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Manufacturing Processes > Forming Processes >Cold Forming

 

Cold Forming

 

Process description

Various processes under the heading of cold forming tend to combine forward and backward extrusion to produce near-net shaped components by the application of high pressures and forces.

Materials

Any ductile material at ambient temperature, including: aluminum, copper, zinc, lead and tin alloys, and low carbon steels. Also alloy and stainless steels, nickel and titanium alloys processed on a more limited basis.

Cold Forming

Process variations

  • Impact extrusion: similar to cold extrusion, but cold billet is plastically deformed by a single blow of the tool. Can be forward or backward extrusion (Hooker process).
  • Cold forming: can be forward, backward or a combination of both.
  • Hydrostatic extrusion: metal forced through die by high fluid pressure. Used for high strength, brittle and refractory alloys.
  • Can incorporate other processes such as: cold heading, drawing, swaging, sizing and coining to produce complex parts at one station.

Economic considerations

  • Production rates up to 2000/h.
  • Lead times usually weeks.
  • High utilization of material (95 per cent). Possible material cost savings over machining can be high. Near elimination of heat treatment and machining requirements.
  • Can be economical for quantities down to 10 000, depending on complexity of part. More suited for high production volumes (100 000+).
  • Most applications in the formation of symmetrical parts with solid or hollow cross sections.
  • Tooling costs high.
  • Equipment costs high.
  • Direct labor costs low.
  • Finishing costs very low.

Typical applications

  • Fasteners
  • Tool sockets
  • Spark plug bodies
  • Gear blanks
  • Collapsible tubes
  • Valve seats

Design aspects

  • Complexity limited. Symmetry of the part is important: concentric, round or square cross-sections typical. Limited asymmetry possible.
  • To avoid mismatch of dies, every effort should be made to balance the forces, especially on unsymmetrical parts.
  • Length to diameter ratios of secondary formed back-extruded parts may approach 10:1; forward extrusion unlimited.
  • Any parting lines should be kept in one plane and placement across critical dimensions should be avoided.
  • Can be used to process two materials simultaneously to produce parts such as steel-coated copper electrodes.
  • Inserts not recommended.
  • Undercuts not possible.
  • Draft angles not required.
  • Maximum section ranging 0.25–22 mm, depending on material for impact extrusion. No limit for cold forming.
  • Minimum section ranging 0.09–0.25 mm, depending on material.
  • Sizes ranging Ø1.3–Ø150 mm, depending on cold formability of material being processed.

Quality issues

  • Inside shoulders require secondary processing to ensure flatness.
  • Cold working offers valuable increase in mechanical properties, including extended fatigue life.
  • Concentricity of blank and punch is important in providing uniform section thickness.
  • Supply of lubrication (commonly phosphate based) to the die surfaces is important in providing uniform material flow and reduce friction.
  • Small quantities of sulfur, lead, phosphorus, silicon, etc. reduce the ability of ferrous metals to withstand cold working.
  • Surface cracking: tearing of the surface of the part, especially with high temperature alloys, aluminum, zinc, magnesium. Control of the billet temperature, extrusion speed and friction are important.
  • Pipe or fishtailing: metal flow tends to draw surface oxides and impurities towards center of part. Governing factors are friction, temperature gradients and amount of surface impurities in billets.
  • Internal cracking or chevron cracking: similar to the necked region in a tensile test specimen. Governing factors are the die angle and amount of impurities in the billet.
  • Surface detail is excellent.
  • Surface roughness ranging 0.1–1.6 µm Ra.
  • Process capability charts showing the achievable dimensional tolerances for impact extrusion and cold forming are provided .
  • Dimensional tolerances for non-circular components are at least 50 per cent greater than those shown on the charts.

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