Corrugated paper grades: what they are and how to choose the right one

These are the six grades used in the board and boxes we produce at Auto Boxes. Available specifications and sizes are listed on Corrugated sheets.

CA belongs in the fluting medium or the inner liner, so it is the one grade in the table to skip when you choose the outer face of a box, whether for strength or for print. It is made to form the wave that holds the two liners apart, and it is designed neither to carry compression directly nor to serve as a printed surface.

A line such as 3-ply, KA150, C flute holds both a paper grade and a flute type, and the two control different things. KA150 describes the paper: the outer liner is KA at 150 gsm. C flute describes the shape and height of the wave at the center of the board, in this case about 4 mm. Every box specification has both, yet many buyers assume the two terms mean the same thing.

Vertical compression is carried mainly by flute height. Changing the grade without changing the flute therefore has almost no effect on stacking strength.

As a rule of thumb when you read a specification or discuss it with a sales team, grade answers questions about the paper: toughness, moisture resistance and printing. Flute answers questions about the structure: vertical compression and shock absorption. For the flute types and their actual heights, see Corrugated board structure: how E, B, C, BE and BC flutes differ.

Each property puts the grades in a different order. The rankings below apply to the paper we use.

Which grade to use depends on where the box will be used:

• Cold rooms and fresh fruit. KH justifies its higher price where moisture is the main threat. It leads on both burst strength and moisture resistance.
• General industrial work that needs strength. KA gives better overall value once price and lead time are both counted. Its raw material is easier to source, it costs less per sheet and it has a shorter production lead time. Those practical advantages, more than performance, explain why the market uses KA far more widely than KH.
• Retail boxes printed to exact brand colors. WS is the right grade, even though it is not the strongest. On the shelf, these boxes are judged mainly by how they look.

Picking a grade on one property alone is the mistake buyers make most often. Specify KH for a 4-color printed retail box because it is the strongest grade, and you pay more for the paper and still get print that falls short of WS.

Line up quotations from three suppliers and you may find three different names for paper with the same properties. The reason is that each paper mill in Thailand sets its own grade codes, and no mandatory standard ties them together. To match an unfamiliar name to a grade, check the second column of the grade table at the top of this article, which lists the common alternative names.

The letters trace back to three types of paper:

• Kraftliner is linerboard made from virgin wood pulp, with long, tough fibers and high strength.
• Testliner is linerboard made mainly from recycled pulp. Its fibers are shorter because they have already been used once.
• Corrugating medium is the paper that forms the center wave.

Most codes start with K for kraft, some start with T for test, and the C comes from corrugating. The second letter is a product name chosen by each mill.

Treat the first letter as a broad indication only. Some mills use K for products they themselves describe as testliner, so a code such as KA can appear under kraftliner or testliner depending on who named it. Some mills also state that their liner has three layers, combining virgin and recycled pulp in one sheet. The leading letter does not show which pulps are blended or in what proportion.

The market uses dozens of codes. The table below groups them by their function in the board, whichever mill uses them.

Three questions identify a paper more reliably than its code. Is it made from long-fiber pulp or recycled pulp? Does the pulp contain a moisture-resistant agent? What is the grammage? The answers also show at once whether a cheaper quotation comes from a lower grade or a lower grammage.

Send your existing specification or a sample box to our Technical Service team if your current grade matches none of the codes in the table, and ask which grade it corresponds to. This is the quickest way to find a match. Guessing from the letters alone can leave you with paper that behaves differently from what you used before.

To read a full specification such as KA150/CA105/CA105, or the short form KA150/M, layer by layer, see What is KA paper: grammages, specifications and typical uses.

Grammage is the weight of paper per square meter, in grams. At 150 gsm, a sheet measuring 1 m × 1 m weighs 150 g. Higher grammage means denser paper, which gives the box surface better puncture resistance and tensile strength.

When you specify grammage, stay with the standard values. For liners the common ones are 125, 150, 185 and 230 gsm, and for fluting medium the lowest standard values are 105 and 125 gsm. Each grade comes in its own set, shown in the grade table at the top of this article. A figure outside the standard list usually means a longer lead time, because paper mills produce only a few standard grammages and anything else has to be ordered as a special lot.

Reading grammage layer by layer

Read a 3-ply specification one layer at a time. KA150/CA105/CA105 means:

• outer liner: 150 gsm
• fluting medium: 105 gsm
• inner liner: 105 gsm

A box therefore has a grammage for each layer, and treating it as one figure is a common misreading. Adding the three layers gives about 360 gsm. That total is handy for estimating box weight and shipping cost, though the real board weight runs slightly higher, because the medium is formed into a wave and uses more length of paper than the flat liners. As a guide to strength the total says little, since strength depends on which layer carries the grammage.

Grammage, flute and strength

KA125 on BC flute usually resists vertical compression better than KA185 on B flute. The lighter paper comes out ahead because the 5-ply structure holds the liners further apart, and this route often costs less than raising the grammage step after step. Box strength comes from paper grade, grammage and flute type working together, so raising grammage alone often buys only a small gain at a higher cost.

Before adding grammage, look at how the box is failing:

• If it collapses when stacked, change the flute and the number of plies.
• If the surface tears or is punctured by sharp-edged products, move the outer liner to a higher grade or grammage.

The table below gives a recommended grade and starting grammage for common types of work.

Two products of the same weight can need different specifications if they are stacked differently. Use the table as a starting point, then account for:

• the product itself
• how the boxes are stacked
• how long they stay in the warehouse
• the shipping method
• the actual weight of the product

KA and KI rank equal on burst strength, which makes them the pair buyers mix up most often. What separates them is moisture resistance and print quality. For a property-by-property comparison, see KA vs KI paper: how the two liner grades differ and which to choose.

Box size and ply count change the price per box and its stacking performance more than moving up one grade does. This article covers grades and grammage only, so for the size, start with internal dimensions that fit your product: see How to measure a corrugated box: internal or external dimensions.

Are CA and M in a specification the same paper?
Yes. M stands for medium, the fluting medium, and CA is the standard grade used for it. KA150/M reads the same as KA150/CA, with the medium grammage left unstated. When you compare prices between manufacturers, make sure you know the medium grammage as well.

Do 2-ply and 3-ply boards use different grades?
Both can use the same set of grades, and the difference lies in the number of layers. A 2-ply board has one liner bonded to a fluted layer and is used mainly for pads or wrapping, without being formed into boxes. In 3-ply board the fluted center has a liner on each side, and those liners carry the load of the box, so grade has a greater effect on strength in 3-ply.

How much does paper grade affect box price?
Grade has a clear effect on price. Moving up one grade, for example from KK to KA, raises the paper cost per sheet. It is one of several factors: the number of plies, the flute type, the box size and the number of printed colors also move the price. For the full picture, see What determines the price of a corrugated box.

Is the same grade from different mills the same quality?
Quality can differ between mills. A grade code indicates the pulp type and broad properties, while actual values such as burst strength depend on the paper mill that made the roll and the lot supplied. Treat the letters on a quotation as a general guide.

Which prints better, WS or KI?
WS for color accuracy, KI for a natural look. The white surface of WS keeps printed colors true, which gives sharper print and more accurate color control, so Pantone-matched work belongs on WS. KI has a light brown surface that tints every printed color with the tone of the paper. For packaging meant to show a natural, brown-board look, that tint suits the design, and KI costs less.