Composite and lamination — hand lamination and vacuum infusion

Not all plastic products come out of a machine in a few seconds. Large, light and strong structures — boat hulls, tanks, cabins and wind turbine blades — are instead built up layer by layer in a mould, where reinforcing fibre is woven together with a liquid resin that then hardens into a fixed, rigid whole. This is the core of what the plastic technician training calls the hardened plastic specialisation, and the techniques differ markedly from injection moulding at a machine.

§From form to finished laminate

Work starts with a mold that is often treated with a release agent and a gelcoat — a thin, colored surface layer that becomes the product's visible exterior. On top of the gelcoat the laminate itself is built up of layer after layer with reinforcing fibers and resin until the desired thickness and strength is reached. When the resin has cured the part is removed from the mold and the mold can be used again for the next part.

§Hand lamination

With hand lamination fibre mats or woven roving are laid in the mould and resin is applied with a roller or brush while air bubbles are pressed out with a ribbed roller. It is a technique that requires practice: too little resin gives dry weak areas too much makes the laminate unnecessarily heavy and air that is not pressed out becomes weak points in the finished workpiece.

• 01The form is prepared with abrasive and possibly gelcoat.
• 02First layer of fiber is laid in and thoroughly wetted with resin.
• 03Air out where possible
• 04The process is repeated until the planned thickness and layer build-up is reached.
• 05The part is allowed to cure, often with a controlled exothermic reaction before demolding.

§Vacuum infusion — a cleaner and stronger method

With vacuum infusion dry fibre layers are first placed in the mould without resin. Then the entire structure is sealed under a vacuum film and a vacuum sucks the resin through the fibre via channels in the structure. The method gives a more uniform ratio between fibre and resin because excess resin is sucked away during the process which gives a lighter and stronger laminate than hand lamination. At the same time the entire infusion takes place in a closed system which significantly reduces styrene evaporation compared to open hand lamination.

Fiber types in composite

Fibreglass is the most common and cheapest reinforcing fibre and is used in most standard composite. Carbon fibre is significantly lighter and stiffer at the same strength, but also more expensive, and is typically used where weight is critical — as in parts of wind turbine blades, sports equipment and racing cars. Aramid fibre, better known under the Kevlar brand, has particularly good impact properties and is used where the structure must withstand shock and penetration.

“A good laminate is not about using as much resin as possible — it is about using exactly the right amount.”

— Professional rule of thumb in the composite craft.