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face. Final melting and full cure of ther- moset powder coatings occurs when the part is further heated to the prescribed metal temperature. With thermoplastic powder coating, the part is further heated to achieve the desired surface smoothness, texture, or both. Film thickness is controlled in this process by the heat of the part and the time the part is in the fluidized bed. Electrostatic fluidized beds use ionized air to charge the powder particles, which are attracted to a grounded and cool part transported either above the bed or into the bed. Varying the time in the bed and the charge on the powder particles controls the film thickness on the part. After the parts are coated, they need to be heated to melt/flow the material and, in the case of thermoset powder coatings, to fully cure. Spray application techniques for pow- der coatings can be broken down into several categories: corona-charging guns, tribo-charging guns, flame- spray guns, corona-charging bells, and tribo-charging discs. All of these meth- ods require powder to be pumped from a vibrating box feeder, fluidized hop- per, or gravity hopper. The pump uses compressed air to draw powder from the box or hopper (Figure 4) through the use of the venturi principle, which propels it to the spray device via a powder feed hose. Both powder vol- ume and transport speed can be ad - justed at the spray apparatus control panel. Atomization of the powder par- ticles occurs at the spray device where the particles are deflected, spun, directed, or air-atomized into a well- dispersed cloud. The powder within this cloud is electrostatically charged or melted in the case of the flame-spray gun. The powder particles are then attracted to a grounded part to form a continuous film between 1.0 mil and 10.0 mils thick, depending upon the desired application constraints. Film thickness tolerances can be controlled to ±0.20 mil in a well-designed and closely controlled process. Corona-charging guns (Figure 5) and bells use an electrostatic generator that creates an electrostatic field between the gun and the grounded part. The powder particles accept the electro- static charge as they penetrate this field and are attracted to the grounded part. Electrostatic voltages are adjustable at the control panel up to 100 kilovolts. The current within this field can approach 80 micro amperes. Some units have automatic feedback systems that vary voltage to maintain a constant current within the electro- static field. This leads to more consis- tent film thickness control and makes it easier to powder coat complex shapes. Corona guns are most com- monly used today to coat a variety of products. The powder pattern with guns is achieved by using conical deflectors, fan spray tips, or pneumatic atomizers. Selecting the right pattern control device will allow for large well- dispersed patterns useful in coating flat areas or narrow and focused patterns designed to penetrate recessed areas. Corona bells are used in applications in which large flat areas need to be coated at a high rate of speed. If you have to spray a lot of powder in a short period of time onto a relatively flat surface, then these are the preferred application devices. Typical bell appli- cations are automotive car bodies and appliance outer shells. The charging technique is the same as the one used in corona guns, with similar effect. Tribo guns (Figure 6) and discs use fric- tional charging techniques to charge the powder particles. Powder particles develop this charge by the rubbing action caused by meandering charg- ing/transport channels molded into the gun/disc body. These channels are normally much longer than those used in corona equipment to ensure that each powder particle has had an opportunity to accept a tribo charge caused by the rubbing action between these channel surfaces and the parti- cle. The amount of charge imparted on the powder particles is directly related to the materials sprayed and the com- position of the channels along with the duration of frictional contact between the particle and the channel surface. These powder application devices are more suited to overcoming Faraday- cage problems than corona-charging application devices because they don't use electrostatic fields to charge the powder particles. Flame spray powder coating equipment is typically used to coat large objects with thermoplastic powders where using an oven is impossible. This equip- ment ignites liquefied petroleum gas (LPG), or other combustible gasses, at the gun tip where the resultant heat combines with thermoplastic powder pumped by compressed air from a feed hopper. The heat melts the powder as it is propelled to the part surface, result- ing in a continuous protective coating (8.0 mils to 10.0 mils thick) that is both durable and highly corrosion resistant. Flame-spray devices are used to apply protective thermoplastic powder coat- ings to very large parts, such as water tanks, tanker railcars, and bridges. Powder spray booths and recovery sys- tems. Powder spray booths differ from liquid spray booths in their filtration techniques and fans. In liquid sys- tems, simple, and relatively coarse, fil- ters stop paint droplets while the sol- vent fumes are exhausted from the plant airspace. Powder booths use much finer filters than liquid booths to stop small powder particles and, in most cases, return the air back into the plant airspace. Because the filters are finer and hold some of the waste powder onto their surface, the fans used in powder booths are typically rated at a higher static pressure than those used in liquid booths. All powder coating booths are de - signed to accomplish the same goals: containment of the powder particulate Figure 6 Tribo-charging spray guns Photos courtesy Nordson. (a) Manual (b) Automatic POWDER COATING, December 2018 27 POWDER COATING OVERVIEW