Flexible Spray Cabins

Knowing what is required and getting the pallets to the correct place is only half the battle. A spray cabin also is required to achieve true flexibility.

It is generally accepted that paint robots provide the most flexible solution. Paint robots have been available for many years but basically, were large units best suited to coating lines for automobile bodies, etc. Their integration into more modest modular systems has been limited. With the advent of the more compact "mini" painting robots, their integration into a modular spray cabin has been simplified significantly.

Not only has the size of the new robots made their integration more simple but also, their mounting possibilities. These new robots may be ceiling- or sidewall-mounted, greatly reducing the congestion within the cabin. The spray target may now be positioned in the middle of the cabin, allowing unrestricted access from all sides (see Figure 1).

Figure 1

Ceiling-mounted robots also give the additional advantages of better water flow across the basin (fewer obstructions) and more uniform airflow across the pallet. The airflow may be so directed as to drop down from the ceiling and pass uniformly over the pallet without encountering the disrupting presence of the robot’s body. Ceiling-mounted robots do have the disadvantage that the robot’s body can cast shadows within the spray cabin.

To complete such a flexible spray cabin requires its own loading/unloading device (object’s defined interfaces). Robots are inherently slow. The time is takes for a robot to ‘fly’ to its start position is time lost for spraying. But on the other hand, robot spraying, when correctly programmed, is very accurate with minimal over-spray, thereby allowing the load/unload device to carry the new pallet in, exchange it with the coated pallet, and carry it out while the robot starts spraying the new pallet (see Figure 2).

Figure 2

Drying and Curing

With the exception of UV coatings, drying and curing usually takes place at elevated temperatures within electrically- (heater battery or infrared) or gas-heated driers or ovens. These devices do not need to be flexible as the temperatures that can be achieved are primarily defined by the substrate being coated.

However, a flexible line must be able to handle a wide range of substrates. Therefore, it is the temperature control of a drier or oven that must be flexible. Precise electronic control using accurate temperature measurement and final positioning devices are necessary. For gas-heated systems, suitably-sized control valves with repeatable positioning and the correct characteristics are an absolute must. For electrically-heated systems, solid-state thyristor control of the heater battery provides the most stable, accurate control. Infrared heating is very good for bringing parts up to temperature; but for some coatings, this heating method is too intense - curing the coating too quickly and resulting in a very poor finish.

To create a uniform temperature within a drier or oven, careful consideration must be made of the air circulation. It is the air circulation that provides the primary mechanism for the heat distribution. Temperature measurements must be made as close as possible to the parts within the drier or oven. It is the temperature that the parts are exposed to which is important, not the upper or average temperature. To ensure that the parts are being exposed to the required temperature for the specified time, the temperature should be measured at a number of points within the drier or oven. From these measurements a temperature profile may be created.

New Coatings

An important part of creating a flexible line fit for the future is to have a good idea of the direction that the coatings are taking. Previously, single component solvent-based coatings were the norm. However, particularly for the coating of automotive plastic parts, two- and three-component coating systems have virtually taken over. A flexible line must be able to cope with all these variations.

Single component coatings represent no problem. All they require is a supply system, a stirrer, and perhaps a timer to alarm just before their pot life expires. Two- and three-component coatings on the other hand are much more problematic. Not only do they require precise mixing but some also have such short pot lives (measured in a matter of minutes) that they must be mixed on demand and applied immediately. Mixing based purely on flow measurements is too empirical. The best way to achieve the required accuracy on a consistent basis is dosing. Dosing valves dose precise quantities into fixed quantity mixing chambers. Mass flow measurement by means of coriolis mass flow meters provides reportable supervision of the mixing process.

At the moment, solvent-based coatings hold sway. However, with the tightening of the volatile organic compounds (VOC) emission regulations on water-based coatings, single and 2/3-components will increase in popularity. Due to the slightly superior finish quality achievable with solvent-based coatings in comparison to the water-based coatings available today, it is not expected that water-based coatings will supersede solvent-based coatings completely.

A truly flexible coating line must be able to operate as well with water-based as with solvent-based coatings. To accomplish this, consideration must be made of their different chemistry in regard to their separation in wet washout systems. With a mixed operation, a combined surface and bottom extraction system is the best compromise. Such systems consist of a tank, which continuously is scrapped by scrappers suspended between two continuous chains. The scrappers first scrap the surface, pushing the flotsam to the extraction ramp, before diving down to scrap the bottom. The scrappers surface from the bottom of the tank pushing the sludge before them, collect the flotsam that has been pushed forward previously, and then push everything together up the extraction ramp and out of the tank. The extraction ramp allows the water that is trapped in the sludge to drain out and return to the tank, minimizing water loss.