Wednesday, 15 October 2008

Sizing Control points, sizing defects

Control of Yarn Stretch

During sizing, the yarns are under tension, this results in a slight permanent stretch in the yarn. It leads to a decrease in extensibility or elongation at break of the sized yarn, which leads to more breakage at the loom shed.

The various zones of stretch control on modern sizing machines are:

1. Creel zone: start- last warper beam, end-Dry nip
2. Wet Zone; Start- dry nip, end- first drying cylinder
3. Drying Zone: start- first drying cylinder, end- last drying cylinder
5. Splitting Zone: start-last drying cylinder, end- drag roll
6. Winding Zone: start- drag roll, end- loom beam

Control of Stretch in Creel Zone

The creel stretch on the existing type of sizing machines has to be controlled manually. The yarn tension in the creel zone increases gradually with reduction in diameter of the warper's beam. To counter this, the tightening of the beam is required to be adjusted suitably as the sizing progresses.

Control of Stretch in the Wet Zone

The control of stretch in this zone can be done with the help of 'positive dry nip'.

On multicylinder sizing machines, stretch can be controlled by synchronising the PIV gear during the cylinders with that driving he finishing squeeze roller in the wet zone.

Control of Moisture in sized yarns

A moisture control of 8-10% should be maintained in the sized cotton yarns. With excessive drying, the size film becomes brittle and harsh.
Very high moisture content is also undesirable because it makes the size films soft and the yarn sticky.

Quality of Sized Beams

A satisfactory weaver's beam should unwind well on the loom. These are some of the important package faults:

1. Density of sized beams: A loosly packed weaver's beam does not work well. The density is mainly influenced by two factors:
a. effectiveness of the friction clutch or the DC drive
b. effectiveness of the bream pressing motion.

2. Broken ends, missing ends, crossed ends, sticky ends

The major sources of all these faults are
a. lappers
b. invisible breaks during sizing

A lapper is an accumulaiton of layers of yarn on the warper's beam.

Those end breaks during sizing that do not form lappers are called invisible breaks.

Both lappers and invisible breaks result in missing and broken ends in the sized beams.

Crossed Ends- these are formed during weaving whenever the leading end is not available in the appropriate place on the beam,and, therefore, the weaver has to knot the trailing end to an end that is far away. This happens because in some cases the leading end of an invisible break migrates to a distant place.

Sticky ends - These are caused when broken ends from the warper's beam migrate to the yarn of another warper's beam.
In order to control these faults, it is necessary to control the incidence of lappers and invisible breaks.

Factors affecting lappers:
- End breakage rate at warping
- Efficiency of warp stop motion at warping
- condition of beam flanges (warper's)

Factors affecting invisible breaks
- High stretch at sizing
- weak places in the yarn

Defective selvedges

There are two types of defective selvedges that cause more difficulty in unwinding during weaving than the ends of the beam of thebeam. These are:
1. Sunken selvedges
2. Bulging selvedges

These defects can be controlled by
a. correctly setting the expandable comb at the headstock
b. Using the correct size of beam pressing roller so that it reaches both the beam flanges.
c. Ensuring that beam flanges are true.

Formation of ridges on the Beam

Ridges on the beam are formed when the ends that are taken in one dent of the comb do not spread out. To minimise the falut the eccentric dancing rollers at the headstock should be adjusted properly.

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