The Cartons Passed Inspection. Three Weeks Into Warehouse Storage, They Collapsed
Carton Compression, Stacking and Drop Test Guide - Why Cartons Get Crushed in Storage and Transit, ECT, BCT, Burst Strength and Drop Tests Compared, Strength by Corrugated Flute, Calculating Required Compression Strength from Stack Height and Storage Time, Compression Tester Calibration, Supplier Board Substitution, Spec Sheet Items, Stage-by-Stage Checks and a Checklist

Hello, this is StartMade.

A Korean kitchenware brand produced 4,800 sets of heat-resistant glass food containers at a glass factory in Zibo, Shandong. Each master carton held four sets and weighed about 12kg. The carton supplied with the approval sample was 5-ply BC flute, and the pre-shipment inspection found no problems with quantity, appearance or printing. The carton also passed a drop test in which the inspector dropped one carton.

The problem appeared in a Korean logistics warehouse. The cartons were stored five high on each pallet with pallets stacked two high. Three weeks later, in the middle of the rainy season, the bottom cartons bulged outward, their corners buckled and they collapsed. As the upper pallet tilted, lids on the glass containers in the bottom cartons broke and some containers cracked. The brand sent the remaining cartons to a test lab. Box compression strength (BCT) came in at about 2,900N, nearly 40% lower than the approval sample carton of the same size (about 4,800N). Cutting a cross-section showed that the outer liner was recycled board color-matched to look like virgin kraft, and each paper grade had been dropped one step in basis weight. The carton plant had changed the board construction to hit its price, and even the glass factory that ordered the cartons did not know.

You cannot see carton strength. A liner 50g lighter, or an outer face switched to recycled board, looks exactly the same once the carton is printed. A drop test only measures a brief impact, so it tells you nothing about whether the carton can bear weight from above for weeks. The difference only shows up as crushed cartons and broken contents after warehouse stacking, container transit and a humid season. This article starts with why cartons collapse, then covers the test types, strength by flute, how to calculate the compression strength you need, test equipment calibration, the shortcuts suppliers often take, and how to put compression testing into the spec sheet and the inspection checklist.

A reply of "we'll use a strong 5-ply carton" or a quick press by hand is not enough. Set the flute type, board construction, minimum ECT and BCT, pre-test conditioning, actual stack height and storage time in numbers in the spec sheet, and test production cartons you pick yourself. That is how you prevent collapses in storage and transit.

1. Why Cartons That Passed Inspection Collapse in the Warehouse

The stacking strength of a corrugated carton is set by the type and basis weight of the paper, the flute type, the carton dimensions and the moisture content. Most of these tie directly to the carton price, so a carton plant under price pressure trims them a little at a time, starting where nobody can see. On top of that, most Chinese manufacturers do not make their own cartons; they buy them from a nearby carton plant (纸箱厂), so the factory you ordered from often does not know when the carton spec changes. The main causes of lost strength are:

• Lower basis weight: Dropping a carton built with a 250g liner and 150g medium to 200g and 120g leaves thickness and appearance almost unchanged, but compression strength falls noticeably.
• Recycled board: Virgin kraft liner (牛卡纸) is replaced with recycled liner (箱板纸, 茶板纸), with only the outer color matched.
• Flute change: 5-ply BC flute is switched to 3-ply C flute or a thick B flute, or the same BC flute is run on equipment that produces lower flutes.
• Crushed flutes: Heavy roller pressure during printing and converting flattens the flutes, which reduces thickness and strength along with it.
• Moisture content: Freshly made cartons, or cartons stored somewhere humid, hold moisture and are much weaker than dry ones.
• Size and stacking mismatch: If the carton is larger than its contents and leaves empty space, or is stacked overhanging the pallet, the same carton can bear far less.

Stacking compression strength is a separate problem from drop impact. Transit packaging drop tests and ISTA procedures are covered in Episode 127, and cushioning and scuffing inside the carton in Episode 129. This article focuses on how long a carton can bear the load pressing down on it.

2. Test Types - ECT, BCT, Burst Strength and Drop Tests Compared

If you only ask a factory to "test carton strength," it will likely send back a single burst strength figure, the number it is used to putting on reports. Decide first what you want to confirm, then choose the test that fits.

Stacking problems call for BCT

Cartons collapsing in warehouses and containers is judged by the compression strength of the finished carton (BCT). ECT is a board value, which makes it good at catching board substitution, while BCT shows how many kilograms a carton made from that board will actually hold. That is why the spec sheet should list both. Reports from Chinese carton plants often show only burst strength, and burst strength does a poor job of predicting stacking strength. There are plenty of cartons with high burst strength and low compression strength.

Corrugated flutes and how their strength differs

Flute refers to the height and spacing of the wave-shaped corrugations in the middle of the board. Taller flutes give better edgewise strength and cushioning, while shorter flutes give a flatter surface for cleaner printing and better resistance to flat crushing.

Even within BC flute, strength varies widely with board construction, so writing only "5-ply BC flute" does not fix the strength. List the paper types and basis weights and the minimum ECT next to the flute name, and you stop the substitution.

3. Calculating the Compression Strength You Need - Stack Height, Storage Time and Humidity

The load a carton must bear starts with the weight sitting on top of the bottom carton. If each carton weighs 12kg and you stack five high on a pallet with pallets two high, the bottom carton carries nine cartons, about 108kg. Add the weight of the pallet itself and it rises a little more. You cannot, however, compare the BCT measured by a lab directly with that weight. The test finishes in a few minutes in a dry room, while real cartons carry the same load for weeks in a humid warehouse. Rough strength losses commonly used in the industry are:

These losses are rolled into a safety factor. Many use around 3 for ordinary warehouse storage and 4 to 5 when ocean freight in a humid season and long storage come together. Using the example above, 108kg × 4 = about 432kgf, which means the BCT should be at least about 4,200N. The carton in the opening case measured 2,900N. It was no surprise it failed after three weeks in a humid warehouse.

4. Compression Tester Calibration and Test Conditions

In a BCT test, an empty carton is erected and placed between the two platens of a compression tester, then pressed at a set speed while the peak load at failure is recorded. The test itself is simple, but many conditions affect the result. It is not unusual for a factory's in-house value and a lab value to differ by more than 30%.

Conditioning and equipment calibration matter most. Cartons piled in a dry corner of the factory office and tested straight away come out much stronger than they will be in a humid warehouse, and an uncalibrated tester gives numbers you cannot trust. When you receive a test record, do not look only at the numbers. Check first that it states which equipment was used, what conditioning the cartons went through, and how many were tested.

5. Common Supplier Shortcuts

Carton strength cannot be checked by looking, which also makes it easy to fudge. Here are the patterns we have actually run into in consultations.

The most common case is a change of board between sample and production cartons. With the outer color and printing matched, the two are almost impossible to tell apart by eye. The only way to catch it is for the inspector to pick production cartons personally, look at the cross-section, weigh the board for basis weight and compare it side by side with the approved carton. For change control at second-tier suppliers such as carton plants, see Episode 115.

6. Defects Caused by Weak Cartons

Insufficient carton compression strength rarely shows at shipment. It appears after warehouse storage and transit.

• Damaged contents: When a carton gives way, the load above lands directly on the contents. Products that cannot take load, such as glass, ceramics and small appliances, break or deform right away.
• Collapsed loads: When bottom cartons fail, the whole pallet can lean or topple, putting other cargo and workers at risk.
• Damaged retail boxes: Even if the product is intact, a crushed retail box is hard to sell as new. In online sales it leads straight to returns and low ratings.
• Rejected receipts: Large retailers and fulfillment centers may refuse crushed or burst cartons, or charge for repacking.
• Insurance and claim disputes: If poor packing is the cause, cargo insurance may refuse to pay, and without carton specs and test records you are at a disadvantage when disputing responsibility with the factory.

Moisture is often part of the cause. Container condensation and moisture damage are covered in Episode 118, and how cargo insurance treats poor packing in Episode 57.

7. Items to Put in the Spec Sheet

A single line such as "5-ply export carton" lets every carton plant choose its own board and flute. Put the items below in a table and attach it to the spec sheet or packaging spec. For how to write a spec sheet, see Episode 59.

It is worth getting the carton plant name and board supplier as well. When the carton plant changes, the board and equipment change all at once, and if you do not know the name you will not even know a change happened. Printing the maximum stacking layers (堆码层数) on the carton also helps stop warehouse staff from stacking higher than intended. For load planning inside the container, see Episode 101.

8. How to Put Compression Testing into the QC Checklist

Sample stage

When you receive the approval sample cartons, send some to a third-party lab in Korea or China for ECT and BCT, separately from the factory's report. Running a drop test with the contents packed at this stage too gives you a baseline for production. Keep the approved carton and a piece of its cross-section to compare against production cartons.

Production stage

When production cartons arrive, get the ECT and BCT test records before packing. Have the tests repeated whenever the carton plant, board or carton size changes. If the record shows the same number every time, just above the requirement, and has no photos, start by questioning whether the test was done at all. Freshly made cartons hold a lot of moisture, so also check whether they are left to dry for a few days before use. For managing carton spec changes, see Episode 119.

Pre-shipment inspection stage

The inspector picks production cartons personally and checks the cross-section, comparing the condition of the liners and medium and any flute crushing against the approved carton. Caliper thickness is measured, and a piece of board is cut out to check basis weight. Carton moisture content is measured with a moisture meter. BCT is destructive and hard to run on site every time, so if anything looks off, send a few empty cartons to a lab. A drop test on a packed carton is done at the same time. For pre-shipment inspection, see Episode 100.

9. Sample Wording to Send the Factory

Putting the wording below in the purchase order or packaging spec takes the carton board and test method out of the factory's discretion, and makes it easier to assign responsibility if cartons collapse.

"本产品外箱为五层BC楞RSC纸箱，面纸为250g/m²原生牛卡纸，瓦楞芯纸为150g/m²，纸箱厂及原纸配置以确认样为准。纸箱须在23℃、相对湿度50%条件下预处理24小时后测试：边压强度（ECT）不低于8.0kN/m，空箱抗压强度（BCT）不低于4200N。每批至少抽取5个空箱做抗压测试、10个试样做边压测试，并提交注明纸箱尺寸、生产日期、预处理条件和测试照片的测试记录，以及测试仪器校准证书。出货前须按ISTA 1A对装好产品的外箱进行跌落测试，产品不得破损。测试样品由我方验货员从大货中随机抽取。如有不合格，须加抽复测；复测仍不合格的批次须更换纸箱后重新包装。更换纸箱厂、原纸或楞型须事先书面通知并重新测试。"
(The outer carton for this product is a 5-ply BC flute RSC carton with a 250g/m² virgin kraft outer liner and 150g/m² mediums; the carton plant and board construction follow the approval sample. Cartons must be tested after 24 hours of conditioning at 23°C and 50% relative humidity, with an edge crush strength (ECT) of at least 8.0kN/m and an empty-box compression strength (BCT) of at least 4,200N. For each lot, at least 5 empty cartons must be compression tested and 10 specimens edge crush tested, and test records showing carton dimensions, production date, conditioning and test photos must be submitted along with the test equipment calibration certificate. Before shipment, packed outer cartons must be drop tested to ISTA 1A with no damage to the product. Test samples will be drawn at random from bulk production by our inspector. If any result fails, the sample must be increased and retested; lots that still fail must be repacked in replacement cartons. Any change of carton plant, board or flute type requires prior written notice and retesting.)

10. Three Real Cases

These are anonymized versions of patterns we often see in consultations. Amounts and figures are examples.

Case 1 - Heat-Resistant Glass Food Containers

This is the case from the opening. The brand sorted out the damaged goods, repacked the remaining stock in new cartons and sent the test results to the factory to hold it responsible. With no ECT, BCT or board construction in the contract, the factory held its ground, insisting it was "the same 5-ply BC flute carton." In the end the brand recovered part of the repacking cost on the grounds that the carton plant change had not been disclosed, but damaged goods, warehouse rework and delayed sales still left a loss of about KRW 28 million. From the next order, the brand put the board construction, minimum ECT and BCT and conditioning terms into the spec sheet, and had the inspector check carton cross-sections and basis weight in person at pre-shipment inspection.

Case 2 - Ceramic Flower Pots

A company importing flower pots from a ceramics factory in Dehua, Fujian had bottom cartons crushed in the container, so it switched from 3-ply C flute to 5-ply BC flute. The stacking problem went away, but returns for pots with chipped rims during parcel delivery went up. The cartons were stronger, but the pots went in with no cushioning inside, so the impact of a corner drop passed straight to the pot. Returns only fell after pulp molded inserts were added and an ISTA 1A drop test was added to the requirements. Raising compression strength does not solve drop impact.

Case 3 - Stainless Steel Vacuum Flasks

A company receiving 18kg master cartons from a vacuum flask factory in Yongkang, Zhejiang opened a container after a summer ocean voyage and found the bottom two rows of cartons completely collapsed. The carton plant's report listed only burst strength, which had passed. On checking, the cartons had been used for packing two days after they were made, and their moisture content exceeded 15% at shipment. The company then added ECT and BCT requirements and conditioning terms to the spec sheet, along with at least three days of drying after carton production and a moisture content of 12% or less at shipment. The carton price went up by about KRW 350 each, and the same damage has not happened again.

11. Common Mistakes

• Not setting strength in numbers, as in "a strong 5-ply carton"
• Accepting only a burst strength report without checking ECT and BCT
• Assuming that a carton that passes the drop test will also stack well
• Judging by dry-carton values without allowing for humidity and storage time
• Testing the approval sample carton once and never checking production cartons
• Not knowing that the carton plant or board had changed

12. Carton Compression Control Checklist

Specification stage

• Flute type, paper types and basis weights are written down
• Minimum ECT and BCT, test methods and conditioning are set
• Required BCT is calculated from actual stack height, stacking pattern and storage time
• Drop test criteria, per-lot sample quantity and what happens on failure are set

Sample and production stage

• Approval sample cartons were tested separately at a third-party lab
• The approved carton and a cross-section piece were kept
• ECT and BCT records for production cartons were received with conditioning details and photos
• The carton plant name, board supplier and change notification terms were confirmed

Pre-shipment inspection stage

• The test equipment calibration certificate was checked
• The inspector picked production cartons personally and checked cross-section, caliper, basis weight and moisture content
• A drop test was done on a packed carton

Wrapping Up - Check With Numbers, Not by Pressing With Your Hand

Every carton looks fine at shipment. Differences in strength show up as crushed cartons and broken contents after weeks stacked in a warehouse and a trip through a humid container. Writing the flute, board construction, ECT and BCT values and test conditions into the spec sheet, and checking production cartons you pick yourself, does not cost much. Set a few hundred won more per carton against a container load of damage, and the answer is clear.

There is also something you can do right now. Ask the factory currently producing for you to send the board construction of the master carton it uses and its latest ECT and BCT test records. If there are none, or only a burst strength figure, draw up a carton compression control sheet and send it before your next order.

StartMade supports you on the ground with packaging specs that reflect your stacking conditions, third-party testing of approval sample cartons, review of carton test records during production, and cross-section and basis weight checks plus drop testing before shipment. If you have already received claims for collapsed cartons or transit damage, we start with testing the remaining cartons and finding the cause; if you are preparing a new product, we start by calculating the compression strength you need.