Common faults found in GRP Mouldings
Only faults applicable to Hand Lay up and Spray applied laminates are covered below:
- WRINKING (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. Wrinking can be avoided by ensuring that the resin formulation is correct, that the gel coat is not too thin and by controlling the temperature and humidity and keeping the work away from moving air-especially warm air.
- PINHOLING ::
Surface pinholing is caused by small air bubbles which are trapped in the gel coat before gelation. It occurs when the resin is not too viscous, or has a high filler content, or when the gel coat resin wets the release imperfectly.
- 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 undulation in the surface when it is viewed obliquely. Poor gel coat before the glass
fiber is laid up or, more generally, by the gel coat being left to cure for too long.
- 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.
- 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.
- 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 moulding is removes too soon from the mould.
- BLISTERS AND BUBBLES ::
The presence of blisters indicate that there is delamination within the moulding 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 moulding, Blisters can also occur if the moulding 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 MKEP 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 has been allowed for the mat to absorb the resin before rolling. Blisters pf this kind can usually be detected by inspection as soon as the moulding has been removed from the mould.
- 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 either by undercure, the use of too much filler, or the use of resin which has been made too flexible.
- STAR CRACKING ::
This is the result of having an over thich gel coat and occurs when the laminate has received a reverse impact. Gel coats should never be more than 0.4mm thick.
- INTERNAL DRY PATCHES ::
This 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.
- 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 the gel coat. When correctly wetted this will have a glazed
appearance because the fibers are coated with resin.
- LEACHING ::
This is a serious fault. Leaching occurs after exposure of the laminate to the weather, and is
characterized by the loss of resin from the laminate leaving the glass fibres exposed to attack by moisture. Leaching indicated either that the resin used has not been adequately cured, or that it is unsatisfactory resin for that particular application.
- YELLOWING ::
GRP laminates yellow after a period of exposure to sunlight. It is generally only sunlight, 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 Stabilisers, which reduce considerably the rate of yellowing. Yellowing does not effect the mechanical properties of the laminate. If a sheeting laminate has been fully cured it has a resin content of at least 75% and if the cellophone is removed before it is exposed to heat or strong sunlight, the degree of yellowing even after prolonged exposure will be negligible.
- TACKINESS ::
The surface of the laminate exposed to air remains tacky due to the undercure of the exposed surface. Atmospheric oxygen disturbes the curing process on the surface, specially when usingan 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 accelerate levels and subsequently the cure temperature, are possible solutions to avoid tackiness.
- 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 in-sufficient resin system applied to the laminate, or poor wetting of the glass mat. Due to a high viscosity of the resin, or contamination of 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.
- SKIN MARKS ::
This 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.
- PULL-AWAY ::
The gel coat releases from the mould before the laminate is applied specially in corners or angles. This is frequently due to styrene vapours settling in the bottom of the mould. This can be avoided by positioning the mould so that the styrene vapours can flow out, or use a suitable extractor system, or both. Also avoid excessive gelcoat thickness in critical areas and resuce catalyst levels.