Sunday, April 25, 2021

Rapier weft insertion system l Types of rapier looms l Weft yarn passage and working principle of rapier weft insertion system


Rapier loom weft insertion system:

This is the most versatile. It has a very wide weft count range. You can insert from very coarse to very fine yarn without making a big change. The journey of weft yarn gets constantly controlled throughout the picking motion. The fabric width can be adjusted within the machine's specification limit. The rapier looms are widely used in the textile industry. The number of weft colours ranges between 8 to 16. 

Classification of rapier looms:

A rapier loom can be categorized into the below classes according to the weft insertion mechanism used.

Single rapier loom:

That rapier loom in which only one rapier gets used to insert the the weft yarn in the shed is called the single rapier loom. The traveled distance of the rapier gets equal to the reed space used in the loom. This kind of loom works at a very low speed. The warp yarn exerts high pressure on the rapier. 

Double rapier loom:

That rapier loom in which the weft insertion in the shed is carried out with the help of two rapiers, This type of loom is known as the double rapier loom. Each rapier travels a half distance of the reed space used in the loom. Double rapier loom runs at very high speed. The pressure of the warp yarn on the rapiers gets reduced up to a maximum extent.

Flexible rapier looms:

In this kind of loom, the grippers are mounted on the flexible ribbon or tape made of Teflon and carbon fabric. Since the length of the ribbon is almost equal to the reed space used in the double rapier loom and almost half of each ribbon length enters into the shed during weft insertion so that the remaining half-length of the ribbon goes back inside the hollow rapier guide when the rapier reaches to the outer dead center position. This remaining length is that length that doesn't enter into the shed during weft insertion. This task gets possible due to the flexibility of the rapier ribbon. Due to the use of flexible rapier ribbons, this kind of loom is called a flexible rapier loom. This loom can run at higher rpm than rigid rapier looms. This kind of looms occupies less space too.

Rigid rapier looms:

If the grippers are mounted on the rigid rapier racks in any rapier loom, it is known as a rigid rapier loom. The rigid rapier runs at a slower speed than the flexible rapier loom. The rigid rapier loom occupies more space than the flexible rapier loom. 

Negative weft transfer rapier loom:

If the weft yarn gets entered between the gripping jaws of rapier grippers with the help of weft tension and an extra mechanism is not used to open and close the gripping jaws of each gripper during the weft transfer cycle, this kind of weft transfer is called negative weft transfer. If the loom has negative weft transfer during weft insertion, that loom is called a negative weft transfer rapier loom.

Positive weft transfer rapier loom:

If the gripping jaws of each rapier get opened and closed with the help of an extra mechanism throughout the complete cycle of weft transfer, this kind of weft insertion is known as positive weft transfer. The loom equipped with positive weft transfer is called a positive weft transfer rapier loom. 

Passage of weft yarn in the rapier loom:

The weft package is mounted on the weft creel. The number of weft packages is used according to the number of colours in weft yarn getting used. The yarn passes first from the thread guide and weft tensioner mounted on the weft creel. Next, the weft yarn passes through the weft accumulator. This weft accumulator ensures continuous yarn supply at a constant tension. When the weft package reaches to near the exhaustion position, the yarn tension gets increased to maximum. The weft accumulator neutralizes this effect and keeps the weft tension constant throughout the package. The weft tensioner mounted on the weft creel prevents the overlapping of adjacent yarn coils on the accumulator. The number of weft accumulators used gets equal to the number of colours getting used in the weft yarn. 

Now the yarn passes through the filling detector. The filling detector consists of either a single weft sensing channel or multi weft sensing channels. The filling detector monitors the complete journey of the weft from one side to another side. If weft insertion gets either failed or felt short in length, it stops the loom immediately. The yarn next passes through the thread guide which helps to keep the weft yarns separate. It also prevents entangling the different weft yarns together. The thread guide consists of many ceramic eyes in it. 
Finally, weft yarn passes through the eye of the weft selector needle. The weft selector needle gets selected according to the weft sequence feed in the control computer.   

Working principle of rapier weft insertion system:

The weft yarn gets held in the catch selvedge. Ten to twenty ends are used to make this catch selvedge. The ends are wrapped either on a separate reel or a catch selvedge creel is used in which required small yarn packages are mounted. A plain weave is always used in the catch selvedge. A catch plate mounted on the temple bracket very close to the end of the reed prevents the weft yarn from going toward the reed.

When the reed beats the last inserted pick, a fresh weft insertion cycle begins. First of all, the weft selector needle falls, and the weft yarn comes in front of the insert gripper. The weft yarn falls on the rapier guide and touches it.

The rapier starts traveling toward the sley center and the receiving rapier also starts to travel toward the sley center. The receiving rapier starts to move first. When the gripper passes under the weft yarn, the yarn gets entered into the slot of the insert gripper. The gripping pressure of the gripper is adjusted in such a way that it allows entering the weft yarn between the upper and lower gripping jaws of the insert gripper. An approproate weft yarn tension is also achieved by adjusting the yarn tensioner. The weft yarn should be at least reached the middle of the gripping jaws. There should be no yarn slippage between the gripping jaws of the insert rapier. When the insert gripper begins to enter the shed, the filling cutter cuts the weft yarn. A weft yarn support guide is mounted at the one end of the rapier guide near the entry point of the insert rapier. This yarn guide makes a required clearance between the reed and weft yarn. it also prevents reed damage.

Both the rapier travels toward the sley center. The receiving rapier reaches first at the sley center and rests there indwell period. The receiving rapier gets ready to receive the weft yarn from the insert gripper and it waits there. Now the insert rapier reaches the sley center and the yarn gripped in the insert gripper cross the tip of the hook receiving gripper. The three mm clearance between the tip of the hook of receiving gripper and weft yarn is ensured during setting. As the insert gripper comes in the dwell period, the receiving gripper starts to travel toward the fabric selvedge. The hook of receiving gripper pulls the weft yarn from the insert gripper. Since the receiving gripper has little more gripping pressure than the insert gripper so that yarn gets entered between the gripping jaws of receiving gripper. The insert gripper also starts to travel toward the fabric selvedge after passing its dwell period. The receiving gripper comes out of the shed first. A gripper opener fitted at the rapier guide opens the gripping jaws and releases the weft yarn outside of the catch selvedge. A suction nozzle sucks the tail of the weft yarn inside it and helps to keep it tight. The catch selvedge ends cross to each other at the same time and holds the weft yarn firmly. The insert rapier also comes out of the shed during this period. Now the weft yarn is beaten by reed and catch plate hold again the weft yarn. This cycle is repeated continuously. 

Wednesday, April 21, 2021

Definition of a stitch in knitting process and different type of stitches in weft knitting process

Definition of stitch in knitted fabric:

The smallest dimensionally stable unit of any knitted fabric is known as a stitch. It consists of a yarn loop that is held together by being intermeshed with another stitch or loop. The stitch is composed of a head, two legs, and two feet.

Different types of stitch formation in the weft  knitting process:

The types of various stitches getting in the weft knitting process are given below:

Knit stitch or plain stitch:

When the needle gets raised enough high by the camming action to obtain the yarn in the hook portion of the needle and the old loop is below the latch. As the needle descends, the knit stitch is formed. The structure of plain or knit stitch is given below:

Purl stitch:

When the legs of the stitch are below and the feet are above the newly created stitch, this type of stitch is called the purl stitch.

Tuck stitch:

When the needle is raised by camming action to obtain the yarn in the hook, it is not raised high enough to clear the previously formed loop below the latch. The needdle obtains two loops in the hook. In this situation, the tuck stitch forms when it knits at the next course. 

The single needle may tuck at successive before being cleared resulting in the vertical tuck which extends along a wale for a number of courses. Four courses are a maximum limit otherwise loops get tightened and yarn breakage occurs. It is also possible to form a horizontal tuck stitch by tucking an adjoining needle. 

Effects of tuck stitches:

The effects of tuck stitch are given below:

1- The fabric having a tuck stitch is thicker than a knit stitch because of the yarn accumulation in stitches at tucking places. 

2 - The structure with the tuck stitch is wider than knit stitches. 

3 - The loop shape has a wider base at stitches.

4 - The tuck stitch structure gets less extensible because at every tuck stitch the loop length is shortened.

5 - The tucks stitched fabric is heavier in GSM than knit stitch fabric d
ue to the thick nature of the tuck stitch.

6 - The tucks stitched fabric is more opened and porous than the knit stitch fabric.  

7 - The tuck stitch is also used to get a fancy effect by using coloured yarn.

Miss or float stitch:

A float stitch occurs only when the yarn is presented to the needle but it is not taken by the hook of the needle. Here the needle is not activated upward to receive the yarn that is presented to it. Hence it will retain the old loop in the hook. The long float in the back is not desirable which causes snagging problem. 

Effect of float stitch:

It is used when an unwanted coloured yarn is to be hidden completely from the fabric surface being used. The unwanted coloured yarn floats at the back instead of tucking. A uniform texture is obtained by floating and yarn can be saved. The main effects of float stitch are given below. 

1 -  Float stitch makes the fabric thinner than tuck stitched fabric. 

2 - There no yarn accumulation takes place in float stitch.

3 - It makes the fabric narrower as there is no looped configuration.   

4 - The whole structure is pulled to maximum width. 

5 - The float stitched fabric gets less extensible than knit stitched or tuck stitched fabric.

6 - The fabric gets lighter in gsm due to the minimum yarn used in the construction.

7 - The float stitched fabric gets less rigid in comparison to other weft knitted fabrics.