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Additive Manufacturing

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Full Length Article

A novel approach for understanding laser sintering of polymers

D. Drummera,b,⁎, S. Greinera,b, M. Zhaoa,b, K. Wudya,b

a Collaborative Research Center 814, Additive Manufacturing (CRC 814), Am Weichselgarten 9, 91058, Erlangen, Germanyb Institute of Polymer Technology (LKT), Am Weichselgarten 9, 91058, Erlangen, Germany

A R T I C L E I N F O

Keywords:Additive manufacturingPowder bed fusionSelective laser sinteringIsothermal crystallizationPolyamide 12

A B S T R A C T

Selective laser sintering (LS) of thermoplastic powders allows for the construction of complex parts with highermechanical properties and durability compared to other additive manufacturing methods. According to thecurrent model of isothermal laser sintering, semi-crystalline thermoplastics need to be processed within a certaintemperature range, resulting in the simultaneous presence of the material both in a molten and solid state, whichis present during part building. Based on this process model, high cycle times ranging from hours to days are athought to be a necessity to avoid warpage.

In this paper, the limited validity of the model of isothermal laser sintering is shown by various experiments,as ongoing solidification could be detected a few layers below the powder bed surface. The results indicate thatcrystallization and material solidification is initiated at high temperatures and further progresses throughoutpart build-up in z-direction. Therefore, a process-adapted material characterization was performed to identifythe isothermal crystallization kinetics at processing temperature and to track changes of the material state overtime. A dual approach on measuring surface temperatures by infrared thermography and additional thermo-couple measurements in z-direction was performed to identify further influences on the material solidification. Amodel experiment revealed that a few millimeters below the surface, components produced by LS are alreadysolidified. Based on these results, the authors present an enhanced process model of isothermal laser sintering,which considers material solidification in z-direction during part build-up. In addition, a new processing strategyis derived to increase the efficiency of LS processes significantly.

1. Introduction

Powder bed fusion of polymers, which is commonly referred to asselective laser sintering (LS), allows for the fabrication of functionalparts and assemblies of highly complex geometries without the need ofadditional tooling [1]. Due to its high accuracy and high mechanicalproperties of the fabricated parts, LS is regarded as one of the mostpromising additive manufacturing techniques for industrial applica-tions [1]. However, certain still present drawbacks first have to beovercome. On the one hand, the reproducibility of part properties, suchas part density or mechanical performance, has to be enhanced, on theother hand, the process time needs to be reduced in order to render thestill young rapid prototyping technology valid for the production ofcustomized serial parts.

2. State of the art

The LS process can be divided into the three process steps, the pre-heating-, building- and the cooling-step. A typical temperature profile

for the LS process is shown in Fig. 1 [2]. While the pre-heating-step is adefined heating phase increasing the temperature from the startingtemperature TS to the build temperature TB, the building-step re-presents the actual build-up of the parts by sequential local melting ofthe material using a laser and repeated application of new powderlayers. During this step, the temperature in the building chamber is keptconstant at TB, apart from the sections, which are selectively laser-he-ated. Furthermore, according to the applied LS system, for polyamide12 (PA12) the temperature of the build cylinder is actively heated to atemperature of 120 °C–150 °C to avoid warpage from irregularshrinkage from cooling. In the final step, respectively the cooling-step,the final setup of layers, which was applied during the building-step, isleft unchanged. The whole building chamber is slowly and evenlycooled down to the part extraction temperature TE. The combinedduration of the building- and cooling-step tB,C depends on the dis-tribution and volume of the built parts and the overall height of thecombined powder layers in z-direction.

https://doi.org/10.1016/j.addma.2019.03.012Received 12 November 2018; Received in revised form 10 March 2019; Accepted 11 March 2019

⁎ Corresponding author at: Collaborative Research Center 814, Additive Manufacturing (CRC 814), Am Weichselgarten 9, 91058, Erlangen, Germany.E-mail address: info@lkt.uni-erlangen.de (D. Drummer).

Additive Manufacturing 27 (2019) 379–388

Available online 11 March 20192214-8604/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

T

2.1. Model of isothermal laser sintering

For the processing of semi-crystalline thermoplastics, the buildchamber temperature is selected according to the model of isothermallaser sintering proposed by Alscher, based on differential scanning ca-lorimetry (DSC) measurements [3] as depicted in Fig. 2. According tothe model, the build chamber temperature must be higher than theonset temperature Tic of recrystallization and below the onset tem-perature Tim of melting to allow for the simultaneous presence of boththe solid and the molten material state in the form of powder and melt.According to the model, the material remains molten until the building-step is completed. During the cooling-step, the previously molten ma-terial recrystallizes homogeneously due to the low cooling rate. [3]

LS as a process is influenced by many factors and especially thecomplex thermal interactions during the building-step, which com-prises the continuously repeating sub-processes of powder coating, laserexposure and consolidation [4]. The diverse and uncontrolled thermalinteractions of these sub-processes influence the material state and leadto a process with limited reproducibility, resulting in a high variation ofthe final component properties such as dimensional accuracy, partdensity and mechanical properties [5–7]. Therefore, the investigationof temperature fields in LS is highly relevant

Based on the literature research in [4], the validity of the model ofisothermal laser sintering was found to be limited to a narrow period of

time and therefore build-height. It is stated that for the fabrication oflow-warpage parts, the build process needs to be sped up [4]. In [8,9], ithas been shown that according to the used material system, isothermalcrystallization is present during processing (above Tic) and the im-plementation of time-dependent phase transitions into the model isnecessary. Amado [10,11] analyzed and modeled isothermal crystal-lization and the degree of crystallization for PA12 and polypropylenefor the first ten layers, considering the influence of powder coating andsupercooling of the molten regions. The simulations showed that withinten layers, the majority of the crystallization has been completed.

2.2. Temperature fields in selective laser sintering

To generate components with a high reproducibility, an accurateanalysis of the temperature fields present during processing in LS andprocess errors, such as curling, originating in temperature variations isof the highest importance [12]. In [13], curling is defined as a localwarpage phenomenon appearing during the exposure of the first layers.A basic requirement for its appearance is the beginning (isothermal)crystallization of molten areas and the resulting buildup of internalstresses due to prevalent temperature gradients. Consequently, the re-action of the components or partially resolidified component parts tothese stresses is an upwards oriented bending. Due to the mechanicalinteraction between the already partially resolidified component andthe coating mechanism, the deformed component regions may experi-ence further deformation or cause the partially resolidified componentparts to be dragged along with the coating mechanism. At worst, thiscan result in process abortion [13]. In order to maximize componentquality and process stability, it is of great importance to reduce thebuildup of the thermally induced internal stresses in the build process.For that reason, aiming for homogeneous surface temperatures ac-cording to the model of isothermal laser sintering and minimizing thedifference between the highest and lowest temperature points of thesurface is a common target in LS [14]. Nevertheless, depending on theapplied material, curling or warpage can occur in deeper layers.

Alongside the temperature gradients on the powder bed surfaceinduced by the various heating systems [14], the temperature of moltenmaterial as well cannot be considered as constant because of repeatedcontact to new and cool powder during deposition of individual powderlayers [15]. In addition, depending on the process time, the tempera-ture of the coating mechanism changes, which reduces the possibility ofthe associated deformation due to curling [16]. Thus, the general va-lidity of the model concept of isothermal laser sintering is limited anddoes not properly represent the actual LS process physics.

Besides surface temperatures, the temperature distribution withinthe molten material and the within whole build cylinder greatly affectthe quality of the resulting parts. The choice of exposure parameters,such as effective laser power or scan speed, has been found to be de-cisive for the resulting surface and melt temperatures and therefore partproperties [16–20]. However, even for optimum exposure parameters,an influence of part positioning inside the build chamber on the partcharacteristics can be detected, even for identical geometries [12,14].According to Josupeit [21,22] and Wudy [2], the thermal history ofeach part during building- and cooling-step is unique. This can be as-signed to an inhomogeneous temperature field present within the buildchamber and especially along the z-axis of most laser sintering systems[2]. These temperature fields lead to a locally different development ofproperties due to locally different material solidification. In [19],Gibson et al. already proposed to improve the heating and coolingstrategy within laser sintering systems to achieve homogeneous me-chanical part properties independently from the manufacturing posi-tion.

Summarizing the state of the art, the validity of the model of iso-thermal laser sintering and the suitability of the current standard setupof laser sintering systems to produce parts with uniform properties canbe queried. For this reason, this work will focus on the crystallization

Fig. 1. Schematic representation of a temperature profile of a standard LSprocess (according to [2]).

Fig. 2. Schematic illustration of a DSC curve for melting and recrystallizationallowing for the derivation of the processing window of PA12 (according to[3]).

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kinetics and the time-temperature dependent material state consideringphase transitions from a mostly viscous flow to a solid state with mostlyelastic properties. For the aim of direct manufacturing, it is necessary togenerate parts free from residual stresses and with constant properties,which can be achieved by means of a homogeneous morphology and aconstant degree of crystallinity [23].

2.3. Hypothesis

The scope of this article is the critical evaluation of the isothermalprocessing model and the setup of common LS machines. Limitations ofthe model contradicting the actual material behavior are shown and animprovement for the model is suggested allowing for the considerationof new processing strategies. For more than 15 years, the researchcommunity working on LS of polymers accepted the previously shownmodel of isothermal laser sintering as the valid process paradigm. Asshown in the state of the art, advancing research revealed an increasingamount of limitations of this model. There are certain open issues thatcall the existing model into question:

1 When is the crystallization of the melt initiated?2 When does the phase transition between flowable and solidifiedstate take place?

3 How long does it take until the phase transition is completed andfree from residual stresses?

We suggest that the current model is only valid for a few layersbelow the powder surface and needs to be reworked in order to be vi-able for the deeper layers in z-direction, featuring vastly differenttemperature levels, as shown in Fig. 3 [24]. Scientific studies, whichsupport this hypothesis of the continuous solidification of a semi-crys-talline material in z-direction during the building-step of LS, will beshown. These findings demand the reconsideration of the process modelallowing for the realization of a completely new system technology.

3. Materials and methods

3.1. Definition of material state

Polymers are commonly known to exert viscoelastic behavior and,depending on deformation speeds, temperature and pressure levels, theviscous or the elastic properties may dominate. The transition from aviscoelastic fluid behavior to a viscoelastic solid behavior is referred toas the Cross-Over-Point, which can be quantified in rheometers as thepoint, at which the loss modulus dominates the storage modulus(G´´>G´) [25]. As the discrimination of these states is essential for thepresented study, they will be referred to as the solidified state, in which

the elastic properties dominate and the material exerts mostly the be-havior of a solid and the flowable state or fluid state, in which theviscous properties dominate and the material exerts mostly the beha-vior of a liquid.

3.2. Experimental methodology

Fig. 4 shows the experimental methodology of this study. First,using a thermocouple, a component temperature measurement duringbuilding- and cooling-step is performed. According to this measure-ment, two questions emerged: 1) What is the actual the material state?2) How does supercooling during powder coating influence the crys-tallization behavior? In the following sections, a process-adapted ma-terial characterization, consisting of rheological measurements and amodel measurement using DSC, is elaborated. Due to inaccessibility, thecrystallization and solidification behavior has to be studied in process-adapted approaches with measuring parameters close to the actualprocessing conditions. Furthermore, in situ measurements inside afreely parameterizable laser sintering system were performed. There-fore, processing influences were studied in situ, observing the tem-perature on the powder bed surface and within deeper layers in z-di-rection during the running LS process. The results were evaluated withrespect to the material characterization.

Fig. 3. Modified model of isothermal laser sintering (left) according to Rietzel [24] and proposed enhanced model with the consideration of a time and temperaturedependency in z-direction.

Fig. 4. Schematic explanation of the experimental plan.

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3.3. Material

The rheological experiments were performed using a PA12 powderPA 2200 from EOS GmbH (Krailling, Germany). A powder mixture of50:50 wt.-% of virgin and overflow powder was used for the shownlaser sintering experiments. The viscosity number of the powder mix-ture was measured to 64ml/g and the bulk density was determined to0.44 g/cm³, indicating powder properties close to those of virgin ma-terial [26]. For an additional model experiment, polyamide 11 (PA11)powder, PA1101 from EOS GmbH (Krailling, Germany) was mixed withPA12 powder at different volume fractions of 5, 10 and 20 vol.-%.Compared to PA12, PA11 has a significantly higher peak meltingtemperature Tpm of 201 °C (measured at 20 K/min), but a similar par-ticle size and morphology.

3.4. Laser sintering system

For the processing experiments, a freely parameterizable laser sin-tering system with a maximum building height of 500mm was used. Ahomogeneous temperature distribution on the powder bed surface, withdimensions of 350× 350mm², is guaranteed by the eight-zoned IRsurface heating system. This is a fundamental requirement for theproduction of components with reproducible properties.

3.5. Process-adapted rheological measurements

To date, the degree of crystallization and the present material stateof semi-crystalline thermoplastics cannot be measured directly.Therefore, a new approach correlating the Cross-Over-Point determinedby time-sweep in rotational viscometry to isothermal DSC measure-ments was applied. The experiments were carried out to study thematerials’ state under process-adapted conditions, which can be de-scribed simplified as pressureless and isothermal. For rotational visco-metry measurements, 680 ± 2mg of virgin powder were pressed at aforce of 70 kN for 3min to a circular tablet with a diameter of 25mm.Due to a defined volume of material, this step increases the reprodu-cibility of the rheological measurements. The powder tablets were driedin an oven at 70 °C under vacuum for one week before testing to avoidresidual moisture in the hygroscopic material. The measurements werecarried out on a Discovery Hybrid Rheometer HR-2 (DHR-2) from TAInstruments using 25mm aluminum parallel-plate. The samples wereplaced on the lower plate, which was set to a temperature 190 °C. Anoscillation time sweep at a gap of 1000 μm was used to determine thestorage (G´) and loss (G´´) moduli Cross-Over-Point. This point is con-sidered as the transition from viscoelastic fluid to viscoelastic solidmaterial for a temperature range. A starting temperature of 190 °C waschosen. The material was cooled down with a cooling rate of 5 K/min tothe designated isothermal crystallization temperature (Tiso) of 166 °C,168 °C and 170 °C, then held isothermal at the temperature. The tem-peratures are chosen close to processing temperatures. The deformation

was set to a low value of 0.5% and the frequency was adjusted to 1 Hzin order to simulate the unpressurized process. For the resulting mea-surements, the Cross-Over-point was evaluated. The chain arrangementformed due to crystallization limits the possible movement of thepolymeric chains, resulting in an increasingly elastic material behavior.When the Cross-Over-Point is exceeded, the elastic behavior dominatesthe viscoelastic behavior and hence the material reacts in form of aviscoelastic solid as opposed to a viscoelastic liquid prior to the Cross-Over-Point.

3.6. Model experiments for simulating powder coating in DSC

Using isothermal DSC measurements, the influence of unmeltedpowder particles on the crystallization time is shown. Measurementswere carried out to determine the isothermal crystallization behaviorusing a differential scanning calorimeter device Q2000 from TAInstruments. The DSC measurements were performed under nitrogenatmosphere and the sample masses slightly ranged between 2.5 and3.5 mg. The isothermal crystallization process was induced as follows.The samples were heated with a heating rate of 20 K/min to the targettemperature of 195 °C. The temperature was held for one minute toeliminate residual crystals. Afterwards, the material was cooled downwith a cooling rate of 60 K/min to the designated isothermal crystal-lization temperature (Tiso) of 168 °C. Then, the temperature was keptconstant for isothermal crystallization for 120min. The temperature of195 °C was chosen as it is above the melting temperature of PA12 butbelow the melting temperature of PA11.

3.7. Stick-drop-experiment

Stick-drop-experiments were performed to analyze the plasticity ofthe molten material according to their position in z-position. Therefore,the in Fig. 5 depicted measuring set-up was invented. The systemconsists of a positioning system, which provides reproducibility to theexperiments, a falling mechanism, allowing for a defined falling heightfor a maximum of nine penetration sticks with adjustable mass andopening angles. The experiments were conducted with five penetrationsticks with an averaged mass of 21.53 ± 0.04 g and an opening angleof 30°.

For the experiments, parts with dimensions of 150× 60×2mm³(length × width × height) were fabricated with increased energy inputto assure melting of the material. For every experiment, one part wasplaced in the center of the build chamber. Y-parallel hatching of thescan vectors without rotation was applied for part generation. Theprocessing parameters can be extracted from Table 1.

The stick-drop-experiment was conducted as follows. The buildchamber was opened, the positioning system was placed onto the re-ference markers and the falling mechanism was triggered. To ensureminimal influences on the temperature balance inside the process, thebuild chamber was closed immediately after triggering the mechanism.

Fig. 5. Photographic image of the set-up for the stick-drop experiments.

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After impact, the parts remained inside the closed build chamber for5min, before the parts were extracted and cooled at room temperature.Subsequently, the adhering powder was removed carefully by manualbrushing. To measure the depth of the imprint, the samples were ana-lyzed by a sub-μ computer tomograph (sub-μ CT) from FraunhoferInstitut in Germany. The measurements were performed with an ac-celeration voltage of 80 kV, a voxel size of 9.7 μm and an exposure timeof 800ms. The maximum penetration depth was determined by mea-suring the distance between two parallel planes after horizontally or-ientating the layer as shown in Fig. 6. To study the influence of z-po-sition, the stick-drop-experiment was performed on setups featuring 5,15 and 25 new powder layers placed on top of the last processed (i.e.molten) layer.

3.8. Temperature measurements

Two experiments were performed using a thermographic infrared(IR) system. At first, the surface temperature during the sub-process ofpowder coating was tracked. Therefore, 100 powder layers were ap-plied without exposure. The powder coating speed (vt) was kept con-stant at a standard velocity of 250mm/s. As even low temperaturegradients can strongly affect the crystallization kinetics, the observationof the powder coating and exposure process is necessary. For that cause,the IR thermographic system Velox 1310k SM from IRCAM GmbH(Erlangen, Germany) was used. The applied system is capable of de-tecting thermal radiation in a range of wavelengths of 1.5–5.5 μm. Theassembly situation was shown in [20]. According to [27], the presentviewing angle was found to be suitable to avoid interferences. A sap-phire glass window, which is impermeable for radiation of wavelengthshigher than 6 μm, was placed in front of the camera to prevent dama-ging of the IR-detector. For higher data acquisition rates, the recordingformat was reduced from 1280×1024 pixels to 416× 416 pixels. Theemission coefficient of the IR camera was adapted and kept constantaccording to the temperature of the build chamber, which was mea-sured by an integrated pyrometer. However, it can be expected, thatdue to higher surface reflections of the rough powder bed, the realsurface temperatures are underestimated, especially during phasetransition. For that reason, shown temperature values can be inter-preted parametrically as different radiation intensities are measured.

The second experiment was performed to correlate surface

temperatures to the measurement of the melting temperature of thefabricated components in z-direction. Therefore, four parts with the sizeof 20×20×1mm³ (length × width × height) were fabricated withstandard exposure parameters resulting in an energy density of 0.4 J/mm³. The laser power was adjusted to 16W and the scan speed to2000mm/s, which is an optimized parameter setup for the used lasersintering system. K-type thermocouples of 2×0.08mm were fixed ontothe build platform by a metal wire to measure the temperature of themolten component during processing. The thermocouple was posi-tioned precisely at the powder bed surface. A temperature profile of thefirst layer during exposure and powder coating in the z-direction wasmeasured and correlated to the IR measurements.

4. Results and discussion

Within the following sections, it will be shown that the model ofisothermal laser sintering is solely convenient for layers close to thepowder bed surface, but an understanding of the process regarding thedeeper layers is necessary. Therefore, an approach to correlate thematerial properties to processing is shown to prove that the solidifica-tion of the component occurs during the building-step.

4.1. Component temperature during building- and cooling-step

The temperature in the first exposure layer of a component wasmeasured by a thermocouple to analyze its temperature progressionduring building- and cooling-step. Fig. 7 shows the measured tem-perature profile. The temperature gradient between the set surfacetemperature and the temperature measured by thermocouple can beexplained by differing measuring positions and principles. For tem-perature control, an optical measurement device (pyrometer) was used,whereas the reduced temperature values detected by thermocouple canbe explained by its positioning below the first powder layer and smallerdeviations resulting from contact conditions and system inertia.

The duration of the total exposure process can be determined to6.5 min. It is followed by 20min of powder coating for the fabricationof 40 additional isolation layers. Interestingly, coating a new layer ofpowder on top of the last laser processed layer already leads to atemperature drop from 170 °C (T1) to 152 °C (T3) resulting in non-iso-thermal component temperatures. This favors early and acceleratedcrystallization processes. After the building-step is finished, the dura-tion of the cooling-step, which is defined as completed after reachingthe extraction temperature of 80 °C, was determined to 35min (Fig. 7).In LS, low cooling rates typically lead to long durations of the cooling-step, which induces homogeneous shrinking and a minimum distortionof the components.

Table 1Processing parameters for specimen fabrication.

Processing Parameter Unit Value

Build temperature °C 172Hatch distance mm 0.2Scan speed mm/s 2000Laser power W 20Energy density J/mm³ 0.5Layer time (empty layer) s 39.8Layer time (exposed layer) s 46.7

Fig. 6. Schematic illustration of the evaluation strategy for measuring the im-print depth of CT images.

Fig. 7. Component temperature during building- and cooling-step measured bythermocouple.

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4.2. Solidification during LS process

4.2.1. Process-adapted rheological measurementsRheological measurements of the polymer describe the viscoelastic

properties originating in the molecular chain structure, whereas crys-tallization can be defined by the melt partially solidifying due to spe-cific spatial arrangement of the molecular chains in the polymer. Inorder to characterize the state transition and the solidification of thestudied material at isothermal conditions, rheological measurementswere performed to determine the Cross-Over-Point. Fig. 8 shows theresults of storage and loss moduli with an oscillatory time sweep onPA12. The Cross-Over-Point was reached after 10min during iso-thermal oscillation, which can be correlated to the isothermal crystal-lization time using DSC measurement at 168 °C, which was measured to50min [28] (see Fig. 9).

In Fig. 9, the degree of crystallization during isothermal DSC isshown for the three different temperatures of 166 °C, 168 °C and 170 °C.The curves were calculated according to [28]. It can be seen that thetime of Cross-Over (tCO) increases from 6.1min to 32.4 min by raisingthe temperature from 166 °C to 170 °C. To a certain extent, the devia-tion of the second Cross-Over-Point at 168 °C can be correlated, tomeasuring inaccuracies, as both measuring methods are performed withvery different sample masses. However, the Cross-Over-Points arereached much earlier compared to the completion of the isothermal

crystallization process, since crystallized areas prevent a chain move-ment. It can be seen that depending on temperature the material can berated as consolidated after a degree of crystallization of only 20% to30%. In the beginning of crystallization, the material remains viscous.During crystal growth, molecular mobility is prevented and materialstate can be interpreted as viscoelastic solid with an incomplete andprogressing crystallization. The modulus of the Cross-Over-Point in thismeasurement is 2.7× 105 Pa, which is solidified into a rubbery sub-stance. Stress relaxation needs to be further investigated in order todefine whether internal stresses already relax inside a given layer orstresses are built up layer-by-layer and possibly transferred into deeperlayers.

In Fig. 10, the correlation of the Cross-Over-Point to the tempera-ture drop from T1 of 170 °C to T3 of 152 °C within of the molten com-ponent according to the temperature profile in Fig. 7 is shown. Thetemperature decrease leads to accelerated crystallization and an earliersolidification of the melt. While at the initial temperature T1, the Cross-Over-Point is not reached before 30min, the lower temperatures T2 andT3 result in Cross-Over-Points appearing even below the five minutemark (according to the exponential fit). This indicates a quick crystal-lization induced solidification within the lower layers in z-position.

4.2.2. In process measurement of solidification by stick-drop-experimentIn order to evaluate the significance of the rheological measure-

ments for the LS process, stick-drop-experiments were performed asdescribed earlier. No immediate curling could be observed afteropening of the build chamber for placement of the positioning systemonto the powder bed, indicating that five covering powder layers were asufficient insulation for prevention of major temperature changes in thelayers of interest. However, the hot components warped after extractionfrom the powder bed. The extend of warpage can be correlated to theparts total time within the build chamber. Photographic images of thefabricated samples with a subsequently executed stick-drop experimentare depicted in Fig. 11. The experiment with five additional layers ofpowder (Fig. 11a) shows deep imprints into the parts surface, indicatingthat the material is still in a viscoelastic fluid state. In addition, a gra-dient along the parts geometry is visible, which can be assigned to theconstant y-parallel hatching and the differing local thermal history. Infuture, to reduce this effect, comparable experiments should be per-formed with a defined angular offset value. For parts with 15 (Fig. 11c)and 25 follow-up layers of powder, no imprints could be detected. Dueto the insignificant differences, the results are not shown for 25 follow-up layers. After extraction, additional powder was adherent to theimpact site of the penetration stick, which could be removed easily by abrush. Correlating the resulting processing times after exposure of

Fig. 8. Temperature-time-dependent storage and loss modulus curves of a timesweep on PA12 using a rotational viscometer.

Fig. 9. Degree of crystallization in dependence on time and isothermal holdingtemperature calculated from isothermal DSC measurements.

Fig. 10. Time of Cross-Over-Point over temperature for three different tem-peratures determined by isothermal DSC measurements during building-step.

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around 10.0 min and 16.6 min for 15 and 25 additional layers to theprocess-adapted measurements. This can be explained by acceleratedand well-advanced isothermal crystallization and the resulting solidi-fication of the material due to a temperature decrease in z-direction.

For that reason, an additional experiment was performed to isolatethe effects of tempering duration and a possible temperature gradient inz-direction. Therefore, the experiment with five follow-up layers wasrepeated and after the last layer of exposure, the build temperature washeld for 6.6min to simulate 15 follow-up-layers. In Fig. 11b), the re-sulting imprints, which are significantly smaller than those withoutadditional tempering time in Fig. 11a) can be seen. This again is anindication for a time-driven crystallization and solidification process.However, compared to the experiments with 15 layers, imprints arevisible, which can be explained be the greater proximity to the buildsurface and the impact of the IR-heaters. A higher melting temperatureand higher fusion times lead to slower crystallization kinetics, whichcould explain a delayed solidification. The evaluated imprint depthvalues can be extracted from Fig. 12, confirming the described effect, asthe highest imprint depth is measured for the experiment with only fiveadditional layers of powder.

After impact in the stick-drop-experiment, the hot parts were ex-tracted from the powder bed after an additional waiting time of fiveminutes. It can be seen that the higher the probability for (isothermal)crystallization and the higher the resulting degree of crystallization,which can either be achieved by a long tempering time or by a lowtemperature, the lower the warping tendency. Compared to the rheo-logical measurements, material solidification due to crystallization canbe observed as well. Nevertheless, compared to the rheological mea-surements, the material solidification is influenced by the continuouslyrepeated sub-processes of exposure and powder coating. The latterleads to a supercooling of the molten material, which might induce anearlier crystallization and material solidification.

4.3. Supercooling of the molten material

In Fig. 13, a magnification of the temperature profile measured bythermocouple at the location of the first layer is shown for ten follow-upbuilding layers. The temperature rapidly increases due laser impact.

After a few seconds, the temperature decreases to the build chambertemperature. In addition, high cooling rates are induced by powdercoating, but the surface heaters reheat the powder bed quickly. Thetemperature fluctuation due to exposure and powder coating is reducedin the lower level. Even after ten layers, the temperature influence fromthe uppermost layer can still be detected. Regarding the averaged curvein Fig. 13, the model understanding of isothermal processing can beseen as largely valid for the first layer as in sum an increase in tem-perature rather originates in the influence of previous layers than in atemperature reduction. However, due to the temperature drops causedby powder coating, cooling of the exposed areas could promote iso-thermal crystallization.

The results of the surface temperature measurement during ex-posure and powder coating are correlated to the measurement in z-direction, which is shown in Fig. 14. It can be seen, that the tendenciesof both measurements can be well correlated. However, due to the highheating rates during laser exposure, high peak temperatures can beobserved by IR measurement, whereas a lower maximum temperatureis determined by thermocouple measurements. This can be explainedby the positioning of the thermocouple below the first layer of powder,the local conditions of contact to the molten material and the higherthermal inertia of the thermocouple. This assumption is also valid forthe temperature decrease caused by powder coating, indicating higher

Fig. 11. Fotographic images of parts after the stick-drop-experiments after a) 5layers b) after 5 layers with 10 layers of waiting time and c) after 15 additionallayers of powder.

Fig. 12. Resulting imprint depth determined by CT for the stick-drop-experi-ments after 5 layers, 5 layers with 10 layers of waiting time (5+ 10) and 15layers.

Fig. 13. Component temperature during the building-step measured by ther-mocouple at the height of the first layer.

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supercooling of the molten material, which favors early crystallization.Thermography measurements during powder coating were used to

determine the surface temperature development after one and up to100 layers. In Fig. 15, the temperature progression is shown ex-emplarily for the first layer. The irregularities during the coating pro-cess can be correlated to the coating mechanism passing through themeasuring area. A temperature decrease from 170 °C to 141 °C can beobserved. In this case, the low tempered particles can act as crystalnuclei and trigger the crystallization process in the melt.

In Fig. 16, the minimum powder coating temperature is shown for100 layers, whereas layer one was fabricated at first and layer 100 atlast. With increasing layer number no clear effect can be seen, whichcan be correlated to the set-up of the used LS system. The powder ispreheated before powder coating and after every passage excessivepowder is disposed into the overflow container. For that reason, ex-tensive powder preheating is avoided. For the present system, it can beseen that the minimum temperature is even below the crystallizationtemperature Tic of 154 °C, increasing the probability of initiating (iso-thermal) crystallization due to supercooling.

For other systems, which are continuously coating the powder bedsurface with the same material and working without overflow con-tainers, an increasing minimum coating temperature can be expected,which could be advantageous for the LS process to delay isothermalcrystallization.

4.4. Unmelted particle as nucleus

The impact of the earlier mentioned supercooling on crystallizationcan be seen exemplarily in Fig. 17. Well-defined spherulitic super-structures can be found near the particles penetrating the previouslymolten material. It can be seen that the lamellae are spreading out fromthe unmelted particles. These results demonstrate that crystallization isactivated during the material coating sub-process and the cold andunmelted particles act as crystallization nuclei.

The influence of unmelted particles penetrating the molten PA12material was reconstructed within isothermal DSC measurements byadding different volume fractions of PA11 polymer powder. PA11 wasused as it has a significantly higher melting temperature compared toPA12. The resulting heat flow curves and the degree of crystallization isshown within Fig. 18. It can be seen that the crystallization kinetics areaffected by the presence of PA11 particles. An increasing volumefraction of PA11 leads to an earlier crystallization and a higher slope ofthe crystallization curve progression even though the particles are atthe same temperature level. Although the PA11 particles are on thesame temperature level as the PA12 particles, the influence can betransferred to the powder coating process as the influence of the colderPA12 particles on molten PA12 can be expected to be similar. Both areexpected to act as nuclei for crystallization. However, the shown effect

Fig. 14. Fitted comparison between IR and thermocouple measurement eval-uated in the center of the component.

Fig. 15. IR-measurement of the build chamber temperature during powdercoating.

Fig. 16. Minimum layer temperature during the recoating process for 100layers.

Fig. 17. Exemplary laser scanning microscopy image of the part surfaceshowing crystallization initiation at the site of particles penetrating the moltenmaterial.

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might even be enhanced by the lower temperature of PA12 particles.According to this result, crystallization can be induced and acceleratedby cooler particles being introduced into the melt by the powdercoating as indicated by [10,11].

In combination with the previously shown results, the authors coulddemonstrate that isothermal crystallization leads to solidification of thematerial in z-direction during the building-step of the process. Thisnecessitates an extension of the currently available process model asproposed earlier. Interestingly, stresses, which are built-up duringcrystallization, can be eliminated during the process. This leads to theassumption that according to a material-specific process control, themolten material can actively be cooled-down without resulting partwarpage. This provides the opportunity to accelerate the cooling-step,as the part cake material of subjacent layers can be cooled-down whilethe upper layers are fabricated under quasi-isothermal conditions.According to these results, the possibility to expand the previously validprocess boundaries arises, resulting in the potential of a continuousprocess with a possible early component extraction.

5. Conclusion and outlook

The investigations of the present paper show that the model ofisothermal laser sintering is only valid for uppermost layers, but needsto be reconsidered for the complete building-step. Finally, the researchquestions will be answered based on the shown process-adapted and inprocess measurements for the standard LS material system PA12:

1 When is the crystallization of the melt initiated?

The crystallization is initiated at build chamber temperature underquasi-isothermal conditions. At a build chamber temperature of 168 °C,the degree of crystallization of 50% is reached after around 22min.Considering a layer time of 40 s, 33 layers could be built in the mean-time. Within LS part generation, the crystallization process is ac-celerated by super-cooling during powder coating. Therefore, futureDSC-measurements will be conducted under process conditions, re-specting high heating rates during exposure and the super-coolingcaused by powder coating.

2 When does the phase transition between flowable and solidifiedstate take place?

According to the rheological measurements, the PA12 LS materialdisplays an earlier solidification at an even lower degree of crystal-lization of far below 20% for a build chamber temperature of 168 °C.

The materials Cross-Over-Point was detected after 10min, which can becorrelated to 15 layers or a z-height of 1.5 mm. Within the process, thiscould be confirmed by the stick-drop-experiment, as the phase transi-tion between flowable and solidified material has already taken place15 layers after the initial exposure.

3 How long does it take until the phase transition is completed andfree from residual stresses?

To define the exact phase transition time, a new measurement set-up, based on a FTIR spectrometer combined with a rheometer wasdeveloped. The FTIR spectrometer is able to evaluate the degree ofcrystallization and rheological measurements, which are conducted inparallel, give an insight into the stress build-up during crystallization orcooling. Therefore, a better understanding of the materials thermalhistory and condition will be established.

Future work of the authors will deal with the determination andadaption of the process temperature profiles to control material soli-dification within the LS process. Taking material specific stress re-laxation into account, accelerated cooling strategies can be performedgenerating parts free from residual stresses. These findings lead to theadaption of the model of isothermal laser sintering provide the oppor-tunity to reconsider the whole processing and temperature conditioningstrategy of the LS process, which will be performed in future work ofthe authors.

In detail, the build chamber temperature control and distributionwill be revised fundamentally. More precisely, the build surface and theexposed layer will be tempered as homogeneously as possible in orderto provide a stable base for isothermal consolidation, which couldmainly be performed by surface or close to surface heaters in z-directionand an adequately adapted exposure process. Heating in z-direction ofthe complete material is no longer necessary throughout the wholebuilding-step. On the contrary, after a certain height threshold, a de-fined temperature gradient in z-direction can be applied, allowing forcontrolled consolidation parallel to the building-step, leading to ahomogenized thermal history of every part. This should on the onehand result in the demanded higher reproducibility of part propertiesand on the other hand lead to larger possible build chamber cross-sections to enhance the efficiency of the future laser sintering processsignificantly. More importantly, the cooling process no longer de-termines the production time and high quality parts are available faster.Furthermore, the active cooling allows for the removal of parts indeeper layers, creating the basis for infinite continuous production.These improvements have the potential to render the LS technologyeven more attractive for industrial applications.

Acknowledgements

The authors want to thank the German Resarch Foundation (DFG)for funding the Collaborative Research Center 814 (CRC 814) –Additive Manufacturing (Grant ID: 61375930), sub-projects A03 andB03.

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