| Cline's Comments |
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| The Importance of Application Temperature |
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| The application temperature of a hot melt to the spine of a book is extremely important. The most commonly recommended hot melt application temperatures range from 325-375°F (163-190°C). Because of this range, the pot is usually set at 350°F (176°C). In practice, the heaters, which are generally located in the bottom of the pot are turned "on" at a 100% level. It makes no difference whether the pot is set at 350° or 450°F (176° or 232°C). Unfortunately, I have seen cases where the pot was turned to 400°F (204°C) and the operator forgot to turn the dial back to 350°F, causing the adhesive to darken rapidly. |
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| Many of the newer binders have digital readout gauges that indicate the internal temperature of the pots. Unfortunately, these readings can be very misleading for a number of reasons. As the metal of the pot heats and cools a number of times, the sensitivity of the thermocouple will decrease. This causes slow response and, consequently, larger swings between the low and high temperatures in the pot. |
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| The most common cause of low readings is contamination or char buildup around the sensor. Char insulates the thermocouple and causes it to send a signal for more heat to the heaters, resulting in overheating of the pot. It is a good policy to check the temperature daily with a calibrated thermometer or pyrometer and record the readings. |
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| Checking the temperature of the spinner is also very important, but more difficult to achieve. Since the spinner is a small round surface, a special probe is necessary for a pyrometer; either an infrared probe or a surface probe that bends around the circumference of the spinner. This temperature of the spinner should be kept approximately 25 degrees hotter than the pot temperature, and in no case should be cooler than the pot. One of the common causes of overheating the glue happens when one of the heaters in the bottom of the pot burns out. Since there are three or more heaters in the bottom of the pot, when one burns out, the others must put out more energy to heat the entire pot. This causes local hot spots in the glue pot and may locally heat the glue to 400°F or higher if the glue wheels are not rotating. This, in turn, causes local charring and insulates the heaters, causing them to work harder to raise the temperature to the level that the thermocouple calls for. |
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| It is good practice to include an electrical check to each of the heating elements during your preventative maintenance program. If an element must be replaced, be sure to use an element with the same watt level to ensure even heating of the glue. |
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Chuck Cline |
| Technical Service Manager - Bookbinding |
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