Changes of dimension after knitting can create major problems in garments and fabrics, especially those produced from hydrophilic fibres such as wool and cotton. Articles knitted from synthetic thermoplastic fibres such as nylon and polyester can be heat-set to a shape or to dimensions that are retained unless the setting conditions are exceeded during washing and wearing.
In the case, of wool fibres, dimensional changes can be magnified by felting shrinkage. When untreated wool fibres are subjected to mechanical action in the presence of moisture, the elasticity and unidirectional scale structure of the fibres causes them to migrate and interlock into a progressively closer entanglement. Eventually, the density of the felted fabric restricts further fibre movement but, long before this point, the fabric properties (including appearance) will have been severely impaired. Fortunately, it is now possible to achieve a shrink/felting-resist finish in wool yarns during spinning so that, as with cotton yarns, little yarn shrinkage will occur during washing and wearing.
Knitted fabrics tend to change dimensions in width and length after being taken off the machine, even without yarn shrinkage, indicating a change of loop shape rather than of loop length. During knitting, the loop structure is subjected to a tension of approximately 15-25 grams per needle from sources such as the takedown mechanism and, in the case of fabric machines, the width stretcher board. Unless the structure is allowed to relax from its strained and distorted state at some time during manufacture, the more favourable conditions for fabric relaxation provided during washing and wearing will result in a change of dimensions, leading to customer dissatisfaction.
In theory, knitted loops move towards a three-dimensional configuration of minimum energy as the strains caused during production are allowed to be dissipated so that eventually, like all mechanical structures, a knitted fabric will reach a stable state of equilibrium with its surroundings and will exhibit no further relaxation shrinkage.
Unfortunately, there are a number of states which may be achieved by different relaxation conditions, such as dry relaxation, steaming, static soaking, washing with agitation, centrifuging, and tumble drying. These states are difficult to identify, define, and reproduce because friction and the mechanical properties of the fibres, yarn, and structure can create high internal restrictive forces and thus inhibit recovery. However, agitation of the knitted structure whilst it is freely immersed in water appears to provide the most suitable conditions for relaxation to take place as it tends to overcome the frictional restraints imposed by the intermeshing of the structure.
A satisfactory relaxation technique applied during the finishing of cotton fabric in continuous length form is the compacting or compressive shrinkage technique. The fabric is passed between two sets of roller nips, with the feed rollers turning at a faster rate than the withdrawing rollers so that the courses are pushed towards each other and the fabric is positively encouraged to shrink in length. This technique can create difficulties with interlock fabric, which tends to buckle outwards three-dimensionally to produce ripples on the surface known technically as 'orange peel'.
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