TPE Melt Flow Index: Why Injection and Extrusion Grades Are Not Interchangeable
Melt flow index is the single number that decides whether a thermoplastic elastomer will run on your line. Injection grades in this catalogue have a median MFI of 73 g/10 min at 230°C/2.16 kg; extrusion grades sit at 7.5 - roughly a tenfold difference. Getting it wrong produces short shots and weld lines on an injection tool, or a profile that sags out of tolerance on an extrusion line.
Key takeaways
- MFI measures how much polymer flows through a standard die in ten minutes under a fixed load. Higher means thinner-flowing melt.
- Injection moulding needs flow to fill the cavity; extrusion needs melt strength to hold profile geometry. These are opposing requirements.
- Across this 49-grade catalogue, injection grades run 7.2 - 367 g/10 min and extrusion grades 0.6 - 32 g/10 min at 230C.
- Two grades at identical hardness can differ by a factor of 99 in melt flow, so hardness alone will not tell you whether a grade suits your tool.
- MFI is quoted at two conditions here - 230C/2.16 kg and 190C/5 kg - and the two are not directly comparable.
What melt flow index actually measures
A melt flow test pushes molten polymer through a standard capillary die under a fixed weight at a fixed temperature, and reports the mass extruded in ten minutes. A high number means the melt flows easily; a low number means it is viscous and holds together.
That single figure stands in for viscosity, and viscosity is what determines whether a melt fills a thin-wall cavity before freezing, or whether an extruded profile keeps its shape on the way to the cooling line.
Why injection and extrusion pull in opposite directions
An injection tool needs the melt to reach the far end of the cavity at speed. Thin walls, long flow paths and complex geometry all raise the flow requirement. A compound that is too viscous gives short shots, high injection pressure, visible weld lines and stressed parts.
Profile extrusion needs the opposite. Once the melt leaves the die it is unsupported until it cools. A compound that flows too freely will sag, draw down, or lose the sharp detail of the die profile. Extrusion grades are formulated to be viscous enough to survive that unsupported span.
Where the FARRPRENE grades sit
The catalogue splits cleanly along the theoretical line:
| Grade | Process | Hardness | MFI 230°C/2.16 kg | Suits |
|---|---|---|---|---|
| FARRPRENE EU401A | Extrusion | 40 Shore A | 0.6 | The most viscous grade here; tall unsupported profiles |
| FARRPRENE EF753AV | Extrusion | 75 Shore A | 0.93 | Firm profiles needing tight dimensional hold |
| FARRPRENE EF657A | Extrusion | 65 Shore A | 1.3 | Detailed sealing profiles |
| FARRPRENE EF651A | Extrusion | 65 Shore A | 1.6 | Weatherstrip and gasket profiles |
| FARRPRENE EF654AU | Extrusion | 65 Shore A | 23 | Higher-flow extrusion; complex dies |
| FARRPRENE IF658AS | Injection | 65 Shore A | 19 | Thicker-section moulding |
| FARRPRENE IF753A | Injection | 75 Shore A | 61 | General-purpose injection |
| FARRPRENE IF751A | Injection | 75 Shore A | 256 | Thin walls and long flow paths |
| FARRPRENE IF700A | Injection | 70 Shore A | 347 | Very thin wall, short cycle |
| FARRPRENE IF900A | Injection | 90 Shore A | 367 | Highest flow in the catalogue |
The trap: same hardness, different tool
Seven grades in this catalogue are 45 Shore A. Their melt flow at 230°C spans 1.5 g/10 min (FARRPRENE EF455A) to 148 g/10 min (FARRPRENE IF454A). That is a factor of roughly 99 between two compounds a datasheet would describe identically on hardness.
They are not alternatives to one another. FARRPRENE EF455A is an extrusion grade; FARRPRENE IF454A is a high-flow injection grade. Specifying "45 Shore A TPE" without the process leaves the most important variable unstated.
Reading the two test conditions
Every grade here is quoted at both 230°C/2.16 kg and 190°C/5 kg. The two conditions use different temperatures and different loads, so the numbers are not interchangeable and the ratio between them is not constant across grades. Compare like with like: use the 230°C figure against other 230°C figures.
All values quoted are typical average values from a limited sample set, not a formal specification. Request the signed datasheet for current values.
Scope note. The guidance on this page is based on published mechanical properties - hardness, tensile strength, elongation, melt flow, specific gravity, base polymer and stabilisation. Food contact, skin contact, biocompatibility, flame retardancy, halogen-free formulation and electrical performance are not characterised in the public catalogue and require a separately qualified compound and testing. Contact us to qualify a grade against any of those.
Frequently Asked Questions
What MFI should I specify for TPE injection moulding?
It depends on wall thickness and flow length rather than on a universal number. Thin-wall parts and long flow paths need high flow - the injection grades in this catalogue reach 367 g/10 min at 230C. Thicker sections tolerate and often prefer a lower figure, which reduces flash and improves part consistency. The median across the injection grades here is 73 g/10 min.
What MFI should I specify for TPE profile extrusion?
Low. The extrusion grades in this catalogue run from 0.6 to 32 g/10 min at 230C with a median of 7.5. The more unsupported the profile is between the die and the cooling line, the lower the melt flow needs to be. For tall or thin-webbed profiles, look at the bottom of that range.
Can I use an injection grade on an extrusion line?
It is generally the wrong choice. An injection grade has far less melt strength, so the profile tends to sag or draw down after the die and loses dimensional accuracy. If you need one compound for both processes, ask about a grade qualified for both - IEU150A is the dual-process grade in this range.
Why are two MFI values published for each grade?
Because melt flow is only meaningful with its test condition attached. The two conditions here are 230C with a 2.16 kg load and 190C with a 5 kg load. Different temperatures and loads produce different numbers for the same material, and the relationship between them varies from grade to grade, so the values must be compared condition-for-condition.
Does higher melt flow mean a weaker part?
Not directly, but the two often move together within a polymer family because the flow is usually raised by lowering molecular weight or adding oil. In this catalogue the relationship is loose: grades with very high flow such as IF900A at 367 g/10 min still publish 5.5 MPa tensile strength. Check the tensile figure rather than inferring it from flow.
Related guides
- How to Choose a TPE Grade: A Specifier's Guide
- Choosing a TPE for Profile Extrusion
- TPE Grade Selection for Two-Shot Overmoulding
- TPE Shore Hardness: A Selection Guide From 15 Shore A to 52 Shore D
About the FARRPRENE range
FARRPRENE is the thermoplastic elastomer (TPE) compound range of Farr Polychem FZCO, a polymer compounder based in Jebel Ali Free Zone, Dubai, United Arab Emirates, supplying the UAE, the wider MENA region and Europe. The range covers injection moulding and extrusion compounds from soft gel-like Shore A grades through rigid Shore D grades, in unfilled translucent and filled natural variants, colour-matchable to a customer reference, with free samples and technical datasheets available on request. Browse the full grade catalogue, see every grade side by side on the comparison table, or request a sample.
FARRPRENE GRADES
Grades commonly specified for industrial applications
| GRADE | HARDNESS | TENSILE (MPa) | TYPICAL USE |
|---|---|---|---|
| IEU150AInjection / Extrusion / Unfilled | 15 Shore A | 3 | Vibration-damping pads and soft mounts |
| IF752AInjection / Filled | 75 Shore A | 5.5 | Bushings, spacers and wear components |
| IF900AInjection / Filled | 90 Shore A | 5.5 | Hard bushings and near-rigid flexible parts |
Typical average values from a limited sample set; not a formal specification. Request the signed datasheet for current values.
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