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This experimental study investigated the influences of two different powder systems (coarse and ultrafine) on particle charging and deposition characteristics during electrostatic powder coating processes. Results disclosed that, despite their differences in particle sizes, the two powders behave similarly in deposition process, commonly resulting in a cone-shaped deposited pattern in the inner portion of the substrate and an increase of deposited particles in the fringe region. However, their different properties lead to the discrepancies in their deposition efficiencies, which account for a higher efficiency with the coarse powder. The study further revealed that the coarse powder is superior to the ultrafine powder in charging in-flight particles, which directly contributes to its higher deposition efficiencies. Furthermore, it was disclosed that the two powders exhibit distinct characteristics in charging deposited particles. Compared to the coarse powder, the ultrafine powder is more uniform in charging deposited particles, mainly owing to its greater particle number and higher specific surface area but less mass. In particular, the charging efficiency of overall deposited particles decreases for the ultrafine powder but increases for the coarse powder with increased charging voltage, closely related to their particle properties. However, both powders decrease in charging efficiency of deposited particles with extended spraying duration due to back corona intensifying with spraying.

A simple electrostatic model is applied to the charge powder coating of a grounded conductor eventually covered by insulating layers. The electric field inside the powder coating and its evolution during the process are established with also the corresponding evolution in the dielectric layer and some practical consequences are also discussed. The thickness of the charged powder layer is limited by two types of process: a self-limiting process related to the vanishing field in the air gap and a field strength process occurring on one of the two sides of the coating–dielectric interface. Inside the powder coating, the electric field induces an increased electrostatic pressure on the powder grains at the substrate–coating interface and a vanishing pressure on the grains at the coating–air interface. This internal field is amplified into air bubbles and it may be responsible for the back ionisation process and of the formation of moon craters via the ionisation of the air molecules followed by the pressure exerted by the ions pushed in the direction of the free surface. For each of these limits, analytical expressions are established and they permit to identify the role of physical properties of the deposited powder (particle size and dielectric constant) as well as the role of thickness, structure, and dielectric constant of the insulating substrate. The present approach explains the difficulty in obtaining thick coatings on thick insulating substrates or thick coatings from the use of too fine powder grains. Finally, different behaviours as a function of the size of the deposited powder grains are deduced from the contribution of the electric field to the velocity of the particles sticking to the surface. Then and for the first time, the present contribution underlines the important role of the subsurface composition and the need to characterize the structure, dimension and dielectric constant of this subsurface for various applications concerning electrostatic spraying and powder painting of plastics and of insulating coatings on metallic work pieces.

Ten powders, differing in protein, carbohydrate and salt contents and ranging from 19 to 165 μm were coated by nonelectrostatic and electrostatic coating. Nonelectrostatic transfer efficiency (TE) increased to a maximum before leveling off with increasing particle size. Electrostatic TE either decreased or increased then decreased with increasing particle size. Powders became more free flowing as particle size increased. Since TE increases as powders become more free flowing, TE increased with particle size for both nonelectrostatic and electrostatic coating. For electrostatic coating, the effect of charge decreases with increasing particle size. Thus, the conflicting effects of ability to pick up charge and flowability caused an increase then decrease in the TE for powders coated electrostatically, and can also explain the exceptions. The average improvement in TE by electrostatic coating was 20%, with the improvement increasing as particle size decreased.

Electrostatic adhesion is a significant force in painting and other industries. However, coating of food items uses powders and targets with properties that are far from ideal for electrostatic coating. In addition, the adhesion must last a long time in order to be useful. The goal of this study is to determine if electrostatic adhesion is significant in food coating. A range of food powders were coated onto different kinds of food samples electrostatically and nonelectrostatically and the adhesion measured. Electrostatic powder coating increased the adhesion of most food powders onto most food targets. Some correlation was found between effectiveness of the process and resistivity or oil content of the target, resistivity of the powder, and particle size. At low relative humidities, electrostatic adhesion lasted for several weeks, long enough to be valuable.

Powder coating has several key advantages over liquid coating, and fine powder coating makes the surface finish quality comparable with liquid coating. This work reports on the numerical simulation of a fine powder-coating process in comparison with coarse powder coating, using a commercial computational fluid dynamic code, Fluent v6.1. The purpose of the study is to understand the gas and particle flow fields inside the coating booth for various operating conditions and the effect of reducing particle size on the coating process. The air and powder particle flows in a coating booth were modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow was calculated by solving the Navier–Stokes equations, including the standard k–e turbulence model with non-equilibrium wall function. The discrete phase was modeled based on the Lagrangian approach. In addition to drag force and gravity, the electrostatic force including the effect of space charge due to free ions was considered in the equation of motion and implemented using user-defined scalars and user-defined functions. The governing equations were solved using a second-order upwind scheme. This study demonstrates that the use of finer particles of size 15 μm or lower can give a very smooth and uniform surface finish, which may serve the requirement of automotive top-clear coating. This also provides useful information about optimum operating conditions such as the airflow rate, the applied external voltage and the powder-spray rate. The numerical model can also be used to optimize the gun-booth design for a better coating efficiency.

Measurements have been made of the deposited powder layer for conventional electrostatic corona powder guns and triboelectric guns. By selectively removing the powder layer under computer control conditions, measurements of the powder thickness, and the adhesive properties of the powder layer have been obtained for variation in the coating process and detailed comparisons have been obtained for both coating systems.

Previous studies on electrostatic coating of food powders using negative corona have showed benefits such as an increase in coating deposition and a more even coating. However, no work has been done on the advantages of positive corona. Thus, this study was aimed to determine whether positive or negative corona produced better coating for different food powders. Twenty-three powders were coated onto five aluminum strips using an electrostatic powder applicator. Transfer efficiency (TE), adhesion and dustiness were measured and correlated to particle size, flowability and tribocharging value. The polarity of the tribocharging value for each powder determined whether positive or negative corona produced higher TE. For most proteins, positive corona produced higher TE than negative corona since proteins tribocharge positive. Most carbohydrates tribocharge negative; thus negative corona produced higher TE than positive corona. Salts had relatively small tribocharge values; thus there was no difference between TE of positive and negative corona. No significant difference was observed between positive and negative corona for adhesion and dustiness, though electrostatic coating produced higher TE and adhesion and less dust than nonelectrostatic coating.

The total forces holding a layer of electrostatically charged powder onto a metal work piece results from the combination effect of an interfacial electrochemical phenomenon, an inter-particle/particle cohesive force and a bulk electrostatic image attractive force. An attempt has been made to obtain the total integrated effect by subjecting the powder coating to a mechanical impulsive force parallel to the metal substrate. This is a destructive measurement which is obtained by dropping the work piece down a vertical chute which incorporates an inductive charge pick up device to measure the charge of the powder coating during free fall. The sample is brought to rest at the bottom of the chute and dislodged powder is collected in a micro balance incorporated inside a Faraday cage. The exact charge/mass ratio of dislodged powder can be obtained and the technique enables the calibration of the inductive probe. All the measurements are taken with the powder layer before fusing. LabVIEW by National Instruments is a user friendly and powerful graphical development environment used for signal acquisition, measurement analysis, and data presentation. It is a package designed to acquire, analyse and present. Data acquired from the unit under test can be fed through DAQ cards, instruments or PXI systems and connected to a PC running LabVIEW for analysis. With multiple built-in functions and libraries that cover functions such as analysing and processing and also data visualization and report generation, LabVIEW is highly useful in this application.

In this paper results of investigations are described aiming to numerically simulate the electrostatic powder coating process using an extended commercial computational fluid dynamics (CFD) code. The fully three-dimensional turbulent flow was calculated. Based on the Lagrangian approach the trajectories of the powder particles were modelled considering electric and aerodynamic forces. In the calculations of the particle propagation both the particle size distribution and the particle charge distribution obtained through experiments have been applied. The model accounts for the space charge effect of the charged particles and the turbulence dispersion on the particle trajectories. It was found that the space charge plays an important role for the final spray pattern shape, also increasing the transfer efficiency. The numerical results, such as velocity profiles, static and dynamic film thickness on the target were in good agreement with experiment.

Measurements have been performed with different electrostatic powder coating systems to measure the electrostatic charging processes. The measurements consist of spraying a fixed mass of powder through a powder gun and then analysing the powder coating to determine the optimum parameters. The techniques, which are presented, include (i) the absolute determination of the charge/mass ratio for a tribo gun, (ii) the analysis of a powder deposition on a metalised glass substrate to determine the powder thickness and the adhesion of the powder to the substrate for standard and modified powder coating guns.

The total forces holding a layer of electrostatically charged powder onto a work piece result from the combination of an interfacial electrochemical phenomenon at the surface, inter particle/particle cohesion and a bulk electrostatic attractive force. An attempt has been made to obtain the total integrated effect by assessing the force that is required to selectively remove the powder layer from a metal substrate. This is a destructive measurement that disrupts the powder layer. All the measurements are taken with the powder layer before fusing. Powder adhesion measurements have been obtained from an impulse drop test, incorporated with a charge pick up device (CPD), Faraday cup, micro balance, Keithley Electrometer and CVI National Instrument Virtual Oscilloscope. Sample results are presented for powder coating using standard tribo and negative corona gun.

A new powder feed system for powder coating is developed with the aim of the application of powder coating to high-grade finishing like clear topcoats for automotive bodies. In the new system, a capacitance-type sensor is installed, and the system for the closed-loop control based on the detected data is constructed. The operation principle of the system is described in this paper. Next, experiments for performance evaluation of the system were carried out. Then, the discussion on the experimental results is also carried out. Concretely, it is confirmed that the accuracy of powder feed is within a preset value ±3%. The quantification of conveying air flow rate is also achieved along with stable power feed. In addition, the response speed within 0.5 s is also realized on the responsiveness of powder feed by the speedup of the valve operation.

The measurements of the temperature distribution were taken inside a charged, 1 mm thick nylon uncured powder layer coating to be cured over a metallic surface with a filtered IR radiation. An optical filter used in the test allowed the transmission of only the IR wavelength below 2.9 μm. The experiments showed that the tested nylon powder layer close to the metallic surface would be initially exposed to a curing temperature lower than the temperature at the surface of the coating. Hence, the cured surface layer and some of the powder may fall off during the curing. Further work to study the temperature distribution which develops on various powder materials and particle size may help the technology when thick cured powder coatings are necessary.

Fluidization of polymer powders is used extensively in the powder coating industry. During the fluidization process the paint particles become electrically charged due to particle–particle and particle–wall interactions. In the course of this tribocharging, or friction charging process, it has been observed that a bipolarity of charged powder exists. The smaller, or finer, particles tribocharge oppositely to the larger size particles; thus, there exist both positively and negatively charged particles within the fluidized bed. A simple technique for charge measurement and separation has been developed whereby a small sample of bipolar charged powder taken from the fluidized bed is poured from a height and a separation occurs between the majority and minority particles of opposite charge. The amount of separation is a function of charge on the individual particles and the height at which the sample is poured. A computational model, based on techniques of molecular dynamics, has been developed to simulate the trajectories of individual charged particles when a sample of bipolar charged powder is poured. The model takes into account electrical, drag and gravitational forces acting on each particle and confirms the statistically analyzed experimental results.

The electrostatic powder coating process is reviewed. Three basic regions of a powder coating system are considered. The charged powder source region embraces corona and triboelectric guns. These are reviewed and particle charging processes considered. The second region where charged powder transport to the workpiece occurs is one in which aerodynamic, space charge, and gravitational forces act to determine particle trajectories. With corona sources ions generally outnumber charged particles by 10 to 1 so that the space charge due to charged particles is negligible compared to that of ions. Charged particle deposition onto the workpiece, the third region, is affected by the electric field of particles already deposited. Back ionisation may occur if ions are present and lead to poor coating finish. The adhesion of charged particles to a workpiece is often greater than expected by consideration of image forces alone.

A numerical algorithm for simulating the electric field distribution and particle trajectories in tribo-charge powder coating systems is presented. It combines the finite element method for the electric field analysis and the numerical integration of the equation for the particle trajectories. The particle-in-cell technique is used to couple both methods, which are applied iteratively until a self-consistent solution is obtained.

The application of electrostatic powder coating technology to the processing of thermoplastic composites is discussed. An electrostatic powder spray gun is used to deposit polymer powder on a continuously moving glass fiber bundle. The glass fiber bundle is locally spread in the region of powder deposition to expose the filaments. A three-dimensional finite element model is presented for predicting the electric field strength due to the applied corona electrode. The model provides the spatial variation of the electric field vector for the specific spray process geometry. Corona voltage, corona electrode height, fiber bundle spreading, and space charge density were varied in the model to predict their effect on the electric field strength. The finite element results display the “wrap-around” effect of the electric field to the bottom surface of the fibers. Experimental results substantiate the electric field distribution along the length of the glass fiber bundle.

The minimum ignition energies (MIEs) of more than 40 coating powders were determined. It turned out that the MIE depends on the particle size distribution or the specific surface area of the powder calculated from it, the chemical substance group of the resin used, the powder agglomeration and the resin content. Minimum values of 1.7 mJ resulted for transparent epoxy coating powders (d 50 · 25 μ m). The evaluation of the results from the point of view of safety showed that no hazardous situation will arise when these powders are properly sprayed using hand-held spray guns tested according to EN 50 050.

A diagnostic instrument has been designed to analyse the adhesive force of an electrostatically deposited powder coat. The adhesive force is directly related to the efficiency of charging the powder and results are presented of the different charging efficiencies for high oltage corona guns, low voltage ion free corona guns and triboelectric guns. The instrument has also been used to assess the characteristics of different types of powders including epoxy and polyester mixes and also the effect of particle classification. In addition tests will be described to show how the diagnostic instrument can be used to improve the coating efficiency including overcoming the Faraday effect of re-entrant objects.

INTRODUCTION Powder coating can be controlled by a number of parameters which can be regulated easily; such as the voltage of the powder gun, the spraying distance, the diameter of the powder cone, the powder and flow rate. Sufficient information has been obtained on the optimal tec~hnological parameters of powder spraying on conveyer belts and by hand (ref. 1,2,3,4). This da~a, however, is not sufficiently generalized. As a result, each practical task of obtaining a coating for commercial purposes requires a large amount of experimental work. In this paper an attempt is made to use experimental data on physical processes taking place in electrosbatic powder coating, the results of technological experiment and practical experience in order to give recomendation for the optimization of powder spraying.

Both corona and tribo techniques have been adopted for particle charging in the powder coating industry. Although corona offers a relatively controllable charge source, the mechanism is inherently inefficient in terms of imparting charge to airborne particles. In addition, the highest density of free ions and high electric fields have been shown to be detrimental to good deposition. Tribo-charging offers some advantages, but is notoriously unpredictable. Many of these operational difficulties may be overcome by combining internal corona augmentation with primary tribo-charging. Free ions are eliminated, no high field exists between gun and workpiece and the tribo-charging characteristics are retained for prolonged periods of operation.

This paper discusses methods of charge injection onto powder conveyed pneumatically in pipes. The conveyance system used was of a commercial type used in the electrostatic powder coating industry. The powder was charged by means of an injecting electrode, a needle located along the axis of the pipe. The charged powder was then carried downstream where it was detected by two electrodes connected to either charge or current amplifiers. Powder mass flows in the range of 0.5 to 6.0 g/s were investigated. The study showed it was possible to distinguish between high and low flow rates using this charge injection method. Comparison between experimental results and a simplified model of the flow showed clearly that a significant amount of charged powder was deposited on the wall of the pipe a short distance from the point of injection. Also some of this deposited powder was later returned to the flow.

An approach to the mathematical model of electrostatic spraying of powder coating and its computing is discussed. The calculated results of the main process characteristics are given. The choosing of optimal specific charge is recommended.

A novel instrument has been designed to diagnostically determine the quality of electrostatically deposited powder coatings. Unfused coatings are analysed by selectively removing test zones of powder with a pulsed air jet and measuring the transmission of an infra-red beam through the depleted zone. This is carried out under microcomputer control. Initial results have been obtained for two different types of industrial coating equipment consisting of; a conventional high voltage gun operating from 50 kV to 90 kV, and a low voltage ion-free gun operating from 10 kV to 30 kV. The instrument is primarily configured to monitor the adhesive properties of the powder layer, consisting of both interparticulate and particle/substrate forces. From initial results it is possible to readily differentiate between the two types of coating system, and to determine many of the conditions necessary for optimum powder deposition in either process. The adhesive properties of the high voltage coatings are considerably larger than the low voltage coatings. An analysis of the results is being made to determine the optimum coating conditions for any powder deposition process.

This paper presents results obtained on charging, transport and deposition of insulating particles in the electrostatic powder coating process. It has been found that a minimum charge to mass ratio of 2 × 10−4 C kg−1 is required before the powder particlles will adhere to an earthed object. There are two separate forces responsible for transporting the powder from gun to object: the aerodynamic force (viscous drag in the air stream) and the electrostatic force. The relative magnitute of the velocity components due to each of the these forces are investigated for various sets of operating parameters. A typical commercial spraying system was operated with an air pressure of 15 p.s.i. and mean gun—object field of 2.4 × 105 Vm−1. In this system, aerodynamic forces produced a particle velocity around 2 ms−1 and electrostatic forces contributed an additional component of 1 ms−1. In the local region of the target, the electrostatic forces become dominant. It has been shown that a higher transfer efficiency (if> 80%) can be obtained by using a lower powder flow rate and a high applied voltage.

This paper describes the development of a diagnostic instrument capable of evaluating the electrical parameters vital in ensuring optimised performance of electrostatic powder coating. The kit consists of an electrometer, Faraday cup and high voltage detector probe and is capable of measuring the electric charge on the powder, the gun voltage at the corona point and the current flow to the workpiece, all of which are critical parameters affecting plant performance. It is also capable of detecting the onset and severity of the very important phenomenon of back ionization.

The electrostatic charge acquired by powders is frequently blamed for industrial problems in the production and use of powders. Both excessive cohesion and the opposite effect of poor packing have been blamed on electrostatic charging. Charged powders are produced intentionally and used in a number of industrial processes including electrostatic powder coating and reprographic systems. Even in these systems where much research has been carried out the mechanism of charge formation and decay, and the effects of the powder charge are not fully understood. In this paper a number of experiments concerning powder charging and its results are reported.