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L. Del Fabbro, M. Campolo, A. Soldati
26 June, Milano, Italy
Numerical evaluation of spray painting efficiency in industrial
applications: the spray painting of glasses
CIFI - Centro Interdipartimentale di Fluidodinamica e Idraulica
And Department of Energy TechnologyUniversità degli Studi di Udine
Trends in Surface Technology 08
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Big ones
… objects to be painted …
… and small ones
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Waste
liquid gas solid
Solvent
Air
Painted product
How difficult is the process? A mass balance approach …
Paint
Transfer efficiency (T.E.)
η=Qtransferred / Qsprayed
T.E. is as low as 8 ÷13 % for glasses !!!
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• Significant overspray
indoor air quality
VOC emissions
• Waste of raw materials
additional costs
Improve the T.E.!
… and consequences of inefficient spraying…
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60
70
80
90
100
8% 9% 10% 11% 12%
Spray Painting Transfer Efficiency
Pai
nti
ng
Co
st
Increase in spray painting efficiency
Savings
Is it worth valued ?
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Larger transfer efficiency
Efficient use of raw materials
Reduced emissions/low costs for end of pipe solution
Same productivity/same quality
WastesSolvent
Air
Painted product
Paint
… the industrial optimum…
A few point % increase in T.E. would be a good starting point …
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2006-2008: Research project to reduce the use of solvent in the spray painting of glasses (in cooperation with Safilo S.p.a.)
2006-2006: Feasibility analysis of research project to reduce the use of solvent
in the spray painting of glasses (in cooperation with Safilo S.p.a.)
2000-2001: Characterization and evaluation of spray painting transfer efficiency in industrial installations
(in cooperation with CATAS)
Our previous experience in the field…
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Present Case-Study: painting glasses
• Complex shapes• Many different materials (metallic/non metalic)• High quality standards
…is there space left for process optimization?
“Fashion and Luxury Eyewear/Sunglasses”
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Step 0: Analyse the industrial process• Evaluate T.E. on the field (T.E. = 8 ÷12 % for
glasses)
Step 1: Analysis of spray painting process for a spray gun• Numerical simulation of transport/deposition
of spray droplets• Evaluation of spray painting efficiency
Step2: Analysis of the industrial spray painting system (spray gun + hood)
Objectives: identify control parameters to improve T.E.
Steps of the analysis
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The spray painting system
Spray gun
Spray hood
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1. Spray gun: flow field …
DNS + Lagrangian tracking:100 h CPU on super computer @ CINECA !!!
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Small droplets
Optimum size droplets
Large droplets
Low T.E.Large overspray
Good T.E.Good surface quality
Large T.E.Reduced overspraySurface defects
2. Transport/deposition of droplets
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glasses
Computational domain
Head of spray gun
Paint mix
Atomizing air
Air to squeeze the jet
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0.1 micron
10 micron
Droplet trajectories
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Transfer efficiency
0102030405060708090
100
0.01 0.05 0.1 0.5 1 5 10 20 30 40
Droplet size [micron]
%… and transfer efficiency …
Optimum size droplets
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Spray paint hood: flow field
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1 micron
η = 13%
Droplet trajectories
Travel time distribution for paint droplets
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0102030405060708090
100
0.01 0.05 0.1 0.5 1 5 10 20 30 40
Droplet size [micron]
%
Droplet size distribution is crucial Use the best possible!
Low T.E.
High T.E.
… hints for optimization …
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Characterize Droplet Size Distribution
(on line in industrial systems)
•spray0.1µm to 2000µm misure on-line
Particle size distribution
dp
%
… and work in progress …