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Electric field analysis of the tribocharged fluidized bed powder coating process
Abstract
Electrostatic powder coating in fluidized beds is a technique that
has been known for more than 30 years. These systems are predominantly
used for coating of wire-shaped objects and rely on corona charging of
the powder. Experimental studies carried out in a tribocharged system
have shown that the intersection points of a wire geometry rack are
susceptible to insufficient coating. This paper attempts to explain why
the intersection points may present coating difficulties. An analysis of
the electric fields produced around a wire object immersed in a
fluidized bed of charged powder is carried out using commercially
available finite-element software. The finite-element results show that
the intersection points suffer from Faraday-cage effects. However, if
the powder is sufficiently charged, then, as powder deposits on the
substrate in areas where the electric field is strong, the electric
field is enhanced in the original weaker areas, thus promoting powder
deposition toward the intersections. If a powder is insufficiently
charged, no enhancement of the electric field occurs
... Since the 1970s, powder coaters have paid particular attention towards fluidized bed coating processes, being considered low cost and environmental friendly alternatives to traditional coating and painting processes [1][2][3][4][5][6][7][8][9]. Early on, fluidized bed processes were mostly in use to coat small batches of large metallic parts, which were first preheated at high temperature (at least 300-350 • C) and, then, dipped in the bed of fluidized powders [3][4][5][6], generally a thermoplastic polymer. ...
... Early on, fluidized bed processes were mostly in use to coat small batches of large metallic parts, which were first preheated at high temperature (at least 300-350 • C) and, then, dipped in the bed of fluidized powders [3][4][5][6], generally a thermoplastic polymer. Later, more sophisticated fluidized bed processes, based on electrostatic techniques, were adopted to coat also large volumes of small and complex shaped parts [7][8][9]. In that case, the coated parts had to be baked in order to melt the layers of electrostatically deposited powders and consolidate them in the form of a continuous film [8,9]. ...
... Later, more sophisticated fluidized bed processes, based on electrostatic techniques, were adopted to coat also large volumes of small and complex shaped parts [7][8][9]. In that case, the coated parts had to be baked in order to melt the layers of electrostatically deposited powders and consolidate them in the form of a continuous film [8,9]. In recent times, fluidized bed coating has been successfully * Corresponding author. ...
This paper deals with the analysis of the evolution of the surface morphology of metal substrates coated with high-performance thermoplastic powders, namely PPA 571 H, by using electrostatic fluidized bed (EFB) process. Attention has been particularly focused on the relationship between baking time and temperature of EFB coated substrates and the morphological characteristics of the resulting polymeric films.First, thermal behaviour of PPA 571 H polymeric powders was characterized by using standard calorimetric techniques. Accordingly, PPA 571 H melting kinetic was experimentally deduced. Based upon experimental findings, predictive analytical model was also developed and employed to trace ‘iso-conversion’ curves out.Second, metal substrates, made from low carbon steel (AISI 1040), were EFB coated and baked at several baking time and temperatures. Combined analyses of scanning electron and confocal microscopes were led to measure the evolution of the films surface morphology under different baking conditions. Accordingly, a relationship between film morphologies and melting degree was sought. Consistent trends of roughness parameters versus baking parameters were found, with smoother finishes of the polymeric films being achieved for higher degrees of melting, that is, for higher baking temperature and time. Full maps and related analytical models of the finishing levels according to baking parameters were also built up, hence providing first useful indications to powder coaters on how to best deal with their settings.
... At the 3-inch distance between the gun and part, the maximum transfer efficiency was reached at 60 kV [12][13][14]. The methods of coating powder are fluidized-bed coating, electrostatic fluidized-bed coating and electrostatic spray coating [15][16][17]. For the fluidized-bed coating, the products, which are preheated above the melt temperatures of the powder, are dipped into the fluidized bed. ...
... This coating method is used to apply heavy coating in one dip with 75-250 µm thickness. It is possible to build a film thickness of 2.5mm using higher preheat temperatures and multiple dips [15]. The product size is limited and inside corners have low film thickness, owing to the well-known Faraday cage effect. ...
This research deals with the optimization of an electrostatic powder coating by optimizing the combination of total volume of air with the powder output. Generally, the coating's thickness and uniformity are not at the optimum level. A thicker coating does not necessarily provide a good surface quality. This phenomenon is caused by the insufficient volume of air to support the powder reaching the workpiece. Furthermore, too much air also causes the air to blow off the powder on the workpiece. This research aims to identify the optimum combinations of variable parameters to produce the optimum thickness with the best quality by improving the transfer efficiency and specifically to minimize the amount of powder usage. The evaluation methods used to determine the level of coating quality were thickness measurement, Buchholz indentation test, cross-cut test, color visual and surface structure analysis. The instruments used to evaluate the testing samples were a coating thickness gauge, Buchholz indentor, multi-blade cutting tools, spectrophotometer and Scanning Electron Microscope. The optimum level was found when using the optimum combination of powder output and total air volume, which produced a complete single layer of coating film, having good quality in terms of thickness, adhesion, uniformity, indentation resistance and surface structure.
... Electrification of particles is achieved through corona charging in which electrodes are located in the bottom of the bed to charge the particles (Barletta and Tagliaferri, 2006a). Alternatively, particle-particle and particle-wall contacts are exploited for particle tribocharging, alleviating the need of a high-voltage electrode system for corona charging and providing greater safety (Ali and Inculet, 2000). The coating is based on adherence of charged particles to the grounded work-piece. ...
Gas-solid fluidized beds, by their nature, are associated with intense and frequent collisions of solid particles with each other and with the vessel wall, causing tribo-electrification. Accumulation of electrostatic charges in fluidized bed reactors can result in severe problems such as agglomeration, wall fouling, nuisance and hazardous discharge, all reducing the process performance and raising significant safety concerns. Tribo-charging of particles in fluidized beds has also been exploited in a number of useful applications. In this review, the characterization methods of electrostatics and the mechanisms of charge generation and distribution in fluidized beds are presented, followed by an account of the interplay between the hydrodynamics and electrostatic phenomena. Furthermore, techniques of electrostatic charge control in fluidized beds are reviewed, and applications of tribo-electrostatic fluidization systems are summarized. Finally, computational fluid dynamics simulations of the electrostatic effects on the hydrodynamic characteristics of fluidized beds are outlined.
... Since then, more sophisticated FB processes were developed to coat also large volumes of small and complex shaped parts. In the past decade, electrostatic FB was applied by Ali and Inculet [6,7] to deposit thermosetting powder coatings onto rotors. In addition, Leong et al. [8,9] developed a wide range of analytical and numerical models to simulate the deposition process inside a FB of thermoplastic powders. ...
Protective layers were deposited on aluminum substrates by dipping them inside a fluidized bed (FB) of wood and polyphthalamide powders. The experimental investigation looked into the influence of the main process parameters (number and composition of superimposed layers, heating temperature, and dipping time) on the visual appearance, scratch adhesion, wear resistance, and thermal insulation of the resulting coatings. Micromechanical and tribological responses of the coatings were significantly improved by the effect of the wooden particles dispersed inside the polyphthalamide binder. An improvement of the thermal insulation was also achieved whatever the setting of the process parameters. Further, the coatings displayed good adhesion to the substrate and wear endurance.
... In nineties, Leong et al. developed experimental and numerical studies about correlation among film properties and substrate materials and geometries in CHDFB coating process [6]. Inculet and co-workers modelled the relationship between part shape and film uniformity in both electrostatic and tribo-charged fluidized bed [7,8]. More recently, Barletta et al. built first approximation models describing the trend of average roughness of polymeric film deposited by EFB [9,10] and CHDFB [11,12] by varying the deposition conditions. ...
This study focuses on the correlation between the thermo-rheological properties of a thermosetting powder coating system with its surface structure build-up. Epoxy powder coating systems, which displayed surface matting and surface wrinkling, were examined. Firstly, the evolution of the complex viscosity was correlated with the cure kinetic. Secondly, the structure build-up on the surface of the coatings was investigated with a combined SEM-CLA profilometry analysis at different stages of curing process for both EFB and CHDFB coating processes. Different finishes were found to characterize the films applied by using EFB and CHDFB coating processes as a result of the different way the film is heated by. Finally, a strict relationship of film morphology to the degree of conversion and to the evolution of the complex viscosity was found out for both EFB and CHDFB coating processes. The surface structure is built up after gelation point and continues to evolve after gelation with a full development of the film fine structure. Differences were observed in the surface structure build-up when different curing temperature was used, thereby indicating an influence of minimum viscosity on achievable finishing.These experimental results lead to further advances in a better understanding of the formation of surface topography and morphology of polymeric films. They also provide important indications for the settings of curing parameters in both EFB and CHDFB coating processes as well as for the development of new powder coating formulations.
... Besides, the running and control of coating processes as well as the settings of operational parameters were entrusted to the experience of skilled technicians (Guskov, 2002), to the politics of small adjustments (Barletta and Tagliaferri, 2006a) and, at best, to the employment of empirical models (Barletta and Tagliaferri, 2006b). Nevertheless, process modelling and optimization are very important issues in coating engineering and coating processes are too complicated to be empirically managed (Guskov, 2002), to justify the use of simple analytical models often based on many assumptions, which contradict reality (Barletta et al., 2006) or to have recourse to complicated finite elements (FE) (Barletta et al.) or volume elements (VE) models (Ali and Inculet, 2000;Ye et al., 2002), which cannot be implemented on line and often require laborious calibration procedures and heavy and recursive solution methods (Ye and Domnick, 2003). ...
This paper involves experimentation on coating process of metal substrates in an electrostatic fluidized bed (EFB). Several operational parameters were covered like coating time, applied voltage and gas flow rate fed to the fluidized bed.First, a design of experiment (DOE) approach was used to define the experimental campaign and a general linear model based on analysis of variance (ANOVA) was used to elaborate and interpret the influence of all the operational parameters on coating thickness trends.Second, the experimental data were modelled using artificial neural networks. Different neural networks and training algorithms were employed to find the best technique to predict the coating thickness trends. The reliability of the best neural network solutions was checked by comparing them with a built ad hoc regression model. The multi-layer perceptron (MLP) neural network trained with back-propagation (BP) algorithm was found to be the fittest model. Besides, a genetic algorithm (GA) was also employed to improve the capability of MLP model to provide the best fit of experimental results all over the investigated ranges.Finally, a verification experimental plan was performed and a related analytical model was developed to check the reliability of the neural network model with GA to predict the whole coating thickness trends according to the operational parameters. A comparison between the neural network model and an analytical model was also carried out.
... These results are in agreement with what is reported in the pertinent literature [8,[28][29][30][31] and, above all, with what found by Barletta et al. in some previous papers [32][33][34][35]. In particular, Barletta et al. found similar decrease in coating thickness at high-applied voltage in electrostatic spraying of both standard and specialty painting powders on metal and plastic substrates [32,33]. ...
Electrostatic spray deposition (ESD) of aesthetic and protective low curable transparent powder coatings onto carbon fibre-reinforced epoxy composites (i.e., carbon laminates) is the matter of the present investigation. An original environment friendly pretreatment of the substrate, based on fluidized bed peening of glass beads followed by a moderate temperature oven baking, has been proposed. Then, the influence of ESD operational parameters on coating performance has been looked into.Design of experiments (DOE) was used to schedule the experimental trials. Coating thickness and its uniformity over the coated substrates upon curing was systematically evaluated. Further, visual appearance of the coatings was analyzed by both optical and stereoscopic microscopy. Finally, analysis of variance (ANOVA) was performed to model the available experimental data. Detailed examinations of the experimental results allowed to define the best settings of ESD process as well as the maximum deposition time before the occurrence of severe electrical breakdown in the powder layer and/or the increase in the incidence of massive back-ionization phenomena. Accordingly, 3D process maps of the coating thickness versus the operational parameters, applied voltage, feeding and auxiliary pressure, were developed, thus supporting the practitioners in their choices and in the identification of processing windows wide enough for practical purposes.
... In fact, just one analytical model is proposed in the literature and it is referred to thermoset coatings (Barletta and Tagliaferri, 2006b). Besides, the available finite elements models are mostly focused on the determination of electrical fields and they do not provide any coating thickness trends (Ali and Inculet, 2000a). Anyway, they can be assessed as first approximation models, plenty of limiting assumptions and lacking of accurate calibration procedures based upon reliable experimental data (Ali and Inculet, 2000a, b). ...
This paper deals with fluidized bed coating of metal substrates with high performance thermoplastic powders (polyftalamide, PPA). Two different experimental scenarios were investigated: the conventional hot dipping fluidized bed (CHDFB) process and the electrostatic fluidized bed (EFB) coating process. The preliminary experimental plan was scheduled employing design of experiment (DOE) technique. Three experimental factors and operative ranges large enough for practical purposes were considered in both of the examined scenarios. In particular, coating time and airflow rate were chosen as experimental factors in both CHDFB and EFB. The third factor was the preheating temperature of metal substrates in CHDFB and the applied voltage in EFB.A general linear model based upon analysis of variance (ANOVA) was used to evaluate the significance on coating processes of each experimental factor. Main effect plots (MEPs) and interaction plots (IPs) of coating thickness were drawn. Trends consistent with the settings of the operative parameters were displayed.The experimental trends were first modelled by numerical regression of the experimental data and, subsequently, by using artificial neural network. The reliability of the neural network solution and of the built ad hoc regression models was comparatively evaluated. Multi-layer perceptron (MLP) neural network trained with back propagation (BP) algorithm was found to be the most valuable in fitting the coating thicknesses trends for both the coating processes. Examining the developed models outside the operative ranges they were designed for, the good generalization capability and high flexibility of the neural network solution was definitely stated.
A comparative evaluation of electrostatic spray and ‘hot dipping’ fluidized bed to deposit two different organic paints belonging to the class of thermoplastic (PPA571, an alloy of acid modified polyolefins) and thermoset (TGIC-free transparent pigmented bronze polyester) powders was performed. Visual appearance of the investigated coatings was evaluated by colour, gloss and coating thickness measurements as well as by the determination of the surface morphologies. Micro-mechanical performance of the coatings was assessed by progressive load scratch tests.‘Hot dipping’ fluidized bed is found a fast deposition technique as, after substrate pre-heating, it takes just few seconds to have the part completely powder coated. On the other hand, electrostatic spray deposition is a potentially selective coating technique, but it lasts longer (generally, 6–15 s) and, moreover, the coated parts must be post-cured for long time (at least, 15 min) and at high temperature (150–200 °C) to give rise to the formation of continuous films. Indeed, whilst ‘hot dipping’ fluidized bed is found particularly suitable for the deposition of thick and smooth thermoplastic coatings, electrostatic spray deposition is found the most viable technique to deposit thinner and highly scratch and wear resistant thermoset coatings.
References on surface modification process simulations from `Coatings and surface modification technologies: a finite element bibliography (1995–2005)'.
In surface treatment of paper substrates the development is towards more compact and all-online surface treatment processes. The dry surface treatment (DST) process combines both the conventional coating and calendering processes. In this process, the wetting–drying cycles of conventional surface treatment processes are excluded. In DST the dry coating powder is electrostatically applied on the paper surface and after that thermomechanically fixed. DST provides possibilities to considerably reduce both investment and production costs for a similar paper quality as with conventional water-based coating methods. The reduced investment costs can be achieved through a more compact process with combined application and surface smoothening in C2S configuration. This study focuses on the simultaneous dry surface treatment of both paper sides (e.g. C2S) and the application process solutions. The electric fields in the application units and particle trajectories in those fields were modelled in this study. A laboratory-scale application unit was constructed to study the behaviour of coating powder in simultaneous electrostatic application on both sides of the paper. The charge of the particles in relation to their size and the coating powder composition as well as the charge decay properties of base papers and coating layers were determined. Rather homogeneous electric field strength distributions were achieved. The effect of the electric field on the particles and on their trajectories was strong, which allows a compact application process. Charge decay properties indicated sensitive electrostatic behaviour and depended mainly on the moisture content of the base paper and the composition of the coating powder. However, decay properties enable electrostatic deposition in alternating process environments in dry surface treatment of paper substrates.
The electrostatic fluidized bed deposition of a single-layer PPA 571 coating onto low carbon steel rods is reported.A full factorial experimental design was developed in order to study the influence of several operative variables on the effectiveness of the coating process and on the coating thickness and uniformity. The operative variables included exposure time, air flow, the applied voltage, attitude, and the radial and vertical location of the work-piece in the fluid bed. After the experimentation, several process maps were developed as a support to identify the best way to lead the coating process. Finally, by using a statistical approach, the reliability and repeatability of the coating process was also established.Experimental trends were consistent with theoretical expectation. A significant growth in achievable coating thickness was obtained by increasing voltage and air flow. Furthermore, at higher values of exposure time and applied voltage, relevant back ionization phenomena occurred, which simultaneously caused a top limit in coating thickness and a worsening of surface finishing.Process characteristics, leading mechanisms, and some practical aspects are also discussed in detail.
This paper gives a bibliographical review of the finite element methods applied to the analysis and simulation of coatings, their mechanical and material properties from the theoretical and application points of view. The surface modification technologies in the context of coatings are also included. The added bibliography at the end of this paper contains 1032 references to papers and conference proceedings on the subject that were published in 1995–2005. The following topics are included: coating property simulations; surface modification process simulations and practical coating applications.
Experimental analysis of the electrostatic fluidized bed coating process of carbon steel thin sheets was performed using two different mesh-sized epoxy–polyester powders. In particular, systematic analysis of the influence of the operative variables on coating and process variability in the field of low film thickness (below 100 μm), was carried out, which for electrostatic fluidized beds has previously not been considered feasible.In this context, the effect of varying both the electrostatic and fluidized bed setting on the amount of the powder layer coating the work-piece surface was studied. In particular, exposure time, applied corona voltage, and airflow rate were found to influence the performance of the coating process significantly.Experimental results produced coating thickness trends consistent with theoretic expectations. In particular, effects on the coating process of both electric and aerodynamic forces, which in turn affect the powder particles drawn up to the work-piece surface could largely be predicted by examining experimental data. A set of 3D maps of the coating thickness achieved are also reported as they provide a useful tool to monitor the process in a 5–15 s time range, applied voltage from 45 to 90 kV, and flow rate from 3 to 11 m3/h.
Space charges play a key role in many industrial and agricultural applications such as electrostatic painting or application of insecticides in orchards and row crops. For generation of large sized space charge clouds, induction charging is the preferred method. Whereas corona charging often involves voltage in the order of 100 kV, induction charging may be achieved with voltages of less than a few thousand volts. The paper presents a novel construction of a spray unit with induction charging suitable for injecting large volumes of ionic space charge into a region. The experimental unit comprises: a flat spray liquid atomizing nozzle with high pressure air; two large size induction electrodes, and, an air curtain surrounding the electrodes to prevent the fine droplets from landing on the electrodes. The two large, flat induction electrodes sandwiching the flat spray have shown to generate excellent induction charging on the spray droplets achieving charge to mass ratios in the order of 4 mC/kg and ionic charge densities of 2 mC/m/sup 3/. The studied variables were: pressure and flow of water supply at the nozzle and the pressure and flow of the curtain The atomized charged spray cloud of droplets with a mean volume diameter of several micrometers, evaporates at certain distances from the nozzle forming a mixed charged cloud of both water ions and finer water droplets.
J A Cross 1987 Bristol: Adam Hilger xii + 500 pp price £50 (I0P members' price £40) ISBN 0 85274 589 3
The specialist field of electrostatics has been relegated to curiosity value for far too long. This is a great pity, especially when so many modern industrial applications rely on fundamental processes of manipulating electrical charge on to particles, droplets and surfaces.
During the transport of insulating powders or granules, a very high electrostatic charge may be generated. The resulting accumulation of charge in storage silos gives rise to electric fields which, when the breakdown field strength in air (3 MV/m) is reached,can lead to a cone discharge. Depending on the surrounding medium, this may result in an explosion or a fire. More exact knowledge of the electric field in silos is therefore of prticular importance. However, since the methods currently in use only permit the measurement of the electric field at particular points, a mathematical model based on the finite-element method [1] has been developed to support the experiments. In close cooperation with the experimenters [2], we simulated the operating conditions, in parallel with the silo measurements. Important qualitative findings were made on the basis of the calculated field and potential distribution and energy considerations.
A fuller understanding of the transient electrical phenomena that
occur during a charged-pollen transfer process is essential to the
successful development of an effective electrostatic pollination system
for greenhouse crops. This paper describes a 3D model for computing the
electric field produced by a pollen cloud having unipolar space charge
as it approaches and enters a flower. The 3D finite element model is
coupled with the dynamic space charge and the resulting transient
boundary potential on the flower surface. These transients were based
upon charge relaxation of individual flower components calculated using
experimental values for component conductivity and permittivity. With an
initial space charge density of 25 pC/m<sup>3</sup>, a model squash
flower and charged-pollen system was analyzed. For instantaneous
introduction of the space charge, results show that the initial electric
field is 38% of the maximum electric field which subsequently occurs as
displacement currents return the flower components to earth potential.
The calculated electric field at the location immediately above the
flower stigma is found to be approximately 3-fold greater than field
values above the petal edges. The demonstrated focusing of the electric
field onto the flower stigma is of considerable biological significance
since this is the desired target for pollen deposition. The results
provide additional important information needed for prediction of the
movement of charged pollen particles within flowers and are useful in
specifying design criteria for engineering an effective electrostatic
pollen deposition process
The Technology of Powder Coatings
- S T Harris
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User's Guide To Powder Coating
- E Miller
E. Miller, User's Guide To Powder Coating, 2nd ed. Dearborn, MI: Soc. Manuf. Engineers, 1987, pp. 60–65.
Powder Coating The Complete Finisher's Hand-book
- L N Liberto
L. N. Liberto, Powder Coating The Complete Finisher's Hand-book. Alexandria, VA: Powder Coating Inst., 1994, pp. 108–110.