Flattening Index
Geometrical changes in yarn cross-section during fabric calendering describe the transition from round threads to elliptical shapes. Finishing mills measure the thread flattening ratio to evaluate the coverage and luster of the calendered linen fabric. This value is calculated by dividing the major axis of the yarn’s elliptical cross-section by its minor axis after the fabric passes through heavy heated rollers.
A higher ratio indicates that the yarns have been flattened extensively, which fills the gaps between threads and creates a smoother, more reflective surface.
Process Variable
Control of roller pressure, temperature, and moisture levels in the calendering machine determines the extent of yarn deformation. The thread flattening ratio of linen is particularly sensitive to moisture because flax fibers are more plastic when damp. By adjusting the moisture content of the greige cloth prior to calendering, finishers can dial in the precise degree of flattening required for different end uses.
Too much pressure can crush the fibers and weaken the fabric, while too little pressure fails to produce the characteristic linen sheen.
Aesthetic Impact
Evaluation of the fabric’s air permeability and light reflection confirms the success of the calendering process. A high thread flattening ratio reduces the openness of the weave, providing better wind resistance and a more premium appearance. This finish is highly prized in export markets for luxury household linens.
