It is well established that knitted fabrics of all constructions and fibre blends are inherently more prone to shrinkage as compared to wovens. As knitted fabrics are elastic, processing by normal methods often stretches the fabric lengthwise, thus increasing shrinkage in that direction. So, it is important to “normalize” or regain that bulk which in turn will reduce shrinkage.
The term shrinkage can simply be defined as a change in the dimensions of a fabric or garment. This dimensional change may be in a positive (growth) or negative (shrinkage) direction for fabric length, width, and thickness. Shrinkage can be further defined as a dimensional change in a fabric or garment caused by an application of a force, energy, or a change in environment that either allows the goods to relax or forces the fabric to move in a given direction.
In case of knitted fabric, shrinkage relates to the loss of the length and/or width dimensions. In garment form, the shrinkage characteristics relate not only to a change in fabric dimensions, but also can relate to other parameters such as seam puckering, torquing, and overall garment fit.
Practically, it is very difficult to manufacture a knitted fabric with no shrinkage, so it becomes important for the dyer and finisher to make an effort to remove as much shrinkage from the product as possible. Knitted fabric also change dimensions with time, handling and with subsequent wet treatments including steaming, and such change can occur even after garment has been produced and sold to the end user.
Types of shrinkage
Construction shrinkage is defined as the amount of dimensional change in a fabric based purely on the construction variables used to manufacture the knitted fabric.
Processing shrinkage is defined as the dimensional change that a process adds to or removes from the construction shrinkage of a fabric.
Elastic shrinkage is defined as a change in dimensions of a fabric as a result of the ability of the fabric to freely relax from tensions experienced during construction and other processing. Residual shrinkage is the amount of shrinkage a fabric contains plus or minus what subsequent processing stresses apply to or remove from the fabric.
Drying shrinkage is defined as dimensional change in a fabric when deswelling of fibre, yarn, and construction occurs in the drying step.
Factors related with shrinkage
The major factors which associate with knitting fabric
Fibre: Cellulosic fibres, particularly cotton, are not as easily stabilised as compare to thermoplastic synthetics, because they cannot be heat set to attain stability. Therefore, the relaxation of knitted fabrics made with cotton fibres requires either mechanical and/or chemical means for stabilisation.
Yarn type: Yarns, of course, are manufactured with fibres and exhibit the same characteristics as the fibre. Yet the manner these fibres are oriented in a yarn will affect certain properties of the fabric including shrinkage. Cotton singles yarns of high twist will usually result in higher shrinkage values as compare to yarns of lower twist levels and will surely create greater skewing in the knitted fabric. Rotor spun yarns do not typically produce significant different length shrinkage values as compare to ring spun yarns, but are usually wider and definitely exhibit less fabric and garment torque. Plied yarns do not impact shrinkage.
Fabric construction: Different knitting fabric constructions can have significantly different shrinkage characteristics. For example, the performance of a single pique is different from a jersey or interlock made from the same yarns. For example, the “tuck” stitches in a pique tend to make the fabric wider and less extensible than single jersey. Typically, pique fabrics have much higher length shrinkage than width shrinkage. Further, fabrics which are knitted tightly, or with low stitch lengths, tend to be heavier in weight and have lower shrinkage along with more consistent shrinkage values. Fabrics which are knitted loosely, or with a higher stitch length, tend to be lighter in weight, have higher shrinkage and are inconsistent in shrinkage.
Chemical processes: Chemical processing of knitted fabric procedures generally exhibit stress on a fabric. Continuous processes during dyeing and preparation for drying usually stretch the length and pull down or reduce the width, sometimes beyond their elastic limit thereby changing the relaxed dimensions.
Finishing procedures: Finishing procedures may reduce or increase the dimensional stability of the knitted fabric. If relaxation dryers, compactors, and/or cross linking agents are used, then the residual shrinkage after wet processing can be reduced.
Garment manufacturing techniques: Garment manufacturing processes often increase the level of shrinkage in a fabric. The laying down of the layers for cutting and the physical manipulation of the panels in sewing are examples of whereshrinkage values can be increased. In fact, garments comprised of different fabric constructions may have some panels relax with handling in cut-and-sew while other panels may grow.
The importance of understanding shrinkage and it causes is key to its control and the best chance to achieve low shrinkage in cotton knitted fabrics is to totally engineer the product from fibre selection through all processing steps. The parameters for success can be outlined as follows:
1. Proper product specifications and fabrication
2. Low tensions during wet processing (dyeing and extraction) But, practically, it is not possible to manufacture a knitted fabric with no shrinkage, so it becomes important for the dyer and finisher to make an effort to remove as much shrinkage from the product as possible. The maximum shrinkage must be appropriate to reach the desired GSM, running length and width in the finished product. In today’s modern finishing plants, various methods are used to attempt to overcome processing shrinkage and reduce construction shrinkage. These methods include relaxation drying, compaction, and/or chemical processes. Relaxation drying and compaction are examples of consolidation shrinkage.
Relaxation drying allows those excessive tensions to be released since the fabric is dried under little or no restraints. The knitted fabric can be relaxed by means of steam or hot water. Relaxation drying incorporates tensionless, mechanical action at production speeds to complete the drying of both open width and tubular knitted fabrics.
Relaxation drying on steaming tables is widely used method for shrinkage control. In this method, shrinkage is controlled to a certain extent via the steam amount and the feeding speed. The fabric is put onto the steaming table with overfeed, taken off tensionless and then rolled up.
Relaxation dryers are based on the belt principle, where the fabric is placed between two belts and then passed through the drying zone. The bottom belt supports the fabric but allows for shrinkage, while the top belt prevents any stretching. In some cases, the bottom belt can be vibrated for additional mechanical action. Air flow is normally directed down and up through the fabric to give a ripple/wave effect. Once the drying is completed under relaxed conditions, those excessive tensions which have occurred during prior processing have been released. If the fabric has been spread with overfeed prior to relax drying, width shrinkage occurs first. This may prevent the length from shrinking initially, but as the drying progresses, both width and length shrinkage occurs. At the exit of the relaxation dryer, the fabric width will be inconsistent and may not be completely wrinkle-free. Thus, calendaring or compacting is necessary to provide a uniform, finished roll for cut and sew.
Many knitting fabric manufacturers rely on compaction as a means of shrinkage control. Compaction is a method whereby the course loops are compressed upon themselves. Different machines incorporate a variety of techniques to accomplish this. Compacting is consists in applying chemical products or mechanical treatments (compacting, sanforizing and stentering machine) in order that the clothing has the minimum dimensional alterations after being manufactured.
The compaction mechanism, along with heat and moisture, forces the length stitches (courses) to be compacted. During compaction, static friction is overcome by physical force. Compaction is the use of compressive forces to shorten the fabric to reduce the length shrinkage. This is achieved by heated roll and shoe compactors or compressive belt systems to force the length of the loop in a knit to become not only shorter, but also more round in configuration thereby resulting in lower length shrinkage values. This process is a consolidation process resulting in “consolidation shrinkage.”
Sanforizing is more efficient method than the compacting since, with an accurate adjustment, it grants approx 1 per cent shrinkage in the washing. The moistened fabric must be compacted through a rubber clothe curved by a roller.
Relaxation by steaming at the stenter feed end allows greater control of the cloth shrinkage by means of overfeed and width adjustment of the stenter frame. A special advantage of this method is the constant fabric width over the entire batch, so that the fabric can be rolled up evenly afterwards.
Chemical crosslinking has been the most used method for stabilizing cotton knit apparel fabrics especially those finished in open-width form. Compaction methods have also been effective but have been mainly used on underwear fabrics and most tubular goods. The advent of wet processes that impose lower tensions on fabric, such as the evolution of relaxation dryers and the improvement of compaction machinery including open-width, have combined to reduce the need for or level of chemical finishing.
Chemical crosslinking affects the swelling of cotton and reduces shrinkage by altering the normal shrinking (swelling/ deswelling) phenomena. In fact, a well-designed crosslinking system will permanently alter the shrinkage thereby altering the relaxed dimensions. Other benefits of a chemical finish would be a better appearance as related to wrinkling after washing and tumble drying, less tendency to pill or form surface fuzz from repeated laundering, and improved colour retention for some dyestuffs. The disadvantages are losses of strength and shorter wear life.
In the next session, we would be discussing
about development process of knitting fabric.