Composite spraying is an open-mould composite manufacturing process which makes use of a manually controlled spray gun to spray a chopped fibre with catalysed resin directly into the mould. After curing, the part is removed from the mould.
Process Steps:
- A mould of the desired final shape is prepared.
- A gel coat is applied to the mould walls and left for some time to cure. Usually two layers are used for this process, and this is done for the easy removal of the finished part from the mould.
- fibre roving is supplied to the gun where it is chopped and mixed with the resin and catalyst before spraying it onto the mold walls.
- After applying the fibre and resin mixture, a manual roller is used to remove air bubbles.
- The mould is left to cure at room temperature and the final part is removed from the mould.
Applications for Composite Spraying
This spraying technique is used for the production of bathtubs, caravan bodies, shower trays, truck fairings and other structures for light loading.
Materials for Composite Spraying
There are two basic components for the production of any composite part: the matrix and the reinforcement material.
Common matrix materials include:
- Phenolic resin
- Polyester
- Epoxy
- Polyurethane resin
- Unsaturated polyester
Common reinforcement materials include:
- Carbon fibre
- Natural plant fibre
- Glass fibre
- Aramid fibre
Process Variations
- Gel, wax or polish can be used on the mold walls for easy removal of final part.
- Chopped fibre and resin with catalyst can be sprayed separately or together in a single sprayed layer.
- Two types of spray gun systems are used: pressurized air and hydraulic pressure.
Design Considerations
- Minimum draft angle should be 2 degrees.
- Under cuts are possible.
- Only large holes can be produced in the product.
- Recommended minimum practical thickness is 1.5 mm.
- Unlimited process-able vertical thickness.
- Minimum inside radius for any product should be 6.25 mm.
- Variation in the thickness of the part should be less than 0.5 mm.
- Bosses and metal inserts are possible.
Economic Considerations
- Very low production rate; several hours per part.
- Only suitable for the low volume production.
- Material, tooling and equipment costs are low whereas, but labor costs are moderate.
- Good material utilization.
Quality Considerations
- Volume fraction of the fibre is difficult to control.
- Poor control over the part thickness.
- Different quality of surface finish on top and bottom of the part.
- Poor control over the mechanical properties of the part.
Advantages
- Low cost process.
- Small investment in machinery.
- Can create small and large parts.
Disadvantages
- Styrene emission is a problem due to the open mold.
- Highly skilled operators are required.
- Poor dimensional accuracy; cannot be used to product highly accurate parts.
