1. Management of Quality in Knitting Part 2: Specification & Control
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2. Management of Quality in Knitting
Identify key control parameters
Establish appropriate specifications
Identify potential sources of variation
Establish reliable monitoring procedures
Variations in grey fabric cannot be eliminated during finishing
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3. Key Control Areas For Each Fabric Quality Yarn Quality
type, count, twist
Average Stitch Length
these determine the Reference Dimensions
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4. Yarn Quality Fibre Quality Yarn Type Count & Count Variation
Twist & Twist Variation
Irregularity & Imperfections
Strength & Strength Uniformity
Friction
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5. Yarn Specification Yarn properties are inter-dependent
Low variation is often more important
than high mean values
High quality usually means high price
Consider the end product
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6. Control of Yarn Quality
Difficult for the knitter
Depends on reliability of ALL suppliers
Good relationships essential
Quality checks must be installed
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7. Yarns from Different Spinners
Specification = Ne 38 ± 2.5%
Measured Yarn Count, Ne 41 40 39 38 37 36 35
Supplier A
Supplier B
Supplier C
Ne CV 0.5%
Ne CV 2.8%
Ne CV 1.9%
Test Number 20 40 60 80
100
120
140
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8. Effect of Yarn Count Variation
Fabric: Interlock 20g 30d 1884n
Knit Spec: Ne 1/38, SL 3.4 mm
Process: Winch-jet: Mid Tension, White
Finish Spec: 14.5 c/cm, 68 cm tub (200 gsm)
Variation due to Yarn Count only
Weight, gsm
195 to 207
Length Shr.%
-4.4 to -5.5
Width Shr.%
-4.4 to -6.0
No allowance for variation in SL or Processing
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9. Yarn Count Variation If fabrics finished to the same weight and width
Range in Length Shrinkage increases
to between -1.6% and -8.5%
Variation in Ne between deliveries
of ± 3% is not unusual
For Ne 38, range is Ne to Ne 36.86
Actual range was Ne 39.2 to Ne 36.9
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10. Monitoring Yarn Quality
Obtain test data from ALL suppliers
Identify and check EACH delivery
for count, twist & friction before knitting
NB moisture content
COMPARE with agreed Specification
Establish PROCEDURES
for out of tolerance deliveries
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11. Monitoring Yarn Quality
Use charts to monitor suppliers
Establish NORMAL variation
for each supplier over time
Establish appropriate tolerances
Establish guide-lines for action
Communicate results to suppliers
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12. Monitoring Yarn Count Measured Yarn Count, tex ? Sample Number
Specification = 19.3 tex ± 2.5%
Measured Yarn Count, tex
20.2
19.8
19.4
19.0
18.6
+2.5%
Spec. ?
-2.5%
Sample Number 10 20 30 40
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13. Control of Yarn Quality
should get more stringent checking
until their reliability is proved
New Suppliers
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14. Control of Yarn Quality
Finally
Pay attention to yarn storage
e.g. temperature and relative humidity
Check for damaged cones or boxes
NEVER MIX LOTS
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15. Good Yarn Makes Good Fabric
Lowest price is seldom the lowest cost
Dependability of supply is valuable
Consistency is important
Limit the number of suppliers
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16. Knitting Specification
For each quality on each knitting machine
the key production control parameters are
Course Length (SL * Needles)
- determines reference dimensions
Courses per Roll (revs * feeders)
- determines roll length & weight
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17. Control of Course Length
Positive (driven) Feed
Instrumentation
Memminger-Iro: MPF_L
Control Procedures
Memminger-Iro: MLT-Wesco
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18. Variation Between Feeders
Negative (passive) Feed
Course Length, inches
125
123
121
119
117
115
113
Mean = in CV = 2.0%
Feeder Number 1 3 5 7 9 11 13 15 17 19 21 23 25
Source: SITRA
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19. Variation Between Feeders
Positive (driven) Feed
125
123
121
Course Length, inches
119
117
115
113
Mean = in CV = 0.32% 1 3 5 7 9 11 13 15 17 19 21 23 25
Feeder Number
Source: SITRA
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20. Control of Course Length
Yarn input tension and take-down tension
Can influence knitting efficiency
and fabric appearance therefore
proper control is important
But they do not affect stitch length
provided positive feed is used
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21. Effect of Yarn Input Tension
Using Negative Feed
Course Length, cm
125
120
115
110
105
100
Waxed
Unwaxed
Input tension, g 4 6 8 10 12 14 16
Source: HATRA
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22. Effect of Take-down Tension
Courses /3cm 50 55 60 45 65
Fabric Tightness Factor = 16.4
High TD tension
Low TD tension
Time in Winch, min
200
400
600
800
Hydro
Finished Reference
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23. Effect of Take-down Tension50
Fabric Tightness Factor = 13.4 45
High TD tension
Low TD tension 40
Courses /3cm 35 30
Hydro
Finished Reference
200
400
600
800
Time in Winch, min
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24. Control of Course Length
Positive Feed alone is not enough
Accurate setting of positive feed drives is needed
Drive wheel markings
not accurate enough
Modern systems should be more accurate
Memminger-Iro: Drive pulley
Memminger-Iro: Modern system
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25. Control of Course Length
Positive Feed alone is not enough
Accurate setting of positive feed drives
drive wheel markings are not
accurate enough
Regular Maintenance
wax and fly contamination
Independent Checks
using appropriate instrumentation
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26. Course Length Instruments: Examples
Direct length per revolution measurement
Wesco (Memminger-IRO)
Unilength (Meiners-del)
Schmidt LMC
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27. Course Length Instruments: Examples
Indirect and integral systems
SDL yarn length & speed meter
Decotex (Memminger-Iro)
Intus (Mayer & Cie)
Knit Master (Barco)
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28. Course Length Instruments
Check Results by measuring
stitch length of yarns taken from fabric
Calibrate Instruments
on a regular basis
Establish Accuracy and Variability
using Control Charts
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29. Course Length Instruments: Variation
28g Single Jersey
Stitch Length, mm
2.64
2.60
2.56
2.52
2.48
2.44
2.40
2.36
+1.5%
Lab
-1.5%
Instrument 1
Instrument 2
Feeder Number 1 3 5 7 9 11 13 15
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30. Course Length & Needles
Number of needles can vary between
different makes and models
Stitch Length depends on Course Length
SL = Course Length / Needles
Needles must also be specified when
Course Length is the Control Parameter
Fabric Width depends on No. of Needles
Width = Needles / Wales
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31. Variation in Cylinder Needles
SJ 30" Diam, 20 Gg
Needles
% Diff
Theoretical
1884
Falmac "B" Series
1848
-1.9
Monarch VXC-73S
1860
-1.3
Pai Lung FS3A/T
1872
-0.6
Vanguard 1SJ4
1920
+1.9
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32. Course Length & Stitch Length
Use Course Length
to set-up and control production
on individual machines
CL may be different, depends on needles
Use Stitch Length
to specify quality and compare
production across all machines
Stitch Length should be the same on all
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33. Monitoring Stitch Length
Target = 2.82 mm
Stitch Length, mm
2.70
2.75
2.80
2.85
2.90
2.95
Mean = : SD = 0.042
Sample Number 10 20 30 40 50 60 70 80
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34. Stitch Length Variation
Fabric: 1 x 1 Rib 14g 20d 876n
Knit spec: Ne 1/30, SL 2.82 mm
Process: Winch-jet: Mid tension, Medium
Finish spec: 17.6 c/cm, 41 cm tub, 200 gsm
Variation due to Stitch Length only
Weight, gsm
194 to 206
Length Shr%
-0.8 to -8.1
Width Shr%
-5.1 to -9.6
No allowance for variation in Ne or Processing
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35. Control of Course Length
Prevention better than Cure
Check average CL before production
and record the results
establish variation between feeders
establish variation between machines
determine minimum number of
feeders to be checked
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36. Control of Course Length
Making Measurements
Regular checks throughout production
to ensure the quality is maintained
Record results so that average values
and the variation can be monitored
Calculate Standard Deviation of means
for each machine, quality & yarn lot
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37. Control of Course Length
Frequency of Checking
Initially may need to be intensive
until true variation established
Reduction only with clear proof
Increase frequency after maintenance,
new parts, new yarn lot
Effective monitoring will quickly detect
deterioration or drift
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38. Control of Course Length
Tolerances and Control Limits
Must be Realistic and Achievable
Optimum levels take time to establish
Normal Tolerance is ± 2 SD
95 in 100 within Normal Tolerance
Action Tolerance is ± 3 SD
1 in 300 outside Action Tolerance
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39. Monitoring Stitch Length
Target = 2.82 mm : SD = 0.042
2.95
+ 3 SD
+ 2 SD
2.90
2.85
Stitch Length, mm
2.80
2.75
- 2 SD
2.70
- 3 SD 10 20 30 40 50 60 70 80
Sample Number
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40. Control of Course Length
Practical Control Tolerances (± 3 SD)
reflect current capabilities
closer tolerances do not improve control
If Practical Tolerances are too high
first identify and reduce variation
then calculate new tolerances
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41. Monitoring Stitch Length
Target = 2.82 mm : SD = 0.042
2.95
+ 3 SD
+ 2 SD
2.90
2.85
Stitch Length, mm
2.80
2.75
- 2 SD
2.70
- 3 SD
Machine Diameter 16 18 17 19 20 22
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42. Stitch Length Sorted by Machine
Diam
SL mm
CV% SD 16
2.850
0.25
0.007 17
2.806
0.76
0.021 ? 18
2.884
0.45
0.013 19
2.785
0.35
0.010 20
2.873
0.39
0.011 22
2.785
0.31
0.009
all data
2.832
1.49
0.042
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43. Control of Course Length
Maintain a Consistent Standard
Check on-line instrumentation regularly
internal lab measurements of SL
external calibration
monitor using Control Charts
QC independent from production
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44. Control of Roll Weight and Length
Consistent roll Weight
planning, monitoring & control
storage & handling
easy composition of dyelots
Consistent roll Length
improves lay planning
reduces cutting waste
reduces costs
Both can be accurately controlled based on
Yarn Count & Course Length
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45. Roll Weight and Yarn Tex Count
Roll Weight (kg) =
tex * CL(cm) * feeders * revs * 10 -8
therefore
tex =
CL(cm) * feeders * revs
roll weight (kg) * 10 8
590.54 Ne
tex =
590.54
tex
Ne =
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46. Roll Length Roll Length = Total Courses / Course Density
Total Courses = Feeders * No. of Revs
Knitting Control Parameter = Revs
Use finished Course Density
to calculate total courses to knit
Don't forget construction effects
e.g. Two-thread fleece
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47. Quality Checks on Grey Fabric
Grey (machine state) dimensions are
Unstable
Unreliable
They are influenced by
Take-down tensions
Stretcher board settings
which do not affect Reference Dimensions
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48. Quality Checks on Grey Fabric
Grey area weight
is NOT a reliable check of fabric quality
Roll Weight
can be useful, but only if
Course length is accurately controlled
Exact no of courses is known
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49. STARFISH Workshop