Fibre Reinforced Facade Encyclopedia: Manufacturing
CNC formwork is a mould machined by a computer numerically controlled router or milling machine directly from the project’s 3D model, rather than built by hand from drawings. It is the standard way of producing moulds for double-curved, folded and freeform facade panels in glass fibre reinforced concrete (GRC, GFRC), fibre reinforced polymer and glass fibre reinforced gypsum, because the geometry that is hardest to build by hand costs a CNC machine no more than a flat sheet.
A 3 by 1.5 metre double-curved mould that would take a patternmaker a week to loft and fair by hand is typically milled from expanded polystyrene in 6 to 14 machine hours, then coated and finished in a further day.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Common mould materials | EPS and XPS foam, polyurethane tooling board, MDF, laminated timber | Chosen by the reuse count required |
| Machine type | 3-axis for 2.5D relief; 5-axis for undercuts and steep walls | 5-axis avoids stepping on near-vertical faces |
| Typical stepover, finish pass | 0.5 to 3 mm | Smaller stepover, smoother face, longer machine time |
| Achievable mould tolerance | about plus or minus 1 mm over a 3 m mould | |
| Reuse, coated EPS foam | 1 to 10 casts | Lowest cost per mould, highest cost per panel at volume |
| Reuse, sealed MDF or timber | 10 to 40 casts | Moisture sensitive; edges fail first |
| Reuse, polyurethane tooling board | 30 to 100 casts | Dimensionally stable, machines cleanly, expensive stock |
| Reuse, GRP tool taken off a CNC master | 100 to 200 casts | Master milled once, production tools laminated from it |
| Source geometry | Rhino, Grasshopper, Revit or CATIA surfaces to CAM toolpath | Surfaces must be watertight and correctly oriented |
The digital workflow
The chain runs from the architect’s model to the machine without a manual re-draw, and each step is a place where geometry can be lost. The architect supplies a surface model, most often Rhino or Revit. The fabricator rebuilds the panel breakdown on that surface, adding what the model does not carry: joint widths, returns, rebates, drainage, anchor positions and the draft angle needed to demould. That becomes the shop model, signed off through the shop drawing process before any material is cut. The mould is then derived from it, as the external face of the panel for sprayed GRC, or as a female tool carrying the gel coat gloss for a laminated FRP panel.
CAM software then generates toolpaths: a roughing pass at a coarse stepdown, then finishing passes at a fine stepover with a ball nose cutter. Stepover is the dominant variable. A 0.5 mm stepover leaves a face needing almost no filling and takes four to six times as long to machine as a 3 mm stepover that will be skimmed and sanded. The right answer depends on whether the mould is producing an off-form concrete finish or a gel coat, and on how many panels it serves.
Mould materials and what each is for
Expanded and extruded polystyrene is the workhorse for one-off and low-repetition freeform geometry: cheap, fast to machine, glued up from blocks to almost any size and light enough to handle without plant. It is also soft, so the face must be coated and the mould will not survive many demouldings. Polyurethane tooling board is the opposite case, dimensionally stable and crisp-edged, but expensive enough to be reserved for production tools and masters.
MDF is the standard for prismatic and single-curved shapes with moderate repetition: clean to machine and inexpensive, but it swells if water reaches the core, so edges and face must be sealed before a wet process. Laminated timber suits large ribbed moulds that must be robust on the shop floor. For high volumes the usual pattern is to mill a master once, take a glass fibre reinforced polyester tool off it, and run production off the GRP tool.
Whatever the substrate, the machined face is coated and sealed: foam takes a sprayed polyurea, epoxy or polyester skim that is sanded and polished, MDF a sealer and a release-compatible topcoat. The coating makes the face non-porous, resistant to the alkaline GRC slurry or the styrene in a polyester resin, and able to strip cleanly. A mould machined well and coated badly will produce poor panels. See mould-making and demoulding.
Tolerance and quality control
Machine positioning accuracy is better than the tolerance that actually matters, which is the accuracy of the mould after coating, filling and sanding, and after several demouldings. Foam distorts as it is worked and as it takes heat from an exothermic laminate; MDF moves with moisture. Production practice is therefore to verify the finished mould against the model rather than trust the machine, by laser scanning or by profile templates cut from the same model.
Because the mould is the sole source of the panel geometry, mould tolerance and panel tolerance are not independent: the panel is the mould plus process variation from spraying, compaction and curing, plus any spring-in on demoulding. Where a facade carries a tight joint tolerance, the chain from setting-out to substructure to panel to mould has to be tolerance-budgeted as a whole. First-off inspection is the usual control, with the first panel off a new mould measured, checked for finish and often presented as a visual mock-up before production is released. See tolerances, quality-control and mock-up.
When CNC is cheaper, and when it is not
A CNC mould is justified by geometry complexity and repetition count, not by default. Machining carries fixed costs a simple mould does not recover: modelling the mould geometry, preparing toolpaths, block stock, machine hours, then coating and finishing, which is hand labour CNC does not remove. For a flat rectangular panel, a joiner with a sheet of MDF and a table saw beats any router.
The case strengthens on three axes. Geometric complexity: a double-curved, twisted or freeform surface cannot be built accurately by hand at any sensible cost, so there CNC is not the cheaper option but the only one. Variation: when every panel differs, the machine reads a different file and charges roughly the same for each, while hand mould making charges again in full for every unique shape. Precision: where panels interlock across a curved surface with a narrow joint, the repeatability of a machined mould removes a class of site problems.
Against that, high repetition of a simple shape favours a durable hand-built or steel mould, because the tooling cost is amortised across many casts. The decision is best framed as cost per panel: mould cost divided by the casts the mould will survive, plus the labour of each cast. That is why a project with 300 identical panels and one with 300 unique panels reach opposite conclusions from the same numbers.
Rationalisation sits on top of all of it. Reducing a freeform facade from 300 unique panels to 40 mould families, by repeating modules, mirroring, or making one mould adjustable with removable inserts, cuts the tooling line item further than any change of mould material. That work belongs in early design alongside parametric-design and panel-size decisions.
Standards and references
- AS 3610 and AS 3610.1 Formwork for concrete, for surface finish classes and documentation
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products, for mould requirements and demoulding
- BS EN 1169 Factory production control of GRC
- ISO 2768 general tolerances, commonly referenced for machined tooling
- ISO 10303 (STEP) and IGES for CAD geometry exchange to CAM
- ACI 347 Guide to Formwork for Concrete (US)
Related terms
Questions specifiers ask
What materials are CNC moulds made from?
Most often expanded or extruded polystyrene for one-off freeform shapes, MDF for prismatic and single-curved work, polyurethane tooling board for production tools, and laminated timber for large robust moulds. High-volume work mills a master and takes glass fibre reinforced polyester production tools off it.
How many panels can one CNC mould produce?
A coated foam mould typically gives 1 to 10 casts, sealed MDF 10 to 40, polyurethane tooling board 30 to 100, and a GRP tool taken off a CNC master 100 to 200, depending on the process, the release regime and handling.
Is a CNC mould cheaper than a hand-built one?
Not always. CNC wins on complex, curved or one-off-per-panel geometry, and where precision matters. A repetitive flat or simple prismatic panel is usually cheaper with a hand-built or steel mould, because the tooling cost is spread over many casts.
How accurate is a CNC mould?
Machine accuracy is better than the practical tolerance, which is governed by coating, hand finishing and how the mould behaves over repeated demoulding. Typical delivered mould accuracy is around plus or minus 1 mm over a 3 metre mould.
Can a CNC mould handle undercuts?
Only with 5-axis machining, or by splitting the mould into pieces that strip separately. Undercuts and insufficient draft angle are the usual reason a designed geometry cannot be demoulded, and they are resolved in the shop model before any material is cut.
Have a freeform facade and no idea what the tooling will cost? Send us the Rhino or Revit geometry and we will assess mould families, reuse counts and rationalisation options. Send us your geometry
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.