Extrusion

Extrusion is the pressing of the feedstock into and through a die of size and cross-sectional shape appropriate to that of the product. The specific steps (Ref 1) used in binder-assisted extrusion are: (1) feedstock/paste delivery into extruder, (2) consolidation and flow of feedstock/paste through the barrel, (3) deformation and shaping of feedstock/paste as it flows through the die, (4) flow of feedstock/paste through the die land, and (5) part ejection. For the feedstock to flow through the die, the pressure generated by the extruder must be greater than the resistive forces associated with the deformation of the feedstock at the die entrance due to the reduction in diameter and the drag forces along the die wall. The pressure drop along the axis of the extruder reflects the amount of work required to overcome the resistive forces. The pressure is highest in the barrel and lowest at the exit of the die landing, as shown in Fig. 3. There is a pressure drop at the exit of the die, which tends to be small in magnitude compared to the drop associated with the deformation and motion of the feedstock. However, if pressure drop upon exiting is too high, there can be significant "springback," or elastic rebound, of the extruded part. Springback can result in axial cracking and warpage of the part after it has left the die. Slower extrusion rates, longer extrusion dies, and the incorporation of short relief sections at the end of the die (i.e., a negative taper) all contribute to minimize the effect of springback (Ref 5).

Fig. 3 Pressure profile in the extrusion die. Source: Ref 5 Equipment

Two common extruders are used in the binder-assisted extrusion process: the screw, or auger-type, extruder and the piston, or ram, extruder. In piston extrusion (Fig. 4), the feedstock, commonly a degassed billet, is inserted in the barrel, and the piston compresses it, forcing the feedstock down the barrel and through a die of the desired shape. As the ram contacts and compresses the feedstock, the pressure increases (Fig. 5). As the feedstock is pushed along barrel by the piston, the pressure falls as a result of drag forces along the barrel wall. As the feedstock enters the die, the pressure once again rises. Constant pressure in the barrel can only be maintained if inverted extrusion is employed (Ref 5). (In inverted extrusion the die moves instead of the feedstock.)

Fig. 4 Ram extruder. Source: Ref 6

Piston travel (at constant speed), in.

Fig. 5 Pressure profile in the barrel of a ram extruder. Source: Ref 5

Two advantages of the piston extrusion process are (Ref 1, 2, 3, 4, 5, 6): (1) very high pressures can be generated independent of the feedstock properties and (2) there is precise control over output flow. As a consequence, piston extruders are commonly used for the production of intricately shaped components. However, a drawback to the use of piston extruders is the intermittent nature of the process and the low capacity inherent with batch-type processes. However, with the aid of a screw extruder to load the piston barrel with the feedstock, as depicted in Fig. 4, piston extrusion can be made to perform semicontinuously.

Screw extruders are used to produce parts in large batches in a continuous process. Potential capacities of screw extruders can be as high as 100 tonnes/h (Ref 1, 2, 3, 4, 5, 6). The screw extruder typically consists of several sections (shown schematically in Fig. 6). The feedstock is fed into a pug mill, which supplies high-shear mixing. The feedstock is then forced through a perforated plate where it is shredded. This shredded material falls into the main extruder chamber. Degassing can occur at this stage, making use of a vacuum system attached between the two chambers. In the extruder chamber, the feedstock is consolidated and forced through the forming die. Screw extruders may use single- or twin-screw configurations. When using a twin-screw system, the two screws are able to rotate in either the same or the opposite direction. Twin-screw extruders force the feedstock through the barrel using positive displacement, whereas in the single screw extruder, the movement of the feedstock is entirely dependent on the frictional forces that develop among the screw, the feedstock, and the barrel (Ref 1, 2, 3, 4, 5, 6). For conveying, feedstock properties are less significant with a twin-screw extruder than for a single-screw extruder. A comparison of the operating parameters for the single- and twin-screw extruders is given in Table 3.

Table 3 Comparison of single and twin-screw extruders

Factor

Single screw

Twin screw

Energy loss

Viscous flow

Heat transfer

Feedstock transport

Wall friction dependent

Positive displacement

Throughput

Pressure dependent

Independent

Shear forces

Larger

Smaller

Power/kg of material

Higher

Lower

Temperature gradients

Higher

Lower

Air evacuation

Simple

Difficult

Capital cost

Lower

Feed Pug mill

Feed Pug mill

Screw extruder

Fig. 6 Screw extruder. Source: Ref 6

Screw extruder

Fig. 6 Screw extruder. Source: Ref 6

In a single-screw extruder pressure builds up as the segmented feedstock is conveyed and compressed, as shown in Fig. 7 (Ref 23). The segmented feedstock becomes continuous in the metered zone, and the pressure continues to rise. Finally, the feedstock is conveyed into the die, where it is deformed. It is at this point that the highest pressures are generated. The pressure in the die is dependent on the screw geometry and the feedstock rheology (Ref 1). Larger and/or tapered screws may be used to generate higher pressures. The number of flights of the screw controls the number of feed columns that are displaced. Larger helical angles increase the potential delivery rate, but reduce the compressive thrust on the feedstock. The required ratio of screw diameter to part diameter increases as the yield strength of the feedstock increases or when the die land area increases. The length of the feed section is commonly five times that of the screw diameter, and the length of the metering section is ten times that of the screw diameter. Typical screw geometries include (Ref 1, 2, 3, 7) a helical angle between 5 and 20°, radial clearance between the screw and barrel of 0.1 %. of screw diameter, and a channel depth ratio of about 3 to 1. Generally, these dimensions are selected based on standard screw geometries developed for commercial extrusions of other products, such as thermoplastics and food articles. However, recent experiment results indicate that for optimal binder-assisted powder extrusion, larger helical angles should used (Ref 24).

Fig. 7 Pressure profile in the barrel of a screw extruder. Source: Ref 23 Flow through the Extrusion Die

The flow through the die is composed of two parts: (1) flow from the barrel into the die land (i.e., flow through the die entry) and (2) flow through the die land (Ref 1, 5, 6, 14, 15, 16, 24, 25, 26, 27, 28). The feedstock deforms--that is, it extends in the axial direction and decreases in cross-sectional area--as it flows through the entrance to the die. For a square entry die, the work required to deform and extend the feedstock in the axial direction results in a pressure drop (A Pi) at the entrance to the die. This pressure drop is given by the following equation (Ref 6, 25):

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