High-Performance Thermoplastics: PEEK, ULTEM and Other Polymers

• 1
  What Makes High-Performance Thermoplastics PEEK ULTEM Different
• 2
  PEEK: Semi-Crystalline, Strong, and Hard to Machine
• 3
  ULTEM (PEI): Amorphous, Stiff, and Dimensionally Stable
• 4
  PPSU, PPS, PTFE and Other Polymers in the Same Family
• 5
  Machining and Annealing Rules That Actually Hold Tolerances
• 6
  Property Comparison of High-Performance Thermoplastics PEEK ULTEM and Others

Commodity polymers like PP, PE and ABS soften in the 80–120 °C range and creep under modest continuous load. High-performance thermoplastics PEEK ULTEM grades hold their shape far above that. The usual working definition is a continuous service temperature above 150 °C, plus a combination of strength, stiffness and chemical resistance that commodity resins cannot reach.

The difference is chemistry, not additives. PEEK, PEI, PPSU, PPS and PTFE all carry rigid aromatic rings in the polymer backbone. Those rings raise the energy needed to move chains past each other, which is why the material stays stiff when hot. Semi-crystalline grades such as PEEK and PPS also form ordered regions that resist creep and fatigue.

That structure costs money. High-performance thermoplastics PEEK ULTEM stock shapes sell for 20 to 60 times the price of POM or PA. So the first question on any project is not which polymer is strongest, it is whether a commodity resin would actually fail. If the answer is no, the high-performance grade is wasted budget.

Three failure modes justify the upgrade: continuous heat above the commodity ceiling, exposure to solvents or steam that attack ordinary plastics, and a need for dimensional stability under load for thousands of hours. Electrical insulation at temperature and low outgassing in vacuum are two more. Everything else usually points back to POM, PA or PC.

PEEK is a semi-crystalline polyether ether ketone with a melting point near 343 °C and a glass transition around 143 °C. Unfilled grades reach roughly 90–100 MPa tensile strength, and 30% glass or carbon fiber versions push stiffness well past that. It keeps useful mechanical properties up to about 250 °C continuous, and short-term excursions higher.

Chemical resistance is the headline. PEEK survives most organic solvents, fuels, hydraulic fluids and hot water, and it resists hydrolysis in steam. Only aggressive acids such as concentrated sulfuric acid and some halogenated compounds attack it. That combination is why it appears in pump seals, valve seats, bearing cages and semiconductor fixtures.

Machining PEEK punishes bad setups. The material conducts heat poorly, so cutting heat stays at the edge instead of flowing into the chip. Carbide tools with sharp, polished flutes, high positive rake and generous clearance work best. Run 150–400 m/min surface speed for roughing, slower for finishing, and keep feed per tooth high enough to avoid rubbing.

Never let the tool dwell. Rubbing generates local heat above the glass transition, and the surface smears instead of cutting. Use air blast or high-pressure coolant to clear chips, because PEEK swarf is light and packs into pockets. Rough, stress-relieve, then finish. Annealing between roughing and finishing typically runs 150–200 °C for 2–4 hours depending on section.

ULTEM is a trade name for polyetherimide, an amorphous polymer with a glass transition near 217 °C. Because it is amorphous, it has no sharp melting point and no crystalline phase to scatter light, so natural grades are transparent amber. It holds modulus well up to roughly 170 °C continuous.

The amorphous structure brings a real advantage on the machine. ULTEM shrinks far less and more uniformly than PEEK, so tight tolerances hold with less trial and error. It also has excellent creep resistance at elevated temperature, which is why it shows up in structural brackets, connectors and aircraft interior parts.

What it does not have is solvent resistance. PEI is attacked by partially halogenated solvents, some ketones and strong bases, and it stress-cracks under load in those environments. It also absorbs moisture, so parts exposed to steam or hot water degrade over time. Choose it for heat and stiffness, not for chemical service.

Cutting ULTEM is easier than PEEK but still abrasive on tooling. Surface speeds of 200–500 m/min with carbide and sharp geometry work well. The main risk is stress cracking after machining, driven by residual stress and contact with cleaning solvents. Anneal at 200–220 °C, and check which degreaser the shop plans to use before the parts are washed.

PPSU (polyphenylsulfone) sits close to PEEK in toughness and steam resistance but costs less. It takes repeated steam sterilization without crazing, which is why it dominates medical trays, surgical handles and sterilisable housings. Continuous use tops out near 180 °C. It is opaque amber and easier to machine than PEEK.

PPS (polyphenylene sulfide) is a semi-crystalline sulfur polymer with good chemical resistance and a continuous ceiling around 220 °C. Unfilled PPS is brittle, so glass or mineral filled grades are the norm for pump housings and under-hood parts. It machines well but generates fine dust that needs extraction.

PTFE is the chemical resistance champion and has the lowest friction of any common polymer, with a service range from about -200 °C to 260 °C. It is also soft, creeps badly under load and has poor wear resistance. Virgin PTFE should not be used as a structural part. Filled grades with glass, carbon or bronze improve stiffness and wear.

Other polymers worth knowing include PAI (Torlon) for high-strength bearing components up to about 250 °C, PBI for extreme heat near 300 °C, and PSU for lower-cost transparent parts. Each one trades something away. There is no single best polymer, only the one that matches the dominant failure mode of the part.

Plastics move after machining. PEEK and ULTEM both relieve internal stress when material is removed, and the part bends toward the cut side. The fix is a roughing pass that leaves 0.5–1.0 mm of stock, an annealing cycle, then a finishing pass. Skipping the anneal is the most common reason a polymer part fails inspection a day later.

Fixturing matters as much as the tool. These materials are elastic, so normal vise pressure can distort a thin wall before the cutter touches it. Use soft jaws machined to the part profile, vacuum chucks for flat plates, and light clamping with support underneath. Measure after the part is unclamped and at room temperature.

Cooling keeps the edge alive. Air blast handles most PEEK and ULTEM work. High-pressure coolant helps clear chips in deep pockets but must be dried thoroughly afterward, because both polymers absorb moisture and can swell or stain. For PEEK, a coolant-free setup is often the safer route.

Thermal expansion is not a small effect. PEEK expands roughly 47 × 10⁻⁶ per °C, ULTEM around 56 × 10⁻⁶ per °C, both several times steel. A 100 mm PEEK part measured at 20 °C and inspected at 25 °C differs by about 0.024 mm. Hold parts to the drawing temperature, or the tolerance callout is meaningless on the shop floor.