Extrusion & Drawing PDF

PROD1334 - Extrusion & Drawing Monday, October 28, 2024 2:37 PM Direct Extrusion - The basic, conventional extrusion process is called direct extrusion or forward extrusion. - A cylindrical metal billet is loaded into the chamber - Powerful hydraulic ram drives the billet throu...

PROD1334 - Extrusion & Drawing Monday, October 28, 2024 2:37 PM Direct Extrusion - The basic, conventional extrusion process is called direct extrusion or forward extrusion. - A cylindrical metal billet is loaded into the chamber - Powerful hydraulic ram drives the billet through a small die at very high pressures - The die opening may be round to make extruded round stock, or shaped. Indirect Extrusion - Less common - Chamber and billet are stationary, and the driven back into the chamber against the billet - Since the billet does not need to move into the chamber, friction is reduced & lower ram forces are possible - Main limitation is that the ram must be hollow to accommodate the extrusion Lateral Extrusion - Ram is vertical & drives the billet horizontally through a die - Less common than direct or reverse extrusion Hydrostatic Extrusion - Uses a billet that is smaller than the chamber ID - Hydraulic pressure is used to squeeze the billet from all sides & drive it through the die - Hydrostatic pressure increases the ductility of some brittle metals - Not common Extrusion Ratio, CCD & Shape Factor - Extrusion Ratio R is a measure of the reduction from billet size to extrusions size (Typically 10 < R < 100) - Size of an extrusion is given by the circumscribing circle diameter or CCD. Diameter of the smallest circle that the shape can fit inside. The starting billet must be larger than this. - Complexity of an extruded shape is given by the Shape Factor S Extrusion Force Force required to produce an extrusion depends on a variety of factors: - Material strength - Temperature - Extrusion Ratio - Friction between the billet & chamber walls - Friction between the metal & die surface - Die angle Metal Flow in Extrusions The way metal flows during extrusion affects the final properties & party quality Metal Flow in Extrusions - The way metal flows during extrusion affects the final properties & party quality - Flow is similar to the flow of incompressible fluid in pipes but at very high pressures - An elongated grain structure results, which (depending on extrusion temperature) may or may not recrystallize into equiaxed grains. Metal Flow in Extrusion with Square Dies - A dead metal zone forms when extruding through square dies - Size & shape of this zone depends on friction, strength, extrusion speed & other factors Extrusion Practicalities - Alloys commonly used in extrusion are aluminum, copper, magnesium & steel - Aluminum parts (most commonly extruded metal) range up to a max CCD of 1m - Steel is harder to extrude, tends to stick to the dies & chambers. Glass is used as a lubricant - Extruded workpieces can be quite long for higher extrusion ratios, but handling difficulties typically limit lengths to about 20 ft - Most extruded shapes need straightening after extrusion, particularly small sections, & section with uneven area distribution, e.g. uneven shapes - Straightening is done by stretching Extrusion Temperature Ranges - Extrusion can be done cold (usually combined with forging), but most commonly performed hot - Hot extrusion permits the use of lower force, with increased metal ductility & reduced die wear Die Design for Shapes with Cavities - Dies for extrusions with thin sections & no internal cavities is straightforward - Extrusions with internal cavities pose challenges - Such shapes are extruded using the welding-chamber process - Specials dies called spider-dies, form the cavities - Extruding metal flows around the dies supports, rejoining downstream of the die support Welding-Chamber Process for Aluminum Ladder Bracket - Lubrication cannot be used during extrusion since it would interfere with the re-welding - Not all materials are suited to this process, aluminum is suitable Lubrication in Hot Extrusion - Lubrication is extremely important in extrusion - Hot metal tends to stick to the cylinder walls & the die - Glass is used as a lubricant in steel extrusions, circular glass pad placed against the die face - Melted by the hot billet & coats the extrusion as it passes through the die, while protecting the die itself - Jacketing or canning is another lubrication method where billet is coated with a softer metal like copper Cold Extrusion Similar to cold forging, finished parts have good grain flow patterns, & good toughness Strain hardening occurs producing increased strength Impact-Extrusion Process - Used to produce aluminum toothpaste tubes (Hooker process) & other collapsible metal containers - Process is used for non-ferrous metals, typically aluminum usually at high production rates Spider Die Drawing - Process similar to extrusion, but the metal is pulled through (drawn) rather than pushed through the die - Amount of area reduction is limited by the strength of the drawing wire. - The available drawing force is limited to the breaking strength of the wire already drawn Drawing is used to make wire for electrical applications, springs, cables, welding wire, coat hangers ,etc Used to make heavier round sections called rod, used for shafts, bolts, etc. - Can be used to further reduce the area of the shape initially produced by extrusion - Drawing can result in straight, strong products - Tubular shapes (such as hollow structural section) are produced by drawing - Drawing of larger hollow sections is done at elevated temperatures - Drawing of wire is done cold mostly - Thin steel wire for springs & musical instruments is made by heat-treating wire after drawing. Tensile strength is extremely high. Drawing Force (Without Friction) Drawing Force (With Friction) Drawing Equipment Two basic types of drawing machines:  - Draw Bench - Bull Block Multistage Bull Block Machine - Machines can contain a number of sequential dies, much like tandem rolling - Very long lengths of finely-drawn wire are produced Tube-Drawing Operations Draw Bench - Contains a single die - Trolley with gripping mechanism is pulled by drive chains to provide the drawing forces - Heavy sections & shapes are drawn with these machines - Length of drawn products is usually limited Tube & Wire Drawing  - Tube sinking does not use a mandrel ○ Internal diameter precision is sacrificed for cost & floating plug is used Extrusion & Drawing Definitions Extrusion - metal forming process in which metal or work piece is forced to flow through a die to reduce its cross section or convert it into desire shape Drawing - metal forming process in which metal or work piece cross section is reduced or changed in shape by pulling it through the die Dead Metal Zone - zone in the extrusion chamber where the metal does not flow through the extrusion die Skull - the outermost surface of the work is not extruded and remains in the chamber. This material will form a thin shell, (called skull), that will latter be scrapped. Draw Bench - single die, design is similar to that of a long, horizontal tension-testing machine. The pulling force is supplied by a chain drive or hydraulic cylinder. This piece of equipment is used for cold-drawing operations. Extrusion & Drawing Review Questions 15.1 How does extrusion differ from rolling & forging? All three processes involve the use of compressive forces on a workpiece to produce a desired final shape. Extrusion typically involves a cylindrical billet being forced through an extrusion die producing a final shape. Forging compressively forces a workpiece into a die to take the shape of the die. Rolling is typically done to reduce the thickness (draft) of a workpiece by a set of rolls. 15.2 Explain the difference between extrusion & drawing. Both metal forming processes that see a cylindrical billet placed through a die to produce a final shape. Extrusion uses compressive forces to force the billet through the die to reduce its cross section, drawing reduces the cross section of the billet by pulling it through the die to produce a desired final shape. 15.3 What is a spider die? What is it used for? Spider Dies are special dies with geometry to support internal mandrels and are used with extrusion dies. Spider Dies are used to create hollow cross sections in extrusions. During extrusion, the metal divides & flows around the supports for the internal mandrel into strands. The strands being extruded then become rewelded, under high pressure in the welding chamber before exiting the die. 15.5 What is a dead-metal zone? A dead-metal zone in extrusion is a zone within the extrusion chamber where the metal does not flow through the extrusion die. The metal at the corners remains stationary. 15.7 Why is glass a good lubricant in hot extrusion? Glass is used as a lubricant in steel extrusions, circular glass pad placed against the die face & is melted by the hot billet & coats the extrusion as it passes through the die, while protecting the die itself. 15.9 Describe the difference between direct & reverse extrusion. In direct extrusion, the billet is placed into the chamber & forced through a die. The die opening can be round or it may have various shapes. With indirect or reverse extrusion, the die moves towards the stationary billet. Reverse extrusion has no billet-chamber friction due to no relative motion. 15.10 What is a land? What is its function in a die? The land in a drawing die is the straight wall that differentiates the angle of the die & the relief angle. The purpose of the land is to obtain the exact size of drawn wire. 15.18 The extrusion ratio, die geometry, extrusion speed & billet temperature all affect the extrusion pressure. Explain.  Higher extrusion ratios affect extrusion force due to the extrusion force being proportional to the deformation resistance of the metal. Higher extrusion rations show deformation resistance increasing. Die geometry affects extrusion force because lower die angles create more friction at the die-workpiece interface. Extrusion speed affects the extrusion force as a result of a rise of temperature caused by the increased rate of working is sufficient to lower the yield stress of the material being worked. Billet temperature affects the extrusion force as a result the temperature increases, the deformation resistance of the metal decreases, and the extrusion force also decreases. 15.20 What is the function of a stripper plate in impact extrusion? The stripper plate strips the extruded parts from the punch as parts tend to stick to the punch. 15.21 Explain the different ways by which changing the die angle affects the extrusion process. Increasing the die angle restricts lubricant flow into the die. A larger die angle increases the redundant work. A dead metal zone may develop at large die angles. 15.22 Glass is a good lubricant in hot extrusion. Would you use glass for impression-die forging also? Explain. It would not be good for impression-die forging because die pattern failure. Meaning the cavities of the die will not be completely filled due to the thick lubricant of glass. Also the solidification of the glass would require higher draft angles in the die cavity. 15.29 Can spur gears be made by (a) drawing and (b) extrusion? Can helical gears? Explain.