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

The advantages of a ‘closed mould’ process are considerable but include the following:

• Volatile emissions (styrene etc) are massively reduced
• It can be a fast, clean and repeatable process
• The laminate thickness can be closely controlled

• The process is far less reliant on the manual skills of the operator
• The ‘B’ surface of the moulding can be accurately defined
• The process can be automated

Dry reinforcement (glass, carbon, aramid etc) is placed between a two-part mould and the mould is clamped shut using mechanical force (hydraulic press, nuts/bolts, heavy duty toggle clamps etc).
The mould flanges compress a peripheral seal which prevents resin leaks from the mould and may also be vacuum-tight.

Thermosetting resin is injected, often centrally, 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. This aspect of the process can become problematic if large scale components are moulded, with tooling sometimes becoming uneconomic simply due to its mass and handling/clamping requirements.