Saturday, December 10, 2016

Knitted Fabric Defects and Remedies

Faults or defects in knitting production can be caused in different ways and quite a few of them cannot be related to just one cause. The following explanations are expected to be helpful in trying to locate the causes of these faults easier.

Reasons of fabric defects:
  • Yarn manufacturing defects
  • Fabric manufacturing defects
  • Fabric processing faults or defects. Such as dyeing faults, printing faults or finishing faults.
Sources of fabric faults:
The sources of faults could be:
  • Faults in yarn and the yarn package
  • Yarn feeding and yarn feed regulator
  • Machine setting and pattern defects
  • Machine maintenance
  • Climatic conditions in the knitting plant
List of Knitting Faults Found in Knitted Fabric:
Knitted fabric faults are very different in nature and appearance and are often superimposed. The following knitted fabric defects are found in knitted fabric production.
  1. Broken ends, holes or cracks
  2. Drop stitch
  3. Cloth fall-out or pressed-off stitches
  4. Snagging or snags
  5. Tuck or double loop or stitches
  6. Bunching-up
  7. Vertical stripes
  8. Horizontal stripes
  9. Soil stripes
  10. Color fly or color tinges
  11. Distorted stitches or deformed or tilted loops
1. Broken ends, holes or cracks:
Holes are the result of cracks or yarn breakages. During stitch formation the yarn had already broken in the region of the needle hook. Depending on the knitted structure, yarn count, machine gauge and course density, the holes have different sizes. This size can therefore only be estimated if the comparable final appearance of a comparable fabric is known.
Broken ends, holes or cracks
Broken ends, holes or cracks
Possible causes:

a) Yarn parameters
  • High yarn irregularity
  • Incorrect yarn input tension setting, yarn running-intention is too high
  • Poorly lubricated yarns
  • Weak places in yarn, which break during stitch formation
  • Knots, slubs etc.
  • Yarn is too dry.
b) If the yarn is trapped between the cheek taper and the closing latch
  • Yarn damage
c) Too small stitches
  • Difficulty in casting-off of the stitches
d) Relation between cylinder and dial loop not correct; yarn feeder badly set; defective knitting elements.

Remedies:
  • Yarn strength must be sufficient to with stand the stretch as well as uniform.
  • Use proper count of yarn.
  • Air humidification.
  • Guide blowing.
  • Correctly set of yarn feeder.
  • Yarn regularity control.
  • Precise yarn - guide resetting.
  • Knot should be given properly.
  • Use of protective filter creel.
2. Drop stitches:
These are the result of a defective needle. They also occur when a yarn is not properly fed during stitch formation, i.e., not properly laid-in the needle hooks. These are the unlinked knitted loops.
Drop stitch
Drop stitch
Possible causes:

a) In accurate insertion of the yarn into the needle hook;
  • Closed latch – a wale of dropped stitches will be produced until the latch is opened either by the operator or due to machine vibration.
b) Broken needle hook;

c) Due to high yarn twist and low fabric take-down-tension the knitted loop could fall out of the hook;

d) Improper setting of the yarn feed angle i.e. badly set yarn feeder
  • The yarn is not caught by the needle hook, Example-low yarn tension and high yarn vibrations
e) Yarn feeder wrongly threaded-in;

f) Dial loop length not properly related to cylinder loop length; the loop jumps out of the needle hook;

g) Bad take-up;

h) Very dry material;

i) Insufficient yarn tension.

Remedies:
  • Correct take-up of the fabric & correct fabric tension.
  • 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.
  • Proper feeding of yarn during loop formation.
  • The fabric tube should be just like a fully inflated balloon, not too tight or too slack.
  • Needle should be straight & well.
  • 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.
3. Cloth fall-out or Pressed-off stitches:
It is an area consisting of drop stitches lying side by side. They can occur either when a yarn is laid-out or when it breaks without any immediate connection. Cloth fall-out can occur after a drop stitch especially when an empty needle with closed latch runs into the yarn feeder and removes the yarn out of the hooks of the following needles.
Cloth fall-out or Pressed-off stitches
Cloth fall-out or Pressed-off stitches
Possible causes:
  • Yarn breaks before the yarn feeder
  • Yarn package winding faults, poor package buildup;
  • Fibre fly block the yarn guides, feeders etc.
Remedies:
  • Needle detectors, should be set precisely, to detect the closed needles & prevent the fabric tube from completely pressing off.
  • Make sure all the latches of needle are closed with feeding yarn after a drop stitch.
  • Proper yarn tension should be maintained, on all the feeders.
4. Needle marks or Vertical stripes:
Vertical stripes can be observed as longitudinal gaps in the fabric. The space between adjacent wales is irregular and the closed appearance of the fabric is broken up in an unsightly manner. Vertical stripes and gaps in the fabric are often the result of a meager setting, i.e., the yarn count selected is too fine for the machine gauge or the stitch size (course density) is not correct. Needles are bent, damaged, do not move uniformly smooth, come from different suppliers or are differently constructed.
Vertical stripes
Vertical stripes
Possible causes:
  • Twisted or bent needle hooks;
  • Stiff latches and needles;
  • Incorrect closing of the hook by the latch;
  • Heavily running needles;
  • Damaged dial and cylinder;
  • Damaged needle latch and needle hooks;
  • Damages on other knitting elements.
Remedies:
  • Yarn count should be selected as machine gauge.
  • Stitch size should be correct.
  • Selection of needle properly.
  • Needle should be straight as well as from broken latch
5. Horizontal Stripes:
These are caused by unevenness in the courses; they traverse horizontally and repeat themselves regularly or irregularly.
Horizontal Stripes
Horizontal Stripes
Possible causes:
  • Deflector in dial cam brought into tuck position.
  • Deflector not completely switched off. Needle can still grip the yarn and forms a tuck loop.
  • Yarn feeder badly set.
  • Differences in the yarn running-intension.
  • Couliering not constant at all feeders.
  • Jerky impulse from fabric take-up.
Remedies:
  • The machine must be mounted horizontally.
  • Needle dial & cylinder must be exactly centered towards one another.
  • Replace that bobbin.
  • Yarn tension & stitch should be controlled uniformly.
  • Yarns of same lot should be used.
  • Check cams positioning
6. Barriness
Barriness defect appears in the knitted fabric in the form of horizontal stripes of uniform or variable width. Actually barre'ness is the periodic lateral irregularities
Barriness
Barriness
A. Structural Barre'ness:

Possible causes:
  • Individual yarns differ with respect to count, properties or structure;
  • Different course lengths in feeders.
B. Colour Barre'ness:

Possible causes:
  • Knitting of yarns which differ in colour;
  • Yarns dye differently during piece dyeing.
C. Shadow Barre'ness:
Possible causes:
  • Shadow like changes in the appearance of the fabric. Very difficult to detect and done by reflected light.
Remedies:
  • Ensure uniform Yarn Tension on all the feeders. 
  • Ensure that the hardness of, all the yarn packages, is uniform, using a hardness tester.
  • 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.
7. Bunching-up or Thick and Thin Places:
Visible knots in the fabric are referred to as bunching up. They appear as beads and turn up irregularly in the fabric. Can build up resulting in a ‘cloudy’ appearance. More irregular the yarn, more pronounced is the ‘cloudy’ appearance.

Bunching-up
Bunching-up
Possible causes:
  • Thick and thin places in the yarn;
  • Fabric take-up too weak.
Remedies:
  • Specify the quality parameters of the yarns to be used for production to the yarn supplier.
  • Preventing count or lot mixing.
  • Maintaining uniform yarn tension..
  • Fabric take-up should function properly.
8. Snagging:
Snags mainly occur while processing filament yarns. The tendency towards snagging can be reduced by using yarns with a coarser single filament count, lesser crimp elasticity and higher twist.

Snagging
Snagging
Causes:
During knitting all mechanical influences, caused by rough surfaces on yarn guide elements, yarn feeders, needles, fabric take-up, etc. have to be avoided. Even after knitting some snags can appear especially during fabric setting, if its storage and further processing has not been undertaken carefully.

Remedies:

  • Inspect & rectify the fabric contact points on all the machines (Soft Flow Dyeing, Tumble Dryer & Centrifuge etc), on which snagging is taking place.
  • Using yarn with a coarser single filament count, lesser crimp elasticity and higher twist.
  • During knitting on mechanical influences, caused by rough surfaces on yarn guide elements, yarn feeders, needles, fabric take-up etc.
9. Tuck or Double stitches:
These occur due to badly knitted or non-knitted loops. They are unintentional tuck loops or floats, also showing up as thick places or small beads in the fabric. At first instance they may also appear as a shadow when the fabric is observed against light.

Possible causes:

  • Fabric take-up is too weak, i.e., fabric take-up is insufficient, must be re adjusted, has a one sided drag on the fabric or is not continuous.
  • The dial is set too high. The dial needles do not support the fabric, which is thus pulled up.
  • The course density or couliering is not set correctly.
  • The loops are too tight, e.g. with interlock. These loops are not removed from the needles.
Remedies:
  • Fabric take-up must be adjusted.
  • The coarse density must be set correctly.
10. Soil stripe:
Soil stripes can appear both in the direction of wales as well as courses. Soil stripes in the direction of the wales are solely caused by the knitting machine. In most cases they are so called needle stripes; they occur when individual needles have been replaced or when the working of mechanical or automatic oiling or greasing devices is defective.

Stripes or soiled places in the direction of the courses were already present usually in the yarn, if not caused by a standing course as a result of machine stoppage. 


Causes:

  • Defective oiling or greasing.
  • Sudden machine stoppage.
  • At the time of defective needle replacing.
Remedies:
  • Consciously oiling or greasing.
  • Being aware of needle changing.
11. Colour Fly:
Colour fIy consists of single fibres, bunches of fibres or yarn pieces in varying colours. It additionally sticks on the yarn or is knitted into the fabric and is very difficult to remove. 


Causes:

  • Hairs with natural dark color present in raw wool.
  • Fly coming from various processing stages during spinning.
Remedies:
  • Should be careful of fly coming at the time of spinning.
  • Certain of hairs with natural dark color are unavoidable & must be tolerated.