The process begins by heating the stock material. It is then loaded into the container in the press. A dummy block is placed behind it where the ram then presses on the material to push it out of the die. Afterwards the extrusion is stretched in order to straighten it. If better properties are required then it may be heat treated or cold worked.
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.
Hot extrusion temperature for various metals Material Temperature [F° (C°)]
Refractory alloys up to 4000
The extrusion process is generally economical when producing between several pounds and many tons, depending on the material being extruded. There is a crossover point where rolling becomes more economical. For instance, some steel becomes more economical to roll if producing more than 50,000 lb.
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 is done above room temperature, but below the re crystallization temperature of the material. It is usually used to achieve the proper balance of required forces, ductility and final extrusion properties.
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 hydro static.
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. 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.)
Surface cracking - When the surface of an extrusion splits. This often caused by the extrusion temperature, friction, or speed being too 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 hydro static tensile stress at the center line in the deformation zone in the die. (A similar situation to the necked region in a tensile stress specimen.)