Melt index (melt flow rate, MFR) testing is the primary quality-control method for characterizing the flow behavior and molecular weight consistency of recycled plastic materials. Performed in accordance with ISO 1133-1, ASTM D1238, and GB/T 3682, the test measures the mass or volume of molten polymer extruded through a standard die under defined temperature and load conditions, providing critical insight into degradation, contamination, and batch-to-batch uniformity of recyclates. This article explains the principle, procedure, applicable materials, and practical applications of melt index determination for recycled plastics.
Technical Overview
Melt index determination, also known as melt flow rate (MFR) or melt mass-flow rate testing, is a capillary extrusion test in which a thermoplastic material is melted in a heated barrel and forced through a standardized die by a piston carrying a specified load. The result is expressed as the mass of polymer extruded in ten minutes (g/10 min) for MFR, or as the volume extruded in ten minutes (cm³/10 min) for melt volume-flow rate (MVR). Because melt flow behavior correlates directly with average molecular weight and melt viscosity, the test serves as a fast, economical indicator of polymer integrity.
For recycled plastics, melt index testing carries particular significance. Reprocessing exposes polymers to thermal, oxidative, and mechanical stress that causes chain scission, reducing molecular weight and typically increasing the melt index. Conversely, cross-contamination with dissimilar resins, filler content, or partial cross-linking can shift the melt index in the other direction or produce erratic, non-reproducible flow behavior. Regular MFR measurement therefore allows recyclers, compounders, and converters to verify incoming material quality, grade recyclate streams, and confirm consistency between production batches.
- Key measurands: MFR (g/10 min), MVR (cm³/10 min), melt density (g/cm³), and flow rate ratio (FRR) measured under two different loads.
- Governing standards: ISO 1133-1 and ISO 1133-2, ASTM D1238, GB/T 3682, and JIS K 7210.
- Diagnostic value: detection of degradation from repeated processing, hydrolytic damage in condensation polymers, and cross-contamination between resin types.
Testing Samples
Recycled Polypropylene (rPP)
Recycled polypropylene sourced from rigid packaging, battery casings, automotive components, and woven sacks is one of the most frequently tested recyclates. It is typically evaluated at 230 °C with a 2.16 kg load. Elevated MFR values relative to the original virgin grade commonly indicate thermo-oxidative chain scission accumulated over multiple processing cycles.
Recycled Polyethylene (rHDPE, rLDPE, rLLDPE)
Recycled polyethylene from bottles, drums, films, and carrier bags is normally tested at 190 °C with a 2.16 kg load; high-load conditions (21.6 kg) may be applied to high-molecular-weight or low-flow grades. Because post-consumer PE streams often contain mixed densities and small amounts of PP caps, MFR testing is used to detect off-specification blends and confirm grade separation.
Recycled PET (rPET)
Recycled PET from bottle streams is highly hygroscopic and susceptible to hydrolytic degradation, which lowers intrinsic viscosity and raises melt flow. Although intrinsic viscosity measurement remains the primary characterization method for rPET, melt index testing under rigorously dried conditions (commonly around 270 °C with a specified load) is used as a rapid screening tool to assess melt degradation and moisture damage.
Recycled Styrenics (rPS, rHIPS, rABS)
Recycled polystyrene, high-impact polystyrene, and ABS from electronics housings, appliances, and display trays are tested under conditions such as 200 °C with a 5.00 kg load for PS/HIPS and 220 °C with a 10.0 kg load for ABS. Rubber phase content and paint or ink residues in post-consumer styrenics often broaden MFR scatter, making multiple determinations per batch advisable.
Recycled Engineering Plastics (rPA, rPC, rPMMA, rPOM)
Recycled polyamide, polycarbonate, PMMA, and acetal require thorough pre-drying before testing because residual moisture causes hydrolysis during the measurement itself. Typical conditions include 300 °C with a 1.20 kg load for PC, 275 °C with a 5.00 kg load for PA, 230 °C with a 3.80 kg load for PMMA, and 190 °C with a 2.16 kg load for POM. ISO 1133-2 applies to materials sensitive to time-temperature history and moisture.
Testing Items
- Melt Mass-Flow Rate (MFR): mass of extrudate in grams collected over ten minutes, determined by Method A (cut-and-weigh).
- Melt Volume-Flow Rate (MVR): volume extruded in cm³ per ten minutes, determined by Method B (piston displacement measurement).
- Melt Density: derived by combining mass and displacement measurements, used to convert between MFR and MVR.
- Flow Rate Ratio (FRR): the ratio of flow rates obtained under two loads at the same temperature, providing an indication of shear sensitivity and molecular weight distribution.
- Batch Consistency Assessment: statistical comparison of MFR values across lots against agreed specification limits.
Testing Methods
Method A – Mass Determination (Cut-and-Weigh)
The apparatus consists of a vertically mounted heated barrel with an internal diameter of 9.550 mm, a die of 2.095 mm bore diameter and 8.000 mm length, and a weighted piston. After the barrel reaches the specified temperature (controlled within ±0.5 °C), a pre-weighed charge of recycled pellets or ground material is loaded, compacted to exclude air, and preheated for a standardized period, typically four to six minutes depending on the governing standard. The load is then applied, and extrudates are cut at timed intervals, cooled, and weighed on an analytical balance. The melt flow rate is calculated as MFR = (m / t) × 600, where m is the average mass of the cuttings in grams and t is the cut interval in seconds.
Method B – Displacement Measurement
Method B determines flow rate from the distance traveled by the piston over a measured time, using an encoder or optical sensor. Because no manual cutting is involved, this method offers better repeatability at high flow rates and enables automated measurement. MVR requires knowledge of the melt density of the material at the test temperature; when melt density is measured directly in a combined A/B sequence, the result also yields MFR.
Standard Test Conditions
- Polyethylene (PE): 190 °C / 2.16 kg (21.6 kg for low-flow grades)
- Polypropylene (PP): 230 °C / 2.16 kg
- Polystyrene and HIPS (PS): 200 °C / 5.00 kg
- ABS: 220 °C / 10.0 kg
- Polycarbonate (PC): 300 °C / 1.20 kg
- Polyamide (PA): 275 °C / 5.00 kg
- PMMA: 230 °C / 3.80 kg; POM: 190 °C / 2.16 kg
Sample Preparation and Practical Considerations
- Hygroscopic recyclates (PET, PA, PC, ABS) must be dried to the moisture level specified by the material standard before charging, since residual moisture lowers the measured MFR through in-test hydrolysis.
- The cut interval is selected according to the expected flow rate so that individual extrudate segments are large enough for accurate weighing; at least three valid cuttings should be collected after discarding the initial purge segment.
- Reported results must state the standard used, temperature, load, method, and any sample pretreatment, because results from different conditions are not interchangeable.
- At least two determinations on separately charged samples are recommended for recycled materials to reflect the inherent heterogeneity of recyclate streams.
Applications
Melt index data for recycled plastics supports decision-making across the entire recycling and converting value chain:
- Incoming and outgoing quality control: verification that purchased recyclate or produced regranulate meets agreed MFR specification limits before further processing or shipment.
- Recyclate grading and classification: sorting of post-industrial and post-consumer streams into flow-rate grades suitable for different end applications such as injection molding, blown film, or fiber extrusion.
- Degradation monitoring: tracking molecular weight loss across repeated processing loops and evaluating the effectiveness of stabilization additives and gentle reprocessing conditions.
- Contamination screening: identification of cross-resin contamination, for example PP in a PE stream, which manifests as abnormal or unstable flow values.
- Formulation development: adjustment of virgin resin, filler, or modifier ratios in compounds containing recyclate to hit target flow behavior.
- Process setup: selection of melt temperatures, injection pressures, and screw speeds for converters running recycled feedstock.
- Certification support: generation of consistent test data for recycled-content declarations, customer specifications, and audits conducted by accredited third-party laboratories.
FAQ
Why does the melt index of recycled plastic often differ from that of the original virgin resin?
Repeated extrusion, washing, and re-pelletizing expose the polymer to heat, shear, oxygen, and sometimes moisture. Chain scission reduces average molecular weight, which lowers melt viscosity and raises the melt index. Contamination, fillers, or partial cross-linking may produce the opposite effect or increase scatter between determinations.
What is the difference between Method A and Method B?
Method A measures the mass of timed extrudate cuttings on a balance and directly yields MFR in g/10 min. Method B measures piston travel distance and yields MVR in cm³/10 min, which can be converted to MFR using the melt density. Method B is more automated and preferred at high flow rates, while Method A remains the classical reference approach.
How should recycled samples be prepared before testing?
Samples should be homogeneous, free of metal and foreign particles, and dried when the polymer is hygroscopic. Pellets, granules, or ground flake can all be tested, provided the material packs consistently in the barrel and the conditioning time defined by the standard is respected.
How much batch-to-batch variation is acceptable?
Specification limits are agreed between supplier and customer and depend on the application. Recyclates typically show wider variation than virgin resin due to feedstock heterogeneity, so many specifications allow tolerances in the range of a modest percentage around the nominal value and require statistical lot control rather than single-point acceptance.
Can melt index testing detect polymer contamination?
It provides an effective screening indicator. A PP-contaminated PE lot, for example, will usually show an abnormal or drifting flow value. When contamination is suspected, melt index results should be complemented by infrared identification and thermal analysis to confirm the contaminant type and level.
In summary, melt index determination is an indispensable, standardized tool for controlling the quality of recycled plastics. Executed under ISO 1133, ASTM D1238, or GB/T 3682 conditions by trained personnel in accredited third-party laboratories or in-house facilities, it delivers rapid, quantitative insight into degradation, consistency, and processability, helping the industry turn recyclate streams into reliable, specification-compliant materials.