How to Prevent Char in Hot Melt Adhesive II
2. Wide applicators and narrow coating
Narrow coating with a wide applicator head is a common source of char creation in hot melt adhesive. This setup creates dead zones in which the adhesive cannot flow, exposing it to constant heat for days or weeks at a time. This then leads to adhesive breakdown, gelling and char. To further complicate matters, when you adjust the applicator to a wider width, char clogs the system and/or deposits onto the substrate, diminishing the product’s quality.
3. Fiberized spray system
Often, when purchasing a fiberized spray applicator, companies will buy a wide-width applicator to handle the widest width product they have, in hopes that it will work for their full range of products. For example, a company might purchase a 72-in. wide applicator, but also run products that are as narrow as 48 in. The natural tendency is to turn off the nozzles not being used when running a narrow product, but that creates a problem. Just because those nozzles are closed does not mean there is no adhesive behind them. In fact, the adhesive is blocked up behind the closed nozzles, breaking down and eventually forming char. When you open these nozzles back up for the wider application, they are often clogged with char and can spray char onto the substrate.
Once again, the problem is preventable. To avoid char-clogged applicators, design the applicator head in section sizes that can be separated and completely turned off, so that there are no dead zones or heated areas. The smaller sections would not only be turned off, but slid out of the way to prevent transfer of heat to the sections that have been turned off.
4. Slot dies or manifold-type dies
When dealing with slot dies, the tendency is to buy a die for the widest part and shim it down for narrower widths. Once again, the hot melt adhesive that sits behind the shim does not flow, thus, it cooks, breaks down, and eventually becomes char. Afterward, any time a wider shim is inserted for wider-width coating, the char either creates streaks in the coating or puts char onto the substrate.
The proper way to handle this is with simple slot dies or precision-contoured manifold-type dies. For narrow widths in simple slot dies, you can close down some of the modules to the die. Another option is to insert pieces of O-ring cord to block off the dead areas inside the die. When using precision-contoured manifold type dies, have the die manufacturer make sets of filler blocks that match the contour of the internal manifold.
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