Deep Drawing

Deep rawing is basically a press forming operation where material is drawn into a die cavity and flow is controlled by a blank holder. Deep Drawing is when draw depth is deeper than the punch width (diameter).

The most critical failure mechanisms for drawing of:

  • Softer work materials ( ≤ 300 HB ) are abrasive/adhesive wear and galling.
  • Harder work materials ( ≥ 300 HB ) are abrasive and adhesive wear, galling, and plastic deformation.

General

Deep Drawing is characterized by a considerable amount of slip between the work material being formed and the active surface of the die. It is a forming operation in which a formed punch pushes against a flat sheet and forces it over a shaped die edge into the die cavity to take the shape of a recessed vessel. The outer edges of the sheet are normally restrained by a holder ring in order to prevent wrinkling of the side surfaces of the workpiece.

Failure mechanisms in cold work tooling

Due to cyclic mechanical loading and sliding contact between work material and tool surface, the active surfaces of the tool are successively damaged. The destruction of the tool will sooner or later lead to quality problems on the drawn parts (out of tolerance or bad surface qualities). The tool has then to be exchanged (in case of total failure), reground or refurbished.

This maintenance procedure means production standstill and accordingly loss of productivity. It is therefore very important that the tools can resist the different types of tool failure mechanisms in order to achieve high productivity and economical production. The selection of the right tool steel is thus directly linked to the resistance of the actual tool failure mechanism for the application.

Common cold work failure mechanisms are:

  • Wear - Results in a material loss from the tooling material and is related to the tooling material's hardness, carbide type and volume. Wear can also be related to the sheet material type and the process conditions.
  • Chipping - Is related to the stresses in the process and the fatigue resistance of the tooling material.
  • Plastic deformation - Occurs if the process stresses are higher than the yield strength of the tool steel.
  • Cracking - Occurs when the process stresses are higher than the tensile strength of the tool steel.
  • Galling - Is a physical / chemical adhesion of the work material to the tool surface. The severity of galling depends on the surface finish and chemical composition of the tool steel and work material.

Method for tool steel selection

  1. Identify the dominant failure mechanism(s)
  2. Select a tool steel with properties that will overcome this / these failure mechanism(s)
  3. Match the steel choice to the length of the production run