Extrusion could be a method in which the metal is subjected to plastic flow by enclosing the metal in a closed chamber in which the only opening provided is through die.
The material is usually treated so that it can undergo plastic deformation at sufficiently rapid rate and may be squeezed out of the hole in the die.
In the process the metal assumes the opening provided in the die and comes out as a long strip with a similar cross-section as the die-opening.
Incidentally, the meta strip produced will have a longitudinal grain flow. The method of extrusion is most commonly used for the manufacture of solid and hollow sections of nonferrous metals and alloys.
Aluminum is an extremely good material for metal extrusion. Copper, magnesium, zinc, tin and some softer low carbon steels, can also be extruded with little complication due to the material. High carbon steels, titanium and various refractory alloys, can be difficult to extrude.
Advantages :
Extrusion will produce variety of shapes with uniform cross-section.
The grain structure and mechanical strength of work piece material are
Improved in cold and warm extrusion processes.
Cold extrusion will provide close tolerances.
Wastage of material is the minimum in extrusion processes.
Extrusion will be performed even for relatively brittle materials.
Type of extrusion :
Hot extrusion
Cold extrusion
Direct extrusion
Indirect extrusion
Hydrostatic extrusion
Impact extrusion
Hot Extrusion :
Hot extrusion is done at fairly high temperatures, approximately 50 to 75 % of the
melting point of the metal. The pressures will range from 35-700 MPa.
Due to the high temperatures and pressures and its detrimental effect on the die life as well as other components, good lubrication is necessary.
Oil and graphite work at lower temperatures, whereas at higher temperatures glass powder is used.
Applications :-
Trim parts used in automotive and construction applications, window frame members, railings, aircraft structural parts.
Advantages
Improvement of the mechanical properties.
This ratio can be very large while still producing quality parts.
Improved physical characteristics of the metal.
Easy to extrude for larger parts.
It has more extreme changes in shape.
Possible for extruding more complex geometry.
Disadvantages
Results in a layer of oxide scale build up on the external surfaces of the work piece.
Scale can affect surface finish
Affect the accuracy of the part
Wear at die metal interfaces.
High maintenance cost
Decreased tolerances, and
Increased die wear
Cold Extrusion :
Cold extrusion is the process done at room temperature or slightly elevated temperatures. This process can be used for most materials-subject to designing robust
Enough tooling that can withstand the stresses created by extrusion.
Examples of the metals that can be extruded are lead, tin, aluminum alloys, copper, titanium, molybdenum, vanadium, steel. Examples of parts that are cold extruded are collapsible tubes, aluminum cans, cylinders, gear blanks.
Advantages :
Process not having to heat the work,
Higher production rate,
No oxidation and scale form on surfaces,
Greater geometric accuracy,
Better surface finish
Ability to strengthen the part by way of strain hardening.
DIRECT or FORWARD EXTRUSION
Extrusion processes can be classified into two categories. These are
Direct extrusion
Indirect extrusion
Hollow extrusions as well as cross sections can be manufactured by both methods. Each method differs in its application of force and is subject to different operational factors.
In Direct or forward extrusion the work billet is contained in a chamber. The ram exerts force on one side of the work piece while the forming die through which the material is extruded is located on the opposite side of the chamber.
The length of extruded metal product flows in the same direction that the force is applied. During direct extrusion metal flow and forces required are affected by the friction between the work piece and the chamber walls.
Particularly in hot working oxide scale build up on the outer surfaces of the work piece can negatively influence the operation.
For these reasons, it is common manufacturing practice to place a dummy block ahead of the ram.
The dummy block is of slightly smaller diameter than the chamber and work piece.
As the metal extrusion proceeds, the outermost surface of the work is not extruded and remains in the chamber.
This material will form a thin shell that will later be removed. Much of the skull will be comprised on the surface layer of oxidized scale from the work metal.
BACKWARD or INDIRECT EXTRUSION
Indirect extrusion is a particular type of metal extrusion process in which the work piece is located in a chamber that is completely closed off at one side.
The metal extrusion die are located on the ram, which exerts force from the open end of the chamber.
As the manufacturing process proceeds the extruded product flows in the opposite direction that the ram is moving.
For this purpose the ram is made hollow so that the extruded section travels through the ram itself.
This manufacturing process is advantageous in that there are no frictional forces between the work piece and the chamber walls.
Indirect extrusion does present limitations. Tooling and machine set up are more complicated hollow rams are not as strong and less ridged and support of the length of the metal extrusion's profile as it travels out of the mold is more difficult.
Indirect extrusion can also be used to produce hollow parts. In this process, a ram is forced into the work material.
The ram gives the internal geometry to the tubular part while the material is formed around it. Difficulties in supporting the ram limit this process and the length of tubular metal extrusions that may be manufactured.
IMPACT EXTRUSION
Impact extrusion is a discrete manufacturing process, in which a metal part is extruded through the impact of a die with the work stock.
The part is formed at a high speed and over a relatively short stroke. In impact extrusions, mechanical presses are most often employed. The force used to form in standard extrusions is usually delivered over a horizontal vector, producing a long continuous product.
But in this method, Force used to form is usually delivered over a vertical vector, producing a single part with each impact of the punch.
Impact extrusion is most often performed cold. Occasionally with some metals and thicker walled structures, the work is heated before impact forming it.
This process is best suited for softer metals, aluminum is a great material for forming by impacting.
In manufacturing operation of impact extrusion, a work piece is placed in a mold and struck with great force, causing the metal to flow into position in an instant. The forces acting on the machinery are extreme, particularly on the punch and die.
Tooling must have sufficient impact resistance, fatigue resistance and strength, for extruding metal by impact. There are three basic types of impact extrusion processes, forward, reverse and combination.
The different categories are based on the kind of metal flow that occurs during the process. In forward impact extrusion, metal flows in the same direction that the force is delivered.
In backward impact extrusion, the metal flows in the opposite direction that the force is delivered. In combination, the metal flows in both directions.
Advantages
Raw material savings of up to 90%
Reduced machining times up to 75%
Elimination of secondary machining operations
Reduction in multi-part assemblies
Improved mechanical properties for material strength and machining due to cold working of the material Significantly reduced total part costs up to 50%
HYDROSTATIC EXTRUSION
In hydrostatic extrusion the work piece is held in a sealed chamber surrounded by pressurized liquid.
Hydrostatic extrusion is actually a form of direct extrusion. The force delivered through the ram is what pressurizes the liquid.
The liquid applies pressure to all surfaces of the work billet. When the ram moves forward, it is the force from the in-compressible fluid that pushes the work through the die extruding the metal part.
A critical aspect of manufacturing by this process is setup. The metal work billet must first be tapered to fit through the die opening thus creating a seal. This I done before adding the liquid, in order to prevent leaking.
Since the liquid is under great pressure, this taper must be precise to create a robust bond. Many different shapes may be manufactured by this process, using a variety of materials.
Liquid pressure from all directions also greatly decreases the chances of buckling of the work.
Hydrostatic extrusion may be performed at room or elevated temperatures, depending upon the manufacturing process. When performed hot, the liquid will insulate the work from thermal gradients between the container and work material.
An advanced variation of this process is called fluid to fluid extrusion. This process is basically the same, except that the part is extruded into a second chamber also containing pressurized liquid.
The liquid in the second chamber is of a lower pressure than the first. Several different kinds of liquids are used when manufacturing by hydrostatic extrusion, including oils, waxes, melted polymers and molten glass.
Hydrostatic extrusion has not had much use in manufacturing industry, due to the complicated equipment and procedures, work preparation, long cycle times and dangers of working with hot, high pressure liquid.
Advantages
No friction amidst the container and billet. This minimizes the force requirements, allowing higher reduction ratios, faster speeds, & lower billet temperatures.
Friction of the die can be largely reduced by a film of pressurized lubricant amidst the die surface and deforming metal.
On applying high pressures, the ductility of material increases.
Even flow of material.
Large billets & large cross-sections are extruded.
Uniform hydrostatic pressure inside the container eliminates the requirement of billets being straightened and extrusion of coiled wire.
No billet residue is left on the walls of container.
Limitations
Increased handling for the injection and removal of the fluid for every extrusion cycle.
Decreased control of speed of the billet & stopping because of potential stick-slip and enormous stored energy in the compressed fluid
Decreased process efficiency in terms of billet-to-container volume ratio Enhanced complications, when extrusion is done at elevated temperatures
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