The goal of thermal profiling is to always increase quality and reduce waste. Three case histories -- covering powder coating, baking and solder reflow applications -- show you how three manufacturers achieved these goals.

Figure 1. The “process window” is where time and temperature come into perfect synchronization.

All heating processes have one thing in common: the “process window” (figure 1). This is the area where time and temperature come into perfect synchronization.

In electronics, it is the point when solder melts to form the perfect electrical connection. In baking, it is where bread reaches the optimum temperature to create a perfect loaf. For powder-coat curing, it is the point where the process melds the coating into a permanent seal with the base material of the product.



Figure 2. A product profiler is attached to the product and passes through the oven, recording the temperatures the product sees during processing. Figure 3 (middle). Used to record temperatures continuously, a thermal process monitor is permanently installed. Probes are installed in the oven tunnel and connect to a data recorder mounted on the oven. Figure 4 (bottom). The machine quality management tool is a self-contained thermal recording pallet used for optimizing and characterizing ovens before use.

Typical types of equipment used for temperature control include:

  • Thermal profiler (figure 2), which records and maps out the temperature of products and process.

  • Thermal monitor (figure 3), which continuously watches for and reports temperature fluctuations.

  • Machine quality management (MQM) tool (figure 4), which is used to qualify the oven profile before running product profiles.

    The goal of thermal profiling is to always increase quality and reduce waste. Today, the need for process control has never been higher. Profit margins are tight, products are more complicated, materials are changing and product life-cycles are getting shorter every day. Manufacturers must be able to make informed decisions for continuous improvement, as illustrated in these case histories.

    Figure 5. A temperature profiler optimized for curing applications is used to profile powder and coating cure processes -- both ambient tunnel air and product temperature.

    Sherwin Williams Helps Customers Improve Profits

    Sherwin Williams covers the full range of powder coating applications. Randy Klassen, territory manager for Sherwin Williams Chemical Coatings in Western Canada says that a major part of his job is helping customers set up curing lines for new products, test new coating materials and improve processing profit margins.

    “Cure ovens are not sophisticated. They have an air box with dampers and simple controls, and one or two internal temperature probes. Many times, the fire box is in the middle of the oven, but the control probe runs down the side wall and off to a corner. It's not necessarily representative of the heat that the parts experience,” Klassen says.

    Operators can use thermal tape, sticking it on parts of different sizes and thicknesses, to measure cure results against the oven's time and temperature. Unfortunately, this “pass or fail” exercise does not tell why certain parts did not cure.

    “I prefer a precise method,” Klassen says. So, Klassen uses a temperature profiler that allows him to see the profile of air-to-product temperature on his computer screen and actual part temperature every step through the oven (figure 5).

    Klassen explains, “I'll run the profiler through the oven attached to areas of various thickness on the parts. This lets me determine whether we are going to be able to obtain 100 percent cure on any specific powder coating over any certain time duration and temperature.”

    In curing processes monitored by temperature-profiling equipment, air probes measure the ambient air. The metal temperature comes up as a result of the convection within the oven. The metal must reach specific temperatures to begin the curing process and then experience the heating cycle for a specific amount of time (process window) to achieve full curing. The software automatically displays this percentage cure based on the published cure schedule of the coating.

    One of Klassen's recent cases involved a company that makes containers that are up to 100' tall. These are rotated on cylinders for powder coating and then moved into an oven that is 40 x 60 x 110', running at 6 million BTUs.

    “The manufacturer wanted to address natural gas consumption costs while improving cycle time. One or two more products processed per day would increase production by 10 percent to 20 percent, depending on the size of the container,” Klassen explains. “Because they produce 5,000 containers a year, that percentage would be a dramatic increase in profit.”

    To develop the curing process with a new low-temperature powder coating, Klassen attached his profiler to a container using magnetic thermocouple probes and sent both through the oven. Profiles of the ambient air within the oven and product profiles were conducted until settings were matched to the desired goal.

    “I import the profile into the software that comes with the profiler. It gives me a wide range of analytical tools that I can use and apply to find solutions. Using the software's prediction feature, I developed a process that reduces cycle time in a cooler oven, saving money from a gas consumption standpoint while simultaneously increasing throughput. The result was a very satisfied customer,” Klassen concludes.

    Profiling proves the effectiveness of products such as low-temperature-cure powder coatings. It allows graphic viewing of the process window which in turn, provides proof of a robust quality cure process.

    Figure 6. A temperature profiler optimized for baking applications monitors both the internal temperature of the baked product and the baking oven ∆T across the tunnel during processing.

    Process Control at Kroger Bakery

    Today, enzyme blends are used in baked products for texture and flavor as well as to enhance shelf life. All have different temperature ranges and contribute to crumb character during different stages of the baking cycle.

    Kroger Bakery's Clackamas, Ore., plant has six production lines that manufacture a range of baked goods. Mike Adams, quality assurance manager for the plant, notes that every time a product formula changes, so does the process to bake it.

    “Learning how enzymes affect the baking process can be a headache, especially when applying this new technology across a broad spectrum of products, as we were,” Adams says. “Hand-held temperature instruments and visual examination were not enough. A loaf of bread would come out of the oven with beautiful color, but when it reached the slicer, the blades would jam because it was still doughy inside. When we first started using enzymes, we were getting some pretty sad results.”

    The answer was to refocus on the internal, both product and oven, and get a better balance of air velocity and heat. Using a thermal profiling system that consisted of a pass-through profiler and software designed to baking industry standards (figure 6), Adams was able to stabilize the process.

    “A real window into baking is a combination of thermocouples inserted into the dough and measuring air temperature (∆T) across the oven tunnel. You have to measure conductive heat and convective heat in the dough and in the oven at the same time,” Adams explains.

    Adams entered monitoring parameters into the profiling program and placed the thermocouples across the oven and inside the product. He entered the time rate for data collection, the movement rate of the oven conveyor, and the length of the oven. The profiler interfaced to the software and generated a color 3-D profile on a computer screen, showing exactly how, where and when temperature changes occur both inside the product and throughout the full length of the oven, longitudinally and in balance left to right. If anything went wrong, the saved picture of the process told him where to look to solve the problem.

    “On a typical test, I'll first use all six profiler channels to check the air temperature on the left side, right side and center of the oven. Then, I'll insert three thermocouples into the center of the product. As my profiler passes through the oven bake cycle, it's recording temperatures every two seconds and shows graphically if I'm getting the thorough bake out required to deactivate the enzymes,” Adams says.

    Adams also found that profiling allowed him to determine the best overall dough-to-pan-volume ratios for a perfect baking cycle when developing new products.

    “Controlling our ovens using process profiling gave Kroger a monthly savings of $13,000 in reduced waste on just one line,” Adams says. “I think it is impossible to use cutting-edge enzyme technology without very good process control.”

    Figure 7. The modern reflow oven purchased by Universal Avionics has integral thermal profiling available from several profiling companies.

    Reflow Oven Offers Built-In Profiling Options

    Every manufacturer eventually has to change capital equipment. Jack Schreiner, manufacturing engineering manager at Universal Avionics Systems, Norcross, Ga., recently faced this challenge -- but with a twist. The new convection reflow oven for solder processing of lead-free surface-mount electronic circuit assemblies came equipped with self-profiling capabilities and an internal profiling suite (figure 7). Theoretically, all the user has to do is choose a profiling package, team it up with the physical profiler, and the oven controller and profiling software combine to reduce the time to set up and validate the new oven.

    The production line runs two shifts with a high product mix. A typical product run can be anywhere from 10 to 500 pieces plus prototypes. Because of the high mix in production, Schreiner hoped that if the profiling process truly was as streamlined as promised, it would be an added benefit for the frequent changeovers in production, each requiring re-profiling.

    Figure 8. Modern temperature profiling offers an array of statistical process control (SPC) software options designed to the specific needs of various industries. Used regularly, this greatly enhances overall process control.

    “Our monthly volume varies from 1,000 to 3,000 boards. Our line may retool as many as four times a day due to small batch runs,” Schreiner says. “Profiling is important, but it shouldn't consume half a day.”

    “I was not happy with the profiling software I'd been using, so the opportunity to select a new method, along with the new oven installation, came at the right time,” Schreiner adds. The profiling system Schreiner selected was from ECD Inc., Milwaukie, Ore., one of the several profiling packages integral to the new oven.

    The goal was to baseline the older machine, decommission it and replace it with the new machine while running production. To do this, the first profile had to be developed as soon as the new machine was powered up and signed off by the installation engineer. The existing oven was baselined using a thermal process monitoring tool to gain its thermal signature and determine its convective efficiency. Clip-on thermocouples were used, which eliminated using a sacrificial board; instead, the board used for profiling could be returned to the production.

    “The profiling software manages all the oven setpoints and changes as I profile. Anybody that has done profiling knows a lot of wasted time is spent waiting for the profiled assembly to cool down after each run, so fewer runs through the oven really speeds up the process. Using the clip-on thermocouples, I can install a fresh board while waiting for the other one to cool,” Schreiner says. “Scrapping assemblies for profiling is very expensive. I don't scrap circuit cards any more -- I just return them to production. Our new oven was set up and qualified in two profiling passes, and product profiling changeover has been cut from two to three hours per assembly to 30 minutes.”

    Conclusion

    Managing the new process requirements of thermal processes such as low-cure-temperature powder coatings, baking enzymes and lead-free electronic solder alloys are impractical without thermal profiling. Users can employ modern profiling gear to optimize existing thermal processes and easily adapt equipment to the demands of changing process windows (figure 8). This creates savings and enhances profitability and growth for any organization. A regular schedule of time-temperature process profiling is the key to overall thermal process control. PH

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