Resistance weld processes are set up at the proper settings to produce good weld nuggets at the beginning of the operation. Unfortunately, spot welding electrodes all wear with use. This means the face expands in size/diameter. This is generally called mushrooming.
SEVERE MUSHROOMING
If the face expands, the initial starting current would now be passing through a larger surface area. The current density will be decreasing. Essentially the weld area will be cooling down. If it cools the nugget gets smaller and eventually the nugget will fail to meet the required specifications.
This is fully described in another article:
HOW DO I DETERMINE TIP DRESSING FREQUENCY?
Due to mushrooming, there are only two options:
• dress the electrode back to original dimensions before the nugget is unacceptable
• increase the weld current at a rate to match the weld face growth
An article that addresses current increase is:
WHEN THE ELECTRODE FACE AREA CHANGES AT WHAT RATE SHOULD THE RESISTANCE WELDING CURRENT CHANGE?
There are several articles on this subject in this blog. Just search for “Dress”
Reference: RWMA – Resistance Welding Manual 4th Edition