Ensuring polymer consistency at scale through melt flow testing

With global plastic production soaring to almost 450 million tons annually, small variations in polymer quality can quickly cascade, leading to scrap, downtime or even recalls. Here, Stephanie Williams, senior product specialist at materials testing giant Instron explores how melt flow testing helps ensure consistency and efficiency in polymer processing.

Melt flow testing (MFT) is essentially a precise “flow speed” test. Molten polymer is forced through a die under standardised conditions, yielding a melt flow rate (MFR) in grams per ten minutes (g/10min).

While high MFRs can ease mould filling and cycle times, they can also signal thermal degradation, especially in engineering materials like polycarbonates or ABS, risking weaker, crack-prone parts. Conversely, low MFRs usually indicate stronger molecular chains but complicate processing through higher viscosity and energy demand.

For quality assurance, however, MFR can also reveal inferior or incorrect products.

Imagine a compounder receiving polypropylene pellets labelled with an MFR of 12 g/10min for automotive parts, but testing shows only 3 g/10min. This suggests a different grade, higher molecular weight or a recycled blend with unknown additives. Without testing, this batch might cause processing delays and defects like poor fill or warping. Instead, the team was able to recognise this and correct it before costly defects occur.

Incorporating MFT into incoming inspections lets QA teams quickly verify polymer specs, avoiding costly production issues and building supplier trust. MFT can also detect batch variability early, allowing adjustments or rejection before production.

Recycled and mixed polymers

With sustainability goals now mainstream, compounders are increasingly using recycled or mixed source polymers with forecasts predicting the global recycled plastic market to reach $107 billion by 2032. With this shift comes variability in composition and degradation, making ensuring consistent processing and performance more important than ever.

MFT offers a quick check. If a recycled HDPE batch shows an MFR of 25 g/10min instead of the expected eight, it may indicate degradation or contamination. While MFT can’t identify exact chemistry, it can flag non-conforming batches before they reach production.

For compounders, tracking MFR helps decide if a batch meets specs, needs blending or should be rejected.

MFT also aids production-floor quality control. For example, a medical device maker faced complaints about hairline cracks in pre-filled syringes after shipment. MFT confirmed the pellets used for the syringes met viscosity specifications and therefore the manufacturer had received the correct batch.

This knowledge enabled it to shift focus to its production processes. Upon investigation, an unevenly heated cavity in an injection press caused poor flow and weak walls.

Without MFT, blame might have fallen on suppliers, leading to wasted material and regulatory issues. Fast, consistent MFR results instead enabled targeted fixes and prevented costly disruptions.

Making MFT easier and more reliable.

Though MFT protocols follow ISO1133/ASTMD1238, usability varies. Some systems need external PCs, different operating systems and desktop apps, doubling training demands and risking inconsistent results across labs.

In high-volume QA or production environments, users often face a double learning curve: one for the instrument, another for the software. This adds setup time, supervision and risk of inconsistent results despite standardised test methods.

Instron avoids this by embedding an intuitive touchscreen Graphical User Interface (GUI) directly on the instrument. Designed to mirror the look and feel of Bluehill® Melt software, it guides users through each step of the test with clear, intuitive prompts. Operators can be trained and testing in under an hour, helping maintain composure and precision across shifts and sites without added training burdens. Systems requiring third-party software add learning curves, slowing productivity and increasing mistakes.

The same applies to force application. Many systems now use load cells, allowed by ISO and ASTM, but Instron still supports dead weight stacks as standard.

This is not outdated though. ASTM D1238 recognises dead weights as the preferred reference method, stating dead weight results take precedence if labs differ. Calibrated metal weights provide constant, fixed force unaffected by signal drift or calibration errors.

Load cells offer flexibility but need frequent calibration and can be sensitive to changes in material state. Instron, experienced in electromechanical systems and testing, could use load cells but prioritises reliability and standards compliance by keeping dead weights central.

With millions of tonnes of plastic produced each year, ensuring consistent polymer quality through melt flow testing is vital to reducing waste and preventing defects. To keep your production on track visit /www.instron.com/en/products/testing-systems/rheology/melt-flow-index-testers and learn more about reliable MFT solutions.