Mandrel vs. Empty Bending: Understanding the Key Differences

From handrails and guardrails to aerospace exhaust pipes and coolant tubes, bent tubes have extensive application across a myriad of industries. These structures are even used in making the simplest home décor items like lamp shades, bicycle tubular frames, automobile exhaust pipes, and many more. Nevertheless, tube bending is a pivotal part of metalworking and fabrication projects. It allows manufacturers to produce intricate bend patterns of the desired design as per the future applications. That’s why choosing the right bending technique is pre-eminent.

Driven by innovation, the metalworking and fabrication industry has adopted multifarious bending techniques since the last few decades, including rotary compression bending, crush bending, and profile bending. Despite having so many options in hand, the two tube bending techniques that have gained immense popularity are empty and mandrel bending. Having said that, we have highlighted their key differences in this following blog post to help you make a better decision for your upcoming bending project.

What is mandrel bending and how is it done?

Mandrel bending is a popular type of rotary draw bending technique that utilizes a strong, solid metal rod, known as a mandrel to maintain the tube’s structural integrity. It is inserted inside the tube’s hollow frame to strengthen it from within and counterbalance the high pressure applied by the bending die. During the bending process, the tube’s material is stretched along the bend’s outer radius. On the contrary, the material accumulates to form a thicker region along the bend’s inner radius. In other words, the longer side of the tube’s bend undergoes a tearing process while the shorter one forms multiple ripples. 

Due to the equivalent distribution of the material around the bend region, the tube retains its structural symmetry. Furthermore, the solid mandrel prevents the structure from collapsing, causing no mechanical and physical damage to the tubes.

What is empty bending and how is the process conducted?

Empty tube bending, on the other hand is just the opposite of mandrel bending. Here, no solid structure like mandrel is inserted into the hollow lumen of the tube to be bent. Instead, the material’s thickness and physical strength are leveraged to minimize structural deformation and counterbalance the external pressure exerted by the bending die. Since the chances of losing structural integrity are more during forming an empty bend, tubes with thicker walls and immense physical strength are used in this technique.

Features and advantages of mandrel bending

1. Prevents structural deformation: As a mandrel is inserted into the tube during the bending process, no structural deformation is observed in the final products. In other words, the tube retains its physical shape, with no minor dents on the lower or upper bend radii. Also, the bends are cleaner, thereby enhancing the product’s physical appearance and aesthetics. 

2. Even material distribution: In mandrel bending, the material becomes thinner along the outer radius of the bend while it accumulates along the inner radius. With such an expert level of material distribution, no wrinkles are formed along the tube’s surface, specifically in the bend regions. 

3. Higher precision and accuracy: Empowered by CNC tube bending technique, manufacturers can achieve an impeccable level of accuracy and precision around the bends. That’s why this process is often employed in projects that require intricate bend formation with minute detailing. 

4. Stops spring-back action: Lastly, the mandrel inserted can handle the immense pressure exerted on the outer walls of the tube. As a result, it becomes easier for the tube in concern to counterbalance the back pressure and stop springing back to its original shape after bending.

Features and advantages of empty bending

1. Lower tooling costs: The empty bending technique doesn’t require state-of-the-art tools, like mandrels, special bending dies, and so on. As a result, the overall tooling cost is reduced substantially, allowing manufacturers to lower their budget and drive higher ROIs.

2. No material thinning: In empty bends, the tube’s material doesn’t undergo any tearing or rippling effect. Therefore, the material thickness remains uniform along both the outer and inner radii of the bend, ensuring no thinning or breakage. 

3. Caters to larger bends: Empty bending is undoubtedly the best option when it comes to forming bends with larger radii in the tubes. As the bending angle is increased, the pressure exerted on the outer tube walls gets distributed over a larger area, thereby reducing the risks of structural deformation. 

4. Versatility: One of the major benefits of this CNC tube bending technique is the wide range of applications. It is often used in forming larger, diverse bend patterns for aerospace, HVAC, automobile, electronics, and other industries  

Challenges of mandrel bending 

● The cost involved with mandrel bending is exceptionally high, especially due to the innumerable tooling involved and the higher costs of individual mandrels. 

● Choosing the right mandrel for the bending operation is not always feasible, especially when there is a knowledge gap. 

● Weaker mandrels can never prevent structural deformation of the tubes, which will further increase the material damage risks. 

● If the mandrel is too small, the tube will form wrinkles and ripples along the bent region.

Disadvantages of empty bending

● Empty bending is not an ideal option for tubes with thin walls or requiring intricate bend patterns. 

● Without any support of a mandrel, the chances of structural deformity increase, especially if there are sliver of alterations in the exerted pressure. 

● The inner diameter of the tube often reduces around the bend region when compared to other parts, thereby distorting the physical shape. 

● Empty bending increases the chances of spring back action where the tube often returns back to its original shape due to the backpressure.

Factors to consider to choose between mandrel and empty bending techniques 

1. Material thickness and strength: If the tube is thin-walled or made from materials having lesser mechanical strength, mandrel bending is a more suitable option. On the contrary, empty bending is apt for thick-walled tubes with immense physical strength. 

2. Bending radii: Mandrel bending is ideal to achieve smaller bending radii with intricate geometrics while empty bending is more suitable for creating larger bends in the tube. 

3. Aesthetics: To enhance the tube’s aesthetics and quality, mandrel bending is a far better option as the bends created are cleaner with no external damage.


Both mandrel and empty tube bending techniques come with their fair set of advantages and disadvantages. As a result, selecting the right bending technique is crucial for you to achieve higher productivity, optimize costs, and minimize material damage. On top of this, the choice of the bending process will also depend on the tube material, bending radii, and the applications.

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