Cockling has been defined as “an irregular surface effect caused by loop distortion”. In general it appears as localised groups of distorted knitted loops which have twisted out of the symmetrical configuration. The fault is usually found in the plain knit structure, which is relatively unstable, and especially in yarns spun from animal fibres such as wool or mohair.

Wool knitwear made from worsted yarns is more prone to cockling/loop distortion because, unlike woollen knitwear, no milling is carried out during finishing which helps to conceal faults under a surface fuzz.

There are three types of cockling:

1. Rib/plain interface cockling caused by a difference in the relaxed widths of the two structures
   
   
   
   
2. Panel-edge cockling caused by a difference in the relaxed dimensions of neighbouring structures. Panel edge loops are stretched in length when fashioning takes place and this tends to result in the contraction of the adjacent plain knit fabric, allowing cockling to take place.
   
   
   
   
3. Random all-over cockling. Almost always this can be assigned to using unsuitable yarn. Yarns which cause random all-over loop distortion also tend to cause rib/plain and panel edge cockling as well.
   
   
   
   

Factors affecting Cockling/Loop Distortion

The main factors which have been linked to cockling/loop distortion include:

Fibre Diameter
There is a direct relationship between an increase in fibre diameter and increase in loop distortion/cockling.

Method of Dyeing
Package dyeing increases loop distortion/cockling because it creates a ‘setting effect’ on the fibre and yarn, making it more rigid and hence resistant to loop formation on knitting.

Yarn Count Regularity
Although yarn count has little effect, yarn count regularity does influence loop distortion/cockling.

Yarn Twist Regularity
Generally accepted that large variations in twist can lead to loop distortion/cockling.

Yarn Rigidity
The more rigid the yarn the greater will be the tendency for loop distortion/cockling. However, the flexural rigidity of a yarn is usually dependent upon the diameter of the fibres. 

Yarn Regain
Dry yarns are relatively more rigid and thus more prone to cockling.

Yarn Steaming
Steaming causes setting and increased rigidity particularly in the case of shrink-resist yarns.

Cover Factor
The lower the cover factor, the less stable the resultant fabric, and the greater possibility of loop distortion/cockling.

Fibre Crimp
Inconclusive, but it is believed that the higher the fibre crimp the less the tendency for loop distortion/cockling to occur.

Fibre Length
Little or no direct effect

Yarn Friction
Little or no effect

Top Chlorine/Hercosett Treatmen
Although some opinions have been somewhat diverse, it is generally regarded that it has no significant direct effect on the tendency for the occurrence of cockling/loop distortion

Avoiding cockling/loop distortion

Where possible the above factors should be considered and if feasible:

1. Use a wool with a finer mean fibre diameter
2. Improve the regularity of the yarn
3. Avoid package dyeing - top or piece dye instead
4. Avoid steam setting the yarn on package
5. Use a wool with more crimp

Unfortunately it is not always possible to avoid the potential for cockling and thus some remedies might need to be employed

a) Anti-Cockle Treatment of the Garment
The treatment should be carried out immediately after knitting, i.e. before relaxation and cockling has begun to take place. Typically the garments are placed in very hot water with little or no agitation to allow the fibre to relax before the yarn starts to relax. Where necessary this process can be more effective if a reducing agent such as sodium metabisulphite is added to the hot water.

b) Decrease the relative tightness of the rib by increasing the number of stitches in the width and doubling on selected needles when transferring the rib to the fully-fashioned machine. This will result in a relaxed width closer to that of the plain area.

c) Increase the relative tightness of the plain knit area so that its relaxed width is closer to that of the rib.