Carton Drop Testing Basics for Knit Bags: Drop Height, Sequence, Conditioning and What Actually Counts as a Pass
How high should a knit bag carton be dropped in testing? ISTA 3A sets the height by weight: about 460 mm for a package up to 9.5 kg, about 380 mm between 9.5 kg and 18.6 kg, about 300 mm to 27.7 kg, about 200 mm to 45.4 kg and about 100 mm above that. ASTM D5276 sets the sequence at ten drops: one corner, three edges and six faces. Conditioning runs 12 hours at 23 degrees and 50% relative humidity first. This article answers the six questions a drop test has to answer.
What Is a Drop Test Actually Measuring?
A drop test measures whether the packaging system delivers the product intact after the impacts a parcel network actually delivers, and it tests the closure and the corners of the carton far more often than it tests the product inside.
That framing is the useful one, because buyers usually read a drop test as a test of the bag and it is almost never that. On a knitted bag programme the outcome is close to predictable: knit absorbs impact, a knit tote dropped from 460 mm inside a carton comes out with no damage at all, and the things that fail are the taped closure splitting, a corner crushing, and in a few cases a hardware component working loose from the impact. The test is a test of the box, and the report should be read that way.
The second thing it measures is repeatability. A drop test is not a proof that a carton design is good; it is evidence that one sample survived a defined sequence. One sample is the minimum under most procedures and three to five is the number that gives a buyer anything worth relying on, because closure failures in particular vary with how the tape was applied on the day and one sample tells you very little about that.
How High Should the Drop Be?
Under ISTA 3A the drop height is set by package weight: about 460 mm up to 9.5 kg, about 380 mm from 9.5 kg to 18.6 kg, about 300 mm to 27.7 kg, about 200 mm to 45.4 kg and about 100 mm above that.
| Package weight | ISTA 3A drop height | Typical knit bag carton |
|---|---|---|
| Up to 9.5 kg | About 460 mm | A single-bag mailer or a small multi-bag carton |
| 9.5 kg to 18.6 kg | About 380 mm | A 60 x 40 x 40 cm carton near half fill |
| 18.6 kg to 27.7 kg | About 300 mm | A full 12 kg carton with a heavier mixed load |
| 27.7 kg to 45.4 kg | About 200 mm | Rare on knit, which is volume-limited |
| 45.4 kg to 68.2 kg | About 100 mm | Not reached on knitted bag programmes |
The inverse relationship surprises people and is worth explaining. Heavier packages are dropped from lower heights not because they are stronger but because handling equipment handles them differently: a parcel light enough to be thrown onto a belt is subjected to more energetic impacts than one heavy enough to be moved by machine. A 12 kg knit carton sits in the second band at about 380 mm, which is a good number to know when writing a specification, because a supplier quoting a 760 mm drop is quoting a procedure the parcel network does not apply.
How Many Drops and in What Order?
The sequence in ASTM D5276 is ten drops: one corner, then the three edges that meet at that corner, then the six faces, and the order matters because the corner and edge drops do the damage the flat drops never will.
The reason is concentration of force. A flat drop spreads the impact over the whole face of the carton and the contents absorb much of it. A corner drop concentrates it on the point of the box, which is where the closure and the board structure are weakest, and an edge drop puts the load along a line that folds the board. A carton that survives six flat drops and fails on the first corner drop is a carton that would fail in a parcel network, which is why testing only faces is a false pass.
The drop itself has a specification beyond height: the package has to be released rather than thrown, and the impact surface has to be rigid and level, normally steel or concrete, with the release mechanism imparting no rotation. ASTM D5276 covers the free-fall method and ISO 2248 covers vertical impact by dropping, and either may be cited depending on whether the programme follows an ASTM-led or an ISO-led specification.
- Procedure, parcel
- ISTA 3A for packaged products up to about 70 kg
- Procedure, free fall
- ASTM D5276, ten drops: one corner, three edges, six faces
- Procedure, vertical impact
- ISO 2248
- Drop height, to 9.5 kg
- About 460 mm
- Drop height, 9.5 to 18.6 kg
- About 380 mm
- Drop height, 18.6 to 27.7 kg
- About 300 mm
- Conditioning
- About 12 hours at 23 degrees and 50% relative humidity
- Impact surface
- Rigid and level, steel or concrete, with no imparted rotation
What Conditioning Is Required First?
Conditioning runs about 12 hours at 23 degrees and 50% relative humidity before the drop sequence, and it is the step most often skipped and the one that changes the result most.
The reason is that corrugated board is hygroscopic. Board conditioned at 50% relative humidity has meaningfully more edge crush strength than board that has been stored in a humid warehouse, and a carton tested straight off a damp floor will pass more easily than the same carton tested dry in one respect and fail in another: damp board absorbs impact better and resists crushing worse. Testing without conditioning produces a number that cannot be reproduced, which is the definition of a test result that is not worth having.
For a knit programme there is a second reason to take conditioning seriously, and it points the other way. The bags themselves are textile, and a knitted shell conditioned at 50% relative humidity behaves differently from one that has been in a container at 80%. A programme that wants to know what happens after a real voyage should test both: a conditioned sample to satisfy the procedure, and a humid-conditioned sample to find out what actually happens. The second is not a compliance test; it is the one that predicts the damage rate.
What Counts as a Pass?
A pass means the product is undamaged and the pack has not lost its contents or its integrity: the carton may show damage, but the closure must hold, the board must not have split through, and the contents must be saleable.
Writing the acceptance criteria before the test is the single discipline that makes a drop test useful, and it is usually done after. A report that says the carton passed without saying what passed means nothing to a buyer who has a different definition. The criteria worth writing down on a knit programme are these: the closure has not opened; no hardware has worked loose; no loop on the knitted surface has been raised or pulled; the bag is undamaged; and any retail packaging inside is still presentable.
Closure intact
The tape has not split and the flaps have not opened. This is the most common failure on a knit carton and the one that produces the most visible damage at the destination.
Board not split
A crush at a corner is acceptable; a split through the board is not. A split means the carton has lost structural integrity and will not survive the rest of the network.
Product undamaged
No raised or pulled loops, no distortion of the handle, no hardware worked loose. On knit this is usually the criterion that passes first.
Retail pack presentable
Where the bag ships in a retail sleeve or a tray, the outer has to be presentable on arrival. A crushed tray corner fails here even though the bag inside is fine.
How Many Samples Are Needed?
One sample is the procedural minimum, three is the practical number, and five is what a programme with a measurable damage-rate target should test.
The reason three beats one is variance in application rather than variance in the board. Tape applied by hand varies in tension, in overlap and in how well it is rubbed down, and tape is what fails. One sample tells a buyer whether one carton, taped once by one operator, survived. Three samples tell a buyer something about the closure. Five make the difference between a pass and a false pass visible, which matters most on the corner drop where closure failure concentrates.
There is a cost side to this and it is small relative to the value. A drop test is a half-day of technician time plus the cost of the samples, and the samples are reusable for compression afterwards. A programme that tests one sample to save a few hundred dollars and then ships 600 cartons of a closure that fails at the corner has spent the saving many times over.
Where Drop Testing Sits Alongside Compression and Vibration
Drop testing is one of three mechanical tests a shipping carton faces, and choosing which to run follows the mode: drop for a parcel network, compression and stacking for a full container load, and vibration for both.
- ISTA 3A: the parcel procedure, combining conditioning, random vibration with and without a top load, and drops.
- ASTM D5276: the free-fall drop method, cited where a programme specifies drop testing on its own.
- ISO 2248: vertical impact by dropping, the ISO counterpart.
- ISO 12048: compression testing of complete filled transport packages.
- ISO 2234: stacking under a static load, which is what a container voyage actually applies.
- ASTM D4169: performance testing of shipping containers and systems, used where a programme wants a distribution-cycle simulation.
A full container load of knitted bags is not dropped, and this is the point where specifications most often go wrong. The container is lifted, stowed, vibrated for several weeks and compressed in a stack. The relevant tests for that mode are ISO 12048 and ISO 2234, and a programme shipping full containers that specifies only ISTA 3A has tested for a hazard it will not meet and not tested for the one it will.
What Knit Programmes Can Conclude From a Drop Test
On a knitted bag programme, a drop test is best read as a closure and corner test, and the conclusion worth acting on is usually about tape specification and corner reinforcement rather than about the bag.
Three findings recur across knit programmes and are worth expecting. The bags pass: knit absorbs impact and the contents criterion is rarely the binding one. The closure is the failure: a 12 kg carton dropped on a corner puts a load on the tape that a 48 mm acrylic tape applied without adequate rub-down does not hold. And the corner is the second failure: a crushed corner that does not split still means the carton leans, and a leaning carton in a palletised stack shifts load onto its neighbour.
The corresponding specification changes cost little. A wider tape, a tape applied in an H-pattern rather than a single strip, and corner boards at about USD 0.06 per carton take a knit carton from marginal to comfortable on the corner drop. None of them is a change to the bag, and none of them requires a second sampling round, which is why testing is done in the sampling window rather than after it.
When the Test Happens and What It Gates
Drop testing is run inside the 7-working-day sampling window on a carton packed as bulk will be packed, and a pass gates the packaging specification for the 30-day bulk production window.
Packing the test sample as bulk will be packed is the requirement that is most often missed and it invalidates the result when it is. A carton packed with a different fill, a different closure or a different number of bags is a different package, and the drop result does not transfer. Once the specification is gated, pack-out is held to it through bulk and inspected under the AQL 2.5 plan (ISO 2859-1) before despatch. Shipments leave FOB Xiamen, with CIF and DDP available.
Arrange a carton drop test
Send the carton specification, the pack quantity and the shipping mode, and the knit team will return a test plan with heights, sequence and written acceptance criteria. Start the test plan
Frequently Asked Questions
What drop height applies to a knit bag carton?
About 380 mm for a 12 kg carton under ISTA 3A. The height falls as weight rises: about 460 mm up to 9.5 kg, 380 mm from 9.5 kg to 18.6 kg, 300 mm to 27.7 kg, 200 mm to 45.4 kg and 100 mm above that.
Why are heavier packages dropped from lower heights?
Because handling differs by weight. A package light enough to be thrown onto a belt receives more energetic impacts than one heavy enough to be moved by machine, so the procedure sets a lower height for heavier packages rather than a higher one.
How many drops does a carton get?
Ten under ASTM D5276: one corner, then the three edges meeting at that corner, then the six faces. The corner and edge drops do the damage the flat drops never will, so testing only faces produces a false pass.
What conditioning is required before drop testing?
About 12 hours at 23 degrees and 50% relative humidity. Corrugated board is hygroscopic, so a carton tested straight off a damp floor gives a result that cannot be reproduced.
What counts as passing a drop test?
The product is undamaged and the pack has kept its integrity: the closure has not opened, the board has not split through, no hardware has worked loose, and any retail packaging inside is still presentable. A crushed corner is acceptable; a split is not.
How many samples should be tested?
One is the procedural minimum, three is the practical number and five suits a programme with a damage-rate target. Variance comes mostly from how the tape was applied, and one sample tells you nothing about that.
Does a drop test tell me anything about the knitted bag?
Rarely. Knit absorbs impact well and the contents criterion almost always passes. The test is really testing the closure and the corners of the carton, and the report should be read that way.
What fails most often on a knit carton drop test?
The closure. A 12 kg carton dropped on a corner puts a load on the tape that a single strip of tape applied without proper rub-down does not hold. A crushed corner is the second most common finding.
Is ISTA 3A the right test for a full container load?
Not on its own. A container is lifted, stowed, vibrated and compressed rather than dropped, so the relevant tests are ISO 12048 for compression and ISO 2234 for stacking under static load.
What is the difference between ASTM D5276 and ISO 2248?
Both cover dropping a loaded container. ASTM D5276 specifies the free-fall method and the ten-drop sequence; ISO 2248 covers vertical impact by dropping. Either may be cited depending on whether a programme follows an ASTM-led or an ISO-led specification.
Should the test sample be packed exactly as bulk?
Yes. A carton with a different fill, a different closure or a different number of bags is a different package, and the drop result does not transfer. This is the requirement most often missed and it invalidates the result.
When is drop testing carried out?
Inside the 7-working-day sampling window, on a carton packed as bulk will be packed. A pass gates the packaging specification for the 30-day bulk window and pack-out is inspected under the AQL 2.5 plan.
What low-cost changes fix a failing drop test?
A wider tape, an H-pattern closure rather than a single strip, and corner boards at about USD 0.06 per carton. None changes the bag and none needs a second sampling round, which is why the test is run in the sampling window.
Does humidity conditioning matter for the bags as well?
Yes. A knitted shell conditioned at 50% relative humidity behaves differently from one that has been in a container at 80%. Testing both a conditioned sample for compliance and a humid sample for prediction is the practice that forecasts the damage rate.
What impact surface is required?
A rigid, level surface, normally steel or concrete. The package has to be released rather than thrown and the release mechanism must not impart rotation, because a rotating package lands on a different face than the one specified.
Can a drop test be reused across seasons?
Only if nothing in the pack changes. A new carton board, a different tape, a different pack quantity or a change from polybag to sleeve all invalidate a previous result, and any of those requires the test to be run again.
People Also Ask
How high is a carton drop test?
About 460 mm up to 9.5 kg, 380 mm to 18.6 kg, 300 mm to 27.7 kg and 200 mm to 45.4 kg under ISTA 3A. A 12 kg knit carton tests at about 380 mm.
How many drops are in a drop test?
Ten under ASTM D5276: one corner, three edges and six faces. The corner and edge drops are the ones that find failures.
What is ISTA 3A?
A parcel distribution test procedure for packaged products up to about 70 kg, combining conditioning, random vibration with and without a top load, and drops at heights set by weight.
Do I need drop testing for a container shipment?
Not as the primary test. A container load is compressed and vibrated rather than dropped, so ISO 12048 compression and ISO 2234 stacking are the relevant tests.
What conditioning comes before a drop test?
About 12 hours at 23 degrees and 50% relative humidity, because corrugated board is hygroscopic and an unconditioned carton gives an unreproducible result.
Why does the closure fail before the bag does?
Knit absorbs impact, so the contents pass easily while the tape takes the corner load. A wider tape, an H-pattern closure and corner boards usually fix it.
Sources and further reading
Every figure on this page is either our own programme data or attributed below. Open the originals before you write them into a specification.
- ISO 2859-1 — Sampling procedures for inspection by attributes, Part 1: AQL-indexed sampling schemesISO The AQL 2.5 plan under which pack-out and closure are inspected once a drop test has gated the specification.
- ASTM standards for textile and packaging testingASTM International The ASTM library from which D5276 free-fall drop testing and D4169 distribution simulation are specified.
- ISO packing and distribution of goodsISO The ISO series covering vertical impact by dropping and the compression and stacking tests used alongside it.
- ISO 12947-2 — Textiles — Determination of abrasion resistance by the Martindale methodISO The abrasion test referenced when a drop result is read alongside the condition of the knitted surface.
- Incoterms rulesInternational Chamber of Commerce The rule set behind the FOB Xiamen term and the CIF and DDP alternatives offered on knit programmes.
Match the Test to the Mode, Then Write the Criteria First
The two mistakes that make a drop test useless are testing for a hazard the shipment will never meet, and deciding what counts as a pass after seeing the result. Both are avoidable in an afternoon.
Choose ISTA 3A for parcels and ISO 12048 with ISO 2234 for containers, write the acceptance criteria before release, and test a carton packed as bulk will be packed. For the carton being tested, see carton specification for bag shipping. For the protective items the result points to, see protective packing for knitted bags.