Quality control and faults in curing

Because the GRP panel industry has a high labour content, the designer/purchaser is totally dependent upon the quality of the labour force employed and the conditions under which they work in the factory. Environmental control on the shopfloor is very important in terms of evenness in temperature and constant relative humidity if the final product is going to be lasting and of good quality. For an illustration of a typical production process, see Fig. 3.6.

Although in its simplest form the laying up of glass fibre in layers looks easy, the process leading to the successful application in large wall elements is much more complicated. There is the initial difficulty of choosing a resin to make a laminate by reference to the resin properties alone. The properties and performance required of the whole matrix must be considered for different resins, and their additives will perform different functions with regard to weathering, flame retardance and colour stability. Depending upon choice, these various constituents may be used in different proportions to each other; and the right control of temperature and humidity limits is essential. The temperature in the workshop should be controlled between I 8°C and 25°C, and ideally should be maintained overnight.The workshop should not be damp, as this causes the glass reinforcement to soak up water; and could lead to delamination of the finished article and a breaking up of the resin/gl ass-fib re matrix. Cleanliness is of importance, as any contamination in the lay-up will degrade appearance and strength, while good ventilation is also a necessity in order to control styrene content in the atmosphere.

Cutting catalyst levels and reducing workshop temperatures will produce inferior laminates, which perform badly. Blemishes can include gel-coat wrinkling, surface pinholing, poor adhesion of the gel coat and resin, spotting of the gel-coat surface, striation in pigment flotation, pattern of fibres visible through a gel coat, patches of pale colour or 'fish eyes', blisters, crazing of surface of resin, and star mat and leaching after leaving glass fibre exposed to moisture.

As the performance and durability of the GRP product depend mainly on the cure of the polyester

3.6 Typical GRP production with large curing box at the rear of the factory.

resin, it is essential that there should be some means of establishing whether or not the laminate is suspect. Such a method is recommended in Appendix A of BS 4549, Part I: I 970 Guide to quality control requirements for reinforced plastic mouldings, the 'Barcol test', which is a form of the impact test.

The weight of the laminate measured against that of the requirements laid down in the specification will give a check as to whether a layer of reinforcement has been omitted, while the Barcol test for hardness, although not giving an absolute measurement of the cure, will differentiate between a really bad laminate and a reasonable one with a little more accuracy than is possible by casual observation. The laminate should have a hardness of around 35 within a few days. Post-curing for I 6 hours at 40°C can improve this to around 40 with decreased water absorption and little effect on the other mechanical properties. Test laminates can be used to check the strength of a specified lay-up, although these may not necessarily be representative of the overall quality achieved in the product.

The heat distortion point for GRP is particularly important for cladding applications where high external temperatures are expected: for example, the use of dark-coloured panels in south-facing elevations. For such applications, resins with higher heat distortion points must be used, while it is vital that test and post-curing are carried out at or above temperatures that the panel may sustain in use. Any creep in the material that can be anticipated can be taken into account in the design by adding stiffeners to the panel.

To achieve a quality product, therefore, it is necessary for the panel to be manufactured in the right conditions, by experienced workers, in the correct proportion, using the right techniques. Ignorance of these controls may lead to what appears to be a satisfactory product on the shopfloor; but one that is unlikely to perform satisfactorily throughout its expected design life. All this leads to a situation where great care has to be taken in quality control during production. It is not outside the designer's field to be able to set out these conditions in the specification, and it is essential to request record sheets from the manufacturer for each unit provided. Manufacturers should therefore be asked whether they:

- keep a record sheet for each unit manufactured, containing a unique reference number; date of casting, mix details, tests carried out, etc.;

- check weighing of each unit after curing;

- have an established procedure for dealing with minor blemishes and defects discovered in the units at various stages.

Some specifiers have found it necessary to have direct quality control over all the units during manufacture.The specification for the works and materials at Mondial House, London (demolished 2006), for example, includes a clause that 'each panel will be inspected dimensionally, and for gel surface defects and signs of poor laminating, by the consultants and marked if approved'. The GRP panels for the Herman Miller factory at Bath were also inspected individually by the architects, Farrell and Grimshaw, in the factory. Although this may still be necessary for prestigious jobs, most reputable manufacturers will now have the correct quality control procedures. Indeed, it could be said that architects' standards for cladding are higher than can be reasonably expected, and it is interesting to reflect that GRP-coffered slab formers are often acceptable with lower standards of workmanship.

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