Think about a carton box packed with goods. It travels in a truck for two days, gets stacked in a godown and then someone drops it while unloading. If the box splits open, who gets the blame? Usually the manufacturer. And most of the time, the problem could have been caught in the lab, much earlier.
That is where a bursting strength tester comes in. It’s a simple machine, but it tells you something very important: how much pressure a material can handle before it gives up.
What Is Bursting Strength?
Bursting strength is the maximum pressure a material can take before it bursts or tears. The pressure is applied evenly, from one side, pushing straight against the surface.
It is used for paper, cardboard, corrugated sheets, fabrics, nonwovens, leather and even plastic films. Basically anything flat and flexible.
Why not just pull the material and check tensile strength? Because in real life, things rarely get pulled in only one direction. A box wall gets pushed from inside by the goods. A fabric gets stretched at the elbow or knee, in every direction at once. Burst testing is closer to that real situation.
Bursting strength tester working principle
The working is easy to follow once you picture it. Most labs use the Mullen type, which is hydraulic.
You place the sample on top of a round rubber diaphragm.
A clamp comes down and holds the sample tight around the edges.
A motor pushes liquid (usually glycerin) under the diaphragm, at a fixed speed.
The diaphragm swells up like a small balloon and pushes the sample with it.
Pressure keeps rising. At one point, the sample bursts.
The machine records the highest pressure just before the burst. That number is the bursting strength.
Two small things people ignore. One, the clamping should be even, otherwise the sample slips and your reading is useless. Two, the rubber diaphragm itself has some resistance, so good machines and good test methods adjust for that.
Bursting strength tester diagram: parts you should know
If you look at a diagram of the machine, you will see these parts from top to bottom:-
Upper clamp ring, which presses the sample down
Sample (paper, board, fabric)
Rubber diaphragm, which carries the pressure to the sample
Lower clamp ring and base plate
Liquid chamber, filled with glycerin or oil
Plunger and motor, which push the liquid at constant speed
Pressure gauge or digital sensor, which shows the peak reading
Bursting strength unit and formula
The unit mostly used is kPa (kilopascal). Older reports may show psi or kgf/cm², so don’t get confused if you see them. For converting, 1 psi is about 6.8948 kPa.
Now about the formula. Honestly, there is no big calculation for the burst value itself, because the machine gives you the reading directly. The formula comes in when you want to compare two materials of different weight. For that, we use burst index:
Burst index = burst strength (kPa) ÷ grammage (g/m²)
Let’s say a paper bursts at 280 kPa and its grammage is 100 g/m². Burst index is 2.8. Now a heavier paper may show a higher burst number, but if its index is lower, it is simply heavier, not really stronger. The index shows you that.
Which standard to follow?
The method changes with the material. Standards exist so that your result in Faridabad matches the result in Germany or the US. Here is a quick look:-
One thing to remember here. ASTM D774 itself says it is not meant for corrugated board or linerboard, because these can cut the thin diaphragm. For these, you need a high pressure tester. So low pressure and high pressure machines are different and you should buy the one that fits your product.
Where is it used?
Pretty much anywhere strength of a flat material matters:-
Corrugated box and packaging units
Paper mills
Textile and garment makers
Leather and plastic film producers
Quality labs and research institutes
Even if you are a trader or buyer, not a manufacturer, you can use burst test results to check what your supplier is sending you.
Manual, digital or automatic?
Manual machines are cheaper and they work fine. But the result depends a lot on the operator. Different person, slightly different reading. Digital and automatic testers fix most of that. Pressure is applied at the same speed every time and the peak value is captured by the sensor and many models can store or print the results. If your lab tests many samples daily, automatic is worth the money. For small labs with limited budget, manual or semi digital is okay.
How to pick a good bursting strength tester manufacturer
This part matters more than people think. A machine will run in your lab for 10 years or more, so the manufacturer you choose also becomes your long term partner. Some things to check:-
Does the machine follow the standard you actually test to (ISO, TAPPI or ASTM)?
Does the pressure range cover both your weakest and strongest material?
Do they give a calibration certificate?
Are diaphragms and other spare parts easily available? You will need them again and again.
What about service? Who comes if the machine has a problem?
In India, you have good local options now. Labzenix Industries in Faridabad makes lab testing instruments, including bursting strength testers. Buying from a nearby manufacturer usually means quicker delivery and easier service, without import headache.
Small care tips that save big trouble
Most wrong readings come from small things we forget to check. The rubber diaphragm gets old and starts stretching differently, so change it on time. Look for air in the liquid too, it throws the pressure off. Keep the clamp rings clean, a little dust is enough to slip the sample. And get the gauge calibrated regularly, then test a known sample sometimes.
Final words
A bursting strength tester looks like a small thing in the lab, but it saves a lot of money. One bad batch going out or one shipment coming back, costs way more than the machine. So understand how the test works and what the units mean and which standard your material follows. Once that is clear, picking the right tester is not difficult and you can trust what it shows you.
