Aluminum is among the most versatile materials available to manufacturers, particularly among the common metals. Not only are there a wide variety of alloys and grades to choose from, but aluminum is also receptive to many different industrial processes. These options allow aluminum to be employed in all manner of applications.
One of the most popular methods of working with aluminum is of course extrusion. Aluminum is suitable for both commonly used methods of extrusion, direct and indirect. Understanding the differences between the two types is essential for getting the right process for your particular application.
How Does Extrusion Work?
Extrusion is defined as a process that creates objects of a fixed cross-sectional profile by pushing material, aluminum for example, through a pre-made mold. The process results in uniform products of a particular shape. By forcing the aluminum through the mold or die, it conforms to the shape of the die and it’s possible to produce even complicated cross-sections that might not be available through other means. Among its many advantages, the process allows for the use of brittle materials because only compressive and shear stresses are applied directly to the metal.
Among its many advantages with regards to aluminum, extrusion actually has the ability to strengthen the metal in many instances. Of course, extrusion is not limited to aluminum, as stainless steel and many other metals are used as well. Additionally, plastics, ceramics, and even concrete are extruded on a regular basis.
Another benefit to using extrusion is that the pieces that are produced can be of any length necessary. Moreover, it’s possible to form complicated shapes, such as extrusions with a hollow center. This technique is commonly referred to as hole flanging. Because a simple flat extrusion die does not work for this, since there is no means for supporting the center barrier as the metal flows through the die, it’s necessary in these instances that the shape profile support a central section. Therefore, during hole flanging, the shape changes internally along the length of the die. When the material is being forced through, it will flow around the support pieces and fuse together on the other side
Direct Extrusion Versus Indirect Extrusion
The most common method of extrusion, known as direct extrusion, can also be referred to as forward extrusion. The process is generally very straightforward. The material to be forced through is known as the billet, and the mold it is being forced through is the die.
To begin with, the billet that is to be extruded is placed in a heavily walled container. The billet is then pushed through the die using a ram or screw. It is known as forward extrusion because the billet and the ram are moving forward in the same direction. A dummy block is placed between the ram and the billet in order to prevent them from touching.
There is one major drawback to this method, which is the amount of force required to force the billet through the die. The frictional forces are relatively high due to the fact that the billet must travel the entire length of the container. As such, the force at the beginning of the process is at its highest, and it slowly decreases as the billet is extruded. Then, near the end, the force greatly increases again, as the material must flow radially in order to push out of the die. This also means that the end of the billet cannot be used as part of the extrusion and must be discarded or recycled.
By comparison, indirect extrusion has a number of advantages. It also goes by the name backwards extrusion and, as the name suggests, it is in many ways the opposite process to direct extrusion. In this method, the billet and container move in tandem while the ram and the die remain stationary. This is accomplished by employing a stem, which must be longer in length than the container. In this way, the billet is forced through the stationary die.
This method eliminates nearly all of the frictional forces. As the billet movement matches that of the container, all the frictional forces are reduced, resulting in a 25 to 30% reduction of total friction. This allows for the extrusion of larger billets at a faster rate. Additionally, smaller cross-sections are also possible. Other advantages include less cracking because there is no heat formation, the container is subject to less wear and tear, and a lower number of extrusion defects and coarse-grained peripheral zones.
On the other hand, indirect extrusion does have some drawbacks. The process is not nearly as versatile as direct extrusion. This is because the size of the cross-section is confined by the stem size. Moreover, any defects in the billet’s surface will greatly affect the finished extrusion, sometimes even destroying the piece. To avoid such calamities, it’s necessary to wire brush, machine, or chemically clean the billet before it is extruded.
As you can imagine based on the above, it’s critical for manufacturers to make the right decision when selecting whether to use direct or indirect extrusion. While indirect extrusion offers a number of advantages, it’s not appropriate for every application. Determining which is best for you will go a long way to deciding what material you’ll require.
Furthermore, it’s necessary to match the right alloy to the process. Not every aluminum alloy is created equal, and the great diversity of properties among the various grades means that they will respond very differently to the two methods.
At Clinton Aluminum, we make it our priority to ensure that every manufacturer has the right alloy for the job. With our knowledgeable and professional staff, we will work with our customers to fully understand their requirements and not only find the right material but make sure they are optimizing their process all the way through to completion. Contact us today to discuss what alloy is right for you.