ALUMINIUM EXTRUSION

Extrusion is a plastic deformation process in which a block of metal (billet) is forced to flow by compression through the die opening of a smaller cross-sectional area than that of the original billet Extrusion is an indirect-compression process. Indirect-compressive forces are developed by the reaction of the workpiece (billet) with the container and die; these forces reach high values. The reaction of the billet with the container and die results in high compressive stresses that are effective in reducing the cracking of the billet material during primary breakdown from the billet. Extrusion is the best method for breaking down the cast structure of the billet because the billet is subjected to compressive forces only.

Extrusion can be cold or hot, depending on the alloy and the method used. In hot extrusion, the billet is preheated to facilitate plastic deformation.

Factors Affecting Extrusion

Shape is a determining factor in the part’s cost and ease with which it can be extruded. In extrusion a wide variety of shapes can be extruded, but there are limiting factors to be considered. These include size, shape, alloy, extrusion ratio, tongue ratio, tolerance, finish, factor, and scrap ratio. If a part is beyond the limits of these factors, it cannot be extruded successfully.

The size, shape, alloy, extrusion ratio, tongue ratio, tolerance, finish, and scrap ratio are interrelated in the extrusion process as are extrusion speed, temperature of the billet, extrusion pressure and the alloy being extruded.

In general, extrusion speed varies directly with metal temperature and pressure developed within the container. Temperature and pressure are limited by the alloy used and the shape being extruded. For example, lower extrusion temperatures will usually produce shapes with better quality surfaces and more accurate dimensions. Lower temperatures require higher pressures. Sometimes, because of pressure limitations, a point is reached where it is impossible to extrude a shape through a given press.

The preferred billet temperature is that which provides acceptable surface and tolerance conditions and, at the same time, allows the shortest possible cycle time. The ideal is billet extrusion at the lowest temperature which the process will permit. An exception to this is the so-called press-quench alloys, most of which are in the 6000 series. With these alloys, solution heat-treat temperatures within a range of 930°-980°F must be attained at the die exit to develop optimum mechanical properties.

At excessively high billet temperatures and extrusion speeds, metal flow becomes more fluid. The metal, seeking the path of least resistance, tends to fill the larger voids in the die face, and resists entry into constricted areas. Under those conditions, shape dimensions tend to fall below allowable tolerances, particularly those of thin projections or ribs.

Another result of excessive extrusion temperatures and speeds is tearing of metal at thin edges or sharp corners. This results from the metal’s decrease in tensile strength at excessively high-generated temperatures. At such speeds and temperatures, contact between the metal and the die bearing surfaces is likely to be incomplete and uneven, and any tendency toward waves and twists in the shape is intensified.

As a rule, an alloy’s higher mechanical properties mean a lower extrusion rate. Greater friction between the billet and the liner wall results in a longer time required to start the billet extruding. The extrusion ratio of a shape is a clear indication of the amount of mechanical working that will occur as the shape is extruded.

Extrusion Ratio = area of billet/area of shape.

When the extrusion ratio of a section is low, portions of the shape involving the largest mass of metal will have little mechanical work performed on it. This is particularly true on approximately the first ten feet of extruded metal. Its metallurgical structure will approach the as-cast (coarse grain) condition. This structure is mechanically weak and shapes with an extrusion ratio of less than 10:1 may not be guaranteed as to mechanical properties.

As might be expected, the situation is opposite when the extrusion ratio is high. Greater pressure is required to force metal through the smaller openings in the die and extreme mechanical working will occur. Normally acceptable extrusion ratios for hard alloys are limited to 35:1 and for soft alloys, it is 100:1. The normal extrusion ratio range for hard alloys is from 10:1 to 35:1, and for soft alloys is 10:1 to 100:1. These limits should not be considered absolute since the actual shape of the extrusion can affect results. The higher the extrusion ratio, the harder the part is to extrude which is the result of the increased resistance to metal flow. Hard alloys require maximum pressure for extrusion and are even more difficult because of their poor surface characteristics which demand the lowest possible billet temperature.

Difficulty factor is also used to determine a part’s extrusion performance. Factor is the perimeter of the shape divided by the weight per foot. Factor = Perimeter of Shape/ Weight per Foot. Weight per foot is of primary importance because of the consideration for profitable press operation. As might seem obvious, a lighter section normally requires a smaller press to extrude it. However, other factors may demand a press of greater capacity such as a large, thin wall hollow shape. Though it has low weight per foot it may take more press tonnage to extrude it. The same reasoning applies to the factor as with the extrusion ratio. A higher factor makes the part more difficult to extrude consequently affecting press production.

The tongue ratio also plays an important role in determining a part’s extrusion performance. The tongue ratio of an extrusion is determined as follows: square the smallest opening to the void, calculate the total area of the shape, and then divide the opening squared by the area. The higher the ratio, the more difficult the part will be to extrude.

In order to help us understand your needs and requirements and service you better, the following is a check list of things to consider when submitting items to an extruder for quoting or new business:

Advantages of the extrusion process:

There are several advantages of the modern extrusion process

1. A variety of shapes are possible, especially with hot extrusion.

2. Grain structure and strength properties are enhanced in cold and warm extrusion.

3. Fairly close tolerances are possible, especially in cold extrusion.

4. Little or no wasted material is created. However, a limitation is that the cross section of the extruded part must be uniform throughout its length.