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Knife Coating


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Profiles of Knives:

y(a) knife type, (b) V type,

y(c) bull nose, and (d)shoe.


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Effect of Blade Angle

If the blade is inclined at an acute angle to the directionof motion of the web, add-on is more due to the wedgeof coating material formed between the blade and the

web. However, if the angle is obtuse, add-on is low.


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Twin knife arrangement


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yThe blade is usually mounted on a verysubstantial

metal girder mounted over the width of themachine.For quick changeover, sometimes two orthree blades of different profile are mounted onthe same girder, which is rotated through 180 ofarc in the case of two

blades, or 120 in the case of three blades, tochange the blade profile.


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Rolling bank

web

rolling bank

knife


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The viscosity and the amount of rolling bank also

contribute to the penetration and coating weightof the end product.

The rolling bank exerts a pressure on the web.

If the height of the roll bank is greater at the center,a heavier coating will be produced at the center.

Similarly, if the height is more at the sides, the

coating will be more in the sides.In case of a pump pouring fluid over the web across

the width, its traverse also results in variation of

coating in an S pattern.


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Mascoes Trough

feeder

adjustable gap

trough

knifecoatingcompound

web


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SpittingD

uring the coating process, many compounds, because oftheir surface tension properties, rise up the back of aknife, accumulate, and drop on the coated surface in anunsightly fashion, called spitting.

This phenomenon is also referred to as blobbing orcreeps .

In the shoe knife, due to the design of the this iscompletely prevented..

For polyvinyl chloride (PVC) pastes and breathablepolyurethane (PU) coatings, the shoe knife is the

preferred type.


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In case the position of the blade is at a point after thecrown of the roll, the blade will be pressing directlyinto the fabric, and a situation similar to a floatingknife is being created


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Gravure Coating

smoothening

rollsGravure roll

backup roll

doctor blade


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Gravure Coating

y Engraved rollers are utilized in gravure coatings to meter a

precise amount of coating on the substrate.y The coating weight is usually controlled by the etched

pattern and its fineness on the gravure roll.

y There are a few standard patterns, including the pyramid,

quadrangular, and helical.y For lighter coating weight, a pyramid pattern is used.

y In a direct, two-roll gravure coater , the coating material ispicked up by the gravure roll and is transferred to the web as

it passes between the nip of the gravure and the backup roll.y The pattern may be self-leveling, dependent on viscosity of

coating material or the coated web is passed betweensmoothening rolls.


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Offset GravureCoater:

gravure roll

rubber-

coveredoffset roll

steel backup roll

doctor blade.


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Offset GravureCoater:The speed and direction of the gravure and offset rollers can

be varied independently.

The arrangement is suitable for extremely light coating (aslow as 0.02 g/m2) and minimizes the coating pattern.

This offset process can handle a higher-viscosity material(~10,000 cps) than can the direct process. By heating thefeed pan, the process can coat hot-melt compounds.

Gravure coating is used for applying laminating adhesives or atop coat on a treated fabric.

The drawback of the process is that the coating weightdepends on the depth of engraving. For different coating

weights, different engraved rollers have to be used.


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Reverse-Roll Coaters

applicator ro

metering roll

backuprubber roll

web

coating pan

doctor

blades

drip pan

Doctor blades


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Parameters to control the thickness of the

gap between the applicator and the metering roll

rotational speed of the applicator roll

web pressure on the applicator roll adjusted by the backuproll.

Thus, a reverse-roll coater has greater flexibility in adjustingthe coating thickness compared to the knife over roll, where

coating is controlled only by gap of knife and the roll.

In the direct-roll coating where the web and applicator rollmove in the same direction, nonuniform coating occurs withthe formation of ribbing due to the film split phenomenon,

while in reverse roll, the coating is smooth

Nip-fed Reverse-Roll Coaters


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Pan-fedreverse-roll coaterbackup roll

doctor blade

applicator roll

metering roll

web


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Drawback ofNip fed coater

yOne difficulty when using the nip-fedcoater is to prevent leaks from thecoating reservoir, particularly with low-

viscosity compounds.yThe pan-fed coater operates using the

same principle as the nip-fed coater but

is more suited to low-viscositymaterials


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Dip coating

dipping tank.

web.

Squeeze Rolls,


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limitations of direct coating

yIt is applicable to closely woven,dimensionally stable fabrics that can

withstand the machine tension, and is not

suitable for excessively stretchable knittedfabrics.

yPenetration occurs in the weave of the

fabric, increasing adhesion, lowering tearstrength, and decreasing elongation,resulting in a stiff fabric.


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Transfer Coating

(1) release paper, (2) first coating head,(3) first oven, (4) second coating head, (5) textilesubstrate, (6) laminating nip rolls, (7) seconddrying oven, (8) coated fabric take-off roll, and (9)

release paper wind roll.


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Rotaryscreen coating

: (1) web, (2) squeegee, (3) screen, (4)whisper blade,and (5) backup roll.Copyright


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