딥드로잉 균열과 주름: 프레스 성형 불량의 원인과 대책

Deep drawing forms a flat sheet blank into a cup or box using a punch and die. As the punch descends, material from the outer flange passes through the die entry and becomes the wall. The essential mechanism is supplying wall area from the surrounding sheet, rather than obtaining all the depth by continually stretching the bottom.

The flange is pulled radially while being compressed circumferentially. Material originally lying on a larger circumference must accommodate a smaller circumference as it moves inward. If the sheet cannot accommodate this compression while remaining flat, it buckles out of plane and forms flange wrinkles.

The wall transmits force from the punch to the flange. A higher force requirement for drawing in material can concentrate tensile demand near the wall-to-bottom transition. Localized thinning, or necking, reduces the load-bearing section, further concentrates deformation and can eventually produce a tear.

At the die shoulder, the sheet bends from a flat region around a curved surface and then straightens into the wall. Resistance here therefore includes the force needed for bending and straightening as well as friction. Deformation over a flat punch bottom may be relatively small. A domed bottom or raised feature also requires the stretching in that region to be evaluated.

Material flow differs between the corners and straight walls of a rectangular part. Corner flanges undergo circumferential contraction similar to a cylindrical cup. Along straight sections, bending and straightening over the die shoulder account for a substantial part of the flow. Equal restraint around the perimeter can therefore leave excess material at a corner while starving a particular wall.

Process review becomes clearer when divided into blank placement, holder contact, initial bending, sustained draw-in, final shape formation near bottom dead center, and ejection and trimming. Identifying the contact and deformation introduced at each stage helps locate the initiating cause. The largest wrinkle or longest crack on the finished part does not necessarily identify where the defect began.

Lubrication is a material-flow variable, not merely an auxiliary measure against scratching. Changes in friction at the holder and die alter flange draw-in even at the same force. Punch contact also influences deformation distribution, so making every surface more slippery in the same way does not always improve the process.

When comparing lubricants, control application amount and distribution, diluted-fluid condition, sheet surface and tool condition as well as product identity. A generously hand-lubricated trial part does not represent the same conditions as automatic application in production. Connect actual lubricant distribution with directional draw-in to distinguish inadequate lubrication in a necessary zone from an uneven application pattern.

At the highly loaded die shoulder, insufficient lubricant film can allow material to adhere to the tool, causing galling. Deposits that score subsequent parts can increase both surface damage and friction. Wall thinning or cracking may then develop under settings that initially produced acceptable parts. Compare recurring scratch locations, tool deposits and force changes on the same timeline.

Increasing production speed changes deformation rate and contact time. Repeated forming can also raise sheet and tool temperatures. Temperature affects lubricant viscosity, film retention and material deformation behavior, so low-speed results cannot simply be transferred to the target production rate. Slower operation does not reduce every crack either. Identify which changes in temperature, friction and deformation are associated with the actual defect.

Validation should cover both cold startup and the period after temperatures stabilize during continuous production. Record press speed, measured tool or part temperature, lubricant supply, force history and defect sequence together to distinguish thermal effects. If defects increase after a particular number of strokes, compare lubricant depletion, contamination or adhesion, and heat accumulation alongside material-lot changes.

Lubricant selection is also connected to downstream operations. A lubricant suitable for forming can increase overall cost if cleaning residues affect bonding, welding, painting or plating. Check compatibility with the material and finish, removability after storage, replenishment frequency and control methods. Approve changes using relevant downstream results as well as formed-part appearance.

If wrinkle-preventing settings cause cracking and crack-preventing settings leave wrinkles, one forming stage may provide insufficient margin. Review how initial depth and shape, redrawing and final refinement can share material-flow and local-deformation demands within manageable limits at each stage.

Redrawing changes an existing cup’s diameter and depth. Earlier work hardening, thickness variation and directionality remain as starting conditions. Multiple operations do not restore fresh-sheet formability at each stage. Transfer material properties, thickness and deformation history between stages together with geometry.

Allowing sufficient initial draw-in before refining corners and dimensions may help. However, poor material distribution in the preform can introduce folds or excessive stretching during the next operation. Approve intermediate cup depth, diameter, rim height and wall thickness alongside final-product requirements.

Ironing has the specific function of reducing wall thickness and must be distinguished from ordinary drawing. It can be used to establish target thickness or extend the wall, but requires tooling and lubrication capable of accommodating through-thickness compression and contact resistance. Incidental ironing caused by insufficient clearance is not an established wall-thickness control operation.

Intermediate heat treatment is a separate option to evaluate against material temper and required performance. Considering only its ability to reduce work hardening can overlook final strength, surface oxidation and cleaning, dimensional changes and material-specific thermal response. Validate permissible conditions for the particular alloy or grade and their effects on subsequent operations. An annealing temperature and time established for one material should not be transferred to another.

Deformation history also matters in process simulation. Conventional strain-based forming-limit curves assume proportional in-plane strain paths. Comparing repeated redrawing or bending-and-straightening regions against a single curve therefore has limitations. Use a material model that can represent the history and an appropriate fracture or necking criterion, then verify results against intermediate and final trial parts.

Additional stages increase tooling, transfer and inspection demands but may reduce scrap and readjustment costs from a narrow single-stage process. Base the decision on geometry, process margin, material utilization, downstream quality and equipment repeatability, rather than a fixed production-volume threshold.

Diagnosis is more effective when failed parts are compared with acceptable parts made immediately before and after them. First establish whether material lot, blank orientation, center position, lubrication and production timing were consistent. Map defects to the part datums and rolling direction. Distinguish defects formed before bottom dead center from damage introduced during ejection or trimming.

Crack diagnosis requires evidence from both shape and thickness. If a thin band appears near the bottom transition, check whether draw-in decreased in that direction. If a crack begins at a cut edge, compare burrs, microcracks, cutting-tool condition and deformation direction. Measurements on an already torn area cannot readily establish its thickness immediately before fracture. Adjacent acceptable parts and specimens from intermediate strokes help identify the cause.

Draw-In, Force and Adjustment Criteria

Draw-in measures material movement into the die and directly supports flow comparisons. Establish reference positions on the blank perimeter and record edge movement by direction at the same stroke position. Comparing trials with different blank sizes or measurement references mixes differences in flow with differences in initial geometry. Keep those references consistent.

Use force curves as diagnostic evidence. A late force increase does not immediately establish excessive holder force. Ironing of thickened material, folded wrinkles and galling can also change the curve. Correlate the stroke where the increase begins with shape, thickness and draw-in instead of relying only on peak force.

Before adjusting conditions, distinguish defects fixed to a particular location and direction from those appearing around the entire perimeter. Changing overall holder force and lubrication before checking off-center placement or local wear can disturb acceptable conditions elsewhere. Initially separate major variables to identify their effects. Then establish the acceptable range using trial combinations that include interactions between holder force, lubrication and blank design.

Acceptance criteria must remain consistent before and after adjustment. Eliminating a visible crack does not justify approval if minimum thickness is inadequate or trimmed height falls short. Conversely, excessive cosmetic requirements in a region that does not affect function or downstream processing can unnecessarily narrow the process window. Clearly distinguish functional drawing requirements from appearance criteria.

Finished-part appearance alone is insufficient for approving deep-drawing quality. Check the thickness profile from the bottom through the punch radius to the wall and rim, together with directional draw-in, cracks, wrinkles and final geometry. The thinnest location is not always at the easily accessible opening. Include risk locations identified through simulation and trials in the inspection plan.

Choose thickness measurement methods that suit the geometry. Use suitable contact measurements where both surfaces are accessible. For tight radii or inaccessible locations, consider sectioned specimens or validated optical measurement. Define measurement direction, location, spacing and force. Use closer spacing where thinning becomes steep so that a minimum between measurement points is not overlooked.

Thinning percentage describes the reduction relative to measured initial gauge. Rather than accepting a part solely because it is below a particular percentage, establish minimum thickness from load support, sealing, corrosion allowance and assembly function. Poor measurement repeatability or positional reproducibility makes process variation difficult to distinguish from measurement error. Confirm method suitability, particularly when acceptance is close to a limit.

Forming Limits and Production Validation

An FLD that overlays measured strains on a forming-limit curve, or FLC, is useful for identifying necking risk. However, one curve cannot assess local fracture at tight radii or cut edges and wrinkling instability. Surface strains measured with a grid or DIC are also influenced by bending and spatial resolution. Check the curve for the relevant gauge and material condition together with suitable measurement and analysis conditions.

Do not approve a forming simulation solely because it matches peak force. Compare directional draw-in, thickness distribution, earing, defect location and onset against trial parts. Check how the assessment changes with friction, material properties, contact and mesh size. Symmetry constraints can suppress wrinkles in particular. Verify that the model permits buckling modes that can occur physically.

Production approval should include target speed, normal lubricant delivery, cold startup, stabilized temperature and expected material-lot variation. Connect grade, temper, gauge tolerance, radii and datums with minimum remaining thickness, wrinkle and crack criteria, surface requirements, trimmed dimensions and measurement state on the drawing. Also define checks for deviations from approved force and draw-in ranges, and the required revalidation after changes to material, tooling or lubrication.

Manufacturing discussions benefit from functional requirements, downstream operations and inspection criteria alongside the final drawing. A press-forming review should connect the roles of initial blank development, forming and trimming. A manufacturing inquiry that includes crack and wrinkle locations, thickness measurements and occurrence conditions provides concrete information for diagnosis. Process maturity should be judged by the range that repeatedly meets required function and dimensions, rather than by a temporary disappearance of defects.