In the process of sheet metal processing, stretching forming is a common technology, widely used in automotive manufacturing, aerospace, home appliance equipment and other fields. As an important parameter to measure the degree of deformation of the material during deep drawing, the stretching coefficient plays a significant role in designing molds and formulating processes. Correct calculation of the stretching coefficient not only helps to improve product quality but also can effectively reduce material waste and mold wear.



# Basic Concept of Stretching Coefficient



The stretching coefficient (Drawing Ratio), also known as the deep drawing coefficient, refers to the ratio of the diameter or area of the workpiece before and after stretching, which is used to measure the deformation ability of the material during the stretching process. It is usually represented by m, and its definition is as follows:



- Diameter stretch coefficient:

\[ m = \frac{d}{D} \]

Where \(d\) is the inner diameter after stretching, and \(D\) is the original blank diameter.



- Area stretch coefficient (area reduction rate):

[ m_A = \frac{A_1}{A_0} = \frac{\pi d^2 / 4}{\pi D^2 /4} = \left( \frac{d}{D} \right)^2 = m^2 ]



The smaller the stretch coefficient, the greater the degree of material deformation, and the higher the material's ductility requirements. Generally, the stretch coefficient for a single stretching operation should not be too small, otherwise it is easy to cause material破裂 or wrinkle.



# Factors Affecting Stretch Coefficient



1. Material properties

The plasticity, yield strength, elongation, and other properties of the material directly affect its stretching performance. For example, soft steel, aluminum alloy, etc., have good stretching properties.



2. Mold parameters

The gap between the punch and the die, the mold radius of curvature, lubrication conditions, and other factors will affect the stretch coefficient.



3. Stretching times

Multiple stretchings can achieve a larger total deformation amount. Usually, the stretch coefficient after the first stretching is larger, and it can gradually decrease thereafter.



4. Workpiece shape

The complexity of the shape of the stretched part will also affect the selection of the stretch coefficient. For example, the stretch coefficient required for cylindrical parts, box-shaped parts, conical parts, etc., is different.



# Reference Range of Stretch Coefficients for Common Materials



| Material type | Single stretch coefficient limit (m) |

|----------|------------------------|

| Soft steel plate | 0.5～0.6 |

| Stainless steel plate | 0.6～0.7 |

| Aluminum alloy plate | 0.55～0.65 |

| Copper alloy plate | 0.5～0.6 |



In practical applications, the appropriate stretch coefficient should be selected according to the specific material and process conditions, and tests should be carried out as necessary for verification.



# Example Calculation



Assuming there is a circular blank with a diameter of 100mm that needs to be stretched into a cylindrical part with an inner diameter of 60mm. Calculate its stretch coefficient:



- Diameter stretch coefficient:

[ m = \frac{60}{100} = 0.6 ]



- Area stretch coefficient:

[ m_A = m^2 = 0.6^2 = 0.36 ]



This stretching reduces the area by 64%, which belongs to a moderate degree of stretching and is suitable for materials with good ductility.



# Conclusion



The stretch coefficient is one of the key parameters in the design of sheet metal stretching processes. A reasonable choice of the stretch coefficient not only helps to improve production efficiency and product yield but also extends the mold life. In actual operation, engineers should comprehensively consider material properties, mold design, and process parameters, and carry out multiple stretching operations as necessary to achieve complex forming. Mastering the calculation method and influencing factors of the stretch coefficient is of great significance for improving the technical level of sheet metal processing.