BMW’s Leipzig plant employs up-to date paint shop technology to ensure each car finishes up to the highest standard and does so with minimal environmental impact
Nowhere is this more obvious than in the paint shop, which backs modern processes with the latest environmental credentials, using only water-based paints with powder-based clear lacquers.
The paint shop has a ‘U’-shaped configuration to help divide ‘dirty’ parts from finished and semi-finished bodies. A storage buffer of cars is held on chain and skid conveyors instead of using the popular Automatic Storage and Retrieval System (ASRS) favoured by many carmakers.
After the bodies come through from the welding cells, the first job is to manually fit the door, bonnet and tailgate or boot ‘open holders’ – re-usable steel fixtures that enable e-coat and paint to be applied to the interior of normally closed areas. The fuel filler flap is fixed to a bracket that hangs from the rear side window frame.
First cleaning of the body is with 60°C plain, untreated water with detergent followed by a rinse with de-mineralised water, first by dipping into a tank and then by spray application. This is followed by a phosphating pre-treatment, another plain water rinse and then passivation.
At this point the main line splits and a side line can be used for maintenance or for taking bodies out for re-work. Chemetall supplies materials for this section of the paint shop, including the e-coat process, where the bath is heated to 28-30°C, partly by the electric current that is passed through the vehicle body. Cold water pipes run through the bottom of the tank to keep temperatures from rising to unacceptable levels.
Efficient coating Material use in e-coat is tightly controlled and Director of the paint shop, Dr Alfred Rutka, boasts of the highly efficient use of the coating material: “We typically achieve 90 per cent use of the coating, we recover a lot from rinse water using a sophisticated filtration system and because of the clever layout of the paint plant.”
After the e-coat application, and before drying the body, the car is moved from a chain conveyor onto a skid. On this it goes into an oven to dry the e-coat for 35 minutes, travelling 130 metres in a temperature of 180°C. Bodies are then cooled down to 20-25°C in a storage-cooling area. While the 400 litres of water used per car may sound considerable, it is very economical when compared with many other paint shops.
Underbody seam sealing is carried out almost exclusively by robot, including wheelhouses; just a few specific sealing points are still covered by hand application using brushes. The automated system requires a manual change of ‘open holders’, to give underbonnet clearance for the sealant applicator robots to gain access to far corners of the engine bay, and to avoid contamination of later paint processes by the previously e-coated fixtures. While the industry trend is towards two-pack sealers, supposedly giving increased durability, Rutka favours a single pack material which is then baked at 140-150°C: “Two-pack sealers are great when everything is working well, but if you get a breakdown or other problem in the applicator guns, you can have a serious problem with clogging.”
On average 50 e-coated bodies are stored on the buffer conveyors but this can go up to 90 using some overflow storage. The total number of cars being processed in the paint shop at any one time is around 450. Only two paint shop workers make a pre-primer inspection; operators from the body-in-white section also examine every car as “there are generally more defects from body-in-white than from the paint shop,” says Rutka. There are two car space rework areas for bodies at this stage, mainly correcting bodyin- white defects.
Bodies then travel through the ostrich-feather dusters, removing dust and dirt, before Dürr robots apply the first primer coats using bells with 12 internal feed pipes. “We achieve 65 to 70 per cent efficiency,” says Rutka. “Most of the paint goes on the car body and very little is caught by the water collection system underneath.” The following booth is extra wide. This allows robots to open the doors, bonnet and bootlid to give the paint robots access for spraying the door shuts, boot underside and the underbonnet area, again in an electrostatically-charged operation.
The thoroughness of BMW’s paint process is exemplified by the next booth, where paint robots, running on air alone, not electrostatic, finish the areas that do not get covered as well during the electrostatic paint process. Before travelling through to the basecoat stations, the bodies are baked at 140°C for 30-35 minutes to cure the primer coats. The basecoat is then applied, first by nonelectrostatic robot paint guns, to door shuts and edges after other robots have opened the doors. This is carried out first to avoid dust (created by overspray) falling on the ‘outer skin’ surfaces.
The body travels through two inner body area painting booths where paint-visible interior parts are painted by robot and then on to two outer body area booths. Here robots cover the class A surfaces using the electrostatic system for the base colour coat and then employ a straight air system for the metallic aluminium flakes. This method is chosen to avoid the uneven attraction of the metallic flakes that would be caused by the electrostatic charge.
To keep the liquid basecoat paint in ‘spray-ready’ condition, it is pumped around the systems constantly, slowing to three circulations per hour in offproduction periods.
Powder coat expertise
Dr Rutka talks proudly of BMW’s use of the powder clearcoat system: “The powder clearcoat installation has taken a large and long-term investment. We are keen on sustainability. We use no solvents [or even water] and achieve almost total material usage; we use 95 to 98 per cent of all the powder.”
This is accomplished using a sophisticated system of underfloor recovery funnels that lead to filters and dry powder pumps. These recirculate the unused powder back up to the robot bell pickups. The booth air is constantly circulated and powder is also recovered from this air. The clearcoat is then baked, making a miraculous transformation from a car that appears to be covered in snow, to a glistening finish with a very smooth surface and none of the ‘orange peel’ effect found after spraying. Five cars every hour are removed from the line and inspected by spectrometer, checking colour accuracy and paint density. Handling and automation in this area is by HYDAC, SAR ‘Make it Easy’ conveyors and ASIS automation controls. KUKA supplies most of the robots throughout the paint shop. A final polishing takes place where any defect spots are polished out – black cars may have up to eight defects and silver cars six defects, before re-painting of the clearcoat is deemed necessary.
Rust never sleeps
To make each BMW car truly ‘sustainable’ – to proof it as much as possible against corrosion – a water-borne, cold-applied cavity wax is injected into box sections and panel inners by robots. Infra red lights heat the bonnet, bootlid or hatch to warm the cold wax and encourage it to flow in to corners and points where water might also flow in use.
As the Leipzig plant was designed from a clean sheet – a true greenfield enterprise – the end of the paint line ends fairly close to the ‘paint on wheels’, final assembly re-work area. Dr Rutka says that it allows a good rapport between final assembly and paint shop: “The guys from both sides can meet easily and talk about any problems and exchange ideas easily.” On new technologies, such as UV curing, Rutka is more sanguine: “It is a great technology but getting the drying effect evenly through a unibody is not easy, I would always think that the inner parts of the body would not cure at the same rate as the exterior.”
The Leipzig plant is an interesting showcase for paint technology as it is in the real world; the use of powder clearcoat, flexibility in electrostatic application and selective unit inspection of colour and density are, to most industry experts, about as far along the technology road as a high-production plant can employ at present.