Extrusion is considered to be among the most efficacious procedures to shape and mold metal and the most used metal in this process is aluminum. We obtain high-quality extruded aluminum by forcing hot raw aluminum through a steel profile for creating a definite shape. Once the extruded material comes out in a single piece, it can be cut as per your requirements.
However, the extrusion process is not easy and understanding the aspects that can influence your success in aluminum extrusion can help you to be more careful in future.
We understand that the shape of the part is important when ease and cost of the extrusion are considered and that a diverse range of shapes can be attained. There are however certain limiting factors that need to be taken into account, and without the limits of these factors, a part cannot be successfully extruded. Shape, size, extrusion ratio, alloy, tolerance, tongue ratio, scrap ratio and finish are as important limiting factors as billet temperature, extrusion speed, the alloy to be extruded and extrusion pressure, and all of them are interrelated in the process.
At our workshop, we have observed that usually, extrusion speed changes with the change in pressure created inside the container and metal temperature, and the change is directly proportional. Again, pressure and temperature are limited by the alloy, which is utilized and the shape that is required. For instance, if the extrusion temperature is low (which can be caused by high pressure), the dimension of the extruded aluminum will be more precise, and the quality of the surfaces will also be good. Owing to pressure limitations; at times, we may come across a point where thrusting out a shape via a specific pass is practically achievable.
The ideal billet temperature is that which offers admissible tolerance and surface conditions and also makes way for the briefest cycle time. So we recommend extracting billet at the minimum temperature allowed by the procedure. Standing apart from this are the press-quench alloys, and most of them belong to the 6000 series. With such alloys, our experts must make sure that the solution heat-treat temperatures are from 930°-980° F at the die exit to create optimum mechanical properties.
When the extrusion speed and the billet temperature is high, the flow of metal becomes more free-flowing. During this time, by selecting the path for the slightest resistance, the metal is more inclined to fill the gaps in the die face and withstand entry into narrow areas. In such an environment, the shape dimensions are prone to go below the admissible tolerances, specifically those of ribs or thin projections.
With the increase in speeds and extrusion temperature, the metal may lose its tensile strength owing to the enhanced temperature, and tear at sharp corners or thin edges. When the speed and the temperature are extremely high, the connection between the die bearing surfaces and the metal will probably be uneven and insufficient. Hence, if there had been any propensity towards twists and waves earlier, it heightens.
If it is observed that an alloy has high mechanical properties, it means that the extrusion rate is low. When the friction between the liner wall and billet is more, the time needed to begin the billet extruding is high. A shape’s extrusion ratio indicates the amount of mechanical working that will take place as the extrusion is done.
From our experience, we can tell you that with a low extrusion ratio, sections of the shape that involves the biggest mass of metal will have little mechanical work implemented on it. This is especially true on almost the first ten feet of the extruded aluminum. Its metallurgical construction is mechanically infirm and shapes that have an extrusion ratio below 10:1 may not have all the mechanical properties.
As expected, the circumstance is contradictory to what happens when the ratio of extrusion is high. A huge amount of pressure is necessary to thrust metal via the smaller openings in the die and the maximum mechanical working will take place. The generally allowable ratios of extrusion for soft alloys is 100:1 while for hard alloys, it is 35:1. Also, the ratio range of normal extrusion for soft alloys is from 10:1 to 100:1 and for hard alloys, it is from 10:1 to 35:1. These limits should never be deemed absolute as the original shape of the extrusion can influence the outcome. The greater the ratio of extrusion, the more difficult it becomes to extrude the part that was an outcome of the high resistance to the flow of metal. Hard alloys have poor surface traits and need huge pressure for extrusion, but the least amount of billet temperature.
The extrusion performance of a part is also decided upon by the difficulty factor.
The weight for each foot is of fundamental significance as profitable press operation is brought under consideration. As might appear crystal clear, a light section usually necessitates a small press to extrude it. But other factors may command a press of greater capability like a huge, thin wall hollow shape. Even though it has a low weight per foot, the press tonnage we need to extrude may be more. The same logic is relevant to the factor as with the ratio of extrusion. A higher factor makes the part tougher to extrude, thus impacting press production.
The tongue ratio also plays a crucial role in ascertaining a part’s extrusion performance. This ratio of an extrusion is decided upon as mentioned here - square the tiniest opening to the gap, measure the shape’s total area, then divide the opening squared by the area. The greater the tongue ratio, the tougher will it be to extrude the part.
As noticed from the above piece, the factors are all woven into each other and keeping them in mind can help you to extrude aluminum with ease.