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Basics of flocking

Know How

Forword

In flocking, fibers cut to the same length are applied vertically to a surface provided with adhesive using the electric field. After drying of the adhesive and cleaning of residual flock, the flock surface is usable. The origin of 'flocking' lies in the Middle Ages. At that time, paper was coated with resin and sprinkled with fibers to be used as wallpaper. The first electrostatic 'flocking' was used in the 1950s to make sandpaper. The hard material chips are shot into the adhesive in an aligned manner by the high voltage. Flock fibers can be made from a variety of materials. Depending on the material, this can produce very different mechanical or physical effects in addition to an attractive look and feel. Flock can insulate, insulate or conduct electricity. Increasing the surface area through flocking can act as a catalyst or filter. The list of possibilities is endless. The original purpose of creating a decorative surface has long been extended by a multitude of technical applications - and new applications are added every day....

1. Steps in the flocking process

1.1. Pre treatment

Surfaces to be flocked must be load-bearing, clean, and free of grease and release agents so that the adhesive layer and flock overlay applied later are optimally bonded to the substrate.
Pretreatment of the surface to be flocked is only necessary in case of possible adhesion problems between substrate and adhesive. The causes of poor adhesion can be found either in the substrate material itself (e.g. silicone, PE, ...), or due to changes in the surface caused by contamination, aging or evaporation of e.g. plasticizers.
Common pretreatment methods are:

  • Wiping with suitable solvent
  • Mechanical roughening
  • Application of primer
  • scarfing, corona treatment, plasma treatment
  • fluorination
    In any case, the necessity of pretreatment should be clarified in advance in a preliminary test. It is also helpful to test the surface tension with test inks. This value should be above 42 dyn.
1.2 Adhesive application

The adhesive is the link between the substrate and the flock overlay. It not only holds the fibers, but must itself be adapted to the subsequent use. A flock overlay on textiles, for example, requires a very flexible adhesive that must also be wash-resistant.
Information on selecting a suitable adhesive can be obtained from the adhesive manufacturers, as can advice on the right product. Preliminary tests with the selected adhesive are also advisable.
The adhesive should be applied in as uniform a layer thickness as possible. As a guide, a layer thickness of the dried adhesive of 10% of the flock fiber length is recommended. This ensures that the fibers are optimally anchored in the adhesive and can withstand stress.
The time between application of the adhesive and flocking should be as short as possible, so that the adhesive does not dry before the fibers are incorporated in sufficient density. The so-called 'open time' of the adhesive depends on the product and is specified in the manufacturer's processing instructions.
The application can be either full-surface or partial, by hand or automated. Common application methods for adhesives include:

  • Brush application (or shaped sponges)
  • Spraying (with flow cup, pressure vessel, pump or airless)
  • rolling on with a painter's roller
  • Squeegee application in continuous flow or with silk-screen printing
  • Dipping
    Not every application method can be used for every product, often the selection is based on the number of pieces required. All methods can be used manually, automated application systems usually require the use of robots in conjunction with spray application. Here, too, preliminary tests help in case of doubt.
1.3. Flocking

Flocking can be carried out either electrostatically or electrostatically-pneumatically, depending on the workpiece geometry to be flocked.
Flat or outwardly curved surfaces can be reached very well by the electrostatic field and are therefore flocked purely electrostatically. This can be done either with a hand-held device or by a continuous flocking machine.
The fiber density that can usually be achieved by purely electrostatic flocking is 8-10 %, with a maximum of up to 14 % possible. This means that 90 - 92 % of the flocked area consists of air. This air space can be used in a variety of ways.
On parts with depressions, the electrostatic field can only have a limited effect. The so-called Faraday cage prevents field formation in depressions, which causes the flocking to be less dense than flocking on the outer geometry.
If parts with depressions (such as a glove box) are flocked electrostatically only, then a very dense and well aligned flocking at the outer geometry results.
However, flocking in the recesses results in only moderate density and negative corners without sufficient radius even remain flock-free.
Supporting fiber transport by means of air flow helps to minimize these effects. These parts should therefore be flocked electrostatically-pneumatically. Various hand-held devices, machines or automated systems are available for flocking.
Electrostatic-pneumatic flocking produces a somewhat lower flock fiber density than purely electrostatic flocking. The reason for this is the effect of the air flow, which causes the fibers to be applied somewhat less vertically into the adhesive.
The positive effect, however, is a more uniform optical appearance of the later flocked surface. Particularly when flocking molded and interior parts, this process results in an optimum visual value impression.

1.4. Pre cleaning

After flocking, the workpiece is pre-cleaned of adhering loose flock. This is done by carefully tapping or blowing off with reduced compressed air in conjunction with a permanently vacuumed workplace.
The purpose of pre-cleaning is to minimize flock carryover to other, subsequent areas. The collected flock can be reused, provided it has not yet passed through a dryer.
In the case of particularly complicated geometries, it has been shown that the flocking result can be improved again after pre-cleaning and further flocking. The reason for this is that any unnecessary accumulation of loose flock that has not been installed would prevent the further installation of fibers in areas of the adhesive layer that are still open.

1.5. Drying

In the simplest case, the adhesive is dried in the hall air. For higher quantities or higher resistance requirements, drying can also be carried out in an oven dryer or in a continuous dryer. The drying parameters are supplied by the adhesive manufacturer.
Drying can be carried out by the following methods:

  • Hot air with continuous air circulation
  • Infrared radiation
  • Vacuum drying
    Each drying process has advantages and disadvantages, which must be weighed against each other in each individual case.
1.6. Cleaning

After drying, the workpiece is intensively cleaned of adhering excess flock and is then ready for use.
Excess loose flock considerably reduces the impression of quality. Particularly with dark flock types, every effort must therefore be made to achieve an optimum cleaning result.
Common cleaning methods are:

  • Blowing off with compressed air
  • Suction cleaning
  • Ultrasonic cleaning
  • washing
    ... or a combination of all these methods.
1.7. Quality control

Quality control and quality assurance are an essential part of the flocking process. As different as the flock applications are, the good adhesion of the fibers to the surface, as well as their durability and color fastness over time are key criteria.
Since the majority of flock applications are in the automotive sector, the very high quality requirements of car manufacturers and their suppliers have become the standard. These standards are also taken into account by flock and adhesive manufacturers.
Appropriate testing equipment for testing the materials, as well as the finished flock surface, can be obtained from specialist companies.

2. Materials

2.1. Substrate

Basically, all materials can be flocked that have a certain inherent strength and to which an adhesive can find sufficient adhesion. In the case of problematic materials, suitable pretreatment with the pretreatment processes already presented above will help.

2.2. Flock fibres

Flock is available in different materials, fiber thicknesses, cut lengths, and finishes.
Common flock materials are:

  • Synthetic fibers of polyamide, polyester (for automotive parts and profiles)
  • Organic fibers from artificial silk, cotton (for textile applications and household)
  • special fibers from Kevlar, carbon (for higher temperatures and strong abrasion)
  • Exotic fibers made of glass, stainless steel, graphite, etc... (for purely technical applications)
    The fiber strength, i.e. the titer, is given in dtex (decitex) and describes the weight of a single fiber of 10,000 m length in grams. The designation 6.7 dtex thus stands for the weight of 6.7 grams for a fiber 1o km long. The lower this value, the finer the fiber tends to be.
    Common titers are:
  • 0.9 dtex for microfibers, very soft to the touch, difficult to process.
  • 1.7 dtex as a common fiber, soft, for textile applications and cosmetics
  • 3.3 dtex as standard fiber for moldings, profiles, interior parts automotive, good to process
  • 6,7 dtex robust, easy to process fiber for consumer goods, tools
  • 22 dtex hard stubbly fiber For floor mats, easy to process
  • 44 dtex hard fiber for insulation elements or cleaning tools
    Common cut lengths of flock fiber range from 0.3 to 4mm, depending on the titer. Short cut lengths below 1 mm are available with almost all titers. Lengths of 2 mm or more should only be used from a titer of 22 dtex. Special lengths up to 15mm are possible, but very difficult to process.
    A typical fiber designation 'PA 6.6 - 3.3 dtex, 1mm' stands for polyamide flock with a titre of 3.3 and a cut length of 1 mm, i.e. a classic molded part flock.
    Flock is usually supplied by the manufacturer in 20 kg cartons; small quantities are available from specialist dealers. It is important to store flock according to the manufacturer's storage recommendations.
    Flock is usually made of non-conductive material. In order to align and accelerate the fiber in the electric field, a defined conductivity on the fiber surface is required. For this purpose, the fiber is chemically prepared accordingly by the flock manufacturer.
    In order for the flock to jump optimally in the electric field, a minimum conductivity of the flock material must be maintained. The conductivity of the fiber is influenced by the ambient humidity. Too dry an environment reduces conductivity, too wet an environment causes flock to clump.
    The manufacturers' processing recommendations for standard flock are an ambient temperature of 21 °C and 65 % relative humidity.
    Deviations are possible, but this will extend the flocking time and the flock surface cannot be flocked with the desired density.
    The conductivity can be measured by pressing the handle with electrode into the flock. The value of the electrical resistance in Ω is evaluated and converted into a numerical value between 0 - 250 with the designation standard scale parts.
    Usual values for a good conductance are 80-100 standard scale parts. If the value is lower, then the flocking room should tend to be set with somewhat higher humidity, or work with technical humidification.
2.3. Adhesives

Adhesive manufacturers offer different adhesive systems to suit different materials.
Basically, adhesives are divided into the following main types:

  • Water-based dispersion adhesives
  • Solvent adhesives
  • Plastisols (one-substance system without volatile components)
  • UV-curing adhesives
    The requirements for the adhesive system can vary. In addition to the best possible adhesion, other requirements are conceivable:
  • Chemical resistance
  • Solvent-free
  • High temperature
  • Food fastness
  • Toy fastness
    It is also helpful to dye the flock adhesive with pigments in the same color that is to be used for flocking. This makes the flock image appear even denser and more valuable.

3. Properties and applications of flocking

3.1. Optics

The fiber surface conveys a higher value than an unflocked surface. Flocking can be either partial or full-surface.
By selecting suitable flock types, further optical effects can be incorporated in addition to the color. Examples of this are:

  • Suede effect on pressure-sensitive artificial silk flocking
  • Metallic effect on appropriately finished fibers
  • Foil embossing after flocking
  • Laser inscription for individualization

Molded parts or other 3D structures can only be flocked with one flock color or type per flocking run. Another flock color can then be applied, e.g. partially, in a second pass.
Textiles, flat parts or plan blanks can be flocked in one color or with several colors in one pass. For this purpose, the part to be flocked passes through several stations, each of which then applies a flock color via appropriately designed screens. This technique is commonly used for flocking T-shirts.

3.2. Haptics

The aligned fibers result in a velvet-like surface that can be adjusted from soft to hard depending on the flock material used and the fiber length.
Fibers with low titer and/or longer cut length have a soft feel. Fibers with high titers and/or short cut lengths tend to have a rough feel.
Synthetic fibers are more robust and therefore tend to be harder to the touch than organic fibers, such as artificial silk or cotton.
Flocking is therefore often used for products that are likely to come into contact with the body during subsequent use. Examples are textiles, PVC shoes, orthotics, air mattresses, outdoor articles....

3.3. Noise avoidance

Due to the flocked surface, no noise occurs on contact.

This is why, for example, glove boxes, storage compartments, center consoles, eyeglass compartments, but also coffee cups and tableware are flocked in cars.

Another application is the flocking of microphones or their covers to prevent wind noise.

3.4. Refraction of ligh-t and sound waves

Flocking has a damping effect by breaking sound and light waves many times in the flocking structure.
This effect is used in the flocking of sound insulation elements, engine covers or the inside of side channel compressor housings.
This effect also minimizes stray light in optical housings or prevents the reflection of numbers on players' jerseys or other lettering during sports broadcasts.

3.5. Adhesion and sliding effect

Flocking can influence the sliding behavior of two surfaces in relation to each other. Flocked rubber profiles in the car allow the window pane to run only on the fiber tips. Freezing of the pane in winter is impossible.
Glass panes in furniture and trade fair construction can be moved more easily and quietly in flocked aluminum profiles. The user feels value.
However, if both surfaces are flocked, then moving is no longer possible. This effect is used for temporary floor coverings, e.g. in trade fair construction.

3.6. Grip surface

Flocking with flock types of high titer and short cut length produces a very grippy surface. The function of tool handles of all types can thus be optimized.
By flocking screwdrivers, for example, a higher torque can be transferred from the hand to the screwdriver. Even with dirty or greasy hands, a secure grip is ensured.

3.7. Tolerance compensation

The flocking acts as a perfect gap filler. In addition to compensating for gaps, e.g. in interior parts in the car interior, flocking prevents unnecessary rattling.
In the case of belt sliders in the vehicle interior, the flocking creates a double effect. In addition to damping running in the guide, rattling noises inside the guides are prevented.
The flocking can either be applied directly to the component or as tape to be stuck onto the required areas.

3.8. Insulation against heat and cold

The flocking covers only approx. 10% of the flocked surface, the calmed air between the fibers creates an insulating effect. Partial flock dots as spacers between several film layers creates an ultra-insulating material.
Flocking the inside of boat hulls or snow groomer cabins shifts the dew point and reliably prevents condensation.
Flocking of linings on heating elements or sauna ovens prevents burns in case of skin contact. Handles on pneumatic tools are flocked to insulate against the cold.

3.9. Depot effect between the fibers

The free space between the flock fibers can be used as a depot for many substances. Typical applications are paint rollers, painting tools or applicators for cosmetics.
Flocked cannula ends for applying dental primer also serve as micro-brushes.
Flocked spatulas or rods are also often used for taking saliva samples, the so-called corona swaps.

3.10. Functional surfaces

The following is a small excerpt of other flock applications:

Gentle material transport:
Conveyor belts, devices, grippers, support points are flocked in order not to damage the material to be conveyed or moved. The contact surface can either be flocked directly, or a flock tape can be partially applied.

Cleaning systems:
Suitable flock fibers can be used to create very effective cleaning systems. Typical applications are floor mats for laying out, but also carpets. Rotating brushes are often used in textile machines for cleaning spinning waste. Partially flocked cleaning cloths or sponges also remove stubborn dirt.

Fouling prevention:
Aquaculture nets become unusable over time due to algae and crustacean fouling. Flocked nets remain without fouling. The same is true for flocking boat hulls.

Bonding:
Flock as an intermediate layer between different partners that are difficult to bond permanently.
Examples are architectural elements made of styrofoam molded parts with plaster overlays, or orthotic parts made of silicone that are to be bonded to plastic. In both cases, the part to which flock is to be applied is flocked beforehand, and the problem partner then anchors itself mechanically in the flock.

3.11. Active surfaces

Fibers can be produced with special finishes. This finish can be chemically adapted to the respective application. Another possibility is to finish the fibers after flocking by an additional application step

Conductivity by metallization:
Flock fibers can be coated with elemental silver. A small admixture of these fibers results in a very good electrically conductive flock coating to the substrate.

Surface area enhancement:
Filter elements are given a larger surface area by flocking. The flocking can serve as an active ingredient depot, or in the wet area as a 'habitat' for microbiology.

4. Usage properties

4.1. Resistances

The requirements for the flocked surface are manifold. These requirements are met by a wide range of suitable materials and processes.
Since a large proportion of flock applications are in the automotive sector, the very high quality requirements of car manufacturers and their suppliers have become the standard.
One important property, abrasion resistance, is given by the fact that each fiber is deeply anchored in the adhesive. This value is measured by the so-called abrasion test.
Another value is provided by the so-called tear-out test. This determines the tensile force required to detach a defined flock area from the substrate. This test can be performed with a tear-out tester.
Further requirements are, depending on the application, light and temperature resistance, or chemical resistance.

4.2. Cleaning

Flocked surfaces are easy to clean. Unlike looped or knitted fabrics, each flock fiber is anchored in the adhesive at only one end. Dirt therefore finds little hold and can simply be brushed or washed out.

4.3. Durability

Flocked surfaces made of robust fibers and a suitable adhesive guarantee a long service life if the flocked surface is evenly loaded. Point loads should be avoided.