Under control: PLC-controlled system helps improve process control, productivity and quality
March 20, 2009 by Mark DeCramer
PLC-based control systems are enabling engineers, production managers and skilled labour to produce higher quality products in less time with fewer resources. This benefits those producing durable goods who are hardest pressed to streamline production while expanding product line depth. Nordson Corporation, an international producer of precision dispensing equipment that applies adhesives, sealants and coatings to a broad range of consumer and industrial products, saw huge benefits when it implemented a PLC-based control system.
“With advanced controls, we’ve developed the Prodigy HDLV Color-On-Demand system, a powder coating product that changes colours in just 18 seconds,” says Nordson’s senior project engineer, Jim Khoury. “That’s several times faster than manual change out. Another major development [is] we’ve eliminated operator error arising from improperly cleaning those lines during a colour switch, leading to higher quality products.”
A manual colour change can cause specks of the old colour in the newly applied colour if lines are improperly purged between colour changes. The Ohio-based company developed a control system that rapidly changes between 28 colours while eliminating cross-contamination of coloured powder through a sophisticated purge cycle. Nordson’s Prodigy HDLV (high-density powder, low-velocity air) Color-on-Demand powder spray system then electrostatically applies coloured powder faster, with high-efficiency coverage in typically difficult-to-coat areas.
“The genesis was the need to shorten line gaps necessary for changing colours, which helps increase productivity,” explains Khoury. “The number of colours used for everyday products is dramatically expanding. Customers are running single part, part family, customer order scheduling – a Lean manufacturing philosophy. Here, the painting operation is typically the bottleneck.”
Breaking the bottleneck
Nordson based the Prodigy HDLV’s Color-On-Demand control system on a WAGO Ethernet Programmable Fieldbus Controller (PFC) and WAGO digital input and output modules. The system utilizes Ethernet MODBUS TCP communications between controller and HMI touch screen, which has a colour display and a user-configurable interface. The HMI resides in front of the spray operator, tracking and displaying key process data.
Items bearing material ID tags are conveyed to an end-user equipped with a manual spray gun. The control system automatically selects one of 28 colours from a code embedded on the item’s tag. Users can also manually select colour via HMI. For example, if a product line, such as oven doors, requires different colours, the system will change out colours in approximately 18 seconds – a significant improvement over the two to three minutes (depending on operator skill) it previously took to manually switch colours.
The touch-screen also plays a vital role in product quality and waste elimination.
Patented rubber pinch valves accommodate high flow rate requirements while being easy to clean. Valve and seal rubber is a commodity item with a finite number of uses; degraded rubber valves can mix powders in the machine. Understandably, even a small amount of mixed powders is unacceptable for the quality levels required by some users. In the past, monitoring the service life of rubber bladders and seals had been difficult – replacement relied on a user’s own tracking system. Solving this mandated integration of a life cycle counter into the machine’s WAGO PFC. The cycle counter tracks rubber life cycle and usage, notifying the system head (the HMI displays a warning) when a replacement valve or seal is needed. This ensures high product quality and consistency while eliminating costs associated with waste and refinishing.
As the earliest Prodigy HDLV models were installed, Nordson identified a few opportunities to improve performance. In addition to optimizing quality assurance testing procedures (uniform observation of LED blink codes and ensuring both Ethernet connectivity and proper machine sequencing) and improving wiring documentation, Nordson replaced the touchscreen to eliminate an emerging electromagnetic discharge issue.
“The powder is electrically charged to tens of thousands of volts,” Khoury explains. “As you can imagine, most off-the-shelf equipment isn’t particularly well-suited to that environment.”
In addition to reducing colour change times by nearly 90 percent, the controls also eliminated a previously substantial area of opportunity for human error – improperly cleaned colour lines. As with degraded seals, improperly cleaned and/or switched valves and lines mix coloured powders (e.g. white powder particles from the previous colour combine with the new black powder, resulting in salt-and-pepper sheet metal and costly scrap).
“The goal was quickly purging the old colour and loading a new colour in the system without cross-contamination,” Khoury says. “If a quick colour change system does not properly clean the powder delivery lines, thousands of dollars of scrap and rework will be created.”
Thus, Nordson’s challenge became programming the proper system purge cycle to clean the entire colour feed line from powder hopper to manual spray gun (which features continuous or pulse purge). This was particularly daunting, since the Prodigy HDLV can accommodate 28 colours. The additional colours provide just-in-time manufacturing for zero inventory, improved efficiency and higher throughput than previous machines.
“We knew there would be a problem with establishing a standard time sequence for cleaning all powders and system configurations; some powders are more difficult to clean than others,” Khoury explains. “Also, the length of the system delivery lines can impact cleaning. Nordson engineering developed the hardware and software that allows the purge timing sequence to be configurable based on the customer installation. Just supporting the large number of colours makes the system complex, mandating custom diagnostic screens so individual valves can be tested.”
As engineers grappled with programming the purge cycle parameters, they built upon the pinch valve technology used in the product’s pump and adapted it to a manifold responsible for powder flow.
“We developed a custom pneumatic manifold to pressurize and expand the valve bladders, which pinch off powder flow passages in the custom quick-change powder manifold,” Khoury says. “When the air pressure is removed, the pinch valve’s bladder collapses, allowing the system to control the powder flow paths.”
With initial Prodigy HDLV deliveries, engineers closely observed performance and how the purge cycle faired. The Prodigy is equipped with 32 valves, and engineers believed increased functionality for valve timing would maximize purge efficiency, ensuring all lines would be perfectly purged every time.
“We learned from an early install that we needed to incorporate variable timing into the Prodigy to accommodate more purge cycles and different line lengths,” explains Khoury.
As part of the controls design, Nordson engineers selected the WAGO PFC because it could also support ancillary components such as the iControl and Prodigy Automatic Powder Spray systems, as well as the gun movers and part identification systems.
Lean methods fatten profits
Nordson estimates the control system and machine provide up to 35 percent increased efficiency on 90 percent of powders sprayed. Additionally, faster colour change capability allows same-day coating and shipping for approximately 80 percent of received parts.
The controls behind Prodigy HDLV enabled Nordson to create an effective workaround, allowing users to efficiently produce several variants within one product line. Khoury revealed Nordson is going one step further by developing a fully automatic Color-on-Demand System that includes the control of gun movement.
“We are actively developing an automatic version that automatically articulates the spray guns (they move in/out, up/down),” Khoury explains. “This is intended for higher-volume applications, such as coating outside furniture, barbecues or anything requiring high-temperature powder paint. The auto system provides faster throughput.”
Among system details Khoury shared, the WAGO-based control system will be adapted from the Prodigy Color-On-Demand system and evolved to supervise the automatic control system, including the articulating guns. For instance, there are X and Y mechanisms responsible for moving the guns up/down and in/out to properly coat large quantities of metal.
“The human factor with the manual Prodigy HDLV can provide that special quality,” he says. “The automatic version, on the other hand, is being developed for manufacturers with production scales calling for a higher-capacity system.”
Regardless of Prodigy variant chosen, Khoury maintains that substantially improved throughput and quality will allow users to embrace Lean principles such as just-in-time production.
“The old way of coating was not economical,” he says. “If you had a 25- or 30-foot long spray booth that’s 10 feet tall, you’d spend hours blowing it out, collecting waste powder for use in non-colour sensitive products. Then you’d have to clean out the booth, wipe out and ensure every speck of black powder was out before switching colours. This is just the booth, then there’s line prep. Imagine powder-coating 1,000 black barbecue grills, then prepping the room for a run of 1,000 white, 1,000 red and 1,000 grey grills.”
In addition to labour savings and eliminated rework and scrap, Khoury points to the Prodigy’s ability to save the actual coloured powder used to coat metal, effectively lowering material costs.
“There’s also powder transfer efficiency,” he explains. “The Prodigy spray gun uses a lower velocity spray, allowing paint particles to collect during dispersal and more effectively coat metal. The metal enjoys a higher-quality coat, while operators save on powder costs because there is 10 percent less wasted powder.”
Mark DeCramer is the WAGO-I/O-SYSTEM product manager for WAGO Corporation. Contact Mark at firstname.lastname@example.org or 1-800-DIN-RAIL.