The primary type of hot extrusion is direct or forward extrusion. In direct extrusion, the hot billet is loaded into a thick wall container and forced through an extrusion die securely. in a holder. The force for extruding is applied by a ramp along with an intermediate reusable dummy block.
Metal flow from the die is in the same direction as the forward motion of the the ram. Since the surface of the billet length slides along the wall of the container, extrusion force depends on the friction between the billet length and the container and the material. The friction portion of the force can be reduced by using lubrication.
Force increases rapidly as the billet is upset to fill the container, then increases further as breakthrough force before extrusion begins. Upon breakthrough, the force declines as billet length decreases until a minimum force is reached. As the billet thins, the force rapidly rises again to continue metal flow radially toward the die opening. Resistance to deformation or force requirements increase markedly as the thickness of the butt or unused billet portion decreases.
Hot extrusion is most commonly performed on horizontal hydraulic presses. Hot extrusion presses are usually rated in force capacity, which translates to the amount of pressure applied to the billet. That pressure depends on billet material and temper, its cross-section and complexity, length and temperature, extrusion speed and reduction, or extrusion ratio. The extrusion ratio equals the cross-sectional area of the container liner divided by the cross-sectional area of the die openings.