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Multi-bend exhaust system is one of the most complex forming processes in tube manufacturing. Instead of one right angle bend there is a great extent of stress buildup, compression in small areas, and material turbulence by multiple bends in the exhaust path. Don't think of wrinkles as an indifferent cosmetic defect: They're itchy, unsightly crease lines that can appear on the bend's inner radius. These affect the flow of exhaust gas, turbulent flow, backpressure and initiation points for thermal fatigue. There should be a systematic approach to simplifying wrinkling – where tools, machine parameters, material behavior and process sequencing are all involved. Baorui, like all manufacturers, has a consistent approach to providing wrinkle-free multi-bend exhaust assemblies: manufacturers have to keep the parts straight while in transit. All manufacturers, even our own at Baorui, have a consistent approach to providing customers with wrinkle-free multi-bend exhaust assemblies: manufacturers need to keep the parts straight in transit.
Optimize Mandrel Design and Positioning for Each Bend
The best ‘defence' against wrinkling is a well-designed and correctly positioned mandrel. For multi-bend systems, such as, bends followed by short straight sections, the mandrel should have sufficient strength to withstand the tube's inner force at the entire bend deformation zone. A flexible ball mandrel with multiple articulating sections is needed for this tight radius, to follow the curve and to not buckle the material inwards. The absolute amount of ball segments and the diameter clearance, however, together with the position of the bend tangent line in relation to the mandrel nose, need to be adjusted for each position of a bend. Problem #1 is that any problem of wrinkling is due to the mandrel being retracted too much, causing the portion subjected to compression to be unsupported. Here at Baorui we mark each bend angle on the exhaust tube, and then add the mandrel stroke by adjusting it for every bend separately with our CNC controls. We also keep a separate set of mandrels for every diameter/exhaust wall thickness, as using a universal mandrel is a sure way to run in the holes with internal ripples.
Control Feed Rate and Dwell Time with Precision
Wrinkling is a phenomenon related to speed. If bending arm spins around too fast material cannot flow plastically and the inner radius has pile-up. Too slow speeds, however, may lead to uneven strain hardening of material. For multi-bend systems, the optimum feed rate should be worked out for each bend, while considering the work hardening effect from previous bends. Operators can set variable profiles, such as deceleration on the last stretch of the bend to final angle, on the advanced CNC hydraulic pipe benders. Moreover, the controlled dwell time at the end of the stroke provides the material with time to relieve its elastic memory, which can minimize the spring back that can cause wrinkling at the end of the stroke. The real-time monitoring of torque at our production floor allows us to detect compressive instability before it starts, the machine can automatically access the appropriate bite and adjust the party to a lower feed rate, ensuring the stability of the process. This dynamic adjustment has practically eliminated wrinkle defects on all of our multi bend exhaust runs.
Select Appropriate Lubrication and Wipe Die Clearance
Pile up friction is the unseen cause of wrinkling. High friction along the extrados (outside) of the tube can cause uneven material lining and may cause a resistance to the natural flow of material through the pressure die along the intrados (inside). It is possible to minimize these difference forces by applying a high performing bending lubricant on the tube surface and a lubricant on the mandrel as well. But, far more important is the clearance setting between the wiper die and tube. The wiper die follows the bend die, and is employed to "wipe" the inner radius off the material as it's formed. Too tight wiper die clearance will "scrape" the tube and too loose will "allow wrinkles to escape" the tube. For any system with multiple bends, we recommend customized relief of the contact face of a wiper die to match with the outside diameter of the tube. For wiper die, we do feeler gauge test in every installation in Baorui, which clearance is 0.05-0.10mm according to the material hardness. We also automatically add a valuable lubricant to the bend points in a metered spray system between the bends as this is one of the most overlooked causes of intermittent wrinkle.
Sequence Bends Strategically and Account for Work Hardening
Last but not least is the manner which you perform multiple bends and its profound impact on the formation of wrinkles. The general rule for what the engineer should do in a typical exhaust where there are three or four bends is to bend the most critical or the least easy bend first, while the material is still fully annealed and ductile. The last bend that is done first is the tightest bend, and material has previously been stretched from previous bends in the manufacturing process, such that it may not be well suited to later bends in the compression zone. Also take the distance between bends into account. If the two bends are too near each other, the “work-hardened” zone becomes attached to the “deformation zone” of the other bend, decreasing the strength of the bend and making it more susceptible to wrinkling. To optimize the bend sequence and intermediate straight lengths, our engineering group at Baorui applies finite element simulation to the design. When the total arc bend greater than 180 degrees on a single tube point, an intermediate stress-relief annealing is also included due to the high risk of wrinkles from exceeding the amount of cold work to 180 degrees. Sequenced along with the three preceding controls, the bending inner radius control will make sure that all the bends in the body come out with a smooth and uninterrupted turn.
The key to preventing wrinkles in multi-bend exhaust systems isn't a single silver-bullet answer, it's an orchestration of mandrel support, bend speed control, lubrication, tool clearance, and bend sequencing into a coherent process. These five disciplines are interrelated and so we have perfected them at Baorui over the past 20 years, consistently manufacturing components capable of performing both flow and durability functions. Removing wrinkles at the bending point removes problems in the field — and that's the best test of good manufacture!