Analysis and countermeasures for common problems when bending pipes by pipe bending machines

Table of Contents

1. The cross-section of the curved section is severely flattened:

Possible reasons: the mold cavity is not round; the position of the mandrel is behind, or the mandrel balls are few, which cannot effectively support the bending section, resulting in collapse; at the tangent point of the bending, the pressure of the die is not enough; when bending, the pipe is in the clamping die Slight sliding, at this point just check the gripping section for scratches.

Solution: Check the mold cavity, and replace the mold if it is not correct; move the position of the mandrel forward, if the tangent point of the curved arc cannot be reached, increase the number of balls; increase the die pressure and boost pressure; if there are slip marks in the clamping section, the clamping pressure should be increased.

2. Excessive collapse of the bending section:

Possible reasons: The mandrel is not supported enough; the rotation speed is greater than the boost speed, and the tube is clamped too tightly.

Solution: Move the mandrel position forward, if it is not enough, increase the number of balls. Or turn the plug into a ball, adjust the rotation speed or boost speed, and reduce the die pressure.

3. Tool indentation:

(1) Section indentation in the clamping die area

   ①Semi-circular indentation at the joints of the inserts

   Possible reasons: the inserts on the wheel die are not well matched, and there are steps.

   Solution: use the integral mold or modify the mating surface of the insert.

   ②Indentation at both ends or one end of the clamp

   Possible reasons: For deep indentation at both ends, the clamping pressure may be too high. If there is an indentation in the axial direction at the same time, it means that the diameter of the pipe is large; for indentation at one end, the clamping die may be inclined in the axial direction. Spend.

Solution: For deep indentation at both ends, reduce the clamping pressure and strictly control the size of the pipe; for indentation at one end, copper skin can be added behind the clamp to adjust the slope; when the straight section of the pipe is smaller than the length of the clamp, in order to increase the clamp Die clamping force, the auxiliary mandrel can be added.

   (2) Axial indentation in the clamping die/die area

    Possible reasons: The clamping force is too large, the size of the matching mold cavity is too large or too small; the center of the matching mold groove is misaligned; this phenomenon also occurs when the pipe diameter is large.

Solution: Use a mold with the correct cavity size; align the center of the mold groove, while reducing the clamping force appropriately; strictly control the diameter of the pipe.

   (3) Axial indentation of bending section

Possible reasons: The center of the die and the wheel die are not vertically aligned or the die cavity is small; this phenomenon also occurs when the pipe diameter is large.

Solution: adjust the vertical alignment of the center of the die, increase the gap between the die and the wheel die, and reduce the die pressure; if there is still an indentation, measure whether the pipe diameter matches.

4. Bend the medial folds:

(1) The entire curved arc is covered with wrinkles and extends to the anti-wrinkle mold area

    Possible reasons: The anti-wrinkle die is located at the back and the size of the groove is large, which cannot effectively support the tube wall; the gap between the die and the anti-wrinkle die/wheel die is large, and the pressure of the die is small; the diameter and position of the mandrel are incorrect.

Solution: Check the diameter of the mandrel and the mold groove, and select the correct mold; narrow the gap between the compression mold and the anti-wrinkle mold or the wheel mold, increase the pressure, and push the anti-wrinkle mold forward; the mandrel gradually (1.5mm) advances forward, until a satisfactory result is obtained.

   (2) There are wrinkles between 45º~90º of the curved arc, and most of them are at the bottom

Possible reasons: The cavity of the wheel model is not round, and the alignment of the positioning ring and the positioning block is not concentric; the gap between the die and the anti-wrinkle die or the wheel die is large, and the pressure of the die is small.

Solution: reduce the gap, increase the pressure, and push the anti-wrinkle mold forward. If necessary, sink the die cavity as a whole, and place a copper sheet after the die.

   (3) Wrinkles appear in the straight line segment after bending the arc

Possible reasons: the inclination of the anti-wrinkle die is too large, and the tube wall cannot be effectively supported; the gap between the anti-wrinkle die and the die is large; the diameter of the straight section of the mandrel is small, and the gap is too large after inserting the tube, and the tube wall cannot be effectively supported; The boost force is large or the boost speed does not match the rotation speed; check the wall thickness of the pipe in the bending section, if the thickness is not reduced much, it means that the pressure of the die may be small; the tip of the anti-wrinkle die is worn, and the root does not fit well with the wheel mold cavity.

Solution: reduce the inclination of the anti-wrinkle mold, reduce the inclination of the anti-wrinkle mold, and reduce the gap between the anti-wrinkle molds; detect the wall thickness of the pipe in the bending section, and increase the pressure of the mold when the thickness is satisfied; detect the core If the diameter of the shaft is small, replace the mandrel; adjust the boost pressure, and adjust the boost speed to match the rotation speed; repair the anti-wrinkle mold, so that the contact surface between the rear root and the wheel mold cavity should be at least 80%, and the distance between the front and the wheel mold should be at least 80%. The cutting point of the wheel die is 2~3mm, and the thickness of the tip should be uniform up and down.

5. Scratches:

(1) Scratches appear in the curved arc and the anti-wrinkle die area

Possible reasons: There are burrs on the surface of the pipe; there are metal slag, chips, or irregular fluff on the surface of the die or the anti-wrinkle mold cavity; the size of the mandrel is small; The material of the tubing does not match.

Solution: increase the inclination of the anti-wrinkle die, the pressure of the die, or the surface roughness; clean or polish the surface of the pipe and the mold cavity; add diesel oil or lubricating oil to the anti-wrinkle die area; adjust the boost pressure or speed to match the rotation speed; Anti-wrinkle mold material compatible with the material of the pipe to be bent;

  (2) There are wrinkles in the arc segment, and straight line scratches appear in the clamping area

Possible reasons: The clamping force of the die and the clamping force of the die do not match; the position of the mandrel and the anti-wrinkle die may be wrong; the roughness of the die cavity is low; there are metal slag, chips, burrs on the surface of the pipe or the surface of the die cavity Or oil stains; excessive clamping force makes the clamping die against the ball of the mandrel, clamping the pipe and causing friction marks.

Solution: adjust the clamping force of the die and the clamping die; check the position of the mandrel and the anti-wrinkle die and the lubricating oil condition; spray carbide spray to the clamping die cavity, or increase the roughness; clean or polish the surface of the pipe or Clamp the mold cavity, no particles, burrs or oil stains are allowed.

6. The wall thickness is smaller than the standard requirement:

Possible reasons: the boosting speed is lower than the rotation speed; the pressure of the die is large and the boosting force is small; the wall thickness of the raw material of the pipe is small.

Solution: increase the boost force and boost speed; reduce the pressure of the die or increase the gap between the die and the wheel die/anti-wrinkle die; detect the circumferential wall thickness of the pipe, and select the relatively thick side to put it out; down, minimize the length of the end of the pipe to facilitate metal flow.

7. A straight line segment appears before the tangent point of the bent end:

Possible reasons: The mandrel fails to effectively support the pipe, and the mandrel pitch is large.

Solution: Re-process the coupling joint of the ball to reduce the pitch.

8. Bend rebound:

Possible reasons: When the pipe is bent and loaded, the pipe wall material is elastically and plastically deformed. After unloading, the deformation due to the elasticity of the material must be restored, that is, bending rebound occurs.

Solution: mainly use compensation method and correction method to control.

The compensation method is to comprehensively analyze the influencing factors of bending spring back, pre-estimate the amount of spring back according to various conditions and spring back trends during bending, and modify the size and geometry of the working part of the convex and concave die when designing and manufacturing the mold. “Over-positive” bends.

The correction method is to take measures on the mold structure, so that the correction force is concentrated at the bend, changing the force state, trying to eliminate the elastic deformation and overcome the spring back. For example, in the stretch-bending process, tension is applied while bending, so that the entire section is under the action of tensile stress. When unloading, the elastic recovery is consistent with the deformation direction, which can significantly reduce the amount of rebound.

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