PEEK and high-temperature mouldings: temperature, cost and process

What makes PEEK different

PEEK is a semi-crystalline aromatic polymer. It is used where a part must hold mechanical properties at temperatures that would soften polyamide or PBT, resist aggressive chemicals, or meet a flammability and smoke requirement without additives. It is also expensive, and it is harder to mould than the engineering plastics used for most electrical housings.

The properties that matter in an assembly are the glass transition temperature, the melting point, the continuous use temperature and the stiffness at temperature. PEEK is often quoted with a glass transition around 143 C, a melting point around 343 C and a continuous use temperature around 250 C. The exact values depend on the grade, the filler and the test method.

Those numbers are not interchangeable. A part can survive a short excursion above its continuous rating and still fail in service, because the failure is often creep or thermal ageing rather than a sudden melt. The design should use the continuous rating with a margin, and treat any peak as a separate case with its own duration and load.

Temperature is a set of limits, not one number

The first limit is deflection under load. A heat deflection temperature tells you where a standard bar bends under a standard load, and it is useful for comparison but not for design. A part under a real load at a real temperature may creep at a much lower value.

The second limit is continuous service temperature. This is a judgement based on retention of properties after ageing, and it depends on the property that matters. A part that only needs to insulate may accept a higher temperature than a part that has to hold a spring force.

The third limit is the short-term peak. Soldering, welding, a fault current or a nearby heat source can expose the part to a temperature well above its rating for seconds or minutes. The design should state the peak, the duration and the number of cycles, because a repeated peak ages the polymer.

The process window is narrow and hot

PEEK melts at a high temperature, so the barrel, the nozzle and the hot runner must run hot, often in the region of 360 to 400 C. The tool also runs hot, often around 180 to 200 C, to allow the polymer to crystallise. A cold tool produces an amorphous part with different properties and a high risk of later dimensional change.

The resin must be dry. PEEK absorbs little moisture compared with polyamide, but the little it absorbs is enough to cause voids, splay and a loss of molecular weight at melt temperature. Dry it to the resin supplier's schedule and keep it dry in the hopper.

Crystallinity is a design variable. A slowly cooled, well crystallised part is stiffer and more chemically resistant. A fast cooled part may be tougher in some grades but less stable at temperature. Annealing after moulding can relieve stress and raise crystallinity, but it also adds a process step and can move dimensions.

Shrinkage is higher than for an amorphous polymer and it is anisotropic. A long glass fibre filler reduces shrinkage in the flow direction and increases it across the flow, so a part with a long flow path can warp. Gate position, wall section and packing all matter more than they do for an unfilled engineering plastic.

PPS, PEI and the cheaper options

PPS is often the first alternative. It has a lower continuous temperature than PEEK but still covers many electrical and under-bonnet applications, and it processes at a lower temperature with a wider window. It is also cheaper, which matters because the material can be a large share of the part cost.

PEI is an amorphous polymer with a high glass transition and good dimensional stability. It is useful where the part must hold tolerance over temperature and does not need the chemical resistance of PEEK. It is transparent in natural grades and has good flammability behaviour.

The table compares the options on the factors that decide a housing or insulator.

Where the cost comes from

The polymer is the first line. PEEK is several times the cost of PPS or PEI per kilogram, and a part with a thick wall uses more of it. Reducing wall thickness is the most direct saving, provided the part still has the stiffness and the creep resistance it needs.

The tool is the second line. A hot tool needs heaters, insulation and a controller. A wear resistant steel adds cost. A hot runner adds cost and complexity. The tool life may be shorter with a filled grade, so the maintenance and replacement cost belongs in the model.

The cycle is the third line. A hot tool takes longer to heat and to cool, and an anneal adds a separate operation. The energy cost of running a hot tool is higher, and the scrap rate at start-up is often higher because the process takes longer to stabilise.

Design rules for a high-temperature part

Keep the wall as uniform as the function allows. A thick section cools slowly and can sink or warp; a thin section may not fill before it freezes. High-temperature polymers generally want a thinner wall than a commodity plastic, but the exact minimum depends on the flow length and the grade.

Radius the corners and avoid sharp internal features. A sharp corner concentrates stress, and a high-temperature polymer is often less ductile than the material it replaces. A generous fillet also helps the melt flow and reduces warpage.

Design the gate for the flow. A gate that is too small creates high shear, a weak weld line and a lot of wear. A gate that is too large leaves a mark that is hard to remove. For a filled grade, the gate position also sets the fibre orientation and therefore the shrinkage.

Treat inserts with care. A metal insert in a high-temperature polymer sees a large expansion mismatch, and the polymer may not have enough ductility to accommodate it. A knurled insert with a generous wall and a preheat can help. A pressed insert should be checked at the high end of the temperature range, not only at room temperature.

Specify tolerances that the process can hold. A high-temperature moulding with a glass filler will move more than an unfilled amorphous part. Do not apply a PA66 tolerance table to a PEEK part without checking the shrinkage and the warpage.

Validate the part in its condition

Test at temperature, not only at room temperature. A tensile or a pull-out test at 150 C tells you more about a high-temperature application than a room temperature test on the same part. Condition the sample at the service temperature for long enough to reach equilibrium.

Add thermal ageing. Hold the part at the continuous temperature for a defined period and measure the property that matters before and after. The result is a trend, not a guarantee, but it is more useful than a single data sheet value.

Check the dimensions after the thermal cycle. A part that grows or warps in service can lose a seal, loosen a clip or fail to mate. Measure the critical features before and after, and include the annealed condition if the process uses one.