Why a Steel Formwork Diagram Is a Manufacturing Drawing, Not a Picture
A steel formwork diagram for a precast mould is a fabrication drawing that tells the workshop how to cut, weld, and assemble the tool. It is not a 3D render for a catalogue. When you request a diagram from a factory, you should receive a set of views that include panel thickness, rib layout, stiffener positions, bolt hole diameters, and the stripping sequence. If you only get a shaded image with overall dimensions, you cannot verify whether the mould will survive 60–100 reuses or distort after 20 pours.
This article explains how to read a steel formwork diagram and use it to choose the right mould for your precast plant. We will cover panel thickness, rib spacing, weld details, tolerance stack-up, and the arithmetic behind cost per reuse. The goal is to help you avoid a mould that looks fine on paper but fails on the casting bed.
Key Elements of a Steel Formwork Diagram
A complete diagram for a precast steel mould should contain at least five views: plan, elevation, section, detail views, and a bill of materials. The plan view shows the mould cavity and the outside frame. The elevation shows the side panels and any attached vibrators. The section view reveals the internal ribs and stiffeners. Detail views call out weld sizes and bolt connections. The bill of materials lists every plate, rib, and fastener with material grade and quantity.
For a standard wall panel mould, the diagram will specify Q235 carbon steel plates. The side panels are typically 3–6 mm thick, while the base plate may be 6–8 mm. Ribs are welded on the back of the panels to resist concrete pressure. The diagram should show rib spacing, usually between 300 mm and 500 mm on centre. If the spacing is wider than 600 mm, the panel will deflect under a pour rate of 2 m/hr.
Look for the stripping sequence on the diagram. It should indicate which bolts to loosen first and in what order. For a typical L-shaped mould, the sequence might be: remove the top tie rods, then the side bolts, then use the built-in jacking screws to separate the panel from the concrete. Without this sequence, workers may pry the mould and damage the edges.
Panel Thickness and Rib Spacing: The Deflection Arithmetic
Concrete exerts hydrostatic pressure. For a pour rate of 2 m/hr and a temperature of 20°C, the pressure at the bottom of a 3 m high mould can reach 50 kN/m². A 4 mm steel panel with ribs at 400 mm centres will deflect about 1.5 mm under this load. A 3 mm panel with ribs at 600 mm centres will deflect over 4 mm, which exceeds the ±0.5 mm tolerance required for many precast elements.
Therefore, the diagram must match the pour rate. If your plant pours at 3 m/hr, you need either thicker panels or closer rib spacing. The table below compares three common configurations for a 3 m high wall mould.
| Configuration | Panel Thickness | Rib Spacing | Max Pour Rate | Estimated Deflection | Reuse Expectancy |
|---|---|---|---|---|---|
| Light | 3 mm | 600 mm | 1.5 m/hr | 4.2 mm | 60 reuses |
| Standard | 4 mm | 400 mm | 2.5 m/hr | 1.8 mm | 80 reuses |
| Heavy-duty | 6 mm | 300 mm | 4.0 m/hr | 0.9 mm | 120+ reuses |
Note that the heavy-duty option costs about 30–40% more than the standard, but it can double the reuse count. If you produce 500 panels per year, the extra cost is recovered in less than 18 months through reduced mould replacement.
Welding Details and Bolt Torque in the Diagram
The diagram should specify weld sizes and types. For Q235 steel, fillet welds of 4–6 mm are common for ribs and stiffeners. Full-penetration welds are used at the corners of the mould cavity to prevent concrete slurry leakage. The diagram must show weld symbols according to ISO 2553 or AWS A2.4. If the welds are not specified, the workshop may use undersized welds that crack after 10–15 stripping cycles.
Bolt connections are equally important. The diagram should list bolt diameter, grade, and torque. For M16 bolts of grade 8.8, the recommended torque is 190 Nm. For M20, it is 370 Nm. Under-torqued bolts allow the mould to open during pouring, causing bulging. Over-torqued bolts can strip threads or yield the steel. The diagram should also show bolt hole clearance: typically 1–2 mm larger than the bolt diameter to allow for alignment.
When you review a diagram, check that the bolt holes are not placed in areas of high stress. If a bolt hole is too close to a weld, it can initiate a crack. A minimum distance of 3 times the plate thickness from any weld toe is a good rule.
Stripping Sequence and Draft Angles
The stripping sequence is the order in which the mould is disassembled. A good diagram will show this with numbered steps. For a typical column mould, the sequence is: (1) remove the top cover, (2) loosen the side bolts, (3) use the jacking screws to separate the side panels, (4) lift the mould off the concrete. If the diagram does not show jacking screws, the workers may use a crane to pull the mould, which can damage the concrete.
Draft angles are also critical. For a vertical mould, a draft of 1–2 mm per 100 mm of height is often used to ease stripping. This means a 3 m high mould would have a 30–60 mm taper. The diagram should show this taper clearly. Without it, the concrete may bind and cause edge spalling.
For complex shapes, the diagram may include a stripping simulation. This is a series of drawings showing the mould opening in stages. It helps the workshop understand which parts must move first. If your mould has undercuts, the diagram must show how the mould segments separate to avoid locking.
How to Use the Diagram to Compare Suppliers
When you request quotes from different steel formwork factories, ask for a preliminary diagram. A supplier who provides a detailed diagram with panel thickness, rib spacing, and weld details is likely to deliver a better mould than one who only sends a price. Use the diagram to compare the following:
- Panel thickness and rib spacing: Thicker panels and closer ribs mean higher cost but better tolerance and longer life.
- Weld specifications: Look for weld sizes and whether full-penetration welds are used at critical joints.
- Bolt grade and torque: Grade 8.8 bolts are standard; grade 10.9 may be needed for high-pressure applications.
- Stripping sequence: A clear sequence reduces labour and damage.
- Tolerance callouts: The diagram should state ±0.5 mm for critical dimensions.
Also check the bill of materials for material grade. Some suppliers may substitute Q195 for Q235 to cut costs. Q195 has lower yield strength (195 MPa vs 235 MPa) and will deflect more under load. The diagram should explicitly state Q235.
Common Mistakes in Steel Formwork Diagrams
Even experienced buyers miss these details. Here are the most common mistakes and how to avoid them:
- Missing rib layout: Without rib spacing, you cannot calculate deflection. Always ask for a section view that shows ribs.
- No stripping sequence: This leads to improvised stripping and damaged moulds. Insist on a numbered sequence.
- Vague weld symbols: "Weld all around" is not enough. Specify fillet size and length.
- Ignoring bolt torque: The diagram should list torque values. If not, ask for them.
- No tolerance table: Critical dimensions should have tolerances. A general note of ±0.5 mm is acceptable for most precast.
If a supplier cannot provide these details, consider it a red flag. A reputable factory will have standardised diagram templates and can produce a custom drawing within 3–5 working days.
Cost, Lead Time, and When to Choose Heavy-Duty
The cost of a steel formwork mould depends on its size, complexity, and panel thickness. As a typical market range, a standard wall panel mould might cost USD 1,200–2,500 per tonne FOB Tianjin. A heavy-duty mould with 6 mm panels and 300 mm rib spacing could be USD 1,800–3,200 per tonne. These are not quotations; they are ranges to help you budget.
Lead time is 7–10 working days for stock moulds and 15–25 working days for customised formwork after deposit. Payment is 30% deposit by T/T, balance against copy of B/L. Documents include CI, PL, MTC, CO, and FORM E. There is no MOQ.
When should you choose heavy-duty? If your production volume exceeds 300 panels per year, or if you pour at rates above 3 m/hr, the heavy-duty option is more economical. For low-volume production (50–100 panels per year) and slow pours, a standard mould is sufficient. The diagram will help you decide: if the calculated deflection exceeds 2 mm, go heavy-duty.
Remember that the diagram is a contract document. Once approved, it defines what the factory will build. Take the time to review it with your production team. A few hours spent checking rib spacing and stripping sequence can save weeks of downtime and thousands of dollars in rework.