Paint and surface treatment processes in the car paint shop are to a large extent automated and performed by robots. Having access to tools that incorporate the flexibility of robotic path planning with fast and efficient simulation of the processes is important, since such tools will reduce the time required for introduction of new car models, reduce the environmental impact and increase quality. The key process in the paint shop, which is also the most demanding from a modelling point of view, is the spray painting of the car body.

In spray painting paint primer, color layers and clear coating are applied through the Electrostatic Rotary Bell Sprayer (ERBS) technique. Paint is injected at the centre of a rotating bell; the paint forms a film on the bottom side of the bell and is atomized at the edge. The droplets are charged electrostatically and driven towards the target car body both by shaping air surrounding the rotating bell and by a potential difference in the order of 50-100 kV between paint applicator and target.

The combination of high physical complexity, large moving geometries, and demands on near real time results constitutes a big challenge. The current situation in the automotive industry is therefore to rely on individual experience and physical validation for improving their processes.

This multi-physics application requires tracing of paint droplets that are two-way coupled to the fluid flow and electrostatic field, from bell to target. The major part of the computational work is done within the flow solver – where the transient Navier-Stokes equations are solved to obtain the air flow velocity field. An accurate solution is necessary in order to predict where paint hits the target surface.

A major improvement in computational speed compared to other approaches has been realized through the development of the incompressible flow solver IBOFlow. IBOFlow is based on a finite volume discretization of the Navier-Stokes equations on a Cartesian octree grid that can be dynamically refined and coarsened. Unique immersed boundary methods are used to model the presence of objects in the fluid. This enables modeling of moving objects (robots or cars) at virtually no additional computational cost.

The IBOFlow solver has been integrated in the in-house package for automatic path planning, IPS. The first version of the software IPS Virtual Paint was released in 2009 and our industrial partners predict that positive effects will include a reduced time required for introduction of new car models, a reduced environmental impact and an increased product quality. In the software, an arbitrary geometry can be painted using a moving, pre-defined electrostatic rotary bell, where the user can set process conditions like paint flow, air flow, electrostatic droplet charge and atomizer bell rotation speed. The process conditions are used to compute physical inlet conditions such as paint droplet size and velocity distributions and applicator inlet air flow. Validations of the film thickness for test plates are in excellent agreement with experimental data.

Currently we are working on further improvements of IPS Virtual Paint as well as additional modules to enable simulation of the other processes relevant to the automotive paint shop such as electro dipping, sealing and cavity wax, and oven curing. Another challenge is the automatic generation of collision free robot paths through 3D scanning of complex components. This technology will mainly target low volume series which have until today been painted manually due to the lead time of programming a paint robot to perform the same operation.