ELECTROSTATIC FLOCKING OF TEXTILES

Electrostatic flocking equipment for T shirt and other textiles is available in three configurations:

·         Automatic carousel for multicolor flocking,

·         Single station flocking unit that usually attaches to one station of a garment press, or

·         Portable hand-held unit for lower volumes.

The cost of hand-held units is less than that of automatic multicolor systems.

The operation of all equipments uses the same basic procedure, and is explained by a law of physics stating that opposite electrical charges attract each other. In flocking, the electrical charge is generated by the use of two electrodes:

·         A high voltage, direct current grid connected to a power generator,

·         A grounded substrate.

An electrostatic charge is generated that propels the fibers at a high velocity on to the adhesive coated substrate. This causes the flocking fibers to penetrate and imbed in the adhesive at right angles to the substrate. This forms a high density uniform flock coating or layer. Controlling the electrical field by increasing or decreasing either the applied voltage or the distance between the electrodes and the substrate controls the speed and thickness of the flocking.

Multicolor flocking equipment has one print station for applying the adhesive and multiple stations for applying the flock. It uses a flat metal screen that is coated with an emulsion and exposed with each of the design elements, the same as it would be for screen printing. The flock is placed on the metal screen, which acts as the high voltage electrode, and a rotating brush precisely dispenses the flocking material. When the screen is lowered to the proximity of the adhesive coated substrate, the flocking fibers are propelled into the adhesive, as determined by the stencil on the metal screen. Since the electrostatic field strength is controlled, and because the metal screen and the adhesive-coated substrate are brought close together, the flocking material is prevented from attaching to the adhesive except where the stencil is located, regardless of the size of the adhesive coated substrate.
HAND HELD UNITS

The hand held units are comprised of a metal plate, a generator and a flocking head. The metal plate must be grounded, and it can be placed where convenient. It is the equivalent of the platen on a textile press. The generator creates the electrostatic charge, and is wired to a canister that contains the loose fibers. A metal screen is mounted halfway inside the canister opening. The open end of the canister is then passed over the adhesive coated substrate, drawing flock fibers from the canister through the screen. The electrostatic charge propels the fibers toward the grounded metal plate. The adhesive coated substrate intercepts the fibers and flocking occurs. The substrate is then cured in a conventional dryer, and the loose fibers are removed by shaking, vacuuming or by using compressed air. Operation of these units requires a degree of skill to obtain the desired results. If the flocking head is held too far from the substrate poor coverage of flocking fibers will occur. The operator must also hold the unit perpendicular to the substrate to prevent the flocking fibers from imbedding in the adhesive at an angle other than perpendicular to the substrate. Hand held units are also messier than automatic systems and leave behind more fibers.

Curing the flocking is also an area that needs to be investigated. Since few screen printers use water-based adhesives, they may not have the proper curing equipment. Water-based adhesives require the use of dryers that have multiple independent heating zones with changeable air flow rates. Even plastisol and catalysed adhesive may require additional time to fully cure.

THE ENVIRONMENT

Having a controlled atmosphere for flocking operations is generally regarded as another essential ingredient for success. Ideally, the flocking area should have a relative humidity of 60% and a temperature of 20⁰C (68⁰F). 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. These changes will have an adverse effect on the process, and will result in flock balling, reduced adhesion and density of the flocking, and an excessive use of flock. Flocking fibers are very sensitive to humidity and temperature conditions. When a new batch of flock fibers is opened, the fibers will give off or receive moisture based on the surrounding environment. Less than 30% relative humidity in the production area will lead to fibers that won't accept a charge. Relative humidity in excess of 65% causes the flock to stick together and flow poorly through the metal screen or plate. For best results the flocking operation should be located in an atmospheric controlled room. To ensure that flocked designs have received a proper cure, sample prints should be subjected to the standard textile wash test.