Yarn Manufacturing Technology (DRAW FRAME)

Draw Frame

Drawing

Drawing is the operation by which slivers are blended, doubled and leveled. In short staple spinning the term is only applied to the process at a draw frame. In drawing slivers are elongated when passing through a group of pair rollers, each pair is moving faster than previous one. This permits combing, drawing and elongating of several slivers to make them strong and uniform.In most modern worsted drawing sets there are 3 passages of pin drafting and roving process.

Objects of drawing

1.      To straighten the crimped, curled and hooked fibers.

2.      To make the fiber parallel to their neighbors.

3.      To improve uniformity of fibers by drafting and doubling.

4.      To reduce weight per length unit of sliver.

5.      To remove dust from slivers.

6.      To blend raw material of same hank perfectly.

Necessity of Draw frame

To parallelization of fiber and blending the carded sliver, draw frame is needed.

In carded sliver, fibers are present in hook form i.e. trailing hooks and leading hooks. To parallel these hooks raw frame is used.

Majority of the fiber hooks in a carded sliver are trailing hooks while leading hooks are comparatively less. Trailing hooks are also known as major hooks, while leading hooks are known as minor hooks.

Tasks of Draw frame

·        Equalizing

·        Parallelizing

·        Blending

·        Dust removal

Equalizing: One of the main tasks of draw frame is improving evenness over short, medium and especially long terms. Carded slivers are fed to the draw frame have degree on unevenness that cannot be tolerated in practice and slivers from the comber contain the “infamous” piecing. It is obscured by draw frame.

Equalizing is always performed by a first process, namely doubling and can optionally also be performed by a second process, namely auto leveling. The draft and the doubling have the same value and lie in the range of 6 to 8.

Parallelizing: To obtain an optional value for strength in the yarn characteristics, the fibers must be arranged parallel in the fiber strand. The draw frame has the tasks of creating this parallel arrangement. It fulfills the task by way of the draft, since every drafting step leads to straightening the fibers.

Blending: In addition to the equalizing effect, doubling also provides a degree of compensation of raw material variation by blending. Their results are exploited in particular way in the production of blended yarns comprising cotton or synthetic blends. At the draw frame metering of the individual components can be carried out very simply be selection of the number of slivers entering the machines.

Dust Removal: Dust is steadily becoming a greater problem both in processing and for the personnel involved. It is therefore important to remove dust to the greatest practical extent at every possible point within the overall process.

Dust removal can only be carried out to a significant when there are high levels of fiber. Since a large function the smallest particles adhere relatively strong to the fibers. High performance draw frame is equipped with appropriate suction removal systems; more than 80% of the incoming dust is extracted.

Actions involved in Draw Frame

Drafting: It is the process of increasing length per unit weight of sliver. It is mainly due to peripheral speed of the rollers.

Doubling: The process of combing two or more carded sliver into a single form is called doubling. In draw frame m/c generally six slivers are fed to convert into one i.e. six doubling.

Drawing: In the cotton industry the term is applied exclusively to processing on the draw frame, where the operation is one of doubling and drafting. Drawing= Drafting + Doubling.

Operating Devices

·        Creel (Sliver Feed)

·        The drafting Arrangement

·        Suction systems for the drafting arrangement

·        Coiling

·        Can charger

Creel

The creel must be designed so that:-

·        False Drafts are avoided.

·        The machine stops upon accordance of the sliver break.

·        Sliver breaks can be dealt with easily, comfortably and safely.

In this purpose it is necessary to provide a rotable roller or roller pair above each can. A guiding device for leading the rollers into the drafting arrangement is also required.

Requirements of drafting arrangement

The drafting arrangement is the heart of the draw frame. Drafting arrangement exerts most decisive influences on sliver quality. The requirements placed on the drafting arrangements are mentioned bellow:-

1.      Simple, uncomplicated construction.

2.      Stable design with smooth running roller. IT means rollers should be centric and smooth running.

3.      A mode of operation giving a high quality product at even high running speeds.

4.      High Degree of flexibility, suitability for all raw materials.

5.      Optimal control over the movement of the fibers during the drafting operation.

6.      High precision in both of operation and adjustment.

7.      Rapid and simple adjustability of roller spacing and draft levels.

8.      Ease of maintenance of cleanings.

9.      Optimal ergonomic design.

Factors dependent upon the fiber materials

1.      Mass of the fibers in the strand cross section.

2.      Degree of order of the fiber.

3.      Shape of cross section of the fiber strand.

4.      Compactness of the fiber strand.

5.      Length of fiber.

6.       Adhesion between the fiber depends upon:-             

·        Surface structure

·        Crimp

·        Lubrication

·        Compression of the strand.

7.      Evenness of distribution of fiber length.

8.      Twists present in the fiber strands.

Factors depending upon the drafting arrangement

1)      Diameter of the rollers.

2)      Hardness of the top rollers.

3)      Pressure exerted by the top rollers.

4)      Surface characteristics of the top rollers.

5)      Fluting the bottom rollers.

6)      Type and form of fiber guiding devices, such as pressure rods, pin bars, aprons, condenser etc.

7)      Clamping distance.

8)      Level of the draft.

** Both input and output of draw frame is sliver, then what is done in draw frame?

Answer:  From a purely commercial point of view draw frame is of little significance. But its influence on quality and evenness is greater. The carded sliver contains fibers which are oriented in various directions and they are crimped, hooked or curled. They entangled with one another and their height per unit length is high. They are irregular and contain some impurities. So they need a process for eliminating these errors.

In cotton spinning line raw frame is the definitive compensation point for eliminating errors. This draw frame:-

·        Straighten the fibers in sliver.

·        Make the fibers more or less parallel.

·        Reduce the weight per unit length of sliver.

·        Remove dust from sliver.

·        Make very perfect blending of fibers.

So, the carded sliver is actually made perfect in quality and evenness of draw frame.

Elements of drafting arrangements

Bottom roller: Bottom rollers are made of steel and are mounted in roller strands or in frames by means of needle or ball bearings. They are positively driven from main gear transmission.

On order to improve their ability to carry the fibers along, they are formed with flutes of one of the following types.

·       Axial flutes

·       Spiral flutes

·       Knurled flutes

Top roller: The top rollers are not positively driven; they are mounted by the means of ball bearings. These top rollers are made of steel and they are covered with thick coating of synthetic rubber. An important matter avoid this coating is its hardness. Soft rubbers coating can grip the fiber strand more perfectly than that of hard one, but soft coating of rubber wear out more quickly.

Hardness of rubber coating on top rollers in specified in terms of degree shore.

Soft coating -> 60-70 degree shore

Medium coating -> 70-90 degree shore

Absorb coating-> Above 90 degree shore

3 over 3 drafting Arrangement

                 

This is probably the most widely used form of drafting arrangement which was first developed by Platt in 1960. The drafting arrangement runs more smoothly with longer rollers. This is applied especially to the front rollers. For a given circumferential speed larger rollers can be operated at lower speed of revolution.

Roller slip

For pressure variation on top rollers gripping of fiber becomes irregular and due to this variation occurs in the draft of sliver. Due to roller slippage in front zone drafting becomes less. Roller slop also occurs if thick slivers are passed through drafting arrangement, because the bulky material cannot be drawn foreword by the rollers with exactitude. Though, more or less 3% roller slip in considerable.

Auto leveler

Without auto leveler we do not correct errors but avoid them. Auto leveler may be off 3 types.

1. Short term auto leveling system (for lengths of product from 10-12 cm)

2. Medium term auto leveling system (for length above 3m)

3. Long term Auto leveling system (for length above 20m)

Whatever be the system, the design of an auto leveler falls into two groups.

1. Open loop auto leveler

2. Closed loop auto leveler

Open loop auto leveler

In open loop auto leveler sliver weight is measured from input material. Here the measurement of sliver irregularity is expressed in terms of voltage and is compared with a desired value (DV). The resulting error signal is amplified (A) and after a suitable time delay (TD) its signal is used to vary the speed of variable speed device (S). A tachogenerator (TG) ensures that the variable speed device accurately follows the signal.

Advantages:

1. Accurate regularity maintaining is possible.

2. Piecing which arises from combing can be partly eliminated by open loop system.

3. Suitable for control the variation of short to medium wavelength.

Disadvantages:

1. If the measuring be sensor is improper, it is not possible to control the regularity.

Closed loop Auto leveler:

In closed loop auto leveler system sliver weight is measuring (M) from the output material and the resulting signal is integrated (I). Then the signal is compared with the desired level. After amplification (A), the variable speed devices (S) vary the speed of the drafting rollers. Because of the absence of any averaging device this would cause irregularities in output.

Advantages:

1. Suitable for compensating errors of long wave length.

2. It can be used in error raw frame.

3. A part of faults can be eliminated by it in first passage.

Disadvantages:

1. It cannot eliminate piecing as finely as open loop auto leveling system.

2. It is very costly.

After going through closed loop auto leveling system the sliver needs a second passage for eliminating other faults.

Necessities of Auto stop motion

Stop motion is a device which provided to ensure in the machine are stopped immediately if: –

1. The sliver run out from the creeled can.

2. Sliver brakes in the creel.

3. Lap up on drafting roller.

4. If sliver is chocked in the sliver collecting plate and funnel.

5. If sliver is chocked in between trumpet and calando roller.

6. If delivery can is filled with sliver.

If stop motion system is absent then the following defects may arise:-

1. Considerable damage may cause to machine.

2. The output sliver may be thick or thin.

3. Lapping up of drafting or calendar roller.

4. Irregular drafting.

5. Excess of full can.

6. Too much waste produce.

7. The yarn strength or roving strength may vary from place to place.

8. Any operator must have to attend the machine full time to stop it at any fault.

Features of modern Draw frame

Modern draw frame manufactured by RETER have the following feature:

® High speed of production.

® High draft.

® Better control of short fiber.

® Auto leveling.

® Auto stop motion system.

® Auto drafting and display unit.

® Electronic control and display unit.

® Improved drafting system.

® Can linking system.

® Rectangular can.

® Fault locating induction lamp.

® Monitoring system.

® Automatic can charger.

® Suction system for drafting arrangement.

Effects of Draft and doubling on sliver quality

We know that due to drawing and drafting the irregularity of sliver increase but due to doubling irregularity decreases.

Let us assume,

Cγm = Coefficient of variation of mass.

D =draft

Now,

Cγm α d

=>Cγm = k.d, where “k” is a constant for particle fiber.