Moulds fall into two groups – male and female. Glassfibre is laminated on the outside of a male mould and on the inside of a female mould. The desired finish whether it be smooth or textured will be on the side nearest to the mould. A female mould would be used for example to produce car body parts, boat hulls etc where the finish has to be on the outside. A male mould would be used for example on baths, shower trays etc where the finish has to be on the inside. Please note workshop conditions for fibreglass moulding service are critical when proceeding into the stage of mould making. Temperatures should be between 18º– 22ºC , direct sunlight should be avoided, any workshop heating should be directed away from work area as any kind of draft, hot or cold, should be avoided. Humidity should be normal and constant.
To produce a mould you need a pattern or former commonly called a “plug” – an exact replica of the finished item. The plug can also be an existing item i.e. motorbike or car panel, canoe, dinghy etc but be wary of infringing copyright. Usually you will have to make the plug from scratch. The plug can be made from almost any material as long as it is made rigid, accurate, and dimensionally stable and set on a solid foundation.
It is necessary to have slight taper on side walls of your plug so that the mould can be removed easily. Typically a large mould would need to be a rigid wood frame covered in hardboard, plywood or MDF, clay or plaster can be used and reinforced with wire netting and Hessian (Pic 1), as toolmakers we tend to manufacture from wood, before applying the primer, fill in any grain, holes, dints and joints as any defects will show on your finished mould. Hammer down any nails using a punch and counter sink, any screws or nails cover using polyester body filler, the surface must be smooth and free from blemishes. Seal the wood with a hard varnish such especially if using MDF, a good sand and sealer or shellac varnish can be used. Then sand down with 60 grit sand paper.
You can then cover with several coats of Durabuild surface primer letting each coat cure before re-appying. The Durabuild is best applied by spray gun, if spraying you will need to add Durabuild thinners to get the right consistency for your gun, add no more than 10% by volume. Add 2% catalyst and mix well before applying the Durabuild. Alternatively you can apply by brush without the need to thin it down however this will leave brush marks that will have to be rubbed