Tuesday, August 6, 2019

Methods To Prevent Barre In Knitted Fabric

Methods To Prevent Barre In Knitted Fabric In textile production, one of the most common and perplexing quality issues is barrà ©. The factors that can cause or contribute to barrà © are varied and diverse. For this reason, when a barrà © problem is detected, the skills of a sleuth may be required to expose the problem and eliminate its cause. Once a cause is identified, steps can be taken to minimize or eliminate the barrà ©, and better quality fabrics can be produced. This discussion will center on knitted fabrics. DISCUSSIONS The noun barrà © is defined by ASTM1 as an unintentional, repetitive visual pattern of continuous bars and stripes usually parallel to the filling of woven fabric or to the courses of circular knitted fabric. In a warp knit, barrà © normally runs in the length direction, following the direction of yarn flow. Barrà © can be caused by physical, optical, dye related differences in the yarn, geometric differences in the fabric structure, or by any combination of these differences. A barrà © streak can be one or several courses wide. A typical characteristic of barrà © is that it generally consists of stripes that repeat. Isolated or intermittent defects may or may not be barrà ©. 1 ASTM D123-96a Standard Terminology Relating to Textiles 1 Various aspects of barrà © are important for consideration and discussion. The first is the identification of barrà ©. Secondly, how can barrà © be analyzed? What are the causes of barrà ©, and finally, how can it be prevented? IDENTIFICATION OF BARRÉ The first step in a barrà © investigation is to observe and define the problem. Barrà © can be the result of physical causes that can usually be detected, or it can be caused by optical or dyeability differences that may be nearly impossible to isolate in the fabric. Barrà © analysis methods that help to discriminate between physical barrà © and barrà © caused by other reasons include Flat Table Examinations, Light Source Observation, and the Atlas Streak Analyzer. Flat Table Examination For a visual barrà © analysis, the first step is to lay a full-width fabric sample out on a table and view both sides from various angles. Generally, if the streaky lines run in the yarn direction that is in the course direction, apparent color differences can be seen by looking down at the fabric in a direct visual line with the yarn or course direction, and the defect can be positively identified as a barrà © defect. Viewing the fabric with a light source in the background will show if the barrà © is physical. Light Source Observation After completing an initial Flat Table Examination, a Light Source Examination may provide further useful information. Full width fabric samples should be examined 2 under two surface lighting conditions, ultraviolet (UV) and fluorescent light. Observations that should be made while viewing under lights are: 1. the frequency and width of the barrà ©, 2. whether the streaks are dark or light, and 3. the total length of pattern repeat. Ultraviolet light, commonly referred to as black light, allows the presence of mineral oils to be more easily detected, due to their radiant energy (glow). When observed under UV light, fabrics with streaks that exhibit glow suggest improper or insufficient preparation. A change in composition or content of oil/wax by the spinner or knitter without appropriate adjustments in scouring can create this problem. Fluorescent lighting simulates the mode of observation that is common to most inspection tables in mills and will highlight whether the barrà © is perceivable in an industry quality control setting. Atlas Streak Analyzer The function of the Atlas Streak Analyzer is to isolate barrà © caused by physical differences. A fabric swatch is combined with polystyrene sheet film, and the Atlas Streak Analyzer produces a plastic impression of a fabric surface by incorporating specific conditions of pressure and heat. The absence of color on the plastic impression ensures that only physical streak effects will be seen. The plastic impression is examined to determine whether the streak alignment matches the 3 streaks observed on the fabric. However, impressions made from spun yarns such as cotton can be difficult to read due to the inherent yarn variation characteristic of spun yarns. Also, a too rapid cooling of the test specimen after making an impression can produce a moirà © pattern. From a valid plastic impression, the barrà © source can be identified as: 1. physical with all streaks showing on the impression, 2. optical or dyeability variations where none of the color streaks are aligned on the impression, and 3. a combination of physical and dyeability differences where some streaks align with those on the impression, and some do not. Fabrics with combination causes present the greatest challenge for analysis. If the streak analyzer indicates the same barrà © pattern as seen in the fabric, then the barrà © stripes are physical in nature. This can relate to several physical causes such as yarn tensions, stitch length, yarn count, twist differences, etc. Yarn tension causes can be found by raveling adjacent courses and measuring the lengths of yarn removed from each course. If all the lengths raveled from the fabric are the same, the pieces can be weighed to determine if the yarn counts are the same. 4 If the plastic replica shows no stripes, then the barrà © is due to chemical causes or to light reflectance differences. Chemical causes relate to improper preparation, and light reflectance differences relate to non-uniform dyestuff penetration or reflectance. The next step is to remove the color and evaluate the stripped sample before re-dyeing to determine if the removal of dye was complete and if the barrà © is still present. An uneven or incomplete stripping can indicate an additional strip. If the color is stripped uniformly and the barrà © is gone, then the sample should be re-dyed. If after stripping and over dyeing the fabric no longer has barrà ©, then the barrà © was caused by improper preparation. If the barrà © remains, then the problem is related to optical or light reflectance problems. PHYSICAL BARRÉ ANALYSIS When the cause of barrà © is determined or presumed to be physical in nature, physical fabric analysis should be done. Physical barrà © causes are generally considered to be those which can be linked to yarn or machine differences. Methods of physical barrà © analysis include fabric dissection, microscopy, and the Roselon Knit Extension Tester. Fabric Dissection To perform accurate fabric dissection analysis, a fabric sample that contains several barrà © repetitions is required. First, the barrà © streak boundaries are marked by the placement of straight pins and/or felt markers. Individual yarns are removed from light and dark streak sections, and twist level, twist direction, and cut length 5 weight determinations are made and recorded. For reliable mean values to be established, data should be collected from at least two light/dark repeats. After compilation of yarn information, the numbers can be compared individually to adjacent yarns as well as by groupings of light and dark shades. Microscopy Microscopic examination is useful for verifying yarn-spinning systems. Yarns from different spinning systems can have different light reflectance and dye absorption properties resulting in barrà © when mixed. Ring-spinning produces yarn that is smooth with all fibers twisted in a tight helix. Open-end spinning produces yarn with wrapper fibers that form a belt around the diameter of the yarn at irregular intervals. Air jet spinning produces yarn with more wrapper fibers that form a continuous spiraling band around the inner fibers that are more parallel to the axis of the yarn. Microscopy can also reveal a shift in loop formation in knitted fabrics when twist direction (S and Z) differences are present. Roselon Knit Extension Tester2 Barrà © produced by knitting machinery is relatively common, is the easiest to see in the greige, and is the easiest to correct. Often uneven yarn tension during knitting may be a cause. To test for uneven tension, the Roselon Knit Extension Tester can be used. For this test, a fabric sample is cut and raveled to yield yarn samples from light and dark streak areas. The yarn ends are taped and clamped 2 Source: Spinlon Industries Incorporated, 18 S. Fifth Street, Quakertown, Pennsylvania 18951 6 to the tester. As each yarn is stretched to the maximum extension point, the points are plotted on graph paper. Comparisons are usually made visually rather than mathematically. CAUSES OF BARRÉ The varied and diverse causes of barrà © can generally be summed up in one word INCONSISTENCY. An inconsistency that leads to barrà © can originate in one or more of the following categories: ! fiber quality/raw material management, ! yarn formation/supply management, ! knitting processes, and ! preparation and dyeing techniques Fiber Quality/Raw Material Management 1. Failure to control fiber diameter (micronaire or denier) from laydown to laydown. 2. Too high a C.V. of micronaire in the laydown for a given mills opening line blending efficiency. 3. Failure to control the fiber color in the mix (grayness Rd, yellowness +b). 4. Failure to control maturity/fineness in a laydown 5. Most, if not all, fiber barrà © can be controlled by the above four items; however, under certain unusual circumstances, it may be beneficial to also select mixes using ultraviolet reflectance information for each bale of cotton. 7 Micronaire Average micronaire must be controlled within a laydown and from laydown to laydown. Controlling average micronaire in the laydown may not be sufficient to completely eliminate barrà ©. Other micronaire related causes of barrà © are: ! > 0.2 difference in micronaire. ! > 0.1 change in mix-to-mix average micronaire. ! > 12.0 % CV of micronaire within the laydown. It may be necessary to change the laydown averages periodically to make use of all the bales in the warehouse. This must be done slowly with no more than a 0.1 change in mix-to-mix averages. Figure 1 shows acceptable changes in micronaire from laydown to laydown. No barrà © from micronaire differences should show in knitted fabrics when yarns are mixed on the knitting machine from consecutive laydowns. However, if laydowns vary more than 0.1 micronaire, for example laydowns #1 (4.3 mic.) and #6 (4.6 mic.) in Figure 1, then barrà © is much more likely to occur. 8 Figure 1: Slowing Changing Micronaire Average in Laydown Acceptable Change in Micronaire From Laydown to Laydown3. NumberMicronarie Figure 2 shows a high CV% in the micronaire for bales within a laydown. The laydown shown has 24 bales with an average of 4.1 micronaire and a CV% of 17.7. The change in micronaire from bale to bale is more than a 0.1 change. Also, the CV% is more than 12.0% and would probably result in barrà ©. High Micronaire CV% in Laydown3. NumberMicronaireAvg. 4.1 CV% 17.7 * Figure 2: Results of Changing the Micronaire Average too Rapidly Max +/- 1.0 mic 9 Maturity and Fineness Although micronaire gives an indication of fiber maturity, it does not directly measure that property. Maturity and fineness related causes of barrà © include: ! Blending cotton from different growth areas or seed varieties. ! Immature fiber content such as white specks, neps, etc. Yarn Formation/Supply Even with proper fiber selection to reduce raw material effects on barrà ©, techniques in the opening, cleaning, blending, and carding areas can have an impact on barrà ©. These variations can occur at carding where different amounts of non-lint content removal from card to card can be a problem. Poor blending of fiber from opening through finisher drawing cannot be overlooked. In ring spinning, the running of different types of spindle tapes and the use of different cots or new and used cots mixed together on the same frame can create barrà ©. Faulty management in the spinning plant can result in the following yarn related causes of barrà ©: 1. Yarn count variations. 2. Yarn twist variation. 3. Wrong yarn size, color, blend level, twist direction. 4. Mixed yarn lots. 5. Mixed shipment dates of same dye lot. 10 6. Uneven waxing or oiling of yarn 7. Improper conditioning. Knitting Processes and Yarn Management The set-up and operation of a knitting machine is complex and requires precision settings to produce first quality goods. Machine related causes of barrà © include: 1. Different stitch settings (stitch lengths). 2. Improper tension at a feed. 3. Faulty cylinder or dial cam settings. 4. Malfunctioning of storage or tape feeders. 5. Improper threading of yarn. 6. Variations at take-down or spreader system. 7. Machine vibration. 8. Dirt, lint, and/or yarn fragments in the camming system, tricks, needles, or sinkers. 9. Variation in oil content. 10. Worn needles, which generally produce length direction streaks. 11. Uneven cylinder height needles (wavy barrà ©). 12. Worn cylinder and/or dial. Even with a properly set-up machine, barrà © can still occur at knitting due to poor yarn management. Examples of poor yarn management at knitting include: 11 1. Mixing yarns of different counts. 2. Mixing yarns from different spinning systems. 3. Mixing yarns with different blend levels. 4. Mixing yarns from different suppliers. 5. Mixing yarns with different twist level/twist direction. 6. Mixing yarns with different degrees of hairiness. 7. Mixing yarns with different amounts of wax. 8. Mercerization differences. 9. Excessive backwinding or abrasion during this process. 10. If yarns are conditioned, then each lot must be uniformly conditioned. Preparation and Dyeing Techniques Normally, dyehouses have standard preparation procedures and formulas that work very well in terms of removal of non-fibrous contaminants and natural fiber pigmentation. However, some fabrics can exhibit barrà © that is caused by either optical or dyeability issues. Often, the barrà © is not created in dyeing, but can be highlighted. In this scenario, the dyeing is not the cause but the messenger of the problem. Typically, barrà © is not a problem in whites, only in dyed fabrics. Therefore, if a fabric is known to have barrà ©, then that roll should be processed into a white. 12 Preparation can help mask barrà ©, but cannot and will not eliminate it. The cause of the barrà © must be eliminated for future rolls to be free of the defect. The scour and/or bleach procedure can be intensified to mask the barrà ©. This can be done by combinations of using a higher temperature or longer processing times. Usually the chemistry will remain the same; however, in some cases, it may have to be increased or changed. If any changes are made in the preparation procedure, the fabric and resultant shade will possibly change in whiteness, strength, and appearance. In fact, the whiteness will almost always change. If whiter, the old dye formula will need to be adjusted. The strength of the fabric may be affected to the point where it becomes a problem. Finally, the surface of the fabric may change so that it may not match previous lots. As a result of more aggressive preparation to cover barrà ©, it is important to evaluate such changes on small samples in the lab or a sample machine. Tension can play an integral role in not only the formation of the fabric, but also during preparation, dyeing, and finishing. As with all variables, it must be controlled to known parameters and be consistent. Any inconsistencies in tension could change the morphology of the cotton (i.e. during mercerization), level of dye pick- 13 up, or surface appearance (i.e. during mechanical finishing such as brushing or sanding). PREVENTION OF BARRÉ As discussed in the text, barrà © is caused by inconsistencies in materials, equipment, or processing. To prevent barrà © from occurring, consistency must be maintained through all phases of textile production. Stock yarns should be properly and carefully labeled to avoid mix-ups. Fugitive tints and/or marked cones can be useful for accurate yarn segregation. Inventory should be controlled on a first in/first out basis. All equipment should be properly maintained and periodically checked. In spinning and knitting before beginning full-scale production, sample dyeings can be done to check for barrà ©. Knit machine operators should be trained to look for barrà © as it occurs in the greige. If spotted, the machine should be stopped until the cause is eliminated. Salvaging a fabric lot with a barrà © problem may be possible through careful dye selection. Color differences can be masked by using shades with very low light reflectance (navy blue, black) or high light reflectance (light yellow, orange, or finished white). Dye suppliers should be able to offer assistance in this area. Also, if the cause of the barrà © is an uneven distribution of oil or wax, a more thorough preparation of the fabric before dyeing may result in more uniform dye coverage. 14 With close cooperation between production and quality control personnel, barrà © problems can be successfully analyzed and solved. Recommendations to minimize barrà © include: ! Knit an entire dye lot from the same knitting machine. ! Use only yarn from the same spinning lot. ! Use only from the same shipment date if possible. ! If yarn shipment dates must be mixed, then use consecutive shipment dates. ! Determine through laboratory analysis and experience if the preparation procedures are sufficient or can they be modified to eliminate the problem. ! Determine if some shades and dyestuffs are less susceptible to showing barrà ©, and apply those to problem fabrics. ! Make use of yarn/fabric analysis systems such as CYROS ®. ! Identify those rolls within a dye lot that have mixed yarn shipment dates so that it is known when the fabrics go through the dyehouse. ! Identify dye lots that have rolls from different yarn shipments before dyeing. CASE STUDY IN BARRÉ A 100% cotton, ring-spun single jersey style T-shirt is knitting in production with a yarn that does not have a barrà © problem. This style has been knitting for numerous days and no more of the yarn is available for creeling on the machine as the yarns are knitting out. Some yarn packages are skinners and others are full five-pound packages. The machine is 24-inch diameter with 80 feeds. 15 When the new yarn is creeled onto one of the 80 positions, the mixing of yarn shipment dates begins to take place. For this discussion, we will assume that this new yarn will cause barrà © when mixed with the old yarn. Once all positions are knitting the new yarn, there will be no barrà ©. How many rolls of fabric will have barrà © when both these yarns are present in the rolls? If the knitter is making 50 pound rolls, then each yarn package will supply 1/80th of the 50-pound roll or 0.625 pounds of yarn. Therefore, the new yarn will make 8.0 rolls of fabric with barrà © (5-pound package divided by 0.625 pounds per package for a 50 pound roll). It is important to realize that while this new package is knitting, other old yarn positions on the creel are also being replaced by new yarns. By the time the first new package has knitted out, all feeds will be using the new yarn. When the last old package is replaced and all positions have new yarn, the barrà © will disappear. Often, many machines will be knitting the same style. If ten machines are knitting this style and are using the old yarn and new yarns are placed on these 80 feed machines, then the total number of rolls knitted with barrà © will be 80. A total of 4,000 pounds of fabric will be made with barrà ©. If the style in question weighs 5.8 ounces per linear yarn, then a total of 11,035 yards of fabric will be made. Further, if it can be assumed that each yard of fabric can make 1.2 shirts, then a total of 13,240 defective T-shirts will be made. This is 1,103 dozen 16 garments. If the cost per shirt is $2.05 per unit, then a loss of $27,142 plus is realized. If open-end yarn was used for this case study instead of ring-spun yarns, then 8-pound packages of yarn could be used. This would result in 128 rolls containing 6,400 pounds and 17,655 yards of fabric. From this, 21,186 T-shirts (1766 dozens) would have been made. Based on a cost per shirt of $1.86, the loss would be $39,405. The result is that bigger packages mean bigger losses. The best methods to reduce the possibility of this catastrophe are listed below: 1. When mixing yarn shipment dates of only one week, try to reduce the number of machines using the old and new yarns. 2. As more machines are expending the old yarn, consolidate the old packages to fewer machines. This means removing the yarns from one creel and using them on other machines that are still knitting the old yarn. 3. Knit dye lots from a single machine if possible. 4. When a roll is known to have mixed yarns, a laboratory dyeing should be done on a swatch from the roll to determine if barrà © is present. If not, then proceed as normal. If barrà © is present, then the roll should be processed in 17 shades that are not known to be barrà © sensitive or prepared with more aggressive chemistry. CONCLUSIONS It is evident that barrà © is a problem that results from inconsistencies and is a result of poor management of fiber, yarn, and/or related knitting processes. The spinner, the knitter, and the dyer must communicate and work as a team to reduce the potential for barrà © to occur. A well planned and executed system of monitoring the spinning, knitting, dyeing, and finishing systems in the mill can provide for defect free fabrics. 18

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