In contrast to piecing after ends down or quality stops, no thread is available from a package in the package holder for piecing after package changes. This means that an „extraneous thread“ has to be used for piecing at the spinning position in order to re-start the spinning process.
When piecing on an empty tube, the thread from a package carried by the robot is used for this purpose. After the full package has been replaced with an empty tube and the rotor cleaned, the thread from the supply package is fed back into the rotor, fiber feed to the rotor is started and the piecing is formed (Fig. 56 – step A). A particular advantage of this system is that the piecing thread and the piecing are extracted by the rotor and fed to the filter (step B). The new original yarn, which also briefly passes into the robot‘s extraction device, is transferred by the robot to the empty tube (step C) through the formation of a fixed thread reserve (the first layer of yarn is covered by the following layers, the end of the thread remains free).
The great advantage of piecing onto empty tubes is that:
- A „piecing-friendly“ yarn can be selected for piecing, e.g. especially strong, not too fine, etc., since the piecing thread (complete with the piecing) is cut off and extracted, and does not reach the package.
- Piecing mass and tenacity can be adjusted so that high piecing tenacity and thus a high piecing success rate are achieved. The length and mass of the piecing are of no concern, since the piecing is extracted. The piecing success rate after package change is in many cases 100%.
- It ensures that only original yarn is wound onto the newly started package and thus no problems can arise in downstream processing with starter yarn and starter piecings.
Piecing with a starter packages: the alternative method for re-starting the spinning process after package change is to place a pre-wound starter package (with 20 - 50 meters of original yarn) in the tube holder and piece with the end of the yarn on this package. In this case piecing after package change is identical to piecing after an end down or quality stop (see above). The starter packages required for this piecing concept are produced on a starter winding station installed at the drive end and transferred via a transfer station to the automatic doffer, which docks onto this transfer station. The number of starter packages carried determines the number of package changes possible in one pass. When all the starter packages have been used, the doffer returns to the transfer station to collect new starter packages. When spinning conditions necessitate very frequent package changes (small packages, e.g. dyeing packages, short running times with coarse yarns) an additional starter shuttle can also be used to supply the automatic doffer with starter packages „on the spot“ while it is in operation.
However, the use of starter packages is not unchallenged, for various reasons:
- Original yarn must be used for the starter packages. For this purpose several packages must either be produced before the machine starts up (which takes time) or reserve packages from earlier final spinning operations must be used.
- The technical effort expended in producing starter packages is relatively substantial (service, maintenance). A starter winding station, a starter transfer station and in some cases a starter shuttle are required.
- The yarn on the starter package is wound in the opposite direction to the spun yarn. In the case of sensitive end products/dyeing methods the differences in direction can be visible.
- A serious drawback is that by virtue of the system every package contains an additional piecing compared to packages produced by piecing onto an empty tube. In the coarse count yarn range, e.g. with denim yarns, the number of starter piecings can exceed the number of spinning- related piecings (after ends down or quality stops).
- Even if technically ingenious piecing systems can produce high-quality piecings, the emphasis must be placed on keeping the number of piecings per package as small as possible. A „poorly“ produced piecing, whatever the reason, is a potential weak point in the yarn.