Although the presentation of a thermal profile has many variations, in its simplest form, temperature data is plotted along the Y-axis of a graph and time is plotted along the X-axis. When the points created along this graph are sequentially connected, what results is a graphic representation, or a thermal profile, of the temperature the product experiences over a given period of time. Thermal profiling serves the following purposes:
- Process Setup. Thermal
profiling is a setup tool that enables the engineer to establish the
optimal process specifications.
- Process Monitoring. Thermal profiling is
used to monitor the process and ensure defect-free pro-duction.
- Process Improvement. Thermal profiling can be an invaluable tool for determining potential areas of process improvement. Everything from process optimization to improved preventative maintenance frequencies can be accomplished with the aid of this information.
Simply put, thermal profiling is a tool used to establish, monitor and improve thermal processes. Figure 1 shows a profile for the powder coating of automotive accessories.
Why Profile?There are as many ways of heating a product as there are industries using heat: conveyorized and batch ovens as well as convection, infrared and direct heating systems. In each application, the equipment is designed to heat and cool the product to match a specific thermal profile. If the actual profile produced by the heating equipment matches the specified profile closely enough, the product will be saleable. But, if the profile that the product sees does not meet the profile specification, the product either will be defective and require rework or else be scrap. Asking several key questions can determine how much effort an operation should dedicate to thermal profiling and monitoring.
- How Valuable Is the Product? Is the potential loss from an out-of-control process significant?
- How Thermally Sensitive Is the Process? For example, aluminum brazing requires product reach a precise temperature, and it is necessary control process temperatures to I5°C.
- What Is the Production Volume? Is the thermal process a point of constraint in the manufacturing process? If so, there may be a potential for increased throughput and process improvement.
- What Is the Defect or Scrap Rate? Are you accepting a high defect or scrap rate because you have always had one?
- What Are Rework Costs? Are they a significant portion of overall production costs?
- Are Field Failures Traceable to the Thermal Process?
- Is the Overall Process Capital-Intensive? If so, minor improvements to any portion of the process, including the thermal portion, may result in significant additions to the overall bottom line.
- Does the Process Need to be Documented? Do product liability issues - for example, those associated with food processing or the production of critical automotive components - exist?
- Is Setting Up And Maintaining the Thermal Process Labor-Intensive? Does it result in significant production stoppages?
If the answer to any of these questions is yes, then some thermal profiling is required. If the answer to two, three or more of the questions is yes, the process is probably a candidate for real-time thermal monitoring.
Strip Charts, Trailing Wires and DataloggersThe strip-chart recorder was the first true profiler. Early models could plot only one thermocouple at a time. The thermocouple would be attached to the product and a long wire trailed behind as the product moved through the oven. Specially marked paper moved through the strip-chart recorder at a preset speed and a pen tracked thermocouple temperature. The profile appeared on the paper in real-time. Many strip-chart recorders are still in service though more sophisticated and user-friendly temperature profiling equipment is available.
Dataloggers were developed to eliminate the trailing wire and allow processes to be profiled during production. A typical unit accepts from three to 12 thermocouples and runs through the process behind the product. As it runs through the process, it stores the temperature profile information in its internal memory. Once the datalogger emerges from the process, the temperature data is downloaded to a PC for analysis.
While dataloggers are an improvement over strip-chart recorders in that they can record more temperature data, they have disadvantages as well. They do not show the product temperature in real-time. This can be a problem in longer processes, where the risk of the datalogger overheating and losing the data, thus requiring another profiling run, is higher.
In higher temperature applications, dataloggers are protected by massive thermal barriers. These barriers cause multiple problems. Outgassing from the barrier can contaminate the process. The weight of the barrier can act as a heat sink, and the size of the barrier can disrupt oven airflow, which distorts the profile and gives an inaccurate representation of actual production conditions. It often is necessary to place dummy product around the thermal barrier to offset the heat-sink effect or interrupt production to take a thermal profile.
Real-Time ProfilingReal-time profiling is now available in two forms. One is a pass-through profiling unit that provides live output to the profiling software via radio frequency. This system speeds up the profiling process by eliminating the need to capture profile data in a datalogger's memory and then return to a desktop computer to export the data and review the results. When used with automated prediction software, a real-time profiler can identify the best combination of zone setpoints and belt speed to process a given product.
For higher temperature processes, a wireless profiler can be used with a thermocouple extension. This approach provides real-time profiling without the problems associated with thermal barriers. Customized to the process, the thermocouple extension is attached to the product and the real-time profiler. The profiler never enters the oven, so a barrier is not required. When the profile is completed, the thermocouple extension is uncoupled from the profiler and passes through the process. Because product can be placed on top of the thermocouple extension, production is not lost while retrieving the thermocouples. This also means that the oven can be profiled fully loaded, ensuring a profile that reflects the process.
This technology may sound to some like a return to the original method of thermal profiling, but significant differences exist between this system and a strip-chart recorder with trailing wires. This system delivers real-time data and allows users to analyze data from up to 12 thermocouples using thermal profiling software, yet it avoids the problems associated with dataloggers and thermal barriers. Users can employ the automated prediction software to determine the optimal profile for a given process, reducing process setup time.
Continuous Automated MonitoringFor processes requiring full-time thermal monitoring, a real-time thermal management system can be used to continuously collects live data. Up to 60 thermocouples are embedded in slim stainless steel probes that are permanently mounted in close proximity to the product (figure 2). The probe thermocouples continuously monitor the process temperature, taking readings as frequently as every 5 sec. Temperature readings are displayed as process profiles on the oven's PC screen. All data is recorded permanently to the hard drive, giving users the ability to review process data from previous production runs.
The real-time thermal manager provides a product profile for every product pro-cessed by creating a mathematical correlation between product profile, as measured by the profiler, and process temperature, as measured by the real-time thermal manager thermocouple probes. A virtual product profile can be calculated every 30 sec, and virtual profile statistics such as peak temperature also are calculated and updated continuously. The real-time thermal manager can detect critical temperature variations that the oven control thermocouples cannot, then reveal these temperature drifts and their locations. If a virtual profile falls outside of the predefined process window, the system will alarm.
Continuous automated thermal management offers users several benefits.
Elimination of Confirmation Profiles. Continuous process monitoring eliminates the need to periodically check the process, which can be time-consuming and interruptive to production. Furthermore, if a problem is detected between confirming profiles, it is difficult to know when the process began to drift out of specification. Continuous monitoring allows users to pinpoint when the process began to drift.
Scheduling of Preventative Maintenance. Continuous process monitoring detects when oven performance begins to deteriorate so maintenance can be done when needed.
Automated Process Documentation and Statistical Process Control (SPC). Profile data can be recorded automatically to the hard disc at selected intervals and also can provide live data output to automate SPC data collection.
Network Process Data. All ovens can be monitored from a single location, and the process can be remotely monitored over the Internet.
Troubleshooting. The thermal process can be instantly assessed and eliminated as the source of the problem. If the process is the source of the problem, the proper corrective action can be determined quickly.
Zero-Defect Production. Continuous process monitoring eliminates potential product defects due to thermal variations.
Real-time profiling and monitoring offer manufacturers using thermal processes an opportunity to improve efficiency, productivity and quality. Recent cost-benefit studies using software simulations of industrial assembly lines have shown that even small time savings - as little as 1 to 2 hr per week - produce significant cost benefits, especially in the form of increased profits realized from increased production. Thermal profiling technology can help users to realize these time savings through more efficient equipment utilization.