Two reasons lead to the issue as below.
1. Checking the issue sample from customer, we find the sink mark on the crack place. After inspecting the production samples from the same batch, we also find the same mark. The sink mark will decrease the thickness of the cup, and reduce the strength. The sink mark comes from the stamping process. The abrasion punch brings scraps on the product, and lead to mark on the pedestal of cup.
2. Inspecting the dimension of the issue sample, the crimping diameter is ∅27.34mm(The specification is ∅27.20±0.1mm), and the crimping height is 4.81mm(The specification is 4.35±0.1mm). The crimping dimension out of specification, and reduce the thickness of the cup.