Temperature Measurement

Automotive paint cure – a formidable task

April 2021 Temperature Measurement

When choosing a new car, the cosmetic appearance of the paint is critical to the visual quality of the product, but the appearance also underlines the important role that coating technology plays throughout the car's operational life.

The process

To provide protection to the car body, a series of coatings is applied to the raw substrate and then thermally cured. The accurate control of the thermal cure process is critical to the performance of the coating both cosmetically and physically. To complete the cure reaction, it is necessary that the part of the car on which the paint is applied achieves the paint supplier's recommended cure schedule.

The dilemma

For successful paint cure, there is a critical need to measure and control the temperature of the car body as it travels through the oven. Today, paint ovens are controlled in a sophisticated fashion with static thermocouples located in each zone that give constant feedback on the ambient temperature of the oven. Although this temperature data is helpful in giving an idea of process control from an oven perspective, it does not tell the whole story. To cure the paint to specification, the critical information required is the peak metal temperature and the time that peak metal temperature is maintained. The control thermocouples in the oven cannot provide this data for the following reasons:

1. Air temperature measured by the control thermocouple is not necessarily an accurate measure of the air temperature at the product surface.

2. The temperature at different locations on the car body may be different for the same oven zone temperature because of varying metal thicknesses, so there are different thermal masses and heating rates.

3. Control thermocouples give constant temperature readout but do not provide any data relating to the transition time of the car body through the heated zone, which is critical to validate that the cure schedule has been achieved.

4. The control thermocouple may not accurately represent the effect of oven loading patterns on the car body shell temperatures, as the total thermal mass of cars in the oven can change significantly.

Temperature profiling

Oven temperature profiling is the only truly accurate method by which the oven paint cure process can be monitored to determine that the paint cure schedule has been correctly achieved. The oven profiling system is designed in such a way that it is able to travel with the car body through the cure oven, continuously measuring the product temperature at selected locations on the car body. At the end of the process, the collected temperature readings create a thermal profile, from which the cure schedule can be measured and validated. An oven profiling system consists of four elements:

* Thermocouples are attached to the product to measure actual product temperature.

* A data logger captures and stores the temperature data from the thermocouple.

* A thermal barrier, an insulated box, protects the data logger from the hostile conditions within the oven.

* A software package provides the end user with tools to review, analyse and generate reports on the temperature profile.

As a leading manufacturer and service provider for temperature profiling systems, Datapaq provides a complete offering for monitoring, logging, and profiling of temperatures in applications involving furnaces, ovens or dryers.

Prior to a production run, the SmartPaq feature can be used to program the system with critical performance criteria (maximum temperature limits, time at temperature, or acceptable Datapaq ranges). The data collected by the logger is analysed against these criteria at the end of the run, and the logger displays whether the specifications were achieved. The SmartPaq feature makes profile qualification as easy as checking for a green 'Pass' LED, and data can be analysed in full later after all runs are completed.

The MicroMag is a probe designed specifically by Datapaq to meet the demands of general automotive paint profiling strategies. The compact sensor can be placed in the tightest of recesses, allowing those difficult areas to be measured accurately and efficiently.

The Datapaq Insight software performs an automatic temperature difference analysis to highlight problem areas within the process. The Insight system has been developed to allow customisation of reports generated, while within the Insight SPC program, SPC (statistical process control) charts are generated without any need for data export or the use of additional software.

For more information contact R&C; Instrumentation, +27 11 608 1551, info@randci.co.za, www.randci.co.za


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