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

Big ones

… objects to be painted …

… and small ones

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 !!!

• Significant overspray

indoor air quality

VOC emissions

• Waste of raw materials

additional costs

Improve the T.E.!

… and consequences of inefficient spraying…

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 ?

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 …

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…

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”

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

The spray painting system

Spray gun

Spray hood

1. Spray gun: flow field …

DNS + Lagrangian tracking:100 h CPU on super computer @ CINECA !!!

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

glasses

Computational domain

Head of spray gun

Paint mix

Atomizing air

Air to squeeze the jet

0.1 micron

10 micron

Droplet trajectories

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

Spray paint hood: flow field

1 micron

η = 13%

Droplet trajectories

Travel time distribution for paint droplets

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 …

Characterize Droplet Size Distribution

(on line in industrial systems)

•spray0.1µm to 2000µm misure on-line

Particle size distribution

dp

%

… and work in progress …