PEEK Injection Molding: High-Temp Material Guide

A PEEK mold that fails on the first trial usually fails in the tool design, not the resin. A cavity compensated for 0.8% shrinkage on a material that shrinks 1.6% produces an undersized seal groove and a steel change order that costs weeks. Cooling channels laid out as water circuits on a mold that has to run at 180°C fail the same way — rubber O-rings soften, flow stops, and the cavity never reaches crystallization temperature. And there is a third failure that catches new PEEK tooling more often than any other: the steel grows. A 300 mm cavity cut and fitted at 25°C runs roughly half a millimeter larger at 180°C operating temperature, which quietly changes slide clearances, ejector alignment, and parting-line flatness. All three failures are visible on the drawing long before the steel is cut — and all three are why this guide reads PEEK from the mold side, not the chemistry side.

PEEK — polyether ether ketone — is a semicrystalline high-performance thermoplastic that carries the highest continuous-use temperature of any melt-processable resin that is practical to injection mold. Its melting point of roughly 343°C and glass transition of about 143°C are the door into a processing window that punishes conventional tooling assumptions: a 150-200°C mold, a 350-400°C melt, shrinkage in the 1.2-2.0% range, and glass-reinforced grades abrasive enough to erode a soft cavity. It replaces metal in medical implants, semiconductor handling hardware, aerospace seals, and automotive transmission components precisely because those applications outlive what lower-cost polymers can survive.

This guide is written for the people who buy and build the tool: mold buyers, tooling engineers, and manufacturing engineers. It walks from the properties that drive mold decisions, through the design rules for PEEK tooling, into the processing window the mold must survive, and ends with defects traced to their mold-side root causes. Processing numbers follow typical published values for injection-molding grades; test methods are cited where they matter.

The Snapshot

• PEEK is a semicrystalline high-performance polymer with a melting point of ~343°C and a glass transition of ~143°C (typical published values).
• Mold shrinkage is 1.2-2.0% — high for an engineering thermoplastic — so cavities are compensated zone-by-zone and post-mold crystallization is designed for, not discovered.
• The mold must run at 150-200°C on oil temperature controllers or cartridge heaters, never plain water circuits — this single decision shapes the whole tool layout.
• Process at a melt of 350-400°C after drying at ~150°C for 3-4 hours to under 0.1% moisture; keep residence short above 400°C.
• Density is 1.30-1.32 g/cm³, water absorption 0.1-0.5%, continuous service temperature 250°C+, tensile strength 90-100 MPa unreinforced — typical published values.
• Glass-reinforced grades wear soft steel: plan on hardened tool steel around HRC 48-52 (S136-class hardened) with TiN/TiAlN coating on slides, cores, and gate inserts.
• DieStrike builds PEEK-capable tooling to IATF 16949 / ISO 9001 systems, with CMM-verified dimensions on every mold.

What Is PEEK Plastic?

PEEK plastic is a semicrystalline aromatic thermoplastic: a polyether ether ketone, built from repeated ether and ketone linkages between benzene rings. The aromatic backbone is the reason it survives where aliphatic polymers fail — the rigid benzene rings give it high thermal stability, chemical resistance, and mechanical retention at temperature. Commercialized in the late 1970s, it became the reference for parts that must hold mechanical properties at 250°C and above, survive aggressive fluids, and still be melt-processable.

Two thermal transitions define PEEK for the tooling engineer. The glass transition (Tg) sits near 143°C — above it the amorphous regions soften. The crystalline melting point (Tm) sits near 343°C, and between them lies the processing reality: PEEK must be melted at 350-400°C, and the mold held at 150-200°C so the melt crystallizes at a controlled rate. The crystalline fraction a part develops — typically around 30-35% at best — decides its heat resistance and dimensional stability. Cool the mold too far below 150°C and the part comes out largely amorphous: it may look fine, but it softens near 143°C and keeps crystallizing and shrinking in service. For the mold designer, crystallinity control is the reason the mold carries heating, not just cooling.

Datasheet test data comes from standard methods. ASTM D638 covers tensile, D790 covers flexural, D648 covers HDT, D570 covers water absorption, and ISO 527, ISO 178, and ISO 75 are the equivalents. Always compare grades using the same standard — numbers from different methods are not interchangeable, and published PEEK values vary noticeably between unfilled, glass-reinforced, and carbon-fiber-reinforced grades.

PEEK Properties That Drive Mold Design

Material selection and tool design are one decision, not two. The table below maps the PEEK properties that matter most to the specific mold decisions they drive. If a datasheet number is missing at RFQ time, this is the list to ask for.

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Values are typical published ranges for injection-molding grades; actual numbers vary by grade, filler, and test method.

Shrinkage and dimensional stability. PEEK is crystalline, and crystalline resins shrink more than amorphous ones. Mold shrinkage of 1.2-2.0% is typical for unfilled injection-molding grades, and the shrinkage is not isotropic: flow-direction and cross-flow shrinkage differ, and glass-reinforced grades shrink less overall but carry more anisotropy into the part. The practical consequence is that cavities are compensated with the datasheet range split by wall-thickness zone, then tuned on the first molding trial — the same discipline as any crystalline resin, with a wider starting range. Water absorption of 0.1-0.5% saturation is low, which is why PEEK parts hold dimensions in humid service. The five shrinkage compensation rules apply directly and cover the zone-by-zone method.

Heat resistance. Continuous service at 250°C and above is the property that justifies PEEK's cost. For the tool, the heat requirement sets expectations: a part that must hold a seal at 250°C needs a crystalline structure, which means the mold must hold 150-200°C so crystallization completes in the cavity rather than in service. That is the direct line from a service-temperature requirement to a heating system in the mold.

High melt viscosity. PEEK melt is viscous, even at 350-400°C. Thin walls, long flow paths, and pin gates are the classic failure combination — the melt freezes before the cavity packs. The mold answers with large gate cross-sections, short flow lengths, and full-round runners, covered in the mold design section below.

Abrasiveness. Unfilled PEEK is abrasive over long runs; 30% glass-reinforced grades are noticeably harder on tooling. P20-class prehardened steel erodes at gates and along flow paths — first as gate wear, then dimensional drift and rough surfaces. Hardened steel and coatings are the mold-side answer, not an optional upgrade.

Mold Design Considerations for PEEK

PEEK is demanding on the machine, but the tool carries the design decisions that decide rejects, cycle, and tool life. The rules below are the ones that matter when the steel is being cut for a PEEK part — several of them invert what works for commodity resins.

High mold temperature: heating, not just cooling. The mold runs at 150-200°C, so it carries heating circuits, and conventional water cooling is off the table — water boils at 100°C and pressurized hot-water systems top out well below PEEK's window. Oil temperature controllers are the standard for the 150-200°C range, with cartridge heaters for localized hot spots like cores and inserts a circulating circuit cannot reach evenly. Channel layouts follow the same balancing logic as cooling design, but the fluid is hot oil, the seals are high-temperature rated, and the mold and platen are separated by insulation plates so heat stays in the cavity instead of walking into the machine. The mold heating and cooling products page covers the hardware side of this decision.

Thermal expansion compensation. Tool steel expands at roughly 11-12.5 µm/m·K, and a PEEK mold runs 130-180°C above the bench temperature where it was fitted. On a 300 mm mold that is roughly half a millimeter of growth — enough to bind a slide, close an ejector clearance, or open a parting line. The design rules follow: fits and clearances are specified at operating temperature, not at 25°C; slide and lifter clearances get thermal headroom; ejector pins and sleeves are sized so they do not seize when the steel grows; and cavity dimensions are cut with both resin shrinkage and tool expansion in mind. This is where holding tight tolerances on PEEK parts becomes a thermal problem, not just a machining problem — the discipline is covered in detail in how to hold ±0.002 mm precision mold tolerances.

Shrinkage compensation. Compensate cavities for 1.2-2.0% shrinkage (unfilled), split by wall-thickness zone, and check the grade's flow-direction data before cutting. Glass-reinforced grades shrink less but anisotropically, so the compensation map follows the flow pattern, and the gate location influences which direction shrinks more. Because PEEK continues to crystallize after ejection, critical dimensions should be agreed on a measurement method — as-molded, or after a post-mold annealing step — before the tool is quoted.

Gating for high viscosity. PEEK needs large gate cross-sections and short flow lengths. Edge, fan, and tab gates sized generously — gate depth approaching 60-100% of wall thickness for unfilled grades — beat pin gates on anything but the smallest parts, because the melt must transfer through the gate before freeze-off. Flow-length-to-wall ratios for PEEK run short; keep flow paths compact and consider multiple gates on large parts. Weld lines are the trade-off of multiple gates, so place them where strength and cosmetics allow. Gate inserts are wear parts on glass grades: make them replaceable and hardened.

Runner system. Full-round cold runners sized generously (8-12 mm range for typical parts) work for short runs and prototyping. Production PEEK tools commonly go hot-runner: the manifold and nozzle tips must be rated for 400°C-class melt — high-temperature heaters, thermocouples, and seals throughout. Valve gates suit cosmetic or high-precision parts. Balance the system across cavities — PEEK's crystallinity and shrinkage are sensitive to melt-temperature differences between cavities.

Mold steel and coatings. The abrasiveness of PEEK — especially glass-reinforced grades — sets the steel floor. Plan on hardened tool steel around HRC 48-52: S136-class stainless, hardened and tempered, is the workhorse for PEEK tooling because it combines wear resistance with corrosion resistance for medical and cleanroom applications. Add TiN or TiAlN coating on slides, lifters, gate inserts, and ejector pins — the coating buys the wear margin where the glass fibers scrub hardest. Unfilled PEEK on short runs can run prehardened 718-class steel at 36-42 HRC, but the hardened option is the safer call for production. The trade-offs between the common mold steels are laid out in the P20 vs H13 vs S136 guide.

Venting, draft, and ejection. Vent depths around 0.02-0.04 mm are the typical guideline for PEEK — deep enough to move the air and volatiles at high melt temperature, shallow enough to avoid flash on a high-pressure resin. Draft of 1-3° per side is the working range; PEEK parts shrink onto cores and leave the mold at 150-200°C, so undersized draft shows up immediately as sticking and ejector damage. Polish cores and cavity walls in the draw direction, use larger-diameter ejector pins or added lifters to spread ejection force, and verify ejection stroke at operating temperature, not on the bench.

Injection Molding PEEK: The Processing Window

PEEK is one of the least forgiving thermoplastics on the machine — the window is narrow and the temperatures are high. The window below is the standard starting point for injection-molding grades (typical published practice); your grade datasheet overrides it.

Drying: ~150°C for 3-4 hours. PEEK absorbs 0.1-0.5% moisture depending on grade and humidity, and at a melt of 350-400°C water becomes steam and voids. The processing target is under 0.1%, ideally below 0.05%. Dry with a hopper dryer at roughly 150°C for 3-4 hours, and keep dried resin covered — PEEK re-absorbs moisture in hours. Wet resin shows as splay, voids, and brittle parts, and none of it is the mold's fault.

Melt temperature: 350-400°C. The practical sweet spot for most grades is 360-390°C. Below the window the melt is too viscous to fill thin sections; above roughly 400-420°C, residence starts degrading the polymer — discoloration, falling mechanicals, and corrosive by-products that attack the barrel, screw, and mold. Keep barrel residence short at the top of the window; PEEK tolerates no long dwells. Screws with generous L/D and low-compression profiles are typical; check with the machine builder before running it on a standard screw.

Mold temperature: 150-200°C. This is the number that separates PEEK tooling from everything else. The mold must hold the part above the crystallization range long enough for a useful crystalline fraction to develop — cool too fast and the part freezes amorphous, with lower heat resistance and continuing dimensional drift. Typical practice runs 160-190°C for most parts, with the high end reserved for glass-reinforced grades and parts that need maximum crystallinity. The temperature is delivered by oil temperature controllers and cartridge heaters, and uniformity across the cavity matters as much as the set point — a 20°C gradient across a part is a warped part.

Pressures and speeds. High viscosity means high injection pressure — typically 100-150 MPa (1,000-1,500 bar), with hold pressure a substantial fraction of injection pressure so the cavity packs before the gate freezes. Fill speed is usually moderate; too fast risks shear heating at the gate, too slow risks freeze-off in thin sections. Back pressure stays low and the shot size is matched so barrel residence stays short.

Crystallinity control. The crystalline fraction — typically 30-35% at best for standard grades — is set by the cooling rate, which the mold temperature controls. Higher mold temperature and slower cooling give higher crystallinity, better heat resistance, and higher shrinkage; lower mold temperature gives a more amorphous part, lower heat resistance, and a part that keeps changing dimensions as it crystallizes in service. For parts that serve at 250°C, the mold runs at the top of its window and the process is validated against crystallinity, not just against dimensions. Post-mold annealing at 200-260°C for a few hours is common practice to stabilize dimensions before final inspection — agree with the molder whether the acceptance measurement is as-molded or post-anneal, because the two numbers differ.

Shrinkage: 1.2-2.0%. The number moves with grade, wall thickness, filler, and process conditions: higher mold temperature and higher hold pressure push shrinkage toward the low end, and glass grades shrink less overall. Because shrinkage is crystalline and direction-dependent, warpage and post-mold drift are the visible risks rather than simple undersize. Cavities are compensated with the datasheet range, then tuned on the first molding trial — at operating temperature, not on the bench.

Common PEEK Applications

PEEK parts cluster in four spaces where resin cost is justified by what the part survives. Medical devices — implantable spinal cages, orthopedic components, and surgical instruments that hold dimensions through repeated steam, EO, and gamma sterilization — push medical PEEK molding toward polished, hardened, tightly toleranced tooling under the discipline of our medical device molding work. Semiconductor fabs use PEEK for wafer-handling components, high-purity seals, and CMP rings, where low outgassing and chemical resistance matter more than cost; the tooling follows the precision and cleanliness requirements of our consumer electronics manufacturing page. Aerospace brackets, bushings, and fire-resistant interior seals exploit strength-to-weight and fire resistance, while automotive transmission thrust washers, seal rings, and clutch components replace metal where lubrication is marginal — the same gearbox parts our automotive injection molding team tools daily. EV and energy applications — battery connectors, busbar insulation, compressor seals — are the fastest-growing bucket, and the EV and energy industry page covers their tooling requirements.

PEEK Defects: Mold-Side Root Causes and Fixes

Most PEEK rejects trace to a mold design decision, not a bad shot. The table below maps each defect to its mold-side root cause and the tooling fix — process fixes that also work are noted where they matter. For the systematic workflow, see how to troubleshoot mold defects.

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Moisture voids deserve their own line because they are the PEEK defect that is rarely a tool problem: internal voids and splay with no obvious mold cause usually mean the resin was wet. The fix is drying discipline — ~150°C for 3-4 hours, target under 0.1% moisture — and voids that reappear mid-run point at a leaking hopper dryer, not at the mold.

Gate wear on glass-reinforced grades is a wear-out signature, not a one-off defect: as the gate erodes, part weight drifts, then dimensions follow, then flash appears. Track gate condition in preventive maintenance, keep replaceable hardened gate inserts on hand, and let coating thickness checks — not part appearance — schedule replacement. The mold repair and maintenance service page covers the inspection rhythm for abrasive-resin tooling.

PEEK vs PPS vs PA46 vs PI: Comparison Table

PEEK is rarely specified in a vacuum — it is usually weighed against the other high-temperature candidates: PPS, PA46, and polyimide (PI). The table below uses typical published values for injection-molding grades; actual numbers vary by grade, filler, and test method.

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Values are typical published ranges for injection-molding grades. Glass- and carbon-reinforced versions of every resin move their rows.

PEEK vs PPS: PPS is cheaper, easier to mold, and dimensionally steadier (0.2-0.7% shrinkage), and it is the default for hot, aggressive electrical environments up to about 200-220°C. But it is brittle where PEEK is tough, and its service ceiling stops well below PEEK's 250°C+. Hot connector — PPS is often the answer; loaded mechanical part at 250°C — PEEK wins.

PEEK vs PA46: PA46 (polyamide 46) matches PEEK's room-temperature tensile strength at a fraction of the cost, and molds far more easily. Its limits are water absorption — 3.0-3.7% saturation versus 0.1-0.5% for PEEK — and a service ceiling near 150-190°C. For dry, warm automotive underhood parts PA46 is a strong value; where moisture, chemicals, or sustained high temperature enter the picture, PEEK is the more stable choice. PA46's moisture behavior also means its tooling needs corrosion protection when idle, unlike PEEK tooling.

PEEK vs PI: Polyimide (PI) outlasts PEEK at the top of the range — 300°C+ continuous — which is why it dominates extreme-heat seals and bearings. But much of PI is not injection moldable at all (it is compression-molded or machined), and injection-moldable grades are difficult, expensive, and limited in part size. PEEK is the practical melt-processable answer for most programs; PI is the specialist's choice when nothing else survives the temperature. The LSR material guide covers the other extreme of the process window.

Frequently Asked Questions

Q1. What is PEEK plastic?

PEEK (polyether ether ketone) is a semicrystalline high-performance thermoplastic with a melting point of ~343°C and a glass transition of ~143°C. Its aromatic backbone gives it a continuous service temperature of 250°C+, chemical resistance, low water absorption (0.1-0.5%), and mechanical retention at temperature — which is why it replaces metal in medical, aerospace, semiconductor, and automotive parts.

Q2. What is the PEEK molding temperature?

PEEK is processed at a melt temperature of 350-400°C (typically 360-390°C) into a mold held at 150-200°C. The mold temperature is the demanding half: it must stay above the crystallization range so the part develops a crystalline structure, which means oil temperature controllers or cartridge heaters instead of plain water cooling.

Q3. What is the PEEK shrinkage rate?

Mold shrinkage for unfilled injection-molding grades is typically 1.2-2.0%, varying with grade, wall thickness, mold temperature, and hold pressure. Glass-reinforced grades shrink less but more anisotropically. Because PEEK is crystalline, cavities are compensated zone-by-zone and parts may continue to crystallize and drift after ejection — agree on as-molded versus post-anneal measurement before quoting.

Q4. Does PEEK need drying before injection molding?

Yes. PEEK absorbs 0.1-0.5% moisture depending on grade and humidity, and at a 350-400°C melt that moisture becomes steam and voids. Dry at ~150°C for 3-4 hours to under 0.1%, ideally below 0.05%, and keep dried resin covered — PEEK re-absorbs moisture in hours.

Q5. What mold steel should be used for PEEK?

For production PEEK tooling, plan on hardened tool steel around HRC 48-52 — S136-class stainless, hardened and tempered, is the workhorse — with TiN or TiAlN coating on slides, cores, gate inserts, and ejector pins. PEEK and its glass-reinforced grades are abrasive, and soft steel erodes first at the gates, then along the flow paths.

Q6. Is PEEK difficult to injection mold?

PEEK is demanding rather than difficult: the window is narrow (melt 350-400°C, mold 150-200°C), the resin is viscous, and the tooling must be designed for high mold temperature, thermal expansion, and abrasion from the start. With a properly designed mold — oil-heated circuits, large gates, short flow lengths, hardened steel — the process runs steadily. The mold design decisions carry the outcome, which is what a DFM review exists to catch before the steel is cut.

The Bottom Line

PEEK's 1.2-2.0% shrinkage, 150-200°C mold temperature, and abrasive glass grades make it the most demanding resin a tool can run — every cavity, gate, and vent decision shows up in the first trial shot. DieStrike designs PEEK tooling with expansion-compensated, hardened-steel cavities and returns a DFM report within 24 hours.