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Hot Tips for Heat Measurement

(January 2001) posted on Fri Jul 27, 2001

A review of temperature-measurement tools and how to use them.

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By Rick Davis

During the flashing process, the flash unit is employed to bring the ink film printed on the garment up to a temperature of 125-275°F (50-135°C) in 4-10 sec, depending on the gel point of the ink. Because the flashing process takes place so rapidly, measuring the heat our prints are exposed to is all but impossible. Of the measurement devices available, only the non-contact pyrometer will give us a reasonably accurate estimate of the temperature that the ink-film surface reaches during flashing. The other tools simply don't react quickly enough to provide accurate temperature data.

Because it occurs in such a short time and temperatures elevate so quickly, there is no practical way to measure the temperature of the ink film during flashing. Instead, we must take a reading as soon as the garment indexes out from under the flash unit.

We point our non-contact pyrometer at the middle of the graphic as it rotates out from under the flashing head. The pyrometer will instantly provide a reading of the temperature and show it dropping as the print cools. For this reason, we need to keep a close eye on the digital readout to determine the peak temperature as the print emerges. The temperature should not exceed 275°F to ensure that the next ink layer will adhere to the underbase properly. If the underbase reaches 320°F (160°C), its surface will be overcured, and subsequent ink layers won't adhere well.

To determine the temperature of the flash panel itself, we have two choices. A non-contact pyrometer will work when the platen moves out from under the flash head, and we can aim the pyrometer directly at the flash-unit's panel. But since the non-contact pyrometer will pick up the slightest variations in temperature, the readings will most likely be erratic.

Using an on-contact pyrometer, we can place the probe in direct contact with the flashing element and determine the actual temperature at any point on the panel. The temperature fluctuations that an on-contact pyrometer experiences are not nearly as great as those of the non-contact pryometer.

Measuring dryer temperature

In the conveyor dryer, it is not as important to know the peak temperature reached as it is to know how long our garments and prints are exposed to that temperature. The exposure time is really what determines if the ink film is cured completely.


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