quality control in the knitting process.pptx

the quality test of the threads

1. Quality control in the knitting process 1

2. Knitted fabric is gaining popularity in the textile and clothing industry. The demand for knitted fabrics is increasing: consumers today are looking for comfort, fashion and style, which results in ever-changing demands on the apparel market. 2

3. The advantage of knitted fabrics is that they are able to meet consumer demand for such properties as a softer feel, good draping quality and wrinkle recovery. 3 AKFM

4. Knitted fabric is therefore an ideal material for manufacturing sportswear, intimate garments and casual wear as it allows for stretch and free body movement. 4

5. 5 Quality Terms

6. Weight per unit area . The mass per unit area of fabric is measured to determine the consistency of the fabric weight of the sampled knitted fabric. The weight deviations of circular knitted fabric should not exceed ± 5% from the stated weight. 6

7. Courses per centimeter/inch and wale spacing . Courses and wales per centimeter are measured by placing a centimeter glass on the fabric, and counting the number of courses and wales contained within the area. Handles. The feel of the knitted fabric to the hand, including the softness or stiffness of the fabric. 7

8. Elasticity. Each knitted fabric has its own unique elastic property, which is measured by bursting strength testing methods. Loop density. knitted fabric quality is usually defined as a number of loops in a square of prescribed dimensions and represented by the number of loops per square inch or loops per square centimetre . 8

9. Loop density is the most important element in defining knitted fabric quality and is directly related to fabric appearance, weight per unit area, dimensional stability, fabric weight, tightness factor, drape and many other factors. 9

10. Quality control in the knitting process Two main causes of inconsistency that lead to fabric defects are raw material management before the knitting process and variation in parameters during the knitting process . Therefore, the best way to improve the quality of knitted fabric is to monitor the knitting parameters and the knitting conditions. 10

11. Yarn appearance. The appearance of the yarn directly affects the appearance of the fabric after the knitting process. Several factors influence the appearance of the yarn, including cleanliness, fluffy texture and colour. A yarn can be labelled as good in appearance when it is free from impurities, contains a reasonable amount of projecting strands and has the minimum level of spinning defects such as short or long yarn slubs . 11

12. Yarn count. Yarn count or linear density is used to express the mass per unit length or length per unit mass of a yarn. It has a direct influence on the weight and dimensional stability of the knitted fabric. The selection of yarn with a proper yarn count is essential in determining the knitted fabric quality, since only the correct yarn count gives optimal knitting performance for a specific machine gauge and structure. 12

13. Yarn evenness. This refers to the yarn irregularity and non-conformity, which directly affects the knitted fabric quality and the knitting performance. Yarn evenness is expressed by standard deviations ( SD %) or commonly by the coefficients of variation ( CV %). 13

14. Yarn elasticity. An elastic yarn is easier to knit and results in fewer knitting faults such as drop stitches, holes and bad selvedges. In a general case, wool yarns perform better than cotton yarns in knitting due to the higher elastic property of wool. 14

15. Yarn twist. The direction of yarn twist plays a decisive role in knitted fabric quality. The yarn twist should be in the same direction, either S or Z in knitting the same fabric roll. In addition, the amount of twist has a significant influence on yarn torque. Excessive or improper yarn twist causes distortion of the finished knitted fabric, i.e. skewed fabric. 15

16. Yarn friction. The coefficient of yarn friction should be set as low as possible in the knitting process. The higher the yarn friction, the higher the knitting tension will be. When the knitting tension is greater than the yarn strength, the yarn will break or cause a fabric fault. In practice, the yarn friction can be reduced by adding lubricants. A good waxed yarn can reduce the coefficient of friction by nearly 50%. 16

17. Quality control during the knitting process In order to produce faultless knitted fabric, precision in the settings of the knitting machine is essential during the whole knitting process. The optimal setting of a knitting machine should be balanced in the following parameters: 17

18. Yarn tension before and after the yarn feeder. The yarn tension should be set at the minimum prior to the yarn feeder or with direct feeding (without yarn feeder). Fabric take-up tension should be set as low as possible . 18

19. • Drawing-in of yarn at the cylinder and the dial . In knitting, a larger distance between cylinder and dial gives a greater chance of obtaining a loosely knitted fabric. • Height of the dial. The tightest setting should be set between cylinder and dial to ensure the fabric can freely pass through without being torn. 19

20. Faults resulting from poor cleaning can be due to the following: lint and/or yarn fragments in the camming system, tricks, needles, or sinkers; variation in oil content for machine and needle track lubrication; worn needles , which generally produce length-direction streaks; worn cylinder and/or dial. 20

21. Parameters of knitting control Loop length Loop length is defined as the amount of yarn used to form one unit loop. The loop length is the absolute quantity of any knitted fabric and is directly related to loop density. 21

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23. Stitch density, tightness factor, weight and thickness are all inversely proportional to the loop length. As a result, it is vital to keep the variations of loop length to a minimum so that loop length is maintained uniformly and consistently throughout the knitting process. The only effective and reliable way to ensure the consistent loop length is by means of a positive yarn feeding system. 23

24. Positive yarn feeding Positive yarn feeding is a system often fitted on circular knitting machines to positively drive the yarn at a fixed rate relative to the surface speed of the needle cylinder . It is currently being considered as a standard quality control installation in all modern circular knitting machines. The main function of this system is to regulate the yarn knitting tension to a desired value , by enabling a predetermined length of yarn to be fed positively and consistently to all the needles for each revolution of the machine cylinder. 24

25. Knitting tension The loop length on knitted fabric is inversely proportional to the knitting tension. Therefore, a uniform loop length can only be produced on a knitting machine with regular knitting tension. By carefully controlling the knitting tension, the variations in loop length can be minimized and the quality of knitted fabric improved. Some parameters affecting knitting tension include yarn-unwinding tension, package diameter and package density: 25

26. 26 Yarn unwinding method . The yarn knitting tension changes during the yarn unwinding process as the yarn is pulled out from the top package layer by layer. This creates a higher unwinding tension at the bottom of the cone, while a lower tension occurs at the shoulder level.

27. Tightness factor . It is defined as the ratio of the area covered by the yarn in one loop to the area occupied by that loop. In simplified formula expression: tightness factor ( K) in SI units, where Tex is the unit of yarn count and L is the stitch length in millimeters. 27

28. Yarn input tension Yarn input tension (YIT) is used to tune the feeding of the yam into the knitting zone. The optimal YIT ranges from 2 to 4 grams. An excessive value in YIT results in yarn breaks and machine downtime, both of which are uneconomical. 28

29. YIT can be used as a means of process control, so that defects can be prevented or quickly detected. An exceedingly high yarn tension can arise from improper threading up, dirt and fluff in the yarn path, tilted cones, poorly wound cones or incorrectly set tensioners . 29

30. Yarn length per stitch: The length of yarn in one stitch is another important factor which permanently affects the quality of a knitted fabric. The yarn length per stitch determines the dimensions and stitch density of the fabric. It is therefore essential to keep the variations of the loop dimension to a minimum. 30

31. Take-down tension : It is a well-known fact that the take-down tension can materially affect the dimensions of knitted fabric and so the fabric quality will deteriorate. An excessively high take-down tension would induce undesirable stretching in the length direction of fabric, resulting in a narrower fabric with a lower value of courses per unit length. 31

32. Machine gauge thus plays an influential role in the choice of yarn count and can have an effect on fabric properties such as weight and appearance. Therefore, it is important to obtain an optimal balance of yarn count and machine gauge in order to ensure the best knitting performance for a specific machine gauge and structure, with high machine efficiency and minimum fabric fault rate. 32

33. STARFISH – Engineered knitted program for cotton circular knits. The name STARFISH is contracted from the phrase ‘START as you mean to FINISH ’. STARFISH is a computer program that resembles a simulator. It models the influence of the major variables in the production and processing of circular knitted cotton fabrics and calculates their effects on the final properties of the finished fabric. 33

34. Using STARFISH, the most appropriate combination of yarn count, stitch length and type of knitting machine necessary to deliver the desired combination of properties in the finished fabric can be calculated in an efficient and accurate way, without using production time or materials to excess. 34

35. STARFISH helps knitting manufacturers to rapidly develop new fabric qualities or optimize existing qualities in an effective way without recourse to expensive trial and error sampling. 35

36. In addition, it also helps to optimize the development process and make direct savings in development time and costs. It further helps to optimize the process management and production control procedures, in order to improve product quality and consistency. 36

37. Mayer and Cie MCTmatic Quality Monitoring System The MCTmatic system uses computer-controlled adjustment and a processor controlled braking system installed on the knitting machine. It is a monitoring system for setting and altering the yarn delivery and tensioning. 37

38. The MCTmatic system allows the motors to be set for feed wheel, central stitch adjustment and fabric take-down. The MCTmatic system is very useful in ensuring knitted fabric quality throughout the production process. When non-conformity is detected, the knitting machine will stop and the knitted faults will be indicated on the MCTmatic display panel. 38

39. Knitting faults can be categorized into horizontal and vertical components . The first category is mainly due to yarn inconsistencies and inappropriate raw material management . The second category usually results from inappropriate knitting conditions, especially incorrect machine settings and maintenance, poor monitoring of the machine performance and improper yarn delivery. 39

40. In order to eliminate or reduce knitting faults, manufacturers endeavour to set up a standard quality control method with the aid of fault detecting devices. Examples include a needle detector to find closed latches for rising needles, and a yarn breakage detector to show up broken yarn during production. 40

41. The online monitoring system is based on the principle of measuring the important knitting parameters in real-time process and comparing these data with predetermined set values for a particular product quality. 41

42. The monitoring system offers the following parameters during operations: yarn input tension (YIT), speed of knitting machine (m/s), yarn delivery speed (m/min), yarn consumption per course, fabric production in kg, tightness factor ( K) and loop length. 42

43. YIT can be measured by a set of sensors. The measurement system is composed of a force sensor, which is installed close to the feeding zone and encoders and an optical sensor. Next, the monitoring software is connected and primarily used for analysing the waveforms of the YIT, such as the MonitorKnit . 43

44. The resulting waveform is analysed by a signal processing technique, which produces a signal when a knitting fault is formed. Therefore, by inspecting and comparing waveforms resulting from normal and abnormal knitting, fabric faults and malfunctioning of the knitting machines can be quantified and identified in an accurate way, which constitutes a major step in reducing repair time. 44

45. The other online monitoring system is that of fabric image acquisition . Samples of different knitted defects are acquired by image-capture equipment . In image processing, the sensed image (e.g. by a video camera) is translated into a digital image (i.e. a two-dimensional array of numbers or grey levels) by an analogue-to digital converter. 45

46. The digital image can then be analysed by using image processing techniques to reflect the knitting faults and defects. The defects can be analysed and identified by the image processing algorithms and filters. 46