Drawing Defects in Drawing Dies and How to Resolve Them

In this article on die-making, we’ll cover the main defects that occur when forming with drawing dies, their causes, and how to resolve them.

Main Drawing Defects

Wrinkling, tearing, and similar defects are never acceptable in parts produced with drawing dies. The most commonly encountered drawing defects and their causes are explained below.

Wall Thickness Imbalance in the Drawn Cup

The part undergoing the drawing process is subjected to bending, compressive, and tensile stresses. This variation across the bending, compression, and tension zones cannot be fully explained. However, the change in the drawn shape’s wall thickness varies depending on the type of the cup material, the drawing depth, the drawing speed, the pressure plate (blank holder) force, the coefficient of friction between the female die and the drawn cup (the lubrication system), and the one-sided die clearance.

Above, the change in wall thickness is shown for a cup drawn from 1 mm thick aluminum sheet material.

Scratching, Marring, and Nicking

During drawing, the sheet material rubs against the surface of the female die. This friction causes scratching, marring, or hard-to-remove nicking on the surface of the cup. To eliminate these types of defects, the die must be properly lubricated, kept clean, and any heat generated during drawing must be dissipated.

Orange-Peel Texture (Roughening)

A quality surface finish cannot be achieved when material grain coarsening occurs on the surface of the drawn cup. This results in a dull appearance on the drawn surface. To eliminate this type of defect, the formed part is polished with felt wheels, or the surface is chrome-plated.

Since this process increases the cost of the formed part, the part is instead tempered or subjected to a normalizing anneal. However, care must be taken during this process to prevent the drawn cup from tearing. Figure 2-c shows the roughening that occurs on the surface of a drawn cup.

Earing (Projections)

This is one of the defects frequently encountered in cups other than those made with flanged drawing. During drawing, the drawn shape elongates in the rolling direction of the material. This is because grain coarsening is greater in the rolling direction, and for cylindrical parts, the number of ears formed around the circumference varies according to this grain coarsening.

This defect is eliminated by using a larger initial blank diameter or dimensions, and trimming the mouth of the cup after drawing.

Stretcher Strains (Marks)

After drawing, patch-shaped streaks (marks) appear on the surface of the cup. These types of marks are called stretcher strains. Diagonal marks prevent the stress distribution on the surface of the drawn cup from being homogeneous. However, this type of drawing defect is generally minor.

Discoloration (Burning)

This type of defect frequently occurs in high-polish drawing operations. To eliminate this and similar defects, the one-sided die clearance is increased.

Wrinkling

Wrinkling can occur in the flange or body sections of the drawn cup material. These wrinkles are called material buildup and can cause the drawn cup to tear. To prevent wrinkling, the one-sided die clearance and the pressure plate (blank holder) force must be properly adjusted. Additionally, the die mouth and punch nose radius must be designed in a way that prevents wrinkling.

Springback

The cup exiting the drawing die opens up as a result of springback. To prevent this opening, the female die is tapered slightly inward starting from the mouth.

Besides these, many other drawing defects can occur, for example: the inability to perform a symmetrical drawing operation, the formation of tear zones at the mouth and base of flanged cups, corner tearing in rectangular or square drawing, and similar defects.

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