How to Prevent Char in Hot Melt Adhesive III
5. Proper hot melt adhesive level
Neglecting the proper adhesive level can cause the greatest amount of char. When the hot melt adhesive level gets too low, the heater grids become exposed, which creates a hot spot that can be oxidized, and the result is char. This error typically occurs when you manually load pellets of adhesive into the melter. With manual loading, it’s common for people to get busy and forget to keep the tank full.
Fortunately, this is one of the simplest fixes. Two items can help solve this issue, say, an automated filling system and a level control system. An automated system is basically a suction wand that communicates with a level control device on the melt tank and automatically feeds the pellets into the melt tank on demand. The level control system automatically monitors the adhesive level in the melt tank and ensures that the level will never get too low and the grids will never be uncovered. This is the single most important thing a company can do to help prevent char.
Many different types of level control systems are available, with one of the best types is a three-point level control system with adjustable set points. The top set point says it is full, no need to feed any adhesive to the tank. The next set point says it is low on adhesive and requires feeding the melt tank either manually or automatically. The third set point is critical, if the low level reaches this point, it means that the required adhesive feeding did not take place. When the system reaches this set point, it is triggered to shut down to prevent charring.
6. Adhesive melt tanks
It is very important to size the melt tanks properly. If a melt tank is too large for the application, adhesive will not turn over frequently enough. The adhesive that is retained in the tank for long periods of time will begin to break down and eventually char.
As mentioned earlier, one of the most imperative steps in char prevention is to set the temperature as low as possible while still accomplishing optimum coating or laminating. A good hot-melt system will have a temperature adjustment screen that allows you to adjust temperatures by zone. These temperature settings should also allow you to set an acceptability range or band on either side of the set point.
For example, let’s say that the intended application temperature is 350°F. Assuming that there are four zones in the melter (one zone for the hose, and four zones in the applicator head), you might set the top two zones in the melter to 325°F and then increase the temperature to 350°F as you get closer to the applicator head. You can also set the bandwidth of acceptability to ± 10°F. Should the temperature drift outside the band of acceptability, the system will set off a temperature alarm and shut down the system. Another device that can enable proper temperature maintenance is an in-flow temperature probe with PID loop capabilities.
In addition, if the system will sit idle for any length of time, the hot melt adhesive should have a set-back temperature to drop temperature significantly during idle times. This will help minimize the possibility of adhesive breakdown.
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