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Maintain PUR Quality In Glue Pot
When using PUR in an open pot binding operation, you must be aware that PUR is affected by the temperature of application, the amount of time exposed to air, and the amount of moisture in the surrounding area.
The temperature of application is the most critical of the three. With the earlier generations of PUR, National specified the application temperature at 250°-265°F. The newest generation, 34-2003, has a higher green strength and slightly higher viscosity, requiring a higher running temperature of 265°-285°F. This lowers the viscosity and allows the adhesive to flow better into the substrates. Here it is critical to measure the pot temperature with a calibrated thermometer and not rely on the dial readings.
The amount of time the PUR is exposed to air is determined by the size of the book being bound. Small books, such as annual reports, use very little PUR, so the material in the pot is not refreshed very often. Thus PUR pots are generally smaller than standard hot melt pots to help replenish the PUR more often. Also, the lower the temperature in the pot, the more air is drawn into the pot, resulting in a foamy appearance.
Moisture in the surrounding area also affects PUR condition in the glue pot. During hot, humid summers, the air drawn into the pot contains more moisture than during dry winters. This causes the PUR to become thicker, but not to the point of "curing" in the pot.
In order to minimize the above effects, keep the glue level at maximum to draw the minimum amount of air into the pot. The other preventive measure is to replenish the PUR at short intervals. Instead of filling the pot after the level has dropped an inch, start the fill after it drops one half-inch. This will keep fresh PUR in the system.
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