Hot Melt Gun Modules are the valves used to cycle hot melt through nozzles onto substrates. The modules are the considered the “wear parts” in a hot melt system. Actually, the wear part is an internal O-ring or seal that contacts the needle valve and separates the hot melt chamber from the pneumatic air chamber that lifts the needle valve.

Most hot melt modules are pneumatically actuated - requiring the separation of the internal air chamber and high pressure fluid chambers in the valve with an O-ring or seal. This seal, which separates these areas within the module, will wear over time - allowing adhesive to flow into the air chamber and the eventual failure of the module.
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Hot Melt Module Internal Seal Wear - by Pierce Covert,
Glue Machinery Corporation
www.gluemachinery.com