Factors that affect the opening angle

In the production of welded pipes on used roll forming machine, the opening angle is one of the most sensitive welding elements. The size of the opening angle directly affects the welding heat, welding speed and weld strength, and involves a wide range of aspects. As can be seen from the figure below, there are at least 7 direct factors affecting the opening angle.

(1) Squeeze roll hole radius. According to the welded pipe production process, the radius of the extrusion roll pass is always smaller than the radius R of the open cylindrical tube to be welded. In this way, different extrusion roll passes (R1≠R2) produce different first force points on the edge of the tube blank. , see Figure 2,

This affects the position and opening angle of the edge of the tube blank. In the figure, the smaller the pass radius of the extrusion roller, the earlier the pass force point B will exert force on the tube blank, and the unilateral advance S is determined by R, R1 and R2:

At the same time, since the force-applying point of the extrusion roller hole type R2 is advanced by S mm, the opening angle is reduced from α to α´, then according to the formula:

The opening angle α´ affected by R2 is:

Obviously, α>α´

When the opening angle becomes smaller, the actual apex of the edge of the heating tube blank will inevitably be advanced, and defects such as over-burning and weld perforation are likely to occur. For the same type of tube, the opening angle will decrease sharply as the radius of the extrusion roller hole becomes smaller, causing the actual confluence point of the edge of the tube blank to move forward, that is, away from the center line of the extrusion roller, and run a series of used roll forming machine. Welding process parameters such as extrusion force, input heat, resistor position, welding speed, etc. have an important impact.

For example: to produce φ50mm tube on 50 units, let R=26.11mm, R1=25.5mm, R2=25.3mm, α=2°~6°, b=6mm, S=0.84mm is calculated. Therefore, the opening angle is reduced to α'=1.73°~5.19°, which is reduced by 13.5%.

(2) The outer diameter of the squeeze roller. Welded pipes of the same specification, different outer diameters of extrusion rollers, and the first force point of the hole pattern to the pipe blank are different. In Figure 2, the extrusion roller with a larger outer diameter touches the tube blank earlier by A1A2 (edge) and a1a2 (bottom diameter) than the extrusion roller with a smaller outer diameter, which in turn affects the opening angle and the position of the actual meeting point.

(3) Thickness b of guide ring. The thickness of the guide ring is proportional to the tangent of the opening angle, the thicker the ring, the larger the opening angle; conversely, the thinner the ring, the smaller the opening angle.

(4) The tube blank springs back. According to Figure 1, the tube blank has high strength and hardness, and the edge of the tube blank rebounds greatly in the elastic deformation zone, and the opening of the tube blank will exceed the thickness of the roll ring, resulting in an actual opening angle that is larger than the theoretical opening angle.

(5) The height of the tube blank entering the extrusion roller. Since the size of the open tube is larger than the size of the pass of the squeeze roll, when the guide roll and the squeeze roll are adjusted according to the horizontal rolling bottom line, the edge of the tube blank is higher than the upper edge of the pass of the squeeze roll, which leads to the upper edge of the pass of the squeeze roll. The pressure point is pressed to the edge of the tube blank in advance, and thus the opening angle is reduced; the higher the pressure, the more obvious the reduction.

(6) The wall thickness of the tube blank. Thick-walled tubes plus deformation blind areas can easily form inner and outer opening angles with the same size but different apex (convergence) positions. According to the principle of proximity effect of high-frequency current, the temperature near the inner opening angle of the tube must be higher than the outer opening. The temperature at the corner makes the welded pipe unable to be welded normally.

(7) Roll gap. Including the guide roll and the squeeze roll gap all affect the opening angle to varying degrees.

Adjustment principle of opening angle

The following 4 basic principles should be followed when adjusting the opening angle of the used roll forming machine

(1) The principle of wall thickness. It means that the opening angle should be reduced as much as possible when producing thick-walled pipes, and the opening angle should be appropriately increased when producing thin-walled pipes.

(2) The principle of nature.That is, after the natural wear of the guide ring and the hole pattern, the guide roller is pressed down in stages, so that the opening angle will be reduced accordingly; after the new guide ring is replaced, the opening angle should be restored to the original state.

(3) The principle of micro deformation. Theoretically, the guide rollers do not undertake the task of deformation; however, from the practical needs of stabilizing the opening angle, it is necessary for the guide rollers to undertake the slight deformation that is limited to the control of springback.

(4) The principle of efficiency. In order to improve the power utilization efficiency and production efficiency, the opening angle must be adjusted as small as possible when the process conditions allow.