The Basics of Yarn Making for Students

This summary covers the yarn making processes used by SANS Fibres, including:

• What the basic raw materials are and why we use them.
• What the main requirements of our products are. 
• What the steps are in the yarn making process.
• A brief description of how each step is carried out and what its purpose is.

GENERAL

• At SANS we make both nylon 6.6 and polyester yarns.
• The polymers are different chemically but the yarn making processes are very similar.
• The process used by SANS for making yarn is melt spinning followed by drawing.
• This summary deals with generic melt spinning and drawing processes used at SANS.

KEY STEPS IN THE MELT SPINNING AND DRAWING PROCESS

POLYMER

What are the characteristics of the yarn we make? 

• It is long and thin. SANS produces continuous filament yarn. (Some other producers make staple fibre which is yarn that is crimped and cut into short lengths. This is blended with natural fibres such as cotton or wool, which are themselves staple fibres, and converted back into yarn by a twisting and stretching process.)
    
• It must be strong enough and have the correct balance of properties (extensibility, shrinkage, etc.) for the end use.
    
• The polymers we use are made of long molecules.
    
• The polymer molecules are aligned along the axis of filaments in the final product. Both the spinning and drawing stages of the process play a part in this. 

• At SANS we use nylon 6.6 and polyester (poly (ethylene terephthalate)).
    
• Molecules are strong - they are held together by covalent bonds which are the strongest chemical bonds.
    
• Molecules cling to each other by means of hydrogen bonding in the case of nylon 6.6 and physical entanglement in the case of polyester.
    
• The polymers we use must be of high purity to allow the formation of thin filaments (down to ~ 10 mm diameter) and avoid weak spots. 

POLYMER PREPARATION

• Polyester must be dried before melting to avoid hydrolysis.
  (Hold chip in hot dry air or nitrogen. Polyester must be kept agitated during initial heating while the surface of the chip crystallises to prevent it from sintering (sticking together). Once it has been crystallised, it can be processed in a column-type drier where there is little movement between chips.)
    
• Polyester can be solid state polymerised.
  (Hold dried chip in hot dry nitrogen or heat in vacuum.)
    
• Nylon 6.6 can be spun as received (approx 0.5% moisture) through a screw pressure melter.
    
• Nylon 6.6 must be conditioned to an appropriate moisture level for spinning through an extruder. If it is too wet it will hydrolyse in the melt and if too dry it will polymerise. This can have a big effect on spun yarn molecular weight if not well controlled.
  (Dry the chip then hold in warm humidified nitrogen.)
    
• Nylon 6.6 can be solid state polymerised.
  (Hold in hot nitrogen or in steam or heat in vacuum.)

MELTING AND MELT DISTRIBUTION

• A consistent supply of molten polymer is required by the yarn formation process.
    
• Typical systems are 
  - screw pressure melter (used on older spinning machines)
  - extruder, manifold and pack boxes
    
• Polymer is metered to the packs by gear pumps which give constant throughput at high pressures.

• Polymer streams for all threadlines on a machine must have the same temperature and residence time for a consistent product.
    
• Conditions must minimise undesirable reactions e.g. degradation, gel formation, etc.

FILTRATION

• Melt for the yarn formation process must be clean and without particulate contamination.
    
• Contaminants fall into three classifications:
  - External contamination (e.g. sand, packaging materials, pipe scale, construction debris)
  - Internal contamination (e.g. TiO2 agglomerates)
  - Degraded material and gel (specifically in nylon 6.6)
    
• Filament diameters are small, down to about 10 mm, and “large” particles bigger than about 1/3 filament diameter are likely to result in filament breaks. 
     
• Typical filter media are sand or alumina powder or shattered metal powder or stainless steel screens (woven or sintered).
    
• All packs incorporate filters.
    
• An in-line prefilter in the manifold may be used to reduce the rate at which the pack filters block. 
  

YARN FORMATION

• The spinneret splits the melt stream into individual molten filaments.
    
• Molten polymer leaves the spinneret at typically 5 - 30 m/min.
    
• The filaments are drawn away at typically 250 - 5000 m/min.
    
• This causes the polymer to be stretched and the filament diameter to decrease. Some alignment of the molecules in the filaments takes place.
    
• At the same time the filaments are being cooled as they move through the air, and become too cool to stretch any further.
    
• The quench air bows the filaments out to tension and steady them.
    
• The spinneret is a steel plate with accurately made holes, typically 200 - 500 mm diameter.
    
• Filaments are normally circular in cross section but the holes in the spinneret can be engineered to produce other cross sections (tape, trilobal, pentalobal, hexalobal and octalobal are commonly made). 
    
• Filament to filament, threadline to threadline and along threadline consistency are important. 

DRAWING AND RELAXING

• We draw yarn to achieve the desired physical properties for the end use.
     
• Drawing increases molecular orientation.

• We relax yarn to achieve a shrinkage low enough to satisfy customer requirements and in some cases to allow acceptable yarn packages to be produced.
    
• We draw yarn by running it over two rollers or godets running at different speeds. The yarn must be above its glass transition temperature to draw satisfactorily. For wet nylon 6.6 this is room temperature or below; for polyester it is about 70 - 80 ºC. 
    
• We relax yarn by overfeeding heated yarn between rollers running at different speeds and allowing it to shrink by a controlled amount.

WINDING

• The purpose of winding is to produce a package of yarn that can be unwound later at downstream processing or the customer.
    
• There are different basic types of winder 
  - Drive roll (friction drive) and traverse. Older type, either whole machine or individual winders
  - Spindle drive and traverse
  - Ring and traveller
     
• On a spinning machine without godets the winding speed must be constant as it determines the decitex.

YARN TREATMENT

This covers a range of things we may do to the yarn in order to assist processing or alter its characteristics.

Commonly applied treatments are:

• Application of spin finish. 
  Spin finish is primarily a lubricant but also assists in filament bundle cohesion and reducing static. In the case of nylon, the water added with the spin finish is important.
     
• Steam conditioning. 
  Nylon yarn may be passed through a steam conditioner tube to assist in getting water into the filaments rapidly.
     
• Intermingling. 
  Used to hold the filament bundle together, mainly for processing at customers.
     
• Texturing. 
  Synthetic yarns tend, at a microscopic level, to look like a bundle of plastic rods. Texturing involves twisting or crimping yarn to make it look and feel more like a natural fibre. 

Written by: J.R.T. Sharpe
July 2002