The rough edges need to be sanded off fiberglass molds.Jupiterimages/Photos.com/Getty Images
Concrete planter pots are strong, waterproof and ideal for outdoor use but concrete is a fluid until set, so to make concrete planter pots you will need a mold. Fiberglass molds are lightweight, strong and can be used to form a variety of simple and complex shapes, including planter pots. You can use fiberglass molds to make concrete planter pots cheaply and efficiently, both at home and on an industrial scale.
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Fiberglass provides several benefits over other materials used to make molds. Fiberglass is strong, it can be re-used several times instead of only once or twice with traditional materials like clay, which can crack when the form is removed from them. The strength of fiberglass means it can be sanded, drilled, and bolted as needed without damaging the overall structure. It is also lightweight, so it can be handled easily.
Fiberglass has a relatively long curing time, so it is unsuitable for very quick mold production. The resin in the fiberglass will take up to 24 hours to set completely, before which time the mold is unsuitable for use in case it deforms during the mold-making process. Once the mold is set, it must also be sanded down using a high-grit sandpaper to remove any irregularities on the surface.
Although fiberglass is lightweight compared to most materials used in mold making, the materials being molded, such as concrete for planter pots, can weigh a lot. Fiberglass resin bends when it is sufficiently heavy and the molds will buckle under their own weight. It is advisable to reinforce molds for concrete planter pots using pieces of wood attached with metal braces, to avoid warping due to the weight of concrete.
Fiberglass warps under heat. While this is not a problem when molding most materials, it is a concern when making concrete planter pots. Concrete sets through an exothermic reaction, meaning it gets warm and emits heat for several hours while it sets. As a result, expect your fiberglass molds to warp slightly after the first use, and need replacing after two or three sessions.