Extrusion

Extrusion A compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. Pros: variety of sections possible (hot extrusion) grain structure and strength enhancement (cold) close tolerance (cold) no material wastage.

Types of Extrusion Direct Extrusion Indirect Extrusion The ram forces the work billet metal to move forward to pass through the die opening. Indirect Extrusion The die is mounted to the ram rather than at the opposite end of the extruder container housing.

Direct Extrusion Friction increases the extrusion force. Hollow section is formed using a mandrel.

Indirect Extrusion Metal is forced to flow through the die in an opposite direction to the ram’s motion. Lower extrusion force as the work billet metal is not moving relative to the container wall.

Extrusion Processes Hot extrusion Cold extrusion Keeping the processing temperature to above the re-crystalline temperature. Reducing the ram force, increasing the ram speed, and reduction of grain flow characteristics. Controlling the cooling is a problem. Glass may be used as a lubricant. Cold extrusion Often used to produce discrete parts. Increase strength due to strain hardening, close tolerances, improved surface finish, absence of oxide layer and high production rates.

Extrusion Analysis Extrusion ratio, Assuming all sections are circular, ideal deformation, no friction, no redundant work: Ram pressure Taking into account friction, where a =0.8 and b =1.2 to 1.5.

Extrusion Analysis For direct extrusion, additional pressure, pf, required by the extruder to overcome the wall friction is related as follows: (1) [equilibrium equation] For the worst case that the friction shear stress at the wall equals to the shear yield strength of the work metal: (2) Subs. (2) into (1), the additional pressure: The total ram pressure: The power required:

Ram Force Variation of Ram Force with ram stroke and die angle.

Extrusion Dies For the case of non-circular extruded section, a shape factor has to be introduced: where Kx = shape factor Cx = perimeter of the non-circular extruded section Co = perimeter of a circle that has the same cross- sectional area as the extruded section. For direct extrusion, the extrusion force

Impact Extrusion Forward backward combination

Impact Extrusion Impact extrusion is performed at higher speeds and shorter strokes than conventional extrusion. It is for making discrete parts. For making thin wall-thickness items by permitting large deformation at high speed.

Hydrostatic Extrusion Using hydrostatic system to reduce the friction and lower the power requirement. Sealing is the major problem.

Extrusion Defects a) Centre-burst: internal crack due to excessive tensile stress at the centre possibly because of high die angle, low extrusion ratio. b) Piping: sink hole at the end of billet under direct extrusion. c) Surface cracking: High part temperature due to low extrusion speed and high strain rates.

Wire and Bar Drawing Reducing the cross section of a bar, rod or wire by pulling it through a die. Bar drawing is generally in a batch mode while the wire drawing is in general in a continuous mode.

Mechanics of Drawing Area reduction in drawing ; Draft No friction and true strain The ideal stress Taking into account friction and die angle, where , , The drawing force

Maximum Reduction Assume perfectly plastic material (n=0), no friction, no redundant work, Then The maximum possible area ratio The maximum possible reduction

Drawing Equipment Good dimensional control Good surface finish Improved mechanical properties economic for mass production

Draw Dies Approach angle about 6 to 20 Back relief angle about 30

Tube Drawing No mandrel Fixed mandrel Floating mandrel