Hot Tips for Heat Measurement
A review of temperature-measurement tools and how to use them.
While heat is an essential component of the garment-printing process, it is one of the most troublesome variables that we face as a screen printers. Not only must we maintain the right conditions in our dryers for each combination of garment, ink, and application we print, we also have to stay on top of the heat emanating from our flash-curing units.
Failure to regularly measure the performance of our flash units and dryers can have severe consequences. These include undercured prints with poor washfastness, over-gelled heat transfers that won't transfer and adhere correctly when applied to garments, overflashed underbases that prevent good adhesion with subsequent ink layers, scorched garments, and more.
To combat these problems, we need to regularly monitor the heat that our garments are exposed to during production. Several measurement devices are available to help us in this endeavor, and this month, we'll take a closer look at each of them and the areas of production they're best suited to measure.
In screen printing, four temperature-measurement tools are typically used to gauge the performance of our equipment and determine the amount of heat our printed garments are exposed to. These tools include the following:
* temperature tapes--small adhesive strips that are affixed to garments and indicate the approximate temperature through a heat-activated color change
* non-contact pyrometers--electronic devices that read temperatures without physically contacting the material being measured
* on-contact pyrometers--electronic devices that read temperatures with a probe that physically contacts the material being measured
* doughnut probes--electronic devices designed to accompany the product through the dryer and continuously record heat conditions
Although none of these devices provide absolute temperature values, they help us establish important guidelines that lead to successful curing on the jobs we produce.
Measuring flash-curing temperature
Of the two heat-generating areas in print production, flash curing is probably the most difficult to control and, hence, very important to measure. The reason flash curing is so difficult to control is due mostly to the extreme temperature changes and the short time span in which the changes occur. These factors combine to make accurate temperature measurement on flash units a challenge. Compounding the problem is the fact that flash units come in a variety of configurations with different heat sources, including continuously operating infrared (IR) panels and quick-starting fused quartz elements that can jump from room temperature to 1400°F (760°C) in seconds.
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.
Since the best time to measure the actual temperature of the ink is while the garment is still in the dryer, both non- and on-contact pyrometers are automatically excluded as options. These tools can tell you the ink-film temperature when the garment exits the dryer, but they cannot tell you the peak temperature or the duration of exposure.
The most simple tool for measuring temperature within the dryer is heat tape, which has been a standard in many shops for years. One or more strips of the tape are affixed to the garment surface near the print before the garment is passed through the dryer. When the garment exits the dryer, a heat-sensitive ink on the label shows the highest approximate temperature reached during the curing process. Once again, however, the strip only tells us how hot it got inside the dryer, not how long the print was exposed to that temperature.
To enlighten us more about the conditions inside our dryers, the best tool we have at our disposal is the doughnut probe. This device rides through the dryer alongside our garments and measures the temperature of either the ink film or the belt at any point within the chamber.
Doughnut probes continuously record temperature data throughout the drying chamber. Not only do they inform us about what peak temperatures were reached, they also tell us how long prints are exposed to the various temperatures within the chamber.
We can download the data collected by a doughnut probe into a personal computer and use it in a spreadsheet program to plot a chart that shows hot and cool spots in the dryer. In IR dryers, these fluctuations in temperature could indicate poor airflow or failing heating elements. In gas forced-air dryers, cool spots may mean clogged air knives. As with the flash-curing unit, we can use an on-contact pyrometer to measure the actual temperature of any radiant heat panels within the dryer.
Even if you had to purchase each of the tools discussed in this article, you could still end up spending less than $500. That's a pretty modest investment for devices that could save you hundreds or thousands in miscured printed garments. But you'll only get the full benefit of these tools by making them part of regular monitoring and maintenance procedures for your flash-curing units and dryers and using them whenever job characteristics (inks, substrates, print type) change. Next month, the discussion continues with a look at how these tools can be used when applying heat transfers to garments.