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.

CONVENTIONAL EQUIPMENT USED FOR FRP

EQUIPMENT FOR GENERAL USE:

1.      Resin mixers

2.      Impregnation equipment

3.      Vacuum equipment

4.      Presses

5.      Miscellaneous equipment

1.         Resin mixers:

Resin mixing equipment runs the entire range from the bent rod in the hand drill motor to the continuous vacuum deaerated, proportional pump, in-line mixer.

The type of mixing equipment depend on the type of mixing manufacturing operation e.g. short runs of resin mix employs propeller type mixers. Some other typical mixers are lightening portable drum mixer, converted bakery dough breakers and other types of rotary and planetary paddle mixers.

For high production moulding and encapsulation, a metered, shot dispensing mixing equipment is used to

a.       Eliminate pot life problems

b.      Provide shot weight control

c.       Allow multi-station dispensing from a single system

d.      Improve house-keeping.

For continuous laminator or extruder, an in-line, proportional metering pump system is used to

a.       Get higher running speeds with hotter catalytic system

b.      Provide steady controllable flow

c.       Provide uniform mixture

For boat and other large area of hand lay up or spray up components, a pressure pot, spray mixing unit is used because of their compatibility with the rest of the processing equipment and the ability to deposit a relatively large volume of resin mix over extensive surfaces rapidly.

Regardless of the system used, they must be accompanied by the mix and/or the base resin if high quality end product is to be attained. 

2.         Impregnation equipment:

Different equipment used for impregnation have basically the same operating principle but they vary widely in the specific details. The four common preimpregnated form of reinforcement are fabrics, rovings, mats and chopped fibers.

Impregnation of sheet material (fabrics, woven roving and SMC material) produce usually a B-staged material or SMC. While the impregnation of BMC material (roving as continuous fibers or as chopped fibers) may or may not result in B-staged stock.

The impregnation equipment basically consist of

a.       Unwind stations for PE support films (top and bottom)

b.      An unwind station for reinforcement

c.       Impregnating area where the resin is applied

d.      A compacting area

e.       Oven curing tunnel or tower for B-staging

f.       Rewind station where the laminate is wound.

However for production of chopped fiber SMC, the sheet is formed at the machine and the impregnation is somewhat more complicated.

Application and control of resin:

It varies widely from station immersion and squeeze rolls to doctor blade techniques and many forms of roll coating. Complex control and read out system must be used in the B-staging area because proper temperature zoning and control is extremely critical to achieve quality and reproducibility. Many types of heating, depending on local economics, are used like gas, electricity, steam etc.

Impregnation of continuous fiber roving:

Here the fibers are simply run through a resin bath, then through squeeze rolls or a compaction die and then the fibers are directly applied to the part.

Preparation of premix BMC:

It consists of mechanically mixing chopped fibers, resin, catalysts, fillers, and other additives by various methods. One method employs a water jacketed chamber and spiral or sigma mixing blades and is used for batch preparation. Such a system contains automatic weighing and feed of the material. Continuous mixing and extruding equipment is also used. Where a rotating screw similar to the conventional extruder is used except that the L/D ration is less to minimize the glass fiber degradation. Automatic feed and metering mechanism assures a steady, proper flow of ingredients to the mixer. The product can be a putty like rope which then can be simply cut to length for moulding, thus eliminating the weighing operation.

Other continuous system for premix include:

a.       Chopping of the impregnated roving and then pelletizing, then running resin and reinforcement through meshing gears.

b.      Chopping the roving and impregnating the fibers on a belt.

3.         Vacuum equipment:

To choose the type of vacuum pump for a reinforced plastic plant, the specific operation must be determined e.g. In plants for vacuum bag lamination, small individual pumps are preffered than a central system because the pump can move with the job. While in a plant where primary de-aeration or vacuum impregnation is used, a central system with large receiver is desirable. In any case, precautions should be taken to insure adequate trap capacity between every job and the pump otherwise the system will not function for long time. Even with the best trap and filter system, some of the resin reach the pump in vapour form. The floating eccentric vane type of cylindrical pump can tolerate this type of service better.   

4.         Presses:

Specialized presses for the reinforced plastic differ from most other types of press in that they have relatively low ram forces compared to their platen area. However the premix moulders are looking for increased tonnage and faster closure speeds and the military job shops, moulding ablative components for rocketry use extremely high moulding pressures on outside components.

5.         Miscellaneous equipment:

Depending on the particular process being used, all or only part of the following equipment can be used:

a.       Various capacity scales for weighing the ingredients

b.      Several types of fiber cutters

c.       Spray equipment

d.      Polishers

e.       Rollers

Barcol hardness tester.