Although liquid painting has not changed dramatically over the years, production demand, coatings and processes have. One of the oldest, most reliable types of applicators for the aluminum extrusion market is a disk-type atomizer. The disk offers excellent transfer efficiency along with superior atomization.
New rotary atomizers for liquid electrostatic application equipment rotate at much higher speeds than older-style applicators. The high rotational speed is capable of atomizing even the toughest coatings and allows for higher production rates. Quick-disconnect features provide easy removal and maintenance.
In addition to rotary atomizers, there are new types of operating controls to monitor system parameters. Setting the operating parameters of a system creates uniformity in the application process and virtually eliminates problems associated with a manually operated system.
Companies that are coating aluminum extrusions face many issues in their application processes, including achieving a uniform film build. On a vertical line, some parts may have a light coating of material at the top of the extrusion and a heavier coat at the bottom. Other problems that extruders face include inconsistencies in paint finish, rejects or repaints, increases in production and an accurate method to track paint and solvent usages.
Over the past few years, many different manufacturers have developed equipment specifically for the aluminum-extrusion market. The new products meet specific demands to increase production, decrease repaints, increase safety for the operator and provide communications to efficiently track paint usage, production rates and paint flow rates.
One of the latest technologies available is based on a programmable logic controller (PLC) system, which allows true process control. The system allows the operator to change process parameters, not only between batches, but also within the same part. If the system is unable to maintain one or more of the required parameters, it will alert the operator before a large number of rejects is produced.
This type of technology can control various parameters, such as fluid psi (or actual flow rate with closed-loop fluid control), voltage, reciprocator positioning or any other parameter that is an important part of the control system. Communication through the system can be by discrete wiring, Allen-Bradley data highway or Allen-Bradley RIO. This type of PLC control system takes the "art" out of finishing and puts the "control" in.
How does it work? The parts are loaded on the line and tracked through the washer. As the parts enter the spray booth, the PLC control automatically adjusts the parameters for the die or style. Parameters such as solvent flush, automatic color change, length of stroke, paint flow rate, disk speed and voltage setting are automatically adjusted and controlled. The PLC control can track and report how much paint is used per die or style, how much solvent is used per color change and production rates.
High-speed disk rotary atomizerHigh-speed disk rotary atomizers, which can rotate at speeds up to 40,000 rpm, can be an important technology for meeting increases in production. Higher speed capabilities allow for higher fluid-flow rates while maintaining the same level of atomization. Up to 1,500 cubic centimeters per minute of fluid can be delivered, and even higher flow rates are possible with dual feed tubes.
This higher speed also allows the atomization of the higher-solid coatings used in many systems. The disk's quick-disconnect features allow for easy maintenance and reduced downtime.
New technology is a great advantage, but it is only good when used correctly. To justify any new investment, it is important to understand the variables-labor, maintenance and filter costs, for example-and how to calculate the return on investment.
A review of some control features is useful, starting with the electronic reciprocator, or stroke controller. The first rule of thumb is to eliminate overstroking and understroking. This will eliminate wasting material when the part is not present. Understroking (not stroking far enough to coat the ends properly) creates light coating on the top or bottom of parts, creating rejects or repaints. Overstroking (stroking too far off of the end of the part) wastes valuable paint. To calculate paint usage, follow this calculation:
• 2 inches of over-stroke on top and bottom = 4 inches of excess spray.
• Part-16 ft.
• Booth 1-1,200 cc per minute
• Booth 2-240 cc per minute
• Stroke Speed-2 feet per second
• 1,200 cc ÷ 60 seconds = 20 cc per second
2 inches of overspray at top of part and 4 inches of overspray at bottom = 6 inches at 12 inches of travel = 10 cc of wasted paint per cycle (Booth 1)
• 240 cc ÷ 60 seconds = 4 cc per minute
2 inches of overspray at top of part and 4 inches of overspray at bottom of part = 6 inches at 12 inches of travel = 2 cc of wasted paint per cycle. (Booth 2)
• Stroke Calculation
16-foot stroke at 2 feet per second = 4 cycles per minute, approximately 4 per minute = 4 x 60 per minute = 240 cycles per hour
240 x 16 hour shifts = 3,840 cycles per day
3,840 x 250 working days per year = 960,000 cycles per year per booth
• Booth 1-At 10 cc per minute x 960,000 cycles per year = 9,600,000 cc per year
• 9,600,000 cc ÷ 3,785 cc per gallon = 2,536 gallons per year
• 2,536 gallons x $35 per gallon = $88,760.00 per year.
• Booth #2-At 2 cc per minute x 960,000 cycles per year = 1,920,000 cc per year
• 1,920,000 cc ÷ 3,785 cc per gallon = 507 gallons per year
• 507 gallons x $35 per gallon = $17,745.00 per year.
• Total: $106,505.00 per year based on two eight-hour shifts and 250 working days per year with two atomizers.
The importance of flow controlMaintaining actual flow-rate set points is critical in the paint process. Closed-loop fluid control helps maintain the set flow rate from top to bottom of the part, overcoming the loss of flow due to gravity, thus reducing rejects. On the other hand, excess paint flow rates equal wasted paint and rejects.
Another valuable calculation with which to become familiar is one that figures flow rates:
• Actual flow rates are 10% greater than targeted.
• Targeted flow rate = 1,000 cc per minute, so, following the example above, the flow rate equals 1,100 cc per minute.
• 100 cc per minute of excess paint x 60 minutes = 6,000 cc per hour of waste.
• 6,000 cc per hour of waste x 16 hour days = 96,000 cc per day.
• 96,000 cc per day ÷ 3,785 cc per gallon = 6,341 gallons wasted.
• 6,341 gallons wasted x $35 per gallon = $221,935.00 wasted per atomizer.
• $221,935.00 wasted per atomizer x two atomizers = $443,870.00 worth of wasted paint per year.
Note: The above equation is based on two eight-hour shifts, 250 working days per year with two atomizers.
How the microprocessor helpsThe microprocessor-type voltage power supply reduces rejects caused by nuisance trip-outs or "angel hair." The voltage is folded back (reduced) momentarily when a grounded part gets close to the applicator and returns to the original set point as soon as the part is at a normal target distance. The parts in the omega loop are not noticeably affected and the painting process can continue, thus reducing rejects or repaints.
Color stacks, or color-change valves, controlled by a PLC can utilize a solvent/air chop controlled by the PLC. Solenoid valves are energized in sequence to create "scrubbing bubbles" that effectively clean and load the next color in the painting process. This also reduces the amount of color-change time and solvent usage.
The painting process has entered a new era. PLCs and computers have improved lives and enhanced communications. There are painting systems with diagnostic capability that can be run from the home over the Ethernet.
The criteria that all new paint-finishing technologies must meet include reducing costs by improving quality, reducing material consumption, increasing throughput and improving the work environment. The new advances in technology provide these criteria.