Appliance industry takes a shine to powder blank technology

Among European, Pacific Rim, and U.S. appliance makers and other manufacturers with large-scale, appliance-grade finishing requirements, the use of powder blank line coating systems has grown steadily during the past 10 years.

A powder blank coating line supplies coated blanks for all washers and dryers made for sale in Europe by one large German manufacturer. With zinc/ aluminum-coated steel as substrate and a nominal line speed of 41 feet per minute (FPM), the coating line uses 12 stationary guns to apply 0.031 inch of polyester powder. The blanks cure in 60 seconds in an oven heated by medium-wavelength infrared radiation (IR) and circulated air.

All of the refrigerators manufactured by one major U.S. appliance maker include powder-coated blanks produced on two lines at one of its Illinois plants. A single line at a supplier's plant in Kentucky provides all of the finished blanks for washing machines manufactured by another major U.S. appliance maker.

Although powder blank line coating systems have gained a measure of acceptance by the major appliance industry, the technology is used or is being considered for a wide range of other industrial applications. These include:

• Lighting fixtures
• Metal furniture
• Automotive components
• Water heaters
• Shelving, racking, and displays
• Air conditioners
• Home furnaces
• Modular building components

Powder blank technology, or powder blanking, is one option that can help manufacturers streamline their finishing, forming, and assembly operations; reduce operating costs; and improve the quality of their products.

The process involves applying a uniform powder coating at high production rates on flat sheets, or blanks, before the metal is formed. Blanks can be stamped or sheared, with all cutouts, holes, and notches punched to final dimensions before parts are formed. Then, the blanks can be pretreated, powder-coated, cured, and finally formed into 3-D parts. Some of the benefits of powder blanking include:

• High production rates
• Low operating costs (energy, labor, chemicals, water, powder, and wastewater)
• Precise and uniform film buildup
• Low floor space requirements
• High levels of automation
• Simplified work-in-process (WIP)
• High-quality finishes
• Reduced hazardous waste

Pretreatment

In powder blanking, precut blanks are stacked at a feeding station at the beginning of the pretreatment line. Blanks can be as large as 16 feet long by 4 feet wide, and they can be fed into the line 1 to 3 inches apart. Pretreatment typically consists of surface preparation to remove dirt and oil; the application of a dry-in-place chrome or nonchrome sealant, or a conventional iron phosphate treatment, to the top and bottom of each blank; and fast, low-heat drying to prevent contamination.

Drying is typically performed by an IR/convection dry-off oven. Air is heated by natural gas using an indirect heat exchanger to minimize the risk of creating combustion by-products that can contaminate the pretreated blanks. Blanks then enter a cooling zone, where they are subjected to high-velocity refrigerated air and maintained at temperatures below 95 degrees Fahrenheit in preparation for powder coating.

Pretreatment is fully automated, including destacking at the feeding station, and control of chemical pH and conductivity, degreasing, temperature, pressure, brushing, volume, blow-off, fault conditions, and material handling. Similarly, the dry-off and cool-down steps are fully automated, including temperature control, turnover, fault conditions, and material handling.

Powder Coating

Powder blank technology permits manufacturers to benefit from of the durability and flexibility of the wide range of available organic powders. Pretreated blanks can be powder-coated with standard application equipment.

Modular powder booths can expedite quick color changes. Multiple booth/ conveyer modules can be built into a line, depending on the speed and frequency of color changes, and individual modules can be taken off-line when a color change is required.

Each booth/conveyor module typically has two conveyors: a conductive belt for conveying blanks during a top coating phase and a pin-type conveyor for dust coating the underside of each blank. These facilitate the movement of blanks through the module and prolong the life of forming tools. A smaller booth/ conveyor module with an integrated IR oven can also be used for full bottom coating.

Because blanks are precut and stamped before they are powder-coated, every face, edge, and corner can be completely and uniformly coated with a 1.4- to 1.6-mil layer of powder. Defect rates typically are less than 0.5 percent. Powder coating requires no filters, masking materials, or solvents (VOCs) or other hazardous materials. It leaves no sludge or painted scrap metal. Water consumption and wastewater contaminants are sharply reduced, and no hangers need to be removed and cleaned.

The powder coating phase of the operation is also fully automated to ensure precise control of flows, temperature, booth operation, gun purge, humidity, part identification, powder supply, power supply (if required), fault conditions, and material handling.

Curing

Powder blanking lines can run as fast as 100 FPM. At such speeds, accelerating cure times with IR energy transfer is the only practical way to achieve a high-quality, durable finish. With IR heat, energy transfer efficiency is typically 98 percent. Fast, controlled temperature ramp-up provided by medium-wavelength IR melts powder so that it begins to flow almost immediately.

Curing of coated blanks can also be accomplished through IR energy transfer and IR-enhanced convection systems. Using IR energy for curing improves coating adhesion and quality, which are critical for metal parts that are formed from coated blanks. IR reacts deeply within the coating, effectively curing from the inside out. This eliminates premature hardening, blisters, pinholes, and other defects and yields a smooth, glossy finish. Either curing combination can be fully automated, including the control of oven temperature, part temperature, part/no part, turnover, fault conditions, and material handling.

In a curing oven, peak material temperatures can reach 480 to 550 degrees F. Before coated blanks can be removed safely and restacked, they must be cooled to about 100 degrees F. A cooling system similar to that used following pretreatment dry-off reduces the temperature of the coated and cured blanks before they are picked up by an automated vacuum system and conveyors for restacking in preparation for forming, assembly, and shipping.

Payback

Higher productivity and lower associated costs may be realized by installing a powder blanking system. Coating, curing, forming, and assembly operations generally become faster, more streamlined, and more flexible, which permit manufacturers to optimize production and handle the demands of just-in-time (JIT) production and delivery.

Powder blanking is suited to highly automated control of part spacing, part detection, crash detection, and synchronization. Color changes can be handled with minimal downtime and labor. Powder blanking also yields faster cycle times because of the rapid curing that occurs, which allows manufacturers to devote significantly less floor space to the process.

Coated blanks can be stored flat until they are formed, so floor space is not taken up by odd-shaped parts that cannot be stacked. In addition, orders for different colors can be accommodated more quickly when precoated blanks are available and ready for forming and assembly.

Many powder blanking lines have achieved payback in 12 to 18 months. However, they are best-suited to lines with 20 million square feet or more of annual production.

Manufacturers around the world are looking for ways to reduce the costs and improve the quality of their products. They are looking for ways to streamline their operations, to increase capacity without adding costs, to increase productivity, and to reduce their exposure to legal liability arising from environmental concerns. Many are looking at powder blank technology to help them achieve those objectives. As a result, powder blank coating lines are in use or in various stages of planning in an ever-growing range of industries for an ever-growing range of applications.