Precision Oven Temperature Measurement - PPG Industries Inc - Brief Article

Automotive Finishing, Fall, 2000 by Steven R. Kline Jr.

Matching the optimal time and temperature in an oven is critical to achieving a good cure of any coating. The use of a data logger and its associated software has helped one coating manufacturer demonstrate product workability and determine curing times and temperatures for its customers...

If the curing cycle is not performed properly, finishers can fall victim to a multitude of adherence problems. However, most of these problems are preventable with precise data on the curing window--the range of object and oven temperatures and curing times for optimal curing.

If temperatures and times are not kept within the curing window, the part's coating will either be underbaked or overbaked. Underbaking creates a soft surface film that allows the coating to be rubbed off, sometimes while still on the assembly line. The soft coating results in low surface durability due to poor coating adhesion. Overbaking, on the other hand, makes the coating too brittle and inflexible. These characteristics allow the coating to be easily cracked or chipped.

While curing departments must ensure that parts are cured within the curing window, they also must cycle as many parts per hour as possible through the curing ovens. Temperature control is crucial to achieving a proper cure and keeping assembly lines moving quickly.

While temperature control is critical, curing ovens are generally not equipped with precise heating control equipment. Consequently, oven users in production environments are often left wondering whether their ovens will properly cure a particular part, especially when testing a new coating or paint.

PPG Industries, Inc., Pittsburgh, PA, is one of the world's largest suppliers of electrocoatings. The company has maintained a coating test facility at its Flint, MI, plant since 1985. The Coatings and Resins Application Lab tests and demonstrates paints, primers and coatings and conducts sophisticated curing analysis for these materials. The central goal of this facility is to demonstrate product workability and to determine curing times and temperatures--procedures that were difficult to monitor with traditional temperature recorders and smoke chart measurement equipment.

The previous temperature recorders used by test technicians were cumbersome and not very accurate, which made testing very time consuming and fraught with guesswork. "With the old recorders, we were forced to manually setup all the data descriptions, which was tedious and also time consuming," explained Mike Masters, a robotics specialist at the PPG test lab. "Compounding the problem was the fact that the software we had to work with wasn't user friendly."

Mr. Masters said that changing the batteries of the measurement devices was also a constant chore for operators. The batteries would have to be recharged overnight after just one day's testing. The recorder also needed to be connected into a transducer to download the info. In a worst-case scenario, the memory pack could lose all its stored data if battery power was lost while it was in operation, forcing workers to reset basic parameters. "The old memory packs were a pain to use because we would have to reformat the device every time we did another test, even if it was similar to the previous one," Mr. Masters explained. "We had to re-input information on the number of probes used, all of the different temperature ranges, what types of curing we were doing, etc., since we couldn't store any of this data at all."

Clearly, PPG had to find a highly precise, easy-to-use measurement instrument that would streamline its test methodology and allow its test technicians to define the curing window with pinpoint accuracy.

After extensive research, PPG discovered that the temp-gard [R] from BYK-Gardner would greatly improve its measuring precision. The device is a compact data logger with a built-in LCD display housed in a stainless steel thermal protective case that allows it to remain in an oven during testing. Up to eight objects or air temperature probes can be connected to the unit at one time for multiple readings.

The data logger comes with the manufacturer's temp-chart[R] software that can document the entire curing process. It can also activate all important customer- and process-specific data and analyze temperature profiles to optimize curing.

"The temp-gard helps us define the curing window very precisely," said Mr. Masters. "This is especially helpful to us because we have a hard time keeping temperatures consistent in our curing ovens. It also helps us determine how wide a window is available for proper curing."

Battery problems also were resolved. The device has special long-lasting batteries good for up to 250,000 readings, allowing operators to function for weeks, even months, without battery changes. A standard PCM memory card is used for easy data storage and downloading of test data to remote PCs.

Memory packs never need resetting with the new measurement instrument. "Now anyone in our facility can setup a specific template for testing and reuse it as many times as they want, even if changes are made after each test," Mr. Masters stated. "After testing is completed, our technicians can access the same test data stored in the PCM card. The PCM card eliminates a lot of setup time and menu hopping with an excellent software package."