Stamping is one of the practical processes for mass forming of parts. Numerous die failures occur during stamping production. As a critical factor among all stamping production elements, die conditions directly affect production efficiency, cost and product delivery lead time. The main die-related troubles are concentrated on die damage, product quality defects and die edge sharpening, which have long plagued the industry. Only by properly addressing these key issues can stamping production proceed smoothly.

1 Die Failures

Die failures are common in stamping production and often lead to production shutdowns and delayed product schedules. Therefore, root causes must be identified promptly for proper repair.

1.1 Die Damage

Die damage includes cracking, fracture and bulging. To resolve die damage, root causes must be investigated from die design, manufacturing processes and die operation.

First, verify whether the die material is qualified and if the corresponding heat treatment process is appropriate. Heat treatment has a profound influence on die performance. Excessively high quenching temperature, improper quenching method and holding time, as well as inappropriate tempering cycles, temperatures and durations, may result in die damage after commissioning in stamping production. Insufficient size or depth design of blanking holes easily causes hole clogging and damages the blanking plate. Insufficient spring force or uneven height of equal-height sleeves may lead to spring breakage and tilting of the blanking plate, resulting in repeated overlapping impacts and part damage. Improper punch fastening or insufficient screw strength may cause punch dropping or fracture.

During die operation, incorrect placement of parts, poor bolt tightening, excessively low working height adjustment, insufficient guide post lubrication, feeding equipment faults or press malfunctions can all trigger die damage. If foreign objects enter the die, parts are overlapped, or scrap clogging occurs without timely disposal, continued production will readily damage the blanking plate, punches, lower die plate and guide posts.

1.2 Die Jamming

Once the die cannot close smoothly or becomes completely jammed during stamping, production must stop immediately to locate and eliminate the root cause. Otherwise, the fault will escalate and cause permanent die damage.

Major causes of die jamming include poor or tilted die guidance, or foreign debris between die plates preventing flat contact; insufficient die strength design or uneven loading leading to die deformation. For instance, insufficient hardness or thickness of die holders and die plates may cause deformation under external impact. Inaccurate die installation with excessive positioning error between upper and lower dies, or poor press precision will cause die interference. Insufficient punch strength or closely spaced large and small punches leads to unbalanced lateral forces. In such cases, punch strength should be increased and guiding protection of the stripper plate enhanced.

1.3 Die Damage and Repair

Dies account for high costs in stamping production, normally 1/5 to 1/4 of the total part cost. This is not only due to high difficulty and cost of die fabrication, but also the high expenses for die repair and sharpening after commissioning. The initial manufacturing cost of a die only accounts for approximately 40% of its total lifecycle cost. Therefore, timely die maintenance and damage prevention can substantially reduce die-related costs in stamping production.

When a die is damaged, a decision must be made between repair and scrapping. Generally speaking, non-natural wear failures of stamping dies — such as damage to non-critical components, fracture of small punches, upsetting and shortening of punches, cracking of die plates, chipping of blanking edges and similar faults — can mostly be fully restored via repair and returned to stamping production.

However, repair is rarely worthwhile when critical die components suffer severe damage, such as simultaneous failure of both punch and die. If one-time repair cost exceeds 70% of the original die manufacturing cost or the die is nearing the end of its service life, scrapping should be considered. Except for large dies and complex progressive dies, if repair work is highly complicated, costly and time-consuming, severely disrupting normal stamping production, scheduled scrapping should be adopted and a new die manufactured.

Under normal conditions, the primary failure mode of stamping dies is excessive wear. After a new die is put into service, it will undergo multiple repairs and re-sharpening. The die can only be scrapped when burrs on stamped parts exceed tolerance, dimensional and geometric accuracy of parts go out of specification, and the die can no longer be repaired or repair is economically unviable.