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 are its ability to create very complex cross-sections and to 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). The extrusion process can be done with the material hot or cold.
The most commonly extruded materials in our application spectrum include metals and polymers.
Hollow cavities within extruded material cannot be produced using a simple flat extrusion die because there would be no way to support the center barrier of the die. Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section. The die shape then internally changes along its length into the final shape, with the suspended center pieces supported from the back of the die.
The process begins by heating the stock material (for hot or warm extrusion). 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. Afterward, the extrusion is stretched in order to straighten it. If better properties are required, then it may be heat treated or cold worked.
The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. One of the main advantages of the extrusion process is that this ratio can be very large while still producing quality parts.
Hot extrusion is a hot working process, which means it is done above the material’s recrystallization 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 230 to 11,000 metric tons (250 to 12,130 short tons). Pressures range from 30 to 700 MPa (4,400 to 101,500 psi); therefore lubrication is required. Lubrication can be oil or graphite for lower temperature extrusions; or glass powder for higher temperature extrusions. The largest disadvantage of this process is its cost for machinery and its upkeep.
HOT EXTRUSION TEMPERATURES FOR VARIOUS METALS
|Material||Temperature °C (°F)|
|Refractory Alloys||up to 2000 (3600)|
The extrusion process is generally economical when producing between several kilograms (pounds) and many tons, depending on the material being extruded.
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.
Warm extrusion is done above room temperature, but below the recrystallization temperature of the material. The temperatures range from 800 to 1800 °F (424 to 975 °C). Warm extrusion 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 these 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.
There are several methods for forming internal cavities in extrusions. One way is to use a hollow billet and then use a fixed or floating mandrel. A fixed mandrel, also known as a German type, means it is integrated into the dummy block and stem. A floating mandrel, also known as a French type, floats in slots in the dummy block and aligns itself in the die when extruding. 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. The solid billet could also be used with a spider die, porthole die or bridge die. All of these types of dies incorporate the mandrel in the die and have “legs” that hold the mandrel in place. During extrusion, the metal divides, flows around the legs, then merges, leaving weld lines in the final product.
Direct extrusion, also known as forward extrusion, is the most common extrusion process. It works by placing the billet in a heavy walled container. The billet is pushed through the die by a ram or screw. There is a reusable dummy block between the ram and the billet to keep them separated. The major disadvantage of this process is that the force required to extrude the billet is greater than that needed in the indirect extrusion process because of the frictional forces introduced by the need for the billet to travel the entire length of the container. Because of this, the greatest force required is at the beginning of the process and slowly decreases as the billet is used up. At the end of the billet, the force greatly increases because the billet is thin and the material must flow radially to exit the die. The end of the billet (called the butt end) is not used for this reason.
In indirect extrusion, also known as backwards extrusion, the billet and container move together while the die is stationary. The die is held in place by a “stem” which has to be longer than the container length. The maximum length of the extrusion is ultimately dictated by the column strength of the stem. Because the billet moves with the container, the frictional forces are eliminated. This leads to the following advantages:
- A 25 to 30% reduction of friction, which allows for extruding larger billets, increasing speed and an increased ability to extrude smaller cross-sections
- There is less of a tendency for extrusions to crack because there is no heat formed from friction
- The container liner will last longer due to less wear
- The billet is used more uniformly so extrusion defects and coarse grained peripherals zones are less likely
The disadvantages are:
- Impurities and defects on the surface of the billet affect the surface of the extrusion. These defects ruin the piece if it needs to be anodized or the aesthetics are important. In order to get around this, the billets may be wire brushed, machined or chemically cleaned before being used.
- This process isn’t as versatile as direct extrusions because the cross-sectional area is limited by the maximum size of the stem.