Powder coating on a large scale

Powder coating offers several advantages over wet coating. It is also more environmentally friendly. But how can complete roof rafters or particularly long steel beams be powder coated in one piece? In Denmark, an XXL powder coating line has been specially designed for this purpose.

The Danish company Grædstrup Stål, headquartered in the east of the Jutland peninsula, is a steel construction company that attaches great importance to quality, efficiency and environmental friendliness. Every year, the company processes over 400 tons of steel into custom-made steel profiles and rafters, which are used in the construction of halls for industrial, agricultural, sports and recreational use. Grædstrup Stål uses modern technology in the production of steel profiles and roof rafters for building construction. To improve the coating of their products and productivity, they installed a specially developed XXL powder coating system in dense phase technology from Nordson.In manufacturing, Grædstrup Stål follows a zero defect strategy - from the first cut or weld to the final product, most production in the 14,000 square meter hall is fully automated.

While others in the industry coat with wet paint, Grædstrup Stål believes that powder coating offers better corrosion protection and high surface quality with minimal environmental impact. "Coating our steel beams and rafters with powder coating has only advantages," says Allan Kristensen, Grædstrup Stål board member. "It makes them resistant and durable even under difficult conditions, much more so than is the case with conventionally wet-painted steel components. Our designs are in demand and approved even in Germany, where regulations are often particularly strict."

Grædstrup Stål started up its first powder coating plant in 2000. At the time, the powder coating plant was one of the largest and most modern on the market. It could process products up to five meters high and 23 meters long. Kristensen describes the start-up as a milestone in the company's development. Although the system has been upgraded over the years, management realized in early 2018 that the previous coating booth was a bottleneck. Color changes took several hours, reducing operational flexibility, and the low conveyor speed forced them to run a 3-shift operation.

At the beginning of 2018, the company management began to look for a better solution to increase productivity. In particular, time savings during color changes, a reduction in powder consumption of the scrap should be achieved. In addition, the coating of difficult weld seams and corners was to be improved and, in the course of the climate discussion, the environmental impact was to be further minimized. For further planning, contact was made with Peter Johansen of Lakteknik, Nordson's Scandinavian regional representative. Greadestrup already knew Peter Johansen from earlier times when modernizations of the old plant were carried out. Together with Graedstrup, Peter Johansen now undertook the planning and organization of the new coating plant. The planning went well and was completed quickly. The installation that followed required no more than four technicians and only two and a half weeks to put this really large system with dense phase technology into operation. "This also resulted from the excellent partnership with Lakteknik" says Kristensen.

Color change reduced to 15 minutes

The new ColorMax E powder coating booth is 4.3 meters long and 7.2 meters high and is designed for fast, convenient color changes, allowing color changes to be made within 15 minutes - a significant improvement over the old system. Patented surfaces prevent powder from sticking. The booth also provides reliable powder containment through its 5.2-meter-high product openings, while ensuring smooth airflow in the spray area. The automatic floor cleaning system reduces powder in the booth to an absolute minimum. The system is complemented by two 6.5 meter high variable stroke hoists, each equipped with four Encore HD automatic powder guns. Automatic cleaning of the pumps and spray systems, and of the Spectrum HD powder center, simplifies color changes. After the 16-day installation and commissioning of the XXL booth, the new powder coating plant started operation in April 2019.

One of the special features of the system is that the conveyor speed can be varied depending on the size of the product. For this purpose, the products are scanned in front of the booth. The PowderPilot HD system control monitors the product height to determine the speed at which the workpiece should be conveyed through the coating area. If a product hangs mostly vertically, which also requires a long lift of the hoists, the conveying speed can be reduced to one meter per minute. If rafter areas are only 400 millimeters high over a length of 20 meters, the conveyor speed can be increased to as much as six meters per minute, achieving maximum productivity while maintaining coating uniformity. The lift height, as well as the lift speed of the hoists and the powder output of the spray systems, are automatically adjusted to the production speed to minimize overspray.

Powder is applied by eight Encore HD automatic powder guns. Nordson's Encore HD powder guns use dense phase technology, which pumps more powder to the powder gun at lower speeds using less air. The resulting 'soft cloud' produces very good first coat efficiency and allows for high process control. Thus, even the most difficult welds and corners are optimally coated due to the minimal spray speed.

The quick set-up was followed by a short training session, during which Peter Johansen familiarized everyone involved with the benefits of dense phase technology and the system control. "This is a complex industrial plant, and of course we had planned for there to be a few teething problems at first," says Kristensen. "But in fact, from day one, everything has worked exactly as it should."

The Grædstrup team is impressed by the uniformity with which the coating results are achieved. "To do this, you have to know that we prepare our steel structural elements differently depending on their subsequent use," Kristensen continues. "Mainly we coat for corrosion protection classes one to three, so we need coating thicknesses between 70-120 micrometers or up to 200 micrometers for protection class three. And the new system, with only eight guns, produces these much more uniformly than the old spray system with ten guns. The increased powder application efficiency, combined with the variable and programmable powder output, thus results in an annual powder savings of 40 percent."

Productivity has also increased. Twice as many steel components are now coated with the same amount of powder as before. "What is immediately noticeable is that everything is so much faster now," Allan Kristensen summarizes his impressions. "We used to work with an average conveying speed of about one meter per minute, but now the plant runs at 3.5 meters per minute."

The fact that color changes now only take 15 minutes instead of up to six hours has reduced machine downtime. The days when coating represented a slowdown in production are gone: On average, Grædstrup Stål has determined, the output of an 8-hour shift now equals what used to take 20 hours. Kristensen continues, "Since we started production, we have spent hardly any time maintaining the system. It just runs. The dense phase pumps are just as good today as they were on the first day of production - we haven't touched them. Also, the support we get from Peter Johansen is very responsive. We feel well taken care of."

Based on the knowledge gained so far and with the experience that Peter Johansen continues to provide, the team at Grædstrup Stål is now embarking on further optimization. "The Nordson system makes it easy for us to gain a deeper understanding of the processes," Allan Kåstrup Kristensen describes the goal. "We have recently started monitoring the coating and are currently experimenting with recipes and conveyor speeds to further improve our productivity and realize additional cost savings. This investment has already paid off for us."

Nordson Germany GmbH

www.nordson.com