Corrugated Board Grades Guide

Published date: 18 May 2021

Corrugated board is one of the most popular materials for packaging boxes. It’s sturdy yet easy to cut, bend, fold or drill. Corrugated boards come in a range of strengths, thicknesses, materials, and performance capacities and serve different purposes depending on what you buy. This guide will teach you everything you need to know about corrugated board to ensure you find the correct cardboard for the job, whatever that may be.

Material composition: What is corrugated board?

Corrugated board is a type of cardboard with a distinct composition. Each sheet of corrugated cardboard has three components: one sheet of corrugated (i.e. ‘ridged’ or ‘crimped’) material glued between two layers of paper, also known as the outer and inner liners. This inner corrugated layer adds rigidity, strength and stability.

Single Walled Corrugate Board Explained

Paper Types: What are the components of corrugated board made of?

Different paper types can be used to form the layers of corrugated board. The paper type used has an impact on the board’s performance.

All three outer, inner and corrugated liners are most commonly made of either Kraft or Test paper.

Kraft Paper

Kraft paper is made from softwood trees. As it’s made from virgin material, it is more robust and easier to print on, making it the most popular option.

Test Paper

Test paper is a double (or duplex) layer of paper made from recycled material, and it’s more commonly used for the inside liner.

Other Paper Options and Grades

While Kraft and Test paper are the most popular liner materials, there are a multitude of paper options available, each with a different grade to recognise them by:

• Kraft (K): Virgin Kraft paper • Test 2 (T2): Partly recycled liner paper • Test 3 (T): Fully recycled liner • Chip (C): Waste based liners • Fully Bleached White (BW): Fully bleached Kraft liner • White Top (WT): White coated recycled liner• Mottled Kraft (MK): Mottled White Kraft• Oyster (OY): Mottled test liner• Semi Chem (SC): Virgin fibres using neutral sulphite semi-chemical process• Waste Based (WB) 100% recycled fibresThis list is not exhaustive, and there are numerous other specialist papers available.

Paper Weights: How is paper weighed, and what is GSM?

Paper is weighed in GSM: grams per square meter.

Common paper weights used for corrugated board are:

• 115/125 GSM
• 140/150 GSM
• 185/200 GSM
• 300 GSM

Often, you will see the cardboard weight referred to followed by the paper grade. For example, 125gsm Kraft paper is simply referred to as 125K.

Wall Types: How are different corrugated boards constructed?

There are different ways of constructing corrugated board. Multiple layers of liner and corrugation can be added, resulting in additional rigidity and strength.

Single-Walled Board

As mentioned in the first section, a single wall (or flute) of corrugated board is composed of the below:

Double Walled Board

Adding an extra flute — i.e. another corrugated sheet and flat paper liner — to the single-walled board creates a double-wall, adding extra sturdiness, as shown below:

Triple Walled Board

If the items you’re transporting are particularly heavy or large, you can even use a triple-walled (or ‘Tri-Wall’) grade of cardboard, as shown below:

Flutes: What different flute options are there?

The flute is the corrugated, wavy section between the two flat liners.

Flute Materials

Flutes are usually made from either recycled material (also known as waste-based fluting or WBF) or semi-chem fluting (SC).

Flute Weight

Like liner paper, fluting is weighed by GSM. Typical weights for fluting are:
• 90 GSM WBF
• 105 GSM WBF – Most common flute standard
• 112 SC and WBF
• 150 SC and WBF
• 175 SC and F

Flute Sizes

The flute comes in different sizes resulting in different strengths and performance of the cardboard:
• A FLUTE – 5mm
• B FLUTE: 2.5 - 3mm — versatile all-rounder for all types of packaging.
• C FLUTE: 3-4mm — more compressed than a B flute, so better for stacking lightweight products.
• E FLUTE: 1-1.5mm — very fine, perfect for printing and small cartons.
• F FLUTE: 2mm

Flute Size Combinations

With double or triple walled cardboard, you can combine different flute sizes. This is beneficial, for instance, if you require a rigid structure for a heavy item but with an outer layer that is easy to print on.

Common flute size combinations are:
• BC FLUTE: Double wall of 5.5-7mm in total combining B and C flutes — good all-round performance for items needing a higher level of transit protection with only basic print.
• EB FLUTE: Double Wall of 4-4.5mm in total combining E and B flutes — utilising a fine and large flute offers transit protection in combination with a good surface for print.

Diagram of Corrugated Board Flute Size Combinations

Board Grade Names: How is corrugated board named and graded based upon the above?

Corrugated board is named based upon all the information in the guide above. The format used is as follows:

Outer Liner Weight and Material / Fluting Grade / Inner Liner Weight and Material

For example, if your outer liner is 125gsm Kraft paper, your flute is grade B, and your inner liner is 125gsm Test paper, your corrugated board overall grade name is:

125K/B/125T

Corrugated Board Grade Recommendations: What corrugated board grade should you use for your goods?

Ultimately it is down to you to decide what corrugated board you use to transport your goods, but the table below can help as a general guide:

Table of Corrugated Board Grade Recommendations

Corrugated Board Glossary

Below is a glossary of common terms to help you understand corrugated board grades and find the suitable cardboard for your needs.

Across Flute - A unit of measurement to measure corrugated boards. This is the measurement of the width of the flute.

Backing Liner - A compressible material adhered to the liner to improve finish, water-resistance and strength.

Blank - A flat piece of corrugated board that has been cut and scored, ready to make a box.

Board grade - A grade given to corrugated board based on the weight and type of outer liner, flute type, and weight and type of inner liner.

Carton Board - Board with a medium to high compression and moisture resistance. Unlike cardboard, it is solid, and not fluted.

Chop Edge, Chops - The length of the board/sheet.

Clay Coat - A thin layer of kaolin (soft white clay) coated onto corrugated board to improve printing.

Corrugated - A material which is shaped into a series of parallel ridges and grooves.

Crush - A measurement of a board’s resistance to being crushed.

Deckle - The width of the board being corrugated.

Double Wall Board - Two layers of corrugated material which are combined to give the overall cardboard extra strength.

Duplex - A double-walled board with a coated exterior to make it more water-resistant.

Edge Crush Test - A corrugated board strength test of vertical crush resistance.

Enhanced Fluting – Fluting that adds strength and performance to the material.

FEFCO – The European Federation of Corrugated Board Manufacturers are a non-profit organisation representing the interests of the corrugated industry.

FEFCO Case Codes – Standard design patterns used within the corrugated industry.

Fibre, Moulded Pulp – The substance packaging materials are made from.

Flute – The central layer in corrugated board separating the liners and providing strength and rigidity.

Fluting Profile – The shape of the flute corrugations.

Grammage (GSM) – The weight of paper in terms of grams per square metre.

Kraft – Paper produced from virgin pulp made out of natural unbleached wood fibres.

Light-weighting – The design of packages to reduce material use, weight, cost and environmental burden.

Liner – The components in corrugated board which surround the flute.

Mottled – A paper liner with an off-white appearance.

Single Face – A piece of fluting glued to only one liner.

Single Wall – Corrugated board consisting of one fluted paper sandwiched by two liners.

Slit Edge – The width of the material.

Test Liner – Recycled liner board.

Tri-Wall – Corrugated board with three flutes sandwiched between liners.

Virgin Material – A previously unused and unrecycled material.

Shop Challenge Packaging’s selection of corrugated board boxes and packaging solutions today.