Superforming Aluminium including cavity, bubble, back-pressure and diaphragm forming

The process of heating a sheet of plastic, draping it over a mould and sucking the air out has been in use for some time.

However, as the speed of the development of new materials increases, more technologies overlap when it comes to both materials and processes. Superforming involves such an overlap, since it brings traditional vacuum forming with plastic to aluminium alloys. The process is achieved through four main methods: cavity forming, bubble forming, back-pressure forming and diaphragm forming, each suited to specific applications.

The common element in all these methods is the heating of an aluminium sheet to 450–500°C in a pressurised forming oven, and then forcing it over, or into, a single surface tool to create a complex three-dimensional shape.

This bike is a good example of the transfer of industrial manufacturing processes into consumer products by experimental projects. The text embossed onto the frame also illustrates the detail that is achievable.

In the cavity method, air pressure forces the sheet up into the tool in a process that can be described as 'reverse vacuum forming'. According to the manufacturers, this process is ideal for forming large, complex parts such as automotive body panels.

In bubble forming, the air pressure forces the material into a bubble. A mould is then pushed up into the bubble and air pressure is applied from the top, forcing the material to conform to the shape of the mould. Bubble forming is suitable for deep and relatively complex mouldings that are difficult to achieve with the other superforming processes.

Back-pressure forming uses pressure from both the top and bottom surfaces of the mould to maintain the integrity of the sheet and allow for the forming of difficult alloys.

Diaphragm forming is a process that allows for 'non-superelastic' alloys to be formed. The non-superelastic material is 'hugged' over the mould using a combination of a sheet of heated 'superelastic' aluminium and air pressure.

At a glace

Volumes of production - At present, production runs of about 1,000 parts are considered large, but mass-production is a possibility, with some car manufacturers starting to use the process on a larger scale.

Unit price vs capital investment - High capital investment, mainly in tooling and material.

Speed - Depends on the material – some alloys can be formed in three to four minutes, while the structural alloys used in aircraft, for example, may need up to an hour to form.

Surface - Excellent surface quality.

Types/complexity of shape - This depends on the specific method you use. Bubble forming allows the greatest degree of complexity in shape, but with all methods the basic principle is about creating three-dimensional shapes from a flat sheet. Draft angles need to be considered in order for parts to be removed from the mould. Undercuts are not recommended.

Scale - Each method is suited to different scales and thicknesses of material, for example, using back-pressure forming, parts can be made up to approximately 4.5 metres square. Cavity forming can only process smaller sheet sizes, although these can be up to 10mm thick.

Tolerances - Typically ±1mm for larger parts.

Relevant materials - This process is specifically designed for use with what are known as 'super-elastic' types of aluminium. However, the diaphragm-forming method enables the processing of non-superelastic materials.

Typical products - A large market for this process is in the aerospace and automotive industries. Designers such as Ron Arad and Marc Newson have applied it to diverse furniture and bicycles. On the London Underground, architect Norman Foster used superforming to produce tunnel-cladding panels for Southwark station.

Similar methods - For plastic - vacuum forming, for glass - slumping, and for metal - inflated stainless steel.

Sustainability issues - The process requires several stages of production each of which uses significant amounts of energy through high heat and pressures. A significant amount of excess material is produced after trimming but can be recycled back into the process or used elsewhere. Additionally, when the formed product reaches the end of its lifespan it can be recycled into new products to reduce the use of raw materials. The nature of the shapes that are thermoformed means components can be nested during transportation, saving on bulk.

Courtesy of Making It: Manufacturing Techniques For Product Design (2nd Edition) By Chris Lefteri, Laurence King Publishing

Chris Lefteri

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