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Before these limitations are reached, however, cost and availability may be overriding considerations. A number of techniques can be employed to minimize extrusion cost:

1) Minimize the size of the smallest circle that encloses the section. Sometimes this can be done by extruding a folded version of the section desired and then unfolding it after extruding by rolling or bending.
2) Avoid hollows where possible. They require more complex dies and are more difficult to extrude.
3) Avoid large differences in wall thickness in different parts of the cross section. For 6063, the largest ratio of maximum to minimum wall thickness should be 3:1; for 6061-T6, use 2:1.
4) Keep the perimeter-to-cross-sectional area ratio as low as possible. Some extruders estimate costs based on this ratio.
5) Avoid sharp corners, using generous fillets or rounding where possible.
6) Ask extruders which changes to a proposed section would reduce cost.

 

 

Extruded structural shapes can perform additional functions by incorporating some of these features :

 

1) Interlocking Sections: Extrusions can be designed to interlock with other extrusions to facilitate connections. Several kinds of extrusion interlocks exist. Some are designed to act like hinges (Figure 5a), with one piece being slid into the other from one end. Other extrusions are designed with nested  interlocks that align parts, which are then fastened together (Figure 5b). Still others are designed as friction fit or snap-lock interlocks, which require the parts to deform elastically to fit together (Figure 5c). Unlike the first two joints, the friction fit joint cannot be disassembled without destroying the parts. None of these interlocks are considered to transmit enough longitudinal shear to enable the parts to act as a unit structurally without other means of fastening (133). To prevent longitudinal slippage at significant loads requires a fit too tight to make with parts extruded to standard tolerances.

 

Figure 5. Examples of extruded interlocks

 

 

2) Skid-Resistant Surfaces: Although the extrusion process is only capable of producing longitudinal features in a cross section, skid-resistant surfaces are routinely made with extrusions. This is achieved by extruding small triangular ribs on the surface (Figure 6), and then notching the ribs in the transverse direction after extruding. Extruders may perform the notching economically by running a ribbed roller over the ridges.

3) Indexing Marks: Shallow grooves extruded in parts can indicate where a line of holes is to be punched or drilled. These marks (raised or grooved) can also be used to identify the extruder, distinguish an inside from an outside surface, or distinguish parts otherwise similar in appearance. Dimensions for a typical indexing mark are shown in Figure 7.

4) Screw Chases: A ribbed slot (also called a screw chase or boss) can be extruded to receive a screw. This may be used, for example, to attach a batten bar to a frame member (Figure 8). A method for calculating the amount of lateral frictional resistance to sliding of a screw in a chase is given in AAMA TIR-A9-1991 (34). This calculation must be performed if the design considers both of the connected parts to act as a continuous section. The dimensions of the screw chase are a function of screw dimensions and strength. If the screw chase is too narrow, the head of the screw may break off before the
screw is fully driven. If the chase is too wide, the ribs of the chase may strip during fastener installation. The maximum screw torque before stripping is fairly sensitive to the chase width. The narrow dimension of the chase should be less than the root diameter of the screw. The dimensions used successfully for a screw chase for a -20 UNC screw are shown in Figure 9.

 

 

Figure 6. Skid-resistant surface achieved by transverse notching of extruded ribs

 

Figure 7. Typical dimensions for an extruded indexing or identification mark

 

 

Figure 8. Use of a screw chase to connect extruded parts. (Courtesy of Conservatek
Industries, Inc.)

 

 

 

5) Gasket Retaining Grooves or Guides: In curtain wall, fenestration, and other applications, elastomeric gaskets are often required between parts. By extruding a groove in the metal that matches a protrusion on a gasket, you can eliminate the need for field assembly or adhesives. Usually the gasket is press-fit into the extrusion in the shop. Dimensions for such a detail are shown in Figure 10. Care must be taken to avoid stretching the gasket during installation to prevent contraction of the gasket in the field to a length shorter than required.

6) Extrusions as a Substitute for Plate: Plate costs about 1 times the cost 1–2 of extrusions, so it’s desirable to utilize extrusions rather than plate wherever possible. Extruding to final width dimensions also eliminates the need to cut plate to the desired width, thereby saving fabrication costs. Extruded bars are available from a number of extruders through about 18 in. [457 mm] widths or more, depending on thickness.

7) Non-prismatic Extrusions: Extrusions may have different cross sections along their length when stepped extrusion methods are used. The smallest section is extruded first, the die is changed, and a larger section that contains the full area of the smaller section is extruded next. This method is used on aircraft wings to minimize the amount of machining needed to produce tapered members. Other tapered members, such as light poles, may be produced by spinning. Set-up costs for these methods are high, so they tend to be used only on parts that will be produced in quantity.

 

 

Figure 9. Dimensions for a screw chase for a 1/4 in. diameter screw

 

Figure 10. Dimensions to allow a gasket to be press fit into an extruded slot

 

 

8) Grooves for Fasteners: Grooves can be extruded to permit screw heads to be flush with the surface of an extrusion to avoid the need for countersinking the fastener hole. Groove widths sized to a bolt head flat width can also be used to prevent rotation of the bolt during tightening of the nut. Another use for grooves is to reduce the loss of cross-sectional area that occurs at holes (Figure 11).

 

9) Integral Backing for Welds: As shown in Figure 12, built-in backing for longitudinal welds along an extrusion edge can be provided, eliminating the need for separate backing and the need to hold it in place during welding.

 

Hollow Extruded Shapes If you’re like most people, you may need a moment of head scratching to imagine how hollow extruded shapes are possible. Extruders use three methods :

 

 

Figure 11. Use of an extruded groove in a line of bolts to secure the bolt heads
from spinning, and to reduce the loss of effective net area in the cross section

 

 

Figure 12 Extruded integral backing for a weld. (Courtesy of the Aluminum
Association)

 

 

1) Solid Billet and a Porthole Die: In this method, the metal is divided into two or more streams by the supports for the mandrel that forms the hollow portion of the shape. The metal must reunite (sometimes referred to as ‘‘welding’’) behind the supports before it flows through the die, outlining the perimeter of the shape.

2) Solid Billet and a Piercer Operating Through a Hollow Ram: This method avoids the seams inherent in the porthole die approach and, thus, produces a seamless extrusion.

3) Hollow Billet with a Die and Mandrel: This method also produces a seamless extrusion, but it is rarely used.

 

The material specification identifies if hollow extrusions must be seamless (for example, ASTM B241 Aluminum and Aluminum-Alloy Seamless Pipe and Seamless Extruded Tube) (53). Do seamless hollow shapes have better structural properties than shapes with seams? The answer is no, not with respect to material properties; the minimum strengths for porthole die produced hollow extrusions are the same (all other things being equal) as those for seamless extrusions. For properly extruded shapes, seams do not appear to have any adverse effect on structural performance, such as burst pressure or fatigue.