What is hand/wet lay up?

The fibre is positioned in or on the mould by hand. If the mould is a complex shape small pieces of mat are cut to fit and then more layers applied to achieve the required thickness. The liquid resin is poured over the fibre and rolled to ensure complete wetting of the fibre and removal of air bubbles. In general the resin cures at room temperature with the use of an accelerator and a catalyst. If a hot cure is used then there is no need to use an accelerator. Post curing of cold cured laminates is recommended (2 hours @ 80°C or 16 hours @ 40°C depending on resin manufacturers instructions).

Prior to application of the fibre and resin, the mould is prepared with either polyvinyl alcohol or non-silicon wax to aid release of the component. Release of the component is achieved by either tapping wedges between mould and component or by the use of compressed air to gently force the pieces apart.

Chopped Strand Mat is the most commonly used fibre although woven roving is used when a stronger and stiffer laminate is required. A gel coat is applied to the mould surface to produce a resin rich smooth surface for appearance and protection purposes. For improved surface finish and corrosion resistance a surface veil is used which is applied with an embedded fabric for reinforcement or mixed with resin for smooth surface.

The advantage of this method is that it is simple and hence is widely used. There is low cost of capital equipment and no requirement for highly skilled labour. It is also possible to achieve relatively high fibre contents in the laminate. The disadvantages are that it is not suited for strength or weight critical primary structure as the fibre orientation and local resin content cannot be well controlled. The nature of the wet lay-up also has health and safety issues because low molecular weight resins can be harmful. The fumes from the curing process, especially with polyester resin requires appropriate extraction systems especially to comply with emission levels for styrene.

Some of the key points of this method include: