Fabrication the mould

The process of fabrication involves laying up successive layers of resin, into which glass-fibre reinforcement is embedded to follow a mould profile (Fig. 3.2). Reid and O'Brien (1973) have described the basic laying-up

3.1 Faults: (a) crazing of GRP surface; (b) internal dry patch.

techniques. The open mould contact process uses either hand lay-up or spray-up application.There are, in addition, some other mechanized processes, such as press moulding (hot or cold), using matching moulds to press out panels so that both sides of the panel

3.2 Methods of GRP preparation: (a) contact moulding; (b) vacuum moulding: (c) press moulding.

have moulded surfaces.The moulds for this process are expensive and are more difficult to adapt to accommodate variations in panel shape. Other processes use resin-injected techniques, in which the glass fibre reinforcement is placed in a mould of the required shape and resin is forced under pressure, or a combination of pressure and vacuum, into the mould space.

Hand lay-up or contact process

The simplest and most common method of building up the reinforcement and resin is by the hand lay-up technique. First, the mould is waxed and polished using a non-silicone-based wax (Fig. 3.3). It is usually sufficient to do this once every three to five usages of the mould. A release agent, the most common type being polyvinyl alcohol (PVAL), is then applied over the entire surface of the mould and allowed to dry thoroughly. When the mould has been 'run in', it is possible to do away with this operation, except at the edges, relying only on the waxed surface for separation.

The durability of a moulding is mainly dependent on its surface. Moisture will attack the glass-fibre reinforcement so it is protected by a layer of resin known as the gel coat.This gel coat is applied to the mould by brush or roller at 450-600 g/m2. Gel coat resins differ from lay-up resins in that they are thixotropic to avoid draining from a vertical surface, and they also gel with a tacky surface where exposed to air to facilitate a good bond to the rest of the laminate.The utmost care and skill are needed in this operation if an even coating is to be achieved without the entrainment of air bubbles or trapping of dirt.The thickness should be carefully controlled, for where the coating is in excess of 600 |jm thick, crazing and cracking, coupled with reduced impact resistance in use, are likely to occur Faults in the surface of the gel

3.3 Mould waxing prior to moulding.

coat will result in water penetration and a subsequent breakdown of the glass-fibre reinforcement, leading to swelling and rupture of the laminate.

For a small moulding, mixing is done by the laminator using a volume-measuring device to measure in the catalyst. In the simplest case it is done by hand. Where larger volumes are required, measuring and mixing would be done by a machine in a separate area under the charge of a responsible person.

The lay-up of the glass-fibre reinforcement and resin can start as soon as the gel coat has hardened sufficiently not to come away on the finger when

3.4 Rolling out the lay-up to remove air bubbles.

touched. The resin is mixed as before and a liberal coat is applied over the gel coat.The optimum quantity of lay-up resin can be calculated from the weight of glass reinforcement. For chopped strand mat the resin:glass ratio should be between 2.5:1 and 2:1 by weight.The first layer of reinforcement is pressed into the mould and consolidated with a brush or roller The lay-up is consolidated with a roller, and subsequent layers of resin and glass mat are applied until the required thickness is built up. Figure 3.4 shows rolling out the lay-up to remove air bubbles.

Spray lay-up

An alternative method of building up the resin and reinforcement in the contact moulding procedure is by the simultaneous deposition of resin and chopped glass fibre by spray-moulding equipment. Spray lay-up considerably reduces the labour content of the process, although rolling out is still required to consolidate the laminate and ensure that the resin is mixed.

Machine moulding

There are several methods of producing GRP mouldings by more mechanized methods than the contact moulding technique described so far These mechanized processes, such as resin injection, vacuum forming and pultrusion (only applicable to long continuous runs) are not normally relevant to cladding units. This is partly because of the limits of size possible with these processes, and partly because of the capital investment in the necessary machinery. Cladding panels are not often produced in long continuous runs, and because architects are normally designing a one-off panel system requiring only a few hundred panels, the capital outlay for relatively sophisticated machinery becomes inapplicable. One of the few exceptions to this would be the roof panels for Covent Garden Market, London, which were produced by Armshire Reinforced Plastics using resin injection and pressing techniques (Holloway, 1975, p. 69).

It is interesting to make a comparison with the boat-building industry, where this 'tooling up' process is done once and is paid for by the production of a large number of products. In cladding this would require a complete development programme every time a new contract was initiated. Glass-reinforced polyester being a complex design material leads to development time for this process to take place, whereas building components, such as cladding, are often subcontracted, there being a short time between the design and requirement of the product, putting the manufacturer under considerable pressure to met specific deadlines in relation to other design work construction processes. Hand lay-ups and spray techniques are slow; the production of one panel per day per mould is the norm, causing the manufacturer to use a number of moulds to meet the requirement. This in turn increases costs, coupled with the fact that a mould may only last 50-100 panels, after which it would have to be discarded.

The mould

Before an item can go into production, a considerable amount of work has to be done to prepare the mould required for its manufacture.The quality of the mould will ultimately dictate the quality of the finished product. For most projects where a number of panels are required, the moulds are formed of GRR Master patterns can be made in materials such as timber, steel or plaster; from which this GRP mould is taken. A GRP laminate during curing may shrink by 0.1 -0.4% (linearly), and as there are two moulding processes between the pattern and the finished product, the net shrinkage may be as much as 0.8% (one manufacturer gave as an example a 9 m long panel being oversized by 1.2 mm to allow for shrinkage).The manufacturer gradually knows from experience how much to allow for shrinkage, depending upon the shape of the unit, the resin and the temperature change expected.Tooling is one of the highest costs in GRP manufacture. Moulders and finishers are considered to be skilled labour; and most of the larger manufacturers have introduced training schemes for apprentices to this trade.

Because of the bespoke nature of the industry and the number of panels involved for each job, the manufacturers cannot afford too many prototypes before a satisfactory target dimension is achieved. A large number of panels will have to be produced for a number of moulds, not all necessarily to the same master pattern.This is a factor affecting manufacturing deviations of panel size (tolerances).

Another difficulty of manufacture is that some of the stresses set up in the matrix are inherent, pro duced by the differential shrinkage between fibre and resin when the GRP composite is cooled to room temperature during fabrication, as a result of differing coefficients of thermal expansion (70-100 x I 0~6oC for resin versus 5.0 x I 0~6oC for glass). Resin additives can alleviate these effects by reducing shrinkage.

Although it is clearly desirable to design for simple moulds, it is possible to develop split moulds with movable sections to achieve return angles. A flash line is then seen on the face of the panel at the junction of the two sections of the mould. It is possible to buff this out in finishing operations, or render it less obvious by designing the 'split' to coincide with a change of profile. It is vital that reasonable access be given to the fabricator in order to lay up evenly all sections of the mould and to facilitate consolidation of the material by means of a hand roller to expel any air bubbles that may be entrained in the laminate.Too large a mould, or one of a complicated shape, means that the fabri-

3.5 Tilting moulds being used for production.

cator cannot get to all the areas in order to lay up an even coating of glass fibre and resin, while at the same time ensuring good compaction and ease of rolling out.The ideal panel width is one for which the operator can reach all parts of the mould easily, say 2 m wide.The domed panels at Castle Park, Nottingham (architects: Nicholas Grimshaw Partnership) were manufactured as one-piece units using a tilting mould allowing access to all areas for the fabricator These tilting moulds are similar to those used in boat production (Fig. 3.5).

Was this article helpful?

0 0
Boating Secrets Uncovered

Boating Secrets Uncovered

If you're wanting to learn about boating. Then this may be the most important letter you'll ever read! You Are Going To Get An In-Depth Look At One Of The Most Remarkable Boating Guides There Is Available On The Market Today. It doesn't matter if you are just for the first time looking into going boating, this boating guide will get you on the right track to a fun filled experience.

Get My Free Ebook

Post a comment