Tension and it's effects on knitting quality GROUP MEMBERS Wijesena.R.N :061062C Wijerathna E.A.C.N.:061061X Wickramarathne T. I.:061060T Gunawardana C.A.:061063F

Role of tension in knitting Selection of the knitting yarn Pre-processing of the yarn Knitted fabric loop length Knitting machine Fabric quality (Fitting for the purpose)

Knitting tension (T k ) Knitting tension T k is the tension needle experience at knitting point. Knitting point :The point at which ten T k is applied knitting needle.

Factors affecting knitting tension T 1 : Input yarn tension T 0 : Exit yarn tension Θ : Angle of contact  coefficient of friction

Input yarn tension (T 1 ) Factors affecting input yarn tension – The feeding device – Yarn guide unit – Snagging of the yarn – Velocity of the yarn – Inertia of the yarn pulley units

Input yarn tension (T 1 ) Cont.. T k increases with the T 1 Non liner relationship between T 1 and T k Degree of variation is high in lower T 1 values.

Input yarn tension (T 1 ) Cont. Variation of input yarn tension with the time.

Output yarn tension ( T 0 ) Output yarn tension is caused by friction and influenced by the yarn contact angle. Reversal of the yarn mass flow direction during the robbing back imposes non linear tension variation.

Output yarn tension ( T 0 ) Cont. When the coefficient of friction and yarn contact angle kept constant in a knitting machine is roughly constant throughout operation. Under these conditions To only depend on robbing back. Robbing back is controlled by adjusting the steps of stitch cam.

Output yarn tension (T 0 ) Cont. Irregular variation of yarn tension due to robbing back.

Angle of contact (Θ) Factors affecting angle of contact – Knitting machine gauge – Needle position in the knitting zone When the knitting tension is measured at the knitting point, angle of contact remain same.

Coefficient of friction (μ) Coefficient of friction mainly depend on the surface properties of the yarn used and properties of yarn rubbing places of the knitting machine (needle stem, sinkers, etc) Effect of coefficient of friction towards the Knitting tension often assumed to be constant.

Coefficient of friction (μ) Cont. T 1 is reduced with the increasing tension.

Other factors influencing T k Knitting machine speed. Type of yarn used for knitting. Thickness of the yarn Flexual rigidity of the yarn

Effect of yarn input tension & cam setting on fabric loop length

Yarn input tension Tension in the yarn immediately prior to the feeding point One of the most important yarn variable influencing the size of the knitted loop For innumerable reasons, it is difficult to control accurately

Cam setting The height of the needle at the knitting point, in relation to the sinkers The stitch and upthrow cams, the basic cams in the assembly enabling the needle to form its loop, were permanently fixed at angle of 45 0 to the horizontal and formed a v-shaped cam system It Control the loop length

Theoretical loop length

Theoretical loop length cont… Length of yarn pulled in to the loop configuration at the knitting point L = (4d 2 +a 2 ) 1/2

Effect of yarn input tension & cam setting on fabric loop length Cam setting vs loop length for a range of input tensions Yarn input tension vs loop length at different cam setting

Cam setting vs loop length for a range of input tensions At any constant tension a vertical upward movement of the cam control always result in a linear decrease of loop length The slope of this relationship depends on the input tension of the yarn The unroved loop lengths are always less than the theoretical loop lengths The loop length value, where the yarn breaks during knitting with increase in input tension at different cam setting, lie on a roughly linear curve with a slope similar to the other curves

Yarn input tension vs loop length at different cam setting The variation between loop length and input tension does not depend on the type of machine used At any given value of input tension, it was found impossible to knit a fabric with a loop length less than 0.12 inch, since at this loop length the fabric become so tight that holes appeared and press-off occurred

Fabric loop length The final loop length of yarn depends on, The needle depth in relation to sinkers at the knitting point The angle of the ‘stitch’ & ‘up throw’ cams to the horizontal & the relative positioning of these cams to one another The effect of yarn friction on knitting tensions & these tension's effect on yarn movement around knitting elements The amount of yarn robbed by one needle from another

Variation in the measured loop length with the input tension Three factors Needle Fling Yarn extension at high knitting tensions Robbing Back

Increase in the input tension through the knitting region Yarn Package Trailing Leading A

Yarn Package θ T1T2 Amonston’s law T2 = T1 eμθ Increase in the input tension through the knitting region cont… A

L3 Total L2 L1 T1 T2 T3

Tension effect in Robbing Back What is “robbing back” ? “Robbing of yarn by the needle at the knitting point from the needles that have completed their loop formation” AB

Tension effect in Robbing Back cont… F 2F 4F8F 16F 32F A B C D Fabric

Tension effect in Robbing Back Cont…

Leading and trailing arms of a loop do not rob simultaneously That is there are two stages of robbing

Robbing back percentage Robbing Back % = (l t – l u ) x 100 l t l t – Theoretical loop length l u – Unroved loop length

Tension and it's effects on knitting fabric properties

Tension variation effect overall process Tension variation effect to the overall fabric manufacturing process, there are warping & knitting, finishing, dyeing and quality.

Categorization

EFFECT ON THE FABRIC PROPERTIES Stretch ability & Flexibility GSM (weight) & Modulus Barrie Plating Breakages Edge curving

Size Variations (seamless garments) Dye variation (un even, off shade….) Surface variation (Creapy, filaments…) Finally cant be achieve final requirement Graph

Compare with loop length

Compare stretch & GSM Under low tension Under high tension

Dye variations

FaultReason Stop markMachine stop while production Pin holeMiss formation of the loop HoleBreak the yarn during the production End outBroken yarn Drop stitchIncreases of take down tension Beam shadingTension variation of the beam StreaksYarn fault or machine fault & tention MoireTension variation CreapyLoop formation fault

Beam Shading

Moire

Warping defect FaultReason FilamentYarn contect damage machine surface Tenision variationMachine fault End outBroken yarn Miss endsBroken yarn Yarn faultsThin, thick yarn

Recap Role of tension in knitting Factors affecting knitting tension (input & output) Effect of yarn input tension & cam setting on fabric loop length Variation in the measured loop length with the input tension Tension effect on the fabric production process Tension defects in the fabrics

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