SUHUI builds recycling and extrusion lines. We do not sell testing instruments, and this article is not about how to run the test. It is about the thing a line engineer actually does with the result when a lot of recycled resin arrives with a melt flow index printed on the docket and a decision has to be made about where it goes.
That is a different question from the one most material published on this subject answers. The measurement itself is well covered and standardised under ASTM D1238 and ISO 1133. What is thinly covered is how to read the number on recycled feed, where it is reliable, and where it will quietly mislead you.
The number is a signal about the batch, not a property of the polymer family.
What Melt Flow Index Measures and Why a Line Operator Cares
Melt flow index is the mass of molten polymer that flows through a standard die in ten minutes under a set temperature and load, reported in grams per ten minutes. Higher means a thinner, easier-flowing melt. For a line engineer its value is that it is a fast, cheap proxy for average molecular chain length.
Chain length is what actually matters, and it cannot be measured on a factory floor in five minutes. Longer chains entangle more, so the melt resists flow. Break the chains and the melt flows more easily. The index is therefore an indirect reading of how much polymer damage a material has accumulated, expressed as a flow number rather than as a molecular weight.
That indirection is the reason for both its usefulness and its limits. It is one measurement at one temperature and one load, which is why every quoted figure has to carry those conditions with it. A number without its test conditions is not comparable to anything.
Why Melt Flow Index Moves When Resin Is Recycled
Recycling puts polymer through heat, shear and sometimes moisture, and all three attack chain length. Every additional pass through a barrel breaks more chains, so the average molecular weight falls and the melt flows more easily. The index rises. That is the dominant effect for the polyolefins that make up most recycled volume.
Moisture deserves separate mention because it changes the mechanism rather than the amount. In hygroscopic polymers, water at melt temperature attacks the polymer backbone directly by hydrolysis, which is far faster than shear-driven scission. This is why moisture-sensitive material is dried before extrusion, and why an undried batch can lose more chain length in one pass than several properly dried passes would cost it.
Three mechanisms, three different rates. Moisture is by far the fastest.
There is a third contributor that has nothing to do with degradation at all. Recycled feed is a blend, and blends carry the flow behaviour of everything in them. A lot containing two grades of the same polymer will read a value that neither component has, and the reading will move as the stream composition moves, without any change in how damaged the material is.
Which Direction the Number Moves in Each Polymer
The assumption that recycling always raises melt flow index is wrong often enough to cause real problems. It holds for polypropylene and most polyethylene grades. It reverses or breaks down for polymers that crosslink or branch under thermal stress, and it does not apply usefully at all to PET.
| Polymer | What happens on reprocessing | Direction of the index | What to watch instead |
|---|---|---|---|
| Polypropylene | Chain scission dominates strongly | Rises, often noticeably per pass | Falling melt strength and impact resistance |
| HDPE | Chain scission with some branching | Rises, more slowly than PP | Melt strength, which matters for pipe and blow moulding |
| LDPE and LLDPE | Scission and crosslinking compete | Can move either way | Gel count, which rises whether the index moves or not |
| PET | Hydrolysis and scission, and the material degrades during the test itself | Not a reliable measure | Intrinsic viscosity, which is the standard for PET |
| PVC | Stabiliser depletion, then dehydrochlorination | Rarely used, since PVC is compounded not indexed | Thermal stability testing and remaining stabiliser |
The index only rises predictably for the polymers where scission dominates.
The PET row is the one most often got wrong, and it explains a genuine industry split. PET is measured by intrinsic viscosity rather than melt flow index because it is hygroscopic enough that it degrades during the melt flow test unless the sample is dried to an extreme standard first. Anyone quoting a melt flow index for recycled PET should be asked how the sample was prepared.
Reading Melt Flow Index as a Degradation Record
A single reading tells you where a batch sits. A series of readings across successive passes tells you something far more useful, which is how fast that particular material is losing chain length in your particular process. The slope carries more information than any individual point on it.
The practical version is to record the index of the virgin grade, then of the first-pass regrind, then of subsequent passes, keeping the process conditions constant. A shallow slope means the material and the process are compatible and the loop can run longer. A steep slope points at excessive shear, excessive residence time, insufficient drying or a depleted stabiliser package.
The slope across passes tells you more than any single reading does.
Two patterns in that record are worth learning to recognise. A jump that appears at one specific pass and not the others usually points at a process event rather than gradual degradation, such as a batch that was not dried or a barrel that ran hot. A slope that is shallow at first and then steepens suggests the stabiliser package has been used up and the polymer is now unprotected.
Matching Melt Flow Index to the Process a Batch Can Feed
Processes sort themselves along the flow scale, and this is where the number earns its place in a recycling operation. Processes that must hold a shape against gravity before it cools need viscous, low-index material. Processes that must fill a cavity quickly need the opposite. A batch qualifies for the processes its index and its melt strength allow.
| Process | Flow behaviour it needs | Why |
|---|---|---|
| Pipe and profile extrusion | Low index, high melt strength | The extrudate has to hold its section between the die and the calibration stage without sagging |
| Blow moulding | Low index | A parison has to hang without drawing down under its own weight before it is inflated |
| Film | Low to medium index | Melt strength governs bubble stability and how far the web can be drawn |
| Injection moulding | Medium to high index | The melt must fill thin sections fast before the gate freezes |
| Compounding and filled applications | Wide tolerance | Reinforcement carries the mechanical load, so matrix flow is less critical |
Recycled material moves down this list as it degrades, not off it.
Reading that table top to bottom is the honest description of what happens to recycled resin over its lives. A material that no longer has the melt strength for pipe still has plenty for injection mouldings, and a material too degraded for that still works as a compounding matrix. Degradation moves material down the list rather than off it.
The same logic sets acceptance criteria on the extrusion side. SUHUI’s PP profile extrusion line documentation names melt flow alongside contamination level, colour stability and dimensional control as the four things that have to stay acceptable before a higher recycled content ratio can be used on non-structural profiles. It is treated as one input to a decision, never as the whole decision.
How Melt Flow Index Changes the Screw and the Extruder Choice
The index does not just decide where material goes, it changes how the line that processes it should be built. Viscous low-index feed generates more shear heat and demands more torque. Easy-flowing high-index feed builds pressure poorly and tends to surge. A wide spread within one batch is a third problem again, and the worst of the three.
| What the index shows | What it means on the line | Configuration response |
|---|---|---|
| Low, viscous melt | High torque and pressure, significant shear heating, degradation risk if held hot | Adequate drive torque, careful temperature profiling, attention to residence time |
| High, thin melt | Poor pressure build, unstable output, weak strands | Water ring or underwater pelletizing rather than strand cutting, which needs melt strength |
| Wide spread within the batch | The screw is being asked to homogenise materials that behave differently | Twin screw rather than single screw, because mixing has to happen along the screw |
| High and rising pass to pass | Active degradation, which produces volatiles as a by-product | Vacuum degassing capacity, and drying discipline upstream of it |
| Unstable between deliveries | Feed rate by volume will not give a consistent mass flow | Gravimetric feeding, and incoming testing on every lot |
The reading is a configuration input, not just an acceptance test.
The third row is the one that decides single screw against twin screw. A single screw conveys material well and mixes it poorly, so a feed whose components soften at different temperatures and shear differently leaves unmelted cores and inconsistent pellet density behind. SUHUI’s rigid plastic granulating pelletizing line uses a high-torque twin screw for exactly that reason, doing the mixing along the screw instead of by holding the material hotter for longer.
The second row explains a choice that puzzles buyers comparing pelletizing methods. Strand pelletizing needs the melt to hold a continuous strand across a water bath, which requires melt strength that badly degraded material may not have. The PP PE film compacting pelletizing line offers water ring pelletizing specifically for soft or sticky materials, alongside strand and underwater options.
Where Melt Flow Index Misleads You on Recycled Feed
On virgin resin the index is close to a complete description of flow behaviour. On recycled feed it is a summary statistic over a mixture, and summary statistics hide exactly the information that causes production problems. Four blind spots account for most of the trouble, and the first is the largest.
| Blind spot | Why the number misses it | What it looks like in production |
|---|---|---|
| Spread within the batch | A blend of high and low material averages to a middle reading that neither component has | Output and dimensions that wander even though incoming testing passed |
| Behaviour at production shear rates | The test runs at a far lower shear rate than a screw imposes | Two lots with matching indices that run differently on the same line |
| Melt strength and elasticity | The test measures flow under load, not resistance to stretching | Sagging extrudate or drawing parisons at an index that looked acceptable |
| Gels, unmelts and solid contamination | Solid particles barely shift a flow measurement | Surface defects and specks with no warning from the docket |
An average over a mixture hides exactly what causes the trouble.
There is a fifth trap that is purely procedural. In a hygroscopic polymer an undried test sample degrades during the test, so the result reads high and the batch looks more damaged than it is. Rejecting good material on that basis is a common and expensive error, which is why sample preparation belongs in the incoming test procedure rather than being left to whoever runs it.
None of this makes the measurement not worth taking. It makes it one input among several. Contamination, colour, moisture and bulk density all have to be assessed separately, because the flow number is blind to every one of them.
What to Do With an Incoming Melt Flow Index Number
Treat it as a screening measurement, not an acceptance test. Its job is to place the lot on the scale of processes it can feed and to flag anything that has moved outside its expected band. What it cannot do is certify a lot, because too many of the properties that matter in production sit outside what it can see.
Screening measurement, not acceptance test. Everything else is checked separately.
Three habits make it far more useful. Always record the test conditions with the value, since a figure at one temperature and load is not comparable to a figure at another. Build a band from your own running history rather than working to a supplier’s nominal figure, because what matters is deviation from what your line has been handling successfully. And test more than one sample from a lot when the material is mixed, because a single sample from a heterogeneous batch measures that sample and nothing else.
The wider point for anyone specifying a recycling line is that flow variation is a design input, not a running problem to be solved later. A line built for one grade of clean post-industrial scrap and a line built for mixed post-consumer feed differ in screw type, degassing capacity, feeding method and pelletizing method, and the melt flow behaviour of the feedstock is what drives all four.
Frequently Asked Questions
What does a high melt flow index mean?
That the melt flows easily under the test conditions, which usually indicates shorter average polymer chains. On recycled material a high or rising value normally means the polymer has taken thermal and shear damage. It also means lower melt strength, so the material suits injection moulding better than pipe or blow moulding.
What is MFI and MFR?
The same measurement under two names. Melt flow index is the older term; melt mass-flow rate, or MFR, is the current designation in the standards, reported in grams per ten minutes. Melt volume-flow rate, MVR, measures the same flow by volume, and converting between them requires the melt density.
Does melt flow index change when plastic is recycled?
Almost always. Heat, shear and any moisture present break polymer chains, so the value typically rises with each processing pass. Polypropylene moves most noticeably, HDPE more slowly. LDPE can go either way because crosslinking competes with chain scission, and PET is measured by intrinsic viscosity instead.
What is a good melt flow index for recycled resin?
The one that matches the process it has to feed, not the highest or the lowest available. Pipe, profile and blow moulding need low values and good melt strength; injection moulding needs higher ones. A value is good when it sits inside the band your own line has been running successfully.
What is the melt flow rate range for HDPE?
There is no single range, because HDPE is supplied in grades tuned to different processes. Pipe and blow moulding grades sit at the low, viscous end and injection grades much higher. The figure for any specific material belongs on its own data sheet or certificate of analysis, quoted with its test conditions.
Can a melt flow index that is too high be corrected?
Not in the sense of rebuilding the polymer on a standard extrusion line. In practice the material is blended with lower-index or virgin resin to bring the mixture into range, or it is redirected to a process that suits it. For PET the equivalent problem is solved differently, by solid state polymerization.
Is melt flow index the same as viscosity?
Related but not equivalent. The index is inversely related to melt viscosity, so a higher value means a thinner melt, but it is one point measured at one low shear rate. A full viscosity curve across production shear rates can separate two materials that report the same index yet run differently.
Which Line Can Run That Number
- Plastic Recycling Pelletizing Line — single and twin screw builds for re-pelletizing feed that lands off-band
- PET Solid State Polymerization System — the PET exception, where IV replaces the index and can be raised again
- Single Screw vs Twin Screw Extruder — why a drifting index pushes the screw choice towards twin
- Twin Screw Extruder — the screw, venting and barrel detail behind that extra tolerance
- Single Stage vs Double Stage Pelletizing — whether one extrusion pass or two is enough to stabilise the batch
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