Binder Removal

The binder needs to be removed from the part prior to final consolidation. For solvent-based binders, removal occurs in two steps: (1) drying to remove the solvent and (2) thermal treatment to remove the organic component of the binder. Drying occurs by solvent evaporation at the surface. Solvent dispersed in the interior of the part must be able to diffuse to the surface to be removed. Solvent evaporation at the surface and subsequent diffusion through the part are accelerated by temperature. Shrinkage will occur during the drying process; hence, care must be taken to ensure that the solvent has enough time to diffuse through and then evaporate, as differential drying rates will result in differential shrinkage rates. Differential shrinkage rates can generate tensile stresses in the part, which may be relieved by the generation of a crack in the extruded part.

Most solid-phase organic binders used in binder-assisted extrusion can be burned off in an appropriate environment (e.g., inert, oxidizing, or reducing atmosphere, depending on the chemistry of the binder) at a temperature between 300 and 1000 °C. The burnoff must be slow to allow the carbonaceous gases generated by the decomposition of the organic species to escape through the interconnected pores. Too rapid of a burnoff rate can crack or fracture the part. If the organic binders used for solvent-based extrusion are not burnt off properly, they will decompose, leaving residual carbon and other contaminants trapped within the part, which will subsequently degrade the performance over time. The exact thermal treatment (i.e., its temperature/time profile and environment) will depend on the actual binder used in the feedstock. Plastic binders for hot extrusion are removed via thermal treatments. The techniques used in PIM can also be utilized for plastic binder removal in the hot-extrusion process.

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