A wrapped pallet standing still on a loading dock tells you almost nothing. The forces that take a load apart happen later: the first hard brake on the highway, the lane change, the forklift that lifts it at an angle, hours of vibration that work boxes loose a millimeter at a time. A load that survives all of that was engineered to, whether anyone called it engineering or not. A load that does not was simply never tested against the forces it was going to meet.
This is the part of packaging that falls between departments. Purchasing buys the film by the roll. Operations sets the wrapper to whatever it was set to last year. Nobody owns the question of whether the finished unit load will hold together at the forces it will see in transit. Here is how to own it.
One scope note. Korpack specifies and supplies corrugated and protective packaging, engineers pallet and stacking configurations, and supplies and services stretch wrapping and end-of-line equipment. Korpack does not operate freight or carrier networks. The subject here is the unit load and how it is built, not the lane it travels.
The Load Is Weaker Than the Boxes That Make It
Start with a fact that surprises people: a corrugated box loses a large share of its compression strength the moment you stack it on a pallet, before anything goes wrong. In peer-reviewed testing of boxes palletized three high, the average loss in compression strength from palletization alone was about 25 percent. The box retains roughly three quarters of its single-box compression strength just from being part of a stacked load, with the bottom tier carrying everything above it.
That is the baseline. Everything that follows subtracts from it. Two of the most common choices on a pallet quietly take more:
- Interlocked stacking: Interlocking the layers, where each tier is rotated so boxes bridge the seams below, improves lateral stability but costs vertical strength, because box corners no longer sit directly on the corners beneath them. In the same testing, interlocked patterns lost anywhere from 4 to 35 percent of compression strength compared with column stacking, depending on box, pallet, and whether a tie-sheet was used. Column stacking, with every box aligned directly over the one below, had the highest compression strength in every case tested.
- Overhang: Letting boxes extend past the pallet edge removes the support under the edge of the box, which is exactly where its strength lives. A one-inch overhang on a three-high load cost an average of 13 percent of compression strength in the same study, and one major carrier’s packing guidance warns that overhang can reduce compression strength by as much as 32 percent. Combine overhang with an interlocked pattern and the losses stacked to as much as 34 percent on individual boxes.
None of this means interlocking is wrong. It means it is a trade: lateral stability bought with vertical strength. The right pattern depends on whether the contents are rigid enough to carry load themselves and how high the stack goes. What is always wrong is making the trade without knowing you made it, which is what happens when the pattern is inherited rather than chosen.
Before you adjust the wrapper, look at the stack. A square, column-stacked load with no overhang is the strongest thing you can put film around. A leaning, overhanging, interlocked stack is weaker before the first wrap, and no amount of film restores strength that the pattern already gave away.
Pre-Stretch Is a Film-Cost Setting, Not a Containment Setting
The most common misunderstanding in stretch wrapping is the belief that turning up the pre-stretch makes a tighter load. Pre-stretch is how far the film is elongated by the wrapper’s rollers before it reaches the load. High pre-stretch means less film per pallet, which is a real cost saving, and it is why the setting exists.
But it is not the same thing as containment. In a controlled study that put unitized loads through a full distribution test sequence while monitoring film force, the data showed no correlation between the percentage of pre-stretch and the change in film force or load containment. Loads wrapped at different pre-stretch levels did not hold together differently in proportion to the pre-stretch. A separate doctoral study that combined film thickness and pre-stretch into a single stiffness measure found only a weak, if statistically significant, relationship to containment force. Taken together, the research points the same way: pre-stretch percentage is a poor predictor of how well a load will hold.
What does hold the load is containment force: the inward pressure the film actually applies to the load, which comes from how many wraps are applied, at what tension, and where. Containment force can be measured on the finished load with a simple gauge, and it is the number that most directly predicts whether the load will survive a transit event. It is also the number most operations have never measured. They know their pre-stretch percentage because it is on the machine. They do not know their containment force because nobody put a gauge on the pallet.
Two operations can run the same film at the same pre-stretch and produce loads with completely different containment force, because one applies more wraps at the base and the top where loads fail and the other does not. If you are specifying a wrapper or a wrap pattern, specify the containment force you need to reach and verify it on the load. Specifying pre-stretch alone specifies your film bill, not your load stability.
Stability Is a Number, and There Is a Test for It
The reason this can be engineered rather than guessed is that the transit forces a unit load has to resist are known, and a load can be tested against them before it ships.
In Europe, the reference method is EUMOS 40509, a dynamic test that subjects a complete unit load to a defined horizontal acceleration, held for at least 0.3 seconds and reached within 50 milliseconds, to simulate the inertia of braking and cornering. The European cargo-securing reference levels it is used alongside are 0.8 g forward and 0.5 g sideways, and the level applied is chosen for the transport case being certified. The load passes if it stays within strict deformation limits: no more than 10 percent elastic deformation during the event, no more than 5 percent permanent deformation afterward, and no damage to the packaging. It is a pass-or-fail answer to the question will this load stay together when the truck brakes hard, answered on a test sled instead of on the highway.
In North America, the same question is addressed inside the distribution simulation standards: the ASTM D4169 distribution cycles include stacked and vehicle vibration schedules that a unitized load has to survive, and ISTA procedures for unit loads apply comparable vibration and handling sequences. The test is a different shape, but the principle is the same: a unit load is a packaging system with a measurable performance, and you can know its performance before you trust it with product.
The honest caveat is that a test result is specific to the load that was tested. Change the box, the pattern, the pallet, or the wrap recipe, and the result no longer applies. That is an argument for testing when the load changes, not an argument against testing.
Where Unit Loads Actually Fail
Across the testing and the field experience, the failures cluster in a few places, and almost none of them are the film tearing.
- The load leaves the pallet. A load that is not locked to the pallet slides as a unit on the pallet deck under braking. The film held the boxes to each other and nothing held the stack to the pallet. Wrapping the base so the film captures the pallet itself is the usual fix.
- The top walks. The top tiers have the least weight on them and often the least film, so they move first. Vibration works the boxes loose and a corner walks outward until the overhang that was not there at the dock is there in the trailer.
- A void opens. A column stack with one tier misaligned, or a stack where boxes were not square to each other, creates a void. Under vibration the void grows, the column above it loses support, and the whole load develops a lean. Leaning loads fail in cornering.
- The overhang takes the hit. Overhanging boxes have unsupported edges and nothing between them and the trailer wall or the neighboring pallet. They tear, crush, and catch.
The pattern across all four is that the film was rarely the first thing to fail. The stack was already compromised, and the film was asked to hold together a load that the pattern, the overhang, or the base wrap had already given up on.
How Korpack Approaches It
Korpack is an engineering-led packaging company, and the unit load is treated as one engineered system rather than a box decision, a pallet decision, and a wrap decision made by three different people. The starting point is the finished load and the forces it will see: how high it stacks, how it will be handled, how far it travels, and what the receiving dock requires.
Practically, that means corrugated specified for the stacking load it will actually carry, pallet pattern and overhang engineered rather than inherited, a wrap recipe specified by the containment force it has to reach and verified on the load with a gauge, and stretch wrapping and end-of-line equipment supplied and serviced to hit that recipe consistently shift after shift. Korpack specifies and supplies the packaging and equipment and engineers the configuration; it does not run the freight. The objective is a load that arrives the way it left.
A pallet is not stable because it looks stable. It is stable because the pattern, the overhang, and the containment force were chosen, and then measured, against the forces it was about to meet.
Unit load stability is an engineering problem with a measurable answer. Korpack’s packaging engineers look at the stack, the pattern, and the containment force together, and specify the packaging and wrapping equipment to hit the number.
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Frequently Asked Questions
Is column stacking or interlocked stacking better for a pallet?
Column stacking, with every box aligned directly over the one below, gives the highest compression strength, because box corners carry load straight down into the corners beneath them. Interlocked stacking improves lateral stability but costs vertical strength; in peer-reviewed testing, interlocked patterns lost anywhere from 4 to 35 percent of compression strength compared with column stacking, depending on box, pallet, and tie-sheet. Interlocking is a reasonable choice when the contents are rigid enough to carry load themselves, and a poor one for boxes that depend on the board for their strength. The mistake is not choosing one or the other; it is inheriting a pattern without knowing what it cost.
Does higher pre-stretch make a tighter pallet?
No, not by itself. Pre-stretch is how far the film is elongated before it reaches the load, and its main effect is reducing film consumption, which is a real saving. A controlled study that put unitized loads through distribution testing found no correlation between pre-stretch percentage and load containment, and a separate doctoral study found pre-stretch, even combined with film thickness, to be only a weak predictor of containment force. What holds the load is containment force, the inward pressure the film actually applies, which depends on the number of wraps, the tension, and where the wraps are placed. Containment force can be measured on the finished load with a gauge, and that measurement, not the pre-stretch setting, is what predicts whether the load will survive transit.
How much strength does a box lose from overhang?
A meaningful amount, and it adds to losses that are already there. In peer-reviewed testing of boxes palletized three high, palletization alone cost about 25 percent of compression strength on average. A one-inch overhang cost a further 13 percent on average, and one major carrier’s packing guidance warns that overhang can reduce compression strength by as much as 32 percent. Overhang combined with an interlocked pattern produced the largest losses, up to 34 percent on individual boxes. Overhanging boxes also have unsupported edges exposed to trailer walls and neighboring pallets, which is where tears and crushing start.
Can you actually test whether a pallet load will survive shipping?
Yes. In Europe, EUMOS 40509 subjects a complete unit load to a defined horizontal acceleration, held for at least 0.3 seconds, at a level chosen for the transport case (the European cargo-securing reference levels are 0.8 g forward and 0.5 g sideways), and passes it only if deformation stays within strict limits (no more than 10 percent elastic, 5 percent permanent, and no packaging damage). In North America, the ASTM D4169 distribution cycles and ISTA unit load procedures apply vibration and handling sequences a unitized load has to survive. The result is specific to the load that was tested, so a change in box, pattern, pallet, or wrap recipe calls for a retest, but the principle holds: a unit load’s stability is a measurable property you can know before you ship.
- Singh, S. P., Singh, J., and Saha, K., Effect of Palletized Box Offset on Compression Strength of Unitized and Stacked Empty Corrugated Fiberboard Boxes, Journal of Applied Packaging Research, Vol. 5, No. 3, July 2011 (peer-reviewed; four box sizes, CHEP and GMA pallets, with and without tie-sheets, tested per ASTM D642). Source for the approximately 25 percent average compression strength loss from three-high palletization, the 13 percent average loss from a 25.4 mm (one inch) overhang, interlocked-pattern losses of 4 to 35 percent versus column stacking (Tables 7 through 10), combined interlocked-plus-overhang losses of up to 34 percent, and the finding that column stacking had the highest compression strength in every case tested.
- Cernokus, E. A., The Effect of Stretch Wrap Pre-stretch on Unitized Load Containment (California Polytechnic State University master’s thesis, 2012); Bisha, J. V., Correlation of the Elastic Properties of Stretch Film on Unit Load Containment (Virginia Tech doctoral dissertation, 2012). Cernokus is the source for the finding of no correlation between percentage pre-stretch and change in film force or load containment across unitized loads subjected to ISTA 3E distribution testing. Bisha is the source for the finding that a combined film thickness and pre-stretch stiffness measure yielded a statistically significant correlation with containment force but with a very low coefficient of determination. Both are university research rather than peer-reviewed journal articles.
- Major carrier palletized freight packing guidance (FedEx, 2026). Source for the warning that overhang can reduce compression strength by as much as 32 percent, the recommendation of column stacking with interlocking reserved for rigid contents, and the requirement that wrapped loads reach a minimum containment force securing the load to the pallet.
- EUMOS 40509 load unit rigidity test method, as summarized in reference material on acceleration testing (2026). Source for the dynamic horizontal acceleration test, the minimum 0.3 second hold reached within 50 milliseconds, the European cargo-securing reference accelerations of 0.8 g forward and 0.5 g sideways under EN 12195 (as cited by accredited test laboratories and load-securing references), and the pass criteria of no more than 10 percent elastic deformation, 5 percent permanent deformation, and no packaging damage.
- ASTM D4169 Standard Practice for Performance Testing of Shipping Containers and Systems, and ISTA unit load test procedures, as summarized by accredited packaging test laboratories (2026). Source for the stacked and vehicle vibration schedules applied to unitized loads in North American distribution simulation.
- Stretch wrapping equipment manufacturer guidance on load preparation and containment force (Lantech, 2026). Source for the definition of containment force as the inward pressure applied by stretch film, its measurement on the finished load with a containment force gauge, and the load preparation failure modes of leaning, shifting under vibration, separating from the pallet, and overhang catching on trailer walls.
- Korpack Marketing Guidelines and Value Propositions, November 2023, and Korpack automation and packaging service descriptions. Source for Korpack’s corrugated and protective packaging, pallet configuration engineering, and stretch wrapping and end-of-line equipment supply and service. Korpack does not operate freight or carrier networks.
Korpack is a technologically advanced packaging materials, contract packaging, and automation supplier that approaches solutions with an engineering mindset. Founded by a packaging engineer, Korpack serves growth-oriented manufacturers and brands across North America from its Chicagoland headquarters. Test figures cited are from the specific studies referenced and depend on box, pallet, and load conditions; validate against your own loads before relying on them.
