Yarn Properties
The material used in combination with the process used to manufacture yarn greatly affects the finished yarn and fabric. Three yarn characteristics that affect the yarn are:
- Filament and Staple fibers
- Plies
- Yarn twist
Filament
Yarns can be made of either filament or staple fibers. Filament fibers are those fibers that are long enough to be yarn by themselves, and are usually measured in yards or even miles.
Most filament fibers are synthetic fibers such as polyester and rayon. These fibers can be extruded from machines in infinite lengths. Silk is the only natural filament fiber since unraveled cocoons can yield up to 3000 yards of fiber.
Filament fibers can be monofilament, which is a single fiber, or they can be twisted together to form multifilament yarns.
Staple
The other type of fibers that used to make yarn are staple fibers. Staple fibers are short fibers that must be twisted together in order to create yarn. Staple fibers are made into yarn during the spinning process by pulling and twisting strands of parallel fibers. For this reason, yarns made from staple fibers are called 'Spun Yarn'. The process of spinning staple fibers into yarn is discussed in Yarn Production.
Most natural fibers are staple fibers including cotton, wool, and flax (linen). Synthetic fibers can be cut up into staple fibers for blending with natural fibers, or they can be homogeneously spun to create yarns that have a natural fiber appearance and hand.
Due to different constructions, spun yarns and filament yarns each have different characteristics and properties. These properties will affect the finished fabric depending on the use of the fibers.
Twist
During the spinning process fibers are twisted into yarn. Twisting the fibers holds them together and gives the yarn strength.There are only two directions that yarn can be twisted clockwise and counter clockwise. Either direction can be used.
A counterclockwise twist is known as "S" twist while clockwise is known as "Z" twist. Single strands of yarn are usually given a Z twist during spinning and plied yarns are usually given an S twist.
The degree of twist required depends on the type of fiber and can vary from no twist to high twist. The amount of twist in a yarn is measured in Turns Per Inch (TPI). Spun yarns with relatively little twist of 2-12 TPI are referred to as 'Soft Twist'. Yarns with 20-30 TPI are referred to as 'hard twist'.
Texturizing Yarns
Filament and spun synthetic yarns are inherently smooth, stiff and lustrous, but their appearance can be altered. Texturizing methods are used to give yarns properties such as:
- Stretch
- Bulk
- Aesthetics of staple yarns
- Heat-setting to reduce shrinking
In general, textured yarns have several key advantages:
- Increased stretch and bulk
- Greater cover
- Enhanced breath ability and absorption
- Better insulation
- Increased softness
- Ascetic characteristics of staple yarns
The main disadvantage to texturizing yarns is they tend to snag easier than regular filament yarns. This increases the chance of damaging the fabric due to the length of the filaments.
Texturizing can be done as the filament is formed for synthetic fibers, or can be done after the yarn is formed. The five most common methods are knife-edge, stuffer box, false twist, air jet, and knit-de-knit.
Knife Edge
In this process the filament yarns is heated and pulled across a knife at an acute angle. When the yarn is released it looks like a spring or curled ribbon.
Stuffer Box
Here the filaments are fed into a box faster than the removal rate. While in the box, the filament is forced into a random wavy crimped pattern and heat set in that state. This adds bulk and texture to the yarn.
False Twist
Yarns are twisted and heat set and then untwisted when cold. This gives the yarns a high degree of stretch.
Air Jet
Filaments are fed at high speed into a chamber where compressed air is blown through the yarn. This makes the filaments in the yarn spread apart and form random loops. The yarn is removed from the chamber at a slower speed. The process adds bulk and stretch to the yarn. No heat is used in this process.
Knit-De-Knit
Filament yarns are knit into a narrow diameter tube and heat set. The yarns are then de-knit, giving them a wavy configuration.
