The bending of metal pipes is widely used in modern industry, mainly for automotive, machinery, boiler, chemical, shipbuilding and other industries.
To ensure the quality of pipe bending processing, it is necessary to design reasonable pipe bending tooling. In the design and production of the dies, we must pay attention to the process parameters.
A standard set of pipe bending tooling mainly consists of bend die, clamp die, pressure die, mandrel, wiper die. The action process is: the mandrel enters, the clamping die clamps the tube and rotates with the bend die, the pressure die presses the tube and following the bending of the tube, and the wiper die is fixed. When the bending angle reaches the set angle, the mandrel rod exits, the pressure die and clamp die are loosened and reset, the entire action was completed.
This show whole movement of the pipe bending process：
Based on HIPPO’s years of work experience, here are the several key points that should be specially considered for the design of pipe bending tooling:
1. Bend Die
The bend die is the core of the entire bending tooling design, and the design generally starts with it. After the product tube outer diameter D, wall thickness T, and bending radius R (we call them “design three elements”) are determined, the rebound of the tube must be considered when designing the bending radius of the bend die, so as to determine the bending radius R’ of the bend die.
At present, Rx=1 is the highest level of industry technology, which is not considered in general design due to high cost and difficulty.
The cavity diameter of the bend die is designed according to the outside diameter of the pipe D. The wall thickness and outside diameter of the pipe determine the strength of the pipe, which directly affects the clamping length of the clamping die. The clamping length of the bend die fits the clamping die, which will be discussed later in the design of the clamping die.
The bend die is subject to frequent clamping impact of the clamping die and the bending force of the pipe, so it requires good overall toughness and good impact resistance, and the cavity surface needs to be wear-resistant. Currently，we adopt the heat treatment process of quenching and tempering and nitriding, and the surface hardness of the cavity can reach HRC55～HRC60.
2. Clamp Die
The design of the clamp die mainly needs to consider the length, which mainly depends on the straight distance between the two bends of the product. If the clamp die length is too short，the pipe is easy to slip when the pipe is bent. On the contrary, if the clamp die is too long, it is easy to flatten and deform the previous bend, which is not allowed in the bending process. Therefore, the length should be selected appropriately. It’s better your designer leave 2～3D for the straight distance. For products with only one elbow, we can consider adding a support handle during clamping to improve clamping stability.
The diameter of the clamping die cavity is designed according to the pipe outer diameter D. To ensure stable clamping and no slippage, the diameter of the cavity is generally designed according to the negative tolerance (the cavity size of the straight section of the bend die with which the clamping is designed is the same), we can adjust the clamping degree of the clamping die to achieve the best state, so as to ensure stable clamping of the bent pipe and meet the appearance requirements. In order to ensure that the outer surface of the pipe is not pinched during the clamping process, the corners of the cavity must have an R-angle design. The clamping die is generally quenched and treated to about HRC50 to improve wear resistance and service life.
Mandrel has various shapes, it is mainly used for supporting, so as to control the pipe deformation and quality of the bent part. Using which shape depends on the product design, pipe specifications and design structure. The straight mandrel is the most common mandrel. Straight mandrel is simple in structure, easy to process and use. It is mainly used for products with thicker pipe wall thickness, larger bending radius, and low bending deformation requirements.
The ball-end straight mandrel is more convenient for loading operations, and has basically replaced straight mandrel at present. The arc straight mandrel is more complicated in processing than the ball-end straight mandrel, but due to its arc plays a certain supporting role in the bending process, the bending part has less deformation, and the quality of the bend is improved.
The connecting mandrel is mainly connected with a plurality of spherical beads, which can maintain the supporting effect of the material during the bending process, so the bending deformation is small, the elbow is full, and the quality is quite high. The number of beads depends on the bending angle and bending radius, and the purpose is to obtain the ideal bending fullness. Too many beads, high resistance, easy to break during the bending process, affecting production efficiency. If the number of beads is too small, the bending deformation is large, the fullness is not good, and the quality does not meet the requirements.
Generally, the mandrel rods are mostly quenched to about HRC50 to resist the bending and abrasion of the pipe, thereby increasing the service life. If the hardness is too high, the toughness of the mandrel will be reduced and breakage will easily occur. The diameter of the mandrel is very important. typically, it is 0.1～0.4mm smaller than the theoretical inner diameter of the pipe to ensure the smooth insertion of the pipe and the quality of the bend. Among them, the gap depends entirely on the product design. Different designs and different pipes have different design gaps, which need to be understood in practice.
4. Pressure Die
In the pipe bending process, the pressure die mainly compresses the pipe and assists in the bending. Theoretically, the pressure die movement speed should be the same as the pipe rotation linear speed. In actual debugging and production, the pressure die assist pushing speed can be adjusted until the ideal product come out. The pressing force is generally determined by experience. If the pressing force is too large, the pipe material will be thinned significantly or even broken. If the pressing force is too small, it is easy to wrinkle.
Usually, the pressure die length is calculated from the length of the unfolded arc. We can add 3 times the pipe diameter to the expanded length of the curved arc. If the pressure die is too long, it will interfere when feeding the tail material or will waste pipe material, which is not acceptable in the process.
The cavity edge needs to be designed with an R-angle transition, which can solve the clamping mark and improve the appearance of the finished part. The pressure die should be quenched to about HRC50 to ensure that the cavity is wear-resistant and increase the service life.
5. Wiper Die
When the ratio of the wall thickness to the outer diameter of the pipe, that is, t/D, is less than or equal to 0.055, we need to design the wiper die to prevent the pipe from wrinkling during bending. The wiper die plays a role of supporting and at the same time, the pipe slides relative to the wiper die cavity. Therefore, the wiper die is required to be wear-resistant, smooth in surface and low in friction coefficient. At present, the commonly used material is aluminum bronze (QAl9-4), which is processed by a machining center, with high precision of the curved surface and smooth surface of the cavity.
Theoretically, when designing pipe bending tooling, the bending torque of the product should be calculated first, so as to determine whether the capacity of the pipe bending machine meets the requirements. In fact, the bending moment when the pipe is bent not only depends on the performance of the pipe, section shape and size, the bending radius, and other parameters， but also has a great relationship with the bending method and the tooling structure used.
Therefore, it is currently impossible to express many factors with calculation formulas. In production practice, it is still mainly based on experience to make estimates.
We only need to understand the bender’s capacity, consider the maximum bending torque to prevent wasteful die design due to insufficient equipment capacity, no need for detailed calculations of the bending torque.
Bending toolings are usually made of 45# steel, or higher strength and toughness die steel. We usually need to do heat treatment to the dies, which can greatly extend the service life of the die. But for different materials, need to carry out the different heat-treatment processes.
For example, 45 steel, generally for the overall quenching treatment is good, if the conditions allow, the contact surface of the bend can be quenching or carburizing treatment. For the demanding surface, there are polishing and chrome plating treatments.
The ultimate goal of bending tooling design is to meet the product requirements with the lowest cost and stable mass production to meet customer delivery requirements. This requires continuous research in the work, continuous improvement of the mold design, and gradual improvement of the bending process to produce high-quality bending products. We hope this article is helpful for your tooling design.