Recent progress on polymer materials for additive manufacturing

3D printing: Principles and pharmaceutical applications of selective laser sintering.

A review of the process physics and material screening methods for polymer powder bed fusion additive manufacturing

Advances in powder bed fusion 3D printing in drug delivery and healthcare.

3D printing technology, which greatly simplifies the manufacturing of complex parts by a two-dimensional layer-upon-layer process, has flourished in recent years. As one of the most advanced technology, polymer powder 3D printing has many advantages such as high materials utilization rate, free of support structure, great design freedom, and large available materials, which has shown great potential and prospects in various industry applications. With the launch of the Multi jet Fusion system from HP, polymer powder 3D printing has been attracting more attention from industries and researchers. In this work, a comprehensive review of the main polymer powder-based 3D printing methods including binder jetting, selective laser sintering, high-speed sintering were carried out. Their forming mechanism, advantages and drawbacks, materials, and developments were presented, compared, and discussed respectively. In addition, this paper also gives suggestions on the process selection by comparing typical equipment parameters and features of each technology.

Current Status and Prospects of Polymer Powder 3D Printing Technologies

Additive manufacturing (AM) is a rapidly expanding framework of production technologies evolving in different directions, following the needs of different industries. Among powder bed fusion technologies, one of the main branches of AM, selective laser sintering (SLS) is the second oldest one. In the last few years, a direct rival has emerged: multi jet fusion (MJF). The purpose of this work is to compare these processes throughout a systematic analysis of powder and final parts made of commercially available polyamide 12 (PA12). Differences have been spotted both on the molecular and powder scale, with end capping of the MJF feedstock together with different thermal properties of the new and recycled materials. On the other hand, flowing properties are similar among the two virgin and recycled powders, with only a significant change in the fraction of fines for SLS material. The parts produced through SLS exhibit higher Young's modulus but lower elongation at break and ultimate tensile strength if compared to the ones obtained using MJF. This confirms once more that the occurrence of postcondensation has a profound influence on the final properties. Also Charpy impact strength according to ISO 179 has been tested, confirming the literature data for SLS, but also showing higher strength in the out-of-plane direction for un-notched specimens coming from MJF. Finally, the evaluation of advanced area roughness parameters such as surface roughness, skewness and kurtosis according to ISO 25178 allows the ascertainment of subtle differences arising in parts with different positioning on the build platform, possibly due to the inks employed in the MJF process.

Selective Laser Sintering and Multi Jet Fusion: process-induced modification of the raw materials and analyses of parts performance

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In-Situ Monitoring of Powder Bed Fusion of Polymers Using Laser Profilometry

Powder bed-based additive manufacturing (AM) is a promising family of technologies for industrial applications. The purpose of this study is to provide a new metrics based on the analysis of the compaction behavior for the evaluation of flowability of AM powders.,In this work, a novel qualification methodology based on a camera mounted onto a commercially available tap density meter allowed to assess the compaction behavior of a selection of AM materials, both polymers and metals. This methodology automatizes the reading of the powder height and obtains more information compared to ASTM B527. A novel property is introduced, the “tapping modulus,” which describes the packing speed of a powdered material and is related to a compression/vibration powder flow.,The compaction behavior was successfully correlated with the dynamic angle of repose for polymers, but interestingly not for metals, shedding more light to the different flow behavior of these materials.,Because of the chosen materials, the results may lack generalizability. For example, the application of this methodology outside of AM would be interesting.,This paper suggests a new methodology for assessing the flowing behavior of AM materials when subjected to compression. The device is inexpensive and easy to implement in a quality assurance environment, being thus interesting for industrial applications.

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Compaction behavior of powder bed fusion feedstock for metal and polymer additive manufacturing

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. 

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

Temperature monitoring during the cooldown in powder bed fusion of polymers is essential to qualify parts, and is an absolute novelty in the field. This is because the direct reading of the parts’ temperature is hindered by the powder. In this work, an innovative temperature monitoring technology based on microwave tomography is used to assess the part cooldown history in an industry-grade EOS P110 machine. So, relative and absolute readings of the temperature field can be carried out even for parts surrounded by powder. For the first time, several cooldown rates could be experimentally measured depending on the different polymer state: 0.56 °C min−1 in the liquid phase, 1.2 °C min−1 during supercooling, 0.83 °C min−1 in the solid phase. This technology will allow the development of a new generation of smart PBF machines that can better sense and hence control the entire PBF process, including the cooldown phase, which is often neglected but has the highest impact on the final mechanical performances of parts. 

Francesco Sillani

Papers

Selective Laser Sintering and Multi Jet Fusion: process-induced modification of the raw materials and analyses of parts performance

In-Situ Monitoring of Powder Bed Fusion of Polymers Using Laser Profilometry

Compaction behavior of powder bed fusion feedstock for metal and polymer additive manufacturing

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

In-situ microwave tomography for parts’ cooldown monitoring in powder bed fusion of polymers