Tips from a job shop
July 26, 2001
Learn how Gauthier Industries, a stamper and powder coater, organized its powder coating operation into control points to help regulate quality and efficiency as parts move through the finishing process. Areas discussed include pretreatment, the paint booth, inspection, and tips for efficiency.
In today's metal finishing industry, quality and efficiency are of equal value to the customer. Quality is important for obvious reasons; efficiency is desired because it keeps prices down and results in short lead-times for customers.
The finishing process typically is the final operation before shipment, so the process must be flexible and reactive. Orders often must be coated and shipped the same day, so scheduling, efficiency, and quality are critical. Reworking is not an option, and time is of the essence.
Gauthier Industries, Inc.—a sheet metal fabrication, stamping, and powder coating job shop—has organized its powder coating operations into control points to help regulate quality and efficiency as parts move through this finishing process.
The first control point is pretreatment during the cleaning process, which involves a six-stage parts washer. The washer must be controlled within the parameters specified by the chemical supplier to ensure adhesion of powder to metal, so the machine is monitored every two to three hours.
Four of the six stages incorporate V-jet nozzles to help maximize the directional pressure of the spray patterns. The third stage, phosphatizing, contains hollow cone nozzles for a flooding action that promotes high-phosphate coating weights. Phosphate acts like a primer—it adheres powder to metal. The sixth stage incorporates misting nozzles for the light rinsing.
Deionized water mist is used for a final rinse in the sixth stage. By removing the particulates of hard water, this rinse reduces the dissolved solids remaining on the parts before they are dried. A dissolved-solids meter helps keep solids levels at less than 10 parts per million. The drain water is relatively pure and is recycled back to stage two, another rinse stage.
A second control point in the powder coating process is the paint booth itself, where painters apply the powder manually. Control at this stage is difficult because of the possibility of human error.
Theories abound for controlling quality in a manual paint booth, but painter training seems to be the key. Application equipment continues to advance, and operators must be familiar with the principles of electrostatic attraction. When changing colors, they must know what is needed to eliminate even the slightest potential for contamination.
Methods for eliminating contamination include cleaning all gun parts with compressed air, vacuuming all powder from the walls and floor, and keeping the outside area of the booth clean at all times. A lead painter, one who has been coating parts for at least one year, trains new painters to clean with utmost quality and efficiency.
In addition, Gauthier rotates painters every two weeks to keep people fresh in the booth. The painters are held accountable for their work; as a result, they check their work frequently by stopping the line and ensuring there is full, consistent coverage on each part. Occasionally, the supervisor will spot-check parts while the line is running. An extra set of eyes is key to controlling quality of parts exiting the booth.
The final control point is inspection. A designated inspector is responsible for completing a visual and physical check of the parts for each job that is coated. An inspection report containing information such as paint thickness, adhesion test results, and cure test results is included with every job. A thickness gauge with portable probe is used to check paint thickness.
In addition, a six-point cross-hatch blade is used to scribe an X to create small squares in the paint. After scribing, the operator applies a piece of Permacel® tape over the squares and lifts it quickly to determine if adhesion passes or fails.
Proper curing of paint to substrate occurs when parts are heated to a specified temperature—usually 375 to 400 degrees F—and held at that temperature for 15 to 20 minutes. Methyl ethyl ketone (MEK) is used to check curing of the powder to substrate. A swab is dipped in MEK and rubbed on the paint 30 to 40 times in a rapid back-and-forth motion.
If the swab is generally free of paint (or contains very little), the paint is cured and passes the test. If the swab softens the paint and rubs to bare metal, the paint is not cured. A gloss meter also is used to measure gloss levels at 20- or 60-degree angles.
From an efficiency standpoint, line density is critical. The company manufactured racking for the finishing operation that allows high-quantity runs to be hung densely on the line to help increase throughput. For this company's operations, the racking design was the most efficient way to hang parts with the least amount of effort.
Because the company's customer product line is diverse, so are the colors applied during powder coating. As a result, color change time affects both quality and efficiency. Not only must a color change be thorough enough to prevent contamination, but it also must be completed in a timely fashion.
To assist in color change, the company uses a pneumatic vacuum designed for dust collection. The high level of suction allows painters to clean spray-to-waste material quickly to prepare for the next color. A mild color change (gray to white) takes about 15 minutes. A major change (blue to white) takes about 20 to 25 minutes.
In addition, parts hangers leave a slight gap in the line to accommodate the color change. This gap acts as a timer for the painters to accomplish the color change.
Finishing plays a vital role in many fabricating and stamping operations. Finding methods for improving efficiency and quality can only add to customer satisfaction.