PEEK vs Ultem: Essential Data for CNC Machined Parts

TL;DR

Choosing between PEEK and Ultem for CNC machined parts involves a critical trade-off. PEEK (Polyether Ether Ketone) offers superior mechanical strength, wear resistance, and a significantly higher continuous operating temperature, making it the premier choice for the most demanding applications. In contrast, Ultem (Polyetherimide) provides a more cost-effective and easier-to-machine alternative with excellent dielectric strength, suitable for moderate-load parts where faster production and budget are key considerations.

An Introduction to High-Performance Polymers: What Are PEEK and Ultem?

In the world of precision engineering and CNC machining, standard plastics often fall short. When applications demand exceptional performance under extreme conditions, engineers turn to high-performance polymers like PEEK and Ultem. These materials represent the pinnacle of thermoplastic technology, providing properties that rival those of some metals while offering significant weight savings and design flexibility.

PEEK (Polyether Ether Ketone) is a high-performance, semi-crystalline thermoplastic known for its remarkable combination of mechanical strength, thermal stability, and chemical resistance. Its semi-crystalline structure, which contains both organized crystalline regions and disordered amorphous regions, is fundamental to its performance. This structure allows PEEK to maintain its integrity and mechanical properties well above its glass transition temperature, making it exceptionally durable in high-heat environments. It is a member of the polyaryletherketone (PAEK) family and is often specified for critical components in the aerospace, medical, and energy sectors.

Ultem (Polyetherimide or PEI) is an amorphous thermoplastic that is prized for its high strength-to-weight ratio, dimensional stability, and outstanding electrical insulating properties. As an amorphous polymer, Ultem lacks a defined crystalline structure, which contributes to its transparency (in its natural amber state) and uniform shrinkage. While it has a lower maximum operating temperature than PEEK, its high dielectric strength is among the best of any commercial thermoplastic, making it a go-to material for electrical connectors, insulators, and medical device housings.

The core difference lies in their molecular morphology. According to Curbell Plastics, this structural distinction—PEEK's semi-crystalline nature versus Ultem's amorphous form—is the primary driver behind their different behaviors at elevated temperatures and under mechanical stress. This fundamental variance dictates which material is better suited for a specific CNC machining application.

Technical Comparison: PEEK vs. Ultem Properties for Machining

When selecting a material for CNC machined parts, a detailed, data-driven comparison is essential. Both PEEK and Ultem are formidable materials, but their property profiles make them suitable for different engineering challenges. The choice often comes down to specific performance requirements related to temperature, mechanical load, chemical exposure, and cost.

Here is a head-to-head comparison of their key properties:

Thermal Resistance and Operating Temperatures

The most significant differentiator is thermal performance. PEEK's semi-crystalline structure allows it to maintain excellent mechanical properties up to 260°C (500°F) in continuous service. As noted by Weerg, even though Ultem has a higher glass transition temperature (Tg), its amorphous nature means it begins to soften above this point, limiting its continuous use temperature to around 170°C (340°F). This makes PEEK the only viable polymer option for applications with stable operating temperatures exceeding 180°C.

Mechanical Strength and Wear Resistance

While both materials exhibit high tensile strength, PEEK has superior toughness, fatigue strength, and significantly better wear and abrasion resistance. This makes it the preferred choice for dynamic components like bearings, bushings, piston parts, and seals that are subjected to cyclic loads or sliding contact. Ultem, especially glass-filled grades like Ultem 2300, offers high stiffness and strength, making it suitable for static structural components where wear is not the primary concern.

Machinability and Ease of Processing

Here, Ultem holds a distinct advantage. Its lower processing temperature requirements make it easier and faster to machine, resulting in less tool wear and potentially lower manufacturing costs. PEEK's high heat resistance makes it more challenging to process, requiring specialized equipment and precise control over temperatures to avoid internal stresses and ensure proper crystallization, as highlighted by Comco Plastics. This complexity contributes to its higher overall part cost.

Chemical Resistance and Cost

PEEK is one of the most chemically inert polymers available, resistant to a vast range of aggressive fluids, solvents, and even steam, making it ideal for harsh chemical and downhole oil and gas environments. Ultem offers good chemical resistance but is susceptible to stress cracking when exposed to certain substances like ketones and chlorinated solvents. The final, and often decisive, factor is cost. PEEK is one of the most expensive polymers on the market, with raw material costs that can be three times higher than Ultem. This price difference makes a thorough cost-benefit analysis essential.

Application Guide: Choosing the Right Material for Your CNC Parts

Translating technical specifications into a practical decision requires an understanding of real-world application demands. The choice between PEEK and Ultem is rarely about which material is "better" in absolute terms, but which is optimal for a specific function, environment, and budget. Partnering with a manufacturing expert can provide crucial insights; for instance, specialists in formative manufacturing services can offer Design for Manufacturability (DFM) feedback to ensure the selected material aligns with production capabilities. For companies navigating this process, XTJ's comprehensive formative manufacturing services offer a partnership approach, helping accelerate product development from concept to functional prototype with a wide range of material options.

When to Choose PEEK

PEEK is the material of choice when performance cannot be compromised, regardless of cost. Its unique combination of properties makes it indispensable in certain high-stakes industries.

• High-Load, High-Wear Structural Components: For parts like bearings, thrust washers, seals, and gears that experience high mechanical stress and friction, PEEK's superior wear resistance and fatigue strength are critical.
• Extreme Temperature Environments: Any application where continuous operating temperatures exceed 180°C (356°F) necessitates PEEK. This includes components near engines, in downhole drilling equipment, or in high-temperature sterilization processes.
• Harsh Chemical Exposure: In the chemical processing, oil and gas, and medical industries, parts that are exposed to aggressive chemicals, solvents, or repeated steam sterilization benefit from PEEK's exceptional chemical inertness.
• Aerospace and Medical Implants: Due to its strength-to-weight ratio, biocompatibility (in medical grades), and ability to withstand harsh conditions, PEEK is used for structural aerospace parts and long-term medical implants like spinal fusion cages.

When to Choose Ultem

Ultem provides a balanced profile of high performance and better value, making it the ideal selection for a wide range of applications where PEEK would be over-engineered and cost-prohibitive.

• Electrical and Electronic Components: With one of the highest dielectric strengths of any thermoplastic, Ultem is perfect for high-voltage insulators, connectors, and semiconductor test sockets where electrical insulation is paramount.
• Moderate-Temperature Structural Parts: For components that require high strength and stiffness but operate below 170°C (340°F), Ultem offers excellent performance at a lower cost. Examples include automotive sensor housings and aircraft interior components.
• Medical and Food-Grade Equipment: FDA-compliant and steam-autoclavable grades of Ultem are widely used for reusable medical instruments, sterilization trays, and food service equipment that require repeated cleaning.
• Complex, Precision-Machined Parts: Because it is easier to machine, Ultem is a great choice for intricate parts where achieving tight tolerances is crucial and the higher cost and difficulty of machining PEEK are not justified. As noted by HONSCN, its high strength-to-weight ratio is a key advantage.

Decision Checklist for Engineers

Ask these questions to guide your selection:

1. What is the maximum continuous operating temperature the part will experience?
2. Is the part subject to significant wear, friction, or cyclic loading?
3. What specific chemicals or fluids will the part be exposed to?
4. Are there critical electrical insulation requirements?
5. What is the target cost per part and the overall project budget?
6. How complex is the part geometry, and are there extremely tight tolerances?

Limitations and Disadvantages: What to Watch Out For

While PEEK and Ultem are high-performance materials, they are not without their trade-offs. Understanding their limitations is crucial for avoiding costly design errors and ensuring long-term part reliability. A balanced perspective that acknowledges potential disadvantages is a hallmark of sound engineering judgment.

Disadvantages of PEEK

Despite its superior performance, PEEK has several factors that can make it a challenging or impractical choice for some applications.

• Extremely High Cost: PEEK is one of the most expensive polymers available. Its price can be a significant barrier, making it suitable only for applications where its unique properties are absolutely essential and no other material will suffice.
• High Processing Temperatures: Machining and molding PEEK require very high temperatures (melting point around 343°C / 649°F), which necessitates specialized equipment and expertise. This adds to the overall manufacturing cost and complexity.
• Susceptibility to UV Radiation: Unfilled, natural PEEK can be degraded by prolonged exposure to UV radiation. While this can be mitigated with additives like carbon fiber or pigments, it is a consideration for outdoor applications.
• Chemical Attack by Specific Acids: While generally inert, PEEK can be attacked and dissolved by concentrated sulfuric and nitric acids, a limitation to consider in very specific chemical environments.

Disadvantages of Ultem

Ultem offers a more accessible price point but comes with its own set of performance limitations that must be respected in the design process.

• Susceptibility to Stress Cracking: Ultem is more prone to environmental stress cracking than PEEK, particularly when exposed to certain chemicals like ketones, esters, and chlorinated hydrocarbons while under mechanical load. This requires careful chemical compatibility checks.
• Lower Thermal Resistance: Its primary limitation compared to PEEK is its lower continuous operating temperature of 170°C (340°F). Using it in applications that experience temperature spikes above this can lead to deformation and failure.
• Lower Wear Resistance: Ultem is not ideal for high-wear applications involving friction or abrasion. In such scenarios, PEEK or a dedicated bearing-grade polymer would be a much better choice.
• Brittleness in Some Grades: While generally tough, some grades of Ultem can be notch-sensitive or more brittle than PEEK, requiring careful design to avoid sharp corners and stress concentrators.

Key Takeaways: Making the Final Decision

The selection between PEEK and Ultem for CNC machined parts is a classic engineering decision that balances performance against cost. PEEK stands as the ultimate choice for applications where failure is not an option, offering unmatched thermal, mechanical, and chemical resistance. It is the go-to material for extreme environments in aerospace, energy, and medical implants. However, this premium performance comes at a significant cost, both in material and processing.

Ultem, on the other hand, provides a highly capable and more pragmatic solution for a broader range of applications. It delivers excellent strength, stiffness, and superior electrical insulation at a fraction of the cost of PEEK. For components in electronics, medical devices, and automotive systems that operate under 170°C, Ultem often provides the perfect blend of performance, processability, and value. The final decision rests on a thorough analysis of the application's specific demands, with temperature, wear, and budget being the most critical deciding factors.

Frequently Asked Questions

1. Is Ultem plastic better than PEEK?

Neither material is universally "better"; their suitability depends entirely on the application. PEEK is superior for extreme temperature environments (up to 260°C), high-wear situations, and harsh chemical exposure. Ultem is a better choice for applications requiring high dielectric strength, easier machinability, and a more controlled budget, provided the operating temperature stays below 170°C.

2. What are the main disadvantages of Ultem?

The primary disadvantages of Ultem include a lower maximum operating temperature compared to PEEK, susceptibility to stress cracking when exposed to certain chemicals (like ketones and chlorinated solvents), and lower wear resistance. It is not ideal for parts that experience significant friction or abrasion.

3. What are the disadvantages of PEEK polymer?

PEEK's main disadvantages are its very high cost, which can be prohibitive for many projects, and its high processing temperature, which requires specialized equipment and adds to manufacturing complexity. It can also be degraded by UV radiation if not properly formulated and is attacked by concentrated sulfuric and nitric acids.

4. Is Ultem 2300 (glass-filled) better than PEEK?

Ultem 2300, which is 30% glass-reinforced, offers significantly increased strength, stiffness, and dimensional stability compared to unfilled Ultem. While it is stronger than unfilled PEEK in some respects, it still does not match PEEK's superior thermal resistance, chemical inertness, or wear performance. Therefore, unfilled PEEK would still be the better choice for applications exceeding Ultem's temperature or chemical compatibility limits.