Working with silicone: bonding, air bubbles, cutting, colouring and release

The practical questions that come up most often when working with pourable and spreadable silicones. Looking for an explanation of the difference between addition and condensation cure, of Shore hardness, or of why silicone won't cure? Those are covered as separate topics in the Information Centre.

Bonding silicone

Silicone is difficult to bond, certainly with standard adhesives. You'll get the best results with a condensation silicone: the 2-component condensation silicones or a 1-component silicone adhesive. Often an ordinary construction silicone sealant will even do the job.

Condensation silicones bond well to themselves and to many substrates. If it doesn't hold, use a primer.

Addition silicones do not bond well to themselves. Your best chance is to cut away a thin layer at the contact face, exposing the grainy, fresh structure of the cured silicone. You can then use this primer on it. Many bonds only hold after a few days, and best after curing at raised temperature.

In a hurry? The silicone adhesive set combines a cyanoacrylate glue with a special primer for silicone. It cures quickly and bonds well, but hardens the silicone slightly at the contact point, reducing flexibility there.

Air bubbles in my mould after pouring

Processed correctly, there should be no or almost no air bubbles. Four causes and their fix:

1. Too much B component. Above 2 percent the silicone cures too fast and the bubbles get no time to escape. For condensation silicones, 1.5 percent B is a good solution: longer demould time, more time for the bubbles. The weights matter fairly precisely; with a dosing syringe you can measure grams accurately (1 ml equals 1 gram here). Addition silicones you slow down the other way round, by cooling the components slightly before use.
2. Too much air beaten in while mixing. Don't whisk, stir.
3. Poured from too low. Pour from around 50 cm in a thin stream. The last bubbles burst during the fall.
4. Want certainty? First pour a thin layer over the model and go over it with a brush. Once no bubbles are visible, fill the rest of the mould.

Will silicone stick to my clay model?

A model of untreated or unvarnished clay, dried or fired, will indeed cause release problems. Use a release agent, or a household solution: petroleum jelly, beeswax, furniture wax or colourless shoe polish. That seals the pores of the clay.

To apply the wax layer as thinly as possible: thin the wax with turpentine and rub the mixture in with a brush or cloth. The turpentine evaporates and leaves a wafer-thin layer of wax.

Cutting silicone

Cutting is straightforward if you follow a few rules:

• Always use a sharp utility knife.
• Preferably take a long knife, meaning a long cutting edge.
• Don't saw; draw the blade through to the required depth in one pass.
• If you need a second pass, follow the existing cut at its deepest point, again without sawing.
• Have a second person hold the cut open. That makes cutting easier and the previous cut easier to follow.
• Where possible, cut in a sawtooth pattern. The mould then closes neatly later and you get less of a seam.
• If the cut stops inside the mould rather than running all the way through, let it end in a rounded shape. That stops the cut from tearing further.

Which silicone can I cut into?

Cutting into a mould is best avoided, because the risk of it tearing through increases. But sometimes it is the simplest way to release a complex model properly.

If you're going to cut and won't be processing food in the mould, choose Condensation Silicone 20. It has superior tear strength. Pour or spread the areas you'll cut later slightly thicker than usual — that adds strength. Make a zigzag cut where possible, so the mould closes properly every time. If needed you can put a rubber band around the mould — not too tight, or it will distort.

Removing flashing and membranes

The best mould is one without flashing. Fix the master model to the surface and seal every crack underneath very carefully with plasticine so no silicone can run beneath it. It takes time, but the result is immediately professional and reworking is often no longer needed.

If that isn't possible everywhere, the membrane has to go:

• Always use a sharp utility blade.
• Hold the membrane with one hand and pull gently, cut with the other and follow the shape.
• Never pull too hard. The mould stretches, and on release you often find you've cut too far.
• Alternative: slide a finger under the membrane, press lightly from above with the blade and pull on the membrane. It tears along the blade. This technique takes practice.

In both cases, take care not to cut yourself.

Colouring and painting silicone

Silicone is virtually impossible to paint with ordinary paint. It only works by abrading the skin and spraying on coloured silicone, for instance dissolved in thinner. In the SFX world, make-up mixed with silicone sealant is also used.

Silicone does colour well, though. Use silicone colourant or satin oil-based paint. Note: always test colourants from outside our shop first. Their solvent power varies, and constituents in paint can inhibit curing or block it entirely. Oil-based paint also lowers the hardness considerably.

Finally: silicone that is naturally white gives a paler, pastel-like colour result than transparent silicone.

Is a release agent necessary?

Often not. Run a simple test beforehand, for example on the underside of the model.

Materials that almost always do need a release agent: dry, fired or untreated clay, cardboard, untreated wood, fabric, paper and other dry or porous materials.

Besides the release agents from the shop, a layer of petroleum jelly, beeswax, furniture wax or colourless shoe polish works well. Thin with turpentine for the thinnest possible layer.

Writing on silicone

Nothing writes on silicone. There is however a way to mark a mould:

Before you pour, write the name or number of the model in mirror writing inside the casting box with a waterproof marker, at the spot where the text should appear. After demoulding the text stands proud in the silicone. It is indelible and dishwasher-safe.

This works well with condensation silicones and high-temperature silicones. With Siliconen Culinair (addition cure) it works less well.

Pouring silicone into a silicone mould

Apply a release layer first. Rub the mould with petroleum jelly, for example, and buff it out with a cloth. Repeat that three times for a good release.

A good silicone for moulds like this is Wacker Elastosil Vario 15 Normal.

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