Thermal rounding of micron-sized polymer particles in a downer reactor: direct vs indirect heating

Abstract

Purpose The purpose of this paper is to investigate the effect of different heating approaches during thermal rounding of polymer powders on powder bulk properties such as particle size, shape and flowability, as well as on the yield of process. Design/methodology/approach This study focuses on the rounding of commercial high-density polyethylene polymer particles in two different downer reactor designs using heated walls (indirect heating) and preheated carrier gas (direct heating). Powder bulk properties of the product obtained from both designs are characterized and compared. Findings Particle rounding with direct heating leads to a considerable increase in process yield and a reduction in powder agglomeration compared to the design with indirect heating. This subsequently leads to higher powder flowability. In terms of shape, indirect heating yields not only particles with higher sphericity but also entails substantial agglomeration of the rounded particles. Originality/value Shape modification via thermal rounding is the decisive step for the success of a top-down process chain for selective laser sintering powders with excellent flowability, starting with polymer particles from comminution. This report provides new information on the influence of the heating mode (direct/indirect) on the performance of the rounding process and particle properties.

Improvement of polymer properties for powder bed fusion by combining in situ PECVD nanoparticle synthesis and dry coating

“…N 2 adsorption measurements were conducted to determine the mass‐specific surface area of the nanoparticles collected in the filter and also determine the Sauter diameter of the primary particle, assuming spherical primary nanoparticles and a density of 2.2 g/cm 3 , which is typical of pyrogenic (amorphous) silica nanoparticles. Nitrogen gas sorption analysis was performed using a volumetric gas sorption analyzer Nova4200e (Quantachrome Instruments) The procedure employed for the nitrogen sorption measurements was the same as the reported by Gomez Bonilla [8] …”

“…Polymer powders have then to be modified to improve the flowability of the powders and to fulfill the requirements of PBF. Methods used to improve the flowability of bulk powders are the particle rounding using mechanical [ 7 ] or thermal [ 8,9 ] approaches and the increase of the surface roughness of the particles at the nanometer scale by dispersing nanoparticles on the surface, [ 10‐12 ] with the last one being the most commonly employed. As explained by Rumpf, [ 13 ] nanoparticles deposited on the surface act as “spacers,” increasing the distance between two surfaces, thus decreasing the magnitude of the van der Waals forces.…”

Abrasion-Induced Acceleration of Melt Crystallisation of Wet Comminuted Polybutylene Terephthalate (PBT)

“…In particular, wet comminution allows one to produce smaller particles than dry comminution, in several cases even below 5 µm [ 15 ], which opens new possibilities for the production of novel composite powders or fine-structured parts using smaller than ‘state-of-the art’ particles, if the powder flowability is maintained. With an additional thermal rounding step, the flowability and bulk density of powders obtained by comminution can be further improved [ 20 , 25 ]. This makes the cold wet comminution process a suitable candidate for extending the limited commercially available range of powder feedstocks for PBF-AM [ 1 , 2 ].…”

A Model for Translation and Rotation Resistance Tensors for Superellipsoidal Particles in Stokes Flow

“…Some of the more advanced models also employ additional shape descriptors [28] and even consider the particle orientation [29]. Examples that can be found in the literature range from the studies of volcanic ash transport in the atmosphere [30,31], as well as investigations of ice crystal trajectories in an aircraft turbofan compressor [32], to simulating the formation of polymer particles in a downer reactor [33].…”