Resin transfer molding.
Transfer molding (TM) (or resin transfer molding, RTM) differs from compression molding in that in TM the resin is inserted into the mold (or tool) which contains the layers of fibres or a preform, whereas in compression molding prepregs or molding compounds are in the mold which is then heated and pressure is applied. No further pressure is applied in TM.
In RTM the resin is injected or drawn into a mold, which contains the fibres, from a homogeniser under low pressure. The mold can be made from composites for low production cycles or with aluminium or steel for larger production. The differences between the two types being that metal has better heat transfer, hence quicker cycle times; metal lasts longer and deforms less, but at a higher cost. The main problem with this production route is that air can be trapped in mold and hence a method must be incorporated for allowing this air to escape. A number of solutions to the problem exist including extending one level of reinforcement beyond the cavity (with a 25% resin loss), appropriate vents and creating a vacuum in the mold (which also improves quality). Larger structures, better properties (less movement of fibres), increased flexibility of design and lower cost are some of the advantage this process has over compression molding due mainly to the low pressure injection. Other benefits include rapid manufacture, not labour intensive, ability to vary reinforcements easily or include cores such as foam and produce low and high quality products.