Tips for beginners and experts

For one-off articles the mould may be made of wood, hardboard, plaster, cardboard e.t.c. which may be destroyed on taking the finished article out. The surface must be fully sealed with release agent (see Release Agents). For production runs a glass fibre mould must be used. This is made from a plug (pattern) made of timber, plaster or urethane foam. The mould should be heavier than the article to be moulded to avoid distortion during laminating.

 

Trimming

Try to catch the laminate when just set and trim in the mould with a sharp knife. When cured cut the laminate with a jigsaw, coping saw or hack saw. Cut from the gel side to prevent chipping.

 

Garden pools.

Line a dug hole with aluminium foil and lay up with two layers of 450 g/m2 chopped strand mat and lay-up resin. Finish by brushing on a coat of Topcoat resin to give a smooth, tack free surface. Check moulding for leaks before bedding down on sand.

 

Sheathing.

The surface should be cleaned down to the bare wood or metal. Metal may be degreased with acetone. Aluminium is not suitable for sheathing. All wood must be completely dry and the surface scoured and scuffed. Paint with a coat of BondaMarine G4 Primer / Sealer or a resin wash consisting of resin with the addition of 10% styrene on a wood or plywood surface.

 

Cloth or woven rovings are usually used over the chopped strand mat to improve the finish. In cases like plywood cabin tops it is usual to use only cloth to give a better finish.

 

Apply on the wet laminate. Wet out cloth with lay up resin. Staple or use drawing pins to hold the cloth in position. Paint on resin to the required finish using plenty of catalyst and not too much pigment. Apply in a warm, dry atmosphere. Make sure the sheath is fully cured before exposing to water or a white bloom will appear on the surface. Use of a Topcoat resin as the final coat will avoid surface tackiness. Always cover the raw edge of sheathing with a lathe or beading.

 

At relatively low temperatures on a thin laminate the catalyst rate may be increased to 20cc per kg. In any case never mould in temperatures below 10°C or in damp atmospheric conditions.

 

Avoid making up more than 2 kg of resin at a time (especially in hot weather) until experienced. When moulding overhead avoid brushing a thick coat on to the object as it will just drip off. Use 300g/m2 chopped strand mat which should first be laid flat on an old board or similar surface and brush on resin. Pick up the mat and push it resin side up on to the overhead object. Get contact quickly using a stippling action with a brush or use a roller. This method is called 'pre-wetting' and can also be used on vertical surfaces.

 

General method

It is usual to have a mould made to the female shape of the finished product. Normally the mould would be glass fibre but it could be timber, plaster or metal.

 

The first step is to clean the mould thoroughly to remove any dust or grease and apply a release agent to prevent the finished article sticking to the mould.

 

The gel resin is then applied and left to cure. A good test is to touch the gel with a finger which should come away clean but the surface should still feel 'tacky'.

 

The chopped strand mat or cloth reinforcing should be 'tailored' to the required shape using a trimming knife or scissors. Catalyst must be added to the lay-up resin at the rate of 1O-15cc per kg. Ensure thorough mixing. Brush the lay-up resin thickly on to the cured gel and position the pre-shaped chopped strand mat in the correct position. Quickly get contact all over between the resin and mat using a stippling action with a brush (or better still a metal roller). The object is to force the bulk of the resin through the mat to drive out any air bubbles.

 

More resin is then brushed or rolled on until the mat is thoroughly wetted out and all air bubbles removed. Additional layers of mat are added in the same way. Resin has about 20 minutes pot life (at 20°C) once the catalyst has been added and it takes approximately 4 hours to set and 7 days to completely cure.

 

These times can vary depending on the particular make of resin in use. Similarly, quantity, temperature and other environmental factors can affect setting and curing times. It is advisable to test a sample before starting. Ideal working temperature is 22°C and the minimum recommended temperature for proper curing is 16°C but occasionally lower temperatures are unavoidable.

Fillite armospheres

These are micro hollow glass spheres which look like sand but being hollow are light weight. Used by mixing into lay up resin to make a light weight filler paste. One of its many uses is for bedding-on decks to hulls and so filling the gaps between the deck flange and hull.

Also used to fill in water trap areas between frames in the bottom of hulls and as a filler in casting resins.

 

Freefix

Paste-like resin which will gel quickly. Useful for positioning stiffeners on mouldings as it will cure while holding object in position. This resin changes colour as it cures. Curing is achieved either with the use of MEK peroxide catalyst (as supplied with lay-up resin) or with benzoyl peroxide paste catalyst. Good adhesive properties.

 

Scoreboard

High density urethane board scored on both sides to bend easily into contours. Used as a core material in sandwich construction allowing resin to run from one skin to the other.

 

Epoxy resin

Generally speaking epoxy resins have less water absorption, greater flexibility, and better adhesive qualities than polyester laminating resins. These resins are usually used in conjunction with high strength cloths made of carbon fibre or Keviar. More costly than their polyester equivalents. If the specific job in mind calls for light weight/high strength properties advice should be sought before starting.

For the first few lifts off new moulds apply at least three coats of wax polish at 6 hour intervals between each coat. After ageing the wax polish is buffed up and a PVA release agent may be applied if required. PVA may be sponged or sprayed on. Apply sparingly to avoid lengthy drying time. After a few mouldings release becomes easier as the surface absorbs the wax. Preparation can then be reduced to just a wax release agent. PVA can be removed with warm soapy water.

 

Waxes must be pure carnauba and must not contain silicon additives.

Chemical semi-permanent mould release systems can be used instead of wax. These are easy to apply, do not build up on the mould surface, do not transfer to moulded part and allow multiple releases between applications.

 

Body filler

May be knifed into dents and used extensively for car body repairs. Easily rubbed down, non-shrinking and quick setting. Hardens in about 5 minutes (depending on amount of catalyst used) and ready for sanding and painting in about 30 minutes.

Only epoxy based fillers are suitable for below waterline usage.

 

Filler with fibres (V11)

Polyester resin with chopped glass fibre strands giving a mulch-like appearance. Used for bridging across gaps or any repair that requires reinforcing strength as opposed to straight forward filling. Hardens quickly with paste hardener supplied.

 

Urethane foam (two can mix)

The foam is produced in situ by mixing two liquid components (A and B) then pouring into the cavity or mould. Mixing is best done with a paint mixing attachment on an electric drill but it may be done with a stick in a polythene bucket.

 

1. Accurately measure parts A and B each into separate containers at a ratio of 1:1.16 by weight or 1:1 by volume. In hand mixing, part A should be pre-mixed for at least 1min to aerate it before mixing with part B.

 

2. Pour into the mixing vessel and start stirring immediately moving the agitator up and down and around to cover all parts of the liquid. This must not take longer than 10 seconds.

 

3. Pour liquid into mould or cavity taking no more than 15 seconds. In buoyancy chambers in boats put adequate vent holes in tank tops especially in extreme corners and rock the boat to get an even cover of liquid in bottom of tank. Cream time is about 35 seconds, full rise about 2 minutes and full cure about 30 minutes.

 

If less than 1 cubic ft. per kg is produced the causes are - insufficient mixing, deviation of the mixing ratio, working at too low a temperature, stirring after bubbles have appeared, or foaming in a restricted mould with cold surfaces. Times quoted are for ambient temperature of 20°C.