Beater bar/gravity method

Flocking material is sprayed using an air compressor, reservoir, and a spray gun similar to the spray painting. It is primarily used when large areas require flocking. It is an untidy process, because some of the flocking fibers become air borne. Flocking is also applied by printing an adhesive on to a substrate, and then rapidly vibrating substrate mechanically, while the flock fibers are dispensed over the surface.

The vibration increases the density of fibers. This is critical to good fiber coating, and causes the flocking fibers to adhere to the adhesive pack into a layer. This process is called beater bar or gravity flocking system and is basically a mechanical process. With this process the flocking fibers are randomly adhered to the surface of the substrate, and each fiber adheres to the adhesive at a different depth, creating an irregular flocked surface. Since the fibers adhere to the surface of adhesive, rather than penetrate or imbed in it, some fiber shedding occurs. Loose flocking fibers generated during production also have a tendency to migrate, so many of these systems are installed in a separate area to prevent fiber contamination of the factory.

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.

METHODS OF APPLICATION FOR DECORATIVE FLOCKING

APPLICATION METHODS:

Decorative flocking is done using any one of the following application methods:

·         Electrostatic

·         Beater bar/gravity

·         Spraying and

·         Transfers.

The electrostatic method is the most viable flocking method.

Flocking is an alternative decorating method for achieving unusual look. It costs slightly more than that produced by a standard screen print, and for textiles, it is less expensive than embroidery. This process uses dyed fibers of nylon that are charged with high voltage and driven into design. This process eliminates fiber shedding, which is common with most other flocked images. Standard flock transfers can also be made in house.

Basically the pre-press is the same as it is for the screen printing with only a few differences. The screens are made from coarser mesh counts and the line thickness of some of the art work is increased. The adhesive is printed, and a crystalline powder is applied to the transfer and then is sent through the dryer at a low temperature. This removes moisture from the adhesive, but does not melt crystalline powder. Excess powder is removed and the transfer is sent through the dryer for second time. This binds the powder chemically to the adhesive.

The most successful method to ensure a good dense coverage is a combination of electrostatic flock application with the use of beater bars to help increase the density of the coating.

MATERIALS FOR FLOCKING

FLOCK FIBERS

Flock can be made from natural or synthetic materials 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 the manufacturing process, milled flock is not uniform in length, and can vary from fine (0~ - 0.5 mm) to coarse (0.4 - 1.1 mm). Milled cotton flock has the advantage of being the lowest in cost and the softest, but has the least abrasion and wear resistance.

Cut flock is produced only from monofilament synthetic materials. The cutting process produces a very uniform length of flock. Lengths can be obtained from 0.3 - 5.0 mm and 1.7-22 dtex in diameter. (One dtex is the measurement of a fiber that weigh; one gram per 10,000 meters of length.) Rayon is a little bit better on wear resistance and 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 compartments, and roofing.

Other flock manufacturing steps:

After cutting, the flock is cleaned of oils that have accumulated during processing. It is vat dyed to any number of colors, and then chemically treated to enable the fibers to accept an electrical charge. Since the fibers are all dielectric, a certain amount of conductivity must be present for electrostatic flocking process to occur. When the process is complete the fibers are spin dried and then oven dried to a specific moisture content. The flocking fibers are never totally dried, because moisture content adds to their conductivity. Finally, the flock is packaged in moisture proof bags that maintain proper humidity.

The fineness of the flock, length of fibers and adhesive coating density determine the softness of the flocking. However, the fine or short flock is difficult to work with, because it has a tendency to ball-up during processing.

ADHESIVES
Flock adhesives are available in wide varieties, both single part and two-part catalysed systems. Adhesives are generally water or solvent based. Some are air drying, others temperature or catalyst curing. Adhesives are usually applied by brush, roller, spray or screen printing.

SCREEN PRINT OF ADHESIVES

Many of the adhesives have the consistency of plastisol ink. Care should be exercised to select a stencil emulsion or film that is compatible with the adhesive to be printed. Proper application of the adhesive is the most important part of the process. A very heavy deposit of adhesive is required, but at the same time the adhesive should not be 'squeegeed' through the substrate. Care should also be exercised not to apply a thin coating. Less adhesive does not give proper adhesion characteristics for the fibers, which will result in low wear resistance. In order to achieve the proper deposit of adhesive, the screen should be made from a 24 - 43 threads/cm (60 - 110 threads/inch) monofilament mesh. Tension should be at 20 N/cm. Extra face coats of emulsion on the print side of the screen may be required for mesh counts greater than 36 threads / cm (96 threads/in). Printing should be off-contact, using a 65 durometer ball-nose squeegee. If difficulty in getting the proper coverage is experienced then, do not thin the adhesive to make it more printable. This will only create a thinner deposit by allowing the adhesive to soak into the substrate. A better solution is to slow the squeegee stroke down to allow the adhesive time to flow through the screen and on to the substrate. The flock adhesion can be tested by subjecting the substrate to the standard textile wash test. If the flocking fibers come loose or fall off, the adhesive may be too thin or the adhesive is improperly cured. If the adhesive is properly applied, then the curing temperatures should be adjusted until the substrate passes the wash test. This is the only safe way to ensure proper curing of the adhesive.

FLOCKING - Introduction

HISTORY

Flock printing is an old technique which was already in use in China around 1000 BC. In Europe it became popular during the Middle Ages, where fiber dust was attached to sticky surfaces. In 1970s with the advent of improved technologies and adhesives the flocking method became popular method for decorating. In 80s and 90s the method lost its popularity. But in last few years the flocking method is once again in demand because the flocked surfaces reduce water condensation, act as good thermal insulators.

APPLICATION:

T-shirts, packaging for perfumes, car glove box, car head liners, floor coverings, model railway-landscapes, Eye liner brush, scrubbing pads, garments, greeting cards, trophies, toys, book-covers etc.

ADVANTAGES OF FLOCKED SURFACES ARE:

·         Pleasant appearance

·         Fibered grip

·         Persistence against bounces and scratches

·         Good sliding effect on even surfaces

·         Many other specialized properties.

THEORY:

Flock printing or flocking is a printing process in which short monofilament fibers of rayon, cotton, wool or another natural or synthetic material are applied to an adhesive-coated surface. This adds a velvet or brush (suede) like texture to the surface. Since the fibers can be dyed, flocking can also add a color to a printed area.

The fibers used in the process are known as flock. They can be applied to paper, plastic, metal, rubber, wood, textiles and even glass.

The diameter of the individual flock strand is only a few thousandths of a centimeter and ranges in length from 0.05 to 5mm. The adhesive that capture the fibers must have the same flexibility and resistance to wear as the substrate. The flocking process requires a special equipment that charges the flock particles causing them to stand erect. The fibers are then propelled and anchored into the adhesive at right angles to the substrate. The application is both durable and permanent.

SUMMARY
Flocking is an alternative decorating method for achieving unusual look. It costs slightly more than a standard screen print, and for textiles, it is less expensive than embroidery. Also with the advanced state of adhesive technology nearly any material can be flocked, making it easy to add new products to the textile production capabilities. Wallpaper, greeting cards, mouse pads, book and album covers, and posters can all be flock printed. The successful use of electrostatic flocking depends on tight control over the process and the environment. Detailed production records should be kept, so the process can be repeated. Retention of production samples is also an important factor for repeat jobs, product reliability, and quality control information.

The RIM Process: (Elements of the RIM process)

The RIM Process: (Elements of the RIM process):

The major equipment components are:

1.       Storage tanks

2.       The chemical system

3.       Metering units

4.       Mixing head

5.       Other miscellaneous components

6.       Mould support

7.       The mould

8.       The press.

1.       Storage tanks:

It should be capable of being maintained at constant temperature of 27-38^o C and must be fitted for recirculation of fluids.

2.       The chemical system:

There is a wide range of chemical system which produces urethane polymers having a wide range of physical properties. The chemical system should react to form a polymer having the physical properties required for the part being moulded, and its viscosity must be within limits imposed by the metering system and the impingement mixer.

3.       Metering units:

The RIM machine should include at least two high pressure metering pumps, capable of delivering the liquid material to the mixing head at pressure of 1500-3000 psi. The pumps must also be capable of completely filling the mould within 2-3 seconds.

4.       Mixing head:

It is a small mixing chamber (1-6 cc). Here two or more liquids at pressure up to 3000 psi impinges and mixes thoroughly and then transferred to the mould cavity. The mixing heads are usually self cleaning, between injections, by means of recirculating action of the pumps. Hence no solvent flush is required to keep the mixing head clean.

5.       Other miscellaneous components:

The RIM machine not only includes metering unit and mixing head but also includes a set of temperature controlled conditioning tanks, the piping, hoses, filters and controls for operation.

6.       Mould support: OR The mould handling system:

It must position the mould to facilitate the expansion process.

7.       The mould:

The moulds can be made of Aluminum, Kirksite (Aluminum + Zinc) or even epoxy because the mould pressures are relatively low (50-70 psi). For high production, the moulds are usually made of polished of Nickel plated steel. The clamping pressures are also low due to the low mould pressures. In RIM, the mould orientation is very important. The mould must be positioned to allow flow entry with minimum turbulence and rapid gas venting. Depending on the compound the mould temperature range from ambient to a maximum of approximately 66^oC.

8.       The press:

The process is ideal for automation because it is designed for rapid production. Most presses are fully automatic in opening and closing of mould, feeding the resin, demoulding etc.

RIM Terminologies

Explanation of terms:

RIM: Reaction Injection Moulding:

It is a variation of the basic injection moulding process involving high pressure impingement (impact) mixing of two or more reactive thermoplastic or thermoset liquid component (either cold or warm) in a mixing head. After mixing, the liquid stream is injected into a closed, warm mould, at low pressure, where the reaction continues and the mixture solidifies. The finished parts can be cellular or solid elastomer, with a wide range of hardness and modulus.

LIM: Liquid Injection Moulding:

It is another term used for the same process – RIM

HPIM: High Pressure Impingement Mixing:

It is used especially with urethanes, which is a most important one in the overall moulding process.