Resin Transfer Moulding

The difference between Resin Transfer Moulding (RTM) and Vacuum Resin Transfer Moulding (VRTM)

Resin Transfer Moulding:

Resin Transfer Moulding (RTM) 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 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.

Vacuum Resin Transfer Moulding (VRTM) or 'Light RTM'

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

Compared with the traditional RTM process, VRTM tooling is relatively lightweight and thus considerably lower cost. The process relies on atmospheric pressure as a mould closing force as opposed to the mechanical clamping methods and heavily reinforced mould structures used in RTM.

Unlike infusion processes that use a vacuum bag, the VRTM process uses a semi rigid upper mould and so can still provide good definition of the moulded ‘B’ surface and extra detail such as encapsulated inserts, cores or surface textures are readily mouldable.

The mould is built with double seals; an outer vacuum seal and an inner resin seal. The flange area between the seals is evacuated to provide the main clamping force. This double seal also provides excellent security against vacuum leaks through the seals.

Vacuum is applied to the mould cavity and the resin is introduced under low pressure (<1 bar) into a peripheral feed channel running around the outside edge of the component. This peripheral injection strategy provides the most efficient route to fill the mould, with the minimum of reaction pressure against the clamping force.

The resin fills the mould cavity and is vented near the centre of the component. Any excess resin is contained within a catch-pot positioned at this point. The vacuum is drawn from the mould cavity via the catch-pot. The catch-pot prevents excess resin from being drawn into the vacuum pump.

The primary mould, usually representing the ‘A’ surface of the component, is built as a relatively stiff structure with a simple framework to prevent distortion.

The upper mould is often a much lighter structure and consists of a 4-6mm semi-translucent laminate over the mould cavity and a reinforced and stiffened flange area. A simple frame can be incorporated to aid handling and to prevent distortion.

Due to the use of vacuum to provide a uniform clamping force, the VRTM process can be successfully used to mould large structures using relatively simple and low-mass tooling.