Saturday, February 22, 2020

PARAMETERS OF COMBING PROCESS


PARAMETERS OF COMBING PROCESS:

The main  factors affecting the combing process are given below:

1 - Quality of feed preparation: 

The quality of preparation of feed( lap) greatly affects the performance of the combing process. The man quality parameters of feed ( lap) are given below:

Degree of evenness of sheet:

The preparation of lap  sheet requires very high careness during this process. The lap sheet is prepared uniform (i uniform) width. The uniform width of lap sheet helps to achieve the maximum gripping of lap sheet between nipper plates. The lap's evenness results in the form of better clamping of lap sheet between the nippers. The improved evenness get increased by using more number of doublings in the web form like ribbon lap machine.



Orientation of fibre hooks in the sheet:

The lap sheet gets fed to the comber in correct direction. The correct feeding direction helps to minimise the good fibres loss in waste percentage. The trailing hooks present in the lap sheet are fed to the comber in such a direction so that these are changed into leading hooks. The correct direction of feeding also affects cleanliness and the opening of hooks both. If the lap gets fed Incorrectly in the combing machine, it results in the form of  increase in  neps percentage in the yarn to be spun. The long and fine fibres always contain more number of hooks than short and coarse fibres. As the fibres get long and fine, the importance of the hooks gets increased.


Degree of parallelisation of fibres in the sheet:

1 - The degree of parallelisation of fibres in the lap is very important parameter in the combing process.

2 - The optimum degree of parallelisation is achieved during preparation of feed stock to the comber.

3 - The waste percentage is reduced by achieving optimum degree of parallelisation.

4 - The over parallelisation of fibres creates problem of lap licking.

5 - The mutual separation of layers within the sheet lowers due to fibre to fibre adhesion force in over parallelised lap sheet.

6 - The retaining capacity of the over parallelised lap sheet is decreased up to great extent. This low retaining capacity of lap sheet, it is not able to hold back the neps as it usually does.

7 - Some of these neps also pass through the top comb. Neppiness of the web is increased.

8 - The sheet does not hold together well due the over parallelisation of fibres in the lap sheet. Over degree of parallelisation leads to increase the hairiness of lap.

Linear density of fibre sheet:

The retaining capacity of the fibres results in self cleaning of the lap during combing process. A thick lap sheet shows better retaining capacity than a thin lap sheet. The effective gripping of nippers gets obtained with thick lap sheet up to certain limit.

Thick sheet has negative effect also. Thick lap sheet develops much load to the comb, thus leads to uncontrolled combing. 

Comber waste percentage:

The higher comber waste percentage results in form of low imperfection in the yarn to be produced. The strength and other quality parameters are improved up to the certain limit of waste percentage. further increase in waste percentage results in quality  deterioration. 

Machine setting: 

Following machine settings parameters affect the quality and performance of combing machine:

Feed length:

The feed length has direct effect on production rate, waste percentage and quality of combing.  The production rate gets increased by increasing feed length but at the same time quality deterioration takes place. The feed length is kept low to achieve higher quality requirement. The staple length of fibre also decides the feed length in combing process.

Effect of type of feeding used in combing machine:

There are two types of feeds in combing machine:

Forward feed (concurrent feed):

Feed of the sheet into the nippers occurs while the nippers move towards the detaching roller

Backward feed (counter-feed): 

Feed of the sheet occurs during return of the nippers In backward feed, the cylinder comb combs through the fibres more often than in forward feed Therefore, the elimination of impurities and neps is always good. However the difference is usually undetectable in modern high performance combers of the latest generation.

Detachment setting: 

The distance between the bite of the nippers and the nip of the detaching rollers is called detaching length. The detaching length has direct influence on the waste percentage. The waste percentage in combing gets increased when the detaching length increases.

Point density of combs:

The needles of the top comb have a flattened cross section. The point density of upper comb ranges from 22 to 32 needles per centimetre. The more number of needles per unit length in the comb results in the form of increase in waste percentage.

Depth  of penetration of top comb:

The depth of penetration of top comb also affects the comber waste percentage. If the comb depth is increased by 0.5mm, approximately 2% increase in waste occurs. When the depth is increased, the main improvement in quality is seen in Neps. Over deep penetration of top comb disturbs fibre movement during piecing which deteriorate the quality.

Piecing:

Since the web from detaching roller is intermittent because of the intermittent functions like feeding, combing and detaching, to have a continuous web from the comber, fibre fringes are laid on the top of each other in the same way as roofing tiles. This process is called Piecing. This is a distinct source of fault in the operation of rectilinear Combing. The sliver produced in this way exhibits a periodic variation.

Influence of combing operation on quality: 

The combing can be applied to a wide range of spinning processes.  The classification of quality of combed yarns can be classified as below:

Semi - combed yarn:

The upgraded higher quality of the yarn results  in comparison to carded yarn. The noil percentage in this yarn ranges between 5 - 10 %.

Normally combed yarn: 

It is better quality yarn in comparison to semi -  combed yarn. The noil percentage in this yarn ranges between 11 - 20 %.

Super combed yarn:

It is the best quality yarn in comparison to semi combed and normally combed yarn. The noil percentage goes over 20% in this yarn.

Combers Noil:

The wastage which is removed from the comber m/c during processing is known as comber’s noel. It is expressed as percentage. It is mainly of short fibres and naps. The noil is used for lower count as raw material.

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