Guide · Kraft paper and packaging
Kraft liner, testlinerand fluting, compared.
Kraft liner is a containerboard made mainly from virgin kraft pulp, testliner is made mainly from recycled fibre, and fluting is the wavy middle layer between the liners of corrugated board. Which one you need depends on the load, the climate and the supply chain the box must survive. Sack kraft is a different product again: it is made for multiwall sacks, not boxes.
The short answer
Three papers,one board.
Corrugated board is built from two kinds of paper. Liners are the flat facings on the outside and inside of the board. Fluting (also called medium or corrugating medium) is formed into waves and glued between the liners, which gives the board its thickness and stiffness. Liners come in two main families: kraft liner, made predominantly from virgin fibre, and testliner, made predominantly from recycled fibre.
Virgin kraft fibre is long and strong, so kraft liner reaches a given strength at a lower grammage and keeps more of that strength in damp conditions. Recycled fibre has already been through the papermaking process; it is shorter and weaker, so testliner needs more grammage for the same performance. It still suits many dry, short supply chains well.
Sack kraft sits beside these grades, not among them. It is a strong, porous, often extensible paper for multiwall sacks of cement, flour, sugar and chemicals, and it is judged by TEA and air permeance rather than by crush tests. See how to specify sack kraft paper.
Side by side
Kraft liner, testliner,fluting and sack kraft.
Typical values used in the trade. Each order sets its own figures.
| Point | Kraft liner | Testliner | Fluting | Sack kraft |
|---|---|---|---|---|
| Main fibre | Virgin kraft pulp, mostly softwood | Recycled fibre from recovered boxes and paper | Semi-chemical virgin fibre, or recycled fibre | Virgin long fibre softwood kraft pulp |
| Role | Outer and inner facings of corrugated board | Outer and inner facings of corrugated board | Wavy middle layer of corrugated board | Plies of multiwall paper sacks |
| Colour | Brown (unbleached) or white top | Brown or white top | Brown | Natural (brown) or white (bleached) |
| Typical grammage | Commonly about 115 to 300 g/m², lighter and heavier grades exist | Commonly about 100 to 250 g/m² | Commonly about 80 to 200 g/m² | 50 to 120 g/m² |
| Key tests | Burst, RCT or SCT, Cobb, moisture | Burst, RCT or SCT, Cobb, moisture | CMT, RCT or SCT, moisture | TEA, tensile strength, stretch, air permeance, Cobb |
| Strength for its weight | Highest of the liners | Lower: needs more grammage for the same result | Semi-chemical: stiff; recycled: moderate | High tear and drop resistance |
| In damp conditions | Holds its strength well | Loses strength sooner | Semi-chemical keeps its stiffness better | Depends on the grade |
Where our grades sit
The codeson our product page.
The grades listed on our Kraft paper and packaging page fall into these groups.
| Group | Codes | What they are |
|---|---|---|
| Kraft liner board | KS | Kraft liner board for the flat layers of corrugated board, also used as a packaging material |
| Unbleached sack kraft | SKE HP, SKE, SKS HP, SKS, SKI | Extensible, semi-extensible and standard improved sack papers, some with high porosity |
| Bleached sack kraft | WSKS, WSKS HP, WSKI | White sack papers for printed food, agricultural and retail sacks |
| Liner and fluting by specification | Kraft liner (brown or white top), testliner, fluting | Specified by GSM, burst, RCT, Cobb, moisture and reel size, agreed for each order |
The tests
What each testtells you.
| Test | Standard | Applies to | What it measures |
|---|---|---|---|
| Grammage | ISO 536 | All grades | Mass per square metre, in g/m² |
| Burst (Mullen) | ISO 2759 | Liners | Pressure needed to rupture the sheet, in kPa; often quoted as burst index (kPa·m²/g) |
| RCT (ring crush test) | ISO 12192 | Liners and fluting | Edgewise compression strength of a strip formed into a ring, in kN/m |
| SCT (short span compression test) | ISO 9895 | Liners and fluting | Edgewise compression over a very short span, in kN/m; closely linked to the stacking strength of the box |
| CMT (Concora medium test) | ISO 7263 | Fluting | Flat crush resistance of fluting corrugated in the laboratory, in N |
| ECT (edge crush test) | ISO 3037 | Finished corrugated board | Edgewise crush resistance of the board, in kN/m; the main input to box compression strength |
| Cobb | ISO 535 | Liners and sack kraft | Water taken up by the surface in a set time (Cobb 60 means 60 seconds), in g/m²; lower means more water resistant |
| Moisture | ISO 287 | All grades | Water content in percent; affects strength, warp and running on the corrugator |
From paper to box
Board strengthin general terms.
Paper tests describe the raw material; ECT describes the board. Box compression strength (BCT, tested to ISO 12048) depends mainly on the ECT of the board, the board thickness and the box perimeter. That is why many buyers now specify liners and fluting by SCT, which predicts ECT more closely than burst does. Burst still says something useful about resistance to puncture and rough handling, so many specifications state both.
Board is described by its wall type and flute. Flute profiles are named by letter: A is the tallest, then C, B, E and F. Taller flutes give more cushioning and stacking strength; smaller flutes give a flatter surface for print. In North America, board is often graded by ECT (for example 32 ECT or 44 ECT) or by its burst (Mullen) rating.
Corrugated board grades
Single, doubleor triple wall.
| Construction | Layers | Typical uses |
|---|---|---|
| Single face | One liner and one fluting | Wrapping and cushioning |
| Single wall | Two liners and one fluting | Most shipping and retail boxes |
| Double wall | Three liners and two flutings | Heavy goods, export cartons, fruit and vegetables |
| Triple wall | Four liners and three flutings | Bulk bins and very heavy or large items |
When to choose which
Match the paperto the job.
- Kraft liner. Heavy or valuable goods, long sea voyages, cold stores, humid climates, fruit and vegetables, and any box that must stay strong when damp.
- White top liner. Kraft liner or testliner with a white face, where the outside of the box carries print or is seen on the shelf.
- Testliner. Dry goods, light to medium loads and short, dry supply chains, when the box meets its compression target at the chosen grammage.
- Kraft outside, testliner inside. A common mix: the stronger, more water resistant liner faces the weather and handling.
- Semi-chemical fluting. Where stiffness and stacking strength matter most, especially in humid conditions. Recycled fluting suits general purpose board.
- Sack kraft. When the package is a filled sack, not a box: cement, dry mixes, flour, sugar, feed or chemicals.
Your checklist
What to put inyour specification.
- 01Product: kraft liner (brown or white top), testliner or fluting, and the fibre basis
- 02Grammage and tolerance
- 03Strength values: burst, RCT or SCT for liners; CMT, RCT or SCT for fluting
- 04Cobb and moisture targets, and the test standards to be used
- 05Reel width, reel diameter and core size
- 06End use: board construction, flute, the goods packed and the transport route
- 07Quantity, destination and preferred Incoterm
Questions
Common questionsabout liner and fluting.
What is the difference between kraft liner and testliner?
Kraft liner is made mainly from virgin kraft pulp, testliner mainly from recycled fibre. Kraft liner is stronger for its weight and keeps its strength better in damp conditions, while testliner needs more grammage to reach the same performance.
Is fluting the same as medium?
Yes. Fluting, medium and corrugating medium all mean the paper formed into waves between the liners of corrugated board. It is made from semi-chemical virgin fibre or from recycled fibre and is tested mainly by CMT, RCT or SCT.
What does white top liner mean?
White top liner has a white surface layer over a brown base, for print and shelf appearance. It exists as white top kraft liner and as white top testliner.
Which matters more, burst or SCT?
For boxes that are stacked, SCT or RCT usually matters more, because compression strength decides how a box survives stacking. Burst still describes resistance to puncture and rough handling, and many specifications state both.
Can sack kraft be used for corrugated board?
Sack kraft is made for multiwall sacks and is specified by TEA, stretch and air permeance, not by crush tests. Corrugated board needs liner and fluting grades; on our product page, KS is the kraft liner board for flat layers.
What grammage of kraft liner do I need?
It depends on the box compression target, the flute, the load and the climate. Start from the box strength you need (ECT or BCT) and agree the liner and fluting weights with the board maker; see how BGI works for how a request moves forward.
Please note
Typical parameters only.
This guide explains parameters commonly used in international trade, for orientation. It is not a specification, an offer or a statement of available goods. Every deal sets its own specification in the written contract.
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