Having a bespoke open section made is a different exercise from buying from a catalogue. Whether the section is designed for roll forming, whether material and coating suit the service environment, how critical dimensions are defined and how the hole pattern relates to assembly all feed directly into tooling cost and into dimensional reliability in series production.
This guide covers what to prepare before approaching a roll former, the design rules worth observing, and how a project runs from technical review to series production. The aim is to start with a complete technical package and keep revision loops to a minimum.
When a custom section makes sense
Standard C, U and Z sections cover many applications. A custom section usually pays off when an integrated channel, interlock or stiffener can replace separate parts; when a hot-rolled section would be heavier than the load requires; or when the same section will be used in high, repeating volumes across a product family.
Thin-walled cold-formed members behave differently from hot-rolled sections: local buckling, distortional buckling and cross-section deformation govern the design. Load-bearing profiles are therefore sized to EN 1993-1-3 (Eurocode 3, cold-formed members and sheeting). Structural design remains the designer's responsibility; the manufacturer contributes engineering support on manufacturability, material and coating.
Design rules for roll forming
Constant thickness, constant section. Roll forming produces a section that does not change along the length. A local change of thickness or a form that varies along the profile needs a separate operation or a different process.
Inside bend radius. The inside radius is generally specified at no less than the material thickness. Larger radii reduce the risk of cracking and coating damage on high-strength grades and coated steel. State clearly on the drawing whether a radius is inside or outside.
Flange and lip proportions. Very narrow flanges are unstable in forming; very wide unsupported flanges are prone to local buckling. A lip on the free edge improves both load capacity and forming stability.
Symmetry. Symmetrical sections form more evenly. Asymmetric sections such as Z purlins have a greater tendency to twist and camber; this is compensated in the flower pattern and straightening stands, but the designer should be aware of it.
Hole position. Holes close to a bend zone ovalise or move during forming. Leave enough distance between the hole edge and the start of the bend; the required distance grows with thickness and radius. For punched holes, a common rule is that the hole diameter should not be smaller than the material thickness.
Developed width. The developed width of the section sets the strip width to be slit from coil. A small dimensional adjustment at design stage can land on a standard strip width and reduce material cost.
Material and coating selection
In roll forming the material is fed from coil and, for coated grades, is coated before forming. The choice follows from load-bearing requirements, the corrosivity of the service environment and any downstream processing. Corrosivity is classified to ISO 9223 (C1 to CX); coating mass is specified with designations such as Z275 or ZM310, giving the total for both faces in g/m².
For load-bearing profiles, use structural grades with a guaranteed yield strength (S250GD, S350GD, S450GD); the number in the grade is the minimum yield strength in MPa. Higher strength reaches the same capacity at a thinner gauge, at the price of more springback and a larger minimum inside radius. See ZM coated sheet and galvanized sheet for more on coated steels.
| Material | Standard | Typical use | Note |
|---|---|---|---|
| DX51D+Z galvanized | EN 10346 | Non-structural, forming-led profiles | Good formability; use a structural grade for load-bearing design |
| S250GD / S350GD / S450GD +Z | EN 10346 | Purlins, solar mounting profiles, racking and load-bearing members | Guaranteed yield; springback rises with strength |
| Zn-Al-Mg (+ZM) coated | EN 10346 | Coastal, agricultural, high-humidity and aggressive outdoor sites | Better protection at cut edges and hole walls |
| Uncoated hot-rolled | EN 10025-2 | Indoor use, parts to be painted or galvanized afterwards | No coating; protection applied after forming |
Tolerances: what to specify, and how tightly
EN 10162 provides a general framework for cross-section dimensions, straightness, twist and length of roll-formed profiles. Referencing it for dimensions without an individual tolerance gives both parties a clear basis. Dimensions that are genuinely critical to assembly should be flagged separately and, where necessary, toleranced more tightly.
Tight tolerances on every dimension add cost and make production harder; in most sections only a few dimensions are functionally critical. The table shows what each type of dimension depends on and how to specify it.
| Dimension type | What it depends on | How to specify it |
|---|---|---|
| Section dimensions (depth, flange, lip) | Tooling design, thickness and strength | Flag the critical ones; reference EN 10162 for the rest |
| Bend angle | Springback, material batch | Specify on faces that mate with other parts |
| Straightness (bow, camber) | Straightener setting, section symmetry | As a deviation over a defined measuring length |
| Twist | Section asymmetry, residual stress | As an angle or deviation over the profile length |
| Length | Cut-off system, measuring reference | Nominal length and tolerance; temperature matters on long profiles |
| Hole pitch | Feed and measuring system | Hole-to-hole and cumulative (from datum end) tolerances separately |
Hole patterns and in-line processing
Holes, slots and notches are applied on the line before or after forming. Pre-punching on flat strip keeps punch tooling simple, but the holes can be influenced by subsequent bending. Post-punching on the formed section gives better positional accuracy but needs section-specific punching tools. The right choice depends on hole position, tolerance and section geometry.
Using slots rather than round holes to absorb site tolerances is common practice. The dimensioning datum matters too: dimensioning every hole from one end controls cumulative error, while chain dimensioning lets error accumulate step by step. On long profiles, agree the total deviation along the length as well as hole-to-hole spacing.
Each hole pattern needs its own punch tooling, so more patterns mean more tooling cost. A pattern that repeats at a fixed pitch across different lengths lets a whole product family run on one set of tools.
What to send for a quote
A complete technical package sets both the speed and the accuracy of the quote. An ideal request contains:
Section drawing: DXF, DWG or PDF with all dimensions, thickness, inside/outside radii and critical dimensions flagged. Material and coating: grade and coating mass (for example S350GD+ZM310), or a description of the service environment. Length and tolerance: a single length or a cut list? Hole pattern: positions of holes and slots, the dimensioning datum and tolerances. Volume: the length for this order and the estimated annual length. Application: where and with which mating parts the profile will be used. Packing and delivery: bundle weight, packing requirements and delivery address.
The application matters more than buyers often expect. Knowing what the profile does lets the manufacturer propose a more economical section, a different gauge or a better-suited coating.
Project steps: from technical review to series production
Technical review. The drawing is assessed for roll-formability, number of stations, strip width and possible improvements. Small geometry changes at this stage can make a marked difference to tooling cost and to dimensional stability in production.
Flower pattern and tooling design. Bends are distributed across stations, springback compensation is built in, and the roll set and any punching tools are designed and machined. Because we design and make tooling in-house, this step is markedly shorter than with outsourced tooling, and section revisions can be turned round quickly.
First article. The line is set up, the first profiles are produced and measured against the drawing. Results are shared with the customer, and series production does not start without approval.
Series production. Repeatable runs on approved tooling and settings, with punching and cut-to-length on the line. Our roll forming lines have a capacity of more than 2,500 tonnes per month.
Packing and shipping. Profiles are bundled by section and length and strapped with steel banding; loading is planned around profile length.
Decisions that drive cost
Most of the cost of a roll-formed profile is material weight, so thickness and developed width are the strongest levers. Moving to a higher-strength grade to reduce gauge can lower weight per metre and therefore cost. Coating type and coating mass also affect material price: more coating than the environment needs is wasted money, while too little shortens service life.
On the tooling side, the drivers are the number and depth of bends (and hence stations), the number of hole patterns and any need for post-punching. On the production side, volume, the variety of lengths and packing requirements affect unit cost. Standardising sections and hole patterns across a product family reduces the number of tool sets and set-ups, and with them total cost.
Shipping, storage and repeat orders
Coated profiles stacked in damp, unventilated conditions can develop white rust. Storing bundles off the ground, in a dry and ventilated area and with a slight slope reduces the risk. For export, profile length should be considered together with the container or truck loading plan.
Once the tooling exists, repeat orders carry no tooling time or cost; the same section is reproduced on approved settings. Sharing the annual requirement in advance allows material purchasing and production planning to follow it. Send your section drawing and project details through the quote form, or see our capabilities on the roll forming manufacturer page.