Silicone moulds for GRP plant containers.

Silicone rubber is often used as a moulding material usually where minute surface detail is required. There is virtually no shrinkage and it is recommended for when a larger number of casts are required such as plant containers.
There are many types of silicone rubber available and this article gives a general introduction to its use.

Mixing and Mould Preparation

The resins used for the products made in silicone rubber moulds can reach a temperature in excess of 160ºC and after the mould is made it should be left to cure for at least 24 hours.
Post curing of the silicone mould for a further several hours at 10ºC higher than the curing temperature of the resin may also prolong the mould life.

Master patterns are made from the shape that you want to copy. The pattern can also be designed and produced by specialist companies. The master shape must be firm enough to be painted with Silicone and has to be prepared with great care, as all surface defects will be reproduced on the finished item. Master patterns made of porous material such as wood, plaster or stone can be sealed with a sealant. If the surface is then polished with a release wax, a high gloss will be imparted on to the silicone mould. Other release agents that can be used are petroleum jelly and polyvinyl alcohol.

A Simple Open Mould

A mould box is used to fit around the master shape. For a custom built square planter it can be made from wood or cardboard or similar stiff material. Any gaps between the floor of the mould box and the master shape should be sealed with Newplast or Newclay. The area under the master shape will eventually be the hole through which the resin is applied to the inside of the finished mould. The mould box should then be coated with a fine layer of petroleum jelly or mould wax.

The silicone rubber in liquid form is activated by the addition of a curing agent which is stirred in to the silicone by hand using a flat stirrer. A disposable vessel is used for this. Extreme care should be taken not to stir in any air, although stirring must be thorough to ensure a complete mix of the two components. It is recommended that the material is “degassed” after mixing by using a vacuum chamber, as it is very difficult to remove air by any other method. If a vacuum chamber is not available paint over the surface of the master pattern with a thin coat first of all to obtain an air bubble free layer. Working time for the mixed silicone is around 30-40 minutes at room temperature.

Allow to stand for approximately 10 minutes before pouring the remaining silicone in to the mould. Hold the container as low as possible and pour in a thin stream. Leave to cure for 24 hours before removing the mould.

Making the product

A release agent is applied to the inside of the silicone mould to stop the fibreglass from sticking to it. For simple moulds the surface is coated with PVA (polyvinyl alcohol) release liquid. For larger or complex moulds, a two stage parting compound of non silicone wax and PVA is generally used.

A special resin called gelcoat is always used as the outermost layer in fibreglass moulding. This un-reinforced resin coating is at least 0.5mm thick and provides a smooth, glossy, protective and decorative layer between the glass fibre and outside moisture. It is often coloured with a pigment paste. At least 4 hours curing time must be allowed to prevent the solvent in the next layer of resin from wrinkling the gelcoat.

When the gelcoat has cured, it is coated with lay-up resin. The first layer of chopped strand matting made of fine glass fibres is applied. In order to fully impregnate the glass fibre with resin, a paint roller or brush is used, stippling the brush over the fabric to avoid dislocating the glass strands with the sticky, resin coated bristles. This is known as consolidating. The process is repeated to build up a strong layer of GRP (glass reinforced plastic).

When sufficiently cured, the moulding will be slightly shrink away from the sides of the mould, this process is assisted by dissolving the blue release agent with water poured along the edges. The final product is now finished.

Repeat this process thirty or so times until the order for the plant display maintenance contractor is ready!