In high-precision steel sheet stamping processes, wrinkling is not merely an aesthetic defect but a critical failure mode. When compressive stresses within the blank exceed a critical threshold and the material lacks sufficient constraint to maintain stability, plastic buckling occurs—a physical phenomenon manifested as wrinkling.
Wrinkling during custom steel sheet stamping directly leads to dimensional non-conformity, compromised assembly integrity, and significantly reduced load-bearing capacity and fatigue life of the final product assembly. Addressing wrinkling in steel sheet stamping requires fundamental root cause analysis grounded in sheet metal forming mechanics, not merely superficial fixes.
This paper systematically analyzes wrinkling in custom steel sheet stamping. Starting from mechanics and plastic forming principles, it delves into the interactions among key variables—blank holder force (BHF), drawbead layout, die clearance, and material properties—to help you understand these systemic factors and ensure reliable performance and cost control for steel sheet stamping components.
Definition of Creasing in Steel Sheet Stamping
In precision metal manufacturing, creasing during custom steel sheet stamping is not merely an aesthetic issue. Technically defined, it occurs when the blank undergoes plastic buckling instability under compressive stress during the steel sheet stamping process, resulting in wrinkles. The core mechanical principle is that when localized tangential or in-plane compressive stress exceeds the critical buckling stress, the material loses stability and wrinkles.
The impact of wrinkling in custom steel sheet stamping manifests in three dimensions: First, in terms of geometric quality, wrinkling directly causes deviations in the flatness and profile of stamped parts. This leads to positioning and clamping difficulties on automated assembly lines, resulting in uncontrolled fit tolerances.
Second, functionally and safety-wise, wrinkles create geometric discontinuities that significantly weaken structural static strength and fatigue life while compromising the integrity of subsequent coating applications. Finally, in terms of cost, wrinkling leads to high scrap rates and increased secondary forming labor hours, directly eroding project profitability. Therefore, controlling wrinkling in custom steel sheet stamping at its root is critical to ensuring product reliability and total cost of ownership (TCO).
Mechanical Analysis of Wrinkling in Steel Sheet Stamping
Fundamentally, wrinkling occurring during custom steel sheet 스탬핑 represents a phenomenon of plastic instability. Its core mechanical basis is as follows: instability occurs when the in-plane compressive stress experienced by the sheet within the forming zone exceeds the critical buckling stress for that material at its current thickness, strain-hardened state, and boundary constraints. This principle manifests in the steel sheet stamping process as the interaction between “material flow” and “geometric constraints.”
예를 들어, 딥 드로잉, material in the flange region flows radially into the die under tensile stress, causing circumferential contraction and generating intense tangential compressive stress. If the blank holder or drawbead provides insufficient constraint to counteract or “absorb” this compressive stress through friction and bending effects, the excess material will bulge outward, causing visible wrinkles in the stamped parts.
As a professional steel sheet stamping manufacturer, Supro translates abstract mechanical principles into actionable process parameters and mold designs through systematic stress analysis and material flow simulation. This approach prevents defects at the source, ensuring dimensional integrity and structural reliability of steel 시트 스탬핑 부품.
Flange Wrinkling
In 강판 스탬핑 processes, this is the most typical wrinkle type occurring in the outer flange region of drawn parts. Its direct cause is the tangential compressive stress generated in the flange during contraction. The critical factor is whether the radial constraint provided by the blankholding force (BHF) or drawbead layout is sufficient to keep this compressive stress below the critical threshold. If constraint is insufficient, the flange edge exhibits the poorest stability, with wrinkles typically initiating here and propagating inward. For custom steel sheet stamping projects, flange wrinkling serves as the most direct signal to adjust the blankholding force (BHF), lubrication conditions, and drawbead resistance.
Die Radius Area/Wall Wrinkling
This wrinkling occurs on the sidewall section already within the die cavity, typically manifesting as longitudinal wrinkles on steel sheet stamping parts. Its mechanism differs from flange wrinkling: it arises not from pure compressive stress but primarily from uneven material flow. If material flows too rapidly or in excessive amounts to a specific area, this “excess” material cannot be sufficiently stretched in the sidewall region (insufficient radial tensile stress). Consequently, it buckles due to slight longitudinal compressive stress or its own instability. In steel sheet stamping processes, this is often associated with uneven die clearance, lubrication issues, or fluctuations in material properties.
Local Feature Wrinkling
When forming steel sheet stamping parts with complex geometric features, wrinkling may concentrate around bosses, ribs, or sharply turned-up features. This occurs because these local features severely impede material flow, causing significant obstruction and accumulation around the feature perimeter. This generates highly localized, directionally variable compressive stress fields. Professional steel sheet stamping manufacturers conduct focused CAE simulations during the tooling design phase to analyze material flow rates and stress concentrations in these critical zones.
Springback Wrinkling
This secondary wrinkling manifests after part removal from the tooling, stemming from uneven residual stress distribution within the component. When external constraints (die surfaces) are removed, parts undergo elastic springback to achieve new stress equilibrium. If this recovery process varies significantly across different regions, it may induce new compressive instability in certain areas of the steel sheet stamping components. This type of wrinkling requires particular anticipation and compensation in high-strength steels or complex multi-stage forming processes.
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Systematic Analysis of Creasing Causes in Steel Sheet Stamping
Specifically, the root cause of creasing during custom steel sheet stamping can be attributed to the disruption of the dynamic equilibrium between “flow” and “constraint.” On the process side, the blank holder force (BHF) setting and drawbead layout constitute the external “active constraint system”; the physical state of the steel sheet stamping dies represents the “passive constraint system,” where deviations in die clearance, radius, and alignment directly diminish constraint effectiveness.
Meanwhile, the material's inherent mechanical properties (such as r-value and n-value) and blank preparation accuracy determine its intrinsic resistance to instability and the initial conditions for flow. Whether due to drift in steel sheet stamping process parameters, progressive die wear, or fluctuations in incoming material properties, any of these factors can trigger wrinkling.
Improper Process Parameters in Steel Sheet Stamping
In custom steel sheet stamping projects, process parameters serve as the primary defense for actively controlling material flow and preventing wrinkling. However, improper process settings are the most common cause of imbalance between “flow” and “constraint,” ultimately leading to wrinkling.
This “inappropriateness” does not refer to simple errors in individual parameters, but rather to core variables—such as Blank Holder Force (BHF), drawbead layout, lubrication strategy, and stamping speed—failing to synergistically match the specific geometry and material properties of steel sheet stamping parts, resulting in a failed control system.
Improper Blank Holder Force (BHF)
Blank Holder Force (BHF) is the most critical and direct parameter variable in controlling wrinkling during steel sheet stamping processes. Its core function is to generate sufficient frictional resistance to manage the rate and uniformity of material flow into the die cavity within the flange zone. When BHF is too low, it fails to effectively suppress plastic instability caused by tangential compressive stress, inevitably leading to flange wrinkling. Conversely, excessively high BHF drastically increases radial tensile stress, potentially causing excessive thinning or even cracking of the material.
As a professional steel sheet stamping manufacturer, Supro determines an optimal BHF range—not a single value—through process window experiments. This range simultaneously suppresses wrinkling and prevents cracking. For complex custom steel sheet stamping projects, employing segmented blankholding rings or CNC hydraulic pads to achieve variable blankholding force (VBHF) control represents an advanced strategy for addressing wrinkling in deep-drawn or asymmetrical parts.
Improper Drawbead Settings and Layout
Drawbeads essentially function as “flow control valves” on steel sheet stamping dies. They precisely increase resistance to material flow into the die cavity by forcing material to bend and counter-bend. If the drawbead’s layout, quantity, geometry (round or square), or resistance coefficient is improperly set, it cannot impose balanced and progressive restraint on the blank’s edges.
For instance, in areas prone to wrinkling (typically where curvature is low or material is excessive), insufficient local drawbead resistance allows premature and excessive material flow, leading to pile-up wrinkles. Optimal drawbead design must align with the shape of steel sheet stamping parts, providing a differentiated circumferential resistance distribution to guide orderly material flow. This constitutes one of the core technologies for optimizing the steel sheet stamping process.
Incorrect Lubrication Strategy
Lubricants influence material flow by altering the friction coefficient between steel sheet stamping dies and the sheet metal. Incorrect lubrication strategies disrupt the delicate balance established by the holding force and drawbead. Excessive lubrication in areas requiring high friction to constrain material (e.g., the outer edge of a flange) reduces effective constraint force, inducing wrinkling.
Conversely, insufficient lubrication in areas requiring smooth material flow (e.g., die radii) may cause excessive stretching and cracking of the stamped part.
Stamping Speed and Tonnage Curve
The speed of custom steel sheet stamping directly impacts material strain rate. Higher speeds may impair material flowability and increase frictional heat, altering local friction conditions and material properties, thereby affecting forming stability. For high-strength steel or complex stamped parts, excessive speed may prevent adequate material flow, increasing instability in compressive stress zones.
Additionally, the press tonnage curve must ensure sufficient and stable tonnage throughout the entire stamping stroke to maintain pressure and complete final forming. Insufficient tonnage or an improper curve can cause constraint force decay during the final stroke phase, leading to wrinkling or dimensional deviations.
Die Design and Condition for Steel Sheet Stamping
A die is not merely a forming tool; it serves as the physical actuator and ultimate constraint system for steel sheet stamping process parameters. Even with perfectly set process parameters, improper die design and poor die condition can directly cause “constraint failure,” becoming the root cause of wrinkling. The core function of the steel sheet stamping die system lies in transforming theoretical material flow control schemes into precise, stable, and repeatable physical reality.
Unreasonable Die Clearance
Die clearance refers to the single-sided distance between the punch and die. A reasonable clearance is typically 7%-10% of the material thickness (depending on the material). Excessive clearance deprives the sheet of adequate lateral support during custom steel sheet stamping, causing buckling and wrinkling in sidewall areas.
Insufficient clearance intensifies friction and wear, heightening the risk of tearing in steel sheet stamping parts and potentially generating unintended material thickening or micro-wrinkles due to compression forces.
Die Radius and Surface Finish
The die radius serves as the critical passage for material flow from the flange region to the side walls. An excessively small radius requires greater bending moments during material flow, significantly increasing flow resistance.
This causes material buildup ahead of the radius, leading to wrinkles during the steel sheet stamping process. Additionally, a rough radius surface (insufficient surface finish) exacerbates friction and scratching, further impeding material flow. In custom steel sheet stamping projects, Supro ensures smooth material flow and minimizes localized stress concentration by optimizing die radius and maintaining high surface finish.
Die Alignment and Parallelism
Steel sheet stamping dies must be precisely aligned and parallel when mounted on the press—ensuring exact registration between the upper die (punch) and lower die (die cavity, blankholder). Poor alignment causes uneven gap sizes—one side larger, the other smaller—resulting in non-uniform blankholding force distribution across the forming zone. The larger gap side experiences insufficient restraint, allowing material to flow in and wrinkle more easily; the smaller gap side may be over-compressed or even damaged.
Similarly, parallelism deviations in the press slide or the steel sheet stamping dies themselves can cause analogous pressure imbalance issues. Such systemic deviations caused by equipment or installation are common sources of regular, asymmetric wrinkling.
Die Wear and Deformation
During prolonged steel sheet stamping production, critical areas like draw beads, radii, and die surfaces undergo normal wear. Reduced draw bead height decreases their resistance coefficient, compromising their original flow control capability. Worn die radii become rough or develop pits, scratching stamped parts and disrupting material flow.
Additionally, periodic high-tonnage impacts may cause microscopic elastic or plastic deformation in stamping dies, altering die surface geometry. These progressive changes cause initially stable processes to gradually “drift,” ultimately triggering quality issues like wrinkling. Therefore, regular die maintenance and condition monitoring are critical.
재료 특성 및 준비
In wrinkle analysis for steel sheet stamping processes, material properties and blank preparation are often treated as fixed static factors. However, they form the dynamic foundation determining the process window width. Even under ideal process and tooling conditions, fluctuations in material performance or defects in blank preparation can become root causes of wrinkling in custom steel sheet stamping.
Fluctuations in Material Mechanical Properties
The mechanical properties of steel sheets, particularly the plastic strain ratio (r-value) and strain hardening index (n-value), are intrinsic factors determining their wrinkle resistance and formability. A high r-value indicates the material resists thinning in the thickness direction while being easily stretched in the plane, resulting in stronger resistance to compressive buckling (wrinkle resistance). A high n-value indicates more uniform work hardening during deformation, promoting stable forming.
Variations in r-value and n-value may occur between different batches or even across different sections of the same coil. In custom steel sheet stamping projects, failing to rigorously inspect incoming materials and implement batch management—resulting in the use of materials with significant performance variations—can cause previously optimized process parameters to fail. This leads to unexplained wrinkling or cracking in steel sheet stamping parts.
Blank Dimensions, Shape, and Trimming Quality
The blank serves as the starting point for custom steel sheet stamping, with its dimensions determining the amount of material involved in deformation. An oversized blank introduces excessive “excess material” that must be drawn into the die cavity or accumulated in the flange zone, increasing compressive stress levels and wrinkling risk. An optimized blank shape (beyond simple rectangles or circles) balances material flow by reducing material in prone-to-wrinkle areas while preserving it in areas requiring supplementation.
Additionally, the quality of blank edge trimming is critical. Excessive burrs, overly thick cold-worked hardened layers, or microscopic tears along edges become stress concentration points and sources of irregular flow during the steel sheet stamping process, triggering localized wrinkling.
맺음말
In summary, the wrinkling issue in custom steel sheet stamping projects is not an isolated process error, but rather a systemic engineering challenge arising from the coupled interaction of three subsystems: process parameters, die condition, and material properties. Its root cause revolves around the dynamic equilibrium between “material flow” and “geometric constraints.” In the steel sheet stamping process, deviations in any variable—whether BHF window settings, drawbead layout, or mold clearance and wear—can disrupt this delicate equilibrium. This disruption allows compressive stress to exceed critical buckling limits, manifesting as flange, sidewall, or localized wrinkling.
Accurate root cause diagnosis is the first step toward effective solutions. For specific solutions addressing the aforementioned causes of wrinkling, please read: “Solutions for Steel Sheet Stamping Wrinkling: From Process Adjustment to Design Optimization.”
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