The Flocking Process
The process of flocking is fairly simple and easy. First a suitable adhesive is applied to the surface to be flocked. The flock is then applied, penetrating the surface of the adhesive to create the desired velvet finish.
Electrostatic flock applicators charge the flock particles which are then attracted to the grounded surface that is to be flocked. Unlike puffer or blown application methods which merely sprinkle a flock layer onto the surface, electrostatic application ensures that the fibres all end up standing at right angles to the surface resulting in a velvet finish.
Electrostatic flocking is used extensibly in the automotive industry for coating window rubbers, glove boxes, coin boxes, door cards, consoles, and dashboards. Rally cars usually have their dashes flocked to reduce reflections and to provide an as new finished to a modified dash.
Flocking is proving successful in a number of artistic ventures including the decoration of jewellery, ceramics and pottery.
Using suitable adhesives flock can be applied to an endless range of materials including plastic, metal, wood, rubber and fibreglass.
A More Detailed Insight
Consumers are always looking for something different and unusual. Suppliers seek the same thing - a special item or product that will increase their market share or generate new business. An example of this might be the recent popularity of mixed media garments in the marketplace. Developing something different is always a top priority, then, and is the driving force behind the recent resurgence of printer interest in learning about flocking.
Flocking for decoration is not new, of course; similar methods were used in the Middle Ages to attach fibre dust to sticky surfaces. It was in the 1970s, however, with the advent of improved technologies and adhesives, that flocking became a popular decoration method. Then, in the 1980s and early 1990s the popularity of flocking faded away and few printers used the process. Even so, while flocking is not the most widely used decorating process, nor is it a well known decorating technique, the average person is aware of its velvet or suede feel.
Over the last several years, however, inquiries about the process have begun to increase, and flocking is once again in demand as a decorating method. Even though flocking may not be most decorators' first choice process at present, it is used widely in many industrial applications. Flocked surfaces reduce water condensation, act as good thermal insulators, and have been used in the automotive industry for years for such items as glove compartment boxes, door mouldings and window trim.
Flocking - What is it ?
In short, the flocking process involves applying short monofilament fibres, usually nylon or rayon, directly on to a substrate that has been previously coated with an adhesive. The diameter of the individual flock strand is only a few thousandths of a centimetre, and ranges in length from 0.25 - 5 mm. Adhesives that capture the fibres must have the same flexibility and resistance to wear as the substrate. The process uses special equipment that electrically charges the flock particles causing them to stand-up. The fibres are then propelled and anchored into the adhesive at right angles to the substrate. The application is both durable and permanent. Flock can be applied to glass, metal, plastic, paper or textiles. Flock design applications are also found on many items such as garments, greeting cards, trophies, promotional items, toys and book covers.
Decorative flocking is accomplished by using one of four application methods: electrostatic, beater bar/gravity, spraying and transfers.
The electrostatic method is perhaps the most viable flocking method, especially for the printer doing more than an occasional flocking job.
Flocking material can also be sprayed using an air compressor, reservoir, and spray gun similar to spraying paint. The resulting finish using this method is similar to a thin felt coating, as most of the fibres will be lying down in the adhesive. It is primarily used when large areas require flocking. It is an untidy process, because some of the flocking fibres become airborne.
Flocking is also applied by printing an adhesive on to a substrate, and then rapidly vibrating the substrate mechanically, while the flock fibres are dispensed over the surface.
Flock application by the vibration method.
Flock application by the electrostatic method.
The vibration promotes the density of fibres, which is critical to good fibre coating, and causes the flocking fibres to adhere to the adhesive and pack into a layer. This process is called a beater bar or gravity flocking system and is basically a mechanical process. With this process the flocking fibres are randomly adhered to the surface of the substrate, and each fibre adheres to the adhesive at a different depth, creating an irregular flocked surface. Since the fibres adhere to the surface of the adhesive, rather than penetrate or imbed in it, some fibre shedding occurs. Loose flocking fibres generated during production also have a tendency to migrate, so many of these systems are installed in a separate area to prevent fibre contamination of the shop.
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.
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). 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 fibre that weigh; one gram per 10,000 meters of length.) The fineness of the flock, length of fibres and adhesive coating density determine the softness of the flocking. It should be noted however, that fine or short flock is difficult to work with, since it has a tendency to ball-up during processing. Milled cotton flock has the advantage of being the lowest in cost and the softest, but has the least abrasion and wear resistance. 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. Besides cutting or milling, flock manufacturing includes several other steps. After cutting, the flock is cleaned of oils that accumulated during processing. It is vat 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 process to occur. When the process is complete the fibres are spin dried and then oven dried to a specific moisture content. Note that flocking fibres are never totally dried, since moisture content adds to their conductivity. Finally, the flock is packaged in moisture proof bags that maintain proper humidity.
A wide variety of flock adhesives are available, 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 fibres, which will result in low wear resistance. In order to achieve the proper deposit of adhesive, the screen should be made from a 24 to 43 threads/cm (60 to 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 you experience difficulty getting the proper coverage, 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 fibres come loose or fall off, the adhesive may be too thin or the adhesive was 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.
ELECTROSTATIC FLOCKING OF TEXTILES
Electrostatic flocking equipment for T shirt and other textiles is available in three configurations: an automatic carousel for multicolour flocking, a single station flocking unit that usually attaches to one station of a garment press, or a portable hand-held unit for lower volumes. The cost of the equipment varies from hundreds or a few thousand pounds for hand-held units to tens or hundreds of thousands of pounds for automatic multicolour systems.
All of the equipment operates using the same basic procedure, and is explained by a law of physics stating that opposing 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, and a grounded substrate. An electrostatic charge is generated that propels the fibres at a high velocity on to the adhesive coated substrate. This causes the flocking fibres 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.
Multicolour 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 fibres 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 fibres. 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 fibres from the canister through the screen. The electrostatic charge propels the fibres toward the grounded metal plate. The adhesive coated substrate intercepts the fibres and flocking occurs. The substrate is then cured in a conventional dryer, and the loose fibres 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 fibres will occur. The operator must also hold the unit perpendicular to the substrate to prevent the flocking fibres 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 fibres.
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.
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 20C (68F). 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 affect on the process, and will result in flock balling, reduced adhesion and density of the flocking, and an excessive use of flock. 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 the metal screen or plate. For best results the flocking operation should be located in an atmospheric controlled room. As stated earlier, in the adhesives section, to ensure that your flocked designs have received a proper cure, sample prints should be subjected to the standard textile wash test.
Flocking is a value-added alternative decorating method for achieving that unusual look. It only costs slightly more than producing a standard screen print, and in conjunction with textiles, it is certainly 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 your 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. While the process requires the use of special equipment, with practice a quality product can be produced.