17 Knitted fabric defects

S. Natarajan

1.   INTRODUCTION

Defects in knitting production can be caused in various 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 defects easier.Faults found in knitted fabric mainly originated from three separate aspects e.g. (i) Faults due to yarn, (ii) faults occur during knitting and (iii) faults occur due to environment. In order to ensure quality of knitted fabric one must ensure coordination of all the three aspects mentioned above

The sources of defects could be

• Defects 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

  2.  TYPES OF DEFECTS

Knitting defects are very different in nature and appearance and often superimposed. The most common knitting defects are

• Broken ends, holes or
• cracks Drop stitches
• Cloth fall-out Snags
• Tuck or double loops
• Vertical stripes
• Soil stripes
• Coloured tinges Bunching-up
• Distorted stitches:

The same type of defects may occur in the fabric due to a variety of different causes e.g. Drop Stitches, Spirality etc.

2.1 CRACKS OR HOLES

Holes are the result of cracks or yarn breakages. During loop formation the yarn had already broken in the region of the knitting zone. Holes have different sizes depending on the knitted fabric structure, yarn count, machine gauge and course density. Possible causes:

a) Yarn parameters

• High yarn irregularity
• Weak places in yarn, which break during stitch formation
• Knots, slubs etc.
• Yarn is too dry.

Large holes are caused by weak places in the yarn, resulting the yarn to break during loop formation. Small holes are often the result of a broken yarn before (or after) a knot or splice, since the yarn end with the knot sits tightly in the last stitch. Normally the yarn breaks before the knot, because this gets stuck in the needle or gives rise to a tension peak.

These problems are not present in exactly the same manner when splices are present in place of knots.

Another reason for yarn breakage during loop formation is when the yarn tension is too high in relation to the structure and the breaking strength of the yarn.

• Yarn strength must be sufficient to with stand the strain during knitting.
• Use of proper yarn count.
• Air humidification.
• Yarn regularity control.

2.2. DROP STITCHES

Definition:

Drop stitches or runners are the result of a defective needle. They also occur when a yarn is not properly fed during loop formation, i.e., not properly laid in the needle hooks Major Causes:

• The yarn is too stiff and jumps away from the needle hook while being laid-in. This could be compensated by a re-adjustment of the yarn feeder or by increasing yarn tension.
• The yarn feeder is not properly set. It presents yarn to the needle’ for being taken-over at an unfavorable angle.
• Fabric take-off is insufficient. Loops already formed could be hung out before the next course is produced.
• Wrong stitch cam

Remedies

• Ensure uniform yarn tension on all the feeders with a Tension Meter.
• proper take-up of the fabric tension
• Rate of yarn feed should be 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.
• The gap between the Cylinder & the Dial should be correctly adjusted as per the knitted loop size

2.3. CLOTH FALL-OUT

Cloth fall-out 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. The yarn is not stitched by several needles lying adjacent to one another.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.

Remedies:

• Ensuring that all the latch needles are closed while feeding the yarn after a drop stitch.
• Ensuring the proper yarn tension at all the feeders.

2.4. TUCK OR DOUBLE LOOPS

Tuck or double stitches occur due to badly knitted or non-knitted loops. They are unintentional tuck loops, 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 or insufficient,
• The dial is set too high. The dial needles do not support the fabric, which is thus pulled up.
• The course density is not set correctly.
• Fabric take-up must be adjusted.
• The coarse density must be set correctly.

2.5. 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.

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

2.6. HORIZONTAL STRIPES

Horizontal stripes are caused by unevenness in the courses; they traverse horizontally and repeat themselves regularly or irregularly. Horizontal stripes are caused to the same extent by the yarn or by the setting of the knitting machine. They do not appear that often with worsted woo! yarns.An irregularly striped fabric during fabric production is solely the result of irregularities in the yarn.

Fabric take-up can also cause horizontal stripes, when a jerky impulse occurs at each machine revolution and take-up is not uniform

Remedies:

• The machine must be mounted horizontally.
• Yarn tension should be controlled uniformly.
• Yarns of same lot should be used.

2.7  BARRINESS

Definition:

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

A. Structural Barriness:

Possible causes:

• Individual yarns may differ with respect to count, properties or structure;
• Different course lengths in feeders. B. ColourBarriness
• Yarns used for knitting may differ in tone or shade
• Yarns dye differently during piece dyeing. Remedies:
• Ensure uniform Yarn Tension on all the feeders.
• Ensure that the hardness of, all the yarn packages, is uniform, using a hardness tester. Soil stripes

Soil stripes can appear both wale and course directions. Soil stripes waledirection are solely caused by the knitting machine and 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.

Causes:

• Defective oiling or greasing.
• unexpected machine stoppage.
• At the time of defective needle replacing.
• Consciously oiling or greasing.
• Being aware of needle changing.

2.8 COLOUR FLY

ColourfIy consists of 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. Fly may come from various processing stages during spinning. It can only be avoided by a careful separation of individual colours during the production. Distorted stitches:

Distorted stitches: Such stitches are usually the result of a bad knitting machine setting, especially unequal coulier depths between dial and cylinder needles. If one views the wales, one can then observe that the heads of the stitches are not round (ideal shape) but lopsided. They also appear to have tilted towards the one or the other side. 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.

Causes

High twist in the yarn.

Remedies

• Twist in the yarn should be in required TPM. Bunching-up
• Visible knots in the fabric are referred to as bunching-up. They appear as beads and turn up irregularly in the fabric resulting in a ‘cloudy’ appearance
• 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

Vertical lines

The defects as stated in the foregoing mainly occur during knitting and observed in grey knitted fabrics. After wet processing (scouring, bleaching, dyeing, printing, finishing), such defects generally become more prominent in one hand and some more new defects are added into the knitted fabric on the other

2.9 SNAGGING

Definition

Snagging occurs almost without exception only while processing fabrics continuous with filament yarns. Besides the specific sensitivity of these yarns, one main cause is mechanical strain during knitting or subsequent processes. Filaments or yarns have been pulled out of the fabric. If these are not removed properly the connection between courses is broken, and this results in an appearance very similar to cloth fall-out. 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:

• Inspection and rectification of 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.

2.10 SPIRALITY

Spirality, a common defect, is generally found in single jersey structures. The first and the main source of spirality is the twist liveliness of the yarn used. Loop formation involves both twisting and bending, resulting in twist redistribution in the arms of the loop. The net result is that all the loops in the fabric take up an inclined position giving the fabric a skewed or spiral appearance and the wale lines are nomore at right angles with the courses. Spirality of more than 5° is clearlyvisible and objectionable. The spirality value generally increases afterwashing due to relaxation of the residual torque of the yarn. Yarn withhigher twist multiplier always produces higher spirality than a yarn withlow twist multiplier.

• The factors other than twist liveliness
• Tightness factor of the fabric
• Feeder density in the machine
• Machine gauge
• Yarn linear density Spirality can be eliminated by setting the twist in yarn or by using balanced twofold yarns where possible.

2.11 DYEING AND FINISHING RELATED DEFECTS:

1. Bowing
2. Dyeing patches
3. Stains
4. Snagging
5. Softener marks
6. Crease marks
7. GSM variation
8. Fabric Width variation Bowing

Definition:

Bowing appears as rows of courses or yarn dyed stripes forming a bow shape along the fabric width.

Causes:

Uneven distribution of tensions across the fabric width while dyeing or finishing the fabric. Remedies: Bowing can be corrected by reprocessing the fabric by feeding it from the opposite end. Dyeing Patches

Definition:

Dyeing patches appear, as random irregular patches on the surface of dyed fabrics. Causes: Inadequate Scouring of the grey fabric is one of the primary causes of the dyeing patches. Improper leveling agent is also one of the causes of dyeing patches.

• Correct pH value not maintained.
• Dyeing machine stoppage due to power failure

Remedies:

• Scour the grey fabric thoroughly to remove all the impurities from the fabric before dyeing.
• Use appropriate leveling agents to prevent patchy dyeing.
• Maintain the correct pH value during the course of dyeing.
• Stains

Definition:

Stains appear as spots or patches of grease oil or dyes of different color, in a neat & cleanfinished fabric surface.

Causes:

• Dyeing Machine is not cleaned thoroughly after dyeing a lot.
• Grease & Oil stains from the careless moving machine parts like; Gears Shafts Driving Pulleys & Trolley wheels etc.
• Fabric touching the floors & other soiled places during transportation, in the trolleys.
• Handling of the fabric with soiled hands & stepping onto the stored fabric with dirty feet or
• shoes on.

Remedies

• Wash & clean the dyeing machine thoroughly after dyeing every dye lot.
• Follow the dyeing cycle of Light- Medium- Dark shades & then the reverse the cycle while dyeing the fabric.
• All the lubricated moving machine parts should be protected with safety guards.
• Make sure that the fabric is neatly packed in or covered with Polythene sheets while transporting or in storage.
• Handle the fabric carefully with clean hands & do not let anyone step onto the stored fabric.

Softener Marks

Definition

Softener marks appear as distinct irregular patches in the dried fabric after the application of softener.

Causes

Softener not being uniformly dissolved in water

Remedies

• Scour the grey fabric thoroughly to remove all the impurities from the fabric before dyeing.
• Ensure that the softener is uniformly dissolved in the water & doesn’t remain un-dissolved as lumps or suspension.
• Use the right softener & the correct procedure for the application.
• Maintain the correct pH value of the softener before application.

Crease Marks

Definition:

Crease marks appear in the knitted fabric, as dark haphazard broken or continuous lines.

Causes:

Damp fabric moving at high speed in twisted form, in the Hydro extractor (Centrifuge)

Remedies:

• Use anti Crease, during the Scouring & the Dyeing process.
• The use of anti Crease, swells the Cellulose & prevents the formation of Crease mark.
• Spread the fabric in loose & open form & not in the rope form, in the Hydro Extractor.
• GSM Variation

  Definition

The fabric will appear to have a visible variation in the density, from roll to roll or within the same roll of, the same dye lot.

Causes

• Roll to roll variation in the, process parameters, of the fabric, like; Overfeed & Width wise stretching of the dyed fabric, on the Stenter, Calender& Compactor machines.
• Roll to roll variation in the fabric stitch length.
• Make sure that all the fabric rolls in a lot, are processed under the same process parameters.
• The Knitting Machine settings, like; the Quality Pulley diameter etc. should never be disturbed.

Fabric Width Variation

Definition

Different rolls of the same fabric lot, having difference in the finished width of the fabric. Causes

• Grey fabric of the same lot, knitted on different makes of Knitting Machines, having varying number of Needles in the Cylinder.
• Roll to roll difference, in the Dyed Fabric stretched width, while feeding the fabric on
• The whole lot of the grey fabric should be knitted on the same make of knitting machines.
• For the same gauge & diameter of the knitting machines, there can be a difference of as high as 40 needles, from one makes to the other make of the machine.
• This difference, in the number of needles, causes a difference of upto 2”-3” in the finished width of the fabric
• The stretched width of the grey fabric should remain constant, during finishing on the stenter.

3. CONCLUSION

Knitting is one of the important fabric manufacturing techniques. The demand for quality of knitted fabrics is increasing rapidly now a day, since customer has become more aware of quality of fabrics. Detection of faults during the production of knitted fabric is important for improved quality and productivity and any variation to the knitted process needs to be investigated and corrected immediately. Human inspection by using knitted fabric inspection machines remains today the most used way to classify faults after knitting and after finishing

REFERENCES and URLs

2. Sadhan C. Ray, “Fundamentals and Advances in Knitting Technology”, WPI Publishing, March, 2012
3. Muhammad Abu Taher, “Study on Different Types of Knitting Faults, Causes and Remedies of Knit Fabrics”,  International Journal of Textile Science 2016, 5(6): 119-131
4. David J Spencer, “A comprehensive handbook and practical guide Knitting Technology”,WPI Publishing, 2001