Practical Aspects of Hand Lay-up

In this class we shall cover the basic procedures to be used in laying up a composite by hand using cloth (woven and/or chopped strand mat) and liquid resins. First some terminology..

Terminology

• Piece: The finished product that you are making, a kayak or sailboard, for example.
• Mould: The thing from which the piece is fabricated. The mould from which you would fabricate a kayak will look very much like a kayak except that it will be smooth on the inside. The inside of the mould will be duplicated on the outside of the piece.
• Plug: When the mould itself is constructed from fibreglass, the starting point is the plug. The mould is constructed from the plug. The plug, used to construct a kayak, will look exactly like a kayak on the outside. However, the plug can be constructed from anything so long as it has a smooth non-porous surface.
• Laminate: A solid constructed of successive plys of resin and fibreglass.
• Ply: A layer of cloth impregnated with resin and allowed to cure (set). Most structures are fabricated from several plies, the plies may have the same or(more usually) different orientations.
• Gel Coat: The outer-most surface of the mould or piece, its purpose is cosmetic. It gives the piece and the mould the colouration, finish and durability desired. The gel coat consists of a thin layer of specially compounded resin.
• Mould Release: The material used to effect a release between a mould and the moulded part.
• Cure: The process in which the liquid resin becomes a solid and bonds to the fibres. This is a chemical reaction between eith different components of the resin or between the resin and a hardner. The cure can be effect by heat or the addition of a catalyst.

Fundamentals of Polyester Resin systems, Including Gel Coats

The curing mechanism of polyester resin is inhibited by air. Therefore if the resin is exposed to air for a considerable time, the surface may remain sticky for several weeks. If the coat is cured rapidly, as in a thick laminate, inhibition will not occur because of the heat generated by the curing reaction. However, when applying thin films of polyester resins, such as surface coats, it is necessary to add styrene wax to the resin to obtain a full cure. The wax rises to the surface during the cure and can be later removed with soap and water.

Wax does not need to be added to gel coats applied against the mould as subsequent layers prohibit air inhibition. Wax is only necessary when the gel coat is exposed to air.

Building The Plug

The plug may be constructed of nearly anything. It is the surface finish that is important. Quite often it is desirable to merely reproduce, in fibreglass, an item already on hand. In such a case the item already on hand will serve as the plug. In most cases, the plug is constructed from scratch from some easy to work with medium. An ideal medium is styrofoam or polyurethane. In these cases the plug must be coated with an epoxy resin rather than a polyester resin (just watch the styrofoam dissolve). After sanding the plug smooth coat with a thin layer of resin and cover with a layer of 35gm/sq.m (1oz/sq.yd) glass fibre cloth. After drying brush on several thin layers of resin, sanding between layers. The surface finish at this point is the most important.

Mould Release

Mould release must be applied to the plug - this is an important step in the process. If the release agents fail to perform the mould will not release from the plug and many hours will be required to fix the damage and develop a smooth surface with the desired geometry. The usual method of applying mould release is 3 layers of carnuba wax. Each layer should be left to dry fully 1-2 hours then buffed to a shine. This is followed by a light coat of PVA film either sprayed or brushed on. Allow to dry overnight before applying the gel coat.

Gel Coat

The gel coat is the first step in making the actual mould. The gel coat should be applied in two coats of 0.4mm (.015") each, allowing 1-2 hours between coats. The gel coat should be tacky but not wet before proceeding. Use MEKP (MethylEthylKetone Peroxide) hardener at 2% by volume with polyEster gelcoats.

Gel Coat Troubleshooting

Application of the gel coat is perhaps, the most difficult aspect of manufacturing in fibreglass. Problems arise due to temperature variation, catalysing and a variety of handling techniques. The following list should help resolve some of the difficulties.

• Wrinkles and Pinholes
  A coating less than (0.005") thick may wrinkle especially when brush marks are present. The preferred thickness is 0.25mm (0.010") to 0.5mm (0.020"). A wrinkle can also occur if the gel coat is not cured enough prior to lay-up. Before lay-up check the surface for tack. The surface should be sticky but not transfer to your finger. Several things can affect a slow cure; solvent or water entrapment, undercatalysation and cold temperatures. If the gel coat contains pinholes, check the spray equipment for moisture in the air lines or dirt in the traps.
• Slow Gel Coat Cure
  Any temperature below 21°C (70°F) will retard the gel time. An under-catalysed gel coat will result in a slow cure but this is not a recommended technique for deliberately lengthening gel time. The usually ratio of initiator (MEKP) to Polyester Resin is 2% by volume. High moisture and humidity will lengthen gel time.
• Sagging of the Gel Coat
  The most common reason for the gel coat to sag is because the application was too heavy. To avoid this, spray in multiple light applications from about 15" distance from the mould. The maximum application to be sprayed at on application is 0.4mm (.015").
• Porosity
  Gel coat porosity (air bubbles) is normally caused by trapped air or moisture. This can be prevented by limiting the gel coat application to about 0.1mm (.005") per pass. Avoid excessive air pressure, 40 -80 psi is adequate depending on resin viscosity.
• Blisters When Immersed in Water
  Blisters are caused by an incomplete cure, improper wetting of the backing fibres, a bad bond between the gel coat and the back up laminate (Usually due to grease) or too thin a gel coat.
• Lifting of the Gel Coat before Laminating
  This is caused by shrinkage of the gel coat because the coat cures too fast because of over catalysation, delays between application of the gel coat and laminating or the mould surface is too hot (usually the mould has been in direct sun light and is over heated.
• Back up Pattern Showing
  Gel Coat is too thin or was not sufficiently cured before laminating
• Gel Coat sticking to the Mould/Part
  This condition is caused by improper releasing of the mould surface. Must use release agents developed for the fibreglass industry.

Laminating

Once the final layer of gel coat has been allowed to dry for several hours the skin lamination may be applied. For making the mould use a lightweight glass cloth, about 25gm/sq.m (0.75 oz/sq.yd). Apply a coat of moulding resin and gently lay the mat into the resin. Careful cutting of the cloth may be necessary to ensure conformance to the surface of the plug. Use a stiff bristle brush to force resin into the cloth and to make it conform to the various contours of the plug. Dabbing is better than brushing, A short mohair roller is good for long gently curving surfaces. All air must be worked out so that the matt is tight against the plug surface. Allow 2-3 hours to dry before proceeding with the next layer of the lamination using a 50gm/sq.m (1.5 oz/sq.yd )cloth. This should be followed up immediately with a 350gm/sq.m (10oz/sq.yd). fabric. The mould should be built up such that its final thickness is about 2 to 3 times that of the parts that are to be produced from it. One the heavy fabric layer has been allowed to cure for about 3 hours a combination of woven roving or chopped strand mat under a 50 gm/sq.m (1.5oz/sq.yd) cloth can be added to build up the required thickness. Use MEKP hardener at 1% by volume with polyEster moulding resin.

Removing the Mould

Allow the completed mould to cure for 48 hours before attempting to remove it from the mould. Care and time are required to get the mould off the plug. Soft plastic wedges can be driven in between the plug and mould to facilitate the release. Once free of the plug, replace the mould on the plug and add bracing to the mould using wood or metal to prevent distortion of the mould. Wait 24 hours before removing from the plug. The mould edges should be trimmed and the PVA washed out with warm soapy water. Any wrinkles or air bubbles in the gel coat should be filled and sanded plug to the mould at this time.

Moulding the Piece

Once the mould has cured for at least 72 hours you are ready to apply the mould release wax and PVA parting film exactly as was done when preparing the plug. Apply one 0.5mm (.020") layer of pigmented gel coat as before, waiting the 2 to 3 hours before continuing with the laminations, employing the same technique and care as used in preparation of the mould.

Safety Considerations

Styrene, the solvent and cross-linking monomer used in polyester resins is a highly volatile and dangerous substance, being both flammable and toxic when inhaled. Styrene will act as a solvent for the fatty tisues of the lungs and results in liquation (drowning) of the interior of the lungs - symptoms are similar to tuberculosis. Work in a well ventilated area only - the great outdoors is perhaps best - but avoid direct sunlight. Use a respirator capable of absorbing organic vapours. Do not have any open flames anywhere near the resin as combustion will be instantaneous. Remember that the temperature must remain above 20°C (75°F) for 24 hours to allow for proper curing. If you must work indoors use a bagging system to contain the vapour during curing.

Multiple Piece Moulds

..... to be continued.

Fundamentals of Epoxy Resin Systems

..... to be continued

Fibreglassing Polystyrene Foam

The two most common types of foam are expanded or pre-expanded polystyrene foam. Expanded polystyrene foam is commonly used in the building industry as an insulation material while pre-expanded foam is used as a packaging material. It is often desirable to apply a skin of durable fibreglass to these foams, however, both are dissolved by the more common and inexpensive polyester resins and as such the more expensive epoxy resins must be used instead.

Materials

Fibreglass cloth comes in many varieties. The most commonly used in skin laminations are:

Epoxy resin is a two component, low viscosity epoxy system for applying a skin lamination. The normal mixing ratio, #88 Epoxy resin to #87 Epoxy hardener is 5:1 epoxy to hardener. Colour pigments can be added to the epoxy resin system. The best way to apply the resin is to use a short nap paint roller for both the priming coat and saturating the cloth, finish coats are best applied with a good quality nylon paint brush.

Methods

• The temperature must be above 23°C (75°F) for at least 24 hours after application.
• Work in a well ventilated and dust free environment - avoid direct sunlight.
• Always add the hardener to the resin and mix well before applying.
• The surface to be fibreglassed should be clean and dust free - a fine grit sandpaper will suffice. Do not use chemical cleaners other than household detergent as they will attack the foam.

Prime Coat

A litre of resin will cover about 3.5 sq.m (35 sq. ft). The initial application should be thick, but not too thick as to cause runs or sags. Allow to dry for several hours before applying the glass. Resin should be mixed in small quantities e.g. 100ml of hardener to 0.5 litre of epoxy resin. This will last about 15 minutes before it hardens.

Glass Cloth

Cut the cloth so that it more or less conforms to the surface to be covered. Final trimming should be done while saturating the cloth. Apply a thin coat of the mixed resin to the surface and immediately cover with the glass cloth. Smooth out the glass cloth. Apply additional resin to the glass cloth so that it is fully saturated and transparent. Make sure there are no blisters, air pockets or dry areas. If more than one piece of glass is used, make sure each is fully saturated before proceeding to the next piece - alloy about 2.5cm (1") overlap between adjacent pieces.

Allow 24 hours to cure. Sand out any laps, seams or other surface regularities with a 100 to 220 grit sandpaper.

If a colour pigment is to be added to the final coat it is a good idea to slightly tint the glass layer. use about 1 teaspoon (5ml) of pigment per 250ml of resin.

Finish Coat

If colour pigment is used add at the rate of 1 teaspoon (5ml) to 250ml of resin. Mix all the resin you will need at one time to avoid any variations in coloration. Thixotropic silica can be added to prevent sags and runs at the rate of 75ml of silica to 250ml of epoxy resin - allow about 10 minutes to thicken - before adding the hardener.