Metal Extrusion

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In Metal extrusion, a cylindrical billet inside a closed cavity is forced to flow through a die with a fixed cross-sectional profile.

What is Metal Extrusion?

Metal Extrusion is a metal-forming manufacturing process in which a cylindrical billet inside a closed cavity is forced to flow through a die of the desired cross-section. These fixed cross-sectional profile extruded parts are called “Extrudates” and are pushed out using either a mechanical or hydraulic press. The process, patented by Joseph Bramah, was first used to extrude lead pipes by Thomas Burr.

Aluminium-metal-extrusion
Figure 1. Aluminium-metal-extrusion

The most commonly extruded materials are Aluminium, Copper, Steel, Magnesium, and Lead. Plastics and ceramics are also extruded extensively but not discussed in this article.

Characteristics of metal extrusion

  • Able to create complex cross-sections and will be uniform over the entire length of the extrudates
  • Factors that affect the quality of extrusion are die design, extrusion ratio, billet temperature, lubrication, and extrusion speed. Check out the detailed design guide for metal extrusion, “How to design parts for direct metal extrusion”, to understand the 5 key design variables of metal extrusion and design for manufacture (DFM) extrusion design tips.
  • Like any other metal-forming process, it can be either hot or cold. However, the process is generally carried out at elevated temperatures to reduce the extrusion force and improve the material’s ductility.
  • Low cost due to reduced raw material wastage and high production rate
  • Brittle material can be deformed without a tear as it only exerts compressive and shear forces in the stock part
  • Parts that are formed have an excellent surface finish which minimizes post-processing machining
  • Metal extrusion tends to produce a favourable elongated grain structure in the direction of the material.
  • The minimum wall thickness of ~1mm (aluminium) to ~3mm (steel) could be achieved.

Types of the metal 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

Direct Extrusion

Direct Extrusion, sometimes called Forward Extrusion, is the most common type of extrusion. As shown in the figure below, the process begins by loading a heated billet (only for hot extrusion, discussed later) into a press cavity container where a dummy block is placed behind it. Then the mechanical or hydraulic ram presses on the material to push it out through the die. Then, while still hot, the part is stretched to straighten. This process is beautifully animated by Core Materials.

Direct-extrusion-process-schematic
Figure 2 – Direct extrusion (source: Kalpakjian & Schmid)

Under direct extrusion, the high friction caused by steels at higher temperatures is reduced using molten glass as a lubricant while oils with graphite powder are used for lubrication for low temperatures. The dummy block protects the tip of the pressing stem (punch or ram) in hot extrusion. When the punch reaches the end of its stroke, a small portion of the billet called the “butt end” cannot be pushed through the die opening.

Direct metal extrusion advantages

  • No billet modification required
  • It can be used for both hot and cold extrusion.
  • Simple tooling compared to other extrusion processes

Direct metal extrusion disadvantages

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

Indirect Extrusion

In Indirect Extrusion, the die is located at the end of the hydraulic ram and moves towards the billet inside the cavity to push the material through the die. This is illustrated in figure 2 below.

In-direct extrusion process schematic
Figure 3 – In-direct extrusion (source: Kalpakjian & Schmid)

This process consumes less power due to the static billet container causing less friction on the billet. However, supporting the extruded part is difficult when the extrudate exits the die.

In-direct metal extrusion advantages

  • Less friction and less power used
  • It can be used for both hot and cold extrusion
  • Simple tooling compared to other extrusion processes

In-direct metal extrusion disadvantages

  • Difficult to support the extruded part
  • The hollow ram limits the load applied

Hydrostatic extrusion

In hydrostatic extrusion, the chamber/ cavity is smaller than the billet and filled with hydraulic fluid, which transfers the force from the ram to the billet, as shown in figure 3. Although tri-axial forces are applied by the fluid, the pressure improves billet formability on the billet. Sealing the fluid must be considered early to avoid any leaking and reduce pressure issues.

Hydrostatic-extrusion-process-schematic
Figure 4 Hydrostatic extrusion process (Kalpakjian & Schmid)

Although the hydraulic fluid eliminates the friction between the wall and the billet by isolating them, due to the specialised equipment requirement, the high set-up time and low production rate limit its usage in the industry compared to other extrusion processes.

Hydrostatic metal extrusion advantages

  • Low power/force requirement due to no friction
  • Fast production rates & high reduction ratios
  • Lower billet temperature
  • Even the flow of material due to the balanced force distribution
  • Large billets and large cross-sections can be extruded
  • No billet residue is left in the container

Hydrostatic metal extrusion disadvantages

  • Billets need to be prepared by tapering one end to match the die entry angle.
  • Only cold extrusion is possible
  • Difficult to contain the high-pressure fluid

Lateral Extrusion

In Lateral Extrusion, the container is in a vertical position, as shown in the image, and the die is on the side. This process is suitable for low melting point material.

lateral extrusion process schematic
Figure 5 – lateral extrusion process schematic (source : Kalpakjian & Schmid)

Impact Extrusion

Impact extrusion is part of the cold extrusion category, very similar to In-direct extrusion and limited to softer metals such as Lead, Aluminium and copper. As the schematic illustrates, the punch is pushed down at high speed and has extreme force on the slug to extrude backwards. The thickness of the Extrude is a function of the clearance between the punch and the die cavity. The Extrudates are slid off the punch using a stripper plate.

Impact extrusion process
Figure 6 Impact extrusion process (source: Kalpakjian & Schmid)

For impact extrusions, a mechanical press is often used, and the part is formed at high speed and over a relatively short stroke

Since the forces acting on the punch and die extremely high, tooling must have sufficient impact resistance, fatigue resistance and strength for extruding metal by the impact. Impact extrusion can be divided into the following three types by material flow.

  • Forward
  • Reverse
  • Combination
Types of Impact extrusion process
Figure 7 Types of Impact extrusion process (source : Kalpakjian & Schmid)

In forward impact extrusion, the metal flows in the same direction that the force is delivered, while it flows in the opposite direction in reverse impact extrusion. As the image shows above, the metal flows in both directions in combination.

Impact metal extrusion advantages

  • Raw material savings of up to 90%
  • Reduced machining times by up to 75%
  • Elimination of secondary machining operations
  • Reduction in multi-part assemblies
  • Improved mechanical properties for material strength and machining due to the cold working of the material
  • Significantly reduced total part costs by up to 50%
  • Hollow thin-walled tubes, closed on one end, are often produced in the manufacturing industry by backward impact extrusion.

Impact metal extrusion disadvantages

  • Produced as long as the part is symmetrical over the axis by which it is formed
  • Many of the parts formed by impacting in industry, will require further manufacturing processes, such as metal forging, ironing or machining, before completion.

Extrusion Defects

Depending on the material condition and process variables, extrudates can develop many defects that could affect the quality of the end product.  These defects can be grouped under the following three defects.

  • Surface cracking
  • Piping
  • Internal cracking

Direct metal extrusion design guide – DFM

Shape, alloy, tolerance, surface polish, and CCD are essential design elements to consider while designing metal extrusion parts.

A thoughtfully designed metal extrusion part will successfully satisfy the final product design’s form, fit and function. Following is a detailed design guide for metal extrusion, which explores 5 key design variables of metal extrusion and provides design for manufacture (DFM) extrusion design tips.

Metal extrusion process
Metal extrusion process ( Source: google images)

What is direct metal extrusion?

Direct metal extrusion is one of 5 main types of metal extrusion processes in which a metal billet is forced through a fixed die to create a specified shape.

This metal-forming process can be performed both hot and cold. But generally, the billets are heated up before pushing through the die to reduce friction and the force required. The dimensions of the extruded workpiece depend on the extruded profile and the amount of material that the workpiece withholds. The parts obtained after extrusion is called extrudates.

Direct extrusion is the most common extrusion process used in profile extrusion, mainly due to its simplicity in the manufacturing design and flexibility in profile manufacturing.

The common metals used for extrusion are steel, copper and aluminium. Aluminium alloy extrusion has many advantages, such as being lightweight, having a high strength-to-weight ratio, corrosion-resistant, heat conductive, fully recyclable, having very tight tolerances, easy to fabricate and being cost-effective. Hence, it is one of the most commonly extruded materials.

For the above reason, this article will be biased towards aluminium alloy extrusions.

Metal extrusion design factors

Making the right design choices will yield significant benefits in terms of extrudability, manufacturing cost and design. During the early embodiment design phase of the part, the following five design variables of metal extrusion should be considered before moving on to the detailed design.

Alloys and their tempers

Pure aluminium, like many other metals, has its limitations. Hence they are alloyed with elements such as copper, and magnesium, to produce alloys with different properties. Different aluminium alloys have different properties and characteristics, hence offering different benefits. Depending on the final product design requirements, choices must be made regarding its mechanical performance, post-processing, quality, and cost to achieve the desired functional part.

  • Mechanical performance
    • Product properties – strength, ductility
    • Workability – extrudability, product yield
  • Heat treatable or non-heat-treatable
    • Heat treatable – 2xxx, 6xxx, 7xxx (HT)
    • Heat treatable – 1xxx, 3xxx, 5xxx (NHT)
  • Corrosion characteristics
  • Alloy composition
  • Cost

Further, depending on the composition of the chosen alloy, it can be strengthened and hardened using various tempering techniques.

Golden tip – 6000 series aluminium alloys are the most common and selected for almost 75% of the aluminium extrusion. 6063 and 6061 are the most frequently used.

This publication from Aluminium Extruders Council has more information on aluminium alloys, their properties and performance parameters on commonly used aluminium extrusion alloys.

Shapes (Extrusion profile)

The aluminium extrusion shape configurations can be grouped into three categories. Hollow, semi-hollow and solid as shown in figure 2.

Metal extrusion shape configuration types (Source:bwcgroup.co.uk)

Solid profiles are cheaper to make as they are the least complex. Semi-hollow profiles are defined by the tongue ratio.

The following should be considered when considering the shape configuration

  • Uniform wall thickness
  • Symmetry
  • Rounded edges
  • Shape constraints

Golden tip – The least complex and easier to extrude a profile, is lower the cost, than a profile that is more complex and has a longer process.

Tolerances

Most of the time, the extrusion profiles will be part of an assembly and need close interfacing with surrounding components. Hence dimensional tolerancing becomes critical. The product designer should be aware of the ISO standards’ dimensional tolerancing to which extrusion parts can be cost-effectively made. These should cover straightness, flatness, twist, thickness, angles, contours and corner radii.

The following Aluminium association’s standard tolerance tables provide key Aluminium extrusion dimensional tolerances:

The following videos from the Aluminium association on how aluminium extrusion tolerances can be accurately measured give you an understanding of what is measured and how to use the above tables.

Surface Finish

When aluminium is extruded, a thin oxide layer forms on its surface to protect it from the atmosphere. Different surface finishes and treatments are performed to increase or enhance the protection on the product. These include:

  • Powder coating (use of solvents)
  • Liquid coating (use of paints)
  • Anodizing (electrochemical process)
  • Chemical finish (e.g. etching, etc.)
  • Mechanical finish (e.g. polishing, sanding, etc.)
  • Pre-treatment (e.g. cleaning, etc.)

Circumscribing Circle Diameter (CCD)

Circumscribing circle diameter is a common measurement of a metal extrusion profile and is the smallest circle that encloses the entire extrusion cross-section. Although it is possible to produce extrusions up to 18” CCD, the cost increases with increasing CCD. Hence, most common extrusions are less than 8” in diameter.

Circumscribing circle diameter
Circumscribing circle diameter (Source: Aluminium extruders council (AEC))

Golden tip – Keep the design so that the profile CCDs are under 8” and the Wt/Ft is 3 lbs or less which would improve the design significantly.

DFM guidelines for metal extrusions

Design for manufacture is about integrating the design with the practicality of manufacturing the part. The goal is to design an extrusion profile that is easily and economically manufactured, which satisfies the part’s form, fit and function.

design for manufacture guidelines
Design for manufacture guidelines (Source:bwcgroup.co.uk)

Design with a suitable wall thickness

It is common for the metal to distort during the extrusion process due to thin or unsymmetrical metal thickness, similar to metal casting processes such as sand and investment casting. Specifying a suitable metal thickness that avoids distortion, depending on the structural needs to satisfy the requirements and make it cost-effective is essential.

Appropriate Metal extrusion Thickness
Appropriate metal extrusion thickness (Source:bwcgroup.co.uk)

The following factors dictate the minimal wall thickness (link) and need careful consideration during the design stages of the new product development.

  • Heat generation – Larger the extrusion thickness, the more it generates heat during the process, which would distort the profile.
  • Tolerances
  • Alloy choice
  • Cross-sectional complexity

The following reference chart shows the minimum wall thicknesses practical from an extrusion standpoint based on the profile’s circumscribing circle diameter.

Table 1 Minimum wall thickness vs CCD

Circumscribing Circle (inches)Class 1
Solids & Semi hollows
(inches)
Class 2
Hollows
(inches)
0.5 ≤ CCD < 20.040.055
2 ≤ CCD < 30.0450.062
3 ≤ CCD < 40.050.078
4 ≤ CCD < 50.0620.094
5 ≤ CCD < 60.0780.11
6 ≤ CCD < 70.0940.125
7 ≤ CCD < 80.110.14
8 ≤ CCD < 90.1250.156
9 ≤ CCD < 100.140.188
10 ≤ CCD < 110.1560.204
11 ≤ CCD < 120.1720.22
12 ≤ CCD < 130.1880.236

Design with uniform metal thickness

One of the advantages of extrusion is that it allows putting extra material where needed, such as mounting hole locations & high-stress areas, and elsewhere, you could have thinner sections. But, the adjacent wall thickness change must be less than 2:1.

Design with a uniform wall thickness (Source: geminigroup.net)

Large non-uniform metal thickness variations would introduce dimensional control problems. So, it is crucial to maintain a uniform metal thickness throughout the process if possible. In case of a difference in thickness, ensure there are streamlined transitions in areas with a sharp change in thickness.

Design with uniform metal thickness
Design with uniform metal thickness

Use metal dimensions when tolerancing

Dimensions across solid profile metal are easier to measure compared to open gaps, hence easier to make it to a tight tolerance. These are called “metal dimensions”, and the designer should specify and rely on these as much as possible for tighter interfacing sections of the aluminium extrusion.

Golden tip – A metal dimension can be extruded to close tolerance while open space dimension is more difficult to hold to close tolerances.

Extrusion metal tolerance
Extrusion metal tolerance (Source:bwcgroup.co.uk)

The DIN ISO 2768-1 tolerance standards can be used for aluminium extrusion as most extrusion manufacturers work to these standards.

Consider surface finishes

Sudden changes in wall thickness and inside profile junctions such as webs, flanges and ribs create visual stripes on the opposite side of the wall. These can be smoothed out by breaking the surface with grooves and serrations.

Extrusion surface finish (Source:bwcgroup.co.uk)

Adding appropriate corner radii also minimises the sink on the outside.

Remove sharp edges

Edges of the profile openings should be rounded to avoid the die tongue snapping off. This will also aid material flow and make it easier to extrude.

Remove sharp edges
Remove sharp edges (Source:bwcgroup.co.uk)

The following corner radii are recommended for corner design:

Corner radii
Corner radii (Source: Google images)
Wall thickness
mm (inch)
Recommended corner radius
mm (inch)
OverUp toR1R2
02 ( 0.08″)2 (0.08″)1 (0.04″)
2 ( 0.08″)4 (0.16″)2.5 (0.1″)1.6 (0.06″)
4 (0.16″)6 (0.24″)4 (0.16″)2 (0.08″)
6 (0.24″)10 (0.39″)6 (0.24″)3 (0.12″)
10 (0.39″)20 (0.79″)10 (0.39″)5 (0.20″)
20 (0.79″)35 (1.38″)16 (0.63″)10 (0.39″)
35 (1.38″)50 (2″)20 (0.79″)16 (0.63″)

Source: Purso

Introduce smooth transitions

As discussed before, areas that involve changes in thickness from thick to thin should be streamlined over a curved radius with smooth transitions to avoid problems related to dimensional control.

Incorporate webs

The hollow portions in a design can be extruded easily, using a web that makes extrusion of thin wall sections without any problem.

Metal extrusion webs
Metal extrusion webs (Source:bwcgroup.co.uk)

Incorporate ribs

Ribs are used in designs because they help reduce the twisting in broad, thin sections and make them flat or straight.

Metal extrusion ribs
Metal extrusion ribs (Source:bwcgroup.co.uk)

Incorporate indexing mark

Incorporating indexing marks can help distinguish between inside and outside surfaces or differentiate between similar profiles. They can also be used as scribe lines for drilling and tapping, assembly alignment lines, etc., during the final assembly.

Metal extrusion index marks
Metal extrusion index marks (Source:bwcgroup.co.uk)

Consider post-processing

When the final design specification of the extrusion profile is known, ensure any post-processing details are considered or shared with the profile designer. For example, very similar to sand casting design, where any close interface machining surfaces would need additional wall thickness.

  • Working allowances for machined surfaces need to be added and incorporated into the extrusion.
  • Dimensioning of screw pockets and functional areas taking profile tolerances into account
  • Interfaces between profile and machining standards
  • Surface treatments: before/after (for example, certain features such as threads might need masking before painting or anodising)
  • The dimensioning of bent components cannot fully comply with the most common machining standards

References and recommended reading

  • Kalpakjian, S., & Schmid, S. R. (2009). Manufacturing Engineering & Technology (Sixth edition ed.). London: Pearson.
  • Extrusion. (n.d.). efunda. Retrieved October 2017, from http://www.efunda.com/processes/metal_processing/extrusion.cfm.
  • Extrusion. (n.d.). Engineers Edge. Retrieved October 2017, from https://www.engineersedge.com/extrusion.htm.
  • Extrusion. (n.d.). Engineers Handbook. Retrieved October 2017, from http://www.engineershandbook.com/MfgMethods/extrusion.htm.
  • Extrusion. (n.d.). The Library of Manufacturing. Retrieved October 2017, from http://thelibraryofmanufacturing.com/extrusion.html.
  • Extrusion. (n.d.). Wikipedia. Retrieved October 2017, from https://en.wikipedia.org/wiki/Extrusion.