The manufacturing technique under investigation is Double-Diaphragm Forming (DDF). Two circular rubber diaphragms, clamped around their edges by a blank holder, sandwich the fabric blank and apply a through-thickness pressure and in-plane tension to it as it is formed. This restraint helps suppress wrinkling.

The process has two stages. In the first, vacuum draw-in, a vacuum of 1 bar is applied between the diaphragms, drawing them inwards and clamping the fabric between them. This stretches the diaphragms, and the amount of pre-strain depends on the geometry of the blank holder. In the second, deep draw forming, a cylindrical punch pushes the fabric and diaphragm sandwich into a matching circular female die, forming a double-curved part.

The diaphragms used so far are 1 mm thick, general-purpose silicone rubber of 40 Shore A hardness. The fabric is a plain weave woven glass fabric (ECK 10), laid up in multi-axial stacks of 0/90° and ±45° plies. Where lubrication is used, a mixture of epoxy resin and hardener is applied at the fabric–fabric and diaphragm–fabric interfaces.

Ongoing work is investigating incremental forming as a way to mitigate wrinkles, replacing the circular punch with a 5-cylinder multi-step tool.