Choosing a Precast Mould: Which Mould for Which Element?

In a precast plant, mould selection follows the production scenario more than the shape of the element. The same beam can be produced with three different mould types; which one is right depends on daily output, surface requirements and the crane and steam infrastructure on site.

Five inputs needed for the decision

Daily / annual output — how many cycles the mould will run
Element section and length — load and mould length
Surface requirement — exposed concrete or to be rendered
Curing method — natural, steam or covered
Plant layout — fixed line or open yard, and crane capacity

Discussing mould type before these five are clear is premature.

Common solutions by element type

Beam moulds

For rectangular, I, L and LT sections, a mould with hinged side panels and a fixed bed is standard. When the side panel is laid outward the element is lifted straight out by crane; the mould itself is never dismantled. Where the section varies, side panels are made extendable with filler pieces.

I beam — mould at the bed and top flange, with an internal form creating the web void
L beam — edge axis beam; bed projecting on one side
LT beam — double projection; the slab seating face is held to tight tolerance in the mould

Column moulds

Columns are generally produced in a horizontal mould: three of the four faces formed, one face open. Corbel and socket details are added as removable inserts. Where corbel positions vary by project, adjustable corbel boxes are placed along the bed so one mould produces several column types.

Slab and wall panel moulds

There are two routes for flat elements:

	Tilting table	Battery mould
Layout	Horizontal, large area	Vertical, small footprint
Area use	Takes much space	6–12 panels in the same area
Surface	One face smooth	Both faces smooth
Curing	Heating under the table	Steam between panels — fast
Investment	Lower	Higher

Where high volumes of panels smooth on both faces are produced, the battery mould is the most area-efficient solution.

Site elements

For culverts, barriers, kerbs, gratings, channels and sleepers, the decisive factor is cycle speed. These moulds typically:

Work as an outer form plus an inner core
Are given a slight taper (0.5°–2°) to ease stripping
Keep lifting anchors and pick-up points fixed in the mould

Tetrapods and special geometry

For three-dimensional, non-symmetrical elements the mould is designed in segments, each released by its own latch. The critical point is that segments seat in exactly the same position every cycle; otherwise element dimensions drift.

Repetitions and mould economics

There are roughly three bands:

Below 500 cycles — light steel sections, simple stripping, moderate cost
500–2,000 cycles — reinforced panels, thickened wear faces, serviceable hinges and latches
Above 2,000 cycles — replaceable contact face, high-strength steel, spare parts list

When unit cost over the mould's life is calculated, what matters is not the purchase price but the cost per cycle. The saving from a cheaper mould disappears when it starts marking the surface at the 300th pour.

What is done in the mould for surface quality

Flatness of the contact plate — panels are straightened after welding to counter weld shrinkage
Tightness of joints — concrete leakage marks the surface and builds up in the mould
Corner chamfers and radii — a sharp corner tires the mould and makes the element brittle
Vibrator mounting points — mounted vibrator positions are determined by calculation, not welded at random

Mould to suit the plant, or plant to suit the mould

In a new plant the correct order is: product first, then the line, then the mould. Once element quantity and length are known the line layout follows; once the line layout is known, where the mould sits, where the crane lifts from and where the steam line arrives are all known. A mould designed with that information wastes neither space nor time in the plant.

In short

In precast mould selection, the element section is only the starting point. Once output, surface requirements and plant layout are known, the right mould type becomes evident. If that information is not yet in hand, the first step is not the mould but working out the production scenario.