Moulds & Formwork

The idea is a clean inversion of everything so far. Instead of milling your object directly, you mill its negative - a cavity in a block, shaped so that its hollow is exactly the form you want in reverse. Then you pour a liquid or plastic material into that cavity, let it cure (set hard), and release a positive: the cast, which is the precise mirror of the milled void. What was subtractive (removing material to make the cavity) becomes the means to a formative end (shaping poured material without cutting it).

Moulds are commonly milled into machinable wax, tooling board (a dense modelling foam), MDF sealed with resin, or machinable resins and metals for durable production tooling. The mould material is chosen for the casting material and the number of casts: a soft foam mould might survive a handful of plaster pulls, while a sealed hardwood or metal mould can produce many concrete casts. This is also where the ball-nose finishing skills of the previous lessons pay off directly - the smoothness of the cast is only ever as good as the smoothness of the milled cavity. Every tool mark in the mould appears, mirrored, on every part you cast from it. A mould, in other words, is a milled part where finish quality matters more than usual, because it is inherited by everything that comes after.

A mould is worthless if the cured part will not come out of it, and this is where beginners are caught. Two things make release possible. The first is draft angle: every wall of the cavity must taper slightly - a few degrees is typical - so the part loosens the instant it starts to lift, instead of gripping the walls all the way up. A cavity with perfectly vertical walls acts like a piston in a cylinder; the cast wedges and will not budge, and forcing it cracks the part or the mould. Designing draft into the geometry from the start is fundamental moldmaking discipline, and it is exactly why the indexed and undercut ideas from the multi-axis lesson matter: a mould must have no undercuts in the pull direction, or the part physically cannot leave (unless you use a flexible mould or a multi-part mould that splits open).

The second is the release agent: a wax, oil, spray or film applied to the cavity before pouring, so the cured material does not bond to the mould. Concrete, plaster and resin all stick to bare surfaces given the chance; a release agent, plus a well-sealed non-porous mould surface, lets the cast part company cleanly. Get draft and release right and demoulding is a satisfying, repeatable pop; get them wrong and you destroy the cast, the mould, or both. The honest rule of moldmaking: no draft, no release, no part. Design the exit before you design the shape.

A single rigid milled mould is perfect for a shape with draft and no undercuts - but real forms are not always so obedient, and there are three standard escapes. The first is the flexible mould: pour a silicone or polyurethane rubber against a CNC-milled master (the positive), let it cure, and peel off a stretchy negative that can flex over mild undercuts to release a part the rigid version would trap. Here the mill makes the master, not the working mould - a common and powerful division of labour. The second is the multi-part mould: split the negative into two or more rigid pieces along carefully-chosen parting lines, so the pieces clamp together for the pour then open to free a part with genuine undercuts. The mill cuts each half; you register them with locating pins so they close accurately.

The third escape is to rethink the geometry so a simple mould works: add draft, remove the undercut, or split the object itself into castable pieces you assemble afterwards. Which route to take is a judgement about quantity, material and detail - silicone for fine detail and short runs, multi-part rigid moulds for many concrete casts, redesign when the undercut was not essential. And moulds combine happily with the rest of the course: a laser-cut or 3D-printed master can be moulded and cast; a milled mould can produce a master that is then silicone-moulded for production. The through-line is always the same file-to-factory logic - only now the machine makes the tool that makes the part, and the design question becomes not just what shape you want, but how it will let go.

Different casting materials suit different jobs, and each behaves differently in the mould. Plaster (and gypsum products) is the studio favourite: cheap, fast-setting, low-hazard, and superb for detail, which makes it ideal for mouldings, cornices, prototypes and study models - though it is brittle and not for structural or outdoor use. Concrete (including fine mixes and glass-fibre-reinforced concrete, GRC) is the architectural heavyweight: it casts facade panels, worktops, cladding, paving and precast structural elements, faithfully reproducing a milled texture at building scale, and it is where CNC-milled formwork earns its keep on real projects. Resin (polyurethane or epoxy) cures hard and tough, captures the finest detail, and can be pigmented or filled to imitate stone, metal or timber - excellent for fittings, decorative elements and durable prototypes, but more hazardous and more expensive, demanding ventilation and care.

Beyond the material there is the craft of the pour: mixing to the right consistency, and above all dealing with air bubbles, which spoil a surface. Vibrating the mould, pouring slowly in a thin stream, or drawing air out under vacuum all help the liquid fill every corner of the cavity and release trapped air. Curing takes time - minutes for plaster, hours to days for concrete to reach handling strength - and rushing it wrecks the cast. The mould defines the geometry; the pour and cure define whether that geometry comes out clean.

The reason all this effort pays is repetition. The expensive, precise, digital work is spent once, milling the mould; after that, each additional cast costs only material, a little labour and time. Mill one mould, pull fifty identical plaster capitals or fifty GRC facade panels - the per-part cost collapses. This is precisely the mass-customization logic of the whole course, applied to casting: and because the mould itself is CNC-milled from a file, you can even mill a family of related-but-different moulds nearly as easily as one, so a facade of subtly varied panels becomes producible. Digital fabrication does not only make one-offs; it makes affordable, controlled repetition of bespoke forms.

At the largest scale this becomes formwork - the moulds that shape poured concrete in construction. Traditionally formwork is built by hand from timber and ply, which limits concrete to flat and simple shapes. CNC-milled formwork (in foam, timber or tooling board) unlocks complex, curved, patterned and double-curved concrete that hand-built forms could never achieve - the sculpted concrete surfaces of much contemporary architecture exist because a machine cut the mould. The same one-mould-many-casts thinking scales up: reuse a milled form across many pours, or mill a set of forms for a repeating structural bay. As always, structural concrete design and its formwork must be signed off by qualified engineers - but the geometry those forms can deliver is now bounded mainly by what you can mill, not by what a carpenter can build.