Can I apply silicone as a separating layer inside a rigid mould

You can, and it is a proven technique. You apply a thin layer of liquid silicone to the inside of a rigid mould, for example a printed plastic mould, a plaster mould or a polyester mould. Once cured, that layer acts as a flexible inner mould with a rigid backing, and the casting releases just as easily as it would from an ordinary silicone mould. Note that this is not a release agent but a permanent inner layer, so you are converting the mould.

Choose an addition curing silicone for this, also known as platinum silicone. It cures without by products, shows virtually no shrinkage and is available in grades carrying a declaration for food contact. Condensation silicones are less suitable, because they contain a tin catalyst, release alcohol while curing and usually contain non reactive silicone oil that is slowly leached out of the rubber by fats and solvents. A hardness of 30 to 40 Shore A works best, because very soft grades are more vulnerable in repeated use and wear faster under abrasive casting materials. Also choose a long working time so you can work without pressure.

Always test for inhibition first. Addition curing silicones will not cure on contact with sulphur, nitrogen compounds, amines, metal salts and latex, and those substances occur in some filaments, resins, lacquers, adhesives and gloves. Apply a coin sized blob to a reject piece of the mould and let it cure fully. If the surface stays tacky, the substrate is unsuitable and must be sealed first.

Work horizontally wherever possible. Place the mould section with the cavity facing up, pour in a small amount and tilt the part until everything is covered. Gravity levels the layer, so you avoid brush marks and trap far less air. Only on steeply overhanging or vertical surfaces should you apply the first layer with a brush. Work in two layers, first a thin print coat that you rub well into the surface and then a second layer up to the required thickness. Apply that second layer within the open time stated in the technical data sheet, so both layers bond chemically. Keep screw holes, vents and parting faces clear.

Plan for a layer thickness of 1,5 mm, with 1 mm as the absolute minimum. Anything thinner will tear in repeated use, particularly along the edges of mould sections. For quantity, allow roughly 1,6 kg of silicone per square metre at 1,5 mm.

Do not thin the silicone with solvent, this causes shrinkage and residue, and do not thin with silicone oil either, since it does not cure along with the rubber and migrates into the casting. Thickening is possible with a thixotropic additive from the same manufacturer, but bear in mind that any addition voids the supplier declaration of conformity for that mixture. Working horizontally usually makes thickening unnecessary.

Silicone does not bond by itself to plastic, plaster or metal. For lasting adhesion, degrease the surface with isopropanol and apply a silicone primer. Alternatively, drill small holes of 2 to 3 mm into the mould so the silicone anchors mechanically. Pay particular attention to the edges, since edge lifting is the most common failure point on moulds that are dismantled every time. Let the silicone run a few millimetres over the edge, or anchor it precisely there.

Where food contact is involved, bear in mind that a declaration under BfR and FDA only applies if you carry out the prescribed post treatment. This is usually two to three hours of post curing at 100 to 140 degrees Celsius to remove residual volatiles. Many rigid moulds will not withstand that temperature, and printed plastic moulds deform well below it. If you cannot post cure according to the data sheet, you cannot regard the silicone as food safe, even though the material itself is certified. You can then cure and post cure the inner layer separately, choose a mould material that withstands the temperature, or limit the application to non edible castings. In all cases, wash the layer with warm water and a neutral detergent, rinse thoroughly and let it dry completely.

Finally, consider the service life. Silicone swells slightly in fats, oils and solvents. Under repeated contact with warm fats or resins, a thin layer gradually absorbs material, softens and may lift at the edges. Treat a silicone inner layer as a part to be replaced in due course, not as a permanent finish.

Read more: which substrates should I seal before applying silicone