Laser Sintering Of Polymers Research Articles

Selective Laser Sintering of Polymers: Process Parameters, Machine Learning Approaches, and Future Directions

Selective Laser Sintering of Polymers: Process Parameters, Machine Learning Approaches, and Future Directions

Laser sintering of polyamide 12 with limited thermal degradation

The paper presents, for the first time, the results of PA12 polyamide sintering using the original method of Dual Beam Laser Sintering of polymers (DBLS). The DBLS method, which is a modification of the standard polymer Laser Sintering (pLS) approach, allows to reduce the thermal degradation of the polymer by using selective (in terms of powder volume and process time) heating the material with laser radiation. The paper presents both the properties of sintered parts and the analysis of the degree of thermal degradation of post-process powders. Examination of the sintered parts included X-ray computed tomography (XCT) to visualize and analyze their porosity and a tensile test. Post-process powders were evaluated by Gel Permeation Chromatography (GPC), Melt Flow Rate (MFR), Dry Laser Diffraction Spectroscopy (DLD) and Powder Rheometer (FT4). The analysis of the result of PA12 sintering with the DBLS method was carried out in comparison with the classic pLS approach. As it has been shown, the DBLS approach allows to obtain comparable results for sintered parts compared to the pLS method. What is particularly important, it was shown that the post-process powder in the DBLS method has almost unchanged properties - the change in molecular weight and viscosity is at the level of the measurement error. Reducing thermal degradation is especially important in order to obtain continuous circulation of the powder in the manufacturing process.

In-situ monitoring of powder bed fusion of polymers using laser profilometry

Process monitoring allows for unique opportunities to capture information at intermediate layers during fabrication for part qualification, but also to improve process robustness by identifying issues before yield or quality is reduced. An industrial-grade laser profilometer has been integrated for the first time in a commercial polymer laser sintering machine and used to monitor powder bed quality and potentially qualify parts during production. Powder layer quality was measured in different conditions and changed drastically as a function of recoating speed, preheating temperature but also particle shape. The effective layer thickness was obtained for the first time in PBF of polymers, and gradually increased with the layer count until reaching a plateau at the nominal layer thickness. Powder layer density could be calculated as well and lies between 34 and 65 %, depending on the powder type and layer count. Finally, curling increased with decreasing preheating temperature, ranging from 70 µm to 350 µm i.e. until collision with the recoater. Based on the measured degree of curling, mitigation strategies could be integrated using a closed-loop feedback control. Effective process monitoring such as the one provided with laser profilometry may enable the development of new materials featuring more complex processing conditions, but also larger machines where thermal uniformity might be an issue.

Influence of bioactive metal fillers on microstructural homogeneity of PA12 composites produced by polymer Laser Sintering

In this paper, polyamide 12 (PA12) blends with three types of metallic fillers are tested, which differ in the type of material and its’ morphology. Low content mixtures are taken into consideration (0.5, 1.0, 2.0, 5.0 wt%), since a low impact on mechanical properties along with obtaining antibacterial properties are desired. The investigation focuses on filler distribution as well as the influence on microstructural homogeneity of the base material after processing with polymer Laser Sintering. Moreover, the influence of the filler content on the mechanical properties and fracture behaviour were examined. Processability PA12 with bioactive metal fillers was confirmed, and no significant changes in ductile behaviour of PA 12 were observed. An in-depth analysis of the effect of the filler on microstructural homogeneity was conducted.Graphical abstract

Mechanical properties and degradation of laser sintered structures of PLA microspheres obtained by dual beam laser sintering method

This paper discusses the influence of process parameters on the degradation and the mechanical properties of laser-sintered polylactide (PLA) microspheres obtained using the novel dual beam laser sintering method (DBLS). DBLS is a technique developed by our team that is a modification of standard polymer laser sintering (pLS), with the potential to reduce polymer degradation during the process. The PLA microspheres were produced using the standard emulsion-solvent evaporation method. The laser sintering process was carried out in a wide range of process parameters to obtain samples with various degrees of sintering. Next a number of tests were conducted to assess the physicochemical properties of these samples, including visualization techniques (SEM, digital microscopy and photography), gel permeation chromatogrphy (GPC), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), dynamic mechanical thermal analysis (DMTA) and static compression tests. The work shows that for different sets of process parameters, it is possible to obtain a product with similar mechanical properties, but at the same time with a completely different degree of polymer degradation. Hence, the hypothesis that when assessing the sinter quality one should take into account not only the mechanical properties of the detail, but also the degree of polymer degradation, which is of great importance, for example, in biomedical applications. It has also been shown that the DBLS method has a potential to reduce the degree of degradation of the sintered polymers and the post-process material outside the sintering zone.

First, do not degrade – Dual Beam Laser Sintering of polymers

In this paper, the method of Dual Beam Laser Sintering of polymers (DBLS) is presented for the first time. In this method, rather than heating the entire volume of the polymer throughout the entire process, a second, additional laser is used to heat the material selectively in terms of volume and time. The principle of operation and the design of the prototype are presented. Using the developed system, an experiment has been carried out in which samples of technical polylactide (PLA) powder were sintered for selected process parameters. This experiment confirms that the proposed method is fully functional and proves that sintering can be controlled in a wide range. A preliminary comparison of the changes in the physicochemical properties of the obtained samples compared to those sintered by standard polymer Laser Sintering (pLS) was carried out using Gel Permeation Chromatography, Fourier Transform Infrared Reflectance and Differential Scanning Calorimetry. Our experiment shows that the method has a potential to limit the thermal degradation of sensitive polymer materials. • The design and principle of operation of the Dual Beam LS station were presented. • The change in PLA properties as a function of process parameters was examined. • The potential of the method to limit the thermal degradation of polymers was shown. • Lower degradation of the post-process powder by the DBLS method was demonstrated.

Investigating the Recyclability of Laser PP CP 75 Polypropylene Powder in Laser Powder Bed Fusion (L-PBF).

In the present study, recyclability of Laser PP CP 75 polypropylene powder from Diamond Plastics GmbH was determined by characterizing and comparing the used powder after each cycle with material from previous cycles and with fresh powder. The melt flow index of Laser PP CP 75 was affected by recycling since it was observed to change by 30.62% after the 8th 100% re-use cycle, a lower value than PA 12 of 66.04%, for the 6th re-use cycle. Parts printed with virgin Laser PP CP 75 had an average dimensional error of 3.02% (virgin material) and 4.06% after the 4th 100% re-use cycle, which raises concerns about the commercial viability of the material. After the 4th re-use cycle, the printed parts had distorted edges and failed to print after the 9th print cycle. Lastly, tensile testing revealed a skewed bell-shaped curve of strength versus the number of recycles with the highest ultimate tensile strength occurring for the second 100% re-use cycle (7.4 MPa). The curves for elastic modulus and percentage elongation were inverted with minimum points for the 2nd 100% re-use cycle. Overall, the experimental work confirmed that the properties of polypropylene powder were affected by recycling in polymer laser sintering, but the powder exhibited superior characteristics upon recycling to those of the predominantly used PA 12 powder.

Process Phenomena and Material Properties in Selective Laser Sintering of Polymers: A Review.

Selective laser sintering (SLS) is a powder bed fusion technology that uses a laser source to melt selected regions of a polymer powder bed based on 3D model data. Components with complex geometry are then obtained using a layer-by-layer strategy. This additive manufacturing technology is a very complex process in which various multiphysical phenomena and different mechanisms occur and greatly influence both the quality and performance of printed parts. This review describes the physical phenomena involved in the SLS process such as powder spreading, the interaction between laser beam and powder bed, polymer melting, coalescence of fused powder and its densification, and polymer crystallization. Moreover, the main characterization approaches that can be useful to investigate the starting material properties are reported and discussed.

Powder Spread Process Monitoring in Polymer Laser Sintering and its Influences on Part Properties

Confidence in additive manufacturing technologies is directly related to the predictability of part properties, which is influenced by several factors. To gain confidence, online process monitoring with dedicated and reliable feedback is desirable for every process. In this project, a powder bed monitoring system was developed as a retrofit solution for the EOS P3 laser sintering machines. A high-resolution camera records each layer, which is analyzed by a Region-Based Convolutional Neural Network (Mask R-CNN). Over 2500 images were annotated and classified to train the network in detecting defects in the powder bed at a very high level. Each defect is checked for intersection with exposure areas. To distinguish between acceptable imperfections and critical defects that lead to part rejection, the impact of these imperfections on part properties is investigated.

A comprehensive characterisation of Laser Sintered Polyamide-12 surfaces

Polymer Laser Sintering (LS) is a powder-based Additive Manufacturing (AM) process known for its ability to produce highly complex geometries. The powder-based nature of the process means it intrinsically produces components with characteristic surface topographies abundant with features, as well as relatively high surface roughnesses, when compared with traditional polymer processing techniques such as Injection Moulding. There are a number of factors which influence the resultant surface topography of LS components and consequently affect their functional performance, particularly when subject to dynamic contact. However, little work has been carried out to date to fully understand these surface determining mechanisms. The scope of this research was to comprehensively characterise the surface topography of LS PA12 specimens and to specifically understand how resultant roughness is a function of applied energy density; XY location across the powder bed; part surface orientation; measurement technique and roughness descriptor. Results showed that the roughness profiles of top and bottom surfaces of cube-shaped samples were distinct in both size and shape. Top surfaces had positive Skewness values and were therefore dominated by asperity peaks, whereas bottom surfaces were neither entirely featureful of peaks nor valleys. Moreover, micro-CT analysis provided insight into how the sub-surface microstructure was affected by part orientation and applied energy density. Resultant binary images revealed the upward-facing section of a cylindrical LS PA12 specimen orientated 45° with respect to the powder bed to be less dense than the downward-facing section of the same sample. This work provides a benchmark for future Polymer Powder Bed Fusion (PBF) studies, specifically when characterising the friction and wear properties of resultant samples.

Development of material-adapted processing strategies for laser sintering of polyamide 12

Laser sintering of polymers (LS) is one of the most promising additive manufacturing technologies as it allows for the fabrication of complexly structured parts with high mechanical properties without requiring additional supporting structures. Semi-crystalline thermoplastics, which are preferably used in LS, need to be processed within a certain surface temperature range enabling the simultaneous presence of the material in both, the molten and solid state. In accordance with the most common processing models, these high temperatures are held throughout the entire building phase. In the state of the art, this leads to high cooling times and delayed component availability. In this paper, process-adapted methods, in-situ experiments and numerical simulations were carried out in order to prove that this drawback can be overcome by material-adapted processing strategies based on a deepened model understanding. These strategies base on the fact, that the crystallization and solidification of polyamide 12 is initiated a few layers below the powder bed surface at high temperature and quasi-isothermic processing conditions. Therefore, isothermal crystallization and consolidation behaviour is analyzed by process-adapted material characterization. The influence of temperature fields during laser processing was analyzed in dependence of part cross-section, layer number and fabrication parameters and correlated to the resulting part properties. Furthermore, the possibility to homogenize the parts thermal history by controlling the part cooling is highlighted by a simulational approach. The authors show that the material-dependent solidification behavior must be taken into account as a function of the geometry- and layer-dependent temperature fields and demonstrate a major influence on the material and component properties. From these findings, new processing strategies for the laser exposure process as well as for the temperature control of the build chamber in z-direction arise, which allow for the acceleration of the LS process and earlier availability of components with more uniform part properties.

Laser sintering of polymer nanocomposites

Laser sintering is a commonly used Additive Manufacturing (AM) technique applicable to a variety of applications in fields such as the automotive industry, healthcare, and consumer goods. As well as offering mechanical properties suitable for end-use part production, polymer laser sintering can produce more complex structures than many other AM techniques since it does not require support structures, and parts can be stacked in the build area for more efficient processing. A wide range of polymers should theoretically be processable by laser sintering. However, in practice this is not the case, with only a small number of polymers currently able to be processed reliably and consistently. This paper reviews research that has been undertaken to increase the processability, mechanical properties and functionality of laser sintering polymers through the addition of a range of organic and inorganic nanofillers. It examines key challenges, including dispersion of the nanophase, and methods that have been developed to overcome them. The effects of the nanophase on processability are explored, as well as the importance of key processing parameters. The latest developments on techniques for production of nanocomposite powders and characterisation of parts are discussed. The final properties of laser sintered parts that have been achieved and their potential applications are highlighted, and the current challenges and potential directions for future research are discussed.

Novel process for suppressing orange peel formation in polymer laser sintering through pretreatment with low-power laser irradiation

PurposeThis paper aims to describe a new process for suppressing the formation of orange peel, which is a polymer laser sintering (LS) process error.Design/methodology/approachThe target for controlling the suppression of orange peel is securing the contact between the molten polymer and the surrounding powder. The authors set the powder bed temperature closer to the melting temperature than that for a typical LS. Alternatively, the authors use a low-power laser to irradiate the powder bed surrounding the parts being built. The surface finish of the built parts was evaluated using a three-dimensional scanner.FindingsBoth approaches were effective in suppressing orange peel. From the viewpoint of reusability of the used powder, the process that includes low-power laser irradiation is practical. The presence or absence of contact between the surrounding powder and the molten polymer determines whether the orange peel is formed.Research limitations/implicationsThe authors have not tested orange peel suppression for complex shapes.Originality/valueThe authors have demonstrated a concrete process that can suppress orange peel formation even for powders with low melt-flow rates. Furthermore, a mechanism for the formation/suppression of orange peel based on the experimental results was proposed.

Thickness-Dependent Permeation Properties of Quenched and Standard Laser-Sintered Polyamide 12 Sheets.

The laser sintering of polymers is an additive manufacturing technology that is becoming increasingly established in the industrial environment. This study investigated the thickness-dependent permeation properties of laser-sintered (LS) polymers as required to design and produce components with a special barrier performance to gaseous substances. Helium and oxygen permeation experiments were carried out on quenched and standard LS polyamide 12 (PA12) sheets generated with two, four, six, and eight layers at a constant powder layer thickness of 100 µm. The structural properties of the sheets were examined by differential scanning calorimetry, light microscopy, and X-ray micro-computed tomography. A reduction in thickness resulted in higher diffusion coefficients for both types of LS sheets. An explanation could be the large volume fraction of poorly sintered powder particles adhering to the surfaces and incomplete melting and low consolidation of the polymer at small thicknesses. The thickness-dependency of the solubility coefficients was the opposite, especially for the standard LS sheets, which might be related to the larger pore volume in thicker sheets. As both effects compensated for each other, nearly constant permeation coefficients for all thicknesses were observed. The results provide further insights into different material characteristics of thin LS PA12 structures and offer new information on factors relevant to their solution and diffusion behavior.

Preliminary Testing to Determine the Best Process Parameters for Polymer Laser Sintering of a New Polypropylene Polymeric Material

Polymer laser sintering is an elaborate additive manufacturing technique because it is subject to process parameters and material properties. In this regard, each polymeric material necessitates a different set of process conditions. To this end, testing was done to determine the most suitable process parameters for a new commercially available polymer (Laser PP CP 60), from Diamond Plastics GmbH. It was established that the material requires slightly different settings from those provided by the supplier for the values for the removal chamber temperature, building chamber temperatures, and laser power to achieve the best mechanical properties (ultimate tensile strength). The preliminary testing indicates that the process parameters that yielded the best mechanical properties for the laser PP CP 60 powder were 125°C, 125°C, 0.15 mm, 250 μm, 4500 mm/s, 34.7 W, 1500 mm/s, and 21.3 W for the removal chamber temperature, building chamber temperature layer thickness, hatch distance, scanning speed fill, laser power fill, scanning speed contour, and laser power contour, respectively.

A review of the techniques used to characterize laser sintering of polymeric powders for use and re-use in additive manufacturing

Additive manufacturing (AM), is one of the key components of the 4th industrial revolution. Polymer laser sintering (PLS) is a subset of AM that is commonly used to process polymers, and which achieves good surface finish, good mechanical properties of finished products and for which there is no need for support structures. However, the requirements for polymeric powder for PLS are strident. Moreover, PLS subjects polymeric feed powders to high temperatures that lead to degradation of their thermal, rheological, and physical properties and is thus an impediment to their recyclability. Therefore, it is imperative to investigate the degree of polymer degradation or aging before re-using the material. This paper reviews the common techniques that are employed to characterize the suitability of polymeric powders for use and re-use in the PLS process. These include, but are not limited to, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), laser diffraction analysis, gas pycnometry, scanning electron microscopy (SEM), and melt flow index (MFI) testing.

Mixing and Reuse of Polymer Laser Sintering Powders to Ensure Homogeneity – A Review

Mixing and Reuse of Polymer Laser Sintering Powders to Ensure Homogeneity – A Review

A Review of Methods Used to Reduce the Effects of High Temperature Associated with Polyamide 12 and Polypropylene Laser Sintering

The polymer laser sintering (PLS) process is one of the most promising additive manufacturing (AM) technologies for polymeric materials. However, the technique has challenges because the physical, mechanical, and chemical properties of the polymeric powder deteriorate due to the high temperatures prevailing in the build chamber during manufacture. These high temperatures cause agglomeration of powder, which leads to a decrease in the flowability of powder. There is also a related drop in the coalescence of the powder granules during PLS, which results in porosity that undermines the mechanical integrity of printed parts. Moreover, the viscosity of the melt increases due to cross-linking of molecular chains. This, in turn, increases the tensile strength of the printed components at the expense of the percentage elongation at break. Thus, high prolonged processing temperatures decrease the reusability of polymeric materials used in PLS. In this paper, a review of the studies conducted to investigate ways of reducing the effects of high temperature on polymeric powders is presented.

Build orientation dependent microstructure in polymer laser sintering: Relationship to part performance and evolution with aging

Laser sintering (LS) is widely used to produce functional polymeric parts; however, the resulting parts are often limited by their porous structure, and performance of the part may be strongly anisotropic. Relating the structural features of parts to build process conditions or powder feedstock has been explored previously. In contrast, little is known regarding how the unique internal structures of LS parts evolve and relate to their performance later in their life cycle, for example, after use in a real-world operating environment. Here, a tightly controlled LS build process and measurement campaign by standard X-ray computed tomography (XCT), supported by high-resolution synchrotron XCT, was used to benchmark the internal microstructure (e.g. porosity) as a function of four different build orientations in printed polyamide-12 (PA-12) parts. The initial performance of the parts after printing was characterized by tensile testing and dynamic mechanical analysis (DMA). Arrays of PA-12 parts in different build orientations were exposed to steam (under pressure) and air across multiple temperature and time points for the purposes of accelerated aging through oxidative and hydrolytic breakdown. Exposing parts to steam dramatically altered the internal microstructure and functional properties such as the glass transition temperature, tensile properties, and damping behavior. Notably, post-aging XCT revealed large microstructural changes after only 40 h of steam treatment relative to the as-printed specimens. Furthermore, pore reorganization and changes in crystallinity occurred regardless of whether the steam temperature was aggressive enough to induce a significant loss in mechanical properties. The build orientation dependence on the aging rate was minimal with the relative trend in performance persisting across most metrics among the different orientations even after aging.

Numerical analysis of the crystallization kinetics in SLS

In the selective laser sintering of polymers, the most widely used powders are based on polyamide 12 (PA12), which is a semi-crystalline polymer. Because the mechanical properties of the printed parts depend largely on the microstructure, knowledge on the crystalline architecture is important. We developed a numerical model based on the finite element method to solve the flow, temperature and crystallization kinetics of PA12 powder during sintering using two different geometries. Our results show that the temperature plays a crucial role in the crystallization kinetics and that simplified 0D calculations can be used to study the crystallization kinetics if the temperature behavior in time at a certain location is known. With our choice of initial and boundary conditions, we found primarily crystals of the α′-phase.