What is resin transfer & resin injection moulding?

Reinforcement material is placed in half of the mould and the second half of the mould clamped over the top. The reinforcement material can be pre-pressed into the mould shape; this is known as "preform". For resin injection, resin materials are machine-mixed prior to being injected under pressure (~ 5 Bar) into the mould cavity. To ensure complete wetting of the reinforcement material either a pinch off device is used (see Cold Press Moulding) or a vacuum can be applied to draw the resin through the cavity. Once the mould cavity is filled, the resin inlet is closed and the laminate allowed to cure. In order to release the component from the mould high temperature release agents are necessary due to the highly exothermic nature of the cure.

There are various forms of resin injection moulding using different techniques to ensure that the resin is fully injected into the mould and the fibres are properly and evenly wetted. These methods include:

• SRIM - Structural Reaction Injection Moulding: uses high pressure rapid dispensing, typically of polyurethanes which have low viscosity.
• VARI - Vacuum Assisted Resin Injection: have moulds that are usually vented. A partial vacuum holds the mould in place and provides the moulding force. The vacuum also aids the reduction of voids in laminates of large area.
• VI - Vacuum Infusion is an extension of VARI where a complete vacuum is applied. This technique can be used either for high quality (low voidage) aerospace parts, or high volume production parts with flexible tooling. Minor variations on this technique include VARTM (Vacuum assisted RTM), RIFT, SCRIMP (Seeman composite resin infusion moulding process), BLIP (bladder infusion process)
• RFI - Resin Film Infusion: This method uses a single mould and vacuum bag. The resin is introduced as pellets of a film along with the reinforcement. The mould in the vacuum bag is then heated under pressure to lower the viscosity of the resin which flows through the thickness as opposed to along the part as with the other methods.

The laminates manufactured using RTM methods have high fibre content with relatively few voids. Further, both sides of the component have a moulded surface with good finish. As the resin is enclosed in a mould it is possible to control the extraction of any fumes, thus improving the health and safety aspects. However, using RTM, it is not possible to determine whether the reinforcement material is properly covered by the resin until after curing so there is a possibility of expensive scrap components. Also the moulds need to be able to withstand high moulding pressures and temperatures whilst being the net shape of the component. Therefore the moulds can be relatively expensive, and therefore unsuitable for low volume or prototype manufacture. Because of these issues, RTM is generally only used for small components. For larger parts the various RIM methods discussed above are favoured.

Some of the key points of this method include: