chain axis. Post mortem X-ray analysis of biaxially drawn samples reveal the development of a planar texture and a loss of the hexagonal symmetry of mesophases and HT forms.
Article
Materials technology is currently a great challenge in selective laser sintering (SLS). The development of new method for the preparation of a variety of materials has drawn great attention from both industrial and academic organizations. In this work, we described a simple strategy to prepare polyamide 12 (PA12) microspheres through a modified phase-separation process. The phase separation was conducted by adding ethanol as a poor solvent into a formic acid solution of PA12 pellets with polyvinyl pyrrolidone as a dispersant. The mean diameters of the obtained PA12 microspheres, ranging from tens to hundreds of micrometers, were well controlled by adjusting the amount of ethanol and the phase separation temperature. Further investigation by differential scanning calorimetry demonstrated that the sintering window for PA12, between the onset temperatures of crystallization and melting, was drastically stretched during the microsphere formation process. The PA12 particle size and morphology, together with the wider sintering window of the microspheres, demonstrated that the obtained PA12 powder was suitable for SLS processing.