Resin Transfer Moulding is the process of producing composite components within a mechanically clamped, rigid, normally two-part mould.

The advantages of a ‘closed mould’ process are considerable and include:

• Volatile emissions (styrene etc.) are massively reduced
• It can be a fast, clean and repeatable process
• The laminate thickness and Vf can be closely controlled
• The process is far less reliant on the manual skills of the operator
• Both ‘A’ and ‘B’ surfaces of the moulding can be accurately defined
• The process can be automated

Thermosetting resin is injected directly into the fibre-pack, and the mould is filled by positive hydraulic pressure from the injection machine. The mould is normally vented at the furthest points from the injection point allowing the air to escape.

Vacuum can also be drawn from the vents to improve laminate quality when necessary. Traditional RTM relies on the mould/clamping structure being stiff enough to withstand the pressure of the injected resin without opening or distorting.

Vacuum RTM (VRTM or RTM Light) is the process of producing composite components within a vacuum-clamped, two-part mould.

Compared with traditional RTM, VRTM tooling is relatively lightweight and thus considerably more economical. Atmospheric pressure is used as the mould closing force, as opposed to the mechanical clamping methods and heavily reinforced mould structures used in RTM.

Vacuum is applied to the mould cavity and the resin is introduced peripherally under low pressure. This injection strategy provides the most efficient method of filling the mould, with the minimum of reaction pressure against the clamping force.

As with Resin Infusion, any moulding process in which the laminate is fully enclosed during the filling and curing process will be higher quality and produce far fewer volatile emissions than an ‘open mould’ process such as hand or spray laminating.