4. Extrusions

1. 4. Extrusions Brief introduction Compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape Process is similar to squeezing toothpaste out of a toothpaste tube In general, extrusion is used to produce long parts of uniform cross-sections Can be hot or cold Hot � steels Cold �soft metals

3. Extrusion Process Direct � Indirect � Hydrostatic � billet is smaller in diameter than chamber and fluid pressure forces billet by a ram Side (lateral) � extrusion forced at right angle to billet

6. Metal Flow in Extrusion Elongated grain structure Can test with grid test

8. Extrusion-Die Configurations

9. Hot Extrusion Dies can be preheated to reduce wear and to reduce cooling of the billet Oxide film can form so a dummy block smaller in diameter is placed ahead of ram Square dies used for non-ferrous metals Work piece has high plasticity and low deformation resistance Used for parts with big sizes and high strength

10. Cold Extrusion Improved mechanical properties by work hardening Good dimensionals Improved surface finish Elimination of billet heating Competitive production rates High stresses on tooling

15. Impact Extrusion Punch descends rapidly on the blank and extruded backward Collapsible tubes Thin walled parts

16. Impact Extrusion

18. Hydrostatic Extrusion No container wall friction High pressure Defects can be reduced by fluid-to-fluid extrusion Vegetable oils used at room temperature Waxes, polymers, glass used for high temp Limited use due to complex tooling, high pressures, special equipment, and long cycle times

19. Extrusion Defects Surface cracking Temperature, friction, or speed too high Sticking along die land Pipe Impurities drawn toward center of billet Can be minimized by controlling flow and by eliminating impurities Internal cracking Tensile stresses Increases with increased impurities Decreases with increased extrusion ratio and friction

21. Extrusion Equipment Horizontal hydraulic press Can control stroke and speed Vertical can be used for cold extrusion

23. 5. Drawing Brief introduction Cross-section of wire or rod reduced by pulling through die Limit is 63% reduction in cross section Shapes such as flat strips can be drawn

26. Drawing Practice Speeds of 200-500 ft/min for heavy to 10,00 ft/min for thin Reduction of 0-45% Bundling � drawing many wires simultaneously Material is usually tool steel or carbide Wet drawing � completely immersed in lube Dry drawing � surface of rod coated

27. Die for Round Drawing

31. Questions What are some of the attractive features of the cold extrusion process? What is the unique capabilities and special limitations of hydrostatic extrusion?

32. 6. Sheet metalworking Brief introduction Cold-working The starting stock is sheet metal Higher deformation forces Closer dimensional tolerances Increased strength High production rate High die cost for small quantities

33. Sheet metal process Shearing Bending Drawing Sheet metal forming

34. Shearing Mechanical cutting of materials without the formation of chips or the use of burning or melting. Shearing process Sheared edges Burr Break Burnished land Roll over

35. Sheared edge improvement Clamping the starting stock firmly against the die Maintaining proper clearance and alignment between the punch and the die Restraining the movement of the workpiece Simple shearing Sheets of metal are sheared along a straight line Shear angle is often used to reduce shear force on larger shear

36. Piercing and blanking Shearing operations where the shear blades are closed In piercing, the punch-out piece is the scrap and the remaining strip is the workpiece. In blanking, the piece being punched out becomes the workpiece and the remaining strip is the scrap. Die design The punch should fit within the die with a clearance between 5% to 7% of the stock thickness. Punches and dies should be in proper alignment so that a uniform clearance is maintained.

37. Bending The plastic deformation of metals about a linear axis with little or no change in the surface area. Bending process The metal on the outside is stretched while that on the inside is compressed. The location that is neither stretched nor compressed is neutral axis. The outside surface undertakes the maximum tensile stress

38. Bend radius The smaller the bend radius, the larger the deformation Should be as large as possible to permit easier forming and wider variety of engineering materials Springback Elastic recovery occurs when the bending load is removed The yield strength of the metal and the bending radius are involved in the amount of springback The metal must be overbent by an amount equal to the subsequent springback

39. Design of bending The smallest bend radius The metal is formed without cracking The value is dependent on both the ductility and the thickness of the metal sheet The process must provide springback compensation when the bend radius is greater than four times of the material thickness The bend axis should perpendicular to the direction of grain flow

40. Drawing (sheet metal drawing) The forming of closed-bottom cylindrical or rectangular containers from metal sheet Drawing process Shallow drawing and deep drawing Shallow drawing The depth of the product is less than its diameter Deep drawing The depth of the product is greater than the diameter

41. Drawing defects Wrinkle and tear Can be limited by multiple operations when deep drawing parts are made Trimming of drawn parts Establish both the size and uniformity of the final part

42. Sheet metal forming Embossing A pressworking process in which raised lettering or other designs are impressed in sheet material Very shallow drawing operation where the depth of the draw is limited to one to three times the thickness of the metal Bulging Locally expand a metal blank or tube outward against a split die Flanging Flanges are rolled on sheet metal

43. Superplastic sheet forming Metals achieve tensile elongations as high as 2000 to 3000% Ultrafine-grain-size, low strain rates and elevated temperatures are required Can be used to form material into large, complex-shaped products with compound curves by single operation Can be extrusion, drawing, forging, etc. The major limitation is the long cycle times

44. Questions What is springback? What determines the amount of springback in bending? What factors determine the smallest bend radius for materials? What measures can be employed to improve the quality of a sheared edge? What is the purpose of having a shear angle on a punch? What is the major limitation of the superplastic forming of sheet metal? What are some of the attractive features?