In-process temperature profiling is an easy and accurate way to verify an optimum cure. Profiling measures the product time at temperature on selected areas of a coated part or test piece while in the oven throughout the entire cure process. Oven controllers are commonly used, but they can't do the job by themselves as they only measure air temperature and can miss potential problems such as cold spots. Since the heating rate of any component is affected by many factors - including product thermal mass, air circulation and oven loading - product temperature measurements are essential to prove finish quality (see Figure 2).
Temperature Profiling - More than Quality AssuranceA temperature profiling system consists of thermocouples, a data logger, a thermal barrier and analysis software. Installation on a typical coating line is simple. Magnetic or clamp thermocouples are attached to a workpiece and connected to the data logger. The logger fits into a thermal barrier that protects the logger while it is suspended in the oven. After a run, the information in the data logger is downloaded and analyzed by the software. Telemetry-capable systems, which use radio frequency technology to allow transmission of temperature data from a data logger in real time, provide live data for those who need it. Such systems are typically used by companies with longer duration processes or batch processes where production throughput can be significantly improved by identifying immediately when cure is achieved.
The software provides a variety of analysis options, and finishing managers are using these features to improve operating efficiency and control of their thermal cure processes in the following areas:
- Quality assurance - confirm that all products achieve the required cure schedule to guarantee coating quality and immediately identify problems that could impact quality.
- Process control - prove that the cure process is being performed in a controlled and repeatable manner. Use statistical process control (SPC) to identify possible future problems so that maintenance can be scheduled at the plant's convenience.
- Process optimization - set up new cure processes accurately and efficiently.
- Improved productivity and efficiency - reduce rejects or rework, maximize throughput, minimize scrap and reduce fuel costs.
- Problem solving - when problems occur, identify the cause and location of the problem promptly. Use the profile information to suggest necessary corrective action, and prove the success of such action with follow-on profile runs.
- Compliance reporting - archive fully traceable and certified temperature profile data and/or create hard copy reports to prove to management, customers, coating suppliers or other regulatory bodies (ISO9001) that a process complies with formal operating standards.
Charting Cure PerformanceEach cured product has its own thermal footprint that coaters want to verify. For example, a typical polyester powder coated product might be heated to 400°F and held there for 10 minutes to achieve optimum cure. Temperature profiling measures and documents specific parameters that finishing managers need to verify. The typical cure schedule checks to make sure the product is at or above a critical temperature for a specific length of time that meets the coating supplier's specification for the coating used. However, a single time-at-temperature number isn't adequate because powder doesn't cure at one fixed temperature but at many time and temperature combinations. Taking this into account, many coating suppliers now provide bake charts or graphical cure windows, which show from laboratory testing the range of time/temperatures that can be applied to give a satisfactory cure. Bake charts are two-dimensional representations of available cure schedules that are often used in the automotive industry as a tool to help operators compare actual time-at-temperature data against the laboratory tested acceptable range by plotting the information on a hard copy chart.
The newest generation of bake chart software automatically creates a digital graphical cure window and eliminates the need for tedious and potentially error-prone manual graphing (see Figure 3). For processes in which multiple coatings are cured simultaneously, it is also possible to identify the optimal cure characteristics by overlapping the bake charts and determining the common cure region.
A major U.S. automotive manufacturer and a coating supplier collaborated with software developers to expand the benefits of the bake chart even further. The software now combines the bake chart cure window with an index of cure - a quantitative score calculated by the analysis software indicating how well the profile matches the cure schedule requirements for the coating, with a score of 100 indicating a perfect match between the practical and theoretical profiles - to make proving optimum cure even easier. The bake chart function translates the time-at-temperature coordinates into index of cure data that describe the boundary limits of the bake chart, thereby giving the acceptable range of index of cure values for a particular process. This advance guarantees a high quality product with optimal cure.
Once this data is established for an individual product, the index of cure can be used alone, independent of the bake chart, to provide a "pass/fail" evaluation of cure. It makes accurate profiling for any shape or size of product as easy as checking that the reported index of cure is within tolerance. In addition, the software also lets the finishing manager set alarms that immediately inform the line operator when the index of cure is out of specification and the cure might be unacceptable. For example, in the graph shown in Figure 4, the out-of-specification range would be <100 or >955.
This information is also critical for process optimization. Once the target index of cure is known, finishing managers can adjust the balance of line speed and oven temperature to attain the highest throughput at the lowest cost. A quick check that the index of cure is within defined limits verifies that an acceptable cure has taken place.