Fiber neps have always presented a challenge to mills and manufacturers. In this article, expert Dr. Yehia Elmogahzy answers frequently asked questions about the effect of neps on the manufacturing process. What are the causes? Can they be controlled? What is the cost of neps to my business? Discover these answers and more.
Disclaimer: This article responds to questions addressed by a number of spinners in Vietnam and Bangladesh regarding cotton fiber neps and seedcoat fragments. The author of this article is solely responsible for the content and the recommendations made. The author welcomes any additional questions on the subject and he will be happy to answer them. Please direct your questions to the author at firstname.lastname@example.org.
Question 1: What are neps, seedcoat neps, and white specks? And what causes them?
Neps, seedcoat neps, and white specks are all entangled and knotted fibers that are formed during cotton harvesting or in the ginning process. Microscopic observations of neps indicate that the number of fibers forming a single nep may range from 5 to 20 fibers knotted together. The following types of neps may be observed in a bale of cotton:
- Pure fiber neps (or mechanical neps): these are neps consisting only of fibers entangled together. This type of nep is very common, and it represents the majority of neps in U.S. cotton. They are mechanically created during harvesting and in the lint-cleaning process after ginning.
- Biological neps: these are serious neps and they may consist of a cluster of immature fibers or fibers entangled with non-fiber materials such as seed coat fragments, leaves, or stem material. These are also caused by mechanical means during seed-cotton cleaning before ginning or due to seed damage during ginning.
Pure fiber neps (or mechanical neps) typically consist of long and fine fibers. Normally, long and fine fibers are more flexible, and they have more tendency to curl and entangle together than short and coarse fibers. Light dense pure fiber neps may be straightened during carding particularly at low carding speeds. Biological neps are more serious than pure fiber neps because they are more difficult to disentangle, and they tend to be progressively fragmented into smaller and smaller neps during spinning preparation.
Question 2: Are there cotton bales free of neps?
All cotton bales, manually harvested, mechanically harvested, saw-ginned, or roller-ginned will have fiber neps. This is because the equipment required to gin, and clean cotton will inevitably result in some fiber entanglement causing neps to occur. The amount of neps per bale will vary depending on the methods of fiber production. Manually-harvested cotton will have fewer neps than mechanically-harvested cotton, stripper-harvested cotton will have slightly higher neps than spindle-harvested cotton due to the need for more cleaning and saw-gin cotton will have higher neps than roller-gin cotton. Thus, the existence of neps in a cotton bale is inevitable. In a best-case scenario, a cotton bale will have about 100 to 200 pure fiber neps per gram. This represents less than 8% of the cotton worldwide. A manageable average nep level in a bale is from 200 to 350 neps/g. A high nep level is from 350 to 500 neps/g. With regard to seed-coat neps, up to 10 neps per gram is considered a best-case scenario, 11-20 is low, 21-30 is medium, 31-45 is high, and > 46 is very high.
Question 3: How can we control the effects of neps during spinning?
There are four stages of processing during spinning in which nep content can be controlled: (a) Cotton mixing, (b) Opening and cleaning, (c) Carding, and (d) Combing.
Neps In the Cotton Mix
The key step to control neps in the cotton mix is to purchase cotton bales that do not exceed 400 neps per gram, or 35 seedcoat neps per gram. In case some of these bales are available in the inventory, they can either be processed separately using slower rates of processing, or they can be dispersed in small amounts over many cotton mixes. It is also important to purchase cotton of high maturity ratio since immature cotton fibers can result in biological neps that are difficult, if not impossible, to remove during processing. Normally, fibers of micronaire value above 3.8 are very mature. More than 80% of U.S. cotton fall in this range.
In spinning mills with different spinning systems and different preparation methods, bales of high neps should be used for medium-quality carded ring-spun yarns or open-end spun yarns. When fine-combed yarns are produced, lower neps in the cotton bales will result in lower waste and lower comber noil. For air-jet spinning, neps represent a more serious matter and careful purchasing of cotton bales is critical. Neps in Opening and Cleaning
The opening and cleaning process will always add neps to cotton fibers. This can be as low as 40% and as high as 90%. But what spinners need to know is that the higher the neps in the cotton mix, the lower the percent added neps during opening and cleaning. This is a long subject that I will be glad to discuss with individual spinning mills. The point here is that the opening and cleaning process is normally kinder with cotton mixes of high neps than with cotton mixes of low neps. It is advisable that spinners periodically measure neps in the chute feed material to determine the severity of adding neps during opening and cleaning.
Neps in the Carding Process
The carding process will always reduce neps. This is because neps may either be removed during carding or straightened in the carding zone. Nep removal efficiency in carding is determined using the following formula [100 × (neps in the chute-feed – Neps in the carded sliver)/neps in the chute feed]. The higher the nep-removal efficiency the lower the neps in the carded sliver. What spinners should also know is that the higher the neps in the chute feed material, the lower the nep removal efficiency. It is advisable that spinners periodically measure neps in the chute feed material and in the card slivers to determine nep removal efficiency. It is also important that these measures are made for all cards. This is because different cards in the same line may have different nep removal efficiency because of the different wiring conditions of card machines. When high variation of neps between cards is observed, better card maintenance and wiring schedule will be required. It should also be noted that the higher the carding speed the lower the nep removal efficiency. This means that carding machine rates should be set in view of nep removal.
Neps in the Combing Process The combing process will always remove neps. Nep removal efficiency in combing is determined using the following formula [100 × (neps in the card sliver – Neps in the combed sliver)/neps in the carded sliver]. The combing process could have nep removal efficiency exceeding 80% depending on the comber noil % used. This means that lower the neps in the card sliver could mean lower comber noil, which can be a major saving to spinners.
Question 4: We receive frequent complaints of white specks and undyed spots in fabrics made from our yarns. What are the causes of these defects?
About 80% of these defects are due to biological neps or white specks that were not dyed in the dyeing and finishing process. However, these defects may also be due to the presence of fiber contaminants during the knitting process as a result of fly generation during knitting. It is important for spinners to diagnose the fabric defect very carefully before making a conclusion about the cause. We perform these diagnostic tests in my company on routine basis, and the problem can either be caused in the spinning process or during knitting.
Question 5: What is the cost of neps and seedcoat neps?
The effect of neps on yarn cost is twofold: (1) neps must be largely removed before yarns can be produced, (2) residual neps can be detrimental to the appearance and the value of yarn. The removal of neps represents a significant added-cost of yarn manufacturing since they may require more frequent wiring of carding machines, or higher comber noils during combing. In a mill using 100,000 bales annually for combed ring-spun yarn, a 1% to 2% less comber noil could mean a saving of about $1 million per year. The residual neps can be as low as 30 neps per gram in carded yarns and as low as 8 or 10 neps per gram in combed yarns. These values can be translated to up to 3,000 neps/1,000 meter in carded yarns, and up to 400 neps per 1,000 meters of combed yarns (with finer yarns having more neps/1,000 m). Neps of large sizes (> 2,000 µm) that manage to survive into the yarn can result in poor yarn appearance and they can appear in the finished fabrics in the form of undyed dark dots (white specks) particularly if they constitute dead or immature fibers.