On a bill of materials, a flat spring clip is often the cheapest line item – a strip of steel, two bends and a hole. It stays that way until the first production lot arrives and half of the parts crack at the bend, while the other half deliver a clamping force a third lower than the design called for. Neither problem shows up on the drawing. Both are decided by choices that are made, or skipped, before the tool is built.
Strip does not behave like wire
Wire is isotropic across its section. Bend it in any direction and it responds the same way. Strip does not.
Cold rolling aligns the structure of the material along the direction in which the strip travels through the mill, so the properties measured along the strip and across it differ in a way that can be measured, not just observed. For a designer used to wire forms, this is the first and most important adjustment.
The practical rule that follows is simple: the bend line should run across the rolling direction. Bending parallel to it raises the risk of cracking on the outer side of the bend, and with strip in the hardened condition it can cause a crack almost immediately.
When the shape of the part forces bends in two perpendicular directions, say so in the documentation and allow a larger radius for the bend that falls unfavourably. The manufacturer will then lay out the blanks on the strip with the rolling direction in mind. What matters is that the supplier knows this is a functional requirement and not an accident of how the drawing was made. A buyer comparing offers from several countries should expect a competent supplier to ask about it; if nobody does, raise the point yourself.
Minimum bend radius depends on the condition of the material
The bend radius is not chosen for appearance. It follows from the thickness and hardness of the strip.
Strip in the soft condition, intended for forming and subsequent heat treatment, tolerates radii close to its own thickness. Strip delivered already hardened and tempered needs radii many times larger, because its capacity for plastic deformation is limited. A radius that works perfectly on soft material can be the direct cause of failures on hardened stock of the same grade.
A bend that is too tight produces a defect that is often invisible at incoming inspection: microcracks on the outer surface of the bend. The part passes every dimensional check, then breaks after a few tens of thousands of cycles at a point where nobody thought to look for the cause. For an OEM, that usually surfaces as a field complaint rather than a rejected delivery, which makes it far more expensive.
Springback and how the tool compensates for it
When the press opens, the part partially returns towards its previous shape. With strip this effect is stronger than with wire, because the layers of material on both sides of the neutral axis work on a longer lever arm.
The amount of springback grows with the yield strength of the material and with the bend radius. Hardened and tempered strip springs back noticeably more than soft strip, and the difference between two lots of the same grade can be large enough to require the settings to be corrected.
The tool is therefore designed to overbend, and where tolerances are tight an additional restriking step is added in the last station. For the project schedule this has one consequence worth planning for: after the die is built, there is still a phase of adjusting the tool on the first parts. That is a normal stage of launching a stamped part, not a delay, and it is better to have it in the timeline from the start than to discover it in the week the samples were promised.
Flatness and residual stress in the strip
Strip comes off a coil and remembers being wound. Blanking adds residual stresses of its own around the cut contour. The result is that a part cut from strip that looks perfectly flat can bow slightly once it is released from the tool.
For parts where flatness has a function – contact washers, pressure plates, sealing elements – the flatness requirement needs to be given as a number, together with the measurement datum. Without it, inspection comes down to laying the part on a surface plate and judging by eye, and that is not an acceptance criterion anyone can defend in a dispute between customer and supplier.
Material: two families of standards
Most flat springs and clips are made from one of two groups of strip, each covered by its own European standard.
| Group | Standard | Typical grades | When to use |
|---|---|---|---|
| Spring steel strip | EN 10132-4 | C67S, C75S | high forces, dry interior, protective coating |
| Stainless spring strip | EN 10151 | austenitic and precipitation hardening grades | humidity, washing, steam, food contact |
EN 10132-4 covers cold rolled narrow strip intended for heat treatment, in thicknesses up to 6 mm. EN 10151 describes stainless strip for springs up to 3 mm thick and less than 600 mm wide, intended for parts exposed to corrosion and moderately elevated temperatures.
Choosing between the two is not only a question of strength. More often the deciding factor is the working environment, and whether the part can be coated at all in the place where it works under friction. Citing the standard and grade on the drawing, rather than a generic description such as “spring steel”, also makes quotations from different suppliers directly comparable.
Delivery condition: form soft or form hard
There are two routes, and they lead to different tools.
The first starts with soft strip. The part is blanked and bent, and only then hardened and tempered as a finished component. This route gives freedom of shape and allows small radii, but heat treatment introduces distortion that has to be anticipated in the tool.
The second uses strip already delivered in the hardened and tempered condition, formed straight to final shape. It is dimensionally much more stable and carries no risk of distortion, but it limits the geometry and needs larger radii.
For flat parts with simple bends, the second route is usually cheaper and more repeatable. For three-dimensional shapes with tight radii, the first one wins. This is one of the decisions best made together with the manufacturer before the design is frozen, because it determines the tool – and changing it later means changing the tool.
Progressive dies and the break-even point
At low volumes the part is made in several operations on separate stations: blanking, bending, embossing. Tooling cost is low and the unit price is high, because labour and handling of the part between several production stations dominate the cost.
At high volumes a progressive die is built. The strip passes through successive stations and leaves the press as a finished part. The unit price drops many times over, but the die is an investment spread over the volume, and it requires a stable product design. Every change of geometry after launch means reworking the die, and in extreme cases the whole set of tooling.
Where exactly the break-even point lies depends on the complexity of the part. The conversation about it should take place before the first quotation, not after. For buyers sourcing from abroad, this is also where the supplier’s in-house capabilities matter: our own CNC toolroom allows die corrections to be made on site, which shortens the launch cycle and lowers the cost of design changes during ramp-up.
Edges and burrs
Blanking always leaves an edge with two different characters: a smooth shear zone and a fracture zone with a burr on the side where the punch exits. This is not a defect. It is a property of the process.
The problem starts when the part comes into contact with a seal, a cable, a paint finish or the hand of an assembly worker. The burr side and its permitted size then have to be stated on the drawing, using the edge designations of EN ISO 13715. A note saying “no burrs” cannot be met as written, and in practice it means an extra deburring operation that nobody has priced.
Coatings and finish
After hardening, spring strip parts have high strength, so the same precautions apply to them as to springs made from wire. Electrolytically applied coatings carry a risk of hydrogen embrittlement and require a de-embrittlement bake carried out soon after plating.
On parts that work under friction – clips squeezed at every assembly, snap fasteners, guides – the coating will be worn away in the contact zone anyway. In such cases a stainless grade can be cheaper over the whole life cycle than carbon steel with a coating, despite the higher price of the material. It is a calculation to run before the material is fixed, not after the first corrosion complaint.
Where these parts work
The largest user is the household appliance sector: snap fasteners and clips in housings, contact pressure plates, spring guides for drawers, fixing elements for heaters and sensors, cable clamps. Volumes run to hundreds of thousands of pieces a year, with requirements for repeatable clamping force and resistance to moisture and cleaning agents.
We make them as part of our steel strip components service, together with secondary operations and heat treatment. A large share goes to the household appliance industry, as well as to construction and lighting manufacturers, where the part has to be cheap, repeatable and resistant to an environment in which it works for well over a decade.
What to include in your enquiry
Beyond the drawing or 3D model, a complete enquiry for a strip part contains:
- strip grade and delivery condition, with the standard
- direction of the bends relative to the rolling direction
- required force at a given deflection, with tolerance
- permitted burr and the side it may be on
- corrosion requirement expressed in hours of salt spray testing
- annual volume, single batch size and the date of the first delivery
With that information on the table, the supplier can propose a route and a tool concept in the first reply instead of the third.
Designing a flat spring, clip or clamp made from strip? Send us your enquiry – we will help you choose the solution and propose the material and process that match your volume.
