What Is Extrusion?- Working, Types, And Application

What is Extrusion?

Extrusion is a manufacturing process that revolves around the idea of shaping material by pushing material (in the form of a billet) through a die that contains the profile to be produced.

The difference between extrusion and other forming processes is that extrusion is capable of producing very complicated profiles, with even brittle materials.

This is the case because the manner in which compression and shear stress are placed on the material differs from the tensile stresses often at play with breakage. The extrusion process may also produce a quality surface finish and a variety of design possibilities at the design stage.

Drawing is a different beast altogether. Drawing involves pulling the material through a die and involves tensile strength.

As a result of this difference in mechanism, drawing generally cannot change shape in one shot over large amounts. Given that if you don’t keep fairly simple, and depending on the large size and shape of the material you wish to produce, it usually requires multiple processes.

This process is usually associated with the production of wire but can be associated with other material shapes, eg metal bars or tubes

Extrusion, however, can be a continuous process (in theory, you can extrude for as long as you want) or a semi continuous process (ie extruding lengths of material into separate pieces)

Extrusion can be done with materials that are both hot and cold. And, surprisingly, extrusion has a fairly diverse variety of materials suitable for extrusion not only metals, but also plastics, ceramics, metals and even food (and concrete) can all be extruded. The term ‘extrudate’ is often used to refer to extruded products.

So if you want to create hollow sections within extruded material, which can also be referred to as hole flanging where you cannot use flat dies because they lack the support for the hollow areas of the section, appears to be a custom die that is made as a deep block.

This die will have an initial profile that supports the hollow area, which in fact gets closed in as the product gets pushed through albeit as a gradual change.

As the material moves and flows through the die, the core portions depicting the hollow areas are supported by material from behind, and the joining layers eventually meld together to make up the hollow section.

Another worthy mention with regard to extruding metal is that it not only allows for the shaping of material, but it also has the effect of increasing the strength of the material.

Characteristics of Extrusion

• Extrusion’s ability to manufacture very complex cross-sections with consistent shape for the detail of the extrusion allows for the production of high performance and complex parts that are robust and reliable. There are many factors affecting the quality of an extrusion; not just the die design, but also the extrusion ratio, the temperature of the billet, lubrication, and the speed of extrusion and these all contribute distinguishable effects. For those who want to pursue the practical side of this, there is a good guide available called “How to design parts for direct metal extrusion” that contains the five main variables and some good design for manufacture (DFM) considerations to know relative to extrusion.
• Like most other metal forming methods, extrusion can be hot work or cold work, but the process is most frequently performed at elevated temperatures. Elevated temperatures not only make the material easier to work with, but it actually reduces the force needed as well as ductility.
• From an economic standpoint, extrusion is preferred due to it’s overall low-cost potential. Much of the cost efficiency comes from reduced waste of raw materials, and the sheer quantity of product that can be profiled in a time efficient manner.
• Of course, even some brittle materials can be extruded. The reason for this is that extrusion primarily applies compressive and shear stresses, and less tensile stress, and therefore less opportunity for the materials to tear during deformation.
• Also, extrusion produces parts with great surface finishes. Most often parts come out of the process so smooth and precise, if at all, there is very little or no need for further machining.
• The extrusion process produces an elongated grain structure in the material flow direction, which is considered a good feature for a variety of applications.
• Regarding the technical limits, extrusions can have very thin wall sections. For aluminum wall sections can be as thin as 1 mm, while steel could be 3 mm, depending on the specifics of each job.

Types of Extrusion Process

Metal extrusion can be subdivided and grouped into the following categories depending on the direction of extrusion flow, the medium used to apply force, working temperature, etc.

• Direct Extrusion
• Indirect Extrusion
• Hydrostatic Extrusion
• Lateral or Vertical Extrusion
• Hot extrusion
• Cold Extrusion
• Impact Extrusion

1. Direct Extrusion

Direct extrusion – or forward extrusion – is the most basic type of extrusion process, and is the most prevalent in the extrusion family.

The process starts with the heated billet (for hot extrusion, which we will discuss later) placed inside of the press cavities, or container. Next we need to place the dummy block in directly behind the heated billet.

After the heated billet and dummy block have been placed in the press and the hog or ram have engaged, either the hydraulic ram or a mechanical ram will apply force to the heated billet, pushing the material in the shape of the die opening.

At this time the extruded part will be hot as it is also emerging out of the die. To ensure the desired straightness of the extruded part, it is even put on a stretch to ensure no bowing could occur as the part exits the die. It is quite nice to check the animation that Core Materials produced to dub this function.

There is one component that is unique to the direct extrusion process and that is friction management. When producing parts and managing friction, at the elevated temperatures steel tend to create quite a bit of friction. So rather than using a lube with high viscosity, manufacturers will use molten glass on the heated billet.

When dealing with the lower temperatures, then the standard lubricating oils with graphite powder will do. The dummy block will act as a protective element which will cover the tip of the ram (or rammed stem), specifically in the hot extrusion.

Another point that can be made is that there is always a small remaining piece of the billet called the “butt end.” This butt end cannot be passed through the die opening within the container when the punch (or ram) completes its stroke.

Advantages of Direct metal extrusion

• There is no requirement to prepare the billet at the outset of the procedure.
• This process is flexible because it is applicable to hot or cold extrusion.
• The tooling for this process is relatively simple when compared to other types of extrusion.

Disadvantages of Direct metal extrusion

• High force requirement due to friction
• Butt end left inside the cavity
• The force required to push the ram changes as the punch moves

2. Indirect Extrusion

Indirect extrusion has a different mechanics than what you might be used to with direct extrusion. In indirect extrusion, the die is attached to the hydraulic ram that moves toward the fixed billet in the container, and the ram forces the billet material back through the die.

An advantage in this method is the total power required is much lower. Because the billet container is fixed in place, comparatively, you have to overcome less friction with this process compared to all other processes of extrusion. Less friction means less energy consumption which is a positive point in the column of efficiency.

With all advantages, of course, comes a disadvantage. Probably one of the biggest challenges is to support the extruded part as it emerges from the die. Indirect extrusion doesn’t create an easy system for catching or directing the newly formed material through the die.

Advantages of Indirect Metal Extrusion:

• The procedure generates lower friction, therefore less energy is required to perform the task.
• It is adaptable indirect extrusion works for hot and cold applications.
• The tooling is relatively uncomplicated than some of there more complex extrusion processes.

Disadvantages of Indirect Metal Extrusion:

• It’s not easy to support the extrudate as it leaves the die, which can complicate the operation.
• The hollow design of the ram places some limits on the maximum load that can be applied.

3. Hydrostatic extrusion

Hydrostatic extrusion process differs from conventional extrusion processes in a number of ways. The cavity that contains the billet is actually designed to be a little smaller than the billet. And instead of being empty, this cavity is completely filled with hydraulic fluid.

In this case, the ram does not directly apply force to the billet; it applies force to the hydraulic fluid, which acts in all directions on the billet, what is called in engineering tri-axial forces. Because of this, the billet resists pressure much better.

With hydrostatic extrusion, there are (at least) two limitations. The first is obtaining a tight seal on the fluid from the very start. Any drops in pressure or leaks will create operational problems.

The second is that one aspect of the process is at least slightly better; the hydraulic fluid between the billet and the cavity provides a barrier against friction that would absorb energy in production and create inferior extrusion.

On the other hand, it can be claimed that this process is limited in special equipment (meaning it is less common), set up first will take longer, and longer production cycles than conventional extrusion processes. While the hydrostatic extrusion process has inherent advantages, it has not dominated the industry because of this.

Advantages of Hydrostatic metal extrusion

• It uses considerably less energy or force, because there is no friction slowing the process down.
• The production can be done in relatively fast time, and you can achieve great reduction ratios (which means pile-down a lot of material in the same reduction stroke).
• The billet maintains cooler temps during the process.
• You get a nice, even material flow due to the balanced stress fields in the pressure distribution.
• You can extrude very large billets, and have large cross-section sizes.
• There is no captive billet material left over in the container after the end of the process.

Disadvantages of Hydrostatic metal extrusion

• You will have to prepare each billet by tapering one end to match the entry angle of the die, an additional step on top of preparing the billets.
• The process is strictly limited to cold extrusion, the process cannot be performed with hot billets.
• Managing and containing that high-pressure fluid is tricky and requires a lot of safety and design considerations.

4. Lateral Extrusion

In the lateral extrusion process, the container is positioned vertically, while the die is situated on the side, as illustrated. This method is particularly well-suited for materials with low melting points.

5. Impact Extrusion

Impact extrusion is a type of cold extrusion and is well-suited for softer metals like aluminum, lead, or copper.

If you can visualize the process, it is not that complicated, as it involves a punch which comes down quickly and has enough force to push the metallic slug backward instead of forward as you might imagine. Since in that respect, it is like indirect extrusion.

One of the main determining factors of the final thickness of an extruded piece, is the stop between the punch and the die cavity. After a part is formed, a stripper plate is used to slide the extruded part off the punch.

Commonly, manufacturers implement a mechanical press for impact extrusion, and ultimately, it is great because the entire forming process takes place uniformly and at a considerable speed with very little stroke distance.

Because of the large forces involved – and especially on the punch and die – it is very important that the tooling is appropriate. It may seem obvious, but tooling must resist not only impact and fatigue, but have enough strength to accommodate the process of metal extrusion.

Impact extrusion is commonly classified as one of three types depending on how the material flows:

• Forward,
• Reverse, and
• Combination.

With forward impact extrusion, the metal flows in the same direction of the punch force. Conversely, reverse impact extrusion provides force to push the metal in the opposite direction.

Advantages of impact metal extrusion

• Significant Material Savings: One of the key benefits of impact extrusion is the potential for material savings – often more than 70-80%, sometimes as much as 90%. Less material means less scrap (and a much greener process as well).
• Faster Machining: The process can eliminate significant amounts of machining time, often as much as 75%. This is a huge boost to shop-floor productivity and shop-floor efficiency.
• Fewer machining steps: Because impact extrusion can be close in shape to the part, less secondary machining steps may be needed, potentially eliminating them and streamlining production.
• Fewer assembly steps: Because it is able to form complicated parts in one piece, part assembly (multiple parts) is reduced. With the reduction of parts the complexity of a multi-part assembly is reduced, creating potentially improved reliability.
• Stronger, machinable parts: Since impact extrusion is cold working, the finished products will typically have superior mechanical properties and will be stronger and easier to machine, if machining is needed afterward.
• Reduction of total costs: The total lives and processes reductions leads to an overall reduction of up to 50% total cost/page – a significant advantage in any industry.
• Good for specialized tubes: Impact extrusion is very well suited for producing hollow, thin-walled tubes, often closed at one end or a requirement in many industries. Backward impact extrusion is often the best production method for this type of product.

Disadvantages of impact metal extrusion

• Shape limitations: Keep in mind one caveat, the best-case scenario for this process is a part that is symmetrical about the axis being used in the process. If the shape you need is more complex or asymmetrical, then you will have to look to some other process for parts that size.
• Additional processing sometimes required: Just because there are all these advantages with impact-extruded parts doesn’t mean there aren’t additional steps in order to really be ready for its intended use, most of the time in the form of forging, ironing, or more machining.

Application of Extrusion

• Extrusion is commonly used to manufacture tubes and hollow pipes. Therefore, in regard to shape, extrusion can easily be utilized to manufacture these shapes consistently.
• Aluminum extrusion has been very popular for structural applications in many industries because it has great versatility and strength.
• Extrusions in the automotive industry will typically produce frames, doors, windows and so forth for the reason that extrusion provides mass production and design flexibility.
• Extrusion is also prominent in making plastic products. Many products that we use today are in the shape of plastics that are extruded.

Advantages of Extrusion

• Extrusion has a high extrusion ratio (i.e., the ratio of the area of the original billet to the cross-sectional area of the finished extruded object), permitting large changes in shape and form.
• One of the advantages of extrusion is its ability to produce complex cross-sectional profiles that could be hard or expensive to produce other ways.
• Extrusion can be applied to both brittle and ductile materials, which gives it a wide range of potential applications in manufacture.
• If using cold extrusion, it may be possible to obtain increased mechanical properties in the final product, which in many engineering applications is a distinct advantage.

Disadvantages of Extrusion

• One disadvantage is that the capital or set up costs for the extruder system may be quite large, which can make it a considerable investment.
• Given that the process does require significant compressive forces the equipment must be strong enough to handle these forces, and also may require considerable energy requirements.

FAQs