We have received complaints and chargebacks from our customers due to crushed cartons, but only during the summer months. We think this is due to humidity being trapped by the stretch wrap because nothing else in our process changes. Is that possible, and what preventive action can we take?

Hello Lenny,

Thanks for your question. If it makes you feel any better, you’re not alone!

The first step is to understand what is happening and why before arriving at a solution.

Your packaging engineer designed your cartons to support the weight of the cartons stacked above them on the pallet without collapsing. Whether they are printed chipboard cartons or heavier corrugated boxes, they are all made from paper pulp (plant-based cellulose fiber), which is naturally, hygroscopic. This means the fibers absorb moisture from the surrounding environment.

Cellulose fiber products act like a sponge, absorbing moisture vapor from the air around it. In high- humidity environments, the carton absorbs moisture and the fibers transfer moisture from the surface of the carton toward its core. In very dry environments, the opposite happens and moisture moves from the core towards the surface, where it evaporates.

Let me put this into context. Moisture content is a critical design factor because it directly affects the strength of paper-based cartons and boxes. As I mentioned, your packaging engineer designs the carton or box to support a calculated weight. An additional safety margin is normally built into these calculations to account for the effects of transportation forces, including lateral forces, product-layer shifting, changes in the center of gravity, and polar moment of inertia, under nominal environmental conditions. For many applications, these conditions are around 40% to 55% humidity.

The fact that paper-pulp based cartons gain and lose moisture as humidity changes is well established. Now we need to understand how those moisture changes affect carton strength so we can determine what to expect in your application.

When humidity is high, cartons absorb moisture from the air. This moisture weakens the bonds between cellulose fibers, reducing the carton’s ability to resist compression. As a result, the cartons become weaker.

As a rule of thumb, I have found that a strength reduction of up to 40% can be a useful starting point when evaluating the potential effects of moisture absorption. The actual reduction depends on the carton material, its construction, and the environmental conditions. Unfortunately, packaging engineers do not always have the budget or flexibility to include enough additional strength in the design to account for this loss.

Now we can understand why your cartons may be at greater risk of deforming during the summer months. This is consistent with what you and your customers have observed firsthand.

Next, let’s turn our attention to the effects of stretch wrapping a pallet of cartons in humid conditions.

Stretch film is made from polyethylene resin, which has a high resistance to moisture transmission. So, to your point, stretch film can slow the release of moisture from cartons through evaporation after wrapping. It makes sense that you would suspect the film of trapping moisture and causing your summertime carton-crushing problems.

But things are not always that simple, and this is certainly one of those cases.

The key is to understand that carton moisture content changes in response to the surrounding environment over time. If your cartons are exposed to high humidity before wrapping, they may have already absorbed enough moisture to lose a significant amount of their strength. Wrapping them does not reverse that damage.

Stretch film can slow moisture exchange, but it is not necessarily the root cause of the problem. Unless the environmental conditions change significantly after wrapping, there may be little moisture movement to prevent. In other words, the cartons may already be weakened before the stretch film is applied.

The next piece of the puzzle is understanding how stretch wrapping affects those weakened cartons.

The purpose of stretch wrapping a pallet is to prevent independent movement of the individual cartons, which can lead to load failures. Stretch film is elastic in nature, much like a rubber band. After the film is stretched and applied to the pallet, it retracts by some percentage, exerting a unitizing force around the load.

This would be like gripping a deck of playing cards with your hand, where the unitizing force of your grip keeps the individual cards stacked tightly together. The same principle applies to cartons. If moisture has compromised their strength, the corners can crease and collapse under the force exerted by the stretch film.

The damage may not be obvious immediately after wrapping. The carton corners can gradually deform under the sustained force of the film. As they collapse, the film can lose much of the unitizing force it applies to the load. This results in a two-pronged issue, because not only do you have deformed cartons around the perimeter of the pallet, but you also have reduced load containment, which increases the risk of a complete load failure.

Spoiler Alert! The solution involves a little science!

We have developed two technology options to address this specific problem.

The first is the Rapid Bander, which uses our filament-reinforced, high-performance stretch film. We pre-stretch the film beyond the point where conventional stretch films would break, significantly reducing its remaining elasticity. As the film is applied to the pallet. The remaining elasticity is further reduced.

The benefits are twofold. First, the film provides the unitizing force needed to secure the load while minimizing the force applied to the cartons. Second, by reducing the film’s tendency to retract against the carton corners, we can help prevent them from being crushed.

In other words, increased load containment without crushing the cartons.

The second option is our Rapid Vent. This technology uses the same principle as the Rapid Bander, but the Rapid Vent’s film is perforated as it is applied to the load. As the film stretches, the perforations expand until they reach the reinforcement filaments. This creates openings that allow greater ventilation through the wrapped load while still delivering maximum load containment.

We have a nationwide network of Field Service Engineers who can optimize your existing equipment and install our device to give you the highest level of load containment without crushing your product, even in the summer.

We use science to understand the challenges our customers face and develop cost-effective solutions that deliver results.

Thanks for asking!

STEVE