The selection of a paint or powder finishing system depends on a number of factors, including the types of parts being coated, the required surface-treatment process, size and weight of the parts, sequence of process steps and the flow of material through the entire fabrication process.
The finishing line layout should be planned based on the space requirement of the process equipment and the available space. Rout the conveyor so as to carry parts from the loading point in the fabrication area through the process system to the unloading point in the assembly/shipping area. This concept will minimize material handling/damaged parts and personnel required.
For liquid-paint-process lines, mainly two types of conveyor systems are used. Overhead monorail conveyors have the load-carrying and the drive function combined in one element – the endless chain. This chain is guided by a rail system based on either an I beam or an enclosed track. A take-up station overcomes chain slack caused by temperature variations or wear. A caterpillar drive or a sprocket in a turn drives the chain. Parts are suspended from the chain by special attachments or load bars. This conveyor can follow only one path – deviations are not possible. This system is simple and efficient but not very flexible.
Overhead power and free conveyors are two-rail systems. The load-carrying element, (trolley) and the driving element (chain) are separated. The trolleys use the lower “free rail” and the chains are in the upper “power rail.” Pusher dogs establish the connection between chain and trolley, and they can be engaged and disengaged as needed. The “free rail” can be equipped with switches, lifting stations or stops to allow the trolleys to follow several different paths, be lifted or lowered, stopped or moved at different speeds through the system. The entire conveyor system is PLC-controlled.
This system is more costly, but it is flexible enough to link fabricating processes, paint lines, assembly and shipping, and to accommodate process variations within these areas. This system will also reduce the number of personnel and the cost of transporting parts through several fabrication steps. Because of this, the danger of damaged parts through various manual handling steps is greatly reduced.
OTHER CONVEYOR SYSTEMS
Besides the two systems previously described, there are several other systems used to lesser extents for finishing processes:
• Automatic hoist systems for dip processes (typically e-coat and dip pretreatment).
• Square-transfer systems (applications similar to those for hoist systems).
• Inverted monorail or power-and-free conveyors.
• Roller conveyors for transporting skids of parts or assemblies.
• Dual-chain and drag-chain conveyors for dolly systems.
Any of the above conveyor types can be combined to provide the optimum paint system for a specific application. Conveyor systems will reduce manual handling of the parts, minimizing labor costs and the potential for damage.
In most cases, a paint or powder coating line consists of two portions: the pretreatment system and the coating section.
Pretreatment systems produce a substrate surface suitable to receive the coating. These systems are intended for cleaning, phosphating and similar treatments. Pretreatment chemicals are normally in aqueous solutions and may be used in steam-cleaning applications. Vapor degreasing/phosphating is also used. Mechanical cleaning, such as shot blasting, is another means for treating parts prior to cleaning.
Each pretreatment process starts with cleaning of the parts, followed by application of conversion coatings such as iron or zinc phosphates or chromates. The process sequence also includes the required rinses. Spray or dip or combinations thereof could apply the process steps mentioned here, depending on part configurations and process steps required.
A dry-off oven typically follows the pretreatment process. The oven normally operates at approximately 300°F and is heated by natural gas. If gas is not available, fuel oil, electricity or steam can be used. Note: dry-off ovens are not built to evaporate water puddles. All water collections must be drained prior to dry-off.
Ancillary equipment for pretreatment systems includes oil-water separators, sludge-removal systems such as filters or centrifuges, heating systems (hot water), chemical feed/metering systems and demineralizers/deionizers.
Different coating processes require specific pretreatment steps. Electrocoat, for example, requires a final rinse with deionized water before entering the e-coat tank.