Although cost is the first consideration of the fiber purchasing process, fiber quality plays a role in determining if the price paid for the cotton is a value or not. Fiber quality determines the quality of the end-product and the productivity and efficiency of the spinning mill. This makes the management of cotton a critical process in the manufacturing of yarn.
Fine, long, and strong fibers allow a mill to produce finer, stronger, and uniform yarn. Additionally, finer, longer and stronger cotton normally results in higher spinning efficiencies.
The order of priority of the most important fiber properties varies for different spinning systems, depending on the yarn formation technology applied. However, in every case the length, length uniformity, strength, and fineness play a major role.
Choosing the right quality cotton at the right price is crucial to a mill’s profitability. In order to select the right cotton, it is essential to know precisely the quality characteristics of cotton. This is a weakness in human judgment of cotton fiber quality and is a primary reason why a mill needs objective methods that eliminates the preconceptions and emotion of human classification of cotton.
Even further, when it comes to managing those bales after purchase in laydowns or mixes, for most markets in Asia a bale is considered as part of a “lot” which can consist of up to 100 bales. The typical practice is for a percentage of the bales in a lot, in most cases 10%, to be tested by USTER HVI (typically by the mill buyer), and the whole lot is assigned the average properties of those 10% bales that were tested.
However, the weakness with this practice is that not all the bales come from the same field or may not be from the same variety. Furthermore, even if they were all from the same field, micronaire alone could vary as could other properties, causing problems in spinning and down-stream processing.
Of course, one of the driving factors of the practice of just testing 10% of each lot has to do with storage (warehouse) space.
Experience has shown that in a 100-bale lot, quality can vary widely and if not caught with 100% testing of bale, will result in variations between laydowns that can result in dye defects like Barré.
In our experiences around the world, we have seen cases where micronaire varies greatly within a 100 bale lot ranging from 3.7-5.1. In this particular case, the mill did not utilize HVI information for 100% of the bales in this lot and assigned bales in this lot with the average value of the 10% of bales that were tested.
In the example mill discussed earlier, they did in fact have widely varying micronaire between laydowns as a result creating issues with dyeing in the mill. As you can see in the chart, the micronaire shifts significantly from one mix to the other as indicated by the blue circles where the average micronaire shifted by more than 0.10 unit from the previous mix.
In this case, a shift like this would cause a Barré effect in yarn knitted and dyed of two yarns with such a significant shift.
Imagine assigning a micronaire value to this particular lot base on testing just 10% of bales! Controlling the variation of micronaire between laydowns in a mill would be virtually impossible. You might get lucky and have a few consecutive laydowns be consistent, but there will be the eventual and frequent “blow-up” every few days or weeks. This is when customer complaints come and start to cost money – money that would have contributed to the margin and profitability of a mill and which would have easily paid for the additional warehouse space and HVI needed to control 100% of the cotton properties for spinning mill mixes.