As an improvement on the hand lay-up and spray-up oricesses, it is possible to insert laid-up moulds into special bags, which are then evacuated and/or placed in an autoclave for cure under controlled heat and pressure.

This vacuum consolidation method produces high-quality mouldings, with complete exclusion of air bubbles and improvement to the inner surface of the moulding, which is not in contact with the mould. The controlled curing conditions also improve quality and consistency and allow superior resin systems to be used, while opening the way to a more rapid cure with faster turn round of  moulds.

Vacuum bag moulding is a modification of hand lay-up, in which the lay-up (necessarily smaller) is completed and placed inside a bag made of flexible film and all edges are sealed. The bag is then evacuated, so that the pressure eliminates voids in the laminate, forcing excess air and resin from the mould. By increasing external pressure, a higher glass concentration can be obtained, as well as better adhesion between the layers/plies of laminate.

Some items for the process can be disposable.

Vacuum bag film

This is a polypropylene film, modified to resist elevated temperatures. The elastic nature of the film offers high formability and resistance to puncture, also permitting the film to be stretched over complex moulds without need for a large number of tucks and folds, so improving the efficiency of the vacuum. The film is suitable for use with prepreg and wet-lay-up laminating systems.

Breather/bleed fabric

This is a 5 mm or 10 mm thick uncompressed felt of temperature-resistant synthetic fibres, treated with mould release. The material has good drapeability, allowing use with large and complicated mould patterns. It can also be used as a bleed fabric to absorb excess resin from wet lay-ups.

Perforated release films

These are made from modified polypropylene and can be rigid (recommended for use on flat or uncomplicated mould surfaces) or elastic (thin and stretchable, for large complex moulds with the need for tucks and folds). The films have good mechanical properties and performance at elevated temperature and are naturally self-releasing, treated on one side to assist adhesion to the breather fabric or a protective film.

Tacky tape

This is a butyl-based vacuum bag sealant giving high elasticity and tenacity. It has exceptional sealant properties, eliminating the risk of imperfect seals often found in the initial phases of vacuum bag application, so improving the vacuum efficiency and reducing labour. It is suitable for polyester, vinyl ester and epoxy laminating systems.