FLOCK PRINTING

The flock printing process involves applying short monofilament fibres, usually nylon, rayon or polyester, directly on to a substrate that has been previously coated or printed with an adhesive. It produces a pile or velvety effect on the fabric similar to the pile obtained by raising the fabric by machines. The process is also called as the “Velvet Printing”. Originally fibre dust obtained from the weaving shed was used as flock but now short fibres about 1mm in length cut from natural or man-made fibres are used because fibre dust is not able to meet the demand.

Flocks can be made up from natural as well as synthetic material such as cotton, rayon, nylon and polyester. There are two types of flock – milled and cut. Milled flock is produced from cotton or synthetic textile waste material. Because of manufacturing process milled flock is not uniform in length. Cut flock is produced only from first quality filament synthetic material. Length can be obtained from 0.3-5mm. Milled cotton has the advantage of being lowest in cost and the softest, but has the least abrasion and wear resistance. Rayon is little bit better on wear resistance and the nylon is the best. For cut flock, rayon is the least expensive with the least wear resistance. Cut Nylon is the best grade of flock and produces a good feel, but is also the most expensive Cut polyester is basically used for industrial applications such as automobile window seals, glove compartment and roofings. Besides cutting and milling flock manufacturing needs several other steps. After cutting, the flock is cleaned of oils that accumulated during processing. It is dyed to any number of colours, and then chemically treated to enable the fibres to accept an electrical charge. Since the fibres are all dielectric, a certain amount of conductivity must be present for electrostatic flocking to occur. When the process is complete the fibres are spin dried and then oven dried to specific moisture content. It should be noted that flocking fibres are never totally dried, since moisture content adds to their conductivity. Finally the flocks are packed in moisture proof bags that maintain proper humidity.

The fabric is first printed with an adhesive paste containing a synthetic polymer which is self-reacting i.e. containing monomer which cross-links to form a 3-D network within the polymer film besides anti-foaming agent like silicon, a plasticizer, a thickener, a catalyst like diammonium phosphate, melamine formaldehyde to lower the curing temperature and glycerin to prevent the adhesive from drying prematurely. When the whole fabric is flocked, the process is known as “Flocking” in which the adhesive is applied by doctor blade to the fabric. The fabric is then dried by radio frequency drying (as it ensures uniform drying) and then cured at 150⁰C for 5 min.

The flock is applied to the printed or coated surface by two methods. Earlier it was applied by hand by sprinkling it on surface or through a sieve on cloth. In this method the flocks fall through air in a loose form and some of it floats in air and is laid on the surface on the cloth as a result of which it dos not produce a pile effect. Further, in sprinkling there is no force, therefore the flock is superficially fixed and can not withstand abrasion and wear. Therefore this method is not used.

Beater bar Method

In this method, the coated fabric is held tight on a conveyer belt, the flock is fed to the cloth by means of hoppers. The cloth is rapidly beated by beater bars and vibrated to make the fibres stand on ends and become firmly fixed in the adhesive. The beater bars below the cloth rotate rapidly and rub against the backing material thereby generating static electricity which makes the fibres stand on ends. The unfixed flock is removed by means of vacuum suction. The number of fibres that are oriented perpendicular to the surface of the cloth by this method is not very high with the result that the flock density is low; on account of this; a thick pile is not produced after the fabric is printed with flock, it is dried and then cured in hot-flue oven or chamber at 150⁰C for 5 minutes.

Electrostatic Method

In this method, tiny flocks are charged with high voltage electricity while the surface of the fabric is earthed thus forming two electrodes having a high electrical potential difference between them. The flock, before feeding it through hoppers is treated with ammonium chloride solution to render it more conductive and also to minimize the adhesive of one fibre to another. The flock so treated is then fed from the hopper and spread uniformly over the positively charged surface of the printed portions of the cloth by means of a revolving brush. The electrical force orients the fibres in a perpendicular direction to the fabric surface and fixes them into the adhesive in that state with the result that a thick pile is produced. After printing and dyeing, the fabric is cured in a hot-flue oven or electrically heated chamber at 150⁰C for 5 minutes.

Beater bar Method Electrostatic Method

Having controlled atmosphere for flocking is essential. Ideally, flocking area should have relative humidity of 60% and temperature of 20⁰C. A small variation in temperature or a change in the percentage of relative humidity can result in a 3 to 4 factor change in the conductivity or electrical sensitivity of the flock and the substrate. Flocking fibres are very sensitive to humidity and temperature conditions. When a new batch of flock fibres is opened, the fibres will give off or receive moisture based on the surrounding environment. Less than 30% relative humidity in the production area will lead to fibres that won’t accept a charge. Relative humidity in excess of 65% causes the flock to stick together and flow poorly through metal screen or plate. For best results flocking operation should be located in an atmospheric controlled room.