Direct metal extrusion design guide – DFM

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

What is Metal Extrusion

A cylindrical billet inside a sealed cavity is forced to flow through a die with the desired cross-section during the metal extrusion manufacturing process. Extrudates are these fixed cross-sectional profiles extruded components that are pushed out using a mechanical or hydraulic press.

How to design parts for direct metal extrusion?

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.

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.

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:

• Length, Straightness, Twist, Flatness, Surface roughness – Wire, Rod, Bar and Profiles
• Round tube – Diameter, Width and Depth of Square, rectangular, Hexagonal and octagonal tube, wall thickness

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.

• How to inspect metal and space dimensions on solid profiles
• How to inspect metal and space dimensions on hollow profiles
• How to inspect the straightness of metal extrusion
• How to inspect the twist of metal extrusion
• How to inspect the flatness of metal extrusion

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.

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 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.

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

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.

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.

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.

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.

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.

The following corner radii are recommended for corner design:

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.

Incorporate ribs

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

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.

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