Machining Polysulfone — PSU and PPSU Guide
Polysulfone (PSU) and polyphenylsulfone (PPSU) machine well on standard metalworking equipment, but they require more careful technique than general-purpose engineering plastics like acetal or nylon. Both grades are stress-sensitive: they can develop subsurface stress cracks from aggressive tooling, inadequate chip clearance, or exposure to incompatible cutting fluids. Annealing stock before machining and using sharp carbide tooling with light feed rates are the two most important steps for producing accurate, crack-free parts.
At a glance:
- Anneal stock at 250–300°F for 2–4 hours before machining — always, not optional
- Use sharp C-2 carbide or HSS tooling; dull tools generate heat and stress
- Light feed rates and shallow depth of cut minimize cutting zone temperature
- No chlorinated coolants — use compressed air, water-soluble (non-chlorinated), or dry
- Isopropanol is not a recommended machining coolant for polysulfone
- Positive rake angles (5–15°) reduce cutting force and heat
- Minimum 0.030" inside corner radius to avoid stress concentrations
- Parts should be re-annealed after heavy stock removal if tight tolerances are required
Why Polysulfone Is Stress-Sensitive
Polysulfone is an amorphous thermoplastic with a relatively high glass transition temperature (374°F for PSU, 428°F for PPSU) and moderate impact resistance. Its stress sensitivity stems from two related characteristics:
Residual molding/extrusion stress: Sheet and rod are produced by compression molding or extrusion, both of which introduce internal stresses as the material cools under constraint. These stresses are not apparent from visual inspection but are released — sometimes violently as cracking — when the material is machined, exposed to solvents, or heated and cooled.
Environmental stress cracking: Polysulfone is susceptible to stress cracking in the presence of certain chemicals, including some cutting fluids. A combination of residual machining stress and chemical exposure can cause parts to crack hours or days after machining — a delayed failure mode that makes root cause analysis difficult.
The solution to both failure modes is annealing before machining and strictly avoiding incompatible coolants.
Pre-Machining Annealing
Why Anneal Before Machining
Stress-relief annealing at temperatures below the material's Tg relaxes internal residual stresses in the stock without melting or reshaping the part. Annealed stock produces more dimensionally accurate parts, reduces the risk of post-machining cracking, and improves surface finish quality.
This step is especially important for:
- Parts that will contact chemicals (disinfectants, solvents, even IPA cleaners) in service
- Precision-tolerance components where post-machining dimensional shift is unacceptable
- Parts with thin walls, sharp corners, or small features that concentrate stress during cutting
Annealing Procedure
Use a calibrated oven, not a heat gun or open flame. Slow cool-down (oven off, door closed) prevents introducing new thermal stresses during the cool cycle. For thick sections (over 1"), extend soak time proportionally.
Tooling Recommendations
Turning (Lathe)
Sharp tooling is non-negotiable. A slightly worn tool that works fine on aluminum or steel will generate excessive heat at the cutting edge when turning polysulfone, leading to local heat buildup, stress, and potential cracking or surface degradation. Replace or re-sharpen tools proactively.
Positive rake angles reduce cutting forces and heat generated at the tool-workpiece interface. Negative rake tools require higher cutting forces and generate more heat — avoid them for polysulfone.
Milling
Two-flute end mills provide better chip clearance than four-flute in plastics, reducing chip packing heat. Four-flute mills work at lower feed rates per tooth with proportionally reduced chip load.
Drilling
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Drill point geometry matters: a standard 118° point angle works adequately. Avoid drills with a flat or parabolic flute design intended for chip packing in soft metals — these can pack chips and generate heat in polysulfone. For precision bores, drill 0.010–0.015" undersized and ream to final diameter.
Coolant Selection — Critical
Acceptable Coolants
- Compressed air — preferred for short operations and finishing cuts; eliminates chemical compatibility concerns
- Water-soluble coolants — non-chlorinated, non-petroleum-based, compatible when used at recommended dilution ratios. Confirm with coolant supplier that the formulation does not contain chlorinated compounds.
- Dry machining — acceptable for light cuts and short cycle times where heat buildup is minimal
Not Acceptable
- Chlorinated cutting fluids — methylene chloride, chlorinated paraffin oils, and other chlorinated compounds attack polysulfone and cause stress cracking. This category includes some metalworking fluids that are not labeled as chlorinated solvents but contain chlorinated extreme-pressure additives.
- Aromatic or ketone-based coolants — toluene, acetone, MEK, and similar solvents dissolve or craze polysulfone
- Isopropanol (IPA) — not recommended as a coolant despite its common use in plastics machining; prolonged contact with stressed polysulfone can cause crazing
When in doubt about a coolant's compatibility, request the full SDS from the coolant supplier and cross-reference against polysulfone's chemical resistance list. The relevant concern is not the bulk solvent but the additive package — some water-soluble coolants contain small percentages of chlorinated extreme-pressure additives that can damage polysulfone.
Feature-Specific Guidance
Inside Corners and Radii
Design all inside corners with a minimum radius of 0.030". Sharp (0° radius) inside corners concentrate stress in both PSU and PPSU and are likely crack initiation sites, particularly in parts that will be autoclaved or exposed to disinfectant chemicals. Where design allows, use 0.060–0.125" radii.
Thread Cutting
Both internal and external threads can be cut in polysulfone. Use a chasing approach with sharp tooling and light passes rather than single-pass thread cutting. Standard thread geometry (60° included angle) works well. Tapped holes should use sharp taps and be cleared of chips between passes.
Self-tapping screws should be avoided in polysulfone — the stress from thread-forming can create crack initiation sites, especially in PPSU which has lower modulus than PSU. Use machine screws in tapped or through holes with nuts.
Thin Walls
Walls thinner than 0.060" require support fixtures to prevent vibration and deflection under cutting load. At these wall thicknesses, heat buildup is also a concern — use air cooling and light finishing cuts. Thin-walled polysulfone parts benefit from post-machining annealing to relieve any stresses introduced during cutting.
Post-Machining Annealing
For components that require tight tolerances after heavy stock removal, a secondary stress-relief anneal is recommended:
- Temperature: same as pre-machining anneal (250–300°F for PSU, 280–320°F for PPSU)
- Duration: 2 hours minimum, or 1 hour per inch of maximum section thickness
- Cooling: slow cool in oven
Dimensional measurements taken immediately after machining will shift slightly after annealing as residual cutting stresses relax. Measure and verify tolerances after the final anneal, not immediately after machining.
Polysulfone's machinability is comparable to polycarbonate and Ultem PEI in practice — all three are amorphous thermoplastics with similar cutting behavior. The key differentiator is chemical sensitivity: polysulfone's coolant restrictions are more stringent than acetal or nylon but comparable to polycarbonate. PEEK machines with slightly less stress sensitivity but is harder on tooling due to its semicrystalline structure and higher hardness.
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