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Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn
through a die of the desired cross-section. The two main advantages of this process over other manufacturing
processes is its ability to create very complex cross-sections and work materials that are brittle, because
the material only encounters compressive and shear stresses. It also forms finished parts with an excellent
surface finish.
Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing
many pieces). Extrusions can be done with the material hot or cold.
Hot extrusion
Hot extrusion is done at an elevated temperature to keep the material from work hardening and to make it easier
to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses that range
from 250 to 12,000 tons. Pressures range from 5,000 to 100,000 psi, therefore lubrication is required, which
can be oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions.
The biggest disadvantage of this process is its cost for machinery and its upkeep.[
Cold extrusion
Cold extrusion is done at room temperature or near room temperature. The advantages of this over hot extrusion
are the lack of oxidation, higher strength due to cold working, closer tolerances, good surface finish, and fast
extrusion speeds if the material is subject to hot shortness.
Materials that are commonly cold extruded include: lead, tin, aluminum, copper, zirconium, titanium, molybdenum,
beryllium, vanadium, niobium, and steel.
Examples of products produced by this process are: collapsible tubes, fire extinguisher cases, shock absorber
cylinders, automotive pistons, and gear blanks.
Warm extrusion
Warm extrusion is done above room temperature, but below the recrystallization temperature of the material.
It is usually used to achieve the proper balance of required forces, ductility and final extrusion properties.
Equipment
There are many different variations of extrusion equipment. They vary by four major characteristics:
- Movement of the extrusion with relation to the ram. If the die is held stationary and the ram moves towards it
then its called "direct extrusion". If the ram is held stationary and the die moves towards the ram its called "
indirect extrusion".
- The position of the press, either vertical or horizontal.
- The type of drive, either hydraulic or mechanical.
- The type of load applied, either conventional (variable) or hydrostatic.
A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure (for steel
alloys and titanium alloys for example), oil pressure (for aluminum), or in other specialized processes such
as rollers inside a perforated drum for the production of many simultaneous streams of material.
There are several methods for forming internal cavities in extrusions. One way is to have the mandrel integrated
into the ram. If a solid billet is used as the feed material then it must first be pierced by the mandrel
before extruding through the die. A special press is used in order to control the mandrel independently from
the ram.[1] Another method is using whats known as a "spider die, porthole die and bridge die". During extrusion,
the metal divides and flows around the supports for the internal mandrel. (This is much like water in a river
flowing around a large rock and rejoining downstream.)
Extrusion defects
- Surface cracking - When the surface of an extrusion splits. This often caused by the extrusion temperature, friction, or speed being to high. It can also happen at lower temperatures if the extruded product temporarily sticks to the die.
- Pipe - A flow pattern that draws the surface oxides and impurities to the center of the product. Such a pattern is often cause by high friction or cooling of the outer regions of the billet.
Internal cracking - When the center of the extrusion develops cracks or voids. These cracks are attributed to a state of hydrostatic tensile stress at the centerline in the deformation zone in the die. (A similar situation to the necked region in a tensile stress specimen.)
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직접압출
간접압출
정수압출
extrusion machine