Because the chemical composition of each synthetic fiber is different, the fiber spinning method is also different, and the spinning method has an important influence on the morphological structure of the fiber.

For example, polyester, nylon and polypropylene are melt-spun; most acrylic fibers, vinylon staple fibers, and polyvinyl chloride are mostly wet-spun; spandex, some vinylon and acrylic fibers are dry-spun. During melt spinning, the molten polymer is extruded through the spinneret. When it is cooled and solidified in the air, the shape of the fiber cross-section is related to the shape of the spinneret hole, and the conventional cross-section is circular. The filaments spun by the wet method are solidified due to solvent precipitation in the solution, and the cross-section is mostly non-circular and has an obvious skin-core structure.

When using the combustion method to identify fibers, it is necessary to focus on observing the state of the fibers when they are close to the flame, contacting the flame and leaving the flame, and pay attention to the odor produced during combustion and the characteristics of the residue after combustion.

The temperature at which the crystals in the polymer disappear completely, that is, the temperature at which the crystals melt is called the melting point. Under the action of high temperature, the structure of intermolecular linking of synthetic fibers changes. First soften and then melt. Most synthetic fibers do not have an exact melting point like pure crystals, and the same fiber has different melting points due to different manufacturers or different batch numbers. However, the melting point of the same fiber is fixed in a relatively narrow range, so the type of fiber can be determined. Natural cellulose fibers, regenerated cellulose fibers, and protein fibers, because their melting point is higher than the decomposition point, do not melt and decompose or carbonize under the action of high temperature.

The melting point method is generally suitable for identifying synthetic fibers with obvious melting point characteristics, but not for natural cellulose fibers, regenerated cellulose fibers and protein fibers. Generally, it is not used as a means of qualitative identification alone, but can be used as a supplementary method for confirmation on the basis of other methods of identification.

The melting point of the fiber is determined by observing the temperature when the fiber is extinct under a melting point apparatus or a polarizing microscope with a heating and temperature measuring device, so as to achieve the purpose of identifying the fiber type. Especially for synthetic fibers such as polyester, nylon and polypropylene, their longitudinal and cross-sectional morphological characteristics and combustion performance are very similar, and the melting point method has great advantages.