Why Are Boxes Popping Open After Leaving My Factory Production Line?

Are your boxes popping open or not staying sealed after leaving your production line? Watch the video below for a quick overview then read on for more info!

In the summer we often get a flurry of enquiries from hot melt operators looking for a solution to the issue of cartons and cases popping open as they exit the compression bars. The familiar story plays out as follows.

• They’ve been using the same adhesive for months if not years with no issues
• The board materials haven’t changed
• The machine settings haven’t changed. Tank, hose and gun temperature settings, line speeds etc
• There’s plenty of adhesive on both glue flaps but it’s not holding them closed

Then often a few red herrings get thrown in.

• “There are some agency workers covering for holiday leave so it must be an operator error,”
• “There’s something up with the latest batch of glue,”
• “Someone’s been messing with the machine settings,”

In actual fact, in this particular scenario it’s likely to be none of these. Something is happening outside the factory that’s influencing the setting speed of the adhesive. To be more accurate 150 million Km away from the factory.

The sun’s come out. 

It really is that simple. 

We must bear in mind the way that hot melt adhesive sets. It is fluid when hot and sets by losing heat. Obviously, this means the setting speed of these adhesives is susceptible to both application temperature and also ambient temperature, (which is also why, when testing for set speed in the laboratory at Alphabond, we always keep the ambient temperature within half a degree of 22°C). Any significant increase in ambient temperature will impede the heat loss of the adhesive and therefore slow down the speed of setting.

Manufacturing facilities such as food factories can be warm places all year round with ovens and other processes pumping out heat. When the weather warms up, especially in a heat wave, the resulting elevated ambient temperatures can cause hot melt adhesive setting speeds to be extended. If the setting speed extends beyond the time the bond spends in compression, this will result in pop opens.

The issue is much more common with outer cases where the thick boards try to force themselves open when leaving the compression bars. If the adhesive is already close to the limit of its setting speed on a fast production line, an extra 4 or 5 degrees of ambient heat can be enough to cause the bond to fail.

This particular issue should not be confused with pop opens which occur after production during storage or distribution. In these cases, the problem is caused by the bonds re-softening either from very high environmental temperatures (normally above 50°C) or more rarely from hot product packed and heat build-up and transfer from the product to the glue flaps.

If product is shipped across the equator in a shipping container these temperatures can become a reality. We test for SAFT here in the lab which stands for Shear Adhesion Failure Temperature to find the limit of different adhesives. This is done by hanging weights from samples and putting into an oven which increases in temperature very slowly until the bonds fail.

The chart above is to illustrate how an increase in ambient temperature can extend the setting time of a hot melt over that required to form a bond.

• The black horizontal line is a typical limit of an adhesive setting time for a case erector (measured using an Alphabond test method). If the adhesive sets slower, it will pop open.
• The green box below the line is where the adhesive set time is shorter than the limit so forms a good bond
• The red box above the line is where the set time is longer than the limit and where pop opens occur
• The line climbing as the ambient temperature increases shows the effect that the temperature has on the setting time. When this line crosses the limit line the issues are likely to start.

As mentioned previously the figures shown here are not set in stone but this shows the principle of how these issues occur.

If you are experiencing boxes popping open or not staying sealed in your factory, use the calendar below to book a short, no-obligation meeting with us.