PEEK Plastic 101｜ Engineering Properties, Fabrication, Applications

PEEK is a high-performance special engineering plastic, known as the material “standing at the top of the pyramid”. With its characteristics such as high temperature resistance, high strength, lightweight, and biocompatibility, it has been applied in many industries. In this article, we will discuss what it is, its properties, fabrication methods, and applications.

What is PEEK?

Polyetheretherketone (PEEK) is a high-performance, semi-crystalline aromatic polymer. Its fundamental chemical structure consists of alternating ether and ketone linkages, incorporating benzene rings within the main molecular chain. As the flagship material of the Polyaryletherketone family, PEEK occupies the apex of the polymer performance hierarchy, often referred to as the “King of Resins.” It is an ideal metal replacement material, distinguished by its outstanding mechanical properties, stable thermochemical characteristics, lightweight nature, and exceptional biocompatibility.

PEEK Material Types

PEEK is categorized based on its properties and processing methods into several key variants:

• High-temperature PEEK: Characterized by superior thermal stability, maintaining excellent mechanical integrity and chemical stability even in high-temperature environments.
• Carbon fiber reinforced PEEK: Enhanced with carbon fibers to significantly boost mechanical strength, stiffness, and wear resistance.
• Glass fiber reinforced PEEK: Enhanced with glass fibers to improve mechanical performance and dimensional stability.
• Conductive PEEK: Modified with conductive fillers (e.g., carbon nanotubes or metal powders) to impart desired electrical conductivity.
• Medical-grade PEEK: Specifically treated and purified to comply with stringent medical device requirements. Medical-grade PEEK exhibits superior biocompatibility, corrosion resistance and often possesses inherent antimicrobial properties.

Properties of PEEK

PEEK is a special-grade engineering plastic with a balance of performance. It offers excellent thermal resistance, resistance to hot water and steam, solvent resistance, superior electrical insulation, and high fatigue strength.

The following table details the key physical, thermal, and electrical properties for unreinforced PEEK and its three main reinforced variants:

	Standard	20% GF	30% GF	30% CF
Density, g/cm3	1.32	1.42	1.49	1.44
Water Absorption (23°C, 24h), %	0.50	—	0.11	0.06
Tensile Strength (MPa)	92	123	157	208
Elongation at Break (%)	50	2.5	2.2	1.3
Flexural Strength (MPa)	170	192	233	318
Elastic Modulus (GPa)	3.63	6.66	10.29	13.03
Notched Izod Impact Strength (J/m)	70	88	98	87
Heat Deflection Temp (0.45 MPa), (°C)	152	285	315	315
Coefficient of Linear Expansion (x 10-5/ °C)	4.7	2.4	2.2	1.5
Volume Resistivity (Ω · cm)	4～9 x 1016	—	—	1.4 x 1016
Molding Shrinkage (%)	1.1	0.7～1.4	0.5	0.1～1.4

Mechanical Performance

PEEK is characterized by its high strength and high modulus. Its tensile strength can exceed 90 MPa, and its flexural strength and modulus are equally impressive. Concurrently, PEEK has good toughness and impact resistance. This combination of stiffness and flexibility ensures that PEEK components resist brittle fracture when subjected to complex stress environments.

Electrical Properties

PEEK possesses excellent electrical insulation properties. The volume resistivity of the PEEK resin typically exceeds 10¹⁶ Ω · cm.

Chemical Resistance

PEEK offers good resistance to nearly all chemicals, with the sole exception of concentrated sulfuric acid. It maintains its chemical stability even at elevated temperatures. Compared to other engineering polymers like polycarbonate, modified polyphenylene ether, and polysulfone, PEEK’s resistance to chemical stress cracking is far superior.

However, when PEEK is insufficiently crystallized, immersion in certain chemicals such as acetone can induce stress cracking. This vulnerability can be eliminated by performing an annealing treatment (e.g., 1 hour at 200 °C) to maximize its degree of crystallinity and improve its resistance to environmental stress cracking.

Thermal Resistance

PEEK has excellent thermal stability, with a continuous service temperature of up to 240 °C. Thermogravimetric analysis confirms this stability: PEEK shows zero mass loss at 400 °C and only 2.5% mass loss at 500 °C.

Hydrolysis and Steam Resistance

PEEK’s resistance to hot water and steam is one of its most defining characteristics. After immersion in 80 °C hot water for 800°C, the material’s tensile strength and elongation remain essentially unchanged. Even in 200 °C steam, there are no significant changes in tensile strength, mass, or appearance, allowing for long-term use in steam environments. Among all mainstream engineering plastics, PEEK offers the highest resistance to steam and hydrolysis.

Processing and Fabrication of PEEK

Injection Molding

PEEK has a high melting point and a relatively high melt viscosity compared to general engineering plastics, necessitating high processing temperatures. Barrel temperatures are controlled between 350 °C and 400 °C. Before molding, the material must be predried, usually under conditions of 150 °C for 3 hours.

As a crystalline resin, PEEK must achieve sufficient crystallization during molding to exhibit its superior properties.

• A mold temperature of 150 °C to 160 °C yields opaque, highly crystalline parts, though the surface layer may remain transparent and low in crystallinity.
• Using a mold temperature of 180 °C yields products with a higher overall degree of crystallinity.

If high mold temperatures cannot be used during the initial molding phase, a post-treatment (annealing) must be performed to increase the final crystallinity of the component. PEEK can be processed using standard injection molding equipment, though large, thin-walled, or complex parts may require screws with a high L/D ratio and a short compression section.

Laminate Forming

PEEK is used as a matrix material with glass fiber, carbon fiber, or hybrid reinforcements to produce high-performance composite laminates. These composite plates maintain a high retention rate of their flexural elastic modulus across a broad temperature range below 300 °C.

Machining and Secondary Processing

PEEK materials exhibit excellent machinability, enabling the manufacture of high-precision components through conventional machining techniques such as turning, milling, and drilling. Precision CNC machining can achieve dimensional tolerances as tight as 0.01mm.

Secondary processing involves machining, ultrasonic welding, electroplating, and sputtering. Bonding can be achieved using adhesives such as epoxy, polyurethane, or silicone. To enhance bond strength, the PEEK surface may require pretreatment, such as etching with chromic acid.

Application of PEEK

As a specialty engineering plastic, PEEK’s combination of excellent heat, radiation, and chemical resistance, along with superior mechanical and electrical properties, makes it highly valued across multiple industries.

Aerospace and Aviation

PEEK’s features—including thermal resistance, inherent flame retardancy, fuel oil resistance, high strength, and rigidity—are exploited in numerous aircraft parts manufactured via injection molding.

• PEEK components like radar parts and radomes exhibit excellent weatherability.
• Engine parts made from PEEK can withstand long-term use at temperatures above 200 °C.
• It is used for various sealing materials on aircraft.
• Carbon- or glass-fiber-reinforced PEEK is used for door handles, cabin panels, control sticks, and helicopter tail rotors.

Advanced composites, such as ICI’s APC-2 (PEEK matrix), exhibit a toughness that is over ten times greater than common epoxy resin composites. This capability allows PEEK composites to replace epoxy in large-scale structures like space station components, aircraft wings, and other demanding applications. A notable cost-saving application is the injection molding of rocket igniter tubes using glass fiber reinforced PEEK, replacing traditional metal parts while maintaining integrity under severe ignition and lift-off stresses.

Automotive Industry

PEEK in the automotive industry focuses on high-stress, high-wear, and high-temperature components.

• Piston skirts and rings
• Rotor brackets and bearings
• Bearing retainers
• Turbine load impellers
• Gaskets and gear support seats
• Tire pressure sensor housings

Mechanical and Industrial Equipment

In mechanical equipment and office automation machinery, PEEK is widely used for gears, bearings, valve seats, piston rings, couplings, washing fixtures, and photocopier separation claws.

Bearings and retainers: Specialized self-lubricating PEEK grades offer high strength, excellent wear resistance, low coefficient of friction, and good toughness, making them ideal for manufacturing ball bearing retainers and thrust bearings.

Piston rings for diesel engines: PEEK has replaced metal in diesel engine piston rings due to its superior thermal and lubrication properties. PEEK rings maintain their performance in 160 °C oil, offer excellent fatigue resistance, and generate significantly less noise than their metal counterparts during operation.

Bolts and fasteners: PEEK’s high strength, stiffness, and superior chemical resistance make PEEK bolts widely used in demanding mechanical and chemical processing equipment.

Pipes and joints: PEEK’s exceptional water, hot water, and steam resistance, coupled with high strength, good fatigue resistance, and minimal impurity leaching, lead to its extensive use in fluid handling components.

Centrifuge parts: PEEK can be coated onto metal surfaces via electrostatic coating or fluid bed immersion to create impellers and other centrifuge components, exploiting its chemical resistance, heat resistance, and excellent adhesion to metals.

Detection sensors: Injection-molded PEEK components are used in conductivity detection sensors for boilers and other systems. This application capitalizes on PEEK’s superior resistance to hot water and steam, excellent dimensional stability, and effective electrical insulation.

About Getzshape

At Getzshape, we are experienced manufacturers of CNC plastic parts. We have produced lots of plastic machined parts, including high-performance PEEK parts. If you are looking for quality PEEK fasteners, bearings, and pistons or other custom PEEK machined components, feel free to contact us.