How come when spot welding, if you leave the plastic on and metal arcs out on welder, why does it blow a hole clear through the aluminum?

-- Laura Huggins (lmh118@yahoo.com), April 08, 2001

Answers

The only plastic that I’m aware of on the material is to prevent oxidation and preserve the surface appearance. If the plastic is left on and the weld tips are brought together on the assembly, I would anticipate that the plastic would be an insulating barrier, and no weld would be made. However, should there be a pin hole in the plastic, the close proximity of the two (2) electrodes to the aluminum without having any direct surface pressure would result in drawing a violent arc which I would anticipate being driven deep enough as to reach the opposing electrodes.

Bob Balla T. J. Snow Company, Inc. 6207 Provence Street Chattanooga, TN 37421

Phone: 423-894-6234 423-308-3161 (direct line) Fax: 423-892-3889 e-mail: bobballa@tjsnow.com

-- Bob Balla (bobballa@tjsnow.com), April 23, 2001.

The resistance weld process requires high currents to produce heat in the part, because the part has very low resistance. An arc welder operates with 1/100th the current, because the voltage of the arc is high. Power is voltage times current. The resistance weld would have only 1 volt across it with 10,000 amps through it, resulting in power of 10,000 watts. However the welding transformer might have to put out 12 volts to force all that current through the circuit. If the part is high resistance due a contamination of the surface, the voltage across the part could be the full 12 volts. If a current path gets established, even a "small" current of 2000 amps would produce 24,000 watts, far higher that the work can stand without vaporizing. This problem exists when welding through sealer in the automotive industry, and we are still trying to get "sealer people" to understand our difficulty with sealer. If this is a constant problem with your process, you might try to detect this condition with a single cycle of current, with an undercurrent fault stopping the welder before the full current is applied. If you performed this test at a low firing angle, say 25%, you could detect the plastic before you destroyed the part. In phase angle control, you cannot go lower than 20% of total available current, so stay above that to avoid faults due to the current regulator being out of range. Modern controls such as ATek, Medar, Robotron, Square D, WTC, all perform current regulation and can be setup to catch a fault as this. By the way, I don't work for any of the welding control suppliers, but I used to... My two aren't even on this list, but they are my favorites.

-- David Bacon (dbacon@updatetechnology.com), April 20, 2002.