Carton Specification for Knit Bag Shipping: Choosing Ply, Flute and ECT, Setting a Weight Limit, and Passing a Drop Test Before Loading

Four Decisions, In Order, Before Any Carton Is Specified

Carton specification is usually done backwards. A buyer asks for a strong carton, a board grade is nominated from habit, and the result is either over-specified, which costs freight, or under-specified, which arrives crushed. Four decisions taken in order produce a defensible answer, and each one narrows the next.

The first decision is the transit mode, because it sets how much abuse the carton has to survive. A carton going by sea in a full container load sees compression from stacking and vibration over several weeks. A carton going by air or by parcel sees more individual handling and more drops. A carton going into a domestic warehouse sees less of both.

The second decision is stacking, which sets the compression requirement and therefore the ECT figure. The third is handling, which sets the weight limit: a carton carried by one person has a lower limit than one moved by pallet truck throughout. The fourth is how the carton is opened at destination, which sets the closure.

Reading them in that order means the numbers come out of the product and the route rather than out of a habit. A programme that ships 60 knitted totes by sea on a pallet has different requirements from one that ships 12 premium bags in individual parcels, and the two should not end up with the same carton by default.

Ply and Flute: What the Board Actually Does

A corrugated board is a sandwich: flat liner sheets on the outside and a fluted medium between them. Ply describes how many sheets there are, and a 3-ply board is one fluted medium between two liners while a 5-ply board is two mediums and three liners. Flute describes the height of the wave, with B flute around 3 millimetres and C flute around 4.

B flute is flatter and denser, resists crushing better and prints more cleanly, and it suits a carton that will be stacked and printed. C flute is taller and gives better cushioning and better compression strength for a given weight of board, and it suits a carton that will be stacked high in a container. For bag programmes C flute is the more common starting point.

Moving from 3-ply to 5-ply roughly doubles the board weight and material cost and it is often unnecessary. Where a buyer wants more stacking strength, the more economical route is usually a heavier liner on a 3-ply board rather than an extra medium, and a competent packaging partner can quote both so the comparison is made on numbers.

ECT and Burst: Which Number to Specify

Two numbers describe board strength and buyers often specify the wrong one. Burst strength measures resistance to puncture, in pounds per square inch or kilopascals, and it is a legacy figure that many specifications still carry. Edge crush test, or ECT, measures the compression strength of the board edge in kilonewtons per metre, and it is the figure that predicts how a carton performs in a stack.

For a shipping carton, ECT is the number that matters, because cartons in a container fail by compression rather than by puncture. A commonly specified figure for a bag carton is 32 ECT, meaning 32 kilonewtons per metre, with heavier or taller stacks going to 44 or above. Where a specification still carries a burst figure such as 200 pounds per square inch, it is worth checking whether the ECT was ever stated.

The ECT figure needed follows from the stack. A carton at the bottom of a stack of eight in a container carries seven times its own weight plus a dynamic factor from vehicle motion, and a board that is adequate for a stack of four will creep and then crush over a six week voyage. Asking for the figure to be calculated against the actual stack height is a reasonable request.

One caveat belongs with ECT, and it is the one buyers most often miss: the figure is measured on conditioned board. Corrugated board loses a substantial part of its compression strength at high humidity, so an ECT figure achieved in a dry laboratory is not the figure the carton will have after three weeks in a humid container. Moisture-resistant treatment or a humidity allowance is the answer.

Carton Dimensions and the Weight Limit

Carton dimensions are set from the pack, not from the container, and the order matters. Decide the unit pack and the fold, then the units per carton, then the internal dimensions with a small allowance so the carton is neither crushed by overfilling nor loose enough for the contents to settle and damage each other.

The weight limit is a handling decision rather than a board decision. 15 kilograms is the widely used figure for a carton that one person may have to lift manually, and it is the one to specify unless the destination has confirmed mechanised handling throughout. A heavier carton is not necessarily stronger; it is harder to move and more likely to be dropped.

Units per carton then follows from both. Sixty knitted totes in a polybag is a typical figure for a mid-sized tote at 15 kilograms, while 12 premium bags in individual boxes may reach the same weight at a fraction of the count. Setting units per carton from the weight limit rather than from a round number is what keeps the carton consistent across a size range.

Board

3-ply C flute as the default for bag programmes

ECT

32 kilonewtons per metre, rising to 44 or above for tall stacks

Carton weight

Not above 15 kg where manual handling is possible at any point

Units per carton

Set from the weight limit; around 60 polybagged totes is typical

Internal allowance

Small internal gap so the carton is neither overfilled nor loose

Closure

Reinforced pressure-sensitive tape in an H pattern, or strapping above 15 kg

Drop test

Ten drops on the corners, edges and faces, height set by carton weight

Pallet

1200 by 800 millimetres, with the stack height confirmed against the container

Marking

Style, colourway, quantity, carton number and country of origin

Recycled content

Declared percentage, with the ECT confirmed on the recycled grade

Closure, Tape and Strapping

The closure is the part of the carton specification most often left to the packer's judgement, and it is where a surprisingly large share of transit failures originate. Two methods cover almost everything: reinforced pressure-sensitive tape applied in an H pattern, closing both the centre seam and the two end seams, and strapping for cartons above about 15 kilograms or for palletised loads.

Tape width and grade matter more than buyers expect. A 48 millimetre reinforced tape is the working default, and a narrower or unreinforced tape will lift at the corners in a humid container. Where a carton is opened and resealed at destination, a tear strip or an easy-open feature is worth specifying, because a knife opening damages the contents more often than it damages the carton.

Staples are best avoided on a bag carton. They puncture the board, they reduce the stacking strength at the closure, and they are a safety hazard at unpacking, which matters to a retailer's own warehouse staff. Where a specification calls for them, tape is usually accepted as an equivalent and is better on every measure.

Drop Testing the Finished Carton

A carton specification is a hypothesis until it has been dropped. The standard screening is a sequence of ten drops covering the corners, edges and faces of one carton, with the drop height set by the carton weight under the ISTA procedures; for a carton under about 10 kilograms the height is commonly around 760 millimetres. The carton is then opened and the contents examined.

The acceptance criterion is worth stating before the test rather than after. The usual one is that the carton may show damage but must not open, and the contents must be undamaged and saleable. A carton that opens during the sequence has failed regardless of whether the bags inside survived, because an opened carton in a container loses its contents into the container.

Testing the finished, packed carton rather than an empty one is the point. An empty carton behaves differently, particularly on corner drops, because the contents provide some internal support and change how the impact is distributed. Where a programme has a size range, test the heaviest and the lightest carton rather than a middle size.

One more test is worth running for sea freight, and it is a compression or stacking test rather than a drop test. A carton that survives ten drops can still creep and crush under a stack of eight over six weeks, and the two failure modes have different causes and different fixes. Where transit damage has been seen before, run both before concluding anything.

Palletisation and Container Loading

The carton and the pallet are one system, and specifying one without the other is how a well-built carton arrives damaged. Three variables decide: the pallet footprint, the stack pattern, and the stack height against the available container height.

The overhang point in the second card is the least expensive fix in the whole specification. A carton overhanging the pallet by even 30 millimetres loses support under that section, and the bottom layer of the stack then carries load through the corner rather than through the full footprint, which is where crushing starts.

Writing the Carton Spec Into the Order

A carton specification needs eight fields and most orders carry two, usually carton size and a vague instruction to pack well. Eight fields: board construction, ECT figure, internal dimensions, units per carton, maximum weight, closure, marking, and the pallet configuration. All eight are checkable at packing, which is what makes the difference.

Adding the test requirement is the ninth field and it is what converts the specification into a verified one: one finished carton drop tested per new programme, and re-tested after any change to the board, the units per carton or the pallet configuration. Three triggers, and each of them can silently invalidate a specification that was validated a season earlier.

Frequently Asked Questions

People Also Ask