Other GRP Moulding Techniques
Hand Lay-Up/Spary Lay Up
Usually involves impregnation of glass fibre mat with a liquid polyester mix. Hand Lay-up is a method of producing composite structures by hand applying composite materials in successive layers on a tool or mould that defines the part geometry. Spray lay up is essentially a variant on this process that involves spraying the matetial into the mould. A 'Gel Coat' can be incorporated provides a good surface on the mould side.
Compression Moulding is perhaps the next most common technique for moulding thermosets. Compound or blend is placed in mould and heated under pressure within the platens of a steam-heated press. When reaction is complete, product is cooled and ejected. Compression moulding is a batch, as opposed to continuous process. Press moulding Material is charged to a large press with flat platens, generally in the form of a 'pre-preg'. Heat and pressure are applied to react the mixture and convert it into the finished product. This technique is used for large mouldings and some sheet products. As with compression moulding, this is a rather slow process.
Resin Transfer Moulding
The molten resin is transferred by pressure from a meltpot into a mould, which is at a temperature above the meltpoint of the resin. This higher temperature is important for the filling of moulds with high flow length, high wall thickness ratios and to allow for shrinkage compensation.
Pultrusion is a continuous process for manufacturing composites in rods, tubes and structural shapes having constant cross sections. After the reinforcement is passed through the resin-impregnation bath, it is drawn through a shaping die to form the desired cross section ; curing takes place before the laminate can depart from the cross section.
Is a continuous, automated fabrication method that can be highly automated and repeatable with relatively low material costs. A long, cylindrical tool called a mandrel is suspended horizontally between end supports, while the "head" - the fibre application instrument - moves back and forth along the length of the mandrel as it rotates on the horizontal axis, placing fibre onto the tool in a predetermined configuration. In most applications, the filament winding apparatus passes the fibre material through a resin "bath," just before the material touches the mandrel. This is called "wet winding." Towpreg - continuous fiber pre-impregnated with resin - also can be wound, eliminating the need for an on-site resin bath. In a slightly different process, fiber is wound without resin ("dry winding"). The dry shape is then removed and used as a preform in another moulding process, such as resin transfer molding (RTM).
Reaction Injection Moulding
These procedures produce polymers made by reaction during moulding such as polyamides (thermoplastics) or polyurethanes, unsaturated polyesters, epoxies and phenolics (thermosets). RIM can be done using injection or transfer moulding.
Injection Moulding of thermosets Basically this is the same process as used for thermoplastics, modified to avoid hardening of the plastic in the machine. The materials used are widely known as Dough or Bulk Moulding Compounds (DMC or BMC). In sheet form they are known as Sheet Moulding Compound (SMC) Applications include electrical switch and fuse boxes, domestic electrical equipment, microwave containers and tableware. It is also used for business machine housings, gas and electricity meter housing and dish aerials.