誤った比較を行わずにポリマー MFI をテストおよび解釈する方法

Polymer MFI is useful only when the material and test conditions travel with the number. This guide shows resin buyers, laboratory teams, and process engineers how to compare melt-flow results, investigate a lot mismatch, and request data that can survive a supplier-to-plant handoff.

2026年8月更新しました

What Polymer MFI Measures, and What It Does Not

Melt flow index records how much molten thermoplastic passes through a specified die during a normalized 10 min interval under a defined temperature and load. Because it’s empirical, the result describes the material’s response inside that test, not every property of the resin or its performance on a production line.

In current technical language, MFI is typically shorthand for melt flow rate measured on a mass basis, or MFR. Measuring the melt flow gives polymer processors a fast consistency signal because the apparatus, force, temperature, and timing can be controlled. With every condition matched, the result can help compare lots of the same polymer grade.

Within polymer science, the relationship between MFI and polymer properties stays contextual: MFI indicates flow under one condition, while mechanical properties, the processing window of a polymer, the broadness of the molecular weight distribution, and other properties of the polymer require separate evidence for specific applications.

Because mass-based MFI is measured from the mass of the extrudate, not simply the weight of the polymer sample, the conventional result is grams per 10 minutes; it records the material’s ability to flow through the device and how the polymer flows under that test, not why its structure produces that response.

Use the result as a condition-bound quality-control parameter, not as a universal material property.

Takeaway: MFI helps with a controlled comparison. It can’t carry an unverified claim about strength, purity, durability, or regulatory status.

Choose ASTM D1238-26, ISO 1133-1:2022, or ISO 1133-2:2011 Before Testing

The material’s governing specification and sensitivity determine which melt-flow procedure applies. ASTM D1238-26 and ISO 1133-1:2022 cover extrusion-plastometer methods for suitable thermoplastics, while ISO 1133-2:2011 addresses materials whose rheological behavior changes significantly through moisture or time-temperature history.

Laboratories should not begin with “run an MFI.” Staff should first identify the material standard or, where no standard supplies conditions, the conditions agreed by the interested parties. Recording that agreement in the purchase specification is a buyer control, not an ISO writing requirement.

1. Identify the material — record family, grade, supplier code, and applicable material specification.
2. Select the method — choose the active ASTM or ISO procedure appropriate to the material.
3. Condition the sample — follow the documented drying and handling history.
4. Stabilize the instrument — bring the barrel and sample to the specified test state.
5. Apply the load — use the stated mass and procedure without substituting a convenient condition.
6. Measure and report — collect timed extrudate for MFR or piston displacement for MVR, then retain the full condition record.

During MFI testing, molten polymer moves through a die with a specified diameter and length; the selected temperature and prescribed load affect measured flow. Even with matching test conditions, MFI measurements can’t reproduce the line’s processing conditions, including production temperature and pressure, or the full thermoplastic melt flow characteristics inside a production screw, gate, or die, and they don’t identify what causes material to flow.

ASTM’s official D1238-26 scope and significance page notes that ASTM and ISO 1133 address the same subject but differ technically. ISO’s Part 2 scope also warns that the method can be unsuitable when rheology changes too severely during the test.

How do you calculate an MFI value?

MFR is obtained from the measured extrudate mass and collection time, then normalized to a 10 min basis according to the selected procedure. Each gram collected is recorded on the balance. In plain form, MFR equals collected mass multiplied by 600 and divided by the collection time in seconds. Still, the calculation isn’t valid on its own: temperature, load, timing rule, die geometry, sample preparation, and the applicable method must remain attached to the result.

Takeaway: select the procedure before preparing the specimen; a correct calculation can’t repair the wrong condition.

The 6-Field Condition Lock for Comparable MFI Data

The 6-Field Condition Lock is a normalization rule: compare two MFI values only after material identity, condition authority and method, temperature, load, sample history, and result type all match. One missing field turns a neat numerical comparison into an unresolved data-quality question.

Procurement sees supplier certificates, the incoming laboratory sees retained pellets, and production sees line behavior. Together, the six fields give all three groups a shared record. Each field below is a stop/go check, not a suggestion.

Takeaway: no team should debate whether “12 is close to 13” until it proves both values came from the same six-field condition.

What High and Low MFI Really Mean

Within one polymer grade and one locked test condition, higher MFI usually means the melt passed through the die more readily, while lower MFI means less mass passed in the same normalized interval. That comparison remains relative; neither label establishes a universal processing window or product performance.

What does high MFI mean?

High MFI means higher measured mass flow only inside the stated test. Within a controlled, like-for-like comparison, its relationship with melt viscosity is often inverse, but “inversely proportional” is too strong as a universal rule. Molecular-weight distribution, long-chain branching, fillers, moisture, and shear rate can change production behavior without preserving a simple one-number relationship.

Takeaway: interpret high MFI or low MFI inside a matched system, then verify the property that actually controls the application.

MFI Usually Means MFR; MVR Is Different

MFI commonly names the mass-based MFR result, whereas MVR records melt volume passing the reference point during the normalized interval. ISO 1133-1 allows conversion only when melt density at the test temperature is known, so mass and volume results should never be swapped by label alone.

What is the difference between MFR and MFI?

MFR is the standardized mass-flow quantity. MFI is the older or informal term many datasheets still use for that same type of result. MVR is not another spelling of MFI: it is a volume result. Filled and unfilled compounds can have different melt densities, so ISO identifies MVR as useful for such comparisons and requires density at the test temperature before converting between MFR and MVR.

High melt flow, high melt flow rate, and lower melt flow rate are comparative labels; calling MFI inversely proportional to viscosity without matching the material and test condition is misleading, while melt volume rate may be useful alongside mass flow when the material’s melt density or filler content changes.

Takeaway: ask the laboratory to spell out the quantity and unit; don’t infer MFR or MVR from an unlabeled “flow index.”

Why PE, PP, PET, and ABS Numbers Cannot Be Compared Directly

PE, PP, PET, and ABS results can’t be ranked on one universal MFI chart because thermoplastic polymers are tested under material-appropriate conditions and carry different structures, histories, and specifications. Across families, a higher number may come from a different temperature, load, procedure, or moisture state than a lower number.

For example, a supplier may report PP at 230 °C and 2.16 kg, but that condition isn’t a default for every plastic. Readers who need PP-specific ranges should use the separate guide to polypropylene resin grade selection. For polyethylene families, the HDPE, LDPE, and LLDPE comparison explains why family and application context must remain visible; across the polymer industry, polymer processing guidance and advances in polymer technology work best when they preserve grade, procedure, and condition rather than reducing several resin families to one rank order.

Takeaway: compare polymer grades inside their governing material and process context, never by stripping the condition from the number.

Where MFI Stops Predicting Production Behavior

The MFI Escalation Matrix separates questions that one low-shear flow result can answer from questions that need density, rheology, mechanical testing, chemical analysis, or a process trial. Two polymers with the same MFI can behave differently when shear rate, temperature, branching, elasticity, filler, and die geometry change.

ISO 1133-1:2022 notes that test shear rates are much lower than those in normal processing. Full viscosity curves or extensional flow data of polymer melts are therefore more informative when die pressure, shear thinning, or scale-up drives the decision.

When not to buy on MFI alone: If the decision concerns impact, tensile behavior, composition, food-contact status, additive content, pressure drop, die swell, or a narrow processing window, require the test that measures that outcome. Supplier flow data can screen a candidate, but it can’t replace evidence for a different property.

Takeaway: matching flow numbers justify a closer comparison, not a claim that two materials are interchangeable.

The Lot-to-Line MFI Investigation Loop

The Lot-to-Line MFI Investigation Loop turns a mismatch into a controlled sequence: preserve the lot, match the method, retest a representative sample, account for uncertainty, apply the agreed conformity rule, and escalate unresolved differences. By design, this loop investigates; it doesn’t create a universal tolerance or authorize release by itself.

Even tight replicate spread can coexist with larger total uncertainty. NIST’s 2026 SRM 1474c certification study reported a 0.016 g/10 min standard deviation across 30 determinations, while expanded uncertainty was 0.20 g/10 min for that one polyethylene material and condition. Those values aren’t supplier tolerances; they show why repeatability and decision uncertainty are different questions.

1. Lock the six fields in the purchase specification.
2. Record the supplier Certificate of Analysis result and stated limit.
3. Retain, identify, and condition a sample that represents the lot.
4. Verify the internal method matches the supplier or material method.
5. Repeat the suspect result before assigning a cause.
6. Review sampling coverage, laboratory uncertainty, and any guard band.
7. Apply the agreed conformity decision rule, not a generic web range.
8. Quarantine and escalate when the mismatch survives the controlled retest.

For broader plant and resin-handling discussions, route the technical handoff through BOSHIYA’s resin-handling and process-integration project scope team or request a scoped technical service review.

Takeaway: quarantine first, then separate material evidence from method, sampling, and uncertainty evidence.

What an MFI Shift Can and Cannot Tell You About Degradation

An MFI shift is an investigation signal, not a diagnosis of degradation, recycled content, moisture damage, or contamination. Depending on the polymer chemistry and material history, the result may move upward or downward; neither direction identifies a mechanism without confirmation from additional evidence.

Moisture-sensitive polymers deserve special care because time-temperature history and drying can change the material during testing. In 2026, a Scientific Reports measurement-system study found that load, temperature, drying, and preheat influenced measurements for the studied polyamide composites. Its reported 3.179% repeatability and 12.208% reproducibility values belong to that study, not to a universal acceptance rule.

Takeaway: preserve the sample and choose a confirmation test; don’t name the damage mechanism from MFI direction alone.

Eight Checks Before You Reject a Polymer Lot

Before rejecting a lot, verify material identity, conditioning, preheat, temperature and load, instrument cleanliness, cut and weighing practice, air or voids, and replicate behavior. These eight checks separate a repeatable material signal from a preventable measurement-system or sample-history problem.

A 2024 study validating the authors’ open-source melt-flow apparatus documents calibration, cleaning, fixed settings, and replicate work. NPL’s technical review adds operator loading, air, cutting, sample amount, density, and low-shear limitations.

Takeaway: a lot decision is stronger when the method record explains why the result belongs to the resin rather than the test setup.

How to Specify MFI on an RFQ and Certificate of Analysis

A usable RFQ names the resin, condition authority, active method and procedure, temperature, load, sample conditioning, MFR or MVR unit, supplier limit, sampling plan, uncertainty, decision rule, and retest path. Returned Certificate of Analysis fields should mirror the request so procurement and quality can compare like with like.

Copy the checklist below into the quote request. Blank fields are intentional: the material standard or an agreement between interested parties should supply the condition, not a generic article.

BOSHIYA can discuss the wider project boundary through its wider resin project-boundary review page and engineering, procurement, and construction services. Those commercial discussions don’t replace resin-specific laboratory evidence.

よくある質問frequently Asked Questions

Bring a complete test record to the technical conversation

Send the six condition fields, supplier Certificate of Analysis, sampling plan, and the unresolved plant question. That makes a resin or process discussion faster and keeps commercial review separate from laboratory proof.

Discuss resin-handling project scope

References and Sources

1. ASTM International, ASTM D1238-26 — active method scope, significance, condition dependence, and limitations.
2. ISO 1133-1:2022 — MFR and MVR procedures, density-gated conversion, and processing-shear limitation.
3. ISO 1133-2:2011 — method for materials sensitive to time-temperature history or moisture, with applicability limits.
4. NIST SP 260-267, SRM 1474c — material-specific repeatability and expanded-uncertainty example.
5. Materials (2024), author-team validation of an open-source melt-flow apparatus — calibration, cleaning, controlled settings, and replicate testing.
6. Scientific Reports (2026), moisture-sensitive composite measurement study — load, temperature, drying, and preheat effects within the studied system.
7. National Physical Laboratory, CMMT(A)231 — operator, loading, cutting, density, low-shear, and extensional-flow limitations.

Related Reading

• Polypropylene resin grade selection
• HDPE vs LDPE vs LLDPE differences
• Resin and chemical plant project scope
• Industrial technical service