Bursting Strength Formula for Corrugated Boxes: How to Use & Calculate

In the world of packaging, a corrugated box is the first line of defense for a product. It must withstand the rigors of stacking, handling, and transportation without failing. But how do manufacturers quantify this strength? One of the most critical and universally recognized metrics is Bursting Strength.

This guide will demystify bursting strength, explaining the formula behind it, how to calculate it, and the essential role of the bursting strength tester in ensuring your packaging is up to the task.

Understanding Bursting Strength

Bursting strength measures the maximum pressure a paper sheet or corrugated box can handle before it ruptures. It shows how well the material can resist internal pressure and stress.

Knowing the bursting strength is critical for:

• Corrugated boxes used in shipping and storage.
• Carton boxes for consumer goods.
• Paper sheets in manufacturing and industrial uses.

A higher bursting strength means stronger material that can withstand stacking, handling, and transportation without damage.

The Bursting Strength Formula and Calculation

Unlike tensile strength, which measures resistance to pulling, bursting strength measures resistance to rupture from pressure. The result is expressed in kilograms per square centimeter (kg/cm²) or pounds per square inch (psi).

The beauty of the test is that the calculation is handled automatically by modern bursting strength tester units. However, understanding the principle is key.

The formula is straightforward:

Bursting Strength (kg/cm² or psi) = (Total Force Applied to Rupture the Sample) / (Area of the Diaphragm)

Wait, what diaphragm? This is the key to the test. The pressure isn't applied directly by a mechanical point. Instead, a rubber diaphragm is expanded by hydraulic pressure (glycerin is the common medium) against the sample, which is clamped over a circular opening.

Since the area of the diaphragm applying the pressure is fixed, the machine's pressure gauge can directly display the bursting strength in the desired units. The machine effectively performs the calculation for you.

How to Use a Bursting Strength Calculator

In practice, you don't need a manual calculator. The tester is the calculator. Here’s the process:

1. You set the test parameters (units, sample size).
2. You clamp your paper or corrugated board sample into the machine.
3. You start the test. The hydraulic system increases the pressure on the diaphragm at a controlled rate.
4. The diaphragm bulges against the sample until it bursts.
5. The machine records the maximum pressure reached at the moment of rupture.
6. This final pressure reading , shown on the digital display or analog gauge, is the bursting strength value.

So, if your bursting strength tester displays a value of 14 kg/cm², that is the calculated bursting strength of your sample. No further math is required.

Key Specifications of a Bursting Strength Tester

When selecting or operating a tester, understanding its specifications is crucial for accurate, repeatable results that comply with bursting strength of paper standards.

Feature	Specification	Importance
Capacity	Typically 0-50 kg/cm² or 0-711 psi	Must be suitable for the expected strength of the materials you test.
Diaphragm	Rubber, elastic and frequently calibrated.	The heart of the tester; its consistent expansion is critical for accuracy.
Clamping Pressure	≥ 70 kg/cm² (as per standard)	Prevents sample slippage during testing, which would give erroneous results.
Test Area	50.0 ± 0.5 cm² (7.8 ± 0.1 in²)	The standardized area under pressure. Must be perfectly circular and smooth.
Pressure Increase Rate	95 ± 5 mL/min (for glycerin)	This is critical. The pressure must increase at a steady, standardized rate (e.g., 170 ± 15 kPa/min) to ensure results are comparable.
Gauge Accuracy	± 0.1% of full scale or better	Ensures the reading is precise. Digital gauges often offer higher accuracy.
Compliance	ISO 2758, ISO 2759, TAPPI T810, ASTM D3786, JIS P8112	The tester must meet the specific standards required for your industry and region.

Standard Bursting Strength Tester Procedure

Following a strict procedure is non-negotiable for reliable data. Here is a simplified overview of the bursting strength tester procedure:-

1. Calibration:- Before starting, ensure the machine is calibrated. Check the diaphragm for wear and tear and the glycerin level. Confirm the pressure gauge is zeroed.
2. Sample Preparation:- Cut a representative sample from the corrugated board. The sample must be larger than the clamping area, typically at least 125 mm x 125 mm (5" x 5"), and free of creases, wrinkles, or obvious defects.
3. Mounting:- Place the sample smoothly over the diaphragm opening, ensuring it is perfectly flat and not stretched. Secure it by lowering and locking the clamping mechanism with the specified force.
4. Initiation:- Start the test. The motor will drive the piston, increasing the hydraulic pressure at a controlled rate (as per the standard).
5. Rupture:- The pressure will build until the sample bursts. You will hear a distinct sound.
6. Recording:- The machine will hold and display the maximum pressure value. Record this value as the bursting strength for that sample.
7. Repetition:- Repeat the test on at least 5-10 samples from different parts of the same batch to get a statistically accurate average value.

Bursting Strength of Paper Standard

Globally, several standards govern this test to ensure uniformity:

• ISO 2758: For paper – Determines the bursting strength of paper.
• ISO 2759: For board – Determines the bursting strength of corrugated fiberboard.
• TAPPI T 810: Standard from the Technical Association of the Pulp and Paper Industry (common in North America).
• ASTM D3786:- Standard Test Method for Bursting Strength of Textiles (sometimes referenced, but not primary for paper).
• JIS P 8112:- Japanese Industrial Standard for testing bursting strength.

These standards ensure that a result of 12 kg/cm² from a lab in the USA is directly comparable to a result of 12 kg/cm² from a lab in Germany or Japan.

Frequently Asked Questions (FAQs)

1. Why is the rate of pressure increase so important in the test?

Ans: The rate of pressure increase is standardized to ensure consistency and comparability of results. A faster rate can lead to a higher recorded bursting strength because the material doesn't have time to stretch and distribute the stress. A slower rate might give a lower value. Adhering to the strict rate (e.g., 95 mL/min of glycerin) ensures all tests are performed under identical conditions, making them fair and reproducible.

2. My box has a high bursting strength. Does that automatically mean it's a good box for stacking heavy weights?

Ans: Not necessarily. Bursting strength measures resistance to perpendicular pressure (like squeezing). For stacking strength, which is a box's ability to withstand constant compressive loads over time (like at the bottom of a pallet), the Edge Crush Test (ECT) is a much better predictor. A box can have good burst strength from high-quality liners but poor ECT if the fluting is weak. Always specify the right test for your distribution environment.

3. What is the difference between Mullen Test and ECT?

Ans: This is a fundamental distinction in the US box industry.

• Mullen Test (Bursting Strength): Measures resistance to rupture. Results are in psi (e.g., 200# test). Historically common and good for boxes containing products that might expand or generate internal pressure.
• Edge Crush Test (ECT): Measures the compressive strength of the fluted corrugated board by crushing it on its edge. Results are in lb/in (e.g., 32 ECT). It is a superior predictor of top-to-bottom box compression and stacking strength. Most modern packaging specifications are moving towards ECT values.

4. How often should I calibrate my bursting strength tester?

Ans: How often you should calibrate your bursting strength tester depends on how frequently it is used and the requirements of your quality system:- such as ISO 9001. In busy labs, monthly calibration may be necessary, while for typical use, checking it every three to six months is usually enough. It is also important to do a quick check before each test using a calibrated reference sample or proving ring. This helps ensure your results are accurate and reliable. Always follow the manufacturers instructions for the best performance.

Get Your Bursting Strength Tester at the Best Price!

Looking for accurate and reliable Bursting Strength Testers at the best price? Contact Presto Group today! Call us at 91 9210 903 903 or email info@prestogroup.com to get a custom quote. Ensure top quality testing for your paper, cardboard, or packaging materials with our advanced, durable machines. Don’t wait—request your price now!