metal extrusion and drawing process and equipment
introduction
in extrusion, large deformation can take place without fracture as the material is under high triaxial compression
products made by extrusion are railings for sliding doors and window frames
extrusion and drawing is used for continuous manufacturing of discrete products from metals and alloys
the extrusion process
4 basic types of extrusion
indirect or reverse
hydrostatic
direct
lateral
exist friction
long length
high force
limited force
limited length
no friction
for less ductile material
no friction
less floor area
limited force
geometric variables in extrusion are the die angle and extrusion ratio
metal flow in extrusion
metal flow pattern in extrusion is important as it influence the quality and mechanical properties of the extruded product
process parameters
lower speeds are preferred
dimensional tolerances in extrusion are in the range from 0.25 to 2.5 mm
extrusion ratios, R, usually range from about 10 to 100
in coaxial extrusion, or cladding, coaxial billets are extruded together
hot extrusion
to reduce forces required, extrusion is carried out at elevated temperature
As the billet is hot, it develops an oxide film, unless it is heated in an inert-atmosphere furnace
metals and alloys do not have ductility at room temperature
Die design
tubing is extruded from a solid or hollow billet
hollow cross section can be extruded by welding-chamber and using varies dies known as a porthole die, spider die and bridge die
square dies (shear dies) are used in extruding nonferrous metals
guidelines for proper die design in extrusion
die design requires considerable experience
avoidance of sharp corners
avoidance of changes in die dimensions
symmetry of cross section
lubricant
lubricant is important as it has effects on
glass is an excellent lubricant for steels, stainless steels and high temperature metals and slloys
surface finish and products quality
extrusion forces
material flow during extrusion
manufacture of aluminum heat sinks
hot extrusion of aluminum is preferred for heat sink applications
tooling for hot extrusion can be produced through electrical-discharge machining
cold extrusion
is a general term for a combination of operations such as direct and indirect extrusion and forging
used widely for components automobiles
cold-extruded part
investigating material flow during the deformation of the slug helps avoid defects.
part is sectioned in the mid-plane, polished and etched to display the grain flow
advantages
good control of dimensional tolerance
improved surface finish
improved mechanical properties
production rates and costs that are competitive
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impact extrusion
similar to indirect extrusion
hydrostatic extrusion
pressure required in the chamber is supplied through an incompressible fluid medium
brittle materials can be extruded successfully as the hydrostatic pressure increase the ductility of the material
extrusion defects
extruded products can develop defects that effect their strength and product quality
some defects are visible to the naked eye while others can be detected only by some techniques
surface cracking
cracks are intergranular caused by hot shortness
can be avoided by lowering the billet temperature and the extrusion speed
high surface temperature can cause surface cracking and tearing
pipe
pipe defect, tailpipe, fishtailing
reduced by having more uniform flow pattern ( decrease extrusion ratio, increase angle die )
type of metal-flow pattern in extrusion will draw surface oxides and impurities toward the center of the billet
internal cracking
center of the extruded product can develop cracks called (-------)
extrusion equipment
basic equipment for extrusion is a horizontal hydraulic press
vertical hydraulic presses are used for cold extrusion
capable of applying a constant force over a long stroke
have less capacity than hot extrusion but take up less floor space
the stroke and speed of the operation can be controlled
crank-joint and knuckle-joint mechanical presses are used for impact extrusion to mass-produce small components