The sagging of the filament of an incandescent lamp is not only related to the content of added elements in the doped tungsten wire in bulb and the processing technology, but also related to the processing technology in the process of making the filament. When the tungsten wire in bulb is drawn into a finished filament, a large amount of internal stress is retained, and when it is wound into a filament, a new internal stress of uneven deformation is generated on the section of the tungsten wire in bulb. These internal stresses must be perfectly eliminated before the filament rack enters the bulb, otherwise the bulb will twist, deform and sag the filament when it starts to ignite. Sagging of the filament can seriously reduce the luminous efficiency of the bulb.

Performance at room temperature. The performance of tungsten wire in bulb at room temperature is reflected in the winding performance. Due to the long processing process of tungsten wire in bulb, if the process management is not good, it is easy to cause many small cracks or local brittleness in tungsten wire in bulb, so that it is easy to break when winding the wire. The broken fracture of the winding wire caused by cracks is in the shape of whiskers, while the fracture caused by the brittle wire material is in the shape of crystal surface flashing.