Defects in the Knitted Fabrics

Introduction:

A defect in the knitted fabric is an abnormality, which spoils the aesthetics i.e. the clean & uniform appearance of the fabric & effects the performance parameters, like; dimensional stability etc.

There are various types of defects, which occur in the Knitted fabrics of all types, caused by a variety of reasons. The same type of defects may occur in the fabric, due to a variety of different causes e.g. Drop Stitches, Spirality. Prime causes of the fabric defects are, as follows;

·        Yarns

·        Knitting Elements

·        Knitting Machine Settings

·        Dyeing

·        Finishing

 Types of Defects:

·        Drop Stitches or Holes

·        Barriness

·        Streakiness

·        Imperfections

·        Snarling

·        Contaminations

·        Spirality

·        Needle Lines

·        Broken Needles

·        Sinker Lines

·        Oil Lines or Stains

·        Broken or Missing Ends

·        Fabric Press Off

·        Surface Hairiness & Piling

·        Snagging

·        Bowing

·        Dyeing Patches

·        Softener Marks

·        Stains

·        Color Fading

·        Shade Variation

·        Tonal Variation

·        Wet Squeezer Marks

·        Fold Marks

·        Crease or Rope Marks

·        Skewing or Diagonal Grain Lines

·        Shrinkage

·        GSM Variation

·        Width Variation

·        Curling of Edges

·        Measurements

Category of Defects:

Yarn related defects

Almost all the defects appearing in the horizontal direction, in the knitted fabric are, yarn related. These defects are mainly;

·        Barriness

·        Thick & Thin lines

• Dark or Light horizontal lines (due to the difference in dye pick up)
• Imperfections
• Contaminations
• Snarling
• Spirality

Knitting Elements related defects

Almost all the defects appearing in the vertical direction, in the knitted fabrics, are as a cause of bad Knitting Elements. These defects are mainly;

·        Needle & Sinker Lines

• Drop Stitches etc.

Machine Settings related Defects

These defects appear randomly in the knitted fabrics, due to the wrong knitting machine settings & that of the machine parts. The defects are mainly;

·        Drop Stitches

·        Yarn Streaks

·        Barriness

·        Fabric press off

·        Broken Ends

·        Spirality

Dyeing related defects

The Dyeing related defects are, as follows;

·        Dyeing patches, Softener Marks

·        Shade variation

·        Tonal variation

·        Color fading (Poor Color Fastness)

·        Dull shade

·        Crease or rope Marks

Finishing related defects

Defects caused, mainly due to the wrong process parameters are;

·        High Shrinkage

·        Skewing

·        Spirality

·        Surface Hairiness & Pilling

·        Tonal variation

·        Snagging (Sharp points in the dyeing machine or trolley etc)

·        Fold Marks

·        Wet Squeezer Marks

·        GSM variation

·        Fabric Width variation

• Curling of S.J. Fabrics

  

Drop Stitches (Holes)

Definition:

Drop Stitches are randomly appearing small or big holes of the, same or different size, which appear as defects, in the Knitted fabrics.

Major Causes:

     

• High Yarn Tension

• Yarn Overfeed or Underfeed

• High Fabric Take Down Tension

• Obstructions in the yarn passage, due to the clogging of eyelets, yarn guides & tension discs, with wax & fluff etc.

• Defects like; Slubs, Neps, Knots etc.

• Incorrect gap between the Dial & Cylinder rings.

Remedies:

     

• Ensure uniform yarn tension on all the feeders, with a Tension Meter.

• Rate of yarn feed should be strictly regulated, as per the required Stitch Length.

• The fabric tube should be just like a fully inflated balloon, not too tight or too slack.

• Eyelets & the Yarn Guides, should not have, any fibers, fluff & wax etc. stuck in them.   

• The yarn being used, should have no imperfections, like; Slubs, Neps & big knots etc

• The gap between the Cylinder & the Dial should, be correctly adjusted, as per the knitted loop size.

Barriness

Definition:

Barriness defect appears in the Knitted fabric, in the form of horizontal stripes of uniform or variable width.

Causes:

     

• High Yarn Tension

• Count Variation

• Mixing of the yarn lots

• Package hardness variation

      

Remedies:

      

• Ensure uniform Yarn Tension on all the feeders.

• The average Count variation in the lot, should not be more than + 0.3

• Ensure that the yarn being used for Knitting is of the same Lot / Merge no.

• Ensure that the hardness of, all the yarn packages, is uniform, using a hardness tester.

Streakiness

Definition:

Streaks in the Knitted fabrics appear as; feeble, irregularly spaced & sized, thin horizontal lines.

     

Causes:

• Yarn slippage on the IRO Pulley, due to the yarn slipping in & out from underneath the IRO Belt, due to a tilted IRO Pulley.

• Worn out IRO belts, yarn guides & eyelets etc

• Faulty winding of the yarn packages

• Yarn running out of the belt, on the IRO Pulley

Remedies:

• Ensure very smooth, clean & obstruction free passage of the yarn, through the eyelets, yarn & tension discs etc.

• No cuts or rough surfaces, in the Porcelain Eyelets, Yarn Guides & the Yarn Feeder holes etc. 

• Flawless winding of the, Yarn Package (The yarn coils should unwind smoothly, without any obstruction)       

• The yarn should be running under the IRO belt, between the belt & around the IRO pulley

Imperfections

Definition:

Imperfections appear on the fabric surface, in the form of unevenly placed or randomly appearing, Knots, Slubs & Neps, Thick & Thin places in the yarn.

Causes:

• Big Knots, Slubs & Neps in the yarn, Thick & Thin yarn (Uneven USTER) 

Remedies:

• Specify the quality parameters of the yarns, to be used for production, to the yarn supplier.

• Specify the number of acceptable Imperfections / Km. of the yarn & the USTER evenness %, while ordering the yarn.

Snarls

Definition:

Snarls appear on the fabric surface, in the form of big loops of yarn getting twisted, due to the high twist in the yarn (Unbalanced twist yarn).

Causes

• High, twist in the, yarn.

• Hosiery yarns are soft twisted. High, twist in the yarn, is the cause of snarling.

• (Snarls cause, fabric defects & needle breakages)

Remedies:

• Ensure using Hosiery Yarns, of the recommended T.P.M. only.

• (Hold a few inches of the yarn in both the hands, in the form of a ‘U’.

• The yarn has a balanced twist, if it doesn’t tend to rotate or turn, in the form of a snarl.

• (Such yarn can be used for Hosiery applications.)

 Contaminations

Definition:

Contaminations appear, in the form of foreign matter, such as; dyed fibers, husk, dead fibers etc., in the staple spun yarn or embedded in the knitted fabric structure.

Causes:

• Presence of dead fibers & other foreign materials, such as; dyed fibers, husk & synthetic fibers etc.      

• Dead Fibers appear in the fabric, as a result of the, presence of excessive immature Cotton fibers, in the Cotton fiber crop.

• Dead fibers do not pick up color during Dyeing.

• Presence of the foreign materials, in the, staple fiber mixing

• (Kitty, Husk, Broken Seeds, dyed fibers & fibers like Poly Propylene, Polyester, Viscose etc)

• Dyed & other types of fibers flying from the adjacent Knitting machines cling, to the yarn being used for knitting & get, embedded in the Grey Fabric.

Remedies:

• Use rich fiber mixing for the yarns, to be used for Knitting, in order to have less dead fibers, appearing in the fabric. 

• Rigid control measures in the Blow Room, to prevent the mixing of foreign matters in the Cotton mixing.

• Segregate the Spinning & Knitting Machines, with Plastic Curtains or Mosquito Nets, to prevent the fibers flying from the neighboring machines, from getting embedded in the yarn / fabric.

Spirality

      

Definition:

Spirality appears in the form of a twisted garment, after washing. The seams on both the sides of the garment displace, from their position & appear on the front & back of the garment.

Causes:

High T.P.M. of the Hosiery Yarn

              

• (Spirality is caused, by the Twisting Torque as a result, of the high yarn T.P.M.)

• (Hosiery yarns are soft twisted, whereas the Warp yarns are hard twisted)

• Uneven Fabric Take down tension, on the Knitting machine.

• Unequal rate of Fabric feed on the Stenter, Calender & Compactor machines.

Remedies:

• Use the Hosiery yarns of the recommended TPM level for Knitting

• (Hosiery yarns are soft twisted, in comparison to the Warp yarns)

• Fabric pull or the Take Down tension, on both sides of the grey fabric tube, on the knitting machine, should be equal. 

• Ensure uniform rate of feed of the dyed fabric, on both the edges, while feeding the fabric to the Calander, Compactor or Stenter machines.