Aluminum Extrusions - Design Considerations
An aluminum extrusion profile allows a designer to take full liberty in defining the shape of the product without the significant constraints associated with stampings, die castings, spin casting, deep forming, etc. Extruded aluminum represents about 20% of the total consumption of aluminum in the USA.
Characteristics of aluminum include lightweight, resilient, corrosion resistant, electrically conductive and heat conductive. Extruded products have an attractive appearance, no seams or sinks, close tolerances, easy fabrication and are fully recyclable.
The two major types of extrusions include solid shapes like a simple channel or complex heat sink and hollow shapes like a simple tube or a complex gear. The most significant market for extrusions is the building and construction segment with door frames and window glazing. Other markets include machinery, electrical components, appliances and the transportation industry.
A basic design rule of thumb is to attempt to keep wall thickness similar across the profile. Although not mandatory, and up to a 10/1 ratio is possible, running characteristics will improve with even flow. Another guideline is the depth of a channel should be no more than 3x the width. The limiting size of the profile is determined by the extruders press opening or "circle size". Beyond 8 inches, dies are expensive and wall thickness increases geometrically.
The ability to meet machining-type tolerances is limited to very small shapes, but can be produced with secondary milling operations. Generally, for an inch in width that can run close to 0.005", estimate about 0.010" additional tolerance per inch as the shape gets larger. Many extruders can improve on tolerances, but getting a 6 inch wide profile to extrude within better than about 0.025" is a challenge.
The most common alloy used for extrusion is 6063-T5 and is used on most custom designed shapes. 6061-T6 and 6005-T5 have 2x strength characteristics but carry a slight cost premium and do not finish well if anodized. Even higher strength alloys are available (7xxx series), but from only a limited number of extruders, and are used for aircraft and critical strength/weight applications.
Secondary operations can enhance the usability of extrusions to allow designers to consider more than length or width. Cutting tolerances of +/-0.005" are not uncommon and available at various angles. Secondary machining such as milling, drilling, tapping, countersinking and punching are relatively simple with easy to cut material. In many cases, screw tubes, circuit board slots and label detents can be extruded in the profile with no additional secondary costs.
While attractive in the raw form, most extrusions are either anodized to retain/prevent oxidation or painted, either with powder paint or traditional "wet" paint.
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