The electrode tips do not move towards each other quickly enough during the forging phase of the weld sequence.
During welding, the temperature of the parent material increases due to resistive heating. This heating causes expansion of the workpiece between the tips, moving the electrodes apart. The parent material temperature continues to increase until the material softens and can no longer resist the applied force of the electrodes. At this point the tips start to move towards each other, forging the softened weld material together. If this electrode movement (follow-up) is too slow, surface and interfacial expulsion may occur, and the granular structure within the button may be flawed.
The follow-up characteristics of a welder are influenced by many factors, including the flowrate capacity of the air/hydraulic feeder system, the weld cylinder characteristics (size, friction, mass), electric gun characteristics (feedback control circuit, motor, gear train), design of the welder, its condition, and the weld force in use.
Detectable evidence may include:
Quality, Workplace Issues, Cost, Downtime, Maintenance, Throughput (cycle time; PPH), are all potentially affected by this condition. Special considerations are noted below:
Workplace Issues: Poor follow-up may result in excessive expulsion.