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Checklist: 7 ways to influence the open time of your hot melt adhesive

If you want to integrate hot melt adhesives into your production process, it is important to be aware of the different factors that impact the open time. If these factors are not kept under control, they can have enormous consequences for the production process and the end product. Fortunately, you can control the open time by following our checklist. 

Checklist: 7 ways to influence the open time of your hot melt adhesive

Do you have an optimal bonding process in your production process? Check this on the basis of this checklist!

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Open time

The use of hot melt adhesives in a mattress production process is fairly easy to automate. The challenges lie mainly in relation to the open time. This is the maximum time between the application of the adhesive and the pressing of the substrates. The open time of hot melt adhesives is relatively short: between a few seconds and a few minutes, depending on the components used, the application pattern, the substrates to be bonded and the points mentioned later in this blog. 

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What you need to take into account

It is therefore important to match the adhesive to the production process. An adhesive with an open time of one minute in an application where 90 seconds are required can lead to poor bonding. Once the open time of the adhesive is over, it is no longer possible to create a strong bond because the hot melt adhesive will then have cooled and cured. At the same time, when assembly is carried out too early, this can also cause problems. The adhesive will then still be too fluid, and tension can pull the substrates apart.

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

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, 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.

 

3. Drafts

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.

 

6. Press

With the use of a press, the 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.

 

7. Assembly

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.