Other issues include shape, size, and configuration; use of extrusion puller equipment; quench distortion; and handling equipment. I’ll address each one separately.
Shape size and configuration
Shapes are symmetrical and have more consistent wall thicknesses extrude (and cool) straighter. If a shape is extruded on a press “not quite large enough” to handle it (either circle size of part or certainly the thickness of tool stack vs. the critical dimensions in the part), it will be more difficult to hold twist tolerances. For example, a 6.5-inch-wide shape with critical twist requirements will likely run better on an 8- or 10-inch press with perhaps a 14- to 16-inch deep tool stack than on a 7-inch press with a 10- or 12-inch tool stack.
Use of extrusion “puller” equipment
Older presses may not have pullers, which basically grab hold of the front of the shape and maintain some limited tension during extrusion. The press might, for example, be pushing with 5 million lbs of force, while the puller will only pull with up to a few hundred pounds, just enough to keep tension on the emerging extrudate. Within reasonable limits, this tends to assist with straightness and twist, due to effects back at the die bearing. It also effectively reduces the die bearing length in contact with the extrudate for those portions extruding a little slower (and thus under more tension).
Depending on the alloy and temper required, the extrudate may be able to be quenched via still air, air fans, air fans with mist, or may require more rigorous water quenching. Generally, 6000 series alloys with more alloying ingredients require more quench. More severe water quenching (including misting for some more delicate extrusions) can lead to distortions, gap issues, or twist due to differential cooling around the shape.
Depending on the equipment vintage and design, the ability to hold close tolerances, avoid twist, etc., can be affected, whether in the discussions above or from other causes, such as the use of older-style liftovers vs. belt or rectalinear cooling table mechanisms, and so on.
– Craig Werner is the chairman of the Aluminum Extruders Council’s AEC Academy program, and president of Werner Extrusion Solutions, an extrusion design and process consulting firm.