Machinery's Handbook, 31st Edition
EXTRUSION OF METALS 1521 nonuniform, and nonsymmetrical sections that would be difficult or impossible to roll or forge. Extrusions in many instances can take the place of bulkier, more costly assemblies made by welding, bolting, or riveting. Many machining operations may also be reduced through the use of extruded sections. However, as extrusion temperatures increase, processing costs also increase, and the range of shapes and section sizes that can be obtained becomes narrower. While many asymmetrical shapes are produced, symmetry is the most important factor in determining extrudability. Adjacent sections should be as nearly equal as possible to permit uniform metal flow through the die. The length of their protruding legs should not exceed 10 times their thickness. The size and weight of extruded shapes are limited by the section configuration and properties of the material extruded. The maximum size that can be extruded on a press of a given capacity is determined by the “circumscribing circle,” which is defined as the small est diameter circle that will enclose the shape. This diameter controls the die size, which in turn is limited by the press size. For instance, the larger presses are generally capable of extruding aluminum shapes with a 25 in. (63.5 cm) diameter circumscribing circle and steel and titanium shapes with about 22 in. (55.9 cm) diameter circle. The minimum cross-sectional area and minimum thickness that can be extruded on a given size press are dependent on the properties of the material, the extrusion ratio (ratio of the cross-sectional area of the billet to the extruded section), and the complexity of shape. As a rule, thicker sections are required with increased section size. The table gives approximate minimum cross section and thickness of some commonly extruded metals.
Minimum Cross Section
Minimum Thickness
Material
in 2
mm 2
in
mm
Carbon and alloy steels Stainless steels
0.40
258
0.120
3
0.45 - 0.70 290 - 452 0.120 - 0.187 3.0 - 4.8
Titanium
0.50
323
0.150 0.040
3.8
Aluminum, Magnesium
< 0.40 1.0 Extruded shapes minimize and sometimes eliminate the need for machining; however, they do not have the dimensional accuracy of machined parts. Smooth surfaces with fin ishes better than 30 μ in. (0.762 μm) rms are attainable in magnesium and aluminum; an extruded finish of 125 μ in. (3.175 μm) rms is generally obtained with most steels and titanium alloys. Minimum corner and fillet radii of 1 ∕ 64 in. (0.39 mm) are preferred for aluminum and magnesium alloys; while for steel, minimum corner radii of 0.030 in. (0.76 mm) and fillet radii of 0.125 in. (3.175 mm) are typical. < 258 Extrusion of Tubes: In tube extrusion, the metal passes through a die, which determines its outer diameter, and around a central mandrel,which determines its inner diameter. Either solid or hollow billets may be used, with the solid billet being used most often. When a solid billet is extruded, the mandrel must pierce the billet by pushing axially through it before the metal can pass through the annular gap between the die and the mandrel. Special presses are used in tube extrusion to increase the output and improve the quality compared to what is obtained using ordinary extrusion presses. These special hydraulic presses independently control ram and mandrel positioning and movement.
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