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In the practice of metal sheet bending, springback is a essential common physical phenomenon. It can directly affect the tolerance of sheet metal parts. As for engineers who are responsible for machining precision metal parts, understand springback and know how to reduce springback is very important. So, we will explain springback meaning, causes and calculating methods for you.

What Does Springback Mean?

Springback is the common and important phenomenon during bending metal materials. It means the part may elastically return to original shape if the force is removed, and the final shape is a little different form the desired. This elastic recovery is called springback. But don’t worry, this recovery is not a defect, but an inherent physical properties of materials.

Is Springback Important for Metal Bending?

Springback decides the bending is successful or not. Generally, parts are bent to form a whole product. It means that the precision of parts is important for parts assembly. In some applications such as aerospace, automotive parts, springback is strictly controlled to ensure that parts are made accurately.

Why Springback Happens?

Let us provide an simple example: You may find when you bend a plastic ruler manually, if you release one side of the ruler, the ruler will snaps back, instead of keeping the shape you bent. Why this happens? Actually, this is because the parts undergo two types of deformation. One is plastic deformation, and another one is elastic deformation. Plastic deformation means that your parts won’t recover original shapes, while elastic deformation means that the parts will return into original shapes after the loading is removed. Then what key factors can affect springback? Let us talk about this in the following points.

Material Properties

Properties of materials are is the first factor that causes springback. During bending process, the yield strength and elastic modulus serve as the key parameters for predicting springback trends.

Thickness of Materials

The thicker the sheet metal plate, the smaller the springback, this rule is suitable for almost materials. Under the determined bend radius, the thicker of plate, the the smaller the springback.

R/t Ratio:

R/t ratio means the ratio of the bending radius R to the plate thickness T. It is well-known that if r/t ratio increase, the springback is larger. Below is the table of common r/t ratio and its influence on springback.

What you need to know is that although the trend is the same, the r/t range varies with materials. Here are some common metals and their r/t ratio range.

Bend Radius and Bend Angle

In generally, if the bend angle is larger, then the springback will be larger. It is not the main reason but it is still an important factor. The greater the bending angle, the longer the deformation zone (bending arc length), and the greater the total amount of accumulated elastic strain and plastic strain.

Forming Method

Different forming methods can definitely influence the springbakc of materials, and in real bending process, they are the common methods used to control and reduce springback. We will introduce influence of bending methods on the springback of sheet metals.

Air Forming

Air forming (air bending) can lead to greater springback because the materials don’t fully conform to the die. The less contact usually leads to higher springback. From the picture below, you can easily understand why air bending can cause higher springback.

Although air forming causes larger springback, it is flexible and usually used for small-batch production.

Bottoming

When metal sheet is bent by bottoming, the materials is pressed fully into the die, and less elastic stress is released, so springback is smaller.

Coining

Coining process can make materials undergo full plastic deformation by applying high pressure. It can result in smallest springback.

H2 Why Do Different Materials Exhibit Significant Variations in Springback?

We have known that springback is the result of elastic deformation of materials after unloading. During bending process, plastic deformation and elastic deformation occur simultaneously. When the load is removed, the material can partially recover its shape. Different materials have different elastic-plastic behaviors, so their springback vary significantly.

Here is a simple practical formula for you to understand:

Springback trend ≈ Yield strength ÷ Elastic modulus

Springback ∝ σ_y / E

First, yield strength means the level of stress when a material begins to undergo permanent plastic deformation. And elastic modulus means the resistance of a material to elastic deformation.

If a material has high yield strength, it needs greater stress to induce plastic deformation, it means greater springback of the material when bending.

However, the actual amount of springback depends on the elastic modulus, as you can see the formula above.

For instance, the yiled strength of high-strength steel is up to 700 Mpa, and its elastic modulus is about 210 Mpa, so its springback is grate. On the contrary, with the same elastic modulus, ordinary low carbon steel would happen smaller springback because of its 200 Mpa yield strength. Let us have a look at other materials.

Springback of Aluminum Alloys

Aluminum alloy is a typical “high-springback” material, even when annealed. Let us take 6061-T6 as an example, its yield strength is equal to steel, but its elastic modulus is extremely low. So, aluminum alloy may cause a springback 3 times greater than steel when they are used to bend brackets with the same requirements. In aerospace application, bending aluminum alloy profiles often requires pre-setting the die angle below 90 degree to achieve a 90-degree part.

Springback of Stainless Steel

Stainless steel, especially 304 and 316 austenitic stainless steels, exhibits unique springback behavior during bending. Importantly, in addition to its yield strength, stainless steel tends to work harden easily during bending, which can also lead to large springback. It means that during bending, the yield strength of stainless steel is becoming higher, and that can results in larger springback than evaluated or desired.

Why Springback Is More Critical in Precision Custom Parts

Custom precision parts always have features including small-volume production, special materials, demanding tolerance, and high value. For example, medical or aerospace components like inconel brackets are always made of high-strength material. When bending these parts, the tolerance needs to be strictly controlled so that the parts can finally meet customer’s requirements. Therefore, it is necessary to rely on accurate numerical simulation and springback compensation.

How Springback Affects Angle Accuracy and Part Tolerances

Springback has direct influence on the precision of parts. If the workpiece is required to be bent with 90°, while the springback is 2°, then the final part’s angle may be 92°or 88°. If the part has complex geometry design, each bending may cause one time springback, and finally, the bent part can’t be used or its shape is definitely different from the desired one.

How to Calculate and Predict Springback

The prerequisite for bending successfully is predicting springback accurately. The main calculating methods include empirical formulas, springback angle calculation concepts, and using FEA simulation tools.

Empirical Formulas and Simplified Estimation

Empirical formulas is the widely-used rapid method without need for complicated calculations or software. The commonly-used formula is:

D ≈ [Ir / (Mt × 2.1)] × Material factor

Explains:

• D: springback degree
• Ir: inside radius
• Mt: Material thickness

Springback Angle Calculation Concepts

The core of this method is calculating the elastic strain energy on the both sides of the bending parts. This method requires accurate stress-strain curves of the material. As for symmetric bending, it is generally believed that the change of bending angle after unloading is proportional to the change of bending radius. It can be explained as: Δα/α ≈ ΔR/R.

The typical formula for calculating springback angle is:

Δθ ≈ θ_i (1 – R_i / R_f)

Explains:

1. R_i: original bending radius
2. R_f:final bending radius
3. Δθ: springback angle

Using FEA Simulation Tools

This is the mainstream method for calculating springback nowadays. The commonly-used software includes AutoForm、PAM-STAMP、DYNAFORM、ABAQUS、LS-DYNA. It is very suitable for complex parts.

However, although FEA technique is advanced, the simulation is not always the same as real parts. This is because properties of materials(especially high-springback material) are different and simulation is sensitive to little change of parameters. Therefore, it will be better to combine the calculating methods mentioned before.

Effective Methods to Reduce or Control Springback

Springback is a common phenomenon during bending process. However, it doesn’t mean that there is no method to deal with it. In fact, optimize design of parts, select ideal die, and optimize manufacturing processes can effectively control springback.

Overbending

Overbending is the widely-used compensation method. It can deepen the die angle based of the prediction of springback value, by doing that, when springback happes, the angle of parts can turn into desired angle.

Select Low Springback Materials

We have explained that properties of materials can affect springback. So selecting low springback materials is also an ideal solution for controlling springback. Without compromising the function of parts, choosing materials with low yield strength and high elastic modulus can effectively reduce springback value.

Tooling Optimization

Tooling optimization is one of effective methods to reduce springback. In detail, optimizing puch radius, die angle, and contact geometry can improve the stress distribution, so that the elastic recovery can be reduced. However, springback can’t be eliminated because it is essentially caused by the properties of the materials and elastic deformation behavior.

CNC Press Brake Angle Compensation

Modern CNC press brake is equipped with real-time angle measurement and compensation system. When bending the first part, the angle of the part can be monitored and the data is sent back to the CNC system. The system can calculate the springback value and automatically adjust the bending angle of the next part. It is important for achieving high precision and consistent  quality.

الخاتمة

Springback is a inevitable physical phenomenon in sheet metal bending. It can be controlled by understanding properties of materials, bending processes and calculating methods. The purpose of controlling springback is to achieve desired shapes and high precision of metal parts.

Tuofa is a professional sheet metal parts manufacturer, we can help you bend desired parts and provide good after-sale service. Any question about bending or Tuofa, welcome to contact us.