Also known as: Linerboard, Kraft Liner, Test Liner, Liner, Facing Paper
Liner paper (linerboard) is the flat facing paper glued to one or both sides of the fluting medium to make corrugated board. Kraft liner is virgin fibre, test liner is recycled. Indian converters commonly run 120–180 GSM liners at 16–35 BF; the outer liner carries print, burst and moisture resistance, while both liners set edge crush strength.
Liners come in four commercial types. Kraft liner is made from virgin softwood or hardwood kraft pulp, has long fibres and grades at 24–35 BF in India, higher from imported mills. Test liner is a multi-ply recycled sheet from old corrugated cartons, typically 14–22 BF, and is the economical inner-liner choice. White-top kraft or test liner carries a bleached top ply for high-quality flexo or offset print. "Semi-kraft" in Indian trade usage means a blend of virgin and recycled pulp sitting between the two in both BF and cost.
Grammage runs from about 100 to 300 GSM, with 120, 150, 180, 200 and 250 GSM the stock grades in Gujarat. Bursting strength follows GSM × BF ÷ 1,000: a 150 GSM 22 BF liner bursts at about 3.3 kg/cm², a 180 GSM liner at 4.0. A common 3-ply build of 150/120/150 at 22 BF therefore lands near 8 kg/cm² board burst, and a 5-ply of 180/120/150/120/180 near 14 kg/cm² — the low ends of the typical ranges for those constructions. Heavier outer and lighter inner liners are normal because the outer face takes the abuse.
Beyond burst, four liner properties matter. Ring crush (RCT) and short-span compression (SCT) feed the edge crush test and therefore stacking. Cobb 60 tells you how well the sheet is sized against water; specify 30 g/m² or lower on the outer liner for monsoon or export cartons. Smoothness and porosity decide how cleanly flexo ink lays and how starch bonds to the medium. Ply bond keeps a multi-ply test liner from delaminating under a taped flap.
The outer and inner liners do different jobs and can be specified differently. The outer liner sees print, rain, forklift tines and strapping, so it earns virgin kraft, higher BF and a Cobb limit. The inner liner sees the product; for machined parts or brass hardware it needs to be non-abrasive and, for ferrous goods, may be replaced or backed by VCI paper or an EPE interleaf. Using 22 BF kraft outside and 18 BF test liner inside is a standard cost-neutral upgrade over 20 BF on both.
A purchase order should state GSM and BF for each liner in the outer / medium / inner order, the liner type (kraft, test, white-top), a Cobb 60 limit where humidity is a risk, and any print requirement. Ask the converter for the mill test certificate of the actual reel, since the same "180 GSM 22 BF" line can be met by papers that differ by 30% in ring crush. For 7-ply boxes replacing wooden crates around machinery, imported kraft liner at 35 BF and above on the outer face is usual.
Liner Paper (Linerboard) at a glance
| Role | Flat facing sheet on each side of the fluting medium |
|---|---|
| Kraft liner | Virgin fibre, 24–35 BF (India), 35+ imported |
| Test liner | Recycled multi-ply, 14–22 BF |
| Stock GSM (Gujarat) | 120, 150, 180, 200, 250 GSM |
| Burst rule | BS (kg/cm²) ≈ GSM × BF ÷ 1,000 |
| Key tests | Burst, RCT/SCT, Cobb 60, smoothness, ply bond |
| Outer liner spec | Higher BF, Cobb 60 ≤ 30 g/m² for export |
| White-top liner for high-quality flexo or offset |
Common questions
What is the difference between kraft liner and test liner?
Kraft liner is made from virgin kraft pulp with long fibres, giving 24–35 BF and the best burst, tear and ring crush. Test liner is made from recycled corrugated waste in several plies, grading at 14–22 BF, and costs less. Most Indian cartons use kraft on the outer face and test liner inside.
What GSM liner is used for a 5-ply corrugated box?
A typical Indian 5-ply export carton uses 180 GSM on the outer liner, 150 GSM on the centre liner and 150–180 GSM inside, with 100–140 GSM mediums, all at 20–24 BF. That build gives roughly 11–12 kg/cm² board burst — the board figure is close to the sum of its liners, each at GSM × BF ÷ 1000. Heavier goods move to 200–250 GSM outer liners or imported 35 BF kraft.
Does liner GSM or BF matter more?
They multiply. GSM × BF ÷ 1,000 gives the liner bursting strength in kg/cm², so 180 GSM at 18 BF and 150 GSM at 22 BF are close in burst. Higher BF buys fibre quality — better tear, puncture and wet strength — while higher GSM buys stiffness and ring crush. For stacking, GSM and the medium matter more; for puncture, BF.
Which liner should face the product inside the box?
The inner liner should be smooth and non-abrasive for finished surfaces; a test liner is fine for most goods. Ferrous parts benefit from VCI paper or an EPE foam interleaf against the liner, and ceramics or glass from an EPE layer. Reserve virgin kraft and the Cobb limit for the outer face that meets rain and handling.
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