PPS and PEEK Injection Molding: Hot Moulds and Crystallinity

When the material costs more than the machine time

PPS (polyphenylene sulfide) and PEEK (polyether ether ketone) sit at the top of the engineering thermoplastics ladder. PPS holds its properties to 200 °C continuously and is inherently flame retardant. PEEK goes to 250 °C, has outstanding chemical resistance, and is one of the few polymers acceptable for repeated autoclave sterilisation and for direct contact with aggressive solvents.

The economics are unlike any other material in this series. PEEK costs €70 – €120 per kilogram; PPS €12 – €25. A 20 g part in PEEK therefore carries €1.40 – €2.40 of material before any processing. At that ratio, the machine time and the tooling are almost irrelevant — what matters is yield. A 5 % scrap rate on a PEEK part can cost more than the moulding.

PPS versus PEEK

Table 1 — PPS and PEEK side by side. The cost difference is the reason PPS is used wherever it is good enough.

The decision rule is straightforward: use PPS unless you need above 220 °C, autoclave sterilisation, or resistance to a specific aggressive chemical. PPS is inherently V-0 without additives, which is a significant advantage — flame-retardant PEEK grades exist but are more expensive and less robust. Most "we need PEEK" requirements turn out to be met by PPS at a fifth of the cost.

Grades

Table 2 — High-temperature grade families. PEI (Ultem) and PPSU are worth evaluating before jumping to PEEK.

The machine and the tool

PPS and PEEK cannot be run on a standard machine without modification. The requirements are specific and they are not negotiable.

Table 3 — Machine and tool requirements. Any one of these missing stops the job.

The most common planning error is the mould temperature control. A tool quoted with "hot oil capability" but drilled with 8 mm channels sized for water flow will not deliver 180 °C uniformly — the oil flow rate is too low, the pressure drop too high, and the tool runs 30 °C cooler at the far end. Conformal cooling or generously sized channels (12 – 14 mm) are needed, and the oil unit must be sized for the tool's heat load, not just its temperature.

Processing windows

Table 4 — Processing windows. PEEK's 400 °C melt is at the limit of most standard equipment.

Drying is often overlooked because these materials absorb so little water — 0.02 % for PPS. But at 380 °C even 0.05 % moisture causes visible splay and a measurable loss of molecular weight, and the part cost means the scrap is expensive. Dry at 150 °C for 3 hours regardless of what the datasheet implies.

Crystallinity: the property you control with the tool

Both materials are semi-crystalline, and the degree of crystallinity determines strength, wear resistance, chemical resistance and dimensional stability. It is set by the mould temperature and the cooling rate, and it cannot be recovered afterwards except by annealing.

Table 5 — Crystallinity versus mould temperature. A cold tool produces a part that looks right and is not.

This is the single most important thing to understand about high-temperature materials: a PEEK part moulded at 120 °C looks identical to one moulded at 190 °C, passes a dimensional check, and has roughly half the wear life and a fraction of the chemical resistance. If a supplier cannot tell you their mould temperature, they cannot tell you what they made.

Design rules

Table 6 — Design rules. The material-minimisation row is unique to this cost bracket.

Cost structure: where the money actually goes

Table 7 — Cost structure. On PEEK, material is the price; on PPS, it is roughly half.

The consequence for design is that a PEEK part should be designed like a machined part: minimise the envelope, core out everything, and accept a more complex tool if it removes material. A 20 % reduction in part volume is a 20 % reduction in the largest cost line, which usually beats any cycle-time improvement.

Where each material belongs

Table 8 — Application map. Most "PEEK" requirements are actually PPS requirements.

Defects and causes

Table 9 — Defects. Four of the eight trace back to mould temperature or moisture.

Designing a tool for a 200 °C mould

A high-temperature tool is a different design, not the same design with a bigger heater. The differences are structural and they have to be in the tool from the start.

Table 9 — Standard versus high-temperature tooling. The insulation and the thermal expansion allowance are the two most often forgotten.

Thermal expansion deserves a number: steel grows about 12 × 10⁻⁶ per kelvin. A 300 mm mould block taken from 20 °C to 200 °C grows 0.65 mm. If the ejector plate, the guide pins and the slides were fitted at room temperature with standard clearances, the tool will seize at operating temperature. High-temperature tools are assembled with the expansion allowance designed in, and often they are checked hot before the first shot.

Annealing and machining PEEK

PEEK parts are frequently machined after moulding, and the sequence matters. Machining a part with incomplete crystallinity produces a part that changes dimension weeks later.

Table 10 — The PEEK mould-anneal-machine sequence. Skipping step 2 causes dimensional drift after machining.

For implantable or medical PEEK, annealing is also part of the validation: the crystallinity level affects the material's mechanical performance and must be documented. A supplier who cannot state the crystallinity of the parts they ship cannot support a medical submission.

Qualifying a supplier for high-temperature moulding

Table 11 — Supplier qualification questions. A supplier who cannot answer the crystallinity question should not be quoting PEEK.

The crystallinity question is the one to press on, because it is where a cheap quote comes from. A moulder running PEEK at 140 °C mould temperature will produce parts faster and cheaper, and they will look identical. They will also have roughly half the wear life and a fraction of the chemical resistance — and the failure will appear in the customer's application, not in the incoming inspection.

Frequently asked questions