Carbon Fiber - 0100 Manufacturing Methods - Closed Molding

Closed 'Bladder' Molding

In closed molding, raw materials (fibers and resin) cure inside a two-sided mold or within a vacuum bag, isolated from air. This process is typically automated and requires specialized equipment. Common closed molding processes include:

  • Vacuum bag molding (hand lay-up)
  • Resin transfer (injection) molding (hand lay-up), e.g., the Quickstep process
  • Compression molding

Vacuum Bag Molding (Autoclave)

This process involves laying out unidirectional fibers or woven cloth to form flat sheets, which are then impregnated with a resin matrix and partially cured to a "tacky" B-stage. Lay-ups of multiple layers, oriented to suit load requirements, can be created by hand or automated methods.

Using prepreg materials ensures excellent fiber wet-out, even resin distribution, and high fiber volumes (up to 80%). The lay-up is placed on an open mold (commonly steel or composite) and vacuum bagged to consolidate the layers and remove volatiles during curing. The entire assembly is then placed in an autoclave, where heat and pressure cure the resin. This method produces parts with minimal porosity and excellent mechanical properties.

An additional, but very similar concept, method is called vacuum infusion and involves drawing a resin into the matrix using the vacuum.

Resin Transfer Molding (RTM)

RTM, also known as liquid molding or resin infusion, involves laying dry reinforcement material inside the mold. This allows for any combination of material and orientation, including 3-D reinforcements. The mold cavity determines the part's thickness.

Fast cycle times can be achieved with temperature-controlled tooling, and the process ranges from simple to highly automated. RTM accommodates a variety of tooling, from low-cost composite molds to temperature-controlled metal molds. Vacuum assistance can enhance resin flow, achieving a high resin-to-fiber ratio. This method is ideal for aerospace-grade composites, such as CFRP spars, often produced via the Quickstep process.

Compression Molding

In compression molding, a molding polymer is compressed into a preheated mold, conforming to the mold cavity and curing under applied heat and pressure.

The process can be automated, offering flexibility in part design, including features like inserts, ribs, bosses, and attachments, while delivering good surface finishes and reducing manufacturing costs.

With cycle times ranging from 1-6 minutes (longer than injection molding), compression molding is suitable for producing large, flat, or moderately curved parts.