Fibreglass Design Tips
Good fibreglass products begin with good design. The shape, surface finish, stiffness, tooling and method of manufacture can all affect the performance, appearance and cost of the finished component.
The following guidance outlines some of the fundamental considerations involved in designing moulded fibreglass products and completing basic fibreglass repairs. For advice specific to your project, talk to the FDL team.
Basic Design Tips
Fibreglass composites can be used to manufacture an enormous range of products, from small covers and enclosures to vehicle components, boats, tanks and large architectural features. Their ability to form complex shapes is one of their greatest advantages.
Most fibreglass components are produced in or over a mould. The design must therefore allow the laminate to be applied effectively and the cured component to be removed without damaging either the product or the mould. Features such as tight corners, reverse angles, deep sections and insufficient draft can make manufacture and demoulding considerably more difficult.
From concept to finished component
The following design considerations can help make a fibreglass component easier to manufacture, release from its mould and finish to the required standard.
Strength, Stiffness & Flat Areas
Strength and stiffness are different properties. A fibreglass laminate may withstand a substantial load without breaking while still flexing more than a similarly sized component made from a stiffer material.
Large, unsupported flat areas can be prone to flexing, distortion and visible surface irregularities. A slight curve can improve stiffness without adding significant weight. Where a flat surface is essential, stiffness can be increased using a thicker laminate, a lightweight core, ribs, top-hat sections or other forms of reinforcement.
The most appropriate construction will depend on the component’s dimensions, loading, weight requirements and operating environment.
Shape, Draft & Corners
The shape of a component must allow both the mould and the finished part to be produced and separated successfully. Straight-sided or deep sections generally require sufficient draft to allow the component to release without damaging either surface.
Undercuts, returns and other complex features may require a split mould with carefully positioned parting lines. These lines can remain visible on the finished component unless they are subsequently sanded and polished out.
Corners should incorporate generous radii wherever possible. Rounded corners allow the reinforcement to conform more easily, improve laminate consistency and reduce the likelihood of air pockets, resin-rich areas and chipped edges. The most suitable radius will depend on the materials, manufacturing process and intended application.
Surface Finish & Colour
Fibreglass components can be produced in a wide variety of colours, textures and gloss levels. The quality of the mould surface largely determines the appearance of the moulded face of the finished component.
Highly polished surfaces can make minor imperfections, fibre print-through and changes caused by ageing more visible, particularly across large flat areas. A semi-gloss or lightly textured finish may provide a more practical and forgiving result.
Dark colours absorb more solar heat and may increase the operating temperature and thermal movement of components used outdoors. Metallic finishes are also possible, although complex shapes can make an even appearance more difficult to achieve and subsequent repairs or touch-ups more noticeable.
Thickness & Tolerances
The thickness of a hand-laminated component will vary to some degree. One side of the component takes its finish from the mould, while the reverse surface is formed by the laminate and may have a more textured appearance.
Thickness variation generally becomes more noticeable as additional reinforcement is applied. Using a core of known thickness can improve consistency, while closed-mould processes such as RTM provide greater control over both surfaces and the finished laminate thickness.
Designs should include suitable tolerances for the selected materials and manufacturing process, particularly where the fibreglass component must fit against other parts.
Tooling Costs
Producing a custom fibreglass component usually involves an initial investment in the plug and mould before manufacture of the finished parts can begin. Tooling costs depend on the component’s size, complexity, required surface finish and anticipated production quantity.
For one-off projects, simpler tooling may be appropriate. Moulds intended for repeat production require greater durability, accuracy and structural support, but their cost can be distributed across the production run. Considering tooling requirements early helps identify the most practical and economical manufacturing approach.
Properties
It is not cost effective to use composites in conventional applications against conventional materials, unless you are exploiting its advantages. If you are going to use it, get the most out of it!
Basic composite laminates, while strong, are not stiff. Do not confuse these two properties. For example a 4mm thick strip of fibreglass may bend twice as much as a strip of 4mm ply for the same load, but it may take 4 times the load before it breaks! (These are indicative figures only).
Thermal Expansion
Fibreglass composites expand and contract as their temperature changes. The amount of movement depends on factors including the resin system, reinforcement, laminate construction and orientation of the fibres.
Fixings and connections for large or long components should accommodate the expected movement rather than holding the part completely rigid. This is particularly important for exterior components exposed to direct sunlight or significant temperature changes.
