COST SAVINGS WITH ALUMINUM EXTRUSIONS
Aluminum extrusions offer opportunities to achieve needed structural and visual effects obtainable with virtually no other combination of material and shape. New forms, attractive finishes and simplified assembly methods are continuously being developed to meet the designer's particular requirements.
The processor extruding aluminum has been compared to squeezing toothpaste from a tube where the paste takes the shape of the tube opening. An aluminum billet, heated to between 800-1000F, is placed in the extrusion press and is forced under pressure through a die. The die may contain one or more openings having the desired finished cross section. Limitations on length of parts are the result of the weight of the extrusion billet and/or capacity of the down stream extrusion equipment.
Selecting extrusions over other methods of achieving a constant wall configuration will often result in significant cost savings. For instance, machining many long parts will consistently cost more than extruding to required cross section. And scrap can be a major cost factor in the machining operation.
Ordinarily tolerances obtainable in extrusions are adequate for most needs. If the section is to be used as an integral part of a precise operating assembly, the extrusion usually can be produced oversize and finish machined to required dimensions. While extrusion tolerances are generally larger than machining tolerances, extruders regularly hold tighter-than- standard extrusion tolerances on selected part dimensions. Specific ranges cannot be given as tolerances will vary with cross-sectional configuration and dimension.
When compared to fabrication for obtaining a complex cross-section, extrusion usually is the preferred method. Cost of welding jigs and fixtures can often outweigh that of the extrusion die, which also eliminates the labor required to weld two or more standard shapes together.
The cost of roll forming dies is substantially higher than for an extrusion die and the rolled forms are usually limited to uniform wall thicknesses.
Sand and permanent mold castings have certain limitations when compared to the extrusion method. Where a good visual appearance is required, the finishing costs for castings can be quite high. Die cost compared to part complexity can rise more rapidly with castings compared to extrusion dies.
Extrusion dies are relatively inexpensive, ranging from approximately $300-$400 for sections up to 3" circumscribing circle size, to several thousand dollars for shapes in the 10" size bracket. Complex and hollow dies can cost considerably more. Shapes can be produced with circle sizes of 30" or more, depending on size of extrusion equipment available, at proportionately higher tool cost.
Some metallurgical advantages in considering the use of aluminum extrusions are:
- High Strength-to-Weight Ratio: Place extra metal only where needed to strengthen part.
- Light Weight: Copper, brass and steel approximately 65-70% heavier.
- Corrosion Resistance: Aluminum forms its own thin protective oxide coating.
- Machineability: Speeds of 600 to 700 ft./min. normal.
- Variety of Joining Techniques Available: Can be welded, brazed, soldered and fastened mechanically by such means as staking, bolting or riveting.
- Formability: Compression and stretch-bending techniques are commonly employed. Alloy, wall thickness and temper all affect the minimum radius of a particular shape.
- Good Electrical Conductivity: For a given size, aluminum carries 65-70% of the current copper conducts.
The close dimensional tolerances and smooth joining surfaces of aluminum extrusions assure low cost assembly and complete interchangeability. Here are a few of the more commonly used types of joints between extrusions:
A hand-held instrument housing of a two-piece design using two pairs of extrusions illustrates a dovetail sliding fit. This approach resulted in a 2/3 savings in extrusion and fabrication tooling versus utilizing a one-piece hollow extrusion. An additional joining technique is illustrated by this part where channels are provided to accept various face plates for the instrument. End caps, which are secured by the use of screws and helicons, hold the face plate in position. Studs are mounted as a secondary operation to mount internal components. In addition, several openings were pierced for controls and readout components. The parts were black anodized for appearance enhancement.
A hand held control device used for programming machinery used two dissimilar shapes joined by the welded double lap joint method. By incorporating precise welding fixtures the part is joined to close tolerances capable of assuring plus or minus .005" between screw bosses. Extrusion tolerances much closer than standard commercial tolerances are also required to assure accurate assembly of the component parts.
Appearance was a consideration here also, so the assembly was etched and anodized. The etch process muted minor surface blemishes and the anodized finish provided a hard, protective, clear coat. Finally, the silkscreened lettering was applied, so that an assembly-ready part could be shipped from the extruder to the customer.
A removable snap-fit used three different extrusions to make a four-channel conduit for wiring in an office furniture application. Lateral movement of one wall must be possible for this type of design. If surface etching is required, the removal of material during etching must be factored into the part dimensions to assure tight fit.
There are approximately 250 extruders in the U.S. and services provided vary from providing raw extrusions only, to providing complete secondary operations. These might include piercing, drilling, welding, brushing, polishing, anodizing, painting (including roll coating, silk screening, etc.), V-notching and mitering. It is important to seek an extruder with capabilities suitable for your needs.
The extruder will, of course, need certain information to prepare a complete quotation for given requirements. The following items should be addressed:
- Part sketch or drawing (the extruder may be in a position to offer design suggestions for decreasing cost if consulted early on in the design stage.
- Specification requirements--federal, military ASTM, etc.
- Alloy and temper.
- End-use length and purchase length.
- Tolerances--standard commercial, per print, other.
- Surface finish--mill, paint, etc.
- Packaging acceptable, maximum and minimum weight/package and shipping and handling requirements.
- Secondary fabrication requirements--mitering, punching, bending, etc.
- Product end use.
- Quantity - this order, prototypes, and estimated annual usage.
- Shipping requirements.
- Special quality considerations.
In summary, the cost and operational savings attributable to the use of an extruded design are based principally on these points:
- Die cost is low.
- Desired elements may be combined as a single extrusion with elimination of joints.
- Complexity of profile is usually no handicap.
- Small parts are often made at marked cost savings.
- Finishing costs frequently are cut drastically or eliminated.
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