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ubytki (zgrzewanego) materiału spowodowane przez elektrodę

tip-pickup = electrode pickup

(g) Good cooling of the Class I electrodes to prevent tip pickup[,] or sticking. [czyli przywieranie]
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Electrode tip pickup problems have limited the number of welds which a given set of electrodes could satisfactorily accomplish in prior art systems for the reasons previously mentioned.
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Electrode Pickup is the contamination of the external surface of resistance welding electrodes by the base metal or its coating during welding.
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d. Foreign materials (for example, tip pickup).
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a. Electrode pickup - it is necessary to brush the electrode after 16 welds (one fuel element) to prevent copper contamination of the...
http://tinyurl.com/43hxrl

Are you using a 3 phase or DC machine? If not, electrode pickup can be more on a single phase machine than a 3 phase or DC machine. The electrodes weld differently with or without aluminum pickup (independent of tip radius). Hand cleaning of weld tips after as few as 10 welds was common in some mil spec aluminum welding in the recent past to keep the copper face bare, without aluminum pickup.
http://tinyurl.com/3pq7ud

Surface Burning and Tip Pickup
* Surface burning is caused by excessive heat on the metal surface under the electrode and is indicated by burned, pitted and discolored welds.
* Excessive electrode pickup is caused primarily by the same factors.
http://tinyurl.com/4vfptw