Deep Drawing Process Explained: How Sheet Metal is Formed into Complex Shapes

Today we will learn about deep drawing process. Deep drawing is a sheet metal forming process. Metal sheets are largely used in manufacturing industries due to its easy operation, light weight and ability to be converted into various shapes. Sheet metal also provides good strength. There are various advantages of sheet metal products which make it an versatile operation.

 

Many manufacturing industries like automobile industries, air craft industries, marine industries etc. are largely associated with sheet metal working. There are many products which we used in our daily like metal doors, metal desks, metal can, roof sheets etc. are sheet metal product.

There are various processes which are used to forming or shaping sheet metal. These processes can be classified into three ways.

1. Sheet Metal cutting process 2. Sheet Metal Bending Process 3. Deep Drawing

In our previous articles we have discussed about sheet metal cutting and bending operations. This article mainly focuses on deep drawing process.Let’s start the discussion.

Deep Drawing Process:

Deep drawing is a process of converting metal sheet into cylindrical or box shaped structure with or without changing its length and thickness. Many cylindrical parts like metal can, pots, container for food and beverages, kitchen sinks, automobile fuel tank etc. are deep drawing product.

Before discussing its working process, first we should learn about main part used in drawing process. These parts are as follow.

Circular Die: Sheet metal blank is placed over a circular die opening. This die is made by either tool steel or cast irons. Sometimes carbides and plastic are also used for die material.

Blankholder: It is a part which hold the sheet metal blank at its required place. It provides necesery holding force during drawing. This force should not be as high which can cause tear of sheet metal during operation and also not too small which can cause tearing problem.

Punch: This part provides necessary downward force at the blank. It travel downward and forces the blank into the die cavity to forming a cup shape. The diameter of the punch can be denoted by D_p

Blank: The sheet metal cut piece which is used for deep drawing is known as blank. It is placed over an circular open die. The diameter of the blank is denoted by Do.

Limiting Drawing Ratio: The ratio of blank diameter to punch diameter is known as limiting drawing ratio. It plays major role in drawing operation.

Working Process:

The blank is placed over an open circular die with the help of blank holder. The blank holder provides a necessary force to hold the blank. The punch which is attached with a mechanical or hydraulic press moves downward and provide a necessary drawing force at blank. This force tends to deform metal sheet and forces it into the die cavity and convert it into a cup shape structure.  If this force is high it causes elongation of cup wall to thin and if excessive, it causes tearing of sheet. So the punch force should remain a certain limit to avoid tearing during operation.

Deep drawing operation depends on various parameters. These are

Blankholder Pressure: If the blank holder pressure is too small it create wrinkling problem and if it is too high it causes the cup wall tear. So this pressure remains in a certain limit which allows proper operation. Normally it is taken as 0.7 % to 1 % of the sum of the yield strength and the ultimate tensile strength of the sheet.

Sometimes drawbreads are provided to control the flow of the blank into the die cavity and reduce the blank holder pressure. These drawbeads restrict the flow of the sheet metal by bending and unbending it during drawing operation.

Clearance between Punch and Die: Clearance between punch and die should be taken as 7 – 14% greater than sheet thickness. If this is too small it sheared or pierced the sheet blank.

Corner Radius of Punch and Die: Corner radius can cause of wrinkling if it is too large and can causes fracture if it is too small. So this radius should take between these two limits.

Lubrication: Lubrication lowers the forces, increases draw ability and reduces defect in the part and wear on the tooling. It does not provide on punch because friction between punch and blank improves draw ability by reduce tensile stress. Mostly used lubricants in deep drawing process are mineral oils, soap solutions, and heavy duty emulsions.

 

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