What tensile strength tells you

Tensile testing describes how a film responds when pulled under defined conditions. Common outputs include tensile strength at yield or break, elongation at break and modulus. These values answer different questions: a film may become stiffer yet lose elongation. For bag applications, tensile results should be considered alongside tear, puncture and seal performance.

Why increasing filler dosage changes behavior

Mineral particles do not deform in the same way as polyethylene. Their effect depends on particle size, dispersion, interfacial adhesion, concentration and how the film is oriented during extrusion. Well-dispersed filler can sometimes increase stiffness, while higher loadings or agglomerates may reduce elongation or create stress-concentration sites. No fixed loading threshold applies to every formulation.

Machine direction versus transverse direction

Blown film has two principal directions: machine direction (MD) and transverse direction (TD). Blow-up ratio, drawdown, frost-line height and cooling influence molecular orientation and the resulting properties. A filler trial must report MD and TD results separately. An average value can hide an unacceptable loss in the direction that matters for the finished bag.

Why a percentage alone is misleading

“8% filler” may mean 8% masterbatch added to the formulation, not 8% pure mineral in the finished film. Actual mineral content depends on the filler fraction of the masterbatch. For example, adding 8 kg of a masterbatch containing 70% mineral to a 100 kg finished blend gives 5.6 kg mineral, assuming the dosage is expressed as a share of the final blend. Confirm the supplier’s specification and the denominator used in every comparison.

Build a controlled trial

Use the same resin lots, machine, die, target thickness and conditioning procedure wherever possible. Record the unfilled baseline, then test conservative incremental dosages. Collect samples after the line stabilizes. Measure thickness at multiple positions, MD/TD tensile strength, MD/TD elongation and relevant tear or puncture properties. Record line pressure, output, bubble stability and scrap rate. If one condition changes, document it.

Use appropriate test methods

ASTM D882 and ISO 527-3 are widely used references for tensile testing of thin plastic films, subject to the applicable specimen and method requirements. Do not combine values from different methods, sample geometries or conditioning conditions without explaining the differences. Your customer specification and relevant product standard should determine the acceptance criteria.

Evaluate economics only after performance

Material cost per kilogram is not the only metric. Compare cost per acceptable kilogram or per thousand finished bags, including rejected film, downtime, sealing failures and any thickness changes. A lower raw-material price is not a saving if the process produces more waste or the film fails end-use requirements.

What to ask your supplier

Request recommended trial levels for your resin, a technical data sheet, the masterbatch mineral fraction, processing guidance and batch traceability. Ask whether the supplier can review MD/TD test data and adjust the formulation if performance drifts.

Conclusion

Filler loading should be optimized through measured trade-offs, not maximized by default. Choose the highest economically useful level that still passes the finished-film specification with repeatable production stability.

Frequently Asked Questions

Is one filler dosage suitable for every blown film line?

No. Resin grade, film structure, gauge, equipment, masterbatch composition and finished-product requirements change the workable range.

Should a supplier’s case-study result be used as a guarantee?

No. Treat it as a starting point for a controlled trial, then verify results against your own film specification.

What is the next step before a bulk order?

Request a suitable sample, document the baseline and run repeatable production tests that measure quality and total cost.

Explore More Technical Resources

For related manufacturing insights, visit the Kovicrea Technical Insights center. To review available formulations and discuss a trial for your resin and process, explore our sodium sulfate filler masterbatch products.