To produce a yarn that has the properties of each fibre.
For example, poly/cotton,
the cotton provides comfort and good looks, while the polyester
provides easy care and added strength.
To create an aestheticly interesting effect.
For example, hairy or fluffy yarn,
in this case two fibres of different coarseness are blended.
The more coarse yarn moves to the outside of the yarn producing a wild hairy effect.
For example, cross dyeing,
in this case two fibres with different dye absorbtion rates are blended,
then spun or woven and dyed to produce a special
colour effect.
To produce a yarn for a specific end use.
For example, high bulked yarn,
high bulked yarns (usually acrylic or nylon) are commonly produced as knitting yarns.
You will also be familiar with the bulked yarn used on an overlocker (serger),
for seaming cut and sew knitted garments,
in this instance the yarn needs to be fine enough to thread through the needle eye,
strong enough to withstand the friction
of sewing,
and bulky enough to cover the edges of the seam to prevent the knit
from unraveling.
Bulked yarns are made by combining relaxed and unrelaxed fibres.
The combined fibres are subjected to a hot-wet process,
which causes the unrelaxed fibres to shrink.
In response,
the relaxed fibres wrinkle up,
thus creating increased bulk in the yarn.
For economic reasons.
For example, combining a luxury fibre with a less expensive one
such as, a combination of cashmire with wool or linen with vicose.
for information about yarn twist
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