Why silicone moulds leave a seam line, and what to do about it

Almost every message we get about a disappointing cast blames the resin. In practice the cause is usually the mould, the room temperature or the release — three things that are easier to fix than they sound.

The seam line

Silicone moulds are made in two parts, and a faint raised line where those parts met is normal on every mould at every price. It is removed after demoulding with 400-grit wet sanding followed by 800, then polished. A mould that arrives with a visible parting line is not defective.

Cloudy or tacky surfaces

• Cloudy through the piece — moisture. Bhopal humidity in the monsoon is enough; work in a dry room and keep the mould dry.
• Tacky on top — the resin is under-cured, usually because the room is too cool or the mix ratio was rushed. Follow the maker’s instructions on the pack.
• Tacky only at the edges — the mould was flexed while curing. Cast on a rigid board.

Storing a mould so it lasts

Store flat, out of sunlight, dust-free. A mould that arrives slightly curved from a flat-packed parcel relaxes at room temperature within a day. Do not use craft moulds for food unless the product page states food-grade use.

What we sell and what we do not

We stock the moulds, the coaster forms and the finishing pieces. We do not sell epoxy resin or hardener, so buy those from a chemical supplier and follow their handling instructions — gloves, ventilation, and no shortcuts on the mix ratio.