Corrugated
Board

Types of paper

A key factor in defining the properties of the corrugated material are the types of paper used. Despite cardboard's widespread use, there are 2 main types of paper that are generally used for the liners. These are known as Kraft liners and Test liners.

Kraft paper is manufactured from softwood trees and due to the “virgin” fibres it is both the strongest type of paper and also the easiest to print on. As a result, it is the most commonly used outside liner when selecting material to produce corrugated boxes and packaging.

Test paper is actually a double layered (or duplex) paper. Having been recycled, Test paper is not as strong as Kraft or as easy to print on however it is less costly than Kraft paper so is commonly used for the inside liner.

Each liner type is actually comprised of two distinct layers. A basic layer is used for adhesion and strength, whilst a finer “cover” layer is used or improved aesthetics and printing.
Despite Kraft and Test liners being the most commonly used there are a number of other options.

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 fibres

The last two listed above are most commonly used for flutes rather than liners.
Please note this list is by no means exhaustive.

Weights and GSM

It is also important to consider the weights of paper. This is measured in the same way for all papers using Grams per square Metre (GSM) so you may have seen 80 gsm on the copier paper you use in your office or home printer.

Common paperweights for corrugated material, include the following:

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

Wall Types

Besides the varying paper weights and types, the type (and number) of fluting must also be considered. At a basic level, a corrugated board is made up of the two liners, plus the single fluting. This is known as a “single wall” board.

However, an additional fluting section – and additional liner – can be added to make what is known as “double wall” material as shown in the image below. This effectively adds extra rigidity and strength, making it suitable for packaging heavier items and adding additional protection for items that may be more delicate or expensive.

If the items you are shipping are particularly heavy or large, then a “triple walled” or “tri-wall” grade can also be used. As the name suggests, this adds another section of fluting and additional liner.

Flutes

The final aspect of corrugated board that can affect its performance, weight and cost; is the flute type. The flute is the “corrugated” section between the two liners, and by changing its height and therefore the size of the flutes, it is possible to achieve different performance characteristics. This can vary from very fine flutes to manufacture lightweight retail cartons, through to a coarser flute more usually used for transit packaging.

It is possible to combine one or more flutes to create a grade such as an “EB” or “BC” flute. This can offer a rigid structure perfect for heavier items, whilst also allowing an improved aesthetic outer appearance and printing surface. The flute itself is usually manufactured from a waste based fluting (WBF – i.e., fully recycled material) or what is known as semi-chem fluting (SC).

Typical paper weights used for fluting are as follows:

90 GSM WBF
105 GSM WBF – (the most common flute standard)
112 SC and WBF
150 SC and WBF
175SC and F

There are also a number of commonly used flute profiles or sizes which are as follows:
A FLUTE: 5mm
B FLUTE: 3mm
C FLUTE: 4mm
E FLUTE: 1.5mm
F FLUTE: 2mm
BC FLUTE: Double Wall (7mm -Combination of B + C flutes)
EB FLUTE: Double Wall (4.5mm -Combination of E + B flutes)

Again this list is not exhaustive and is complicated by the fact that many producers of corrugated material make very similar products which are given arbitrary names exclusive to that manufacturer.

E Flute

E flute is approximately 1mm to 1.5mm in thickness, providing a very fine flute. This not only gives it excellent compression strength and crush resistance, but also means it provides a high quality surface for printing. As a result, it is commonly used smaller cartons, where good quality print is required, plus die cutting applications.

B Flute

B flute is amongst the most commonly used types of cardboard used for packaging applications. Being 3mm in thickness, it is exceptionally versatile and can be used in both die cutting and regular case-making processes. It gives good all-round performance in all types of packaging and typically has a total of 47 flutes per linear foot and a thickness of 1/8th".

C Flute

C flute, between 3.5mm and 4mm in thickness, offers greater compression strength than B Flute. This means it provides better stacking strength when used for lightweight products however, it can, if used in the wrong application, be prone to crushing. C-flute has 39 flutes per linear foot and a thickness of 3/16ths".

BC FLute

BC Flute. Being 6mm to 7mm in thickness, this double wall material combines both B and C flutes together. This gives a good all round performance, making it common for producing shipping boxes that offer a higher level of transit protection.

EB Flute

Around 4mm to 4.5mm thick, EB flute combines E and B flutes into a double walled material. Utilising both the very fine E and relatively large B flute, this grade provides an excellent balance between transit protection, strength and print finish. Due to this, only basic print (such as flexographic) tends to be added.

Flute Weights

As it is possible to vary a number of parameters when specifying the board type for packaging – such as paper weights, flute types and number of walls, it is actually possible to achieve similar performance with different board grades. There are typical performance guidelines for the varying corrugated board grades that are often quoted as in the tables below.

Technical Terms

Across Flute – a unit of measurement which is used to measure corrugated (or other fluted material such as Correx®), the measurement is the opposite direction of the flute (so width). Backing Liner – A compressible paper material which, compensates for any irregularities on the surface which is sealed. This compressible material (usually pulp or news board) is usually adhered to the liner and provides a better finish (appearance), water resistance and extra 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 three elements; firstly, the weight and type of outer liner, secondly the type of flute and thirdly the weight and type of inner liner.
Burst Damage – the term given to packaging containers which split or “burst” due to too much pressure (e.g. if stacked to high, the boxes lower in the stack may burst).
Carton Board – Stiffer and thicker than boxboard. It has 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 coated onto corrugated board in order to improve its printing surface. Used on unbleached or natural Kraft paperboard.
Corrugated – A material which is shaped into a series of parallel ridges and grooves.
Crush – An edge crush test measure the cross direction crushing of corrugated board. It provides information on a board’s resistance to being crushed.
Deckle – The width of the board being run on a corrugator.
Double Wall – A combination of two layers of corrugated material (potentially different flute sizes) to provide a material with extra strength.
Duplex – A type of paperboard made up of two layers, the exterior is often coated which makes it more water-resistant, often used for paper cups and plates and in the pharmaceutical industry
Edge CrushTest – A corrugated board strength test of vertical crush resistance
Enhanced Fluting – A substitute to standard fluting that adds strength and performance to the material
FEFCO – The European federation of corrugated board manufacturers are a non-profit organisation which represents the interests of the corrugated industry.
FEFCO Case Codes – A set of standard design patterns used within the corrugated industry. Fibre – packaging materials are made from moulded fibres, otherwise called moulded pulp.
Flute – Paper that provides the central layer in corrugated board (cardboard). It separates the liners and provides the strength and rigidity. Please see appendix 1 further details of the most common flutes.
Fluting profile – The shape of the corrugations within the corrugated material
Grammage – Also referred to as GSM, the weight of paper specified as g/m2 (grams per square metre)
GSM – Acronym for the measurement of grams per square meter.
Kraft –brown paper or paperboard which has been produced from virgin pulp in the pulping process. It is made out of natural unbleached wood fibres.
Light-weighting – The design of packages which reduce material use, reducing weight and cost but also, reducing environmental burden.
Liner – One of the paper materials which, make up one part of the components in corrugated board. There is an inner and outer liner; the outer is usually of higher quality, due to being used for print finish.
Mottled – A paper liner with an off white appearance
Single Face – One piece of fluting glued to one liner only
Single Wall – or double faced, single walled board consists of one part fluted paper (in the middle) and then two materials (usually paperboard) which is bonded on either side of the fluted paper to give it strength.
Slit Edge – The edge of the sheet that has the flute running parallel with it. It is also known as the width of the material
Test Liner – recycled liner board which can be manufactured as a sheet where the fibres are similar throughout. Known as Homogeneous/Simplex or, as a combination of two layers with the outer layer being better quality recycled fibre, this is known as Duplex or Multi-Ply.
Tri Wall – is triple fluted corrugated board which is used for its strength, flexibility and eco- friendliness.
Virgin Material – A material which has not been processed in any form other than its original manufacture.