Variables during Hot Melt Adhesive Application
Generally, to maximize hot melt adhesive bond strength while minimum cost, you may consider the following variables.
Application Temperature: the temperature of the adhesive as it exits the nozzle.
As a rule, hot melt adhesive application temperature should be as high as necessary to achieve the desired wet-out and penetration of the substrate. Furthermore, spray applications usually require higher temperatures due to the heat loss as the adhesive fiber travels from the nozzle to the substrate. And other considerations to make include adhesive heat stability, substrate heat tolerance, and equipment limitations in terms of open time/compression time.
Add-On (coat weight): the amount of hot melt adhesive applied to make a bond.
An application that requires a higher coat weight means the adhesive will take more time to set up. Set up is the process of the hot melt losing its heat and solidifying. This is because more heat is present when more adhesive is applied to the substrate, and the heat must be dissipated for the adhesive to set up.
The amount of hot melt adhesive affects the primary and secondary substrate bond, especially in bead extrusion applications. The more adhesive applied to the primary substrate, the more area it will cover when molten and compressed. Having more adhesive enables it to stay molten for longer and wet out better to the second substrate.
What’s more, the coat weight also determines the surface area of the finished bond.
Compression: bringing the two substrates into physical contact by the use of pressure.
Compression mainly affects the bond to the secondary substrate. It forces the hot melt adhesive to flow into the primary and secondary substrates. Compression time must be long enough to allow the adhesive enough time to develop the cohesive strength to overcome substrate memory or stress. Insufficient compression time results in an improper or inadequate bond.
To facilitate maximum wet-out and bond surface area, compression should take place as soon after adhesive application as possible.
Open Time: the time (seconds) measured from the point of adhesive application to the point of compression with the secondary substrate.
Ideally, open time should be as short as possible to achieve maximum wet-out and penetration of the secondary substrate. However, keep in mind there are other considerations, such as the set speed of the adhesive, physical space on the line, and pressure sensitivity of the hot melt adhesive.
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