What Are Corrugated Board Grades?
A detailed guide to corrugated board grades / cardboard types
If you are using high volumes of packaging, chances are you will be familiar with corrugated cardboard.
If you don’t, it is impossible to go about everyday life without being exposed to cardboard at some point.
However, do you know the elements that determine how strong – or how expensive – corrugated cardboard is? What is it manufactured from? And what impact can this have on the success of your packaging?
This guide aims to explain and clarify everything you could need to know about cardboard.
From flutes to gsm, liners to the number of walls, everything is explained.
Composition of material
Before going into the more technical details, it is useful to highlight the way in which corrugated board is formed (if you would like a full overview of how cardboard is made, please click here).
Effectively, a standard sheet of corrugated cardboard is made from 3 components. These are a sheet of corrugated or “fluted” material in the centre, placed between 2 layers of paper – the outer and inner liners.
Each side of the fluted material is glued to the flat paper liner during manufacture. The glue fixes the flutes in position and adds rigidity and stability to the papers.
You can see a basic diagram highlighting the fluting and liners below.
Types of Paper
One of the key factors that define the properties of the corrugated material is the types of paper used.
Despite cardboards’ widespread use, there are 2 main types of paper that are generally used for the liners. These are known as Kraft and Test liners.
Kraft paper is manufactured from softwood trees. 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. Being recycled, Test paper is not as strong as Kraft, or easy to print on, so is commonly used for the inside liner. It is, however, less costly than Kraft paper.
Each liner type is actually comprised of two distinct layers. A basic layer is used adhesion and strength, whilst a finer “cover” layer is used mainly for improved aesthetics and to aid printing.
However, despite Kraft and Test liners being so popular, there are in fact a number of other options.
The available paper grades for outer and inner liners when manufacturing cardboard are as follows:
- 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 final 2 paper types listed here are those most commonly used for flutes, and less often used for liners.
Please note however this list is not exhaustive, and there are a number of other specialist papers available.
Paper Weights & GSM
Once you understand the types of paper available and commonly used, it is also important to consider the weights of paper.
This is in fact measured in the same way for all papers, meaning you may have seen “80 gsm” on the copier paper you use in your office or home printer.
In essence, if you took a square meter of the paper in question, and weighed it, you would get the grams (i.e. weight) of that paper for one square meter.
Therefore, if you are using a corrugated board that has a 125 gsm Kraft paper liner, you will often see it referred to as 125K.
Common paperweights – when used for corrugated material at least, include the following:
- 115/125 GSM
- 140/150 GSM
- 185/200 GSM
- 300 GSM
Again, this list is not extensive, and other weights may be available.
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. You can see a diagram indicating this below.
However, an additional fluting section – and additional liner, can be added to make what is known as “double wall” or “double walled” material.
This effectively adds extra rigidity and strength, making it suitable for packaging heavier items and adding additional protection for items that may more delicate or expensive.
You can see an example below.
Finally, if the items you are shipping are particularly heavy or large (for example automotive parts, industrial equipment etc.) then a “triple walled” grade can also be used. As the name suggests, this adds another section of fluting and additional liner.
Also referred to as Tri-Wall (which is the brand name of the manufacturers too), you can see a basic diagram of this below.
The final aspect of the corrugated board that can affect its performance (and 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 such as “E” to manufacture lightweight retail cartons (being very good to print onto), through to a more coarse flute such as an “A” or “B” flute more usually used for transit packaging.
As in the examples above, with double walled material, 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 – Most Common Flute Standard
- 112 SC and WBF
- 150 SC and WBF
- 175 SC 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 – 6mm -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 (and are exclusive to that manufacturer).
Corrugated board descriptions / explanation
All of this information allows you to create an abbreviation that describes the type of board being used.
For example, if your material grade was using a 125gsm Kraft outer liner, a 125gsm Test inner lining, and a B fluting, this would be written as…
Similarly, a 150 Test outer lining, plus a 150 Test inner lining, with an E flute, would be described as…
This terminology would be easily understood by virtually any packaging manufacturer (or at least should be), and also allows you to fully understand the type of material your cartons are manufactured from.
Individual flute uses / info
Simply knowing how a corrugated board grade is manufactured, and its’ component parts, doesn’t necessarily allow you to make an informed choice on the type of material you should be using for your packaging.
As such, the below information provides a little more detail on each of the most commonly used flute types.
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.
E-Flute has 90 flutes per foot and a thickness of 1/16th
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.
It typically has a total of 47 flutes per linear foot including a thickness of 1/8th”.
Between 3.5mm and 4mm in thickness, C flute 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”.
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.
Due to this, only basic print (such as flexographic) tends to be added.
Around 4mm to 4.5mm thick, EB flute (as the name suggests) combines E and B flutes into a double walled material. Due to utilising both a very fine (E) and relatively large (B) flute, this grade provides an excellent balance between transit protection, strength and print finish.
Flute type weight tables
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 differing board grades.
However, there are typical performance guidelines for the varying corrugated board grades that are often quoted.
These can be seen in the tables below
B or E Flute
200 K/ 300 T
BC or EB Flutes
200 K/ 300 T
Technical Terms Associated with Corrugated Board Grades
As a final point of reference / explanation, below is a common list of terms relating to board grades and corrugated material in general.
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 – Carton Board is 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 (or just 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 board – 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 crush test – 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 – an 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 – in packaging, light-weighting is 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.
Weight – refers to the density of a material, see GSM for further information.
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Understanding corrugated board grades
Hopefully this guide has provided answers to any questions you may have had regarding corrugated board grades.
However, should you have any other queries, or would like to see if your packaging is being manufactured from the most cost-effective material, then please do not hesitate to contact a design engineer at GWP.