Measuring product time at temperature in thermal paint cure applications has been an accepted industry practice for more than 40 years. The earliest temperature profiling systems plotted data on a chart that process engineers had to laboriously analyze. Today, profiling systems that are both sophisticated and easy-to-use can give a clear pass/fail finding, prove regulatory compliance, alert operators when and where ovens are trending out of acceptable limits, or show that a process is within specifications. In addition, regular and frequent temperature profiling can help increase throughput, reduce rejects and lower energy costs.
Temperature profiling is a means of compiling product and process heating and cooling data over time. A profiling system generally consists of four elements:
- Thermocouples, which are either attached to the product or used to measure ambient air temperature.
- A datalogger, which captures data from the thermocouples.
- A thermal barrier, which protects the datalogger from the heat and cold.
- A software package, which is used for data analysis.
Temperature profiling gives engineers power over their process. They can identify and correct hot and cold spots, reduce fuel costs by increasing line speed and/or lowering oven temperatures, optimize their process, prove adherence to customers’ specifications and prove quality without the need for destructive analysis techniques. With the variety of temperature profiling system configurations and components offered, almost every thermal processor can benefit from routine profiling.
Industrial Oven Profiling Procedures
Processors with multiple paint lines most often want an easy-to-use yet sophisticated profiling system with all the bells and whistles. Process engineers in such plants need to know that product quality is the same on all paint lines. They want data that will help them schedule preventive maintenance, and they require documentation certifying regulatory compliance. Time-saving features are especially critical because temperature profiling is routinely required many times throughout the day on many different coating processes.
When considering the size of many large-scale manufacturing plants and the increasing erosion of manpower, an efficient profile process is essential and can decrease labor costs significantly. Some profiling developments allow routine monitoring of different processes using a single system without having to return to the quality assurance (QA) office to download profile data. With the multiple run capability and process pass/fail indication direct from the profiler, the operator can confirm QA at the oven exit.
For instance, one system uses a green light to show that the process is in control, so the operator can continue operations with confidence. A red light, however, warns there may be process issues that must be investigated further. Under these circumstances, the profile run would be downloaded to the PC and analyzed in detail. Also, some suppliers’ software can be set up to assess the process against key user-selected parameters such as peak temperature and time at temperature.
To see how these full-featured profilers work in a typical application, consider an example of sequential profile testing on production car bodies as they travel through each coating process. In this application, the barrier must not only provide thermal protection but be lightweight enough to ensure easy, safe installation and retrieval from the car body. Installing the system into a car on a moving production line requires skill and efficiency. To help with this, magnetic surface and air thermocouples can be used directly on the car body skin. Typically small in size -- at approximately 1" high and 1" dia. (25 mm high and 25 mm dia.) -- the micro-magnetic probes can be fitted into small recesses or on curved surfaces yet remain fixed throughout the production cycle.
Another important requirement of the thermal barrier is the absence of any silicone contamination. To avoid catastrophic paint contamination issues such as paint craters, the barrier must be designed to use insulation material that does not contain silicone or any silicone derivatives.
One innovative approach to profiling for multiple automotive coating lines is the single-pass method. Rather than testing each oven separately, the profile system is configured to travel the whole paint line in one operation. In the single-pass approach, the profiler is installed in test cars built and used solely for profile testing. Thermocouples are attached permanently to the test body at strategic points to ensure that readings are consistent run after run. To provide the thermal protection necessary to handle multiple ovens, one after the other, a high-performance thermal barrier is used that is capable of protecting up to 392°F (200°C) for nine hours. This gives the quality assurance manager the confidence needed -- even with line downtimes or delays. In practice, it is possible that the system can be installed at the start of day one and retrieved at the end of the line at the start of day two.
Likewise, for the operator, it is simple and safe. While the system travels through the production line like any other car, the operator can be doing other work. At the end of the run, the memory portion of the datalogger is retrieved from the now cold barrier, and the whole process is downloaded to the PC in one single step. If necessary, the single process trace can be separated for the individual ovens and analyzed and reported separately. It is estimated that the single-pass method could save up to 60 percent in labor costs when compared with established profiling strategies, where ovens are tested individually.
Converting Raw Temperature Data into Process Information
Collecting oven temperature data is only part of the story. Although important, the data needs to be reviewed and analyzed in such a way that it can be used to determine what is happening to the product being cured. The analysis software complementing the profile system is a key part of the complete system. Most analysis software is password protected with two-tiered access. Line operators are guided through profiling setup and start procedures while process engineers can drill down to more closely examine analysis functions. Manager-created process files function as templates, identifying critical process and analysis parameters.
The software also gives instant analysis with an alarm. For instance, if the user’s custom-defined parameters fall out of tolerance, an alarm alerts the operator to take corrective action. Quality issues are immediately addressed, minimizing rejects.
Additional software features often used in these kinds of applications include integrated statistical process control (SPC) and index of cure functions. Integrated SPC displays the variations in key process parameters -- peak temperature, time at temperature, and index of cure -- collected from individual profile runs. It will immediately identify when a process goes out of tolerance and can predict future process failures by watching for gradual changes. Maintenance then can be scheduled, avoiding costly unexpected shutdowns.
The index of cure calculation is used to quantify cure via analysis performed by the software. Simply put, index of cure is a sum of all time and temperature contributions taken from the profile and calculated into a single index value. A value of 100 indicates a cure that exactly meets specifications: higher than 100 indicates overcure and lower than 100 indicates undercure.
Another feature offered by some profilers is one that proves a coating’s cure cycle conforms precisely to the paint supplier’s bake window specifications. This feature automatically scrutinizes the profile data, providing analysis, cure specification and results. It also provides full traceability of report information back to the test equipment and raw profile data.
Although oven profiling has been used for many years in the coatings industry, it has come a long way from its humble beginnings. Updated technologies provide a range of benefits that allows the paint technologist or production manager to understand, control and optimize oven cure processes with even more accuracy and efficiency. The paint technologist is now able to maximize both the coating quality of his finished product, ensuring customer satisfaction, and the efficiency and productivity of the paint finishing operation.Editor's Note: This article was originally published with the headline, "Revolutionize Your Oven Profiling Strategy" in the March 2010 issue of Process Heating.