For pipes with large bending diameters, but with small bending radius and thin pipe walls, crease proof molds and ball mandrel rods should be used. When the pipe is bent, the material inside its center line is compressed. When bent to a certain Angle, the material will be pushed back and past the cut point due to the high compression force of the material. If the pipe wall in this area is not supported by the bending mold, wrinkles will be formed. Even if it is flattened between the mandrel and the bending die later, the wrinkles cannot be eliminated. And when the bending is over, the tube will form a big wrinkle.
The front end of the wrinkle-proof die should be made very thin, stretched to the cut point of the bending die, supported by the bending die, and fixed to form a closed cavity. In order to reduce the resistance when bending, wrinkle-proof die in the installation, should be slightly a very small cone. The pressure of the die should be adjusted appropriately. When installing and adjusting the anti-wrinkle die, it is best to use the "adjusting mandrel", which is done with the clamping die clamped.
Using a crease die, you can increase the support in this area so that the tube wall thickens evenly after compression without creasing. However, due to improper adjustment, the anti-wrinkle mold cannot eliminate wrinkles after they have been created. The shape of the anti-wrinkle mold is very important. The groove through which the tube slides should be slightly larger than the outer diameter of the tube, the value can be 10% of the wall thickness, and the tube should be carefully polished to prevent scratches.
An appropriate layer of oil should be applied to the anti-wrinkle die during bending. But too much or too thick oil can also create wrinkles in this area.
A ball mandrel is often used at the same time as an anti-wrinkle mold. Anti-wrinkle mold is used to prevent the pipe from wrinkling when bending; And the ball mandrel can keep the pipe from collapsing after leaving the mandrel support point.
When bending tubes with thin walls, large diameter and small bending radius, the following points should be paid attention to in order to avoid wrinkles:
① Reduce the resistance between the mandrel and the inner wall of the tube when bending. The inner chamber of the tube is wet blown sand before bending.
② The machine tool and spindle should have enough rigidity.
③ Mandrel rod should have enough rigidity, as far as possible to eliminate the trembling and stretching caused by the rod.
④ The radial gap between the inner diameter of the mandrel and the tube should not be greater than 10% of the wall thickness.
⑤ Clamping die, pressing die, mandrel and wrinkle-proof die should be adjusted properly, especially the pressure of pressing die should be carefully selected and adjusted.
Bending large diameter pipe, clamping die clamping is also a prominent problem. This requires the straight section between the two bends of the pipe to be a little longer, and the clamping die to be wider, so as to increase the clamping force and ensure the smooth bending. If the clamping die is too narrow and there is not enough clamping force when bending, the tube will slip and not follow forward, so that the bending cannot be carried out.
⑥ Choose the right amount of lubricating oil.
⑦ The supply of materials is also very important. For stainless steel pipe, the hardness of HB=180 ~ 220, HB=180 is the most appropriate.