Weld is the second of the three main resistance welding functions. They are squeeze, weld and hold. During Squeeze the proper pressure builds up prior to current initiation. Weld is the period when the current flows and the weld nugget is formed. The Hold period marks the end of weld current and the beginning of the weld cooling and solidification under pressure.
The amplitude and time of current flow is critical to forming the proper size nugget. The weld control controls the weld function and is able to accurately repeat exactly the same process each weld cycle for each part.
Reference: RWMA – Resistance Welding Manual 4th Edition