Conclusions

Binder-assisted extrusion is a commercially used powder forming technique. In this technique, powders are mixed with a binder to produce a feedstock with rheological characteristics that facilitate extrusion. The process consists of four basic operations, each step is critical for producing a defect free product. The steps include (1) feedstock formulation and preparation, (2) extrusion for shaping, (3) binder removal, and (4) sintering. Feedstock preparation involves mixing the powders with the binder and is a critical step in producing homogeneous mixtures. Both the characteristics of the powders and the binder influence the rheology of the feedstock. The feedstock is then extruded through a die to form a part with a particular cross section. Care must be taken to avoid flow defects during extrusion. Postextrusion processing includes binder removal and consolidation. At this stage, it is important to completely remove the binder so residual contaminants are not left behind that can degrade the physical and mechanical properties of the final product. Finally, binder-removal cycles should be designed to prevent cracking.

Binder-assisted extrusion is used to produce a variety of industrial parts. These parts may be ceramic or metallic in nature. This technique is amenable to producing long objects such as tubes, bricks, and honeycombed structures, as long as the parts have regularly shaped cross sections. Table 6 compares and contrasts the various powder forming approaches.

Table 6 Comparison of powder forming techniques

Forming method

Shape of product

Advantages of method

Potential difficulties and disadvantages

Die pressing

Two-dimensional; flat objects

Economics; readily automated; products can be directly sintered

Limited sizes and shapes; potential density gradients

Doctor blade

Thin sheets

High productivity; secondary operations can be used to produce laminates and multilayered substrates

Binder removal may be difficult

Slip casting

Thin-walled products; irregular shapes

Utilizes simple equipment; can create relatively complex shapes

Distortions occur during castings; drying of slurry and slurry control can be difficult

Binder-assisted extrusion

Long objects with regular cross sections

Continuous production possible; no limitation on length of product

Objects with large cross sections require large equipment; binder removal can be difficult

Injection molding

Complex shapes

Mass produce, complex parts; readily automated; dimensional control; good surface finish

Economics (mold costs are high); binder removal can be difficult

(a)

Adapted from Ref 39

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