Sunday, 8 August 2010

How to Control Skew on Single Jersey Circular Weft Knitted Fabrics

Ideally courses and wales should be at right angle to each other. Skew occurs when wales are displaced from their vertical position when it is called wale skew. It also occurs when courses are displaced from their horizontal position when it is called course skew.

Skew on 100% cotton single jersey is related to the level of yarn twist, the spinning system used, the strand configuration, the tightness of the knitted stitch, the number of feeders on the knitting machine, the rotational direction of the knitting cylinder and the finishing techniques used.

Normally skew caused by yarn is wale skew and that caused by feeders is course skew.

Skew is measured using a proposed test method developed by AATCC. In this test the samples are marked with a square before washing and tumble drying. If the fabric skews after five wash and dry cycles, the square can be measured for percent skew.

 The method uses a mathematical formula for shear distortion (skew) and is shown below: % skew = 2(AC-BD) x 100/(AC + BD) Where AC and BD are the diagonals of the square.

On a knitting machine making single jersey. For each feed of yarn, one revolution of the machine will make a course of fabric. The more the number of feeders, the more courses are made in one revolution of machine. Which means that the courses are stacked on top of each other for each revolution. This creates a spiral line as shown in the figure. The distance between the spiral lines represent the production of courses for one revolution of cylinder. Thus for example if in one revolution of cylinder there is a formation of 1.5 inches of linear meter of cloth then there will be 1.5 linear inches of skew in the course that is generated. Machines with large numbers of feeders can create substantial skew in the fabric.

It is important to note that skew from the yarn and the skew from the number of feeders in the machine can combine together to create more skew or can offset the skew. Thus while selection of yarn twist according to the direction of rotation of cylinder is very important. In general, yarns with Z twist gives less skew on a machine of counterclockwise rotation. It is due to the fact that fabric coming from the counterclockwise machine have courses with LH skew while yarns with “Z” twist yield fabrics with RH skew. This offsets the two skews and the resulting fabric is more balanced.

It is said that best skew qualities result by alternating feeds of S and Z twist. Taking plied yarn instead of single yarn can also control the skew. If single yarns must be used, then resin finishing offers reasonable control of skew.

Also it is found that higher the twist multiple, the greater is the tendency to skew. All Z twist cotton yarns exhibit skew in a direction referred to “right-hand skew”. It means there is a wale loop distortion that leans to the right. All S twist yarns yield a left hand skew.

In general open-end yarns result in less skew than the ring spun yarns.

Also the tighter the stitch means more the number of stitches per revolution, the less is the skew.

Source: Technical Bulletin- Knit Fabrics and the Reduction of Torque

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