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Quality in GRP

Many complaints concerning the appearance and performance of GRP products and mouldings stem from the basic cause that the resin used is under cured. There are, however, several problems, which are met from time to time which deserve much closer examination. The remedy in each case will become more apparent from the analysis of one or more of the following causes.

This is caused by solvent attack on the gel coat by the monomer in the laminating resin due to the fact that the gel coat is under cured. Wrinkling can be avoided by ensuring that the resin formulation is correct, that the gel coat is not too thin and also controlling the temperature and humidity in the workshop, also keeping the mouldings away from moving air – especially warm air. If the workshop is equipped with hot air blowers they should be directed away from the moulds.
Surface pinholing is caused by small air pockets that are trapped in the gel coat before gelation occurs. It happens when the resin is too viscous, or has a high filler content, or again when the gel coat resin wets the release agent imperfectly.
Poor adhesion of the Gel Coat
Unless the adhesion of the gel coat to backing laminate is very poor, this defect will only be noticed during the handling and pieces of gel coat will flake away. Area of very poor adhesion can only be noticed sometimes by the presence of a blister, or by undulations in the surface if it is viewed obliquely. Poor gel coat adhesion can also be caused by inadequate consolidation of the laminate, contamination of the gel coat before the glass fibre is laid up, much more common is when the gel coat is left too long to cure, always apply a laminate onto the gel coat with twenty four hours of application.
This fault takes the form of very small spots all over the gel coat surface on the laminate. It is commonly due to the ingredients of the resin formulation not being adequately dispersed.
This common fault is caused by pigment flotation and is most likely where the colour used is a mixture of more than one pigment; thorough mixing is the remedy or the use of another pigment in replacement.
Fibre Pattern
This occurs when the fibre glass reinforcement is visible through the gel coat. This occurs when the gel coat has been applied very thin or when the reinforcement has been applied before the gel coat has hardened sufficiently, it can also happen when the moulding is lifted from the mould prematurely.

Fish Eye’s
On a very highly polished mould, the gel coat sometimes ‘de-wet’s’ from certain spots leaving areas where the gel coat is non-existent . This will show up as patches up to ¼” diameter, it can also show up in long straight lines following the strokes of a brush in application.

The very presence of blisters shows that they is a presence of delamination within the moulded part. Blisters that are larger will indicate that the resin is undercured, sometimes this type of blister will not form until some months after moulding.
Crazing can happen immediately after moulding or on the other hand it may take a few months to develop. It will appear as fine lines on the surface of the resin / gel coat. Crazing is sometimes associated with resin rich areas and is sometimes caused using an unsuitable resin or gelcoat. The addition of extra styrene in the gel coat (often done in spraying) is a common cause.
Star crazing / cracking
This is the result of impact from the rear side of the laminate or sometimes just an over thick gel coat. Gel coat should never be more than 0.6 mm thick, gel coat thickness gauges are available.
Dry Patches
Often caused by over ambitious laminating, i.e. trying to impregnate more than one layer of matting at a time, you can test a laminate by using a coin, dull sounds will indicate poor impregnation.
One of the more serious faults in fibreglass manufacturing. Leaching occurs after exposure of the laminate to the weather, and is mostly characterised by a loss of resin from the laminate, leaving glass fibres exposed and open to attack by moisture. Leaching indicates that either the resin used has not been adequately cured, or the resin used has not been designed for the application.

Inspection of GRP parts.
The success or failure in producing high quality fibreglass parts depends to a large extent on the fabricators understanding of the nature of polyester/epoxy fibreglass parts and the importance of the various stages during fabrication.
The essential difference between fibreglass and all other structural materials, i.e. steel / aluminium are mainly determined by the manufacturer, with GRP (Glass Reinforced Plastic), the fabricator determines these properties himself.
Inspection will therefore cover a much wider field than usual and has to be considered at every stage during and after fabrication.
When mouldings are inspected visually the following points should be scrutinised:
  1. Surface imperfections and overall general appearance.
  2. Check they are no air pockets trapped within the laminate.
  3. Ultimate tensile strength.
  4. bend strength (cross-breaking or flexural strength).
  5. Modulus in bending.
  6. Impact strength.
Too much importance should not be attached to high tensile strength figures. With some type of fibreglass reinforcement it is possible to obtain laminates with a high glass: resin ratio. Although such laminates have a high tensile strength they are usually too thin to be rigid enough and are therefore deemed unsatisfactory. Minimum thickness is to be considered. A fibreglass laminate that has a low resin content will have poor overall weather resistance.
Most tests, chemical and mechanical, are destructive, i.e. a section must be taken from the laminate. Cut this piece at least 1” from the edge since variations in resin content are particularly exaggerated near the edge. It is also important for all tests to be carried out when the laminate has reached maturity and is stable. Samples post cured for three hours at 80 oC reach stability more rapidly, allow 24 hours after manufacture before attempting any post curing.
The mechanical and chemical properties of a fully cured laminate are affected more by the resin: glass ratio than any other single factor. As a general guide, a high glass content will give a higher strength laminate, whilst a high resin content will give a laminate with better chemical, water and weather resistance.
The resin: glass ratio is found by weighing a small piece of laminate, 1 centimetre square is normally suffice. Use a crucible, ashing it over a Bunsen burner and then reweighing it.
Controlling variables
Polyester / Fibreglass laminates are not homogeneous structures and unlike most metals there is a considerable degree of variability in the physical properties. It can, however be kept to a minimum if the following factors are controlled.
  1. Variations in resin content, good rolling is essential; it should consolidate the reinforcement without disturbing or breaking the glassfibre strands into filaments.
  2. Draughts. These cause excessive styrene loss leading to undercure.
  3. Gel time. If this is too long the styrene loss through evaporation may be excessive.
  4. Ambient temperature. This should be kept constant, if they are variations the setting time should be controlled by adjusting the accelerator content, not the catalyst content.
  5. Proper mixing of curing agents.
  6. Variation of strength in liquid catalyst.