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Technical Articles
Home >> Technical Articles >>Extrusion equipments vary by four major characteristics
Extrusion equipments vary by four major characteristics
Time: 2009-02-11
Movement of the extrusion with relation to the ram:
If the die is held stationary and the ram moves towards it then it's called "direct extrusion". If the ram is held stationary and the die moves towards the ram it's called "indirect extrusion".
The position of the press is either vertical or horizontal. The type of drive is 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 (often used for steel and titanium alloys), 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 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 dies. 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 and flows around the legs, leaving weld lines in the final product.
Direct extrusion
Plot of forces required by various extrusion processes.
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 need 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 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 radically to exit the die. The end of the billet, called the butt end, is not used because of this reason.
Indirect extrusion
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.
Hydrostatic extrusion
In the hydrostatic extrusion process the billet is completely surrounded by a pressurized liquid, except where the billet contacts the die. This process can be done hot, warm, or cold, however the temperature is limited by the stability of the fluid used. The fluid can be pressurized two ways:
Constant-rate extrusion: A ram or plunger is used to pressurize the fluid inside the container.
Constant-rate extrusion: A pump is used, possibly with a pressure intensifier, to pressurize the fluid, which is then pumped to the container.
The advantages of this process include:
No friction between the container and the billet reduces force requirements. This ultimately allows for faster speeds, higher reduction ratios, and lower billet temperatures.
Usually the ductility of the material increases when high pressures are applied.
An even flow of material.
Large billets and large cross-sections can be extruded.
No billet residue is left on the container walls.
The disadvantages are:
The billets must be prepared by tapering one end to match the die entry angle. This is needed to form a seal at the beginning of the cycle. Usually the entire billet needs to be machined to remove any surface defects.
Containing the fluid under high pressures can be difficult.
Most modern direct or indirect extrusion presses are hydraulically driven, but there are some small mechanical presses still used. Of the hydraulic presses there are two types: direct-drive oil presses and accumulator water drives.
Direct-drive oil presses are the most common because they are reliable and robust. They can deliver over 5000 psi (34.5 MPa). They supply a constant pressure throughout the whole billet. The disadvantage is that they are slow, between 2 and 8 ips (51 to 203 mm/s).
Accumulator water drives are more expensive and larger than direct-drive oil presses, plus they lose about 10% of their pressure over the stroke, but they are much faster, up to 15 ips (381 mm/s). Because of this they are used when extruding steel. They are also used on materials that must be heated to very hot temperatures for safety reasons.
Hydrostatic extrusion presses usually use castor oil at pressure up to 200 ksi (1380 MPa). Castor oil is used because it has good lubricity and high pressure properties.
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