QC in Knitted Production Part 2: Process Control

1. QC in Knitted ProductionPart 2: Process Control Jimmy Lam Institute of Textiles & Clothing

2. Learning Objectives • What are the major elements for processes control? • Fabric properties and loop length • Relationship between loop length and yarn tension • On machine control • Yarn speed meter, yarn tension meter and yarn length meter • Positive Feed device • Trip-tape positive device, multi-coil positive feed, • Positive feed with intermittent yarn consumption • Storage Feed device

3. Remember this model What are the major control elements for process control PROCESS INPUT OUTPUT CONTROL

4. Closed-loop Control ModelQuality Control Process KNITTING YARN FABRIC INSPECTION

5. Knitting Control • This is a process control starting from yarn feeding to fabric manufacturing. Please name the factors that will affecting the knitting fabric quality during production stage

6. Loop Length Control (I) • The loop length is the basic element (cell) in the knitted fabric and is defined as the amount of yarn used to form one unit loop • Loop length = course length / no. of loops • Loop length will affect the following parameters • stitch density; tightness factors; fabric weight; • Fabric width; panel length; dimensional stability

7. Loop Length and Fabric Properties

8. Loop Length Control (II) • Experiment shows that fabric thickness, weight, tightness factor and stitch density are INVERSELY PROPORTIONAL to the loop length • Setting of stitch cam (loop length control) and variation: • Loop length is controlled by the setting of stitch cam in the cylinder machine. However,how accurate is the indicator on the stitch cam is the common question most knitters will ask • Conclusion • The variation on poor machine setting, yarn input tension and knitting tension, all account for loop length variation

9. Yarn Knitting Tension Control • Please show the relationship between the loop length and knitting tension • Please indicate what factors will affect the knitting tension

10. Knitting Tension & Package size

11. Knitting Tension and Inertia

12. Knitting direction and yarn tension

13. Knitting Tension (I) • Yarn unwinding method • The yarn tension is different from the first layer to the bottom layer • Package size • The size of the package (cone diameter and cone height) will affect the winding tension • Package density • Unusual high package density will give higher friction to yarn between coils, results in higher unwinding tension

14. Knitting Tension (II) • Yarn fault • Yarn faults mean sudden increase yarn diameter with results in change of yarn tension in knitting process • Spring wire on tensioner • This will affect on V-bed machine when the cam cartiage changing direction from RHS to LHS and vesa versa. • Diagrams • Relationship between package and unwinding tension • Change of knitting tension on V-bed machine • Analysis on the peak tension

15. On machine Quality Control Equipment • Knowing the factors affecting the fabric quality (loop length variation), what possible equipment can be used to minimize it or to eliminate it ? • If not, what equipment can be used to monitor any loop length variation.

16. Yarn Speed Meter • Yarn speed meter is used to check the yarn consumption speed on the knitting feeders • The results is good for circular machine without positive feeders. • Yarn speed is affected by loop length, structure and machine speed

17. Yarn Tension Meter • Tension meter can be used to check the yarn winding tension, unwinding tension and knitting tension. • The usual tension (medium to fine gauge) on circular machine is 2-5 g/m • High knitting tension = smaller loop length • Fluctuation in tension = variation on loop length • Tension lower than 2 g = drop stitches • Low and even tension = good quality fabric • What should we do if we find unusual high/low knitting tension ?

18. Yarn Tension Meter

19. Yarn Length Meter • The yarn length meter can be fixed on machine to record yarn consumption for a number of courses or machine revolutions. • The results can be used to calculate the loop length between machine of the same gauge.

20. Dial Height Gauge Control • The gap between the cylinder and dial will affect the fabric quality on : • fabric thickness • fabric weight • dimensional stability

21. Now we found the way to measure the loop length variation. But can we actually to control or eliminate such variation ? Can you suggest any methods ?

22. Positive Feeders • The function of positive feeders for circular knitting machine is to regulate the yarn knitting tension to desired value (low and even value) and results in better fabric can be produced • The positive feeders are almost standard equipment for high speed non-jacquard machine • (Why only non-jacquard m/c ?)

23. Yarn Tension with/without Positive Feeders

24. Trip-tape Positive Feeder

25. Trip-tape positive feeder • It is the simple measured yarn speed feeding system. The yarn speed is controlled by speed tape pulley which in turn, controlled by machine gearbox. • Two or three speed tapes are equipped which machine can have two or three yarn feeding speeds for the fabric. • When machine is properly adjusted, the yarn speed is equaled to tape speed, and the tape speed can control the suitable yarn tension and results in regular loop length

26. Multi-coil Positive Feeder

27. Multi-coil positive feeder • Due to the slippage between the tape and the yarn, the efficiency of trip-tape positive feeder method is not 100% • yarn speed IS NOT EQUAL TO tape speed • The multi-coil method is used to achieve this target. • The yarn instead of directly driven by the tape, is first to the pin wheel for 10 coils before feeding to the needles. • The principle is the yarn coils on the wheel can neutralize the yarn tension variation from cone and avoid and yarn slippage.

28. Positive feed for intermittent yarn consumption • So far, the positive feed discussed is only used for continuous yarn consumption, it cannot be used on the following machines: • Cotton’s fully fashion machine; • V-bed machine • Circular hoisery machine; • Circular machine with stripers • Please discuss WHY?

29. Positive Feeder for stripping fabric

30. Positive Feed for intermediate yarn consumption • A special positive feed has designed for circular machine with stripers. • The working principle is to utilize the coil friction between the yarn and the friction roller. • When the yarn is not selected, the tension wire will bring the yarn away the roller and thus, it cannot contact with the tape and remain in idle position • When the yarn is selected, the yarn tension will increase and pull the torsion wire towards the yarn guide, and this friction in the coil will move the yarn to roller speed. • However, the slippage for this type of feeder is about 5-8%

31. Storage Feeder • Positive feeder device is an important device to control the yarn tension and thus to minimize the loop variation in knitting. • However, positive feeder can only be used on non-jacquard fabric such as interlock, pique but not on jacquard fabric • Please discuss WHY

32. Storage Feeder

33. Storage Feeder • Positive feeder is designed for regular loop fabric. For fabric with different loop length in each course, a new concept of storage feed has employed • To eliminate the yarn tension variation, all yarns are rewound onto a drum of equal diameter, under equal tension and speed. • The amount of yarn on drum is automatically controlled so there will be no difference in package diameter. • The yarn is consuming by the knitting needles at variable speeds, thus different loop length in each course is possible

34. Discussion • What are the major objectives for process control ? • Why loop length variation cannot be avoided during knitting process • What are the difference between positive feeder and storage feeder

35. Process Control The process control is to maintain a regular loop length for the whole production process.

36. Multi-coil Positive Feed

37. Positive Feed for striping fabric

38. Storage Feed for Jacquard Fabric

39. Where is the positive feederin this diagram ?