Choosing a thermoplastic elastomer grade comes down to four decisions, in this order: the process you will run it on, the hardness the part needs, the base polymer the service environment demands, and whether a filled or unfilled compound suits your cost and finish targets. Hardness is the number most people start with, and it is the one that tells you least on its own. This guide walks the four decisions using published data from the 49-grade FARRPRENE catalogue.
Key takeaways
- Pick the process first. Melt flow separates injection and extrusion grades by roughly tenfold, and a grade built for one will disappoint on the other.
- Hardness does not predict strength. Across the 46 Shore A grades in this catalogue the correlation between hardness and tensile strength is only 0.57.
- SEBS and SBS are not interchangeable. SBS costs less and processes readily; SEBS resists UV, heat and oxidation far better.
- Specific gravity tells you the filler loading, which drives cost per part, dimensional stability and colour reproduction.
- Two grades at the same hardness can differ tenfold or more in melt flow, so the grade code matters as much as the durometer reading.
Decision 1: which process will run the part?
This is first because it eliminates roughly half the catalogue immediately. Injection moulding wants a compound that flows freely enough to fill the cavity before it freezes off. Extrusion wants the opposite: enough melt strength for the profile to hold its shape between the die and the cooling line.
That difference is visible in the data. Across this catalogue the injection grades have a median melt flow index of 73 g/10 min at 230°C/2.16 kg; the extrusion grades sit at 7.5. That is close to a tenfold separation, and it is why the grade code carries the process in its first letter - I for injection, E for extrusion, IE for a compound qualified for both.
| Process | Grades | MFI 230°C range | Median |
|---|---|---|---|
| Injection | 27 | 7.2 - 367 g/10 min | 73 |
| Extrusion | 21 | 0.6 - 32 g/10 min | 7.5 |
| Both | 1 (FARRPRENE IEU150A) | 25 g/10 min | 25 |
If you are extruding a tall, unsupported profile, you want the stiffer end of the extrusion range - FARRPRENE EU401A at 0.6 g/10 min or FARRPRENE EF753AV at 0.93. If you are filling a thin-wall injection tool with long flow paths, you want the other extreme: FARRPRENE IF900A at 367 g/10 min or FARRPRENE IF700A at 347.
Decision 2: what hardness does the part need?
Shore A hardness is a proxy for how the part feels and how it behaves under load. The practical bands:
| Band | Feel | Typical parts |
|---|---|---|
| 15 - 30 Shore A | Gel-like, deforms under thumb pressure | Cushioning pads, vibration damping, soft inserts |
| 35 - 50 Shore A | Soft rubbery | Compression gaskets, soft overmoulds, flexible closures |
| 55 - 70 Shore A | Firm but compliant | Handle grips, weatherstrip, seals, strain reliefs |
| 75 - 90 Shore A | Firm, load-bearing | Bushings, pedal pads, structural flexible parts |
| Shore D | Rigid | Rigid profiles, structural trim, semi-rigid sections |
One warning that catches people out: Shore A and Shore D are different scales and are not comparable. A compound at 40 Shore D is considerably harder than one at 90 Shore A. See the Shore D range if your part needs rigidity rather than flexibility.
Decision 3: SEBS or SBS?
This is the decision most specifiers never get asked to make, and the one most likely to cause a field failure. SBS - styrene-butadiene-styrene - keeps unsaturated double bonds in its butadiene mid-block. Hydrogenating those bonds converts SBS into SEBS, styrene-ethylene-butylene-styrene. Removing them substantially improves resistance to UV, heat and oxidation.
Five grades in this catalogue are SBS-based, identifiable by the S suffix: FARRPRENE IF658AS, FARRPRENE IF701AS, FARRPRENE IF750AS, FARRPRENE IF800AS and FARRPRENE IF901AS. They are economical and process readily. For a part that will see sunlight, warm service or a long design life, start from a SEBS grade instead.
Decision 4: filled or unfilled?
Specific gravity is the giveaway. Unfilled compounds in this catalogue sit between 0.880 and 0.900. Filled compounds run from 0.940 to 1.270, and the higher the figure the more mineral filler is present.
Unfilled grades - the U in the second position, supplied translucent - give the cleanest colour reproduction and more parts per kilogram. Filled grades, supplied natural, buy dimensional stability and a better cost position per part. If you are colour-matching to a customer reference, start from an unfilled grade such as FARRPRENE IU450A or FARRPRENE EU502A.
Putting it together
A worked example. Say you need a soft-touch overmould on a polypropylene handle, injection moulded, 55 Shore A, indoor use, colour matched to a brand reference.
- Process: injection, so the I-prefix grades.
- Hardness: 55 Shore A.
- Environment: indoor, so SEBS is comfortable but not critical - SEBS anyway for the ageing margin.
- Colour: matched, so unfilled and translucent.
That lands on FARRPRENE IU550A: 55 Shore A, injection, unfilled, translucent, 6 MPa tensile, UV and heat stabilised. Compare it against the full range on the grade comparison table.
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 is the most important property when selecting a TPE?
The intended process, because it eliminates most of the catalogue immediately. A compound formulated for extrusion has roughly a tenth of the melt flow of an injection grade, so running it in the wrong process gives either short shots or a profile that will not hold its shape. Hardness is the second decision, not the first.
Can I substitute one TPE grade for another at the same hardness?
Not safely. In this catalogue seven grades share 45 Shore A and their melt flow at 230C spans 1.5 to 148 g/10 min - a factor of about 99. They are formulated for different processes and tools. Match the process and the flow, not just the durometer reading.
How do I know whether a TPE grade is filled?
Check the specific gravity. Unfilled SEBS compounds sit around 0.88 to 0.90. Anything materially above that carries mineral filler, and the higher the figure the greater the loading. In the FARRPRENE grade codes the second letter tells you directly: U for unfilled, F for filled.
Does a higher tensile strength mean a harder grade?
No. Across the 46 Shore A grades here the correlation between hardness and tensile strength is only 0.57, which is a weak relationship. As an example, two grades at 90 Shore A differ by more than threefold in tensile strength. Hardness describes indentation resistance, not strength.
Which TPE grade should I use if I am replacing vulcanised rubber?
Start by matching the hardness and the process, then check the service environment. TPE eliminates the vulcanisation step, allows regrind and shortens cycle times. Vulcanised rubber still performs better at sustained high temperature and under long-term high compression, so evaluate against the real duty cycle before switching a primary seal.
Related guides
- TPE Melt Flow Index: Why Injection and Extrusion Grades Are Not Interchangeable
- TPE Shore Hardness: A Selection Guide From 15 Shore A to 52 Shore D
- SBS vs SEBS: Which TPE Base Polymer Your Part Needs
- Filled vs Unfilled TPE: What Specific Gravity Tells You
- Shore A vs Shore D TPE: When Your Part Needs to Be Rigid
- Why Hardness Does Not Predict TPE Strength
- UV and Heat Stabilised TPE: What the Callouts Mean
- TPE Grade Selection for Two-Shot Overmoulding
- Choosing a TPE for Profile Extrusion
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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