Dirt, oil, burrs or scale on the surface of the parts being welded can affect resistance welding. They can cause expulsion from the surface, the interface or cause defects in the weld nugget.
If one alters the weld time from the desired range and exceeds the extreme either too long or short you risk surface expulsion, electrode mushrooming, and excessive indentation. If weld times are short you risk small nuggets and low weld strength or no welds.
Improper weld current selection can cause problems during resistance welding. The problems normally show up in the form of expulsion, indentation, electrode sticking, mushrooming or low weld strength. There is an ideal current range for every resistance weld set up. This will produce good weld nuggets with little or no expulsion, modest indentation and minimal electrode wear or sticking. Whenever you push the extremes of the current in the weld window either high or low you risk problems.
If the weld equipment is not properly cooled especially the electrodes weld failure is likely to occur. Poor cooling at the electrodes can cause them to mushroom prematurely which reduces the current density at the weld face and the nugget size will reduce. Poor cooling can be caused by low flow, warm water, improperly located cooling tubes, clogged/restricted tubes and no flow.
Spot welding can decrease in weld size as you weld across the part. One cause for this is increasing magnetic material in the throat of the welder as you move across the part. Most parts are low carbon steel which is a magnetic material. In AC welding applications, if after each weld more material is moved into the throat of the machine, the weld current will decrease.
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