7 ways to influence the open time of your hot melt adhesive
You can control the open time by taking account of:
1. Ambient temperature during hot melt application
The ambient temperature has a major impact on the open time of hot melt adhesives. The adhesive temperature falls as soon as the adhesive leaves the nozzle. So in a cold production area, the adhesive will cool faster, resulting in a shorter open time. In a hot production area, the adhesive temperature will fall more slowly, as a result of which you can extend the open time.
2. The temperature of the substrate
Like the ambient temperature during hot melt application, the temperature of the substrate is also important. If you apply the adhesive to a cold substrate, the open time will be reduced and there will be less time for assembly. You are therefore advised to store the substrate to be bonded in the production area a day in advance. In this way, the substrates can be brought to the ambient temperature overnight.
The influence of drafts is often underestimated. Moving air draws a great deal of heat energy out of the adhesive. Drafts from open doors or air conditioning can have an enormous impact on the open time, even if they last for only a short time. You should therefore make sure that your production line never runs between two sliding doors or ventilators and is never exposed to drafts in other ways.
4. Distance between the nozzle and the substrate
As already mentioned, the adhesive temperature falls as soon as the adhesive leaves the nozzle.The greater the distance between the substrate and the nozzle, the greater the heat loss and the shorter the open time. In practice, a distance of three centimeters proves to be enough. A small distance also has another advantage: it means that the machine can be controlled considerably more precisely.
5. Nozzle size
With an automatic application the diameter of the nozzle determines the amount of adhesive that reaches the substrate. The nozzle must provide the substrate with a consistent volume of adhesive. If too small a nozzle is used, the adhesive is applied to the substrate with a great deal of force. The result is that the exit speed is too high in relation to the conveyor belt, creating an uncontrolled zig-zag pattern. In general, a nozzle of 1.5 millimeters is the most suitable size for bonding soft foam in automatic applications.
With the use of a press, the hot melt pressure sensitive adhesive is pressed into the structure of the foam. A high quality press and its correct use are required for realizing the ultimate strength. The foam should therefore be pressed for at least 30 seconds. This may seem like a long time, but in practice we often find that this time is easily available because assembly has to take place. Often, a mattress can be pressed for even longer! Longer pressing creates a stronger bond and faster handling.
A major advantage of hot melt adhesives is that they build up initial tack very quickly during cooling.This makes fast assembly and processing possible. All the same, the substrates can also be assembled too quickly, which increases the open time. The adhesive then has no chance to cool off quickly. The two substrates ensure that the adhesive stays fluid for longer, as a result of which the process is slowed down and the bond reaches its end strength later. In order to create an optimal bond, make sure that the entire bonding process does not take place too fast or too slowly.