Quantitative theories of sintering started with Frenkel's model (1945) for two equal sized spheres, based on the balance of surface tension and viscous forces, which was applicable for the early stages of the process. This was extended to completion by the first author and his co-workers (AICHE J, 43,3253(1997)). It predicts the rate of neck growth as a function of dimensionless time and it has been successfully applied in additive manufacturing (SLS and FFF) by several other authors. An extension to sintering of two equal diameter cylinders was recently presented (PPS2019, Pretoria, S.A.) by the authors. Also a non-isothermal version for both spheres and cylinders was recently developed (accepted in J. Rapid, Prot.(2020)). The most recent work involves the sintering of four square-packed cylinders. The results are used for approximating the overall densification rate of packed beds. Further, the two equal sized particle model is easily extended to unequal sized spheres. It is argued that these mathematical models can be used for optimizing additive manufacturing processes and products.