THE ELECTROSTATIC POWDER COATING - SFEG

1 THE ELECTROSTATIC POWDER COATING1. IntroductionThis text introduces you to oneof the most environmentallyfriendly and economical tech-nologies for surface COATING ,which permits the carrying outof a wide range of applications,easily and cleanly. POWDER coat-ing has been used since theearly Sixties. Initially, its pro-duction and application werenot without problems. Today,however, POWDER COATING is anestablished and acknowledgedprocess, because powderpaints are:- environmentally friendly- energy saving- safe to process- very economical2. The basic principleThe elementary idea behindelectrostatic POWDER COATING isbased on the fact that partswith opposite electrical chargesattract each other. Therefore,most conducting, and thermal-ly stable solids are suitable forpowder COATING . Above all to-day, metal objects are electro-statically POWDER coated in anever increasing volume. Themost usual examples are:- Household appliances- Office furniture - Garden furniture - Automobile accessories - Fittings - Wire goods- Sections - Cladding elementsDaily, everyone comes intocontact with the results of thistechnology.

2 The dry coatingpowder is filled into a powderhopper, fluidized and conveyedby means of compressed air tothe spray gun. A low-voltage of10 V is converted in the powdergun to high-voltage, accordingto the cascade principle. Oneor more electrodes on the frontof the POWDER gun charges thepowder to 60 - 100 kV whensprayed. An electric field iscreated between the powdergun and the grounded work-piece. The POWDER particles fol-low these field lines andremain adhered to the objectdue to its the residual so processed workpiecescan then be conveyed eithermanually or automatically tothe curing oven, where theorganic POWDER is melted to asmooth film at 160 - 200 C(320 - 392 F) and then hard-ened. Because of its chemicalcharacteristics enamel powderrequires much higher tempera-tures, namely 780 - 830 C(1436 - 1526 F). The coatingthicknesses with organic pow-der for decorative purposes areapproximately 30 - 80 m andfor functional purposes 200 -500 m.

3 Enamel POWDER canbe used in a COATING thicknessrange between 80 - 200 m . 3. The organic powdersEpoxy/polyester POWDER :It is the most widely distributed POWDER with the biggest marketshare. It has very good chemical resistance and excellent mechani-cal characteristics. Its gloss durability and its colour appearanceare, however, mediocre. Its fields of application are industrial equip-ment panelling, office furniture, machine tools, water heaters, radi-ators and POWDER :This POWDER has good chemical and mechanical characteristics, butrenders colour and gloss only poorly. This if further diminished bythe UV rays in daylight. It is used for the underfloor COATING of auto-mobiles, pipe COATING , fittings of all types, in the food processing,and chemical polyester POWDER :It has good chemical resistance and very good mechanical charac-teristics. Its gloss and colour durability are good. It is readily usedfor open-air furniture, farming machinery, air-conditioners, fenceposts, and aluminium sections and architectural POWDER :The POWDER has good mechanical and chemical and colour are very durable.

4 Polyurethane POWDER is used forautomobile wheels, panelling, playground equipment and fluores-cent building POWDER :It is well known for its good mechanical and very good chemicalcharacteristics and for its very good colour, and gloss machines, oven parts, microwave ovens, refrigerators andautomobile bodies (clear coat) are coated with acrylic POWDER :Caution is required when normal and structured powders are usedin the same plant. Contamination can severely affect the coatingquality if the cleaning of the plant is not carried out very carefullywhen changing POWDER :Basically, there are two types of metallic powders. With the first,the metal pigments are mixed together, dry. This POWDER achievesa very nice metallic effect, but a danger of segregation exists. Thispowder is inclined to give difficulties when being recovered. Withthe other metallic powders the metal pigments are extruded in/bonded. With these powders the metallic effect tends to be me-diocre, but the powders do not have problems when being pro-cessed.

5 Metallic powders must never be used with normal Coronapowder guns, as this will lead to short-circuiting, without specialmetallic nozzles. The ion reducing accessory (SuperCorona ) is tobe removed before using metallic powders. As a rule, working withTribo systems is not recommended. Using the same metallic pow-ders with different systems, different plant or also only with differ-ent nozzles give differing colour results! Clear coat POWDER :Thin film POWDER :As long as the plant is set correctly and carefully monitored, thinfilm POWDER does not give any processing problems. The coatingresults to be achieved depend, as with no other POWDER , directly onthe care taken by the operating personnel when using thin filmpowder. Cleanliness in processing and care with the continuousmonitoring of the plant parameters are a must when using best known, but also the most demanding example is the clearcoat POWDER COATING of the BMW car bodies.

6 Clear coat POWDER istechnically not more difficult to process than other POWDER types,but requires considerably cleaner conditions. Clear coat powdercan be processed, by all means, without the clean room conceptwith no problems, when the personnel operate the plant with thecorresponding POWDER :Enamel POWDER is due to its composition very abrasive and requi-res special COATING equipment. After stoving at 780 - 830 C an ena-melled surface is present, which can be heated to a temperatureover 450 C (842 F), and which is colour-fast and has a limited colour palette, but the surfaces finished bythis means display outstanding mechanical and chemical charac-teristics. That is why POWDER enamelling is enjoying an ever in-creasing popularity. The favoured applications of POWDER enamel liewherever especially high requirements are placed on a surface forresistance, such as on cooker panels, kitchen equipment, bathtubsor shower The COATING plantA plant for automatic powdercoating normally comprises thepre-treatment, water drier, COATING zone, curing oven andconveyor system.

7 Such com-plete Industrial plants are moni-tored centrally. According tothe layout more booths withcorresponding conveying cir-cuits may be necessary. Forsmall production volumes or forspecial colours a small boothfor manual COATING is often suf-ficient, which from the price,and space requirements can befitted into any production conveying equipment:Conveying equipment for workpiece transport serves to automatethe COATING process, whereby the primary distinction is made bet-ween overhead, and belt conveying. Many transport problems,especially with not too large or very heavy workpieces, can besolved with a single train or circular conveyor. When long, large orbulky workpieces are to be coated or narrow working spaces arepresent, the Power & Free conveyor is ideal, as this permits lon-gitudinal, and transverse travel as an alternative. Because of thecomplicated design it is more expensive, but permits more flexibleadaptation to continuous and automatic COATING , in cyclic operationduring loading, removal and other processes.

8 A further advantageof the P & F conveyor is possibility of chain formation for opti-mum material flow. Even the lifting and lowering stations can easi-ly be combined. A special observation must be made in connectionwith the lubrication of the conveyor system. Temperatures in thecuring oven may reach 250 C (482 F), therefore heat-resistantlubricants or alternatively, externally ventilated chain protectionchannels must be hanger fixture design: For smooth conveyor functioning suitably designed hangers are es-sential for efficient production. How the workpiece hangers shouldlook depends on the application arrangement. Sufficient stabilityguarantees problem-free COATING and safeguards the productionprocess without notable faults. The hangers are partially coated bythe ELECTROSTATIC , for this reason round material is to be order to coat rationally, two complete sets of hanger are requir-ed: If the COATING on one set of hangers is too thick, the second setare to be used while the COATING is being removed from the firstset.

9 The pre-treatment:Before the workpieces can be coated, they must be freed of allcontamination such as grease, oil This takes place in the multi-zone pre-treatment plant. After cleaning follows mostly, accordingto material, the pickling, phosphating or chromatizing. Generally, adistinction is made between steel sheet, which is phosphated forcorrosion protection, galvanized steel sheet which is pickled to im-prove adhesion, and aluminium sheet which is chromatized. Work-pieces, such as steel containers, wire goods, turned, punched orpressed parts, bicycle parts and accessories pass through the dif-ferent treatment zones on the chain conveyor. The pre-treatmentmust be accommodated to suit the workpieces. An over dimen-sioning means wasting of energy and raw materials. The final coat-ing results depend significantly on the cleaning and for effective environmental protection, and recycling ofthe raw materials are to be principally integrated into the pre-treat-ment residual water dryer:When the workpieces have passed through all the pre-treatmentstations on the chain conveyor, any residual moisture must beremoved.

10 This takes place in the residual water drier. This is similarto the POWDER dryer, but are designed only for temperatures up to120 C (248 F). Depending on the type of workpiece, blowing withnormal air with nozzles may be sufficient; but it always depends onindividual cases, what outlay is required. Pre-treatment for enamel:A prerequisite for the successful use of POWDER enamel is absolu-tely perfectly pre-treated steel. In order to achieve the necessarylevel of quality, pre-treatment lines with up to 20 stages are possi-ble. As a rule, working is reduced to 6 stages (3x degreasing, 2xrinsing, 1x soak cleaning), whereby, this saving is only possiblethrough the choice of suitable COATING materials, as only high qual-ity steel can be pre-treated with only 6 The charging processIn principle three differentcharging processes are usedfor POWDER COATING : electrosta-tic charging, low-grade ioncharging, and Tribo air ion reduction is achiev-ed by practically all POWDER gunmanufacturer s through a spe-cial accessory (SuperCorona )on the POWDER gun.