Feedstock Preparation Mixing

The purpose of mixing the powder and the binder is to produce a uniform feedstock suitable for extrusion where all of the powder particles have been coated with the binder. Areas of inhomogeneity (e.g., pockets of uncoated powder and/or pure binder) can result in strength-limiting defects in the final product. Obtaining uniformity of the feedstock can be problematic because of the stiffness of the final extrudable mixture. Mixing of the powder and binder consists of several steps. For a solvent-based binder system for extrusions made at room temperature, the solid components (i.e., the powders, clays, organic polymers) are first mixed in the dry state. The liquid solvent is then added to the thoroughly mixed powders. To maximize homogeneity of the feedstock, wet mixing occurs first by slurry mixing and then through high-shear mixing. During this stage, it is common practice to add extra solvent to lower the viscosity of the feedstock in order to facilitate homogeneity during mixing. The excess solvent is then removed by the partial drying of the feedstock mixture prior to extrusion. In an alternative approach, the liquid solvent and powders are mixed to form a low-viscosity slurry, to which the solid component of the binder is incrementally added until the mixture possesses the desired viscosity.

High-shear mixing can be performed in a pug-type mill, in which two rows of shafts rotate in opposite directions. The purpose of this type of mixing is to break down the agglomerates inherent in the starting powders and any formed during subsequent mixing. It is important to avoid separation of the liquid and solid phases during high-shear mixing.

In the hot-extrusion process, the specific components that make up the plastic binder are compounded. The powder and binder are then dry mixed. The material is then run through a high-shear, screw-type pug mill, where the powder/binder is mixed, then melted and remixed under high-shear conditions. It is finally pumped through a die to form a strand. The strands are pelletized and fed into the extruder. Similar procedures employed during feedstock preparation for PIM can be applied in the feedstock preparation for hot extrusion.

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