Transcript Process Control

QC in Knitted Production
Part 2: Process Control
Jimmy Lam
Institute of Textiles & Clothing
Learning Objectives
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What are the major elements for processes control?
Fabric properties and loop length
Relationship between loop length and yarn tension
On machine control
– Yarn speed meter, yarn tension meter and yarn length meter
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Positive Feed device
– Trip-tape positive device, multi-coil positive feed,
– Positive feed with intermittent yarn consumption
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Storage Feed device
Remember this model
What are the major control elements for process control
INPUT
OUTPUT
PROCESS
CONTROL
Closed-loop Control Model
Quality Control Process
YARN
FABRIC
KNITTING
INSPECTION
Knitting Control
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This is a process control starting from yarn
feeding to fabric manufacturing.
Please name the factors that will affecting the knitting
fabric quality during production stage
Loop Length Control (I)
The loop length is the basic element (cell) in the
knitted fabric and is defined as the amount of
yarn used to form one unit loop
 Loop length = course length / no. of loops
 Loop length will affect the following parameters
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– stitch density; tightness factors; fabric weight;
– Fabric width; panel length; dimensional stability
Loop Length and Fabric Properties
Loop Length Control (II)
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Experiment shows that fabric thickness, weight, tightness
factor and stitch density are INVERSELY
PROPORTIONAL to the loop length
Setting of stitch cam (loop length control) and variation:
» Loop length is controlled by the setting of stitch cam in the
cylinder machine. However,how accurate is the indicator on
the stitch cam is the common question most knitters will ask
– Conclusion
» The variation on poor machine setting, yarn input tension
and knitting tension, all account for loop length variation
Yarn Knitting Tension Control
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Please show the relationship between the loop
length and knitting tension
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Please indicate what factors will affect the
knitting tension
Knitting Tension & Package size
Knitting Tension and Inertia
Knitting direction and yarn tension
Knitting Tension (I)
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Yarn unwinding method
– The yarn tension is different from the first layer to the bottom
layer
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Package size
– The size of the package (cone diameter and cone height) will
affect the winding tension
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Package density
– Unusual high package density will give higher friction to
yarn between coils, results in higher unwinding tension
Knitting Tension (II)
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Yarn fault
– Yarn faults mean sudden increase yarn diameter with results
in change of yarn tension in knitting process
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Spring wire on tensioner
– This will affect on V-bed machine when the cam cartiage
changing direction from RHS to LHS and vesa versa.
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Diagrams
– Relationship between package and unwinding tension
– Change of knitting tension on V-bed machine
– Analysis on the peak tension
On machine Quality Control
Equipment
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Knowing the factors affecting the fabric quality (loop
length variation), what possible equipment can be
used to minimize it or to eliminate it ?
If not, what equipment can be used to monitor any loop
length variation.
Yarn Speed Meter
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Yarn speed meter is used to
check the yarn consumption
speed on the knitting feeders
The results is good for
circular machine without
positive feeders.
Yarn speed is affected by
loop length, structure and
machine speed
Yarn Tension Meter
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Tension meter can be used to check the yarn winding
tension, unwinding tension and knitting tension.
The usual tension (medium to fine gauge) on circular
machine is 2-5 g/m
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High knitting tension = smaller loop length
Fluctuation in tension = variation on loop length
Tension lower than 2 g = drop stitches
Low and even tension = good quality fabric
What should we do if we find unusual high/low
knitting tension ?
Yarn Tension Meter
Yarn Length Meter
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The yarn length meter
can be fixed on
machine to record yarn
consumption for a
number of courses or
machine revolutions.
The results can be used
to calculate the loop
length between machine
of the same gauge.
Dial Height Gauge Control
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The gap between the
cylinder and dial will affect
the fabric quality on :
– fabric thickness
– fabric weight
– dimensional stability
Now we found the way to measure
the loop length variation. But can
we actually to control or eliminate
such variation ?
Can you suggest any methods ?
Positive Feeders
The function of positive feeders for circular
knitting machine is to regulate the yarn knitting
tension to desired value (low and even value)
and results in better fabric can be produced
 The positive feeders are almost standard
equipment for high speed non-jacquard
machine
 (Why only non-jacquard m/c ?)
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Yarn Tension with/without Positive
Feeders
Trip-tape Positive Feeder
Trip-tape positive feeder
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It is the simple measured yarn speed feeding system.
The yarn speed is controlled by speed tape pulley
which in turn, controlled by machine gearbox.
Two or three speed tapes are equipped which machine
can have two or three yarn feeding speeds for the
fabric.
When machine is properly adjusted, the yarn speed is
equaled to tape speed, and the tape speed can control
the suitable yarn tension and results in regular loop
length
Multi-coil Positive Feeder
Multi-coil positive feeder
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Due to the slippage between the tape and the yarn, the
efficiency of trip-tape positive feeder method is not
100%
– yarn speed IS NOT EQUAL TO tape speed
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The multi-coil method is used to achieve this target.
The yarn instead of directly driven by the tape, is first to
the pin wheel for 10 coils before feeding to the needles.
The principle is the yarn coils on the wheel can
neutralize the yarn tension variation from cone and
avoid and yarn slippage.
Positive feed for intermittent yarn
consumption
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So far, the positive feed discussed is only used for
continuous yarn consumption, it cannot be used on the
following machines:
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Cotton’s fully fashion machine;
V-bed machine
Circular hoisery machine;
Circular machine with stripers
Please discuss WHY?
Positive Feeder for stripping fabric
Positive Feed for intermediate yarn
consumption
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A special positive feed has designed for circular machine
with stripers.
The working principle is to utilize the coil friction between
the yarn and the friction roller.
When the yarn is not selected, the tension wire will bring the
yarn away the roller and thus, it cannot contact with the tape
and remain in idle position
When the yarn is selected, the yarn tension will increase and
pull the torsion wire towards the yarn guide, and this friction
in the coil will move the yarn to roller speed.
However, the slippage for this type of feeder is about 5-8%
Storage Feeder
Positive feeder device is an important device to
control the yarn tension and thus to minimize
the loop variation in knitting.
 However, positive feeder can only be used on
non-jacquard fabric such as interlock, pique but
not on jacquard fabric
 Please discuss WHY
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Storage Feeder
Storage Feeder
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Positive feeder is designed for regular loop fabric. For
fabric with different loop length in each course, a new
concept of storage feed has employed
To eliminate the yarn tension variation, all yarns are
rewound onto a drum of equal diameter, under equal
tension and speed.
The amount of yarn on drum is automatically controlled
so there will be no difference in package diameter.
The yarn is consuming by the knitting needles at variable
speeds, thus different loop length in each course is
possible
Discussion
What are the major objectives for process
control ?
 Why loop length variation cannot be avoided
during knitting process
 What are the difference between positive feeder
and storage feeder
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Process Control
The process control is to maintain a regular loop length
for the whole production process.
Multi-coil Positive Feed
Positive Feed for striping fabric
Storage Feed for Jacquard Fabric
Where is the positive feeder
in this diagram ?