Design, CFD simulation and testing of special-purpose nozzles for powder coating guns
D. T. Pham, N. B. Zlatov, S.S. Dimov, O. A. Williams
Manufacturing Engineering Centre, Cardiff University, Queens Buildings, CF24 3AA
1. Introduction
This paper discusses a methodology for increasing powder deposition efficiency in through the use of efficient gun-nozzles for powder coating guns. Figure 1 presents 3-piece tinplate aerosol cans being powder coated in pilot equipment using three corona spray guns. In the present system each of the three corona gun is used to powder coat different regions of the work piece. The first gun powder coat the bottom part of the cans, the middle gun powder coat the middle part of the cans while the third gun powder coat the top part of the cans. It was observed that the existing standard gun-nozzle design produces significant powder waste when used to spray the top, middle and bottom of the cans with height 132mm and diameter 40mm. This paper therefore investigates the existing process and presents a design methodology for designing and evaluating the efficiency of gun nozzles. It also presents experimental results and Computational Fluid Dynamic (CFD) validation of the gun nozzle designs. Figure 2 illustrates the design methodology implemented in this work.
2. Evaluation of the efficiency of the base powder coating gun-nozzle
The design work implemented in this paper makes use of reverse engineering to modify the existing gun nozzle design in order to design new efficient nozzles that are capable of achieving increased powder deposition efficiency. Figure 3 presents the 3D model and the Computer Aided Design (CAD) drawing of the existing nozzle, while in figure 4 the spray pattern of the existing gun nozzle is presented. The powder deposition test conducted was conducted on a flat plate with constant gun pressure and voltage.
3. Design of Virtual Prototype Powder Coating Nozzle and CFD Validation
In sections 3.1 and 3.2 brief description of the CAD design of a new prototype nozzle and it validation using CFD code, Fluent, are presented respectively. The results of the CFD simulation of the spray process of the prototype nozzle under different inlet and static pressures are also presented.
3.2.1 Simulation of Inlet Gun-Nozzle Pressure
The pressure of outlet powder feed from the fluidising bed and venturi pipe plays critical roles in deciding the powder flow rate of the inlet fluidised powder to the powder gun coating. Fluidised powder travelled from the fluidised bed through the venturi pipe to the gun nozzle. The venturi pipe ensures that fluidised powder is supplied to the powder feed-hose at a consistent rate. In order to monitor the flow condition in the, the outlet pressure was simulated using Fluent. Figure 6 presents the CFD model of the venturi pipe with the air and powder inlets and the powder feed outlet area.
Fig.6: CFD model of venturi pipe and atomising inlet air
Fig.12: Path lines from powder particles with inlet pressure At 1000 Pa
The simulation result of the prototype nozzle
indicated that increased transfer efficiency is
achieved when the inlet pressure is set at
1000Pa with more than 90% of sprayed charged
powder particles deposited on the target surface.
6.0 Conclusions
In this paper, a methodology for increasing powder deposition efficiency in corona applications has been presented. The methodology demonstrated an approach to nozzle design with a view of reducing over spray. A computational fluid dynamic code, Fluent, was used to simulate inlet pressure into gun nozzle from the venturi pipe. The CFD simulation offered an opportunity to investigate wide range of nozzle parameters with a view of finding an optimal candidate nozzle design for the powder application process. Experimental results of the candidate gun-nozzle indicated that powder deposition efficiency improved significantly.
