Effects of particle characteristics on homogeneity of green bodies in powder injection moulding

Abstract

A homogeneous powder and binder distribution in the green body in powder injection moulding (PIM) is important. In the present study, the mould filling model of PIM has been developed, based on the multiphase fluid theory, viscosity model of feedstock and powder-binder drag force model. The particle Reynolds number is influenced by the particle size and density, resulting in the different drag force between powder and binder. Furthermore, the varied velocity of binder and powder will be obtained with numerical calculation of the continuity equations, leading to the change of green body homogeneity. CFX was used to simulate the mould filling in PIM. The results showed that the homogeneity of green bodies was relative to the filling patterns, which varied with different powder densities. The powders were not suitable for PIM when the particle size was bigger than 20 mm, and the fine powders were beneficial to improve the homogeneity.

Effect of Particle Size and Shape on Wall Slip of Highly Filled Powder Feedstocks for Material Extrusion and Powder Injection Molding

“…Furthermore, it is in accordance with the effect of small particles in reducing powder/binder separation and improving a product homogeneity reported in. 51 , 57 At this point it should be pointed out that highly concentrated compounds for PIM and MEX undergo the changes of the particle shape and size during processing as recently reported by Bek et al, 58 which might be also favorable in reducing a slip.…”

“…It is known that the slip of highly concentrated materials is connected to the separation of powder and binder—particles migrate to the middle of the flow channel due to the shear rate gradients during processing, and the low viscosity polymer layer forms near the die wall. 11 , 51–53 In this perspective, the spherical particles seem to be more sensitive to shear rate gradients; as they do not have any preferential orientation dimension (aspect ratio is near 1), they can more easily start to rotate upon shearing as described by Thornagel 54 and move to the center of the die allowing for the formation of the polymer slip layer. This, as mentioned in Introduction, may lead to an anisotropic shrinkage and other associated defects inside final sintered products.…”