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. A billet remnant or a plug of a tougher material must be left at the end of the extrusion to prevent a sudden release of the extrusion fluid. Drives 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 35 MPa (5000 psi). They supply a constant pressure throughout the whole billet. The disadvantage is that they are slow, between 50 and 200 mm/s (2–8 ips). Accumulator water drives are more expensive and larger than direct-drive oil presses, and they lose about 10% of their pressure over the stroke, but they are much faster, up to 380 mm/s (15 ips). 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 1400 MPa (200 ksi). Castor oil is used because it has good lubricity and high pressure properties. Die design The design of an extrusion profile has a large impact on how readily it can be extruded. The maximum size for an extrusion is determined by finding the smallest circle that will fit around the cross-section, this is called the circumscribing circle. This diameter, in turn, controls the size of the die required, which ultimately determines if the part will fit in a given press. For example, a larger press can handle diameter circumscribing circles for aluminium and 55 cm (22 in) diameter circles for steel and titanium. The complexity of an extruded profile can be roughly quantified by calculating the shape