YARN CRIMP

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Crimp:

Due to the interlacement of warp and weft yarns, a certain amount of waviness is imparted to the warp and weft yarns in a fabric. This waviness is called crimp. Hence the apparent length of a thread as it exists in the fabric is less than its straightened length.

Crimp Percentage:

It is defined as the mean difference between the straightened thread length and the distance between the ends of the thread while in the cloth, expressed as a percentage.



Influence of Crimp on Fabric Properties:

Warp and Weft crimp percentages are two factors that have an influence on the following fabric properties:

  1. Resistance to Abrasion

  2. Shrinkage

  3. Fabric Behavior during Strength Testing

  4. Faults in Fabric

  5. Fabric Design

  6. Fabric Costing



Resistance to Abrasion:

The abrasion resistance of fabric will be more if the crimp in the yarn is more. The yarns with high crimp take the burn of abrasive action. This is because curves formed as the yarn bends around a transverse yarn, will protrude from the surface of the fabric and meet the destructive abrasive agent first. The other set of yarns lying in the center of the fabric will only play their part in resisting abrasion when the highly crimped threads are nearly worn through.

Shrinkage:

When the yarns are wet, they swell and consequently say a warp thread has a longer bending path to take a swollen weft yarn. The warp yarn must either increase the length or alternatively the weft yarns must move closer together. An increase in the length of warp yarn requires the application of tension and therefore when the tension is absent equilibrium conditions will be attained by the weft yarns moved closer together.

The largest amount of shrinkage is that represented by an increase of crimp. Yarn shrinkage takes second place and generally it is just less than the increase in the crimp. Since shrinkage is mainly due to yarn swelling and the resulting crimp increase, mechanical means of controlled pre-shrinking have been developed such as sanforizing and Rigmel processes.



Fabric Behavior During Strength Testing:

When a strip of fabric is extended in one direction crimp is removed and the yarns are straightened. This causes the yarns at right angles to the loading direction to be crimped further i.e., when the load is applied along the warp threads crimp in the warp yarns is removed and that in the weft yarns is increased. This is known as crimp interchange. The sample loses its original rectangular shape and the middle portion to the strip contracts. This is known as waisting. Due to the removal of the crimp, the load-elongation curve will show a relatively high extension per unit increase in load in the early stages of strength testing of a strip of fabric.

Faults in Fabric:

Variation in crimp can give rise to faults in fabric, eg, reduction in strength, bright picks, diamond bars in rayons, strips in yarn-dyed cloths, etc. The crimp variation is mainly due to the improper tensions on the yarn during yarn preparation and weaving.



Fabric Design:

Control of crimp percentage is necessary when a fabric is designed with a degree of extensibility. Some fabrics require control of crimp in the finishing processes to give the correct crimp balance between warp and weft so that the finished appearance is satisfactory. Therefore, the tensions applied must be carefully controlled.

Fabric costing:

Since crimp is related to length, it follows that the quantity of yarn required to produce a given length of fabric is affected by the warp and weft crimp percentages. Therefore in calculating the cost and the yarn requirements, the values of crimp play an important role.



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