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Purpose This study aims to develop mathematical models for the determination of the effects of heating or cooling on neck growth in Selective Laser Sintering (SLS) and Fused Filament Fabrication (FFF). Two particle shapes are studied: spherical and cylindrical. Design/methodology/approach The time required for the coalescence (sintering) process is determined by balancing the work of surface tension forces and viscous dissipation. Heating and cooling effects are studied by incorporating temperature dependence of viscosity in an exponential form. Heating by a laser, convective and/or radiative heat transfer is assumed. It is also assumed that there are no temperature gradients within the coalescing molten polymers (lumped parameter heat transfer analysis). Findings The models predict faster sintering with heating and slower with cooling, as expected because of the effect of temperature on viscosity. For the isothermal case of pairs of cylinders, the present model predicts significantly longer time for completion of sintering than a previously developed and frequently cited model by Hopper. Originality/value An isothermal sintering model for two spheres was reworked for two long cylinders, and for the first time it has been compared to other models available in the literature. The mathematical models are capable of predicting neck growth under non-isothermal conditions for both spheres and cylinders. They are useful in assessment of bonding in selective laser sintering and fused deposition fabrication.

A 3D flow analysis package is used to simulate the flow phenomena in a short section of the metering zone of a single screw extruder. The assumption of stationary screw and rotating barrel is used. For sharp flight root corners, in addition to the primary flow field a secondary flow was determined in front of the root of the pushing flight and behind the root of the trailing flight, akin to what is known in the fluid mechanics literature as Moffatt eddies. Fluid particles caught in Moffatt eddy regions require travel times which are at least two orders of magnitude larger than the average residence time. This implies high probability of degradation and explains the deposits of thin carbonaceous films and gels, which have been discussed in several publications in the technical literature. The computer simulations show the existence of Moffatt eddies in the presence or absence of pressure gradients. When the screw roots have relatively wide angles no Moffatt eddies are predicted, in agreement with recommendations for good screw design practices.

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A coalescence model is developed for pairs of unequally sized particles, assuming surface tension driven flow opposed by viscosity. The flow field is extensional, biaxial for spheres and planar for...

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