Fiber Twisting

TPM, twist per meter, or TPI, twist per inch, is the numerical value used to specify how many turns are imparted in each meter or inch respectively. The direction of the twist is also critical. An “S” twist represents a yarn that is turned from right to left while a “Z” twist signifies the yarn has been twisted from left to right. For example, 3.5 TPI Z represents yarn that has 3.5 twists per inch in the left to right direction.

While this twist multiplier calculation is helpful to determine the optimal twist level for a fiber, different fiber types behave differently due to the chemical structure of each individual fiber. Fiber twist levels can be optimized to increase breaking strength and tenacity or can be designed to increase flexibility and abrasion resistance. Typically, a lower twist will add some strength to a fiber compared to the same untwisted fiber, while a higher twist will be more flexible, but will lose some strength.

Twisted Fiber Products

The process of twisting fibers can be used to enable new functionality of multifilament fibers. A single twist can be imparted into a yarn to achieve the characteristics mentioned above. Another function enabled through yarn twisting is yarn plying. Plying yarns together is the process of taking multiple ends of fiber and bringing them together carefully under appropriate tensions and twist levels to achieve a balanced multi-end yarn product, designed to achieve higher strength or larger diameter requirements. For example, it is common in the wire and cable industry to ply 6 ends of 1420 denier para-aramid fiber to create an 8,520 denier yarn which is utilized as a strength member within a wire or cable construction. These types of products typically have less twist to optimize filament interaction and balance within the larger fiber bundle.

Another product type that can be produced by fiber twisting is the creation of a twisted cord or rope. These products are produced in a multi-step twisting operation where individual ends of multi-filament fiber are twisted with a high twist level in one direction, and twisted again in the opposite direction with a lesser twist. This is commonly known as the “open” and “close” twist. It is imperative that these operations are done under proper tension and calculated correctly to ensure a torque balanced line. If done incorrectly, the newly twisted cord or rope will contain catenary which can cause many issues in your product or process.