What are appropriate temperature settings for hot melt tanks, hoses, and guns? These are the most frequently asked questions by hot melt users.
Many hot melt applicator and adhesive providers suggest users set the initial temperature at 325° for every zone of the hot melt system. Users frequently ask if they can set a temperature higher than 325° when the viscosity of a selected hot melt adhesive is very high and is hard to apply.
With higher temperatures the double bond (or π-bond) of a polymeric molecule tends to open, creating two free radicals (when the temperature exceeds 325°). Those free radicals are very unstable or reactive; and will spontaneously react with the surrounding oxygen from the air. As a result, hot melt adhesives are oxidized and aged upon heating in the melting tank.
It is noticeable that most hot melt adhesives basically do not form free radicals at temperatures lower than 325°. When hot melts in the melt reservoir are protected with a nitrogen blanket or under vacuum, they will not be oxidized even though free radicals exist in the adhesives.
To insure precise application and limit adhesive degradation with high viscosity adhesives, set the tank at a maximum of 325° and raise the temperature of the hose and gun first. Since adhesives inside the hose and guns are not exposed to air or oxygen; they will have no chance to be oxidized. Actually, users can also raise the temperature of melting tank to certain extents, but should always maintain the adhesive level at 80-90% of the total tank depth.
Most melting tanks are heated with heating pads, cartridge heaters or cast in heaters. These are typically focused on the lower portion or the bottom of the melting tank. If users constantly charge cold adhesive into melting tank to maintain an adhesive level, the solid room temperature adhesive will also lower the temperature of the molten adhesive particularly at the surface level. In fact, the temperature of those adhesives on the top layer, which are exposed to the air, will not exceed the tank set temperature. Consequently, adhesive users may increase the setting temperature for the melting tank as well without worrying too much about overheating of adhesives. To prevent oxidation, always keep in mind that only those adhesives exposed to air (surface level) should be maintained at a temperature below 325°.
Melt On Demand tanks are extremely effective at keeping adhesive in the tank at lower temperatures prior to use and should be considered.
In summary, we recommend that adhesive in a melter reservoir does not exceed 325° if it exposes to air. If hot melt adhesive users can ensure the temperature of the adhesive at the top or surface of the tank is lower than 325° through consistent filling/use, then the tank can be operated at higher temperature levels than 325°.