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COMMON FAULTS FOUND IN GRP MOULDINGS
Only faults applicable to Hand Lay up and Spray applied laminates are covered below
I WRINKLING (ELEPHANT SKIN) :
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 undercured. Wrinkling can be avoided by ensuring that the resin formulation is correct, that the gel coat is not too thin and by controlling temperature and humidity and keeping the work away from moving air - especially warm air.
II PINHOLING :
Surface pinholing is caused by small air bubbles which are trapped in the gel coat before gelation. It occurs when the resin is too viscous, or has a high filler content, or when the gel coat resin wets the release agent imperfectly.
III POOR ADHESION OF THE GEL COAT RESIN :
Unless the adhesion of the gel coat to the backing laminate is poor, this defect will be noticed only when the structure is being handled and pieces of gel coat flake off. Areas of poor adhesion can be detected sometimes by the presence of a blister, or by local undulations in the surface when it is viewed obliquely. Poor gel coat adhesion can be caused by inadequate consolidation of the laminate, contamination of the gel coat before the glass fiber is laid up or, more generally, by the gel coat being left to cure for too long.
IV SPOTTING :
This fault takes the form of small spots all over the gel coat surface of the laminate. It is usually due to one of the ingredients of the resin formulation not being properly dispersed.
V STRIATIONS :
This fault is caused by pigment flotation and is most likely where the colour used is a mixture of more than one pigment. The remedy is thorough mixing or the use of a different pigment paste.
VI FIBER PATTERN :
The pattern of the glass fiber reinforcement is sometimes visible through the gel coat or prominently noticeable on its surface. This usually occurs when the gel coat is too thin or when the reinforcement has been laid-up and rolled before the gel coat has hardened sufficiently, or when the molding is removed too soon from the mould.
Fiber pattern visibility in case of translucent roofing sheets or panels is due to incompatibility of resin and glass fiber i.e. the refractive indices of resin and glass are not matching.
VII FISH EYES OR CRATERS :
On a very highly polished mould, particularly when silicon modified waxes are used, the gel coat sometimes `de-wets' from certain areas leaving spots where the gel coat is almost non-existent. This shows up as patches of pale colour usually up to 6 mm in diameter. It can also occur in long straight lines following the strokes of the brush during application.
VIII BLISTERS AND BUBBLES :
The presence of blisters indicates that there is delamination within the molding and that the air or solvent has been entrapped. Blisters which extend over a considerable area may also indicate that the resin is undercured and this type of blister may not form until some months after molding. Blisters can also occur if the molding is subjected to an excessive amount of radiant heat during cure. A possible cause of this defect is the use of an unsuitable grade of MEKP catalyst. If on the other hand the blister is below the surface, the cause is likely to be imperfect wetting of the glass fiber by the resin during impregnation. This would be due to the fact that insufficient time had been allowed for the mat to absorb the resin before rolling. Blisters of this kind can usually be detected by inspection as soon as the molding has been removed from the mould.
IX CRAZING OR CRACKING :
Crazing can occur immediately after manufacture or it may take some months to develop. It appears as fine hair cracks in the surface of the resin. Often the only initial evidence of crazing is that the resin has lost its surface gloss. Crazing is generally associated with resin rich areas and is caused by the use of an unsuitable resin or resin formulation in the gel coat. The addition of extra styrene to the gel coat resin is a common cause. Alternatively the gel coat resin may be too hard with respect to its thickness. In other words, the thicker the gel coat, the more resilient the resin needs to be. Crazing which appears after some months of exposure to the weather or chemical attack is caused either by under cure, the use of too much filler, or the use of a resin which has been made too flexible.
X STAR CRACKING :
This is the result of having an over thick gel coat and occurs when the laminate has received a reverse impact. Gel coats should never be more than 0.4 mm thick.
XI INTERNAL DRY PATCHES :
These can be caused by attempting to impregnate more than one layer of mat at a time. The presence of internal dry patches can be readily confirmed by tapping the surface with a coin.
XII POOR WETTING OF THE MAT :
The cause of poor wetting of the mat is either the use of insufficient resin during lay-up or inadequate consolidation of the lay-up. This defect is normally apparent on the reverse face of the laminate only,i.e. the side without a gel coat. When correctly wetted this will have a glazed appearance because the fibers are coated with resin.
XIII LEACHING :
This is a serious fault. Leaching occurs after exposure of the laminate to the weather, and is characterized by a loss of resin from the laminate leaving the glass fibers exposed to attack by moisture. Leaching indicates either that the resin used has not been adequately cured, or that it is an unsatisfactory resin for that particular application.
XIV YELLOWING :
GRP laminates yellow after a period of exposure to sunlight. It is generally only slight, but can be considerable on translucent roof sheeting and white pigmented laminates. It is a surface phenomenon due to the absorption of ultra-violet radiation. For this reason most sheeting resins contain UV stabilizers which reduce considerably the rate of yellowing. Yellowing does not affect the mechanical properties of the laminate. Laminates with a high resin content discolor less rapidly than those having a high glass content. If a sheeting laminate has been fully cured, has a resin content of at least 75 % and if the Cellophane is removed before it is exposed to heat or strong sunlight, the degree of yellowing even after prolonged exposure will be negligible.
XV TACKINESS :
Air inhibition is always a possibility with unsaturated polyester resins especially in open moldings. The surface of the laminate exposed to the air remains tacky due to under cure of the exposed surface. Atmospheric oxygen disturbs the curing process on the surface especially when using an open mould. To reduce tackiness, the lay up should be avoided under cold, damp conditions. Using an air drying resin for the final coat and/or adding paraffin or wax to the top coat, or increasing the catalyst or accelerator levels and subsequently the cure temperature, are possible solutions to avoid tackiness.
XVI DELAMINATION :
This can occur between two layers of reinforcement, particularly if two layers of woven rovings are used without chopped strand mat between them. Delamination can also occur due to insufficient resin system applied to the laminate, or poor wetting of the glass mat due to a higher viscosity of the resin, or contamination at the interface which may occur where the first layer of laminate has been allowed to cure prior to application of the next layer, or over cure of the first layer of the laminate prior to application of the second layer.
XVII SINK MARKS :
These can occur over ribs or inserts due to shrinkage of the resin system during cure. They can be overcome by allowing the laminate to partially cure before moulding in the ribs or inserts.
XVIII PULL-AWAY :
The gel coat releases from the mould before the laminate is applied especially in corners or angles. This is frequently due to styrene vapour settling in the bottom of the mould. This can be avoided by positioning the mould so that the styrene vapour can flow out, or use a suitable extractor system, or both. Also avoid excessive gel coat thickness in critical areas and reduce catalyst levels.
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