Volumes of production
Different manufacturers have different minimum lengths, but extrusion can be a cost-effective process for both batch and large-scale runs. It is definitely not for one-offs—unless your one-off is 150 feet long.
Unit price vs. capital investment
Conventional extrusion requires a low investment in tooling when compared with injection molding (see p.196), for example.
Speed
Up to 65 feet per hour.
Surface
Excellent.
Types/complexity of shape
No problems in making complex shapes with varying wall thicknesses, just as long as the shape is the same along the whole length. Flat sheet can also be produced.
Scale
Depends on the type of extrusion. Most manufacturers have an average maximum size of 10 inches in cross-section. The length is limited by the size of the factory.
Tolerances
Difficult to maintain high tolerances due to a wearing of the die.
Relevant materials
Extrusion is a versatile process and can be used for materials including wood-based plastic composites, aluminum (shown in this example), magnesium, copper, and a wide variety of plastics and ceramics.
Typical products
Everything from architectural and furniture components, lighting, and accessories, to pasta, and sticks of English rock candy with place names written through them.
Similar methods
Pultrusion (p.99), calendering (p.90), coextrusion (multiple layers of extruded material in the same component), laminating (two, or more, materials bonded together), roll forming (p.104), and impact extrusion (p.146).
Sustainability issues
There are multiple forms of extrusion, but both hot and cold extrusion require either high temperatures or pressure, which can be energy intensive. Extrusions can crack internally during forming when too much heat or pressure is applied, so parts must be monitored closely to prevent waste of materials. The nature of the long lengths of extrusions also means parts need post-cutting in order to convert them into usable shapes.
Further information
www.heatherwick.com
www.aec.org
Pultrusion
Product |
sample of pultruded composite profile |
Materials |
glass fiber and polyester resin composite |
Manufacturer |
Exel Composites |
Country |
UK |
These profiles illustrate two of the key properties of pultrusion: first, its ability to produce shapes in plastic having similar properties to metal profiles; secondly, its capacity to have molded-in colors.
Pultrusion is much less common as a plastic-processing method than its more familiar relative, extrusion (see p.96). The processes are similar in that they allow continuous lengths of a set and an unchanging profile to be formed, but one of the main differences between them is that extrusion can be used for aluminums, wood-based composites, and thermoplastics, while pultrusion is used in the forming of composites that use long strands of fiber as reinforcement.
As the name suggests, the process is based on pulling the blended materials of the composite through a heated die. This differs from extrusion, which is based on pushing the material. The continuous lengths of reinforcing fibers, which can be made from glass or carbon, are saturated with a liquid resin mixture as they are pulled through the die, which, besides shaping the component, also acts to cure the resin as it is heated. Sometimes, pre-impregnated (“prepreg”) fibers are used, removing the need for a resin bath.
Manufacturers of plastics have, in recent years, experimented with many applications that traditionally used metals, and pultrusion is a typical example of the benefits such experimentation can bring. Pultruded plastics display an increased range of physical properties that can benefit both engineering and design applications, because they offer the toughness of metals with the advantages of low weight and corrosion-resistance. Pultrusions are incredibly dense, hard, and rigid sections—they even “clank” like pieces of metal when you knock them!
