Pre-preg Laminating vs Wet Lay-up
Pre-preg laminating is a highly specialised method of composite manufacture mainly used in aviation. The laminate comes in the form of a cold sheet, and is moulded to the shape of the mould using specialist tooling. A prepreg consists of a reinforcement material – such as fibreglass – preimpregnated with a resin matrix. The resin is partially cured and is then used to lay up the finished part before leaving it to cure completely. The heat and pressure required for curing will vary with the resin system and the intended application.
The hazards in this are mainly to do with the temperature of the sheets. When the laminate sheet is heated up, there is a risk of mild burns, so it would be suitable to wear gloves to prevent these. There is also a small risk to when the laminate sheet is chilled – the cold could cause damage to hands if the employee is constantly carrying them around. The use of chemicals and resins during the process can cause skin conditions such as dermatitis, so it is important to wear gloves. The chemicals and resins must also be stored and handled correctly, as covered by the Control of Substances Hazardous to Health (COSHH) Regulations 2002.
This is probably the most common composite shaping process. Wet lay-up laminating is also known as hand lay-up due to the fact that the process is done by hand. Dry fibres are positioned in a mould to which a polyester resin is poured over it. It is then rolled to ensure complete wetting of the fibre, and to make sure that all air bubbles are removed. It is then left to cure and, if necessary, a second coat of resin is applied.
The main hazard in this process is the risk of skin conditions such as dermatitis from the resin and other chemicals during wet lay-up. This can be prevented by ensuring that the employees wear gloves at all times during the process under COSHH. COSHH also appeals to the correct methods of storing chemicals, which is especially important to avoid the risk of ill health.
Comparing the two processes
There are many advantages in using either shaping process. Pre-preg laminating is preferred since it is a much cleaner process than wet lay-up, replacing the messy rollers for more specialist equipment. Typical wet lay-up laminates, even when vacuum bagged, have a significant amount of excess resin left over from the process. This excess resin increases the brittleness – and overall reduces the properties – of the laminate. With pre-preg laminating it is much easier to achieve the right resin content, and the process increases the uniformity of the parts produced significantly. With wet lay-up, because the resin is poured on top of the fibres and then rolled by hand, there is a risk of missing parts where it is still dry, and it’s near impossible to have a uniform thickness. There is also the risk of drips and air bubbles, which can ruin the aesthetics of the product.
The problem with pre-preg laminating is the cost. Even when the costs for the resin, cure and fabric needed for wet lay-up are added up, pre-preg still costs more. Also, a heat cure is required for the process. You must be able to achieve a minimum temperature of about 135°C and sustain it for a minimum of four hours. At the same time, heat cure temperatures can reduce the shelf life of the material, contradicting its use. Keeping the material cooler will help extend the shelf life.