Fundamentals and Operation of Stretchers in Aluminum Extrusion

摘要

As extruders are well aware, the flow of metal in the extrusion process is not uniform. This non-uniformity is due initially to differences in fric-tional drag between extrusion container and billet, and further in the process to internal friction in the billet, and, finally, to frictional differences between metal and extrusion die. There is considerable frictional drag along the lands of the extrusion die. The drag is greatest in areas of high surface area, i.e., long bearing lengths cause more drag than short and complex shapes have higher drag than flat faces. Metal flows fastest in the center of the cross section than it does at the surface. Die correctors try to compensate for this uneven flow but, nevertheless, differential flow is to some degree inevitable, resulting from internal stresses within the extrusion as it emerges from the die. If these stresses are small or balanced, the extrusion will be smooth and straight. However, it frequently happens that the stresses are not balanced, and the extrusions are bent or otherwise distorted.