Aluminum alloy extrusion offers designers a freedom from ‘standard’ shape restrictions that is unrivaled among structural materials.

With aluminum extrusion, you don't have to compromise your design to accommodate standard shapes - you can design exactly the shape you need and let the production process accommodate itself to you. Careful design is the way to gain maximum benefit from the many valuable characteristics of aluminum alloys and from the flexibility of the extrusion process.

The Functional Approach

To make the most of aluminum extrusion's almost unlimited opportunities, set high expectations for a product or design component and then figure out what it takes to meet them. Clear your mind of preconceived shapes. The first question is not "What should this part look like?" but "What would I really like this part to do?" When that question is correctly answered, the shape will follow naturally. The basic approach is simple:

• Identify the part's essential functions
• Determine what shapes and dimensions those functions require
• Visualize how these elements relate to each other in space; sketch them in place
• Sketch in the secondary functional elements necessary to connect, support and strengthen the primary elements of the design
• Then develop and refine the design in detail

Design Parameters

Five major factors should be considered in the detailed development of an aluminum extrusion design:

• Shape configuration
• Tolerances
• Surface Finish
• Alloy
• Circumscribing circle size

These parameters are interrelated in their effect on the extrusion design and its application.

Shape Configuration

The designer's first priority is to satisfy a specific need. Extrusions can be designed to aid in assembly, improve product appearance, reduce or eliminate forming and welding operations, and achieve many other purposes. Basically, aluminum extrusion allows you to design the shape that best meets your structural and aesthetic requirements.

Users of computer-aided design programs will find aluminum extrusions a uniquely satisfying product, because cross-sections can be profiled to meet optimum structural requirements.

Extruded shapes are described in three general categories - semi hollow, solid, and hollow. Dies to produce solid shapes are the least complex. But the difference between a solid shape and a semi hollow shape may not be obvious at first glance. It's easier to describe and understand all three categories by working in reverse, starting with hollow shapes.

A hollow shape is simply an extruded shape which, anywhere in its cross-section, completely encloses a void. The void itself may have any sort of shape, and the complete profile may include a variety of other forms.

Example of a hollow shape.

A solid extruded shape is any shape that is not a hollow or a semi hollow. This covers a wide range including, for example, compact cross-sections with or without projections; angular or curved shapes; and those wrap-around shapes whose void area/gap' ratios are too low for the semi-hollow-class.

Example of a hollow shape

Example of a solid shape

Tolerances are critical. A designer should be aware of the standard dimensional tolerances to which extrusions are commercially produced. These tolerances generally cover such characteristics as straightness, flatness, and twist, and such cross-sectional dimensions as thickness, angles, contours and corner or fillet radii. Aluminum extrusions are often designed to minimize or eliminate the need for machining. If desired, extrusions can be produced to closer-than-standard tolerances, generating cost savings in secondary operations; such savings may range from modest to very large, depending on circumstances.

Surface Finish

One advantage of aluminum extrusions is the variety of ways the surface can be finished, and this offers another range of choices to the designer. As-extruded, or "mill," finish can range from "structural," on which minor surface imperfections are acceptable, to "architectural," presenting uniformly good appearance. Other finishes include scratch finishing, satin finishing and buffing. Aluminum can also be finished by clear or colored anodization, or by painting, enamelling or other coatings.

Alloy Selection

Aluminum extrusions are made in a wide variety of alloys and tempers to meet a broad spectrum of needs. Selection is made to meet the specific requirements in strength, weldability, forming characteristics, finish, corrosion resistance, machinability, and sometimes other properties. The complete list of registered aluminum alloys is quite long, but in practice a few alloys are chosen repeatedly for extrusion because of their versatility and highly suitable characteristics. Extruders generally stock the three or four most frequently used alloys. When their specialized markets justify it, individual companies include in their inventories additional alloys which will vary with the needs of their major customers. Thus, a substantial variety of extrusion alloys is readily available.

Circumscribing Circle Size

One measurement of the size of an extrusion is the diameter of the smallest circle that will entirely enclose its cross-section - its "circumscribing circle." This dimension is one factor in the economics of an extrusion. In general, extrusions are most economical when they fit within a medium-sized circumscribing circle: that is, one with a diameter between one and ten inches.

Good Extrusion Design Practices

At this stage in the development of an extruded product, the designer has determined its functional shape and size, and considered appropriate tolerances, surfaces finishes and alloys.

Before proceeding, it makes sense to review the extruder's available standard shapes. It may be possible to adapt a standard shape to the needs of the product, with little or no modification. If a standard shape is not readily adaptable, the design can be completed as a custom shape perfectly suited to the requirements of the product. Here are a few tips on good practices in custom-designing aluminum extrusions.

Specify the Most Appropriate Metal Thicknesses

Specify metal thicknesses that are just heavy enough to meet your structural requirements. Even in low stress areas, however, keep sufficient thickness to avoid risking distortion or damage. Some shapes tend to invite distortion during the extrusion process (such as an asymmetric profile or thin details at the end of a long flange).

Keep Metal Thicknesses As Uniform As Possible

Extrusion allows you to put extra metal where it is needed - in highstress areas, for example - and still save material by using normal dimensions elsewhere in the same piece. Adjacent-wall thickness ratios of less than 2-to-1 are extruded without difficulty.

Visualize The Die And The Metal Flow

Remember what an extrusion die does: while it lets metal flow through its shaped aperture, it must hold back metal all around that aperture against great force. When you design a shape for extrusion, you are simultaneously designing a die aperture and you must take extrusion forces and metal flow into account. For example, a U-shaped channel in an extrusion corresponds to a solid "tongue" in the die, attached at only one end. Flexibility in this tongue can alter the aperture slightly under the pressure of extrusion; the deeper you make the channel, the longer you make the tongue and the more difficult it becomes to regulate the extruded dimensions. Visualize the shape of the die that must produce your design, and try to minimize shapes that would weaken the die or impede metal flow.

Fabricating Properties

The ease of forming, machining and joining aluminum extrusions is an important advantage in fabricating more complex structures.

Forming

Extruded aluminum tube, pipe and shapes can be formed on conventional bending equipment. The minimum bending radius of a specific extrusion will depend on its size, its alloy and temper, the complexity of its shape, and the characteristics of the available equipment. An alloy is usually selected for reasons other than the demands of forming. But when formability is a significant question, several properties of the alloy and temper should be considered, including: elongation, hardness, and the spread between yield strength and ultimate strength. Alloys with high elongation, low hardness and maximum spread between yield and ultimate strength are the easiest to form.

Machining

Aluminum extrusions may be machined rapidly, with some modification of conventional practices. Smooth surfaces can be obtained by finishing the cutting tools with considerably more side and top rake than is used for cutting most other metals. The softer alloys in particular, such as 3003 and 6063, require large rake angles and light cuts. Parts that are to be machined and asymmetrical shapes that will be cut to size should be ordered in a stressrelieved temper, such as TX511, if the final parts must meet close tolerances.

Joining

Aluminum extrusions may be joined by most standard metal-joining methods. And fabrication may be further simplified by thoughtful design features, for example:

As an aid to riveting, a guideline can be extruded into the surface marking the precise location of the rivet line.

Fillet welding can be facilitated by incorporating a weld preparation, such as a vee groove, into the extruded shape.

Screw slots can be incorporated in the extruded shape to eliminate drilling and tapping.

Economics of Design

The economic advantages of extrusions are influenced by several important variables, including:

Quantity

Even in short production runs, aluminum extrusion may "break even" and become more economical than alternative processes - particularly when secondary savings such as reduced machining, finishing and assembly are factored in. A product may be manufactured initially using standard extruded shapes; but as product volume increases, it often becomes more economical to redesign components as custom extrusions, matched more precisely to product needs. At large volumes, manufacturers can benefit from the volume-prices available on large mill runs of a shape.

Product Shape

Shape also influences product economics: in general (although not always), semihollow extrusions are more economical than hollows; solid shapes are more economical than semihollows; and symmetrical shapes are more economical than asymmetrical shapes. Extrusions, however, can often save a manufacturer money in "hidden" ways. A more complex extrusion may be well worth some moderate expense for the savings it creates in reducing machining, forming, joining, assembling, shipping or other costs.

Tolerances

Extrusions produced to industry standard tolerances are more economical than those requiring special tolerances. Product fabrication and assembly techniques sometimes change, making special tolerances less necessary. Periodical review of an extrusion design may reveal an opportunity to reduce costs by easing or eliminating special tolerances.

Alloy

For an extrusion design there may be several alloys and tempers that would be suitable for production. The selection is usually made on the basis of structural or fabrication requirements; but it's a good idea to look, as well, for the most economical alloy among several that may be functionally equivalent in the specific application at hand.

Surface Finish

Careful production, handling and shipping can deliver extrusions with premium-quality surface finishes. But a premium finish may not be needed on all faces of an extruded shape, and as circumstances change the need for premium finishes may diminish or disappear. It may be possible, upon review of an extrusion design, to reduce or eliminate premium finish specifications.

Length

At relatively low product volumes, fabricators often purchase extrusions in economic warehouse lengths. As production volume increases, however, even greater savings may be gained by purchasing extrusions in exact required lengths or multiples of exact lengths, to reduce scrap.

This is an excerpt from "THE SHAPE MAKERS" a publication of the Aluminum Extruders Council, 1000 N. Rand Road, Suite 214, Wauconda, Il 60084. Tel 847.526.2010 - Fax 847.526.3993