Fiber Preparation for Spinning
In any spinning operation, the requirements of the end product, or of the consumer of the yarn, will be the dictating forces in determining the fiber quality and properties that are best suited for the most economic situation. Using fiber that is of better quality than required will prove unprofitable.
Likewise, using fiber that is of poorer quality than required will result in losses as well. Most mills then find themselves in the precarious position of following a very narrow path of success where fiber quality is concerned. Correct decisions regarding the most suitable fiber properties for a given operation are paramount for maintaining profitability.
Specific fiber properties vary in their importance according to the spinning system employed. These differences in importance are due to the mechanical design of the particular spinning system and the physics involved.
Opening
After the appropriate fiber properties have been decided on, the selected bales are prepared and arranged for their placement into a laydown. A typical laydown may include as few as 40 bales or as many as 90 or more.
This configuration ensures proper dispersion of the many different variables from bale to bale and also places them in a position that the top feeder or bale plucker can use them. Usually, this is in a linear fashion but other methods of feeding may also be found.
The bale plucker moves back and forth along the laydown removing just a small amount of fiber from each bale on every pass. This ensures adequate blending by minimizing the amount of fiber removed from each bale and maximizes the number of bales fed per unit time.
From the plucker, the opened tufts are moved along using fans, airflow, and a series of ducts, much like those used in the heating and air conditioning systems in residential and commercial buildings. The fiber flows in this fashion and is directed to the next machine in this process.
To further ensure good blending, a multi-cell blender may be used. This machine randomly fills and then empties its chambers back into the process flow.
Cleaning
From the opening and blending processes, the open fiber tufts are transported to the first of what may be multiple cleaning machines or steps. The first cleaning machines are usually designed to be coarse cleaners. That is, they are designed to remove the larger and heavier trash particles. Fine cleaners will be found last in the cleaning line and just before carding.
Cleaning machines use a number of very basic principles in order to accomplish their goal. The trash or unwanted contents in the opened tufts of fiber are normally denser and heavier than the good fiber.
These principles take advantage of this. The machines’ designs use gravity, centrifugal force, and airflow combined with some type of beating action.
Usually, grid bars may be found under the beaters in most cleaning machines. The unwanted trash falls out between the bars and is sucked away to a central collection point. The grid bars can be adjusted to take out more or less trash depending on the fiber being used, the spinning process, and the end product requirements.
Batt Formation
After opening and cleaning, the fiber is transported to the carding line. Before the fiber gets to the carding machine, it must be prepared for that process by going through batt formation.
The batt former is more commonly known as a chute feed. The chute feed further opens the fiber tufts and forms a continuous matt-like (batt) structure. This operation ensures that the carding process will receive a consistent weight of material in order that the output of the card will be as consistent and level as possible.
Carding
Many have called carding the foundation process of yarn manufacturing. This is partly due to the fact that it is at the carding stage that the previously loose, unoriented cotton fiber first takes on a textile form sliver. The card is made up of a series of cylinders, which are wire-wound. This wire has tiny teeth cut into it and each tooth serves to transport a small number of fibers through the machine. There are also wire-covered surfaces called flats that come in close proximity to the main cylinder. This is the area where most of the carding action takes place. The wire-wound cylinders acting against one another, in conjunction with the flats, are the actions that accomplish carding’s main purpose to straighten, align, and parallel the fiber.
Preparation for Spinning
After the card sliver has been produced, there are several more steps to prepare it for the spinning process.
Drawing
Drawing’s main purpose is to further align and parallel the semi-oriented fiber from carding. In accomplishing this, blending (or doubling) is also achieved. From 6 to 8 card slivers are combined to be fed to the initial drawing process, sometimes called breaker drawing.
A series of pairs of rollers are used to reduce the multiple slivers being fed back down to the approximate weight of one sliver. These roller pairs consist of one rubber-covered top roller and one fluted steel bottom roller. Spring or air-generated pressure is used to hold the slivers firmly.
There are surface speed differences from the back pair of top and bottom rolls to the front set, with the back pair turning more slowly and gradually building in speed through to the front set. This mechanical speed difference, and the slipping action imparted to the gripped fibers, is called draft or drafting. This is where the drawing process gets its name. The machine draws out the slivers being fed and reduces their total weight back down to the approximate weight of one sliver. In doing this, the trailing ends of the gripped fibers, which may not be completely straight, are straightened out by the slipping and pulling action imparted to them.
As the drafted fiber web exits from the front set of drafting rolls, it is condensed back into sliver form using a trumpet-like device and the resulting sliver is coiled into a can. Once again, this is for temporary storage and for transport to the next process. Depending on what spinning system will be employed, the end product, and other process considerations, the output sliver from breaker drawing may be drawn once more (called finisher drawing) or it may be used directly for the spinning process. This is the point that the breaker-drawn sliver may also be directed to the combing process.
Combing
Combing removes short fibers from the yarn-making process. Depending on the cotton fiber being combed, and the settings of the comber itself, “short” can have more than one meaning, but generally, it means fibers that are half an inch or less in length.
The percentage of these fibers removed can vary greatly, again, depending on the cotton being used and the settings of the comber. The combing process starts by preparing what is known as a lap. This is simply a number of individual, once-drawn slivers that are wound in a ribbon-like fashion onto a spool. This wound lap becomes the input for the combing process.
For each combing cycle, a measured amount of the lap is fed out and held fast for the pass of the half-lap. The half-lap is a wire or needle-covered section mounted on a rotating shaft that passes its teeth or needles through the held beard—the measured lap that has been fed out and held fast. The fibers that are not held fast are removed by the half-lap and collected as waste. These short fibers may be recycled and used elsewhere in a less critical yarn. The combed webs are condensed back into sliver and coiled into a can for temporary storage and transport to subsequent processing.
The reasons for combing vary greatly depending on the end product but generally, it is done to increase the yarn count range of the material being processed. A finer yarn can be made from combed cotton which tends to result in an end product with extra sheen, drape, and hand. Combed yarns and the resulting products tend to be softer and yield more durability. From an engineering standpoint, combing may be required for some yarn counts and some end product performance standards. For other end products, it may just be a price point decision.
Roving
The roving process is a preparatory process used only for ring spinning. Finisher-drawn carded or combed slivers are fed into a simple roller drafting system where the linear weight is reduced to a size that is optimum for the yarn count to be spun. Since the linear mass is now drafted to a very lightweight, some twist insertion is also necessary to give the roving enough integrity to be pulled from the bobbin on the ring-spinning frame. The roving is wound in a precise manner onto a bobbin that varies in length from around one foot to over 15 inches. A full bobbin can weigh over 5 pounds.
