Making a brush on silicone mould

What is a brush on mould

With a poured mould you place the original inside a mould box and fill it with liquid silicone. With a brush on mould you apply thickened silicone directly onto the original using a brush or spatula, in several thin layers, until you reach a wall thickness of roughly five to ten millimetres. The mould then follows the contour of the object.

Because a brush on mould is only a thin skin, it almost always needs a support shell to hold its shape. This support shell, also called a mother mould, is made from plaster, polyurethane, epoxy with glass fibre or rigid PU foam.

When to choose a brush on mould and when to pour

Which silicone to choose for a brush on mould

For a brush on mould you use an RTV 2 silicone rubber that you thicken yourself with a thixotropic additive, or a silicone that is already spreadable from the factory. Both work, but the choice between condensation and addition cure determines whether your project succeeds.

Thicken it yourself or buy ready to spread

Thickening it yourself gives you full control over the viscosity of each layer and is more economical. A ready to spread silicone saves time and gives a consistent result, which is helpful on large surfaces and for on site work where time is short.

• Silicone C 20 set, universal condensation silicone
• Thixo C, thixotropic additive for condensation silicone
• Ready to spread silicones
• Fillers, primers and additives

What you need

Lay everything out before you start. The working time of a mixed batch is limited, often no more than twenty to sixty minutes, and during that time you cannot walk away to look for something.

Materials

• Silicone rubber in the right hardness, usually Shore A 20 to 30 for moulds
• Thixotropic additive to make the silicone spreadable
• Release agent suited to the original
• Sealing lacquer or sealer for porous or chemically active substrates
• Silicone pigment to colour each layer differently and verify mixing
• Reinforcement scrim or fibre for large or heavily loaded moulds

Tools

• Digital scale accurate to at least one tenth of a gram
• Mixing cups in several sizes, generously dimensioned
• Spatulas and stirring sticks with a flat tip to scrape the bottom and sides
• Brushes, including a fine brush for corners and detail
• Mould box material for the support shell and for any edges
• Vacuum chamber to degas the contact layer, optional but highly effective
• Gloves and other personal protective equipment

For the support shell

• Plaster with hessian or glass fibre for a classic mother mould
• Laminating epoxy with glass fibre for a light and very rigid shell
• PU foam for a fast and lightweight support shell
• Acrylic resin as an alternative to plaster, stronger and lighter

Preparing the original

Ninety percent of failed moulds trace back to preparation. The silicone reproduces every detail, which includes dust, fingerprints, scratches and leftover release agent.

1. Clean and degrease. Remove dust, grease, old wax and loose particles. Let the original dry completely.
2. Repair. Fill damage and unwanted imperfections. Whatever you do not repair now will repeat itself in every casting.
3. Assess undercuts. Decide in advance how you will get the mould off the original. For a closed shape, plan a cut line or a split in the support shell.
4. Seal where needed. Porous and chemically active substrates get a sealer or lacquer first. Let it cure completely.
5. Apply release agent. A thin and even film. Too much release agent clogs fine detail and can pool in recesses.
6. Run a drop test. Apply a small amount of mixed silicone to an inconspicuous spot and let it cure fully. If it stays tacky, the substrate is unsuitable or not properly sealed.

Which substrate needs which preparation

The layer build up

A good brush on mould consists of at least two and often three functional layers. Each layer has its own job and its own viscosity. Never mix all the silicone you need for the entire mould in one go, because the first batch will have cured before you reach the final layer.

Layer 1, the contact layer

The contact layer determines the quality of the mould. This layer is barely thickened, roughly zero to one drop of thixotropic additive per hundred grams, so that it flows out thinly and picks up every detail. Aim for a film of about half a millimetre.

• Apply with the tip of the brush and stipple it into the surface
• Work fine detail and internal corners first, then the flat areas
• Do not brush back and forth, because that folds air into the layer
• Pop visible bubbles immediately with a needle and smooth the spot
• Degas the mixed silicone under vacuum first if you can, which is only possible while the silicone is not yet thickened

Layer 2, the build up layer

The build up layer is clearly thickened so that it stays put and gives the mould thickness. Apply this layer as soon as the contact layer is tacky, meaning sticky but no longer liquid. Within that window the layers bond chemically into one body.

• Apply in thin passes of two to three millimetres
• Work from the bottom upwards and always in the same direction
• Press the silicone on with the spatula instead of dropping blobs
• Do not tear the contact layer, work with controlled pressure
• Use a different pigment for this layer so you can see exactly where you have not been yet

Layer 3, the finishing layer

The finishing layer brings the mould to its final wall thickness and gives an even surface for the support shell to sit against. On large or heavily loaded moulds you embed a reinforcement scrim or fibre mat into the still wet silicone at this stage and spread a final layer over it.

Vertical walls and internal corners

This is where most brush on moulds fail. On a flat horizontal object almost any technique works. On a vertical wall, inside a tub shape or in a deep internal corner, only the correct handling works.

Why it goes wrong there

On a vertical wall the thin contact layer sags under its own weight. Thin or open spots appear at the top and material accumulates at the bottom. Lay the thickened build up layer over that and you trap air between the two layers, precisely where the mould is most visible.

In internal corners, where the floor meets the side wall, air is at its most persistent. The brush never truly reaches the corner, the silicone bridges over it and an air channel is left behind.

How to do it correctly

1. Apply the contact layer twice on vertical sections. First a wafer thin layer, wait three to five minutes so the silicone starts to grab, then apply a second thin layer. This way the layer stays put without any thickening.
2. Stipple the internal corners first. Use a fine brush with the tip and push the silicone into the corner with a dabbing motion. Only then draw the layer out towards the flat areas.
3. Reduce the internal corner if you can. A sharp ninety degree corner is always a problem zone. If you can add a small fillet to the original, the problem largely disappears.
4. Work bottom to top on the build up layer. Material that sags then lands on an already filled zone instead of an empty one.
5. Press on, do not scoop. A blob of silicone wiped off the spatula almost always carries air with it. Place the spatula flat and draw it out.
6. Build in thin passes. Two to three millimetres per pass. Several thin layers are stronger and more bubble free than one thick layer.
7. Check with raking light. Shine a lamp across the surface. Trapped air shows as a slight bump or a matt patch.

Undercuts

A brush on mould tolerates more undercut than a poured mould because the thin silicone skin is flexible. There is still a limit. For deep undercuts, plan a cut line in the mould and a split support shell so you can open the mould when demoulding. Make the cut in a zigzag rather than straight, so the mould closes back together precisely.

Preventing air bubbles, the ten main causes

1. Air introduced during mixing. Stir calmly and with a scraping action, not whipping or at high speed. A mixing cup with a generous base helps.
2. Thickened silicone can no longer degas. As soon as you add thixotropic additive, trapped air stays put. So degas before thickening, or deliberately keep the contact layer virtually unthickened.
3. No rest period after thickening. Let the mix stand for one to two minutes so larger bubbles can rise, then stir once more calmly.
4. Contact layer applied too thickly. Above one millimetre bubbles can no longer escape to the surface. Keep to half a millimetre.
5. Brushing back and forth. Every return stroke folds air under the layer. Work in one direction and stipple in detailed areas.
6. Internal corners bridged instead of filled. Dab the corner separately before working on.
7. Sagging on vertical walls. See the previous section, apply the contact layer there in two passes.
8. Working on the previous layer too late. A fully cured and dry layer produces inclusions at the transition. Stay within the tacky window.
9. Porous substrate outgassing. Plaster, concrete and wood release air as soon as the silicone sits on them. Sealing solves this.
10. Working too cold. Below eighteen degrees the silicone is thicker and holds on to air. Bring both the silicone and the original up to room temperature.

Thickening with a thixotropic additive

A thixotropic additive makes silicone non sagging without materially changing the properties of the cured rubber. The silicone becomes thin as soon as you move it and thick as soon as it rests. That lets you apply it to a vertical wall without it running off.

Procedure

1. Weigh out component A and component B and mix them fully first, including the bottom and sides of the cup.
2. Only then add the thixotropic additive, drop by drop or in small portions.
3. Mix vigorously until the mass is homogeneous.
4. Let the mix rest for one to two minutes so the additive can work and air can rise.
5. Stir once more calmly and start applying.

Dosage guidance

The practical test: the silicone is thickened enough when you can apply it about one centimetre thick to a vertical wall with a spatula without it sagging within a minute.

The support shell or mother mould

A brush on mould five to ten millimetres thick has no dimensional stability of its own. Without a support shell it sags, deforms while filling and your castings will not be dimensionally accurate. Only on very small and shallow moulds can you leave the support shell out.

When you definitely need one

• On any mould larger than roughly twenty centimetres
• On tub shapes, trays and anything with high side walls
• For heavy castings such as concrete, plaster or filled resins
• On moulds that are used frequently or need to be stored

Procedure

1. Let the silicone mould cure fully before making the support shell.
2. Apply a release agent to the outside of the silicone mould, certainly with plaster and epoxy.
3. Decide on the split. For a closed or deep shape, make the support shell in two or more parts using a dividing wall of clay or sheet material.
4. Create registration keys, meaning recesses or bosses on the flanges, so the parts can only fit back together one way.
5. Build the support shell in your chosen material combination, for example plaster with hessian or epoxy with glass fibre.
6. Give the support shell a flat base or feet so the mould stands stable while casting.

Support shell material choice

Curing, demoulding and aftercare

Conditions

Work at eighteen to twenty five degrees and at a relative humidity of forty to seventy percent. Condensation silicones need atmospheric moisture to cure. In a dry and cold room the cure takes considerably longer than the stated guide time, especially in the core of a thicker layer.

When to demould

Guide times in product documentation apply to ideal conditions and to a single poured layer. In a brush on mould you build several layers, and the bottom layer was applied first but cures through last. So deliberately allow extra time.

• Wait at least twenty four hours before taking the mould off the original
• At low temperature, low humidity or with a thick build up, wait forty eight hours
• Full final strength and tear resistance is only reached after three to seven days
• Do not load the mould heavily in the first days and do not yet cast heavy or strongly exothermic materials

Demoulding

1. Remove the support shell first, in the correct order of the split.
2. Ease the edge of the silicone mould loose carefully and work evenly all around.
3. Never pull at a single point. A brush on mould is thin and tears under point loading.
4. Blow air between the mould and the original if needed to break the vacuum.

Aftercare and storage

• Clean the mould with soap and water or with isopropanol and let it dry completely
• Put the mould back into its support shell for storage, that is the best protection against deformation
• Store dark, dry and free from pressure, and never place anything heavy on the mould
• Use a release agent suited to the casting material on every cast, this extends mould life considerably

Troubleshooting

Tips from our specialists

• Use a different pigment for each layer. You immediately see where you have not been and whether your wall thickness is even.
• Mix a separate batch for each layer and calculate it generously. Half a batch short in the middle of a layer costs you the entire tacky window.
• Always weigh, never measure by volume. A scale accurate to a tenth of a gram is one of the cheapest insurances in this craft.
• Test on a sample piece whenever you use a new substrate or a new release agent. Half a gram of silicone costs almost nothing, a failed mould costs a day.
• Work with two brushes, a fine one for detail and corners and a wider one for the flat areas.
• Have the support shell materials ready before you start. On a large mould you want to make it straight after the final layer.
• Record the mix ratio, the dosage and the ambient temperature for each project. On the next mould you will know exactly what to adjust.

Frequently asked questions

Can any silicone be thickened until spreadable

Virtually any RTV 2 silicone can be thickened, provided you use the correct additive for the correct type. Use a thixotropic additive suited to condensation silicone for condensation cure, and a suitable additive or fumed silica for addition cure. Ask us if you are unsure about a specific product.

How much silicone do I need for a brush on mould

Calculate the surface area of the original in square centimetres and multiply it by the desired wall thickness in centimetres. That gives the volume in cubic centimetres. Multiply that by roughly 1.1 to 1.2 for density and add twenty percent for losses. For a tub of thirty by twenty by ten centimetres with an eight millimetre wall you arrive at approximately one and a half kilograms.

Can I make a brush on mould in a single layer

Technically yes, but the result is almost always worse. A thick thickened layer applied straight onto the original traps air and gives a dull and inaccurate mould surface. The separate contact layer and build up layer are not a luxury, they are the core of the technique.

Why does my silicone not cure against the original

With addition silicone this is almost always inhibition, caused by sulphur, tin, latex, styrene or residues of certain adhesives and modelling clays. With condensation silicone the cause is usually an inaccurate mix ratio, incomplete mixing or a very dry environment.

How long does a brush on mould last

That depends on the casting material and on the use of release agent. With plaster or acrylic resin a condensation mould typically yields several dozen to well over a hundred castings. With polyester or polyurethane the mould wears faster due to chemical loading. A platinum mould lasts considerably longer.

Can a damaged brush on mould be repaired

Yes. Clean the damaged area, abrade it lightly and apply fresh silicone of the same type. For reliable adhesion to fully cured silicone, use a silicone adhesive or a primer. A tear running through the full wall thickness can be repaired but will remain a weak point.

Is a brush on mould suitable for food contact

Only when you use a silicone certified for that purpose, which in practice means addition cure silicones. Condensation silicones are not suitable for food contact. Contact us if you want to make a mould for foodstuffs and we will advise on the correct product and the accompanying documentation.

Related information

• Guide, one part poured or spatula mould of a stone strip
• Silicone selection guide, find the right product for your application
• Information centre with all technical documentation
• Overview of all applications