How Temperature Profiling Can Improve Your Process
In regards to temperature profiling, one of the most significant technological breakthroughs in recent years has been the introduction of real-time monitoring (figure 1). Radio telemetry allows data to be both recorded by the datalogger as it travels through the oven and transmitted directly to a receiver connected to a PC running the profiling system's operating software. With such a setup, product and oven temperature can be viewed and analyzed live within the oven. The obvious benefit of this capability is that decisions can be made immediately -- without the wait involved with conventional datalogging.
Real-time monitoring can benefit your process by allowing you to:
- Compare real-time data with predefined process and cure criteria before the temperature monitoring system has left the oven.
- Run repeat profiles without having to open the thermal barrier or download data.
- Identify the exact moment cure criteria are achieved.
- See immediately when problems occur and use the information to correct them.
- Use live data to speed up oven optimization and balancing by viewing the effect of parameter changes instantaneously.
In many situations, data analysis of a temperature profile has been limited to calculating time-at-temperature. At best, comparing profile data vs. the coating supplier's cure schedule at a single temperature provides only a rough estimate of whether the correct cure conditions have been achieved. The overriding deficiency of evaluation using only time-at-temperature data is that it ignores any contribution to curing from temperatures above or below the critical temperature. To get an accurate calculation of full cure conditions, it is essential to include the whole profile.
Quantifying Cure QualityA cure index can quantify the degree of cure experienced by a product for a specific coating and thermal process. Every powder coating has a specified cure schedule: a defined temperature/time profile at which optimum cure is achieved. Use of the cure index allows immediate identification of the quality of cure on any product irrespective of its shape, thermal mass or orientation within the oven.
From defined cure schedules, it is possible to define a curve-linear function. Any point along this curve is equivalent and would provide 100% cure. An appropriate kick-off temperature is set to define a minimum point at which crosslinking will take place. Additionally, a maximum temperature should be assigned at or above that which would cause overcure and possible coating damage.
Actual temperature profile data is analyzed by the software, and the fit between it and the theoretically perfect cure curve is evaluated and quantified using the cure index. Generally, a pass/fail range of acceptable cure index values are derived from testing of cure quality by independent physical quality assurance testing. In most situations, the acceptable cure index tolerance band range is between 90 and 115, but it may be lower in other situations. An example of such a case involves infrared heating. Employing infrared heating, powder coating undergoes curing well before the substrate temperature rises to the theoretical cure temperature. Under such circumstances, cure index values in the range of 60 to 80 have been proven by differential scanning calorimetry to provide fully cured components. Once an acceptable cure index tolerance band is defined, the values obtained on a routine basis can be monitored as part of quality assurance statistical process control activities.
Effective ControlIn the past, temperature profiling was performed only when problems were encountered on a finishing line. For many reasons, the thinking regarding temperature profiling is changing, with a general industry understanding that prevention is better than cure.
To control a process, it is essential that regular testing be performed. The increasing demands placed on quality assurance managers require that the actual profile run be undertaken by those working on the shop floor. To help with this requirement, it is essential that a system make it easy to collect raw data yet at the same time provide the analysis and reporting capability to allow full interpretation. Software customization is one way in which a single standard software package can be setup to provide the functionality required by the individual using it.
By collecting regular profile data, users can gain a better understanding of how the oven or process changes over time. If profiling daily, it may be possible to apply statistical process control procedures. Trending functionality allows the monitoring of specific parameters and can identify slow deterioration in process performance. Identifying problems early allows corrective action to be implemented before a crisis situation is reached.
Temperature profiling, now widely used in the finishing industry, has come a long way from its simple beginnings of a basic chart recorder temperature trace. It is no longer performed only in situations where the quality of the cure on a product is in question. Developments in both software and hardware are allowing accurate profile data to be routinely collected by those working on the shop floor with all the traceability and certification necessary of quality standards.
Profile data, enhanced analysis and review capability allow more understanding, control and optimization of oven operation. Through regular use, collected profile data can be used proactively to highlight potential problems before they become critical.