Characterising Ultrasint PP Nat 01 Polypropylene to Examine Its Feasibility in Powder Bed Fusion

Abstract

The current study examines the feasibility of Ultrasint PP nat 01 polypropylene material in powder bed fusion through powder characterisation. The results obtained are also deemed to be pertinent when developing or validating analytical and numerical models of Polymer Laser Sintering, which were not within the scope of this paper. The following critical characteristics were examined: powder morphology, powder particle size distribution (PSD), bulk density, tapped density, melt flow index, thermal characteristics of the material, degree of crystallinity, and optical properties. Ultrasint PP nat 01 powder has a PSD in the range of 20–80 µm, which is within the recommended particle size distribution. The Hausner ratio, tapped density, and bulk density of the material were calculated and measured as 1.230 ± 0.05, 0.455 ± 0.02 g/cm³, and 0.370 ± 0.03 g/cm³, respectively. The melt flow index of Ultrasint PP nat 01 was measured as 15.8 g/10 min. The initial melting point of the material was determined to be 133.8 °C. The powder used had a relatively high sintering window of 30.7 °C, a degree of crystallinity of around 31.8%, and a high thermal stability of around 461.52 °C. The material was found to attain full fusion of particles at around 170 °C. Fourier Transform Infrared Spectroscopy indicated that Ultrasint PP nat 01 powder has poor radiation absorption, but high transmission properties.

1. Introduction

Polymer Laser Sintering (PLS), a subset of Powder Bed Fusion (PBF), has failed to be implemented on a full scale in various sectors (aerospace, automotive, and medical), mostly because of limited materials and lack of process repeatability for new materials [1]. Polymeric materials used in PLS should possess suitable intrinsic and extrinsic properties to ensure successful processing. As a result, considerable attention has been directed to this area. Recently, researchers have taken a keen interest in developing analytical and numerical models of PLS. Analytical and numerical modelling can be used to qualify materials for PLS and to determine process parameters [2]. The aim of such models is to facilitate the introduction of new materials by assisting researchers and powder manufacturers to determine suitable material properties, as well as optimum process parameters, without the need for extensive experiments. Presently, research in this area is still at an early stage.

Critical material properties should be identified to process or validate analytical and numerical models of PLS. Adequate knowledge of the most critical material properties for polymers that affect the PLS process and the quality of printed parts is still lacking. Rudloff et al. [3] noted that it is difficult to introduce new materials for PLS because of limited understanding of the material properties that impact the process and the quality of the final product. Moreover, there is a need to ensure process repeatability for the successful uptake of PLS for different industrial applications [1]. Therefore, numerous studies have been conducted to characterise different polymeric materials to determine their suitability in PLS [4,5,6,7,8,9]. Despite notable attention from academia and the industrial sector, the phenomenon of PLS is still not yet well understood.

Soundararajan et al. [1] stated that more research is needed to provide further information on some of the PLS mechanisms, such as heat absorption, phase change, and solidification of particles. Analytical and numerical modelling of PLS are crucial tools since they can be used to link material properties, process parameters, and component characteristics [10]. However, some material properties and their behaviours should be quantified before developing the necessary analytical and numerical models for PLS. In addition, analytical and numerical models for PLS should be validated using experimental results.

Normally, the process of PLS is considered to have three steps, which include pre-heating, building, and cooling phases [2]. According to Mokrane et al. [11], despite the simplicity of the concept, the process of PLS involves numerous complex multi-physical transient phenomena. Some of these phenomena include interactions between a laser beam and powder on the powder bed, powder spreading, thermal diffusion, polymer viscous flow and particle coalescence, solidification and crystallisation, layer deposition, and volume shrinkage [9,11]. These steps and occurrences are influenced by different material properties. For instance, powder spreading is subject to powder particle size distribution and shape. The interaction between a laser beam and powder on the powder bed is affected by the optical properties of the powder. Thermal diffusion, polymer viscous flow, and particle coalescence depend on material characteristics, such as melting point, surface tension, viscosity, heat capacity, and thermal conductivity.

According to Rudloff et al. [3], pertinent information on important material properties, which are needed for developing analytical and numerical models for PLS, is still insufficient. The authors undertook an investigation to determine the changes in temperature over time and sintered volume of monolayers for different surface energy densities, during the processing of polyamide 12 (PA12), through experimental and simulation results. They established that the most crucial material properties for this undertaking were powder particle radius, solid density, powder bulk density, heat capacity, thermal conductivity, thermal expansion, latent heat, melting/liquidus temperature, solidus temperature, offset temperature, activation energy, liquidus viscosity, solidus viscosity, and surface tension. Li et al. [10] developed heat-transfer, thermo-mechanical, and material crystallisation kinetics analytical and numerical models to examine the impact of different process parameters on the distribution of temperature, residual stress, as well as the shrinkage and warping of parts printed using PA12. The authors identified temperature-dependent density, melting point, heat conductivity, power absorption coefficient, degree of crystallinity, thermal expansion coefficient, and specific heat as the main material properties required to develop the heat-transfer, thermo-mechanical, and material crystallisation kinetics models for PLS. Olivier et al. [12] studied results from thermal models to examine the widths of melt tracks. The authors considered the following material properties when developing the models: thermal conductivity, specific heat, and the absorptivity of materials.

In this analysis, it was suggested that nine analytical and numerical models (Figure 1) could be used to comprehensively define the entire process of PLS. A previous study (Mwania et al., [13]) outlined the following material properties as the most crucial for the different models outlined in Figure 1: the size, shape, and distribution of particles, optical properties of powder (absorption, reflection, and transmission), viscosity of the melt, surface tension of the melt, bulk density, melting point, specific heat of a polymer, and degree of crystallinity. Pertinent experiments to examine the above material properties are necessary for new polymeric materials. The results obtained are deemed to be useful when developing or validating analytical and numerical models for PLS, which were beyond the scope of this paper. In this regard, this study is a combination of experimental investigations deemed necessary to provide pertinent insights into material properties crucial in establishing the feasibility of a polymeric material in PLS. The research focused on a particular commercial polypropylene powder (Ultrasint PP nat 01).

2. Materials and Methods

2.1. Materials

Ultrasint PP nat 01 from Badische Anilin und Soda Fabrik (BASF, Ludwigshafen, Germany) was purchased from a local agent in South Africa (Reach 3D, Pretoria, South Africa). It is a polypropylene-based powder for laser sintering, and according to the supplier, the material can be used as an alternative to polyamide 12, which is the leading feedstock polymer in PLS. According to the supplier, the material has excellent plasticity, high elongation, low moisture absorption, resistance to most acids and bases, and high durability. Moreover, they note that it can be used to print components for automotive, electrical, sports, health, and orthopaedic applications. It is also claimed to be perfect for manufacturing hinges and clips. The physical and mechanical properties of the material provided by the supplier are summarised in

Table 1. Material properties of Ultrasint PP nat 01.

Properties	Value	Unit	Test Method
Average particle size D50	60–70	μm	Laser Diffraction
Bulk density	330	kg/m3	DIN EN ISO 60 [14]
Melt volume flow rate	14	cm3/10 min	ISO 1133 (230 °C, 2.16 kg) [15]
Printed part density	890	kg/m3	ISO 61 [16]
Melting temperature	140	°C	ISO 11357 (10 K/min) [17]
Crystallisation temperature	100	°C	ISO 11357 (10 K/min)
Tensile strength (X-, Z-direction)	28	MPa	ISO 527-2 [18]
Tensile modulus (X-, Z-direction)	1400	MPa	ISO 527-2
Tensile elongation at break	30 (x-direction) 10 (z-direction)	%	ISO 527-2

. The material was characterised in this study, in its as-received state, without further modification.

2.2. Methods

Different material parameters were identified as crucial for analysis in the present work based on the models outlined in Figure 1. These material properties included powder morphology, powder particle size distribution, bulk density, tapped density, melt flow index, thermal behaviour of the material (melting point, crystallisation point, enthalpy of melting, sintering window, and degradation temperature), degree of crystallinity, and optical properties. The experimental characterisation undertaken in this study was limited by the availability of equipment and was, thus, in no way exhaustive.

2.2.1. Powder Morphology and Particle Size Distribution

A Scanning Electron Microscope (SEM) (JSM-6610 Microscope from JEOL Ltd., Tokyo, Japan) was utilised to examine the surface morphology of the Ultrasint PP nat 01 powder at an accelerating voltage of 20 kV. The powder was carbon-coated first, using a Q150T E carbon coater (from Quorum Technologies, East Sussex, England) to increase conductivity and reduce the phenomenon of charging effects, which affects the quality of the obtained images. The size distribution of the particles was estimated from a previous study (Wu et al., [19]) using a Zeiss AxioCam optical microscope (from Carl Zeiss AG, Oberkochen, Germany).

2.2.2. Tapped and Bulk Density Testing

Both tapped and bulk density testing were performed using a Tap Density Tester, Model ETD-10120X (from Electrolab India PVT. Ltd., Mumbai, India), following the ASTM D7481-18 test method [20]. The experimental data obtained were used to determine the Hausner ratio in order to investigate the flowability of the material.

2.2.3. Melt Flow Index Testing (MFI)

Melt flow index (MFI) testing was performed using a Melt Flow Junior Tester (from Presto Group, New Delhi, India) at a temperature of 190.0 °C, based on the melting point of the material and the recommendation of the operator, using a load of 2.16 kg, which is a standard weight for ASTM D1238 [21]. The mass of the material that was extruded in 600 s was measured and recorded. A total of five trials were undertaken, and the results obtained are recorded.

2.2.4. Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) was undertaken using DSC 6000 equipment from PerkinElmer (Shelton, CT, USA) to determine the melting range, sintering window, enthalpy of melting, and degree of crystallinity of Ultrasint PP nat 01. The heating and cooling cycles were applied at a temperature rate of 10.00 °C/min. Heating was performed from 30.0 °C to 180.0 °C, whereas cooling was performed from 180.0 °C to 30.0 °C, with a hold of 1.0 min at 30.0 °C. Three cycles were undertaken for each of the samples of approximately 7.0 mg in an environment of nitrogen.

2.2.5. Thermogravimetric Analysis (TGA)

The thermogravimetric analysis (TGA) was conducted using an STA 6000 thermal analyser from PerkinElmer (Shelton, CT, USA). In this process, around 17.00 milligrams of powder was placed in a crucible and then heated from 30.0 °C to 665.0 °C at 10.0 °C/min, with a one-minute hold at 30.0 °C. The experiment was undertaken in an atmosphere of nitrogen.

2.2.6. Fourier Transform Infrared (FTIR) Spectroscopy

Fourier Transform Infrared (FTIR) spectra of the polymer were obtained using a Spectrum 100 FT-IR Spectrometer from PerkinElmer (Shelton, CT, USA). The spectrum was averaged over 8 scans at a resolution of 4.00 cm⁻¹.

2.2.7. Hot-Stage Microscopy

Hot-stage microscopy was undertaken using an HFS91 microscope from Agar Scientific Ltd. (Sheffield, UK) Images were captured using ScopePhoto software 3.1, which involved a set-up process with the following steps (Acquire–Twain Aquire–Framtech USB PC camera). The format of the images was selected as JPEG with an output size of 640 × 480. The sample of powder was spread and sandwiched between two glass cover slides and placed on the sample holder. The behaviour of particles of powder was observed at different temperatures, and images were obtained for analysis. Hot-stage microscopy is a crucial tool for validating crucial numerical models for PLS on the basis of the fusion of particles, the impact of different cooling rates, and the effect of high packing density and uniformity on the powder bed.

3. Results

3.1. Powder Particle Shape, Morphology, and Particle Size Distribution

Figure 2 illustrates the powder morphology of Ultrasint PP nat 01, as depicted in micrographs that were obtained in the present work.

It is clear from Figure 2 that the Ultrasint PP nat 01 powder from BASF consists of a myriad of powder particle shapes, ranging from rod-like, “potato-shaped”, almost spherical, flakes, and irregular shapes. It would be correct from the foregoing statement to expect the powder to have poor flowability, powder packing density, and fluidisation behaviour. According to Sutton et al. [22], the regularity and irregularity of powder particles affect how the particles interact and how they flow as a bulk entity. In addition, according to Berretta et al. [23], suitable materials for PLS should be nearly spherical with smooth surfaces, which ensures good flow, a high packing density, and a high rate to reach fluidisation. The particles also have relatively rough surface features. Powder particles with smooth surfaces are preferred in PLS because they provide small amounts of inter-particle friction and result in uniform and densely packed powder. The flowability of the material was further assessed by establishing the Hausner ratio (HR).

Results from a previous analysis (Wu et al., [19]) showed that Ultrasint PP nat 01 has a wide particle size distribution ranging from 10 µm to 80 µm, which lies within the recommended distribution of 20–80 µm according to Schmid and Wegener [24].

According to Drummer et al. [25], particle size distribution and geometry greatly affect the porosity of printed parts. In addition, particle size distribution, morphology, and shape affect the powder bed packing density, the deposition of thin and homogenous layers, sintering kinetics after exposure to a laser beam, surface roughness, and the density of printed parts [26,27].

Powder particle shapes and PSD should be considered when developing analytical and numerical models for the fluidisation and powder spreading processes in PLS. Polymeric materials with numerous different shapes of the particles of powder, such as the ones for Ultrasint PP nat 01 shown in Figure 2, are said to be complex with respect to their geometric shapes and are difficult to model. Considering all particles of powder to be spherical leads to inaccurate analytical and numerical models. Parteli [28] proposed using the multi-sphere method when modelling powders with complex geometrically shaped particles. The method combines different irregularly shaped particles and then defines them using spheres of known diameters (marked by letter D), as illustrated in Figure 3, from previous work by Tan et al. [29].

Berretta et al. [23] also developed a circularity versus roundness chart, as illustrated in Figure 4. Most of the particles for Ultrasint PP nat 01 are similar to the “potato-shaped” particle encircled in red in Figure 4. Hence, the circularity and roundness of Ultrasint PP nat 01 can be estimated as 0.5 and 0.5, respectively, for application in analytical or numerical modelling, since most of the particles are either “potato-shaped” or almost spherical.

3.2. Hausner Ratio

The tapped and bulk density of the Ultrasint PP nat 01 powder were measured as 0.455 ± 0.02 g/cm³ and 0.370 ± 0.03 g/cm³, respectively. Thus, the Hausner ratio for Ultrasint PP nat 01 was calculated as 1.230 ± 0.05, from the ratio of tapped density to bulk density. The Hausner ratio provides pertinent flowability information for powders [30]. Tan et al. [31] determined the HR values for Laser PP CP 20, Laser PP CP 60, and PA 12 powders to be 1.28, 1.22, and 1.13, respectively. Laser PP CP 20 is an isotactic polypropylene homo-polymer, while Laser PP CP 60 is a propylene–ethylene copolymer. The HR value for Ultrasint PP nat 01, considered in this study, is close to the value for Laser PP CP 60. Tan et al. [31] added that powders that have values of HR less than 1.25 exhibit the free-flowing behaviour that is required in PLS. Hence, it can be determined that Ultrasint PP nat 01 is suitable for PLS since the material seems to have good flowability based on the HR value. This can be attributed to the high percentage of particles that are almost spherical, as seen in Figure 1. Sommereyns et al. [26] stated that spherical or “potato-shaped” powder particles promote flowability and are suitable for PLS.

Analytical and numerical modelling can define different PLS phenomena, such as powder deposition and powder spreading. Powder flowability is a pertinent material property required in this undertaking. However, it is difficult to measure viscosity and flowability for powder. It is proposed in this study that the Hausner number can be used to represent these material properties. A study by Spierings et al. [32] supported this claim that the Hausner number can be used as an indicator of powder flowability. This comment was also affirmed by Santomaso et al. [33], who classified the flowability of powders based on the Hausner number, as outlined in

Table 2. Classification of flowability based on the Hausner number.

#	Flowability	Hausner Number
1	Non-flowing	>1.4
2	Cohesive	>1.4
3	Fairly free-flowing	1.25–1.4
4	Free-flowing	1–1.25
5	Excellent flowing	1–1.25
6	Aerated	1–1.25

. Based on the information provided in Table 2, it can be suggested that the flowability of powders increases with increasing values of the Hausner number between 1 and 1.25.

Powder density is a crucial material parameter because it affects the density, dimensional accuracy, and mechanical strength of printed parts [6]. Hence, it is an essential material parameter when modelling PLS and the properties of printed parts.

3.3. Melt Flow Index Testing

Table 3. Experimental MFI values for Ultrasint PP nat 01.

#	MFI (g/10 min)
Trial 1	14.8
Trial 2	15.0
Trial 3	15.6
Trial 4	16.7
Trial 5	17.1
Average	15.8
Standard deviation	0.102

represents the MFI value of Ultrasint PP nat 01 for five readings and the overall average.

The MFI is representative of the rheological properties of a material. The average experimental MFI for Ultrasint PP nat 01 was determined as 15.8 g/10 min. According to Berretta et al. [34], MFI is related to the viscosity of the material being investigated such that lower values of MFI indicate higher viscosity and vice versa. In a situation where it is not possible to measure the actual viscosity of a material, the values of MFI can be used to develop analytical and numerical models that consider the viscosity of a material.

3.4. Differential Scanning Calorimetry

Differential Scanning Calorimetry (DSC) provides crucial thermal information on polymeric materials that can be used to establish their suitability for PLS. Data from DSC also gives pertinent information that can be used to develop analytical and numerical models for PLS. Yaagoubi et al. [35] stated that the thermal exchange between a laser beam and the powder particles is the most important step of PLS. Hence, it is crucial to determine material properties, such as the melting point and specific heat capacity, when modelling PLS. The initial point of crystallisation, the terminal point of crystallisation, peak crystallisation point, initial melting point, terminal point of melting, and peak melting point of Ultrasint PP nat 01 were determined for Ultrasint PP nat 01 powder from DSC, and the results are shown in

Table 4. Measured DSC thermal information on Ultrasint PP nat 01 powder.

	Sample 1	Sample 2	Sample 3	Average
Initial point of crystallisation (°C)	103.1	103.1	103.1	103.1
Terminal point of crystallisation (°C)	95.3	95.0	95.3	95.2
Crystallisation point (peak value), ${\mathit{T}}_{\mathit{C}}$ (°C)	99.3	99.0	99.4	99.2
Enthalpy of crystallisation (J/g)	−73.4	−82.9	−76.1	−77.5
Initial point of melting (°C)	133.7	133.8	133.8	133.8
Terminal point of melting (°C)	143.4	143.9	143.6	143.6
Melting point (peak value), ${\mathit{T}}_{\mathit{m}}$ (°C)	138.9	139.2	139.0	139.0
Enthalpy of melting (J/g)	60.5	71.1	65.8	65.8

.

The melting range of Ultrasint PP nat 01 was determined as 133.8–143.6 °C with a peak melting point of 143.6 °C. Hence, when modelling for heat transfer or the interaction between a laser beam and powder, the build chamber temperature varied between 130 °C and 133 °C because the build chamber temperature should be set close to the initial point at which the material under consideration melts [36].

The sintering window is the difference between the initial point of melting and the initial point of crystallisation. The sintering window is a crucial factor to consider in PLS because it determines the tendency of a printed sample to shrink and curl, which might lead to the failure of the process when parts are scraped and detached from the build by a recoater blade [27]. The experimental value of the sintering window for Ultrasint PP nat 01 was calculated as 30.7 °C (as shown by the shaded region in Figure 5). This value is within the range recommended by Wencke et al. [37] of 30.0 °C.

Suitable polymers should have a relatively wide sintering window (equal to or greater than 30.0 °C) to prevent the rapid cooling of printed parts [37]. According to Goodridge et al. [38], the shrinkage tendency of parts printed using certain polymeric materials can be regulated by optimising the laser power, build chamber temperature, and removal chamber temperature. Therefore, it is essential to consider the sintering window of a material when developing models to optimise laser power, build chamber temperature, and removal chamber temperature. In addition, the sintering window should be considered when modelling rates of shrinkage for layers and finished parts printed using different polymers.

Degree of crystallinity is another critical material parameter because it determines the mechanical behaviour of printed parts [39]. The value of this factor for Ultrasint PP nat 01 was calculated using Equation (1) as follows:

$${\mathrm{X}}_{\mathrm{c}}(\%)= \frac{∆{\mathrm{H}}_{\mathrm{m}}}{∆{\mathrm{H}}_{\mathrm{m}}^0}\times 100\%$$

(1)

where

• ${\mathrm{X}}_{\mathrm{c}}(\%)$ = degree of crystallinity;
• $∆{\mathrm{H}}_{\mathrm{m}}$ = enthalpy of melting (65.8 J/g);
• $∆{\mathrm{H}}_{\mathrm{m}}^0$ = heat of melting of 100% (HDPE) (207 J/g) [40].

$${\mathrm{X}}_{\mathrm{c}}(\%)=\frac{65.8}{207}\times 100\%=31.8\%$$

3.5. Thermogravimetric Analysis

Thermogravimetric analysis is used to investigate the thermal stability of polymeric materials. Since PLS is a high-temperature process, materials for use in it should have high resistance to thermal loads [41].

Table 5. The onset, end, and peak degradation temperatures for Ultrasint PP nat 01 powder.

Parameter	Value
Onset	407.4 (°C)
End	461.52 (°C)
Peak	434.47 (°C)

illustrates the onset, end, and peak degradation temperatures for fresh Ultrasint PP nat 01 powder that were obtained in the present work.

Ultrasint PP nat 01 shows high thermal stability based on its high degradation temperature of 434.47 °C, thus making it suitable for PLS without the risk of disintegration and material loss.

3.6. Results from Fourier Transform Infrared (FTIR) Spectroscopy

Optical properties are critical for polymeric materials suitable for PLS because they determine the amount of energy absorbed by the materials, which subsequently affects the fusion of particles and layers [42]. Figure 6 is an FTIR spectrum for Ultrasint PP nat 01 powder that was obtained in the present work.

Most of the available PLS machines make use of CO₂ laser beams with a wavelength of 10.6 µm. Figure 6 shows that Ultrasint PP nat 01 powder has around 98% transmission of infrared radiation at around this wavelength (shown by the black dotted circle), which indicates that the material has less absorptivity behaviour. This means that the sintering process using standard CO₂ lasers could be inefficient or would require optimisation of the laser energy density to achieve sufficient laser energy density without causing extreme ageing of the powder. This information can be used to model the optical properties of polymeric materials used in PLS.

3.7. Results from Hot-Stage Microscopy

3.7.1. Effects of High Packing Density and Uniformity of a Powder Bed on a Built Layer After Melting and Cooling Powder Particles

In this analysis, it is suggested that hot-stage microscopy can be used to determine how the uniformity of layers of powder and the packing density of a powder bed affect the quality of a built layer. Figure 7 shows crystallised particles after heating and cooling a (Figure 7a) less compact powder on a powder bed (with a low packing density and uneven powder distribution), and a (Figure 7b) compact powder on a powder bed (with a high packing density and uniform distribution of powder).

The less compact powder bed exhibited more irregularities after heating and cooling compared to the more compact powder bed. The percentage area of the irregularities was estimated using ImageJ software (Windows version) as 11.8% and 13.4% for the relatively more and less compact powder beds, respectively. The inclusions seen in Figure 7 could be either pores, trapped air bubbles, or spherulites (the dark spots). Such imperfections are expected to undermine the mechanical strength and physical qualities of finished parts [43,44]. The number of inclusions for the relatively less compact powder bed was higher, and the inclusions were notably larger compared to the case for the relatively more compact powder bed. Hence, it is evident that a suitable PLS polymeric material should achieve a uniform powder distribution with a high packing density for each new layer. Such an experiment can be used to validate numerical models and illustrate the impact of uniform layers and packing density of powder on a powder bed and the fusion of particles.

3.7.2. Fusion of Particles During Sintering

Particle fusion is pivotal for the success of PLS and also determines the quality of finished parts [11]. The fusion behaviour of polymeric materials can be simulated experimentally using hot-stage microscopy. The results obtained can also be used to validate analytical and numerical models of particle fusion, which form the basis for PLS. In the present work, hot-stage microscopy was utilised to investigate the fusion of Ultrasint PP nat 01 particles, with the results shown in Figure 8.

The behaviour of different sets of particles (encircled using black, red, purple, and blue circles) was examined at 25, 130, 140, and 180 °C. The obtained results illustrated that the particles remained unmelted between 25 and 130 °C, and started to melt and fuse at 140 °C (as the particles started to move close to each other), and full fusion was attained at 170 °C. The data also shows that Ultrasint PP nat 01 has good wetting properties since some of the particles (particles encircled by red, black, and purple circles) were able to attain almost perfect circular shapes at the end of the sintering process at 170 °C. Such results can be used to validate analytical and numerical models illustrating the fusion of particles and wetting properties of different polymers at different temperatures. The temperature at which full fusion is attained is also a critical factor that can be used to optimise volumetric laser energy density, which is among the most crucial process parameters in PLS.

3.7.3. Impact of Different Cooling Rates on Formed Layers

Cooling rate is a crucial factor in PLS because it affects crystallisation, which, in turn, impacts shrinkage and curling [45]. Hot-stage microscopy can be used to investigate the impacts of cooling on the crystallisation of sintered powder particles. Figure 9 shows crystallised powder particles at different cooling rates (5, 20, 35, and 50 °C/min).

The results shown in Figure 9 illustrate that a low cooling rate (5 °C/min) generates a more consistent crystallised phase (the section that appears yellow in colour). The crystallised phases for the other cooling rates appear to have inclusions (the dark spots). Peyre et al. [46] stated that crystallisation rates should be kept low to limit distortion of printed parts. This experiment can be used to model crystallisation and the impact of cooling rates on the quality of printed layers after melting powder particles and subsequently cooling them.

4. Conclusions

Polymer Laser Sintering is a pivotal technology in the additive manufacturing of polymers. However, uptake of this technology is still lacking due to limited feedstock materials, as well as stringent material properties and process parameter requirements. Considerable research has been performed in the field of PLS over the last three decades. However, there are still gaps in understanding different phenomena of PLS. Significant effort has been made to provide crucial information on this process, with the aim of expanding the menu of feedstock materials for PLS. The aim of this research was to examine the suitability of a new polymeric material (Ultrasint PP nat 01) in PLS. Several experiments were carried out based on the availability of the equipment and the importance of target material properties. The following conclusions were drawn from the results obtained:

• Ultrasint PP nat 01 has a powder particle size distribution in the range of 20–80 µm, which is suitable for PLS. The material has a wide range of particle shapes, ranging from rod-like, “potato-shaped”, and almost spherical, to flakes and regularly shaped particles of powder. “Potato-shaped” and almost spherical particles were found to be in the majority, the latter of which is ideal for good flowability.
• The Hausner ratio, as well as the tapped density and bulk density of the material, were calculated and measured as 1.230 ± 0.05, 0.455 ± 0.02 g/cm³, and 0.370 ± 0.03 g/cm³, respectively. The value of the Hausner ratio confirmed good flowability of the material, which is pertinent for PLS.
• The initial melting point of Ultrasint PP nat 01 was measured as 133.8 °C, which indicated that the build chamber temperature for the PLS of the powder should be varied between 130 and 133 °C.
• Ultrasint PP nat 01 was found to have a relatively high sintering window of 30.7 °C, which makes the material good for printing using PLS, as it is expected to have reduced cases of curling and shrinkage.
• The material was found to have a high thermal stability of around 461.52 °C, thus making it suitable for use in PLS.

5. Recommendations

It is recommended that further experiments be performed to determine the physical and mechanical properties of additively printed components using this polymer.