When specifying corrugated board, “standard cardboard” is not a specification. Engineers specify corrugated board using three variables: Wall Types, Flute Profiles, and Strength Grades (ECT/Mullen).
This guide defines the standard grades and how to read a corrugated specification.
1. Wall Construction Types
Describes how many layers of medium/liner exist.
- Single Face: One medium glued to one liner. (Flexible, used for cushioning/wrapping).
- Single Wall (Double Face): One medium between two liners. This is the standard “box” material. (Fibre Box Association)
- Double Wall: Two mediums, three liners. Used for heavy industrial shipping and stacking.
- Triple Wall: Three mediums, four liners. Used for gaylord bins / bulk bins.
2. Flute Profiles (The Geometry)
Flute letters (A, B, C, E) do not go in size order. They are historical.
| Flute Profile | Thickness (Approx) | Flutes per Foot | Primary Engineering Use |
|---|---|---|---|
| A-Flute | ~1/4” (4.8mm) | 33 +/- 3 | Best Cushioning / Good Stacking (Rarely used alone now). (Fibre Box Association) |
| B-Flute | ~1/8” (3.2mm) | 47 +/- 3 | Crush Resistance / Good Print Surface / Die-cutting / Canned Goods. |
| C-Flute | ~5/32” (4.0mm) | 39 +/- 3 | The Standard. Good balance of stacking and cushioning. ~80% of US boxes. |
| E-Flute | ~1/16” (1.6mm) | 90 +/- 4 | High Print / Retail / Pizza Boxes / Small Mailers. (Fibre Box Association) |
| F-Flute | ~1/32” (0.8mm) | 128 +/- 4 | Substitution for folding carton (Microflute). |
Common Combinations (Double Wall):
- BC-Flute: ~7mm thick. Very common industrial standard.
- EB-Flute: Good surface smoothness plus strength.
3. Strength Specifications: Mullen vs. ECT
You must choose one testing standard. They measure different things.
1. Mullen (Burst) Test
- Measures: The force required to punch through the face of the board (psi).
- Relevant for: Containment. If you ship loose parts, sand, or heavy objects that might poke through the wall.
- Designation Example: “200# Single Wall” (Min burst strength 200 psi).
2. ECT (Edge Crush Test)
- Measures: The compressive strength of the board edge-to-edge (lbs/in).
- Relevant for: Stacking. If you ship boxes on pallets. ECT is the input for McKee Formula compression strength calculations.
- Designation Example: “32 ECT Single Wall” (Min edge crush 32 lbs/in).
Why ECT took over: ECT correlates directly to box stacking strength (BCT). Modern paper mills optimize for stiffness (ECT) using less fiber than they need for Burst. 32 ECT is generally lighter/cheaper than 200# Burst, despite being a functional equivalent for stacking.
4. Standard Grade Equivalencies (Approximate)
Warning: These are market equivalencies, not strict technical converts. A 32 ECT box might have a Burst of 170# or 210# depending on the paper recipe. (Fibre Box Association)
| Single Wall Standard | Burst (Mullen) | ECT (Minimum) | Max Gross Weight (Rule 41) |
|---|---|---|---|
| Light Duty | 125# | 23 ECT | 20 lbs |
| Standard | 150# | 26 ECT | 30 lbs |
| Standard Plus | 175# | 29 ECT | 40 lbs |
| Industrial Std | 200# | 32 ECT | 65 lbs |
| Heavy Duty | 275# | 44 ECT | 95 lbs |
| Double Wall Standard | Burst (Mullen) | ECT (Minimum) | Max Gross Weight (Rule 41) |
|---|---|---|---|
| Standard Ind | 200# | 42 ECT | 80 lbs |
| Heavy Ind | 275# | 48 ECT | 100 lbs |
| Heavy | 350# | 51 ECT | 120 lbs |
5. Paper Weights (Basis Weight)
Specification insiders talk about “Basis Weight” (lbs per 1000 sq ft, or MSF). Traditional Standard Recipe (The classic reference for 32 ECT C-Flute): 35-23-35
- 35# Liner (Outside)
- 23# Medium (Flute)
- 35# Liner (Inside)
Higher ECT grades use heavier paper recipes (e.g., 42-23-42). Note that modern high-performance mills may achieve 32 ECT with lighter liner combinations, so always verify the specific recipe with your supplier.
A) Glossary (short)
- Corrugated fiberboard: structure formed by bonding one or more sheets of fluted medium to one or more flat facings of linerboard. (Fibre Box Association)
- Wall Construction: number of layers (single face, single wall, double wall, triple wall).
- Flute Profile: geometry of the corrugation (A, B, C, E, F) defined by height and flutes per foot. (Fibre Box Association)
- Mullen (Burst) Test: test measuring force required to rupture the board face; relates to containment and handling. (Fibre Box Association)
- ECT (Edge Crush Test): test measuring edgewise compressive resistance; relates to stacking strength. (Fibre Box Association)
- Basis Weight: weight of paperboard generally expressed in lbs/1000 sq ft (MSF). (Fibre Box Association)
Citations included from Fibre Box Association as noted in text.