Methods for Preventing Wrinkling in Deep Drawn Parts: Using a Blank Holder
The simplest method for eliminating wrinkling in deep-drawn parts is using a blank holder. In most deep drawing processes, a constant blank holder pressure is applied throughout the entire drawing action.
Variable blank holder pressure, however, has been employed with some success. A pneumatic or hydraulic blank holder cushion can vary the blank holder pressure linearly over the stroke of the machine. This provides some increase in the allowable die cavity depth.
A numerically controlled (NC) die cushion can be used to provide a variable blank holder pressure over the course of drawing action. In an optimal blank holder pressure force profile, the initial force is large so as to provide initial deformations.
The cushion drops off to pull material into the die cavity, and then slowly increases back up to ensure strain hardening in the drawn part. An NC die cushion can dramatically increase the allowable die cavity depth while preventing both wrinkling and cracking.