Metallic card Clothing - Page 2
HARDNESS OF WIRE:
The cylinder wire needs to be hard at the tip of the tooth where the carding action takes place.The hardness is graded from the hard tip to the soft rib. High carbon alloy steel is used to manufacture a cylinder wire and it is flame hardened. Rib should not be hardened, otherwise, it will lead to mounting problems.
The design or type of clothing, selected for the fibre to be carded is important,but it is fair to
state that within reason, an incorrect design of clothing in perfect condition can give acceptable carding
quality whereas a correct clothing design in poor condition will never give acceptable carding quality.
There is no doubt that the condition of the clothings is the most important single factor affecting quality
at high rates of production.
Wire condition and selection of
wire are considered to be the two most important
factors which influence the performance of modern high
production carding machines.
The condition of the clothing may be defined as the collective ability of the individual teeth of the
clothing to hold on to the fibre against the opposing carding force exerted by other teeth acting in the carding direction. For a given design of clothing the condition of the teeth determines the maximum acceptable production rate that can be achieved at the card.
The speed of the main cylinder of card provides the dynamic
force required to work on separating the fibres
fed to the card but it is the ability of the carding teeth on
the cylinder to carry the fibre forward against
the opposing force offered by the teeth of the tops which
determines the performance of the card. Increasing
cylinder speed increases the dynamic forces acting upon the
carding teeth and thus the condition of teeth
becomes more important with increased speed.If the condition and
design of the cylinder wire is poor, the
teeth will not be able to hold onto the fibre through the
carding zone, thus allowing some of the freed
fibre to roll itself into nep.
The doffer is a collector and it needs to have a sharp tooth to pickup the condensed mass of fibres
circulating on the cylinder. It also requires sufficient space between the teeth to be efficient in fibre
transfer from the cylinder, consistent in the transfer rate and capable of holding the fibre under control
until the doffer's stripping motion takes control.
A standard doffer wire has an overall height of approx. 4.0 mm to facilitate the deeper tooth
which must have sufficient capacity to collect all the fibre being transferred from the cylinder to meet
production requirements. Heavier webs require a deeper doffer tooth with additional collecting capacity
to hanndle the increased fibre mass.
The doffer wire's front angle plays a very important part in releasing the fibre from the cylinder wire's
influence. A smaller angle has a better chance of enabling the doffer wire's teeth to find their way under
the fibres and to secure the fibre's release from the cylinder with greater efficiency.
A 60 degree front angle for Doffer has been found to give the optimum performance under normal carding conditions. Too small an angle results in cloudy web and uneven sliver whilst too large an angle results in fibre recirculation and nep generation.
Having collected the fibre, it is important for the doffer to retain it until it is stripped in a controlled
manner by the doffer stripping motion. The tooth depth, tooth pitch and rib width combine to create the space available for fibre retention within the doffer wire. Thus they directly influence the collecting capacity.
If the space is insufficient, fibre will fill the space and any surplus fibre will be rejected. When
the surplus fibre is left to recirculate on the cylinder, cylinder loading can take place. Unacceptable nep
levels and fibre damage will also result. In severe cases pilling of the fibre will take place.
The point of the doffer wire normally has a small land which helps to strengthn the tooth. The extremely
small land of around 0.05 mm ensures that the doffer wire height is consistent, has no adverse effect on
fibre penetration and is considred essential for efficient fibre transfer from the cylinder. The land has
micropscopic striations which are created during manufacturing or grinding.
The striations help to collect
the fibres from the cylinder and keep them under control during
the doffing process.
It has been found that a cut-to-point doffer wire penetrates the
fibre better than does the landed
point wire but is less likely to keep the fibre under control during the doffing process. Sometimes a cut-to-point doffer wire is accompanied by striations along one side of the tooth for this reason.
Until recently 0.9mm rib thickness is standardised for doffer wire, regardless of production and fibre
characteristics.This rib thickness has been found to give optimum results. However doffer wires with a 0.8mm rib thickness have been introduced for applications involving finer fibres.
In general 300 to 400 PPSI(points per square inch) has been found to perform extremely well under
most conditions. Doffer wire point population is limited by the wire angle and tooth geometry. Higher
population for doffer does not help in improving the fibre transfer.
As the production rate rises, the doffer speed also increases. The doffer is also influenced by the
centrifugal force, as is the cylinder.But cylinder wire front angle can become closer to counter the effect
of centrifugal force, to close the front angle on a doffer wire would reduce its collecting capacity and result
in a lowering of the production rate. The solution is to use the wire with striations, which will hold the
fibre until the doffer is stripped.
The hardness of the doffer wire is a degree lower than that of the cylinder but sufficiently hard to
withstand the forces generated in doffing and the resultant wear of the wire. The reason for this slightly lower hardness requirement is the longer and slimmer tooth form of the differ wire.
The fibres which are not able to enter the wire will lay on top, i.e.completely out of control.
There fore instead of being carded by the tops the fibres will be rolled. Similarly a fibre buried too deep
within the cylinder wire will load the cylinder with fibre, weaken the carding action and limit the quantity of
new fibres the cylinder can accept. Therefore, the production rate would have to be reduced.
Licker-in with its comparatively small surface area and small number of carding teeth, suffers the hardest wear of all in opening the tangled mass of material fed to it.
Successful action of the Licker-in depends upon a penetrating sharp point rather than a sharp leading
edge as with the cylinder wire. Therefore the licker-in wire cannot be successfully restored to optimum performance by grinding.
The most satisfactory system to adopt to ensure consistent performance is to replace the licker-in
wire at regular intervals before sufficient wear has taken place to affect carding quality.
The angles most widely used are 5 degrees negative or 10 degrees.
There is no evidence to suggest recommendation of a tooth pitch outside the range of 3 to 6 points per inch.
It is better to use Licker-in roller without groove. Interlocking wires are used for such type
of licker-ins. This avoids producing the eight precise grooves and to maintain them throughout its life.
Interlocking wire is almost unbreakable and thus no threat to the cylinder, tops and doffer in the event of
foreigh bodies entering the machine.
The flat tops are an equal and opposite carding force to the cylinder wire and it should be sharp,
well maintained and of the correct design.
The selection of flexible tops is very much related to the choice of cylinder wire, which in turn is related
to the cylinder speed, production rate and fibre charactersitics, as previously stated.
The modern top is of the semi-rigid type, having flexible foundation and sectoral wire. The points are
well backed-off and side-ground to give the necessary degree of fineness. The strength of the top from a carding point of view is in the foundation and is affected by the number of plies and the type of material used.
The position of the bend in the wire is determined by stress
factors, at around 2:1 ratio along the length of the wire
The modern top is made from hardened and tempered wire to increase wear resistance , thus improving
the life of the flat top.
Life of the cylinder wire depends upon
Material being processed
Wear is the natural and unavoidable side effect of the work done by the vital leading edge of the metallic
wire tooth in coping with the opposing forces needed to obtain the carding action which separates fibre from fibre.
When the leading edge becomes rounded due to wear, there is a loss of carding power because the point condition has deteriorated to an extent where the leading edge can no longer hold on to the fibre against the carding resistance of the flats. This ultimately leads to fibres becoming rolled into nep with consequent degradation of carding quality. Therefore it is important to recognise that, due to the inevitable wear which takes place during carding, metallic wire must be reground at regular intervals with the object of correctly resharpening the leading edge of each tooth.
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