In high frequency welding of tube & pipe, there is a phenomenon called "Strip Edges Meeting Too Soon". It means that the two strip edges can meet several inches ahead of the center line of the weld box. When this phenomenon happens, the risk of weld defect goes up.
There can be two possible causes for this phenomenon, too small a vee angle or too much squeeze out. People can easily understand the influence of too small a vee angle, but rarely realize the role of the squeeze out. Theoretically speaking, when the squeeze out is none, the two strip edges will meet at the center line of the weld box. However, a squeeze out is necessary for the high frequency welding of tube and pipe. When the squeeze out is bigger than zero, the two strip edges will meet ahead of the center line of the weld box. The bigger the squeeze out is, the sooner the two strip edges meet.
To achieve a good weld, a certain amount of squeeze out is needed. Typically the squeeze out is directly related with the strip thickness. The thicker the strip is, the bigger the squeeze out should be. People often use the difference of girth out of last fin and weld as the squeeze out. Since the girth can grow between the last fin and the weld, this way of calculation may result in too much squeeze out.
Additional attention should be given to tube of heavy wall. Since it needs a bigger squeeze out, the strip edges will meet sooner if the vee angle is still the same. In reality people just use one set of fin rolls for all the gauges. This set of fin rolls might work fine for light gauges but have trouble with heavy gauges. So, a bigger vee angle is needed for heavier gauges.
The vee angle is related with the following factors: the last fin thickness, the tube diameter, the distance from last fin to the center line of weld box, the wall of tube, the yield strength of the strip material, the constraint of the weld rolls, and even the amount of fin reduction. The last fin thickness is set in the roll design. If it is found too narrow, it can be widened by a redesign and regrind. The wall of tube and the yield strength can affect the spring back after last fin and the closing process before the weld rolls. Large diameter weld rolls applies a stronger constraint to the tube and will reduce the vee angle. When the upper weld rolls have a narrow working width, the lower weld rolls can apply a strong constraint to the tube and thus reduce the vee angle. The amount of fin reduction can affect the spring back after last fin. The last fin thickness, the tube diameter, the distance from last fin to center line of weld box, and the constraint of weld rolls are set by design, but other factors can vary with each setup. So, the vee angle can vary from setup to setup.
To get rid of the phenomenon of "Strip Edges Meeting Too Soon", you first have to check whether you are doing too much squeeze out. If it is fine, you will have to increase the vee angle.
