PSU Injection Molding: Sterilizable Medical Material Guide

Shot 7 of a 16-cavity PSU valve mold tore 0.5 mm deep at the knit line, and cavity 12 filled only 80%. The tool carried a cold runner and 0.6 mm gates on a cavity held near 60 °C. That layout runs fine for nylon and fails for polysulfone. PSU needs a hot cavity, full runners, and gates sized for a thick melt.

PSU, or polysulfone, is an amorphous high-heat thermoplastic sold as Udel by Solvay and Ultrason S by BASF. Its glass transition (Tg) sits near 185 °C, with HDT at 170 to 175 °C under a 1.82 MPa load. Continuous service lands near 150 °C, which trails PEI and sits far below PEEK. Amorphous means no crystalline melting point. Parts get low, isotropic shrinkage, traded against a thick melt and a mold that must run hot.

This guide reads PSU from the mold side for mold buyers, tooling engineers, and manufacturing engineers. It covers the properties that set cavity compensation, the hot-mold design rules, the processing window, and defects traced to mold-side causes. Values are typical published figures for injection-molding grades. Verify your grade datasheet before cutting steel.

The Snapshot

• PSU is amorphous with a Tg near 185 °C, HDT of 170 to 175 °C at 1.82 MPa, and continuous service near 150 °C.
• PSU parts survive 1,000+ autoclave cycles at 134 °C, and gamma doses near 25 kGy pass without chemical attack.
• Run the melt at 330 to 380 °C, typically 340 to 370 °C, with a mold at 100 to 160 °C. Dry pellets 3 to 5 hours at 120 to 150 °C to under 0.02% moisture.
• Mold shrinkage is 0.6 to 0.7%, low and isotropic, with some grades at 0.5 to 0.8%. Unfilled density is 1.24 g/cm³ and tensile runs 70 to 80 MPa.
• High melt viscosity shortens flow, so gates stay large, runners full, and parting-line vents 0.02 to 0.04 mm deep.

What Is PSU and Why It Molds Differently

Polysulfone's backbone alternates sulfone, ether, and phenylene groups. That backbone resists hydrolysis, which is why PSU dominates steam-sterilization hardware. It also survives acids, bases, and hospital detergents across most cleaning pH ranges.

For the mold designer, two numbers matter most. The Tg near 185 °C sets how hot a part runs before it softens. The HDT of 170 to 175 °C at 1.82 MPa shows where it sags under load. Mold shrinkage runs low at 0.6 to 0.7%, about half of a semicrystalline resin like PEEK.

PSU molds differently because the melt is thick and the cavity must be hot. Viscosity stays high across the whole window, so flow resistance is a design input, not an accident. Flow length per unit of pressure stays short, and thin walls demand big gates. Amorphous parts also keep weak weld lines, because no crystalline phase grows back to heal the knit.

Grades split into unfilled and glass-reinforced types. Unfilled PSU is light amber and transparent, which helps inspection see through fluid paths. At 30% glass, parts turn opaque, density rises to about 1.45 g/cm³, and stiffness climbs at the cost of abrasive wear on steel. Shrinkage drops with glass, so never cut a cavity with unfilled numbers on a filled grade.

Part measurement adds a second temperature lesson. A PSU part ejected at 140 °C keeps shrinking as it cools to the 23 °C measurement standard. Measure after full conditioning, and compare against the shrinkage rate used in the cavity. Gate freeze, packing pressure, and mold temperature all shift final dimensions by fractions of a percent.

Properties That Drive Mold Design

Steam autoclaving is where PSU earns its medical role. Parts hold up through 1,000+ cycles at typical 134 °C conditions, which covers years of instrument reprocessing. Gamma sterilization at standard 25 kGy doses and EtO exposure both pass without chemical attack. Repeated gamma can darken the amber tint, but mechanical properties hold.

Hydrolysis resistance is excellent, so long hot-water exposure does not crack or weaken PSU. Detergents, disinfectants, saline, and steam condensate all sit inside its resistance envelope. The exception is solvent stress cracking. Ketones, chlorinated solvents, and some aromatic hydrocarbons attack stressed parts, so confirm cleaning agents early in the program.

Notch sensitivity is the mechanical trap. Unnotched impact is tough, while notched Izod lands near 70 J/m on typical unfilled grades. A sharp corner or a 0.1 mm tool mark concentrates stress exactly where parts fail. Design internal radii of 0.5 mm or more and keep ejector scars off load paths. A polished ejector pin face also removes the stress raiser that starts cracks on thin ribs.

Tensile strength runs 70 to 80 MPa unfilled, enough for structural housings at normal wall thickness. PSU also earns V-0 flammability and keeps stable dielectric properties, so electrical housings share the same tooling logic. Wall thickness does the rest: keep sections uniform and transitions gradual.

Dimensional behavior is friendly once the cavity compensates. Shrinkage of 0.6 to 0.7% is low and isotropic, which simplifies cavity math. Thermal expansion runs 4 to 5 times tool steel, which sets hot-fit clearances for slides, pins, and inserts. Amorphous parts shrink evenly in all directions, unlike glass-filled semicrystalline resins. Creep at 150 °C continuous duty is low for an amorphous resin, but validate rib geometry under long-term load.

Sterilization validation belongs in the mold conversation. Gamma and EtO cycles are process steps, but autoclave cycling is a fatigue test on molded-in stress. Parts molded on a cold cavity develop microcracks that only appear after cycle 200. Mold-side records of cavity temperature and gate balance become evidence for the sterilization dossier.

Mold Design Considerations

A PSU mold is a hot mold, and every component must tolerate the heat. Cavity and core run at 100 to 160 °C, so standard low-temperature O-rings and seals fail fast. Fit oil, pressurized-water, or cartridge heating to the cavity plate. Insulation plates between mold and platens cut heat loss and protect machine hydraulics and sensors.

Insulation plates earn their cost on high-cavity tools. A plate of 10 to 20 mm holds cavity temperature steady against a machine bed near ambient. Keep water fittings, sensors, and connector routing off the hottest faces. Thermal breaks also protect the ejector assembly, which cycles between a 100 to 160 °C cavity and a cool machine.

Ejection happens hot. Parts leave the cavity at 100 to 160 °C, well above room temperature. Plan draft of 1 to 2 degrees per side on vertical walls. Ejector pins need polished surfaces and generous area, because a warm amorphous part grips steel more than a cooled one. Add air-assist or larger pins on deep boxes, where vacuum holds the part on the core.

Glass-filled grades change the steel specification. Thirty-percent glass compounds abrade soft cavity steel and round gate edges over production. Run hardened tool steel in cavities, cores, and gate inserts, with wear coatings on slides. DieStrike builds these tools with hardened cavity steel at HRC 62 for abrasive medical grades.

Gate and runner design follows melt viscosity. Use full-round runners sized for the flow length, with draft so the shot releases cleanly. Edge and tunnel gates need generous lands and a depth of 60 to 80% of wall thickness. Gate location decides weld lines, so place the knit away from seal faces and loaded features.

Venting is a quality gate for PSU. Vent depth runs 0.02 to 0.04 mm at the parting line, and shallower on ejector pins. Undersized vents cause burn marks and short shots at these melt temperatures. Keep vent lands polished and short, because PSU flash welds to the part and resists removal. Vent land width of 3 to 5 mm is typical on medical tools, and the land is polished like the cavity face.

Hot operation changes fits from the drawing. A cavity and slide fitted at 25 °C moves when the mold runs near 140 °C. Allow thermal expansion in slide clearances, ejector alignment, and core-pin fits, or parts stick and pins bind. Cooling layout must keep cavity temperature even, because a 20 °C gradient across the plate bows every part the same way.

Shrinkage compensation is where tolerance lives. Cavity steel is cut at room temperature for a part that cools from 140 °C. The compensation number comes from the grade datasheet, not from habit. Molders who reuse nylon numbers on PSU cut every cavity undersized. Our 5 shrinkage compensation rules walk through cavity math for rates from 0.3% to 2.0%.

Hot-runner systems shorten cycles and open gate placement options. PSU runs through open or valve gates when the manifold keeps no dead spots above 380 °C. If the program cannot justify a hot runner, keep the cold runner large and the sprue drafted. Either way, shrinkage compensation follows the grade data, and cavity math decides the tolerance that survives validation. Gate balance across 8 to 32 cavities decides part-to-part consistency, so flow analysis pays for itself.

Processing Window

The processing window is wide but unforgiving at its edges. Melt temperature runs 330 to 380 °C, with 340 to 370 °C the typical working range. Below 330 °C the melt is too stiff to fill thin sections. Above 380 °C, residence time must stay short or the resin degrades.

Mold temperature holds 100 to 160 °C, matched to wall thickness and flow length. Cold molds freeze in stress, weaken weld lines, and shift dimensions. Hot molds relax orientation, fill thin walls, and reproduce the shrinkage the cavity assumed.

Drying is non-negotiable for PSU. Dry pellets 3 to 5 hours at 120 to 150 °C to under 0.02% moisture. Wet resin hydrolyzes at melt temperature and shows up as splay, streaks, and brittleness. An under-dried silo can scrap a full shift before the first part is read.

Residence time is the quiet killer in PSU molding. Balance shots so melt does not linger above 380 °C in barrel or manifold. Injection pressure runs high because viscosity is high, so machine capacity must cover the fill. DieStrike stocks standard hot-tool components and ships them in 3 to 7 days for fast-track programs.

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Table: Typical published processing values for PSU injection-molding grades. Confirm with your grade datasheet before cutting steel.

A cavity pressure curve is the best PSU process monitor. Repeatable fill and pack curves predict dimension and autoclave survival better than shot weight alone.

Common PSU Applications: Sterilizable Medical Devices

Sterilizable medical devices are PSU's home turf. Typical parts include instrument trays and lids, reprocessing valve bodies, fluid-path housings, and surgical instrument handles. They must hold dimensions and toughness through 1,000+ autoclave cycles, so the resin debate closes early. The debate that remains is the mold.

In medical device tooling, every PSU part is a hot-mold problem first. Gates size for thick walls, weld lines move off seal faces, and draft suits ejection at 100 to 160 °C. Tray walls often run 2 to 3 mm, so flow length and gate count are settled before steel is ordered. Surface finish inside fluid paths mirrors the cavity, so polish the steel to Ra 0.4 µm or better where flow contact matters.

Tray and lid tooling follows the steel logic in our medical mold steel guide, because thin walls and wet autoclave duty favor corrosion-resistant cavities. Deep fluid ports push core pin specification, with 0.02 to 0.04 mm venting and polished surfaces against sticking. Where a device adds a soft seal, the rigid PSU half and the silicone half are molded to different rules.

Defects: Mold-Side Root Causes

PSU defects trace to four mold-side root causes: cold cavities, small gates, thin vents, and wet pellets. When a defect shows up, inspect the tool before blaming the resin.

Short shots follow restricted flow. Undersized gates, skinny runners, or a cavity below 100 °C stop the melt early. Fix with larger gates, full-round runners, and a hotter cavity. A quick flow check on runner diameter before steel saves a full tool revision later.

Flash follows vents cut too deep or a worn shutoff. Hold parting-line vents at 0.02 to 0.04 mm and re-cut lands that have opened up. Check clamp tonnage too, because PSU fills at high injection pressure.

Weak weld lines follow gate placement and a cold mold. Amorphous PSU cannot heal a knit, so treat every weld as a design seam. Move gates to push the weld off seal faces. Raising mold temperature toward 140 to 160 °C also strengthens the knit.

Splay, streaks, and brittleness usually mean wet resin. Moisture under 0.02% is the target; hydrolysis at 340 °C and above turns pellets into bubbles and short chains. Re-dry the material, check the dryer, and purge the barrel after any idle period.

Black specks follow degraded resin. PSU holds above 330 °C for the whole cycle, so manifold dead spots and long residence carbonize droplets. Purge with a cleaning compound and open every stagnant flow path.

Gate blush and stress marks follow high shear at a small gate. Enlarge the gate and polish the land, because rough steel tears the melt at 340 °C. Cold cavities add frozen-in stress, and parts later crack at the gate line.

Sticking and pin marks follow hot ejection. Parts leaving at 120 °C and above need draft of 1 degree or more and mirror-polished cores. Add ejector area on deep ribs, and time ejection so the part releases before it grips.

Warpage follows uneven cooling in a hot mold. A cavity at 160 °C on one side and 100 °C on the other bows every part identically. Balance the heating layout and keep thick-to-thin transitions under a 2 to 1 ratio.

Autoclave microcracks are the late-stage defect. Parts that look clean at trial crack after cycle 200 when molded-in stress was high. Hot cavities, balanced packing, and generous radii are the only mold-side defenses. Every fix in this section is a drawing change, which is why PSU quality starts at mold design, not at the press.

PSU vs PEI vs PEEK: Choosing a Sterilizable Resin

PSU is the entry point among the three sterilizable workhorses. PEI (Ultem) and PEEK cost more, and each earns its premium in heat and load. This comparison uses typical published values for unfilled grades.

PEI is the closest neighbor to PSU. It shares the amorphous behavior and adds roughly 30 °C of Tg, with HDT near 200 to 210 °C. Choose PEI when parts sit close to heat sources or carry higher continuous loads. Its mold rules are nearly identical, so our PEI material guide pairs with this one.

PEEK is a different animal. It is semicrystalline, with a melting point near 343 °C and continuous service to 250 °C and beyond. PEEK wins on chemical resistance, wear, and hot strength, but its resin price runs far above PSU. Its shrinkage of 1.2 to 2.0% also makes cavity math harder than PSU's 0.6 to 0.7%. Steam sterilization is the overlap zone, and PEEK enters when PSU cannot hold load at temperature.

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Table: Typical published values for unfilled injection-molding grades. PEI and PEEK columns reflect figures covered in our PEI and PEEK guides. Resin cost is a typical market range and varies by grade, volume, and region.

For a sterilizable device at room-to-warm service, PSU usually wins on cost and processability. Pick PEI when the device sees sustained heat near 170 °C class duty. Pick PEEK when nothing else holds at 250 °C or in aggressive chemicals. Price tiers follow the same order. PSU is the baseline, PEI runs 2 to 4 times its price, and PEEK runs 5 to 10 times.

The mold verdict stays simple. All three resins need hot, hardened, well-vented tools. PSU asks the least of the steel but still demands the full hot-mold discipline. Buyers who compare datasheets without comparing mold requirements miss the real cost driver. Ask the molder for the grade's published shrinkage and HDT figures up front.

FAQ: PSU Injection Molding

Q1. How many autoclave cycles can PSU parts survive?

Reports and datasheets cite 1,000+ cycles for PSU parts under typical 134 °C steam conditions. Cycle count depends on part geometry, molded-in stress, and water quality, so validate your own device. Dimensional checks every 100 cycles catch drift before it matters.

Q2. Why must a PSU mold run at 100 to 160 °C?

A cold cavity freezes in stress and produces weak welds, warpage, and off-spec shrinkage. PSU's Tg sits near 185 °C, so the part must cool slowly enough to relax orientation. A hot mold also lets a thick melt fill thin walls before it freezes off.

Q3. What shrinkage do I use to compensate the cavity?

Start with 0.6 to 0.7% for typical unfilled grades, then confirm the exact value on the grade datasheet. Some grades list 0.5 to 0.8%, and glass-reinforced versions shrink less. PSU shrinks isotropically, so the same rate applies across the cavity when flow is balanced. Record the compensated cavity size on the drawing so trials and validation compare against the same number.

Q4. Can I cool a PSU mold with plain water circuits?

Not above 100 °C, where water boils at atmospheric pressure. The 100 to 160 °C range needs pressurized water, oil, or cartridge heaters. Whatever the medium, keep cavity temperature even, because a 10 to 20 °C hot spot shows up as warp or sink.

Q5. Why do PSU parts crack at corners and after cleaning?

PSU is notch sensitive, so sharp corners concentrate every load. Design internal radii of 0.5 mm or more and avoid gate or ejector marks at stress points. Cracks after cleaning point to solvent stress cracking, so verify detergents against the resin's chemical resistance guide. Annealing trials on a rejected lot can confirm whether stress or chemistry caused the cracks.

The Bottom Line

PSU is the sterilizable resin that asks the mold to behave. The whole game is a hot cavity at 100 to 160 °C, gates sized for a thick melt, and vents at 0.02 to 0.04 mm.

Compensate cavities for 0.6 to 0.7% shrinkage, dry to under 0.02% moisture, and 1,000+ autoclave cycles take care of themselves. Mold buyers who specify these numbers up front skip the trial failures that repurposed nylon tools deliver. Ask for the mold book: cavity temperature records, vent depths, and gate balance data prove the tool was built for PSU.

DieStrike builds PSU and medical tooling under IATF 16949 across 120+ machines. Molds ship in 2 to 5 weeks with geometry at ±0.005 mm standard and ±0.002 mm precision. Connector OEMs including TE Connectivity, Amphenol, and Luxshare run programs on the same discipline.