Existing techniques for forming a double thread chain stitch type 401 using three types of looper with an eye at the tip to fill the bottom thread.
The existing technologies for forming the double thread chain stitch type 401 use three types of loopers, each with an eye at the tip for threading the bottom thread.
Double Chain Stitch Formation Principles.
By analyzing all types of chain stitches, we can understand the fundamental principles behind the technology used to create them. The core principle of all chain stitch formation is the «loop in loop» mechanism, meaning each subsequent loop must pass through the previous one. In the double thread chain stitch type 401, the bottom loop first passes through the top loop. Then the top thread loop passes through the bottom thread loop, and the top thread loop is pulled to the underside of the material to ensure the interlocking of threads on the back side of the material.
Due to this chain stitch formation principle, all chainstitch seams unravel from the end of the chainstitch seam and from the skipped stitch towards the beginning of the seam. Therefore, the primary requirement for all chain stitch seam types is the absence of any skipped stitches and the secure fastening of the end of the chainstitch seam to prevent it from unraveling if the chainstitch seam end is left open, i.e., not closed by another seam.
Types of Loopers for Double Chain Stitch 401.
The existing technologies for forming the double thread chain stitch type 401 use three different types of loopers, each equipped with an eye at the tip for threading the bottom thread:
1. A looper capable of performing complex movements.
2. A looper that only performs oscillating movements in the vertical plane in conjunction with a spreader.
3. A looper that performs oscillating movements in the horizontal plane.
Looper Movements and Functionality.
The looper that performs complex movements executes two different actions: oscillating movement perpendicular to the direction of material feed and oscillating movement parallel to the material feed direction. In contrast, the looper that only oscillates in the vertical plane moves parallel to the material feed direction, while the spreader moves perpendicular to the material feed direction. Their respective paths cross above the looper.
The looper that oscillates in the horizontal plane has a semicircular shape, with the axes of the needle and looper aligned along a line perpendicular to the material feed direction.
Three Principles of Chain Stitch Formation.
The three technologies for forming the double thread chain stitch type 401 use three different types of loopers, which also differ in the position of the eye for threading the bottom thread. Despite these differences, all three types of loopers follow the same fundamental principle for forming the double thread chain stitch type 401, consisting of three principles:
1. Pulling the bottom thread loop through the top thread loop.
2. The needle passing the top thread loop through the bottom thread loop.
3. Tightening the top thread loop on the underside of the material.
First Principle.
To implement the first principle, which involves pulling the bottom thread loop through the top thread loop, loopers threaded with the bottom thread through the eye must enter the top thread loop formed by the needle when it rises from its lowest position. As the needle continues to move upwards, it exits the material, leaving the top thread loop on the looper. The material is then fed the stitch length before the needle pierces the material again.
Second Principle.
To implement the second principle, where the top thread loop passes through the bottom thread loop, the needle threaded with the top thread makes a second puncture in the material while the top thread loop remains on the looper. The needle continues descending, pulling the top thread through the material.
Looper Types and Thread Triangle Formation.
To pass the top thread loop through the bottom thread loop, the tip of the needle must pass through the thread triangle formed by the looper body, the bottom thread exiting the looper eye, and the top thread loop on the looper as the needle approaches the looper body. The ends of the bottom thread and the top thread loop converge at the point of the previous needle puncture.
The size of the thread triangle depends on the stitch length. As the stitch length decreases, the size of the thread triangle also decreases, increasing the likelihood that the needle may not pass through the thread triangle. This can result in a skipped stitch, which is unacceptable for all types of chainstitch seams, as chainstitch seams unravel from the skipped stitch towards the beginning of the seam.
The looper capable of complex movements forms the thread triangle by oscillating in a direction parallel to the material feed. The looper that only oscillates in the vertical plane creates the thread triangle using a spreader that captures the bottom thread exiting the looper eye and extends it horizontally, allowing the needle to penetrate the thread triangle. The looper that oscillates in the horizontal plane forms the thread triangle due to its semicircular shape.
Once the needle passes through the thread triangle, the looper continues moving in the opposite direction and releases the top thread loop from its body. As a result, the needle, threaded with the top thread, passes through the bottom thread loop, thereby implementing the second principle.
Third Principle.
To implement the third principle, the top thread loop from the previous stitch must be tightened to the underside of the material being sewn. This is a complex task for the three existing technologies, as the quality of performing this third principle directly determines the quality of the seam produced for various types of materials.
Two-Stage Top Thread Loop Tightening Process.
To address this challenge, the process of tightening the top thread loop occurs in two stages. In the first stage, the top thread loop is pre- tightening, allowing the bottom thread loop to freely tighten through it.
This stage is performed by the needle, which continues moving downward, consuming the top thread until it reaches its lowest position.
For the needle to begin pre- tightening the top thread loop after it is released from the looper body, the thread take-up lever feeds the top thread into the needle until the top thread loop is released from the looper body.
As the needle descends and reduces the size of the top thread loop, friction occurs between the material and the top thread, as the top thread moves inside the sewn material. To minimize this friction, a special needle with two long grooves is used, allowing the top thread to move within the needle groove.
However, friction also occurs at the previous needle puncture point, as the top thread moves within the material, and it is impossible to reduce friction between the top thread and the sewn material at this point, which is a major problem for all three stitch formation technologies.
In the second stage, the final tightening of both the top thread loop and the bottom thread loop is carried out by the looper during material feed for the stitch length. The looper, with the top thread loop on its body during material feed, moves in the opposite direction relative to the material feed.
Limitations of Double Chain Stitch 401.
This two-stage process of tightening the top thread loop from the previous stitch does not allow for strong tightening of the top thread loop, preventing the stitch from tightly securing the joined materials.
Therefore, at present, a seam made with the double thread chain stitch type 401 cannot provide a tight join of materials. For this reason, the lockstitch type 301 is primarily used for this purpose.
Additionally, due to the insufficient tightening of the top thread loop in the stitch, the underside of the chainstitch seam becomes relatively thick, reducing the seam’s wear resistance.
Furthermore, this two-stage process of tightening the top thread loop from the previous stitch makes it difficult to sew material combinations like textile-leather or textile-plastic due to the significant increase in friction between the material and the top thread at the previous needle puncture point, which cannot be reduced. In such cases, the needle may draw the top thread from a large spool through the tensioning device, where the top thread tension is low, leaving the top thread loop in the previous stitch untightened. This untightened top thread loop cannot be fully tightened in the second stage during material feed by the looper.
As a result, the lockstitch type 301 is currently the primary stitch used for sewing such material combinations.