Impact extrusion consists of subjecting metallic materials to very high pressures at room temperature. Under these pressures the metals become "plastic" and assume predetermined shapes. Whereas in coining this process takes place within a closed die, typical impact extrusions allow a portion of the metal to be "squirted" or "squeezed" out of the die cavity, thereby forming an integral part of the desired shape.
From a practical point of view, impact extrusion of suitable parts may result in substantially lower unit manufacturing costs because it permits:
1. High production rates
2. Substantial material savings (up to 75%)
3. Little or no machining
4. Low initial tool costs and long tool life
5. Bright and smooth surface finish ready for decorating
There are two types. Impact extrusion of the slug takes place between punch and die.
Under pressure the metal may be forced to flow counter to the direction of punch travel (backward extrusion) or in the direction of punch travel (forward extrusion). Frequently, parts require a combination of both types.
Starting Material
The raw material for the process is usually referred to as a slug. Slugs may be blanked from sheet or plate, sawed from bar stock, or cast. The cross section of a slug — round, oval, square, or rectangular — fits into the die bottom; its height is determined by the volume of metal required to produce the part.
Equipment Selection
The pressures necessary for impact extrusion are available on mechanical or hydraulic presses. Mechanical presses are usually preferred when higher production rates are required. Both toggle and crank presses are used — depending on load and performance characteristics required. Hydraulic presses have primarily been used for forward extrusions requiring particularly high pressures, and heavy cross sections. Factors bearing on the selection of equipment for impact extrusion are dimensional characteristics of the part to be produced, tonnage, and speed.
Dimensional characteristics of the part (diameter, height, etc.) determine die size of the press and length of stroke required.
Tonnage required for metal flow must be carefully determined in advance. It is predicated on the relative plastic deformation required for the part, i.e., the ratio of the cross-sectional area of the extruded part to the area of the slug. Maximum limits of plastic deformation vary from 90 to 95% for 99.5% aluminum, lead, and tin, to 70 to 80% for mild steel and brasses.
Design Criteria
Impact extrusion should be specified by the designer primarily for:
1. Parts that are essentially hollow shells consisting of a wall and a bottom or partial bottom section. While in drawing the ratio between height and bottom diameter is limited to approximately 2 to 3:1, it is possible to impact-extrude (backwards) up to 8:1. Forward extrusions many times as long as the diameter have also been made.
2. Parts with straight, no-draft walls.
3. Parts requiring high-strength characteristics.
4. Parts with longitudinal ribs, flutes, splines, etc. or with bosses, cavities, etc. in the bottom section.
5. Parts made in large quantities calling for low unit cost.
Applications
It is significant for this process that the flow of metal takes place primarily in a direction parallel to punch travel. Parts produced by impact extrusion are essentially longitudinally oriented, e.g., collapsible tubes, cans, etc.
Originally used only on soft materials (lead, tin, etc.) to make collapsible tubes, the process has, in recent years, found rapidly increasing applications in the field of metallic containers, as well as in the production of a wide range of automotive, electrical, and hardware components. Aluminum and its alloys, copper, high brasses, and mild steel are impact-extruded commercially in large quantities.
Since it is a "chipless" metalworking process, impact extrusion competes in many applications with the automatic screw machine, deep drawing, die casting, hot forging, cold upsetting, and other operations. To achieve optimum results, it is important that likely parts be designed with an understanding of characteristics of metal flow, die design, and proper distribution of pressures in impact extrusion.
