Transcript Knitted Fabric Geometry Review

Knitted Fabric
Geometry
Introduction
• The earliest work on the physical and
geometrical properties (knitting science) on
the knitted structure can be traced back to
Tompskin in 1914.
• A major advance in this field occurred when
Doyle and Munden (1959) reported that for
an extremely wide range of relaxed plainknit fabrics, the fabric dimensions (width
and length) are completely determined by
the knitted loop length
Introduction 2
• The following equations were found:
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cpi =kc/l
wpi=kw/l
R=cpi/wpi; R=kc/kw (loop shape factor)
S=cpi x wpi =ks/l2
• Kc, Kw, Ks are constant and termed
as the fabric dimensional parameters.
• The above relations apply only for plain
knit fabrics that are in their relaxed
state.
• During the knitting process, strains are
imparted to the fabric as it comes off the
knitting machine, and subsequent recovery
from these strains occurs until the fabric
has attained its relaxed or stableequilibrium state.
Relaxation Staes
• Mudnen (1959) defines two baisc relaxed
states: the dry relaxed state and wet
relaxed state.
• The yarn in a dry-relaxed fabric is in a
condition of stress and when unravelled
from the fabric tends to return to its
natural stress-free configuration.
• The stresses in the loop are largely
released during wet relaxation. Setting
takes place during drying, and as a result,
the yarn retains its loop configuration when
unravelled from a wet relaxed fabric.
K-value for two relaxed
states
Dry-relaxed
Wet-relaxed
Kc
5.0
5.3
Kw
3.8
4.1
Ks
19.0
21.6
Kc/kw
1.3
1.3
K-value
• For a wide range of plain knit wool fabrics,
Munden obtained the average numerical
values of the fabric dimensional parameters
as shown in above Table.
• He concludes that on relaxation, the
knitted loop tends to take up a definite
equilibrium configuration, which is
independent of the tightness of
construction of the fabric and of the
physical properties of the yarn.
K-value 2
• This loop configuration is the same for
all plain knit fabrics in a given of
relaxation, and it governs the
numerical values of the fabric
dimensional parameters.
• The values of these parameter are
constant, and once they are
determined, the fabric dimensions are
function of the loop length.
• Although the dimensions of relaxed
knitted fabrics were found to be
dependent only on the knitted loop
length and are independent of the
yarn count, it is expected that other
physical properties (such as
mechanical properties of the fabric,
fabric stiffness, pilling) will depend on
the tightness of construction.
Conclusion 1
• On relaxation, the knitted loop tends to
take up a unique configuration which is
independent of the loop length and of the
physical properties of the yarn.
• This unique configuration is determined by
the condition of minimum strain energy of
the loop and it governs the numerical
values of the fabric dimensional
parameters, Kc, Kw and Ks
Conclusion 2
• The practical importance of this work, is that
there is only one variable, namely the loop
length, that affects the quality and dimensional
properties of a knitted fabric in its relax
state.
• This implies that careful control of the length
of yarn being fed into the knitting machine is
all that is necessary for controlling the quality
of fabric.
• It is on this principle that HATRA Yarn speed meter and Yarn length
meter have been developed.
Conclusion 3
• Prior to the work of Doyle and Munden, many yarn
and machine variables were recognized as having
an influence on knitting quality and the dimensions
of a fabric in its relaxed state.
• However, Munden has shown that a change in any
one of these variables affects the knitting quality
and fabric dimensions either by altering the
knitted loop length or by temporarily distorting
the fabric.
• In the first case, the effect on fabric dimensions
is permanent, but in the second case, the effect
is only temporary as the fabric tends to recover
its stable loop configuration on relaxation.
Conclusion 4
• Adjustment of the stitch cam setting on the knitting
machine and change in the yarn tension at the knitting point
are invariably accompanied by a change in the loop length.
• Nutting has shown that an increase in kinetic yarn friction
causes an increase yarn tension during knitting, so
producing a decrease in loop length or tighter fabric.
• Thus a change in stitch cam setting, yarn tension or yarn
friction has a permanent effect on fabric dimension.
• Adjustment of fabric take down rollers and stretcher
boards usually has little influence on the loop length and so
does not affect the dimensions of the relaxed fabric.
• These variables, however, may have a considerable
temporary effect on the dimension of the fabric in the
form of distortion.