PEEK (Polyetheretherketone) sits at the absolute top of the injection moldable thermoplastic hierarchy. With a continuous service temperature of 260°C, chemical resistance that rivals fluoropolymers, and mechanical properties that compete with aluminum, PEEK is the material of choice when nothing else will work. But molding it is a different game entirely — standard injection molding equipment and parameters simply will not work. This guide covers everything you need to know about PEEK injection molding processing.
PEEK Material Overview
PEEK is a semi-crystalline aromatic polyetherketone with a unique combination of properties: glass transition temperature (Tg) of 143°C, crystalline melting point (Tm) of 343°C, and continuous service temperature of 260°C. It requires processing temperatures of 360–400°C — well beyond the capability of standard injection molding machines.
The crystallinity of PEEK is its defining processing challenge. PEEK can exist in amorphous or semi-crystalline states depending on cooling rate, and the difference in properties between the two states is dramatic: amorphous PEEK has lower chemical resistance, lower modulus above Tg, and poorer wear properties. Achieving the right crystallinity requires precise mold temperature control at 160–200°C.
| Property | Amorphous PEEK | Semi-Crystalline PEEK |
|---|---|---|
| Tensile Strength (MPa) | 90 | 100 |
| Modulus (GPa) | 3.6 | 4.0 |
| Chemical Resistance | Moderate | Excellent |
| Wear Resistance | Fair | Excellent |
| Appearance | Transparent amber | Opaque beige/tan |
Critical Processing Parameters
Molding PEEK requires precise control across every parameter. Here is the recommended profile for unfilled PEEK (Victrex 450G or equivalent):
| Parameter | Recommended Value | Notes |
|---|---|---|
| Barrel Rear Zone | 350°C | Feed zone — must be above Tg but below excessive degradation |
| Barrel Middle Zone | 360–370°C | Compression zone — melting and homogenization |
| Barrel Front Zone | 370–380°C | Metering zone — final temperature stabilization |
| Nozzle Temperature | 380–390°C | Prevent cold slug and ensure smooth injection |
| Mold Temperature | 160–200°C | Critical for crystallinity — oil heating recommended |
| Injection Speed | Moderate to fast | Prevent premature freezing in thin walls |
| Holding Pressure | 70–100 MPa | Sufficient to compensate for shrinkage |
| Back Pressure | Low (2–5 bar) | Excessive back pressure causes degradation |
| Screw Speed | 50–100 rpm | Moderate to minimize shear heating |
| Material Drying | 150°C for 3–4 hours | Target moisture <0.02% |
Why Mold Temperature Matters Most
Mold temperature is the single most important parameter for PEEK. The mold surface temperature directly controls the cooling rate, which determines crystallinity. At mold temperatures below 150°C, PEEK cools too quickly to crystallize fully, resulting in an amorphous skin layer with poor chemical resistance. At 160–200°C, the cooling rate allows sufficient time for spherulite growth, producing semi-crystalline PEEK with optimal properties.
The mold temperature must be maintained uniformly across the entire cavity surface. Temperature variations of more than ±5°C create differential crystallinity zones, which manifest as warpage, inconsistent properties, and visible color variation (amorphous regions appear transparent amber while crystalline regions are opaque tan).
Equipment Requirements
Standard injection molding machines are not suitable for PEEK. The minimum requirements are:
- Barrel and Screw: High-temperature barrel with ceramic band heaters rated for minimum 450°C continuous operation. Bimetallic screw with hardened flight tips (Colmonoy 56 or equivalent) to resist the abrasive wear of PEEK at processing temperature.
- Compression Ratio: 2.0:1 to 2.5:1 screw compression ratio. Higher compression ratios generate excessive shear heat and can cause degradation.
- Mold Heating: Oil-circulating mold temperature controllers capable of 200°C minimum. Electric cartridge heaters are an alternative but require careful multi-zone control for uniformity. Standard water-based mold temperature controllers are useless — they max out at 95–120°C.
- Insulation Plates: Thermal insulation plates (typically 10–15mm thick phenolic or ceramic composite) between the mold and machine platens are essential. Without insulation, heat from the 180°C mold conducts into the machine platens, causing thermal expansion of the tie bars and inconsistent clamp force.
- Drying: Desiccant dryer capable of 150°C with dew point below -40°C. PEEK moisture target is <0.02% — hydrolytic degradation at processing temperature is extremely rapid if moisture is present.
Mold Design for PEEK
Gate Types: Direct sprue gates and edge gates work well. Submarine gates and tunnel gates are problematic because PEEK’s high viscosity at normal processing temperature makes it difficult to shear through small gates. Hot runner systems must be rated for 400°C and use externally heated manifolds — internally heated hot runners create cold spots.
Venting: Vent depth must not exceed 0.01–0.02mm (PEEK has low melt viscosity at processing temperature and will flash through larger vents). Vent land length should be 0.5–1.0mm before the vent channel opens to full depth.
Runner Sizing: Full-round runners of 6–8mm diameter are recommended. PEEK solidifies quickly, and undersized runners freeze off before packing is complete.
Draft Angles: Minimum 1° draft, preferably 2° or more. PEEK adheres strongly to mold steel at elevated temperatures, and brittle at room temperature — insufficient draft leads to ejection cracking.
Mold Steel: H13 (SKD61) hardened to 52–56 HRC. P20 is acceptable for prototype molds but will wear more quickly. Stainless steel (S136/420SS) is recommended for medical applications due to corrosion resistance during steam sterilization of the mold between runs.
Common Defects and Fixes
| Defect | Appearance | Root Cause | Fix |
|---|---|---|---|
| Black Specks | Dark spots in translucent parts | Thermal degradation in barrel | Reduce barrel temperature, residence time, or screw speed. Purge thoroughly. |
| Warpage | Part distortion after ejection | Uneven crystallinity from mold temperature variation | Improve mold temperature uniformity. Increase mold temp to 180–200°C. |
| Sink Marks | Depressions at thick sections | Insufficient holding pressure or premature gate freeze | Increase holding pressure and time. Enlarge gate. |
| Burn Marks | Brown/black discoloration at flow end | Trapped air compression (diesel effect) | Improve venting. Reduce injection speed. |
| Incomplete Fill | Short shot, missing features | Premature solidification in thin walls | Increase injection speed and mold temperature. Add flow leaders. |
PEEK vs Other High-Temperature Thermoplastics
| Parameter | PEEK | PEI (Ultem) | PPS | PSU |
|---|---|---|---|---|
| Tg (°C) | 143 | 217 | 88 | 187 |
| Tm (°C) | 343 | Amorphous | 280 | Amorphous |
| Processing Temp (°C) | 360–400 | 350–400 | 300–340 | 330–380 |
| Mold Temp (°C) | 160–200 | 135–165 | 135–150 | 120–160 |
| Continuous Service Temp (°C) | 260 | 170 | 220 | 160 |
| Chemical Resistance | Excellent | Moderate — stress cracks in solvents | Excellent | Moderate — attacks by ketones |
| Material Cost ($/kg) | 80–120 | 25–40 | 15–25 | 20–30 |
Applications for PEEK Injection Molded Parts
- Medical: Spinal fusion cages, dental implants, surgical instrument handles. PEEK’s radiolucency (invisible on X-ray) and biocompatibility (ISO 10993 certified grades available) make it the dominant material for implantable devices.
- Aerospace: Brackets, bushings, seals, and wire management components. PEEK replaces aluminum in weight-critical applications, saving 70% weight with comparable mechanical performance below 260°C.
- Semiconductor: Wafer handling components, CMP rings, etch chamber fixtures. PEEK’s combination of high purity, chemical resistance, and ESD-safe grades makes it essential for semiconductor manufacturing environments.
- Oil and Gas: Seals, backup rings, valve seats. PEEK’s resistance to H₂S, CO₂, and high-pressure steam at temperatures up to 200°C is unmatched by other thermoplastics.
Cost Breakdown
PEEK parts cost 5–10 times more than equivalent PA66 parts due to three compounding factors:
- Material cost: PEEK resin at $80–120/kg vs PA66 at $3–5/kg — a 20–40× material cost multiplier.
- Mold investment: Hot-oil heated molds with H13 hardened steel cost 2–3× more than standard P20 water-cooled molds.
- Cycle time: PEEK cooling in a 180°C mold takes significantly longer than nylon cooling in a 60°C mold. Cycle times of 60–120 seconds are typical for PEEK parts weighing 20–50g, compared to 15–30 seconds for equivalent nylon parts.
Conclusion
PEEK injection molding is a specialized discipline that requires investment in high-temperature equipment and deep processing expertise. It is not a material you can run on a standard molding machine with a few parameter tweaks. But for applications that demand the highest combination of temperature resistance, chemical resistance, and mechanical properties available in a thermoplastic, PEEK is simply unmatched. The key to success is treating mold temperature control as your primary quality parameter, investing in proper drying, and working with a molder who has verified PEEK processing capability — not just a claim on a website.
Frequently Asked Questions
Can PEEK be molded in a standard injection molding machine?
No. Standard machines typically have barrel temperature limits of 300–350°C and use water-cooled molds that max out at 95°C. PEEK requires 360–400°C barrel temperature and 160–200°C mold temperature. At minimum, the machine needs ceramic band heaters rated to 450°C, a bimetallic screw, and an oil-circulating mold temperature controller. Retrofitting a standard machine is possible but often costs more than the machine itself.
What is the minimum mold temperature for PEEK?
The practical minimum is 160°C for unfilled PEEK. Below this temperature, the cooling rate is too fast for full crystallinity development, resulting in an amorphous structure with poor chemical resistance and lower mechanical properties above Tg. For thin-walled parts (under 1mm), the minimum increases to 180°C to prevent premature freezing. For carbon-fiber reinforced PEEK, 180–200°C is recommended.
How do I prevent PEEK degradation during molding?
Degradation prevention relies on four factors: (1) Dry material to below 0.02% moisture (150°C, 3-4 hours in desiccant dryer). (2) Minimize residence time — keep it under 5 minutes at 380°C. (3) Use low back pressure (2–5 bar) to reduce shear heating. (4) Purge the barrel with a high-temperature purging compound or a thermally stable polymer (like PPS) between PEEK runs. Black specks are the first visible sign of degradation — if you see them, reduce temperature and residence time immediately.
Is PEEK injection molding more expensive than CNC machining PEEK?
For volumes above 500–1,000 parts per year, injection molding is typically more cost-effective than CNC machining from PEEK stock shapes. The injection molding mold investment ($20,000–$80,000 for a PEEK-capable mold) is amortized over volume, while CNC machining costs remain linear. However, for prototyping and volumes under 500 parts, CNC machining from PEEK rod or plate is almost always cheaper. Many programs start with CNC prototypes, then transition to injection molding for production volumes.
