Metal stamped parts are components formed from flat sheet in dies and presses by blanking, piercing, bending and drawing. If you need the fundamentals first, see our explainer What Is Metal Stamping and our custom stamping parts overview. The broader materials and sourcing picture is in Metal Stamped Parts: The Complete Engineering, Materials & Sourcing Guide. We will not repeat that here.
One of our customized products shows the problem clearly. The Galvanized Base Profile for Auto Windshield is an interior trim part for a commercial vehicle. Its specification, as published on the product page [2], is summarized below.
| Item | Published value | Engineering implication |
|---|---|---|
| Dimensions (L × W × H) | 1,195 × 41 × 30 mm | Length is about 29 times the width. A small stress imbalance across the section shows up as visible bow. |
| Flatness requirement | 0.6 (we read this as a 0.6 mm tolerance zone) | Over 1.2 m, this is demanding and drives the tooling and checking strategy. |
| Material and finish | Galvanized steel | The zinc layer has to survive forming without cracking or being scraped off. |
| Equipment | 300-ton stamping press plus CNC bending machine | Press for blanking and forming, CNC bending for the long, tightly controlled bends. |
A flat blank is stable because its internal stresses are balanced. Forming a channel or L-section changes that balance. Metal on the outside of each bend is stretched and metal on the inside is compressed. When the tool opens, elastic recovery (springback) releases part of that stress. On a short bracket this is a few tenths of a degree. Along a 1.2 m profile, the same release can turn into bow, twist or a sideways sweep that no longer fits the mating surface.
Four contributors tend to matter most:
| Defect | Typical cause | Preventive measure | How to verify |
|---|---|---|---|
| Longitudinal bow (camber) | Springback imbalance, coil crossbow | Over-bend compensation, restrike or coining stage, leveled coil | Checking fixture or CMM scan of the full length |
| Twist | Uneven die pressure, worn die steels, stress release after piercing | Die maintenance schedule, piercing before final forming | Surface plate with feeler gauge at both ends |
| Zinc flaking at bends | Radius too tight for coating weight, heavy coating, cold material | Match radius to thickness and coating designation, bend test on incoming coil | Magnified visual check of the outer bend surface |
| Scratches and zinc pickup on tools | Zinc transfer to die surfaces, inadequate lubrication | Suitable lubricant, polished tool surfaces, scheduled die cleaning | First-piece and periodic surface inspection |
| Burrs | Excess punch-to-die clearance, dull tooling | Set clearance by thickness, plan burr direction away from the visible or sealing side | Edge inspection against the drawing |
Note: Table 2 summarizes general stamping practice. Exact settings depend on material grade, thickness and tooling design.
"Galvanized" covers several different production routes. They behave very differently in a forming shop.
| Route | Coating applied | Main advantage | Main watch-out |
|---|---|---|---|
| Pre-galvanized coil | Before stamping | No thermal distortion after forming, consistent coating, good for volume | Cut edges are bare steel. Bends can crack the coating if radius is too tight. |
| Electro-galvanized | Before or after stamping | Thin, uniform, low heat input | Thinner layer, so less corrosion allowance. Needs a passivation step. |
| Post-forming hot-dip | After stamping | Heavy coating, covers cut edges | Heat can distort long thin parts. Not ideal when flatness is tight. |
Two practical points that drawings often miss. First, heavier coating is not automatically better for a formed part. A very heavy zinc layer is more likely to crack or flake in a tight bend and adds galling risk. Specify the coating designation the design life actually needs. Second, cut edges on pre-galvanized sheet are bare steel. Zinc nearby protects a narrow exposed edge sacrificially, but you should still decide whether a post-stamping treatment is needed for a harsh environment.
For a profile 1.2 m long, the forming strategy matters as much as the press. ACRO makes this type of part with a 300-ton press and a CNC bending machine [2]. A typical sequence, described here as general practice rather than a fixed recipe, runs as follows:
Because the die defines how repeatable this is, in-house tooling shortens the loop. ACRO designs and builds single-station and progressive dies in its own tooling workshop [3], and offers a manufacturing feasibility analysis before production begins [2]. If a die needs adjustment after first samples, that happens on site. You can see the equipment on our product equipment page and the tooling team on our tooling manufacturing page. For a wider look at process choices, see 9 Metal Stamping Processes to Consider.
A flatness requirement is only meaningful if everyone measures it the same way. GD&T defines flatness as a zone between two parallel planes (ISO 1101) [5]. The question that matters on a 1.2 m part is how the part is supported while it is measured, because a thin profile will sag under its own weight on the wrong supports. Agree the datum and support points with your supplier in writing.
One more practical observation. ACRO's published coordinate measuring machine has a range of 1000 × 1200 × 800 mm [4]. A 1,195 mm part sits very near the upper limit of that range, so a dedicated checking fixture for routine shape checks is the sensible approach, with CMM used for first-article and periodic verification. This is our own reasoning from the published figures, not a statement about a specific job.
| Check | Method | What it catches | Typical timing |
|---|---|---|---|
| Incoming material | Certificate review, thickness, composition and hardness testing | Wrong grade, out-of-range strength that changes springback | Each coil lot |
| Coating thickness | Coating thickness gauge | Under- or over-specified zinc layer | Incoming and sampling |
| Overall dimensions and hole positions | Projector, gauges, CMM | Die wear, forming drift | First article, then in process |
| Flatness and straightness | Checking fixture, surface plate, CMM | Bow, twist | First article, then periodic |
| Corrosion resistance | Neutral salt spray (ISO 9227, ASTM B117) | Coating damage, poor passivation | By agreement or per lot |
Treat salt spray hours as a comparison tool, not a life prediction. Real-world corrosion depends on climate, road salt, moisture traps and joint design. ACRO's in-house laboratory covers material composition, coating thickness, hardness, torque and salt spray testing, plus CMM measurement [6]. More detail is on our quality inspection page.
Stamping has a front-loaded cost structure. The die is a one-time investment, and the unit price falls as volume rises. We cover that logic in Is Metal Stamping a High-Cost Investment?. For long profiles there is an additional layer: how tightly you specify shape changes tooling complexity, the number of correction steps, fixture needs and scrap rate. A few examples:
| Decision | Cost effect | Practical advice |
|---|---|---|
| Tight flatness over the full length | Higher: restrike stage, fixtures, more scrap | Apply the tight value only where the part mates. Allow a looser value elsewhere. |
| Coating called out as "galvanized" only | Unclear: quote differences, disputes later | Name the coating designation (for example under ASTM A653) and any passivation requirement. |
| Bend radius tighter than the coating tolerates | Higher: rework, rejects, tool wear | Ask for a feasibility review early and accept a modest radius change if function allows. |
| Cosmetic requirement on all surfaces | Higher: protective handling and packaging | Mark the visible zones only. |
| Very low volume, new tooling | High unit cost: die cost spread over few parts | Consider whether laser cutting and bending suit prototypes before committing to a die. |
Send these items with your drawing and you will get a faster, more comparable quote:
What to look for in a supplier. Check that the quality system is certified and current. ACRO holds IATF 16949:2016 and ISO 9001:2015 [6]; IATF 16949 is the automotive quality management standard overseen by the International Automotive Task Force [7]. Ask whether tooling is made in-house, whether first-article reports include shape data, and whether sample lead time is stated. ACRO's stamping page lists samples in 3 to 7 days and supply capacity of 300,000 pieces per month for its stamping range [8]. Confirm the figures that apply to your own part.
About the manufacturer. Founded in 2003, ACRO Metal Products Ltd. makes stamped, welded and assembled parts for the automotive industry, mechanical equipment, kitchen utensils, office supplies and door and window hardware. The company has more than 200 employees, including a 20-person R&D team and a 20-person quality inspection team, with production bases in Jiaxing, Zhejiang and in Cambodia [9]. Its more than 100 processing machines include stamping presses from 16 to 500 tons, CNC punching and bending machines and welding robots [10]. Read more on our company profile page, or see related automotive work such as auto stamping parts and the guide Auto Stamping Parts: A Complete Guide. Other galvanized examples include the galvanized end cap for the garage door and galvanized brackets for freezers.
Forming leaves residual stress in the metal. When the part leaves the die, elastic recovery releases some of it unevenly, and over a long length that appears as bow or twist. Leveled coil, over-bend compensation and a correction stage reduce it.
For flatness-critical parts, pre-galvanized sheet is usually safer because no thermal cycle follows forming. Post-forming hot-dip galvanizing gives a heavier coating and covers cut edges, but heat can distort thin, long parts.
It can if the radius is too tight for the coating weight and base steel. Specify a suitable coating designation, match the bend radius to thickness, and check the outer bend surface on first samples.
Use a checking fixture or surface plate with feeler gauges for routine checks, and a CMM for first-article reports. Agree the support points and datum in advance, since a thin part flexes under its own weight.
At very low volumes the die cost is spread over too few parts, and laser cutting with press brake bending can be more economical. Stamping pays off as volumes grow.
