When manufacturing carbon fiber, it is not produced one by one, but in bundles. Therefore, the number of carbon fibers contained in each bundle may vary, but it can usually be divided into 1000, 3000, 6000, 12000, which is the familiar concepts of 1k, 3k, 6k, 12k, etc.
Unidirectional Carbon Fiber Cloth has no bending state and can withstand stress well. The woven fabric bundle needs to be bent up and down in the orthogonal direction, resulting in a significant decrease in strength. So when fiber bundles are woven up and down to form a fabric, the strength decreases due to the curling in the bundles. When you increase the number of filaments in a standard tow from 3k to 6k, the tow becomes larger (thicker) and the bending angle also becomes larger. One way to avoid this situation is to unfold the fine threads into wider bundles, which is called unfolding bundles, and the resulting fabric is also called spread tow Carbon Fabric, which has many benefits.
The curling angle of the widened tow is smaller than the weaving angle of the standard tow, thereby reducing cross defects by increasing smoothness. A smaller bending angle will result in higher strength. Compared with unidirectional materials, the spread fiber bundle material is also easier to use and still has considerable fiber tensile strength.