Fibreglass can offer many benefits when compared to metal fabrication. The ability to achieve almost any shapes and curves, high strength to weight ratio, corrosion resistance, self colouring are but a few of these. Unfortunately very often, when the fibreglass part is replacing a metal one, designers and engineers, due to a lack of experience at dealing with composites, tend to adhere to metal fabrication principles. This not only loses the possible benefits, but also can create problems as metals and composites are formed in very different ways. For example, small corner radii formed by stamping sheet metal are not suitable in fibreglass mouldings, but strengthening ribs required on metal stampings can often be eliminated on fibreglass mouldings. To achieve the maximum benefits from fibreglass or any other composite, the product has to be designed for composite construction.
– Get a good understanding of the advantages and shortcomings
Although fibreglass offers the designer many advantages, it does also have some shortcomings which can cause problems if they are not taken care of in the design process and well understood. The major one is the fact that there will be a volumetric shrinkage during the curing (hardening) process. This will vary depending on the resin system and can be as high as 8%. This being a volumetric shrinkage, it will shrink more in the easiest direction which usually is the thickness, but nevertheless the part will get smaller. This needs to be accounted for, as for metal castings, when the patterns and moulds are produced. This shrinkage will also tend to cause shape distortions. For example “U” or “L” shaped parts will have a tendency to close and therefore need to be reinforced with metal or fibreglass box section to keep them in shape. Thickness variations in the part, such as a rib or boss will also tend to “print through”.
– Realistic and relevant tolerances will save money and avoid unnecessary frustration
It is common for designers and engineers to specify general tolerances even if these are not relevant to the function of the product. With fibreglass, as previously described, it is not always easy to keep to these tolerances and unnecessarily attempting to do so will increase the cost considerably. All dimensions should be individually evaluated and toleranced accordingly.