What is a Mullen test?

In today's booming global trade and e-commerce, products travel a long and complex logistical journey from production line to consumer hands. During this journey, packaging serves as the first and most important line of defense for a product. Its strength and reliability are directly linked to the value of the product and the reputation of the merchant.

Among numerous packaging testing methods, the Mullen test, also known as the bursting test, has become a cornerstone of quality control in the packaging industry, particularly for paper-based packaging materials, due to its intuitiveness, efficiency, and high degree of simulation.

What is a Mullen test?

The Mullen test, officially known as the bursting test, is a standard method for measuring the bursting strength of materials such as paper, paperboard, and corrugated boxes. Named after its inventor, John Mullen, the test's core purpose is to assess a packaging material's maximum resistance to rupture when subjected to uniformly increasing multi-directional pressure perpendicular to its surface.

Simply put, it answers the question that most concerns packaging engineers and logistics managers: "How much force can this carton withstand before it breaks under external compression and impact?"

Why is the Mullen test so important?

A common misconception is that packaging is strong simply because it's thick enough. However, the reality is much more complex. The challenges faced by packaging during storage and transportation aren't limited to a single direction:

Stacking pressure: In warehouses, cartons are stacked layer upon layer, subjecting the bottom cartons to significant static pressure.

Rough handling: During sorting and loading and unloading, cartons may be thrown, stepped on, or collide with other hard objects.

Transport vibration: On trucks, ships, or trains, constant vibration can cause fatigue stress on the box structure.

The Mullen test simulates this type of concentrated impact and pressure from all directions. It ensures that the packaging material is not only inherently strong, but also that its composite structure (such as the multi-layer construction of corrugated cardboard) effectively distributes localized pressure, thereby protecting the product inside.

A carton that passes a specific Mullen strength standard largely guarantees reliable product protection, reducing returns, claims, and brand damage caused by packaging damage.

Core Law of Physics: Pascal's Law

The design of the Mullen tester perfectly implements Pascal's Law. This law states that "a pressure applied to a closed fluid is transmitted by the fluid in all directions in the same proportion to its original magnitude." This means that in a closed hydraulic system, a pressure change applied at one point is transmitted to every part of the system without loss.

Precision Construction of the Tester

A standard Mullen burst tester consists of the following key components:

Upper and Lower Platens: These firmly clamp the test specimen, creating a sealed test area.

Rubber Diaphragm: A flexible membrane located below the lower platen, filled with a pressure-transmitting medium.

Pressure-transmitting medium: Methyl silicone oil is typically used because of its chemical stability and excellent viscosity-temperature characteristics, ensuring precise and smooth pressure transmission.

Piston and Motor: The motor drives the piston forward at a constant speed, compressing the silicone oil, thereby applying pressure to the rubber diaphragm.

Pressure sensor and processing chip: Monitors and records pressure values ​​in real time, locking the maximum value at the moment the sample ruptures.

LCD display: Intuitively displays test results.

Why "One Test, One Size Fits All" is not a universally applicable method

The Mullen test is not a one-size-fits-all approach. Organizations such as the International Organization for Standardization (ISO), the American Society for Testing and Materials (ASTM), and the Technical Association of the Pulp and Paper Industry (TAPPI) have developed detailed standards for materials of varying material quality and intended use.

Testing Differences Between Corrugated Cardboard and Regular Paper

Although the basic principles are the same, there are significant differences in the standards and parameters used for testing corrugated cardboard and regular paper:

Testing Regular Paper

Applicable Standards: Typically follows ISO 2758 or TAPPI T403.

Sample Characteristics: Thin materials, typically less than 1 mm thick, such as printer paper, kraft paper, and writing paper.

Test Pressure Range: Relatively low, typically within the 100–500 kPa range.

Corrugated Box Testing

Applicable Standards: Specifically adhere to ISO 2759 or TAPPI T810. These standards take into account the multi-layered structure of corrugated board, ensuring that pressure is effectively transferred through the liner to the corrugated core during testing.

Sample Characteristics: The material is relatively thick, consisting of multiple layers (face paper, corrugated core, and liner), typically greater than 3 mm thick.

Test Pressure Range: Due to the stronger material, higher pressures are required, typically ranging from 500 to 1200 kPa or even higher.

Choosing the right standard is crucial. Using the wrong standard or clamping pressure can significantly deviate from the true value, leading to erroneous quality judgments.

The Real-World Significance of Burst Strength

How does a value in the laboratory translate into real-world packaging reliability? This is where the Mullen test truly shines.

Quantifying the "Protection Power" of Packaging

Mullen test results provide a scientific, quantitative basis for packaging selection. For example:

The burst strength requirement for a single-wall corrugated box may be 12–14 kg/cm² (approximately 170–200 psi).

The burst strength requirement for a double-wall or heavy-duty corrugated box may be 20 kg/cm² (approximately 285 psi) or higher.

These values ​​are not set arbitrarily; they are calculated based on factors such as product weight, number of stacking layers, expected transportation distance, and the complexity of the logistics environment. Heavy, high-value products (such as electrical appliances or high-end cosmetics) naturally require packaging with higher burst strength.

A Critical Role in the Logistics Chain

In the real world of logistics, cartons undergo a battle. They must withstand:

Static stacking pressure: When cartons are stacked in a warehouse, the bottom box constantly bears the weight of all the boxes above it.

Dynamic shock: In sorting centers, automated sorting systems or manual handling can cause collisions between cartons.

Transport vibration: Long-term road or rail transportation generates continuous vibration that can cause structural fatigue.

Climate Challenges: Humidity fluctuations can affect the strength of cardboard, so burst tests are conducted under controlled temperature and humidity conditions to eliminate environmental interference.

A carton with insufficient burst strength can break at any stage, leading to damage or loss of the product inside, resulting in direct financial losses and customer complaints. Conversely, a carton that meets the required strength can effectively mitigate these risks, ensuring that products arrive at their destination unscathed. This is an essential component of supply chain management for industries such as fast-moving consumer goods, electronics, pharmaceuticals, and precision instruments.

Wide Applications

Although the Mullen test is most commonly used for paper-based packaging materials, its applications extend far beyond this. Any sheet material that needs to be evaluated for integrity under multi-directional pressure can benefit from similar burst tests.

Textiles and Fabrics: Used to test the tear and burst resistance of tents, canvas, industrial fabrics, and other products.

Packaging Films: Evaluate the strength of plastic films (such as PE and PP) when subjected to internal inflation or external compression.

Leather and synthetic materials: Used in products such as footwear and luggage, ensuring the surface material's durability.

Nonwovens: Used in applications such as medical protective clothing and filter media, where integrity is paramount.

In these applications, the test principle is similar to the Mullen test, but the instrument's clamping method, diaphragm type, and pressure range may be adjusted based on the material's properties.

FAQ

1. Can the Mullen test be used to test metals or rigid plastics?

A: Generally not. The Mullen tester's design and range are designed for materials with specific deformation characteristics, such as cardboard. Metals and high-strength plastics typically require other types of testing, such as tensile testing or impact testing, to assess their mechanical properties.

2. What is the difference between burst resistance and edge crush strength (ECT)?

A: These are two crucial and complementary concepts.

Burst resistance measures the resistance of a cardboard surface to rupture and is closely related to the quality of the pulp fiber.

Edge crush strength measures the resistance of a corrugated cardboard vertical edge to collapse and is more dependent on the stability of the corrugated structure. In modern packaging design, ECT is often more relevant for predicting the stacking strength of cartons, but burst resistance remains an important indicator for assessing the overall quality of the material itself.

3. How often should we conduct the Mullen test?

A: We recommend that each batch of newly arrived packaging materials be inspected upon arrival. For products with high production volumes and high quality requirements, spot checks may even be necessary during each shift or daily. Establishing a continuous quality monitoring system is the most effective way to prevent batch packaging problems.

4. What might cause unstable test results?

A: Possible causes include:

Improper sample clamping: too loose or too tight.

Aging or damaged rubber diaphragm: Regular calibration and replacement are required.

Bubbles in the pressure medium: This affects the accuracy of pressure transmission.

Sample non-uniformity: This may include uneven moisture content, uneven thickness, or defects.

Non-standard operating environment: Testing was not conducted under standard temperature and humidity conditions.