Moulding processes
Five moulding routes, weighted the way construction work actually falls: hand-made and cast moulds first, then vacuum casting, compression, injection. Plus draft, backing shells, release agents and taking a mould off a building.
Start here
It gets brushed on by hand, poured, poured under vacuum, pressed, or injected. Every concrete mould, formliner, GRC mould and cast stone tool we make comes out of one of those five routes, and for construction work the first two account for the great majority of it.
Printing, CNC machining and laser cutting come before all of that. They are how the pattern, the pattern plate and the backing frame get made. Useful, used daily, and not the product.
The rest of this page is the detail a precast yard manager or a conservation architect asks about on the phone. If you want the material side instead, that is on silicone and polyurethane mould materials for concrete.
From the architect's detail to the first pour
A formliner enquiry usually arrives as an elevation and a texture reference. This is what happens between that and a panel coming out of your bed.
Scroll sideways to see the whole diagram.
The shape of a liner programme
We do not quote lead times on a website. What is useful before you ring is the shape of the programme: where the time actually sits, and which stages wait on you.
Scroll sideways to see the whole diagram.
The five moulding routes
Read the first two closely if you cast concrete. The last two matter if the moulded component itself is your product.
A hand-made mould is built up in layers. Silicone is brushed or spatulated onto the pattern, a first thin coat to pick up the texture, then thicker coats with an open-weave reinforcement worked in where the mould will be stretched. It is slow. It is also the only route available when the pattern will not lie flat, cannot be dammed, or cannot be moved at all.
For architectural restoration moulds this is the default. A weathered corbel on a listed elevation stays exactly where it is while the mould is built on it. Same for string courses, tracery, capitals, carved relief. On big work it is often the only sensible answer even in the workshop, because a two-metre panel pattern would need an uneconomic volume of silicone to block-mould.
The other advantage is local control. We can put 8 mm of rubber where the mould has to peel out of an undercut and keep it thinner where it needs to fold. A hand-made mould nearly always wants a rigid jacket over it, because on its own it will not hold shape under a concrete pour.
Most formliners and most precast unit moulds are cast. The pattern goes face-up in a dammed frame, the silicone is mixed and poured in a single controlled stream from one corner so the material walks across the face and pushes air ahead of it, then it cures and the mould comes off as one piece with a flat back ready to be bonded to a backing shell.
This route gives the most repeatable face of the five. Thickness is even, the back is flat and predictable, and a matched set of liners for one elevation can be poured off one pattern so every panel on the building reads the same.
We also cast the part rather than the mould where that is the job. A one-off replacement unit in resin or a cast-stone sample for approval can be poured here, which helps when a conservation officer needs something in their hand before signing off an elevation.
Vacuum casting is the same pour with a step in front of it. The mixed silicone goes into a chamber and the entrained air is pulled out before it goes anywhere near the pattern. It costs a little more time per mould.
The face decides if it is worth that. Rough split-stone texture hides everything, so we would not charge you for it. Cast lettering, a company logo in a spandrel panel, fine ashlar joints, a sharp arris: on those, one pinhole in the mould face is one blemish on every unit for the life of the tool, and somebody has to make it good by hand every time.
Compression moulding presses solid silicone or rubber into a heated steel tool. The material is weighed as a preform, loaded, closed under pressure and cured with heat. That gives a dense, abrasion-resistant part with properties a poured mould cannot reach.
In construction this is where the moulded item itself is the product. Rubber moulds for concrete paving and kerb production, tile production tooling, gaskets, bearing pads, fixings, trims. Abrasion resistance drives the specification here, and the tool cost only makes sense against a repeat quantity.
Injection is the volume end. Liquid silicone rubber is metered, mixed, then injected into a closed heated tool, with rubber injection for the harder compounds. Cavity count is set against your annual number rather than against a catalogue.
For a construction customer this normally arrives late in the relationship, once a component has proved itself on site and the volume justifies a hard tool. At that point the work usually moves across to TCI Products, our volume silicone and rubber moulding division.
Comparing the five routes
Bands rather than figures, because the honest numbers depend on your mix and your yard.
Moulding routes for concrete and architectural casting
| Route | Typical construction use | Tooling cost | Pulls it suits | Face detail | Where it struggles |
|---|---|---|---|---|---|
| Hand-made moulds | In-situ restoration moulding, large panel patterns, deep relief, vertical faces | Low to medium | Low to medium | High, limited mainly by the original surface | Thickness varies by hand, so a jacket is essential and a matched set takes longer |
| Cast moulds | Formliners, precast unit moulds, cast stone, matched liner sets for an elevation | Medium | Medium to high | High and very repeatable across a set | Pattern must lie flat and be dammed, so it is a bench route rather than a site one |
| Vacuum casting | Lettered and logo faces, fine ashlar, sharp arrises, approval samples | Medium | Medium to high | Highest of the poured routes, effectively bubble free | Chamber size caps the mould size, and it adds nothing on coarse texture |
| Compression moulding | Rubber moulds for kerb and paving production, tile tooling, gaskets, pads | High, a steel tool | High | Medium. Dense and abrasion resistant rather than fine | Needs a repeat quantity to justify the tool, and undercut is limited |
| Injection moulding | Moulded construction components at repeat annual volume, including LSR | Highest, a hard tool | Highest | Medium to high, consistent part to part | Only sensible once the component design has stopped changing |
Cost, pull-count and detail columns are indicative bands rather than measurements, given for comparison between routes. The Shore hardness figures used elsewhere on this site are indicative too. Your own figures are confirmed in writing with the quotation, against your mix and release regime.
How the pattern and the tool get made
None of this is a service we sell on its own. It is how the thing you do buy gets built.
A pattern is whatever the mould is taken from. Sometimes it is the original stone. Sometimes it is a timber and plaster bench pattern. Increasingly it is printed: resin on the SLA and DLP machines for carved relief and repeating texture tiles, filament where the pattern is large and the surface will be worked afterwards anyway.
Printing changed restoration work more than anything else we do. A scan of a weathered carving can be repaired digitally and the corrected geometry printed as a pattern before any silicone is mixed.
Flat repeat work runs off a machined pattern plate. CNC gives the flatness and the edge quality a cast mould needs, and it repeats exactly if a second plate is wanted later.
Backing shells, ribs, frames plus handling jigs are cut on the laser and assembled here. Ribbing is set by span and relief depth rather than by habit. This part of the job is invisible in the quotation and decides whether panel forty matches panel one.
Before a mould leaves, the moulded profile is checked against the drawing. Material, hardness, batch, backing detail and joint positions are recorded against your order and the pattern is retained.
That record is the reason a replacement liner two years later is a replacement rather than a fresh argument about matching. It is also what lets us diagnose a problem over the phone, because we can see what you were sent.
Backing and stiffness
This is the most common specification error we are asked to fix, and the most expensive, because the failure is gradual. A deep formliner or a GRC mould with nothing rigid behind it bows under the pour or moves under a roller. Nobody notices at panel three. By panel thirty the elevation has a problem.
So the backing gets engineered with the face. Plywood for flat repeat liners, welded steel where the span demands it, a ribbed resin and glassfibre backer for curved surfaces.
Draft, undercut, architectural detail
Less than a rigid one. The limit is where the rubber tears rather than where it binds.
A flexible mould peels off the set unit rather than sliding off it, so the drafted side walls a GRP or steel mould needs are often unnecessary. Board marking, split stone and carved relief keep their full depth instead of being shallowed out to suit the tool.
Deep undercut is exactly where silicone earns its price. What matters is the rubber behind the undercut: a thin neck of material will split there eventually, sooner if the unit is lifted out instead of the mould being flexed off it.
On a unit with one awkward feature we will run a softer face around that feature and keep a harder material, indicatively 40 to 60 Shore A, across the flat area that has to hold dimension. It costs a little more and it saves the tool.
Sharp external arrises wear first and are worth a conversation before the pattern is cut. Air has to escape at the top of any deep pocket, or the panel arrives with a blowhole in the same place every time.
Release agents and surface defects
Four failures cover nearly every complaint that reaches us about a concrete mould.
Almost always an incompatible release agent. Petroleum-based and chemically reactive agents swell a silicone face, which then picks up and tears. Send us the product name and the wash-down routine and we will check compatibility.
Cure inhibition. Sulphur-bearing modelling clay used to build a dam, latex gloves, old release residue on a pattern, uncured polyester or epoxy resin in a backing shell. The symptom is a soft sticky skin only where the silicone touched the contaminated surface.
Usually air with nowhere to go, or a face with pinholes of its own. Venting and pour sequence fix the first. Vacuum casting the mould face fixes the second, which is why a lettered panel is worth degassing and a rough stone texture is not.
Alkaline residue left sitting on the face, aggressive cleaning, or plain abrasion from the aggregate. Fine texture goes first while the liner still looks serviceable, which is why storage guidance goes out with every mould.
Restoration
The brief on a listed elevation is almost always the same. Match the original, do not damage it, and preferably do not remove it. In-situ hand-made moulding does all three, and it is the reason a conservation architect rings a mould shop rather than a stonemason first.
The sequence: survey and access, then a patch test on an inconspicuous area, then consolidation if the surface is friable, then damming and support, then the mould built up in layers with a jacket over it. Everything comes off in one lift. Access windows and scaffold arrangements change the method more than the carving does, so tell us those early.
Choosing a route
If it can, a cast mould is usually cheaper and more repeatable. If it cannot, because it is on a building or it is too large or too fragile, the answer is a hand-made mould and the rest of the specification follows from that.
Coarse texture takes a straight pour. Lettering, fine ashlar and sharp detail justify vacuum casting the mould so a pinhole does not print into every unit you make.
A mould to cast concrete in sits in the hand-made and cast half of this page. A moulded rubber or silicone component goes to compression or injection.
One replacement corbel and a rolling annual programme of facade panels are tooled differently. Give us the annual number and we will price the tool against it.
Frequently asked
We will not put a headline cast count on it, because the honest answer is a range that depends on your yard. A cast-in platinum-cure formliner running flat panels on a covered bed, released without a reactive agent and washed gently, lasts a long time. Take the same liner, add table vibration, steam curing, a petroleum-based release agent and outdoor stacking, and it can be finished in a fraction of that. We record the material and hardness against your order and give you an expected life against your own specification, in writing, with the assumptions listed.
Hand-made, almost always. Brush-applied silicone can be built up on a vertical face, over a carving that cannot be moved, or around detail that a poured block mould could never reach. It also lets us vary thickness, putting material where the mould needs to stretch and keeping it thin where it has to fold back. A cast mould is the better answer only once the original can come to the bench.
The pour is the same. Vacuum casting degasses the mixed silicone in a chamber first, so air that would otherwise sit against the pattern face comes out before it can print. On a rough split-stone texture you will never see the difference. On a lettered spandrel panel, a sharp arris or fine ashlar, one pinhole in the mould face becomes one blemish on every single unit, which is a hand-finishing cost repeated for the life of the tool.
A shallow, small liner sometimes can. Deep relief and anything over a modest span cannot. Silicone is a face material and it has no useful stiffness of its own, so the pour pushes it out of shape and the panel geometry drifts. We design the backing shell at the same time as the face: plywood for flat repeat work, welded steel where span or loading demands it, plus resin and glassfibre with ribs for curved surfaces. One detail catches people out. A resin backer has to be fully cured before a platinum-cure face is cast against it, or the face will not cure where it touches.
Less than you would for a rigid mould, and often none. That is the main reason to use silicone. The limit is not draft but tear: a deep undercut with a thin neck of rubber behind it will eventually split there, and it splits sooner if the panel is being lifted rather than the mould flexed. So we look at pull direction and local thickness, sometimes running a softer material just around the undercut and a harder material across the rest of the face.
Both. In-situ moulding takes a mould off the building itself, with a patch test first because old paint, consolidant or release residue will all inhibit platinum-cure silicone. Working from a legacy mould is also routine, but there is a one-way door to plan around: tin-cure silicone will cure against a platinum-cure surface, and platinum-cure will not reliably cure against tin-cure. Tell us which you have and we will sequence it, sometimes through an intermediate pattern.
One group, four specialisms
The same engineering team runs all five moulding routes for every division, so a release problem solved on a bakery tool can end up fixing a precast bed.
Not sure which route fits?
Send a drawing, an elevation, a photograph of the original or a sample panel, and we will come back with a route, a material, a backing approach and a price.
