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. 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 means 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.

Thanks, Mehmood for your general discussion about the effect of process parameters like container temperature, billet temperature, extrusion ratio, extrusion speed etc to produce shapes with better quality surfaces and more accurate dimensions. My questions still require proper answers.

Regards

without proper dimensional inputs (billet dia, required dia, number of passes, required surface finish in microns, etc) ... it is not easy for anyone to answer your question ...