The open-time of an hot melt adhesive is defined as; elapsed time between application of the adhesive onto the first substrate and compressing the second substrate at which adhesion occurs.
Open-time is specially important for hot melt applications, as the applied hotmelt starts cooling down the moment it comes into contact with the first substrate.
When the open-time of the formulation is passed, creating a bond is no longer possible as the hot melt has cooled down.
It is important to always make sure that the machinery open-time is shorter than the adhesive open-time.