Burst Factor, Tear Index, and Porosity: A Paper Testing Guide for Packaging Converters

Base paper quality is the single biggest variable in your reject rate. A converter running a well-maintained die-cutter, flexographic press, or bag-making line can still produce defective output if the incoming paper is inconsistent. The machinery does not compensate for weak paper. It amplifies the problem.

Since India’s single-use plastic bans expanded the paper packaging market, the number of paper suppliers has proliferated. Quality standards across these suppliers are uneven. Relying on supplier-provided test certificates transfers your quality responsibility to someone else’s laboratory, and that is a risk most converters cannot afford.

Three parameters separate usable paper from problematic paper for food packaging converting: burst factor, tear index, and porosity. Testing these before accepting a consignment catches quality problems at the gate rather than on the converting floor, where the cost multiplies.

Key takeaways

• For 40 to 80 GSM kraft paper, minimum burst factor of 25 to 35 kPa·m2/g separates acceptable stock from problematic material. Below this range, bag rupture and container failure rates increase measurably.
• Tear index imbalance between machine direction (MD) and cross direction (CD) above 1.6:1 signals handle-failure risk in bag-making operations. Many suppliers report only combined or CD values, hiding MD deficiencies.
• Porosity (Gurley method, IS 1060 Part 3) directly influences print consistency. Paper below 10 seconds/100 cm3 is a red flag for multi-colour flexographic printing: ink absorbs unevenly, producing fuzzy edges and colour drift.
• A documented pre-purchase testing protocol reduces reel rejection rates by catching quality drift at the gate. The testing equipment investment (INR 2.5 to 4.5 lakh for a complete setup) pays for itself with the first rejected consignment caught before converting.
• Three additional tests — tensile strength, Cobb value, and grammage verification — complete the quality picture for food packaging paper.

Burst factor: the structural integrity test

What it measures

Burst factor quantifies the paper’s resistance to rupture under uniform pressure. The calculation is straightforward: burst factor = bursting strength (kPa) divided by grammage (g/m2). A burst factor of 30 kPa·m2/g means the paper can withstand 30 kPa of hydraulic pressure per unit grammage before rupturing.

Testing follows IS 1060 Part 1 (aligned with ISO 2758) using a Mullen-type hydraulic burst tester. The tester clamps a circular specimen and applies increasing hydraulic pressure against a rubber diaphragm until the paper ruptures. The maximum pressure at rupture is the bursting strength.

Acceptance thresholds by GSM

Paper consistently below the minimum for its GSM range will produce bags and containers that rupture under normal use conditions. Your weakest bag is only as strong as the weakest point in the paper.

Testing protocol

Test a minimum of 10 specimens per reel, drawn from at least 5 positions across the reel width (both sides and centre). Condition specimens at 23 ± 1 degrees C, 50 ± 2% RH for at least 4 hours before testing. Record the mean, standard deviation, and minimum values. The minimum value matters more than the mean because it represents the weakest point in the consignment.

For consignments of 10 or more reels, test at least 20% of reels. If any reel fails the minimum threshold, test all reels in the consignment.

Tear index: the directional strength test

What it measures

Tear index quantifies the paper’s resistance to tearing once a tear has initiated. The calculation: tear index = tearing resistance (mN) divided by grammage (g/m2). Testing follows IS 6490 (aligned with ISO 1974) using an Elmendorf pendulum tester with a pre-cut notch that initiates tear propagation.

Why directionality matters

Paper fibres align predominantly in the machine direction (MD) during manufacturing. This creates different tear resistance in MD and CD (cross direction). CD tear is usually higher because the tear crosses the fibres. MD tear is lower because the tear runs along the fibres.

The MD/CD ratio matters because most bag-making operations orient handles along the machine direction. If MD tear index is too low, handles tear out under load. An MD/CD ratio above 1.6:1 (meaning CD tear is 60% or more higher than MD tear) signals handle-failure risk.

Target values for 60 to 80 GSM kraft

• MD tear index: 5.0 to 7.5 mN·m2/g minimum
• CD tear index: 6.0 to 9.0 mN·m2/g minimum

Many mills report only combined or CD values, which hides MD deficiencies. Always test both directions. Always test from the specific reel you intend to run, not from a generic sample provided by the supplier.

Porosity: the print quality and adhesive performance test

What it measures

Porosity, measured by the Gurley densometer method (IS 1060 Part 3), records the time in seconds for 100 cm3 of air to pass through a fixed paper area under specified pressure. Higher Gurley numbers mean denser (less porous) paper. Lower numbers mean more porous paper.

Why it matters for converters

Print quality. High-porosity paper absorbs ink rapidly and unevenly. On flexographic and gravure lines, this produces fuzzy edges, excessive dot gain, and colour inconsistency between print runs. For branded food packaging where colour accuracy matters, porosity directly affects output quality.

Adhesive performance. Water-based and hot-melt adhesives behave differently on porous vs dense paper. Excessive porosity risks adhesive strike-through (adhesive migrating through the paper to the other side). Insufficient porosity can reduce adhesive bonding strength.

Target ranges

For printed carry bags and branded food packaging, paper below 10 seconds represents a reject condition.

Three additional tests worth running

Tensile strength (IS 1060 Part 2 / ISO 1924)

Measures the maximum force per unit width the paper can withstand before breaking. Low tensile strength causes web breaks during converting, reducing line uptime and increasing waste. Recommended minimum: 40 Nm/g in MD for 60 to 80 GSM kraft.

Cobb value (IS 1060 Part 4 / ISO 535)

Measures water absorption in grams per square metre over 60 seconds. High Cobb values mean the paper absorbs water rapidly, causing structural loss when bags encounter moisture, rain, or wet food. Target: below 30 g/m2 for standard carry bags, below 25 g/m2 for food-contact or wet-goods applications.

Grammage verification (IS 1060 Part 5 / ISO 536)

The most elementary test and the most frequently overlooked. Consistent under-grammage means you are paying for weight that is not delivered, and the resulting paper is weaker than specification. Declared GSM should fall within ± 5% of the tested mean across a minimum of 10 specimens.

Building a pre-purchase testing protocol

Step 1: Define acceptance criteria

Write down minimum values for each parameter by GSM grade, using the reference tables above as a starting point. Adjust using your own reject data: if your reject rate for a specific product exceeds 2%, tighten the relevant parameter threshold.

Step 2: Standardise sampling

Discard the first 3 to 5 outer wraps of each reel (these are exposed to transport and storage damage). Sample from at least 3 positions across the reel width. Test a minimum of 10 specimens per parameter, per reel. For consignments of 10+ reels, test at least 20%.

Step 3: Invest in basic equipment

This investment pays for itself with the first rejected consignment caught at the gate rather than on the converting floor.

Step 4: Document and track results

Record every test result against reel number, supplier, date, and consignment. Over 3 to 6 months, patterns emerge that identify consistent suppliers and quality drift indicators. Use the data in supplier negotiations and acceptance decisions.

Step 5: Share specifications with suppliers

Communicate your acceptance criteria in writing before ordering. Include minimum values for each parameter, sampling protocols, and the consequence of non-compliance (return, replacement, or re-negotiation). Suppliers who resist transparent specifications are suppliers you do not want.

Industry standards referenced

• IS 1060 (Parts 1 to 5): Burst, tensile, porosity, Cobb value, grammage
• IS 6490: Tearing resistance
• IS 1848: Kraft paper specification for packaging
• ISO equivalents: ISO 2758, ISO 1924, ISO 5636-5, ISO 535, ISO 536, ISO 1974

Frequently asked questions

What is burst factor and why does it matter for food packaging paper?
Burst factor is bursting strength divided by grammage (kPa·m2/g). It measures the paper’s resistance to rupture under uniform pressure. For food packaging, low burst factor means bags and containers rupture under normal filling, transport, or consumer handling conditions.

What tear index targets should I set for kraft paper bags?
For 60 to 80 GSM kraft: MD tear index of 5.0 to 7.5 mN·m2/g and CD tear index of 6.0 to 9.0 mN·m2/g. The MD value matters most for bags because handles are typically oriented along the machine direction. An MD/CD ratio above 1.6:1 signals handle-failure risk.

How does porosity affect print quality on food packaging?
High porosity (Gurley below 10 seconds/100 cm3) causes rapid, uneven ink absorption on flexographic and gravure lines. This produces fuzzy edges, excessive dot gain, and colour inconsistency. For branded food packaging, target Gurley values above 25 seconds for multi-colour printing.

Which Indian Standards cover paper testing for food packaging?
IS 1060 covers the core tests: Part 1 (burst), Part 2 (tensile), Part 3 (porosity), Part 4 (Cobb/water absorption), Part 5 (grammage). IS 6490 covers tearing resistance. IS 1848 is the overall kraft paper specification for packaging. These align with ISO equivalents.

How many specimens should I test per reel?
Minimum 10 specimens per parameter per reel, sampled from at least 3 positions across the reel width. For consignments of 10+ reels, test at least 20% of reels. If any reel fails, test the entire consignment.

How much does a basic paper testing setup cost?
A complete in-house testing setup (burst tester, tear tester, densometer, Cobb tester, balance) costs INR 2.5 to 4.6 lakh. This investment typically pays for itself within the first quarter by catching defective consignments before they reach the converting line.

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