Metal Stamping

Stamping dies are highly specialized tools used to shape, cut, and form sheet metal parts with precision. The development of these dies begins with detailed designs created using  computer-aided design (CAD) software and finite element analysis (FEA) programs. These technologies enable engineers to simulate and optimize the forming process, ensuring accuracy, durability, and material efficiency, while also identifying potential issues such as springback or material thinning.

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After the designs are finalized and validated through virtual testing, they are converted into physical dies. Diemakers use CNC machining, EDM (Electrical Discharge Machining), and other precision manufacturing techniques to ensure good stamping dies.These dies are then mounted in stamping process.

Once the process begins, multiple quality checks are performed to ensure that the stamped parts meet the specified dimensional accuracy and quality standards. Long maintenance cycle & Lower repair frequency.

Through reasonable molds, the generation of waste can be reduced, and the use of advanced mold design technology can improve production efficiency and reduce costs

How to assess the quality of A Die

1. Dimensional Stability: The die must maintain dimensional accuracy, typically verified through CpK analysis.
2. Production Efficiency: The die should not hinder production efficiency or cause downtime, ensuring smooth and efficient operations.
3. Durability: The die should be made from materials that provide durability and withstand long production runs without significant wear, which demands high processing precision.
4. Safety and Error-Proofing: The die should incorporate safety features and error-proofing mechanisms to prevent mistakes during production.
5. Ease of Maintenance: The die should be designed for easy maintenance, allowing for quick cleaning, adjustments, and repairs to minimize downtime.
6. Material Utilization: A well-designed die maximizes material usage, reducing waste and lowering production costs.
7. Cooling and Lubrication Systems: The design of the die’s cooling and lubrication systems should support stable production and maintain product quality, particularly in high-volume manufacturing.
8. Consistency of Finished Products: The die must produce consistent quality across large production runs, avoiding inconsistencies caused by wear or other issues.
9. Surface Quality of Finished Products: The die should ensure high-quality surface finishes on the parts produced, reducing the need for additional post-processing steps.

Common Defects in Stamping and Their Causes

This analysis will cover three key areas: common defects in Punching, Bending, and Deep Drawing of Large Curved Parts. Each section will outline typical issues, their causes, and potential solutions.

1. Common Defects in Punching and Causes

Punching is a stamping process that separates sheet metal using a die. Common defects in punched parts include burrs, warping, and dimensional inaccuracies.

1.1 Burrs

Burrs are often unavoidable in sheet metal punching, but improving part manufacturability and punching conditions can reduce them. Burr formation is caused by several factors:

• Clearance Issues: Excessive, insufficient, or uneven clearance between the punch and die can lead to burrs. Causes include:
  • Die manufacturing errors: Misaligned die components, improper parallelism.
  • Assembly errors: Loose guide clearance or misaligned punch and die.
  • Press accuracy issues: Poor parallelism between the ram and die, excessive clearance in the press guides.
  • Installation errors: Incorrect installation or non-aligned die components.
  • Die structure: Insufficient die stiffness or unbalanced punching force.
  • Warped steel sheets: Uneven sheet material.
• Dull Cutting Edge: A worn or damaged cutting edge leads to burrs. Contributing factors include poor material quality of the die, inadequate lubrication, and lack of timely sharpening.
• Incorrect Punching Conditions: Improper contact between the workpiece and the die or misalignment during trimming can cause burrs.
• Material Defects: Incorrect material thickness or grade can cause burrs due to inappropriate clearance.

1.2 Warping

Warping occurs due to the combined effects of stretching, bending, and compressive forces during punching. Causes include:

• Large Clearance: Excessive clearance increases stretching and bending forces, leading to warping. Using a hold-down plate during punching helps mitigate this.
• Reverse Taper in the Die: This causes compression at the part’s edges, leading to bending.
• Complex Part Shape: Uneven shear forces due to complex part geometry can cause warping. Increasing hold-down force can help.
• Residual Stress: Internal stresses from material rolling may surface after punching, causing warping. Flattening the material during the decoiling process can reduce this.
• Contact Issues: Oil, air, or poor surface contact between the die and part can also result in warping, especially with thin or soft materials.

1.3 Dimensional Inaccuracies

Dimensional errors can arise from:

• Die Manufacturing Errors: Incorrect die cutting edge dimensions.
• Springback: Parts may deform due to springback after punching.
• Workpiece Shape: Poor consistency between the workpiece and the die can cause deformation during the punching process.
• Multiple Processes: Improper adjustment in earlier processes can lead to size changes in subsequent processes.
• Positioning Issues: Misalignment during operation or poorly designed positioning mechanisms can lead to deviations.

2. Common Defects in Bending and Causes

Bending defects include shape and dimensional inaccuracies, cracks, surface scratches, deflection, and twisting.

2.1 Shape and Dimensional Inaccuracies

These issues often arise from springback and poor positioning. Solutions include:

• Pressing the Blank: Using a cushion, rubber, or springs to hold down the blank tightly before bending.
• Reliable Positioning: Two common positioning methods are:
  • External Shape: Easy to operate but less accurate.
  • Hole-based Positioning: More accurate but harder to operate.

2.2 Bending Cracks

Factors causing cracks include:

• Poor Material Plasticity
• Incorrect Grain Orientation: For single V-bends, the bend line should be perpendicular to the grain direction.
• Small Bending Radius
• Poor Blank Quality: Burrs or cracks in the blank may lead to cracks during bending.
• Worn Punch/Die Radius
• Inadequate Lubrication
• Material Thickness Variations

2.3 Surface Scratches

Causes include poor material selection for the die, low hardness, worn die radius, poor blank quality (e.g., rust), and lack of lubrication.

2.4 Deflection and Twisting

3.Common Defects in Deep Drawing of Large Curved Parts and Their Causes

3.1 Characteristics of Deep Drawing for Large Curved Parts

3.1.1 Deformation Characteristics
Large curved parts exhibit a combination of drawing on the edges and bulging in the interior. The surface shape is supported by material flow from the edges, while the interior stretches to meet the bulging requirements. Due to varying drawing depths and complex shapes, controlling material flow and speed is crucial. Both wrinkling and cracking are common in these parts.

3.1.2 Stable Blank-Holding Force
In addition to drawing force, large curved parts require a stable blank-holding force throughout the process. These parts often feature large contour sizes and significant depth, demanding both high drawing and blank-holding forces. Single-action presses with cushion systems provide limited blank-holding force (around 1/6 of nominal tonnage), making them unsuitable for these parts. Instead, double-action presses are commonly used, as they can deliver over 40-50% of total drawing force for blank holding, ensuring uniform deformation and preventing defects.

3.1.3 Sufficient Stiffness
As large curved parts are often used as machine enclosures, they must exhibit sufficient stiffness to prevent vibration and noise. The material should experience uniform tensile stress to minimize elastic recovery and prevent deformation or cracking. The goal is to maintain consistent stress distribution, ideally with biaxial tensile stress.

3.2. Common Defects and Causes

3.2.1 Cracks and Fractures
These occur when localized tensile stress exceeds the material’s strength limit. Causes include:

• Inadequate stamping properties of the material.
• Sheet thickness variation causing improper clearance during forming.
• Surface defects like scratches or corrosion increasing stress concentration.
• Excessive blank-holding force, leading to feeding difficulties.
• Excessive local drawing depth beyond material deformation limits.
• Misaligned placement of the blank during operation.
• Insufficient lubrication increasing resistance during feeding.
• Improper die installation or low press accuracy causing uneven material flow.

3.2.2 Wrinkles and Folds
Wrinkles are caused by local buckling due to uneven material flow. Reasons include:

• Poor formability of the part, with incorrect stamping direction or blank-holding surface design.
• Insufficient blank-holding force causing excessive material flow.
• Poor contact between the blank-holding surface and the material.
• Excessive lubrication leading to uneven material flow.
• Misalignment of the outer slide resulting in uneven blank-holding pressure.

3.2.3 Unclear Ridges
Unclear ridges occur when the press fails to apply sufficient pressure during the final stages of deformation. This can result from inadequate press force or uneven die clearance.

3.2.4 Poor Stiffness
Poor stiffness often arises from insufficient plastic deformation, due to low blank-holding force. Increasing blank-holding force or modifying draw beads can help improve stiffness.

3.2.5 Surface Scratches
Scratches occur when the die radius lacks sufficient smoothness, or when contaminants enter the die. Other causes include poor lubrication or misaligned die inserts.

3.2.6 Surface Roughness and Slip Lines
Surface roughness can be attributed to large grain sizes in the material.