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Before the mould is used, the surface should first be washed to remove remains of the release agent and then coated with Honeywax or Diamond wax and thoroughly polished, three or four treatments being necessary. For the first few mouldings, release agent No. 3 should also be applied, but after several mouldings have been removed, this treatment becomes less essential, provided that the use of wax polish is maintained. See instructions on use of mould polishes and release agents.
When the release agent has dried, a gel coat resin can be applied. The gel coat forms the surface of the final moulding and consists of a layer of unreinforced, fairly flexible polyester resin which performs a variety of functions. In addition to preventing the protusion of glass fibres from the moulding surface, the gel coat provides a smooth, resilient coating which can be coloured and which provides improved protection against weathering or chemical attack. General purpose gel coat resins are pre-accelerated and are always highly thixotropic, in rather the same way that modern non-drip paints are thixotropic, so that their application to vertical or inclined surfaces is not followed by drainage. Gel coat resins are coloured by adding a suitable quantity of the appropriate colour paste. The amount of colour paste added may be up to 10% by weight on the gel coat and must be thoroughly mixed to ensure even distribution. At the same time, it is necessary to avoid the introduction of air bubbles which could show up as pinholes in the surface of the finished moulding, a folding action often giving the best results in this respect.
The catalyst is added after the colour paste, at the rate of 2%, and is mixed in thoroughly in the same way, so as to avoid the introduction of air bubbles. The catalysed gel coat resin is applied to the mould surface by means of a soft brush, but since the object is to obtain uniform coverage by applying only one fairly thick layer of gel coat, the material should be spread on the mould and not brushed out, a 'painting' action being avoided. The final cured gel coat should have a thickness of about 0.4mm (0.015") corresponding to the application of 600 g/m2 (2 oz/ft2).
Provided that the curing conditions are satisfactory, with the temperature exceeding 15°C (59°F) and the humidity low, backing up of the gel coat can be commenced between one and two hours after its application. This period will however, be longer in some circumstances, when, for example, the design involves deep cavities where air circulation is restricted and curing of the gel coat is consequently much slower. Inversion of the mould sometimes helps on such occasions. The backing up process involves the successive application and impregnation with polyester resin of several layers of glassfibre. Chopped strand mat (CSM) is almost always used, although several other types of glass reinforcement are available, including woven glass cloth, for example, which are employed for special applications.
Chopped strand mat (CSM) is produced in several different weights, those commonly available being 300, 450 and 600 g/m2 (1, 1 1/2 and 2oz/ft2) surface tissue or 300g/nf CSM is often appled directly behind the gel coat, because it is essential to ensure that all trapped air is removed and that no voids, such as those produced by the bridging of corners with glass fibres, are present between the gel coat and the back-up layer.
In the later stages, a heavier weight of glass mat can be employed to achieve a rapid build-up in the thickness of the moulding, using sufficient polyester resin to give an overall resin/glass ratio between 2 : 1 and 2.5: 1.
1 % of catalyst is added to the resin and is applied generously to the back of the cured gel coat and the first layer of previously tailored glass mat is laid over the resin. Impregnation of the glass mat with resin is then achieved by rolling, so as to bring the resin through the glass and remove air at the same time. Full impregnation of the glass mat with resin is best achieved by the use of spiral bristle or paddle rollers.
During the laminating process, it may be necessary to apply more resin to the upper surface of the mat but it is always preferable to apply plenty of resin first and bring it up through the glass. Forcing of resin into the mat from above is always liable to introduce unwanted air bubbles. When a coloured gel coat has been used, it is better to colour the first application of backing resin to the same shade because this will assist in concealing any variations in the thickness of the gel coat.
When the first layer of glass mat has been fully impregnated, further layers of resin and reinforcement are successively applied using the same technique, but for thick mouldings it is necessary to proceed in stages, allowing the resin to cure after each stage. The reason for this is that a thick layer of reinforcement, fully impregnated with liquid resin will become unmanageable and the whole laminate will tend to move on the mould surface during consolidation. A further factor to be borne in mind is that considerable heat can be evolved as resin cures due to the 'exotherm' and if this is excessive, warping and discolouration of the moulding can occur and even damage the mould.
At any stage in the moulding process when operations are suspended or when a change from gel coat resin to lay-up resin or coloured to uncoloured resin is made, the brushes and rollers should be washed thoroughly in acetone to remove any surplus resin before this has gelled.
After lay-up of the moulding has been completed, but before final curing has taken place, the laminate can be 'green trimmed' with a sharp knife. If this is not practicable, other trimming techniques can be employed on the cured moulding after extraction from the mould. These include the use of hacksaws, files and diamond wheel trimmers.
As already mentioned an 'exotherm' occurs as the polyester resin cures and all the heat generated in this process should be allowed to dissipate before the moulding is removed from the mould. It is preferable to allow the moulding to mature for several hours and even better to leave it overnight at this stage. When fully matured, the moulding is extracted using techniques similar to those described for removing the mould from the pattern, any trimming needed being carried out soon after extraction.
Fault Finding and Remedies
Many thousands of G.R.R mouldings are produced every year, most of them without problems, but the process involves a chemical reaction, and the departure from the recommended conditions may lead to faulty mouldings. This section offers a guide in tabular form to the faults most likely to be encountered in G.R.R moulding and to the remedies likely to be effective in overcoming them.
Many of the problems encountered in G.R.R production can be traced to undercure of the polyester resin often due to damp operating conditions or low workshop temperatures. It is important to ensure that the workshop temperature does not fall below 15°C (59°F) and that the humidity is not excessive. The occurrence of rejects is often associated with congested working conditions. It is, for example, almost impossible to produce high quality mouldings in a workshop where the laying down of gel coats and trimming or sanding operations, leading to dusty conditions, are being carried out simultaneously.
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