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PART 3
Automation will help break the finishing bottleneck
In this five-part series, Cefla will provide readers with some of the automation choices available to make the transition from manual application of finishing materials to automation, if even on a limited basis.
The last article in this series discussed the various types of automatic spraying machines used in the industrial wood finishing industry. Companies that install automatic spraying machines are frequently at the level of production that requires more finishing machinery than a single stand-alone automatic spraying machine.
Traditionally, the thought was that a company needing to make the transition from hand spraying to automation needed to install a very large and very expensive complete finishing production line with the idea that 100 percent of the finishing process would be completed on this line: Load white wood on one end and pack on the other.
As lean manufacturing methods have become adopted and resulting demand to finish "to order" has become the norm, the idea of finishing in large lots to be placed into inventory for later assembly has been replaced by small groups of finishing machines. Instead of one major finishing line, the trend is toward one or more small lines that will do special applications and/or afford a great deal of flexibility of color and process steps. The descriptive term "work cell" is currently used to differentiate this concept from a full process finishing line.
The simplest finishing work cell is a stand-alone automatic spray machine to replace hand spraying. Parts are manually loaded, passed through the spray machine, manually unloaded. The wet parts are then placed on racks or trolleys and allowed to air dry.
At some point, the manufacturer finds that he needs to use coatings that require more energy than air drying or the air drying process is too slow to keep production flowing smoothly through the plant (lean — just in time). The next logical addition to the stand-alone spray machine is the addition of an oven system that accelerates the drying process and/or allows the use of more high-performance coatings. There are many variations of ovens on the market. The simplest of these is an insulated and heated enclosure (room) where the drying racks are pushed into the heated room for a period of time.
The next stage up from the warm room is commonly called a "carousel" system where the drying racks are connected to an oval-shaped towing chain. The attached rack of parts is then towed through a hot air drying tunnel.
Note that both of these simple oven systems will achieve temperatures to about 125 degrees F and are quite satisfactory for drying stains, lacquers, acrylic, waterborne or other coatings that do not require high temperatures or high-energy lamps, such as UV or infrared radiation. These ovens are not physically attached to the spray machine, but they have the advantage of drying wet coated parts that may be coming from other sources such as hand spray booths.
If the work cell needs to be more automated, there are ovens that are designed to be attached directly to the outfeed conveyor of the spray machine, such that the part is coated and cured with a single load and unload operation.
There are two general types of hot air convention ovens found in this application: Linear ovens are mounted on conveyors usually in two zones. The first is a flash-off zone where the solvents (or water) are evaporated. The second zone will be much higher in temperature and air movement to allow the coating to "cure." It is in this last zone that IR lamps of different wavelengths may be added to facilitate the curing process. Note, however, that while very efficient and maintenance-free, these ovens can become quite long. Consider a coating that may take "only" 12 minutes to cure; if your work cell is running even a slow 10 fpm, the two-zone linear oven system will be 120 ft. long!
Other hot air convection solutions are available but are rarely used in a "work cell" environment. Note that the hot air source for all these types of hot air convection ovens is usually hot water from a low pressure boiler.
As the work cell continues to grow, the next logical piece of machinery to be installed is a panel brush machine. The brush machine or panel cleaner is always attached directly to the infeed conveyor of the spray machine. The purpose of this machine is to remove any dust or trash from the part before the coating is applied. The use of this type of machine does not make sense if the part is left to air dry in ambient conditions or the box/carousel ovens that are not attached to the spray machine. If, however, an oven system immediately follows and is attached to the outfeed of the spray machine, then the use of a panel cleaner will save significant labor and dust in the air. The quality level of the finish will also approve dramatically.
These three components make up a complete finishing work cell. As noted above, it is easy to see that all three are not necessary on first installation, but growth and productivity can be added as needed. Also note that a full production finishing line is nothing more than multiples of these three components tied together with conveyors or other material handling devices.