Pipe bending die design
The design of the pipe bending die is a special technology for bending metal pipes (such as steel pipes, copper pipes, and aluminum alloy pipes). It is widely used in the fields of automotive oil pipes, air conditioning pipes, and furniture brackets. It can achieve a bending angle of 90°-180° and a bending radius of R5-R100mm. The roundness error at the bend is less than 5%, and the wall thickness reduction rate is less than 10%. It is suitable for pipes with a diameter of φ6-φ50mm and a wall thickness of 0.5-3mm. The core of the design is to solve the problems of pipe flatness (ovality) and inner wrinkling during bending. In the early stage, the size of the bending die, clamping die, and anti-wrinkle die of the mold needs to be determined according to the pipe diameter and bending radius. For example, a φ20mm steel pipe is bent into 90° and R30mm, and the groove radius of the bending die is set to 10.5mm (0.5mm larger than the pipe radius) to ensure smooth fitting of the pipe.
The die structure consists of a bending die, a clamping die, an anti-wrinkle die, a guide die, and a push mechanism. The bending die is made of quenched and tempered 45 steel (HRC 28-32) and chrome-plated (0.01mm thick) for enhanced wear resistance. The working surface roughness is Ra 0.8μm or less. The bending die features a semicircular groove that matches the pipe material. The groove bottom radius is 2-3 times the pipe wall thickness (2-3mm for a 1mm thick pipe) to reduce friction damage during bending. The clamping die is hydraulically driven and works in conjunction with the bending die to clamp the pipe. The clamping force is calculated based on the pipe diameter (5-8kN for a φ20mm pipe), ensuring that the pipe does not slip during bending. The clamping die is constructed of QT600-3 ductile iron, which offers excellent shock absorption. The anti-wrinkle mold is installed on the inner side of the bend and is made of polyurethane rubber (hardness 70-80ShoreA). A spring applies a pressure of 0.5-2kN to suppress wrinkling of the inner material. The length of the anti-wrinkle mold is 1.5 times the length of the bend to ensure wrinkle prevention throughout the entire process.
Bending process parameters must be optimized based on the pipe material and bend angle. For mild steel pipes, the bending speed should be controlled at 10-30°/s, and for copper alloy pipes, 5-15°/s to avoid cracking caused by excessive bending speeds. The ratio of bend radius to pipe diameter (R/D) must be ≥1.5. If the R/D is less than 3, a mandrel should be used for support. The mandrel diameter should be 0.5-1mm smaller than the pipe’s inner diameter, and the insertion depth should be 1.2 times the length of the bend section to prevent internal collapse of the pipe. For complex pipe fittings requiring multiple bends, an intermediate annealing step (600-650°C for one hour) is required to eliminate work hardening and prevent cracking during secondary bending.
The mandrel and lubrication system are key to ensuring bending quality. Mandrels are available in rigid (for R/D ≥ 3) and flexible (for R/D < 3). The flexible mandrel consists of multiple ball joints that deform with the tube, ensuring uniform stress distribution along the inner tube wall. Lubrication is achieved using a specialized bending oil (viscosity 30-50 cSt). Before bending, an oil sprayer evenly applies the oil to the inner and outer tube walls, creating a 5-10 μm thick oil film. This reduces the friction coefficient to below 0.1, minimizing die wear and tube scratches.
During commissioning and maintenance, attention should be paid to bending accuracy. During mold trials, measure the bending angle (tolerance must be <1°) and roundness. If roundness exceeds tolerance, increase the core rod diameter (by 0.1mm at a time). If wrinkling occurs on the inside, increase the anti-wrinkle die pressure (by 0.2kN at a time). After every 500 cycles, clean iron filings from the bending die and core rod surfaces, and inspect the anti-wrinkle die for wear (replace if the rubber thickness decreases by >1mm). Regularly calibrate the thrust of the ejector mechanism (tolerance <5%). Perform magnetic particle inspection on the bending die after every 1000 bends to prevent fatigue cracks and ensure safe operation.
