A detailed guide to corrugated cardboard grades

If your business uses large quantities of packaging, you are likely familiar with corrugated cardboard. Even if you don’t, it’s nearly impossible to go through life without encountering it in day-to-day life.

However, do you know what makes corrugated cardboard strong or expensive? What materials are used to make it? And how do these factors affect your packaging’s performance?

In this guide, we’ll cover everything from flute profiles and paper types to wall structures and GSM (grams per square metre). By the end, you’ll have a clear understanding of how to choose the best cardboard for your packaging needs.

1. How is corrugated material made?
2. Types of paper used in corrugated cardboard
3. Paper weights & GSM
4. Wall types
5. Corrugated flutes
6. Board grade naming
7. Applications
8. Weights
9. Terminology
10. Summary

Before discussing the technical aspects, let’s briefly explore how a corrugated cardboard is formed.

A standard sheet of corrugated cardboard is made up of three components:

• Fluted material: The wavy, corrugated section in the middle.
• Outer linger: A flat sheet of paper on the outside.
• Inner liner: A flat sheet of paper on the inside.

The fluted material is glued to both liners during production, providing strength and rigidity.

The type of paper used significantly affects the strength, printability, and cost of corrugated material. The two main types of paper used for liners are:

• Kraft paper: Made from softwood trees, Kraft paper offers the most substantial and print-friendly surface, making it ideal for outer liners.
• Test paper: A recycling, double-layered option. Test paper is more affordable but not as strong or easy to print on, and is often used for inner layers.

Other liner options:

• Chip (C): Waste-based liners.
• Fully Bleached White (BW): High-quality white Kraft liner.
• White Top (WT): White-coated recycled liner.
• Mottled Kraft (MK): Mottled white Kraft.
• Oyster (OY): Mottled test liner.
• Semi Chem (SC): Made using a semi-chemical process with virgin fibres.
• Waste Based (WB): 100% recycled fibres, typically used for fluting.

The weight of paper (measured in GSM) influences the strength and application of the corrugated material. Typically, ranges of GSM weights for packaging include:

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

A higher GSM usually means greater durability and weight capacity.

Corrugated boards can vary in wall construction.

• Single wall: One layer of fluting between two liners, suitable for lighter packaging needs.
• Double wall: Adds a second flute and liner for increased strength and rigidity.
• Triple wall: The strongest option for heavy-duty applications like industrial or automotive parts.

Flutes are the wavy middle layer of corrugated cardboard, contributing to strength, cushioning, and print quality. Common flute types include:

• A flute: 5mm, high stacking strength.
• B flute: 3mm, versatile for die-cutting and regular packing.
• C flute: 4mm, good compression strength.
• E flute: 1.5mm, ideal for printing and retail packing.
• F flute: 1mm, ideal for small, detailed print applications.
• BC & EB flutes: Double wall, combining benefits of different flute types.

Corrugated board grades are often abbreviated for simplicity. For example:

• 125K/B/125T: 125GSM Kraft outer liner, B flute, 125 GSM Test inner liner.
• 150T/E/150T: 150 GSM Test liner on both sides with an E flute.

These abbreviations help manufacturers and buyers easily understand the material specifications.

Each board grade and flute type is suited to specific packaging needs. For example:

• E flute: Great for lightweight retail cartons.
• B flute: A versatile choice for most standard packaging.
• C flute: Best for improved stacking strength.
• BC & EB flutes: Ideal for heavy-duty shipping boxes.

There are typical performance guidelines for the varying corrugated board grades that are often quoted. These can be seen below.

Understanding industry terms can help in selecting the correct board grade:

• Across flute: Measurement across the width of the board.
• Backing Liner: Adds strength and improves the finish.
• Blank: Pre-cut corrugated board ready for assembly.
• Burst Damage: When a box fails under too much damage.
• Edge Crush Test (ECT): Measures vertical strength of the board.
• FEFCO Codes: Standardised design patterns for corrugated boxes.
• Grammage (GSM): The weight of the paper per square metre.
• Kraft Paper: Strong, natural brown paper made from virgin fibres.
• Single/Double/Triple Wall: Refers to the number of fluting layers.
• Virgin Material: Newly manufactured material, not recycled.

Understanding corrugated board grades can help you select the best material for your packaging needs. Whether you prioritise strength, printability, or cost, knowing the components and terminology of corrugated materials will empower you to make smarter decisions.

For tailored advice or to ensure your packaging is as cost-effective as possible, don’t hesitate to contact a design engineer at Suttons Performance Packaging today.