Cold welding is the most dangerous defect in the electro-fusion (EF) joint of polyethylene (PE) pipes. A proprietary method was developed to detect the degree of cold welding by using an Eigen-line which was discovered in our previous study. To understand when, where and how the Eigen-line occurs, the forming mechanism was investigated in this paper. Three factors, i.e., micro-air-bubbles, difference of acoustic impendence of PE in the melted and un-melted region and small-crowded crystals may cause the appearance of the Eigen-line. It was found that the number of Eigen-lines was the same as that of welding times when re-welded with decreasing welding time, and only one Eigen-line could be observed when re-welded with increasing welding time. The result showed that small-crowded crystals may be the dominating factor. This was then verified by the temperature analysis and differential scanning calorimeter (DSC) tests, and the forming process of the small-crowded crystals was discussed in detail.

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