PRINCIPAL YARN MANUFACTURING UNIT - 4 (DRAW FRAME)

DRAW FRAME:

Draw  frame  straighten  the  fibers   and  also  reduces  the  unevenness   of   sliver  with   the   help  of   doubling   and  drafting process . 

The fibers in  card slivers  are  separate individuals 

but they are   arranged   in  a  disorganized   and  

 random  way .  It is necessary  to  improve  their 

 straightness  and  alignment if strong,

 commercially  useful  yarns  are  to  be  produced.

 

Objectives of Draw-frame:

 

Reduction in unevenness by way of doubling and drafting process besides lowering weight per unit length of sliver.

 

Straightening out the fibres and improve the fibre extent and parallelization of fibers to the sliver axis.

 

Blending of slivers of different types of fibers in desired proportion by doubling and drafting process.

 

Removal of short fibers and dust from the material. 

MAIN PARTS OF THE DRAW-FRAME

 

v    Creel

v    Creel roller

v     Sensing roller

v   Bottom drafting roller

v   Top drafting roller 

  vtrumpet

v  Coiler calendar roller and coiler

v   Auto leveler

v   Can changer

CREEL:

 

Creel is a plain polished steel surface.  Slivers from the cans (received from previous process) are passed over it and taken to the drafting zone.

 

CREEL ROLLERS:

 

Creel rollers are cylindrical in shape, made of steel and are mounted on the creel in pairs.  Each sliver is passed through bottom and top rollers and taken to drafting zone.  Creel roll's function is to keep sliver under tension and help in moving it forward.

 

SENSING ROLLER/SCANNING ROLLERS:

 

These rollers scans the thickness of the sliver and then total draft of the machine is regulated automatically.

 

BOTTOM DRAFTING ROLLER:

 

Bottom drafting rollers are made of steel having parallel grooves on the surface called flutes.  Drafting of sliver is done by these rollers.

 

TOP DRAFTING ROLLERS:

 

Top drafting rollers are made of steel.  A thick synthetic rubber is mounted over it, which is called 'Cots'.  These rollers help in drafting.

 

TRUMPET:

 

Trumpet is also made of steel.  Different hole size trumpets are used for different hank slivers.

 

COILER CALENDER ROLLERS AND COILERS:

 

After the sliver is passed through drafting zone and trumpet,  it is passes through coiler calendar rollers. Finally the sliver is deposited in the cans kept over the can plate.

 

AUTO-LEVELLER :

 

Auto leveler reduces or increases main draft and removes unevenness in the feed slivers.

 

CAN CHANGER:

 

When doff is full, can changer takes out the full can and replace empty can below the coiler head.

 

Working Principle:

The working principle of draw frame is as follows:

• Equalizing

• Parallelizing

• Blending

• Short fibres & Dust removal

Equalizing: One of the main tasks of draw frame is to improve evenness over short, medium and especially long terms. Equalizing is done by doubling and drafting process. It  consists  of   doubling   of   ‘n’  number  of   slivers  and   drafting  them  to produce a single sliver. The  doubling  &  drawing of  sliver  reduces  the irregularity  present   in  the individual  slivers .

Parallelizing: To obtain an optimum value of strength in the yarn, the fibres must be arranged parallel in the sliver axis. The  drafting  action  improves  fiber  alignment,  straightening  the fibers  and aligning  them  with  the  sliver  axis.  

Blending: In addition to the equalizing effect, doubling also provides an opportunity of blending to different types of slivers made from natural & synthetic fibres. In  case  of   blending  of  combed  cotton   with  carded   polyester, blending  can only  take  place  during  this process, as  the  fibers  have  followed  different  processing  routes  up  to  this  point.

Short fibres & Dust Removal:  High performance draw frame is equipped with appropriate suction systems; for effective removal of short fibres and dust and more than 80% of the incoming dust is extracted.

Drawing:

Drawing is the term applied to the operation involving the doubling and roller drafting of slivers.

Doubling is the combination of several slivers that are then attenuated by a draft equal in number to the slivers combined, thereby resulting in one sliver of a similar count.

Roller drafting is the process of attenuating the count of a material using a combination of pairs of rollers. It gives a reduction of sliver thickness and also contributes to improved fibre orientation. 

 

    Factors dependent upon the drafting arrangement:

q    diameter of the rollers;

q    hardness of the top rollers;

q   pressure exerted by the top rollers;

q   surface characteristics of the top rollers;

q   fluting of the bottom rollers;

q   type pressure rods, aprons, condenser etc.;

q   clamping distances (roller settings);

q   level of draft;

q   distribution of draft between the various drafting zones.

    Drafting systems:

   There are different types of drafting arrangements which have be used by various machine manufactures in draw-frames.

        I)   3-over-3 roller drafting system.
      II)   3-over-4 roller drafting system

   III)   4-over-3 roller drafting system.

   IV)  4-over-4 roller drafting system.

Bottom Drafting Rollers:

Bottom rollers are made of steel and are mounted in roller stands or in the frame by means of needle, roller or ball bearings. They are positively driven from the main gear transmission or by a separate drive. In order to improve their ability to carry the fibres along, they are formed with flutes of one of the following types:

v axial flutes (a),

v inclined flutes (spiral flutes) (b),

v knurled fluting (c).

v

v

v

Normally the diameters of bottom rollers are between 25 and 50 mm.

TOP ROLLERS:

The top rollers are not positively driven. Ball bearings are used almost exclusively in the roller mountings. The thick coating forming the roller surface is made of synthetic rubber. An important characteristic of this coating is its hardness.

Soft coats surround the fibre strand to a greater extent than harder ones and thus guide the fibres better. On the other hand, they wear out more quickly.

A soft coating is therefore used where good guidance is necessary, i.e. where few fibres have to be moved with high draft levels (e.g. at the front rollers of the ring spinning machine). Where this is not required, harder coatings are mostly used. Hardness is specified in terms of degrees Shore. The following ranges are defined:

soft: 65° - 75° Shore

medium: 75° - 80° Shore

hard: above 80° Shore

Normally the diameters of top rollers are between 25 and 40 mm.

Weighting in Draw-frame (Top Roller Pressure):

To clamp the fibres, the top rollers must be forced at high pressure toward the bottom rollers. This pressure (loading) can be generated by means of:

v spring weighting (the most usual form);

v pneumatic weighting (Rieter);

v hydraulic systems (hardly used);

v magnetic weighting (was used by the former Saco Lowell company).

Nowadays only spring weighting and pneumatic weighting are used. The first is very simple, robust and easy to handle when dealing with machine faults; the second is in some cases more regular, and allows easy and quick changes of roller weighting exactly according to requirements as well as partial unloading during longer machine stoppages. It also avoids the need to adjust the weighting to a new roller diameter after roller grinding, which is sometimes necessary for spring weighted systems.

 

Draft Distribution:

Three-line drafting arrangements, with two draft zones, are generally used in the short staple spinning mills. The task of the break draft  zone is straightening and extending of the fibres to such a degree that the main draft can immediately cause fibre movements, without doing preparatory work (straightening and extending) . In this way, the main draft can be effected with less disturbance.

The main draft must be applied according to the drafting conditions, mainly the fibre mass in the drafting zone and the arrangement of the fibres in the strand. Since the fibres in card sliver are relatively randomly oriented, the draft in the first draw-frame passage should not be too high, whereas the draft can then be increased at the second passage and so on continually to the ring spinning machine.

     Draft:

In most spinning mills today, the first intermediate product is a card sliver. It contains about 20,000 – 40,000 fibres in cross-section. This number must be reduced in several operating stages to about 100 in the yarn cross section.

During drafting, the fibres must be moved relative to each other as uniformly as possible by overcoming the cohesive friction. Uniformity implies in this context that all fibres are controllably rearranged with a shift relative to each other equal to the degree of draft.

Drafting is effected mostly on roller-drafting arrangements. The fibres are firmly nipped between the bottom steel rollers and the weighted top pressure rollers. If the rollers are now rotated in such a way that their peripheral speed in the through flow direction increases from roller pair to roller pair, then the drawing apart of the fibres, i.e. the draft, takes place.

This is defined as the ratio of the delivered length (LD) to feed length (LF), or the ratio of the corresponding peripheral speeds:

where v = peripheral speed of cylinder, D = delivery and F = feed.

The drafting arrangement illustrated has two sub drafting zones, namely:

a break draft zone (B): VB = v2 / v3, and

a main draft zone (A): VM = v1 / v2

The total draft is always the product of the individual drafts and not the sum:

  Total draft = Break draft x Main draft

      Drafting Force:

When the fibres come in contact with roller surfaces of the drafting system, they move with the surface speed of the rollers. The transfer of the roller speed to the fibres can be effected only by friction, but the fibre strand is fairly thick and only its outer layers have contact with the rollers; furthermore, various non-constant forces act on the fibres.

The forces acting on a fibre (f) in the drafting arrangement will be considered here. The fibre is bedded at its trailing end in a body of fibres (B1) which is moving forward slowly at speed v2. The leading end is already in a body of fibres (B2) having a higher speed v1. In this example, a tensile force FZ acts on the fibre f; this arises from the adjacent fibres of the body B2 already moving at the higher speed and the retaining force FR exerted by the fibres of the body B1. To allow acceleration of the fibre f and finally a draft, FZ must be greater than FR. Permanent deformation of the fibre strand could not be achieved if FZ is only slightly greater than FR. In this case, straightening and elongation of the fibres would produce a temporary extension, which would immediately disappear on removal of the extending force. The drafting takes place in following stages:

qstraightening of the fibres (decrimping);

qelongation of the fibres;

qsliding of the fibres out of the surrounding fibre strand.

The effective drafting force can be represented by the curve form shown below:

Up to point (m) at which the fibres begin to slide apart, the curve climbs steeply. This is the straightening and extending stage. From point (n) onwards, by which stage many fibres are already sliding, the curve falls slowly with increasing draft. The reduction of the drafting force with the increasing extent of draft is easy to explain – there is a continuously declining number of fibres to be accelerated, i.e. to be drawn out of the slowly moving strand, since a higher degree of draft implies fewer  fibres in cross section.

The spinning limit can then be calculated approximately by transposition of the equation: The number of fibres in cross section of yarn (nF):

  nF = tex yarn/tex fibre

    

BESIDES THE NUMBER OF FIBERS IN THE CROSS-SECTION, THE DRAFTING FORCE IS ALSO HEAVILY DEPENDENT UPON:

q the arrangement of the fibres in the strand (parallel or crossed, hooks);

q

q cohesion between the fibres (surface structure, crimp, finish, etc.);

q fibre length;

q nip spacing.

     Sliver Coiling:

Two rotary movements are required for coiling of the sliver. On the one hand, the rotatable plate is to be rotated above the can, while the can itself must rotate, at a considerably slower rate, below the plate.

A sliver tube is provided on the plate as a fixed part to guide the sliver from the calendar rollers into the Can. This tube extends from the centre of the plate to its periphery. It is important for the coils that the circumferential velocity at the deposition point (sliver exit point) is somewhat higher than the delivery speed, so that blockages of the sliver in the tube are avoided.

However, the difference should not be too large, otherwise noticeable false drafts arise in the sliver. Due to the very high delivery speeds of modern draw-frames, coiling is becoming increasingly critical. Therefore the shape of the sliver tube is no longer straight, but is now curved exactly to correspond to the movement of the coiling sliver. On the Rieter draw-frame a honeycomb-structured, high-grade steel sheet is also provided on the underside of the rotating plate to prevent depositions of spin finish when processing synthetic fibres.

Change gears are provided to permit adjustment to requirements. The plate is usually driven by toothed belts and the can turntable by gear wheels or an individual drive. The sliver may be laid in the cans in small coils (under-centre coiling) or in large coils (over-centre coiling) depending on the size of the cans.