Polysulfone (PSU) Injection Moulding: Properties & Design Guide
Polysulfone (PSU) is an amorphous, semi-transparent high-performance thermoplastic with a continuous service temperature of 150–160°C and exceptional resistance to steam, hot water, and chemical disinfectants. It is the first choice for injection-moulded components that must survive repeated autoclave or boiling-water sterilisation cycles without dimensional change, stress cracking, or loss of transparency. Nordmould recommends PSU for medical instrument components, laboratory equipment, and water treatment fittings where PC reaches its thermal and hydrolytic limits.
What are the mechanical and thermal properties of polysulfone?
PSU belongs to the sulfone polymer family, which includes PESU (polyethersulfone) and PPSU (polyphenylsulfone) as higher-performance variants. All three are amorphous, share the sulfone linkage that delivers thermal and hydrolytic stability, and can be processed on standard high-temperature injection-moulding equipment. The table below covers standard PSU alongside its higher-performing relatives for direct comparison.
| Property | PSU | PESU | PPSU | Test Standard |
|---|---|---|---|---|
| Tensile Strength | 70–75 MPa | 83–90 MPa | 70–73 MPa | ISO 527 |
| Flexural Modulus | 2,700 MPa | 2,600 MPa | 2,400 MPa | ISO 178 |
| Izod Impact (notched) | 65 J/m | 80 J/m | ~80 J/m | ISO 180 |
| Heat Deflection Temp (1.82 MPa) | 174°C | 203°C | 207°C | ISO 75 |
| Continuous Use Temp | 150–160°C | 180°C | 180–220°C | — |
| Density | 1.24 g/cm³ | 1.37 g/cm³ | 1.29 g/cm³ | ISO 1183 |
| Mould Shrinkage | 0.5–0.7% | 0.5–0.7% | 0.6–0.7% | ISO 294-4 |
| Water Absorption (24 h) | 0.30% | 0.43% | 0.37% | ISO 62 |
| Flame Rating | UL 94 V-0 | UL 94 V-0 | UL 94 V-0 | UL 94 |
PPSU's impact strength is substantially higher than standard PSU, making it the preferred choice in the family for autoclaved medical components that must also survive mechanical handling loads.
Where is polysulfone injection moulding used?
PSU's hydrolytic stability, steam resistance, and compliance with medical and water-contact regulations define its core application areas. It occupies a niche between PC (lower thermal limit) and PEI or PEEK (higher cost, higher temperature).
Medical and dental: Sterilisable instrument handles and trays, dental processing equipment, fluid manifolds, dialysis machine components, surgical scope components, and any housing that cycles repeatedly through 134°C steam autoclaving. PSU and PPSU are among the few polymers that maintain dimensions and surface quality through hundreds of sterilisation cycles.
Laboratory and analytical equipment: Cuvettes, centrifuge rotors, filter holders, chromatography components, and pipette handles. The combination of chemical resistance, thermal stability, and transparency (for visual inspection) is well-matched to laboratory environments.
Plumbing and water treatment: Valve bodies, fitting components, and membrane filter housings for hot-water circuits. NSF 51 and NSF 61 compliant PSU grades are qualified for drinking water contact at elevated temperatures. PSU withstands continuous contact with hot water at 95°C+ where cheaper materials fail.
Aircraft and aerospace interiors: Overhead bin latches, seat components, and cabin fittings that must meet FAR 25.853 flame, smoke, and toxicity requirements. PSU's inherent UL 94 V-0 rating and low smoke generation qualify it for these applications.
Electronics: Circuit board substrates, high-temperature connector housings, and coil formers where dimensional stability under thermal cycling is required.
What are the moulding characteristics of polysulfone?
PSU is a processable high-temperature amorphous thermoplastic. Its amorphous nature means that, unlike PEEK or PPS, there is no critical mould temperature for crystallisation — this simplifies process development. The key challenges are its high melt temperature, moisture sensitivity, and tendency toward stress cracking in service under solvent exposure.
Melt temperature: 320–380°C. Temperatures above 390°C produce thermal degradation evidenced by discolouration. Purge sequences between PSU and lower-temperature polymers are required to avoid degraded material.
Mould temperature: 70–120°C. Higher mould temperatures (100–120°C) reduce internal residual stress significantly and are recommended for parts that will be exposed to solvents or chemicals in service. Reduced residual stress substantially lowers stress cracking susceptibility. Heated tooling (water or oil circuits) should be used for parts above 50 g or where thin wall sections require good flow.
Injection pressure: Moderate — PSU flows well at melt temperature. Moderate fill rates are recommended; high-speed filling through thin gates increases shear stress and can produce a stress-cracking-sensitive microstructure in the gate area.
Drying: 135–150°C for 3–4 hours to below 0.05% moisture. Dehumidifying dryer required; hot-air ovens are not adequate for PSU.
Tooling: Hardened P20 or H13 tool steel is recommended. PSU's high processing temperature accelerates wear in aluminium tools. Gate land areas must be hardened. Sharp corners in the gate should be avoided as PSU is sensitive to stress concentrations.
Shrinkage: 0.5–0.7%, isotropic due to amorphous structure. Predictable and consistent — a key advantage for precision assemblies such as filter housings and instrument components with mating seals.
Post-moulding annealing: For parts exposed to aggressive chemicals or high residual stress, annealing at 150–160°C for 1–2 hours after moulding significantly reduces stress-cracking risk. This step is commonly specified for medical PSU components.
Which polysulfone grades and variants are available?
| Grade | Key Feature | Primary Application |
|---|---|---|
| Standard PSU | Baseline, amber-transparent, ISO 10993 | Medical instruments, water fittings |
| Medical PSU | Low extractables, validated | Sterilisation equipment, medical devices |
| GF-PSU (10–30%) | Higher stiffness, lower creep | Structural housings, precision brackets |
| PESU | Higher HDT (~203°C), better chemical resistance | Continuous higher-temp service |
| PPSU | Higher toughness, best hydrolytic stability | High-impact medical components |
| NSF-compliant PSU | Hot water contact rated (NSF 51/61) | Plumbing, water treatment |
The choice between PSU, PESU, and PPSU is primarily determined by temperature requirement and impact loading. PSU is the first choice when cost matters and temperatures remain under 150°C continuous. PPSU is specified when impact resistance and repeated sterilisation cycles at 134°C are both required — the premium over standard PSU is justified in reusable medical devices that must survive hundreds of cleaning cycles in hospital use.
What are polysulfone's advantages and limitations?
Advantages:
- Outstanding hydrolytic stability — survives thousands of autoclave sterilisation cycles at 134°C
- Continuous service temperature 150–160°C — significantly above PC, nylon, and POM
- Natural semi-transparency useful for visual inspection windows and fluid-level indicators
- UL 94 V-0 inherently across all grades
- Predictable, isotropic mould shrinkage simplifies tolerance management
- Broad compliance: ISO 10993, USP Class VI, NSF 51/61 grades available
- Low creep under sustained load at elevated temperatures compared to PC
Limitations:
- Poor UV resistance — yellows outdoors without stabilisation or coating
- Susceptible to stress cracking with aromatic solvents, ketones, and esters — chemical compatibility must be verified
- High melt temperature (320–380°C) requires high-spec equipment
- More hygroscopic than PEEK or PPS — stringent drying required
- Higher material cost than PC, nylon, and POM
- Amber colour tint limits truly optical-clarity applications
When should you choose PSU over alternative materials?
PSU vs PC: Choose PSU when the part must withstand repeated steam sterilisation at 134°C, hot water above 100°C continuous, or chemical disinfectant exposure. PC's HDT of 130–145°C and hydrolytic degradation under prolonged steam make it unsuitable for autoclave-cycled components. Choose PC when cost is the primary driver and sterilisation is not required.
PSU vs PEI: PEI offers a higher continuous temperature (170°C vs 150°C) and better structural stiffness. Choose PEI when the application also requires high structural loading or a higher temperature ceiling. Choose PSU when natural transparency is needed, or when cost is a meaningful factor between the two.
PSU vs PEEK: PEEK offers higher temperature, broader chemical resistance, and biocompatible implant-grade options. PSU is a cost-effective alternative for applications where the service temperature does not exceed 150°C continuously. For long-cycle medical devices or applications with specific ketone or halogenated solvent exposure, PEEK should be evaluated.
PSU vs PPSU: If the application involves impact loading combined with repeated autoclaving — typical of reusable surgical instruments — PPSU's substantially higher impact strength over PSU justifies the premium.
Is polysulfone recyclable?
PSU is not collected in consumer recycling streams. Industrial regrind from clean production scrap can be reprocessed at up to 20–25% blend with virgin material for non-medical applications. For medical-grade PSU, regrind is generally not used in production components without full re-qualification. The material's long service life in medical and industrial applications partially offsets end-of-life limitations from a lifecycle perspective. Nordmould can advise on sustainable sourcing options and alternative materials where end-of-life recyclability is a design requirement.
Frequently asked questions
Why is polysulfone (PSU) used in medical device components?
PSU is transparent, dimensionally stable under repeated steam sterilisation and chemical disinfection, and meets ISO 10993 biocompatibility requirements. Its continuous use temperature of 160°C means it survives autoclave conditions (134°C) without creep or deformation. Nordmould can source medical-grade PSU for instrument housings, fluid manifolds, and sterilisation trays.
What is the continuous use temperature of polysulfone?
Standard PSU (bisphenol-A polysulfone) has a continuous use temperature of approximately 150–160°C and a heat deflection temperature of 174°C at 1.82 MPa. Higher-grade polysulfones in the same family — PESU and PPSU — extend this to 180–220°C and offer improved toughness.
Does polysulfone need to be dried before injection moulding?
Yes — PSU is hygroscopic. Drying at 135–150°C for 3–4 hours in a dehumidifying dryer to below 0.05% moisture is required before processing. Undried PSU produces splay, reduced toughness, and surface defects. As an amorphous material it is sensitive to hydrolytic degradation at melt temperatures if moisture is present.
What are the melt and mould temperatures for PSU injection moulding?
PSU melt temperature is typically 320–380°C. Mould temperature of 70–120°C is recommended. Higher mould temperatures improve surface finish and reduce residual stress. PSU is amorphous so there is no crystallisation threshold; mould temperature can be adjusted within this range to balance cycle time and part quality.
How does PSU differ from PESU and PPSU?
PSU (bisphenol-A polysulfone) is the entry-level grade — lowest cost, adequate for 150°C continuous, good transparency. PESU (polyethersulfone, e.g. Solvay Veradel) raises the HDT to around 210°C and improves chemical resistance. PPSU (polyphenylsulfone, e.g. Solvay Radel R) offers the highest toughness and best hydrolytic stability of the three, at the highest cost.
What are the limitations of polysulfone for outdoor or solvent-exposed applications?
PSU has poor UV resistance and yellows on prolonged outdoor exposure — UV-stabilised grades or protective coatings are required. It is also susceptible to stress cracking in contact with aromatic hydrocarbons, ketones, and esters. Residual moulding stress combined with solvent contact is the most common failure mode in service.
Is polysulfone optically transparent?
Yes — unfilled PSU is naturally translucent to amber-transparent. It is not glass-clear in thick sections due to the amber tint, but provides sufficient transparency for visual inspection windows, fluid-level indicators, and laboratory instrument components. True optical-clarity applications typically use PC or PMMA instead.
What are the main applications of polysulfone injection moulding?
PSU is used in medical sterilisation trays and instrument handles, laboratory and analytical equipment, plumbing and water treatment fittings (NSF 51/61 compliant grades), aircraft interior components, and membrane filtration housings. Its hydrolytic stability and steam resistance define most of its application space.
Submit your STEP file to Nordmould for a free DFM review — our engineers will confirm whether PSU, PESU, or PPSU is the right grade and identify any stress-cracking or thermal risks early in the design process.