What is metal extrusion?

Stock material and custom metal work come in a wide variety of cross-sections, many of which will have been created using the process of metal extrusion. The importance of metal extrusion can’t be understated as products that are made this way can be found in a range of industries such as construction, manufacturing, retail, information technology, and more. 

Having even a surface level understanding of some of the key metal fabrication and manufacturing methods as well as their capabilities can be helpful to engineers. Therefore, in this article, we’ll be exploring what exactly metal extrusion is, a quick history of it, types of extrusion processes etc. 

Brief history of metal extrusion 

The process was invented by Joseph Bramah in 1797 when he pushed soft metal through a die using a hand-driven plunger to create metal pipes. Joseph Bramah also went on to develop the very first hydraulic press for Thomas Burr in 1820. This hydraulic press produced the first lead pipes that transformed the metal manufacturing industry. 

What is metal extrusion? 

The extrusion process involves forcing a metal (hot or cold) through a die which then imparts the die shape to the extruded metal as it moves through the cavity. When the material emerges from the die it is referred to as “extrudate”. The metal experiences compressive and shear stress in order to be moulded into the die shape.  

The nature of these forces and the increased temperatures mean materials can be formed with otherwise brittle properties using this process. It’s not just metals that are suitable for extrusion, non-metals like ceramic, plastic, clay, concrete, and polymers can be used in the process. 

Key features 

• Extrusion is an inexpensive process thanks to less waste and having a high rate of production. 
• It can make brittle materials because of only applying compressive and shear forces on the billet. 
• Products possess elongated grain structure in material direction and a smooth surface, reducing post-treatment.  
• Thin wall thicknesses achievable via extrusion: 3mm steel, 1mm aluminium. 

• The process forms very complex cross-sections with a uniform wall thickness throughout the product. 

What is the process of metal extrusion? 

The extrusion has evolved and been through a number of changes since its initial invention. This process is used to give consistent material input for 3D printing and other additive manufacturing applications, in addition to extruding final products. The material is then deposited one layer at a time to form the desired product. Extrusion is very important and beneficial when it comes to bespoke metal fabrication. Read on to see the steps of a generic metal extrusion process below. 

Preparing feed metal 

The billet/ingot is the feed metal that functions as the raw material. It’s important to note that feed metal has to adhere to specific standards set by the designers. Normally it will have a circular or square profile, but it can have other shapes too. The feed metal itself is formed with methods like hot rolling or continuous casting. 

Getting ready for extrusion 

The raw material goes into an extrusion machine such as a press, heated (hot extrusion) or not (cold extrusion) depending on the method used. 

Extrusion 

Next the extrusion itself can take place by putting compressive force that pushes the material towards the die, which has a small opening. In response to the high pressure, the metal leaves the die through that opening, taking the shape of the die during the process. As soon as this is done, pressure is released, and the next step can begin.  

Heat treatment and post-processing 

Heat treatment is the next part of the process as the product needs to have its properties improved and get it ready for its service conditions. The heat treatment process will differ depending on the metal. For example, if using aluminium, the extruded part is cooled first, then stretched and cut to the required lengths. The pieces then go through ageing where they are heated to 350 degrees Fahrenheit and held for four hours so that they harden. 

What are the different types of extrusion processes? 

There are several different types of metal extrusion forms that can be used, and while the main principles stay the same, they are applied differently to successfully create a range of bespoke metal fabrication products. All metal extrusion processes can come under one of the following categories.  

Hot extrusion 

In hot extrusion, the metal is fed above its recrystallisation temperature to soften the metal and allow it to flow through the die opening. The high working temperature stops the material from going through work hardening and the pressure level means that it is necessary to use a lubricant. Setup for hot extrusion is costly to buy and maintain, so it is only really profitable for large scale products. 

Cold extrusion 

When extrusion is done at room temperature it is known as cold extrusion. The method stays the same as hot extrusion except the material isn’t heated at all, or only marginally before the process is started. Advantages of cold extrusion include shorter batch timings, finer tolerances, a smoother surface finish, and lack of oxidation. Disadvantages are the potential need for more power due to the material being difficult to work with, the mechanical properties of the material can change during the process, and it takes longer at lower temperatures. 

Warm extrusion 

Warm extrusion is when the process is completed between room temperature and recrystallisation temperature of the metal. Compared to the previous processes, warm extrusion provides benefits like more control over extrudate properties e.g., ductility. The temperature never goes above the critical melting point at any time during the metal forming process. 

Friction extrusion 

The heat generated by friction between the die and feed metal is used to heat the latter during friction extrusion. Therefore, the feed metal doesn’t need any pre-heating, but it uses the internal energy to increase the extrusion temperature. Despite being previously overlooked, this type of process has gained more attention recently thanks to its benefits in additive manufacturing. 

Microextrusion 

This process produces parts of the small, sub-meter range that are intended for special applications, but it does require small dies and rams, which is difficult with stringent accuracy requirements. The end result of microextrusion will fit within a 1mm square. Other issues arise when working with small products like grain boundaries and structure, deformation defects, and creating stability. 

If you need expert metal fabrication in Sheffield, contact us at FEM today. Our team have extensive experience in the industry and strive to create the highest quality products for our customers.