Shape is a primary determining factor in the part's cost and ease with which it can be extruded. While a wide variety of shapes can be extruded, there are a number of interrelated limiting factors to be considered, including 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. Other considerations include extrusion speed, temperature of the billet, extrusion pressure and the alloy being extruded.

As a general rule, 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. Lower extrusion temperatures will usually produce shapes with better quality surfaces and more accurate dimensions, but also 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. However:

certain alloys (notably the 6000 series) require solution heat-treat temperatures within a range of 499°-527° C 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, causing the metal to fill the larger voids in the die face and resist entry into constricted areas (this particularly affects profiles having thin projections or ribs and for these shapes, tolerance may not be maintained).

At excessive extrusion temperatures and speeds, the metal may tear 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, higher mechanical properties of an alloy means lower the rate of extrusion as 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 three metres 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 (normal range being 10:1 to 35:1) and for soft alloys, it is 100:1(normal range being 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 in inches divided by the weight per foot.

Factor = Perimeter of Shape in inches/ 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.