Thursday, 22 May 2008
Integrated Finishing for Denim- my Notes
Preshrinking of Textile Fabrics- or compressive shrinkage
Shrinkage can be
- Natural Wash shrinkage- When the fibres swell in the presence of water and tensions induced during Spinning, weaving and processing of fabrics
- Compressive or Mechanical Shrinkage
It can be compared to the action of press. The effectiveness of ironing depends upon:
Temperature of the press
Amount of Moisture present in the fabrics
Amount of Physical Pressure Applied
Time duration of application of press.
In case of pressing, the new memory is set by drying, in compressive shrinking, it is the palmer unit which dries the fabric, thereby setting its new memory
Amount of preshrinkage left in the fabric is adjusted by varying the relative speed of the palmer to that of the rubber belt unit.
The temperature affects in the following way the various components of the palmer/ compressive shrinkage unit
- Rubber Belt Cylinder
o Lower Temperature- Fabric appearance is affected- Sharp crease
o Higher temperature- rubber belt life is shortened
- Palmer Cylinder
o If lower temperature: No dry Stuff; Elongation of preshrunk fabric- also stretching of inspection and rolling operation.
o The purpose of palmer cylinder is to dry a fabric to a level of about 4% relative humidity. If there is higher temperature, there is elongation with natural moisture regain.
- Temperature of fabric as it enters rubber belt unit
If properly controlled, then high production. Most fabrics will shrink more easily if heated before entering the rubber belt unit.
- 100% cotton denim may require as much as 14% moisture to permit effective pre shrinkage
- Moisture must be uniform thoroughout the length, width, and depth of fabric
How to ensure moisture uniformity
- Use of heated cans
- Apply needed moisture of fabric vial cooling water applied to rubber belt surface. But it also depends upon the condition of the rubber belt. Freshly grinded rubber belt carry more water à water removal roll of the rubber belt unit
Penetration of moisture applied to the fabric is very important. If insufficient moistureà Innermost dry layers of yarn will act like spring and cause the fabric to elongate.
Maximum amount of rubber belt compression should not be greater than 25% of the actual belt thickness.
More heavy the fabric, more potential, more compression will it need
If above three factors are maintained and we have a sufficiently large palmer unit, we can compress durably a fabric to its ‘zero’ potential.
Its important to use cooling cans at the exit of the palmer
Fabric Scray: Use of exit scray allows additional time for fabric cooling as it is impossible to roll fabrics without the use of lengthwise tension.
Guider: The function of the guider is to keep the fabric to its full width.
Skyer: It is a sort of time delay device allowing time for moisture to penetrate into the fabric without the need to increase the machine length.
Heated Can: purpose: it is to drive the surface moisture into the fabric and to preheat the fabric.
Dry the fabric and set shrinkage
Adjust the shrinkage
To compare incoming and outgoing fabric tension and determine fabric shrinkage
Less dense the felt, greater is the drying capacity
- To relax and cool;
- To prevent hot stop marks. It increases the rubber belt life
- To facilitate shrink environment
Why Wet Finishing for Denim
- Moisture doesn’t penetrate in the core- in foam finishing
- Its better to shrink fabric with a low moisture content than those which are bone dried
In integrated Machine
- squeezing the moisture
- application of heat
Once it is shrunk the fabric is thoroughly dried by palmer
- Hand can be adjusted in padder use of starch, lubricant
- Width can be controlled by adjusting tension between the padder and dry can
- Higher speed
- Even ness of the moisture content- Residual moisture after leaving palmer should be 4%
Drying depends upon the pressure of the steam, m/c speed, size of palmer , construction of felt
Rubber Belts: 36-40 deg. Shore
Harder- crack and lesser shrinkage capacity
Softer- require replacement frequently
Thicker- more grinding
- more wear and tear to machine parts
- energy consumption
Rubber belt: Inside circumference- 3.962m
- Rubber surface width should exceed fabric width by at least 6” and preferably 8”
How to increase the life of the Rubber belt
- Nip pressure used on rubber belt should be optimum
- Over tension in the rubber belt should be avoided
- Belt should be run with sufficient cooling water in its interior and exterior surface
- Belt should be run with lowest possible operating temperature 115 deg- 140deg
- Frequency of grinding of the rubber belt should be optimum ie should be enough and at sufficient depth.
- Grind when density of belt surface has varied by 10% of its original hardness
- Water removal roll pressure adjustment is very important to insure max. belt life. Water acts not only to cool the rubber, but is also a lubricant
- Product machine stops or “hot stops” should be avoided to the maximum extent possible. One of the best ways to eliminate is to install scrays at the entrance and exit of the shrinking machine.
- Foreign objects should be avoided. Knot size for joining fabrics should be smaller.
- Regular cleaning and inspection of rubber belt.
- Use correct belt width
- Be careful during installation and maintenance of rubber belt, avoid use of chemicals.
Function of Felt Palmer
- It is required to maintain the preshrunk fabrics in intimate uniform contact with the surface of the heated cylinder in order to ensure uniform smooth drying of the fabricà new dimensionally stable memory.
- Fabric drying depends upon, palmer cylinder temperature, shrinking machine speed and permeability of the drying felt.
- It helps in precise fabric shrinking adjustment. It acts as a fabric puller to precisely control tension on the fabric
- Provides a pressing and calendaring effect on the preshrunk fabric