What is spray lay up?

Glass fibre (Roving) is fed into a chopper on the spray gun and the resulting strands are blown into a stream of liquid resin. The spray is directed at a mould and the resulting composite has a random array of glass fibres. The resin must be liquid at room temperature to achieve adequate handling, wet out and fibre impregnation. In addition to this the gel time of the resin must be sufficient to allow complete lay up of the part before solidification occurs. The laminate is rolled to conform the laminate to the mould surface and remove any air bubbles. There are two cure systems used, twin-pot and catalyst injection. Both systems begin the reaction process at the gun, which means fast curing resin systems may be used.

Prior to application of the fibre and resin, the mould is prepared with either polyvinyl alcohol or nonsilicon 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.

Glass fibre is particularly suitable for this method as it is easily chopped into strands, has low static and fast wet through. Orthophthalic and isophthalic resins such as vinyl ester or polyester are commonly used owing to their high reactivity.

The spray method has the advantage on being a fast application method. It achieves better wetting of the fibre with fewer voids than with hand lay-up. This results in parts having better physical properties. However, it produces parts that are less uniform, particularly in thickness than hand lay-up. The laminates tend to be resin rich and hence heavier.

This method requires a low molecular weight resin and the styrene emissions can be high requiring tight health and safety control. Further the gelling and curing occur via chemical reactions; if the reactions are too rapid then the resulting exotherm may be too great and damage the part or mould.

If the ambient temperature is warm, e.g. desert climates, then the resin may cure prior to completion of the spraying application.

Some of the key points of this method include: