Introduction

BINDER-ASSISTED EXTRUSION is a plastic forming process in which a highly viscous feedstock, that is, a powder mixed with a binder and other rheology modifiers (also termed the paste), is forced through a die to form a shaped product. The binder-assisted extrusion process is used commercially to produce ceramic parts, such as furnace tubes, bricks, insulators, pipes, tiles, tubular capacitors, catalyst supports, magnets, heat exchangers, wires, and springs (Ref 1, 2, 3, 4, 5, 6, 7, 8). However, binder-assisted extrusion has also been used to fabricate metals and high-temperature superconductors (Ref 9, 10), as well as being a technique for aligning reinforcement in the processing of advanced composites (Ref 11, 12, 13). Depending on the powders being shaped, the extrusion process can be performed at room temperature with the utilization of a solvent-based binder, or at elevated temperatures with a plastic binder system.

The basic principles of binder-assisted extrusion are similar to powder injection molding (PIM) in that both processes consist of four very similar steps: (1) feedstock or paste preparation; (2) product shape forming (powder extrusion in one case, powder injection in the other); (3) binder removal; and (4) the consolidation step, that is, sintering. During the feedstock/paste preparation stage, the powders are mixed with the constituents of the binder to obtain the proper flow characteristics. Feedstock preparation may consist of several steps (e.g., dry mixing, wet mixing, and high-shear mixing) to remove agglomerations, to produce a uniform mixture, and to coat the powder particles with the binder. During the forming step, the feedstock/paste is extruded through the appropriately shaped die. At this point in the process, the part must possess enough structural integrity to be manipulated while maintaining its shape. In the finishing step the part is cut to size, dried, and thermally processed. Table 1 summarizes the steps generally followed in making a part using the binder-assisted extrusion process. Knowledge and control of each step in the process are crucial to the production of a sound part.

Table 1 Stages of binder-assisted extrusion

Processing stage

Purpose

Potential difficulties

Dry mixing

Uniformly mix all solid components

Regions of poorly mixed solids

Wet mixing (for solvent-based binders systems)

Uniformly distribute particles in liquid

Air entrapment; liquid rheology may prevent mixing; adherence to surfaces

High-shear mixing

Break down agglomerates; ensure binder surrounds individual powder particle

Binder--powder separation; incomplete deagglomeration

Degassing

Remove entrapped gas

Hard to achieve in some equipment

Extrusion

Shape feedstock

Binder--powder separation; excessive pressure drop; lamination flow defects between confluent streams; surface defects

Extrudate handling and cutting

Remove product from extruder and cut to desired length

Damage to weak product; surface damage; closure of internal holes during cutting

Binder removal

Remove binder; aid shape retention

Dry cracks from solvent-based binder system; incomplete binder burnout resulting in residual contaminates in product

Sintering

Generate strength in product; remove porosity

Temporary weakness in extrudate

Adapted from Ref 6

Adapted from Ref 6

Binder-assisted extrusion differs from PIM in that the extrusion usually consists of a constant cross section. Although parts with complicated cross sections can be produced, a selection of which is shown in Fig. 1.

Fig. 1 Schematic of typical cross sections produced by binder-assisted extrusion. Source: Ref 2

Binder-assisted extrusion and its derivatives have many advantages and some important disadvantages when compared to other powder processing techniques, for example, powder compaction in dies, isostatic pressing, slip and tape casting, and so forth. According to Benbow and Bridgwater (Ref 6), the primary advantages of using the binder-assisted extrusion processes include:

• The formation of complicated cross sections

• The small amount of binder/liquid added to the solids

• The ability to shape very hard powders

• A uniform density distribution

• The construction of long, thin sections

• Competitive capital and operating costs relative to other processes

The disadvantages include:

• The final structure is dependent on the powder properties.

• The strength is not imparted during the extrusion process itself.

• Structural defects may be introduced.

• Structure or strength are not generally adjustable by changing the extrusion conditions.

• Binder and rheology modifiers can adversely affect product properties, and the rate at which the binder and rheology modifiers can be removed from the extrudate is slow.

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