PEI (Polyetherimide / Ultem) — CNC Machining Material Manual

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Rating legend — ★★★★★ best · ★☆☆☆☆ worst. For machinability/wear/heat resistance more stars = better; for cost, fewer stars = cheaper.

📋 Material Quick-Reference Card

┌──────────────────────────────────────────┐
│  Material Name: Polyetherimide (PEI)      │
│  (Ultem / transparent amber plastic)      │
│  Category: Amorphous high-performance     │
│            engineering thermoplastic      │
│  Density: 1.27~1.29 g/cm³                 │
│  Tensile Strength: 90~105 MPa             │
│  Flexural Strength: 145~160 MPa           │
│  Glass Transition Temp.: ~217 ℃           │
│  Long-term Service Temp.: 170~180 ℃       │
│  Machinability: ★★★★☆ (stable, precise)   │
│  Heat Resistance: ★★★★☆                  │
│  Flame / FST Performance: ★★★★★          │
│  Chemical Resistance: ★★★☆☆              │
│  Cost: ★★☆☆☆ (expensive, < PEEK)        │
│  Keywords: Ultem, amber transparent,      │
│            flame-retardant, low smoke,    │
│            high-temp dimensional stability│
└──────────────────────────────────────────┘

1. Material Overview

1.1 Introduction

PEI (Polyetherimide) is a high-temperature, high-strength, inherently flame-retardant amorphous engineering plastic. It is widely known by the brand name Ultem (SABIC; sometimes misspelled SABIG), and is recognized by its typical transparent amber appearance. PEI is especially valuable when CNC parts need dimensional stability, electrical insulation, heat resistance, and excellent FST performance (flame, smoke, and toxicity), making it common in aerospace, electrical, semiconductor, and medical applications.

• English Name: Polyetherimide / PEI
• Common Nicknames: Ultem, amber high-temperature plastic, PEI resin
• Famous Brand Names: Ultem (SABIC), TECAPEI, Sustapei

1.2 Two Main Types ⭐ Important

💡 Unfilled Ultem 1000 is the most common CNC PEI grade for precision insulating and aerospace parts. Glass-filled Ultem 2300 is selected when stiffness and low thermal expansion are more important than tool life or edge toughness.

1.3 Raw Material Forms

Common forms for CNC machining:

• PEI Rod (round bar): turned parts, bushings, insulators, small precision components
• PEI Sheet/Plate: milled brackets, fixtures, semiconductor nests, aerospace interior parts
• Common colors: transparent amber/natural, black; glass-filled grades are usually opaque amber/brown

2. Composition & Physical Properties

2.1 Material Composition

PEI is an amorphous thermoplastic polyimide-family resin containing imide and ether groups in its polymer backbone. The imide structure provides high heat resistance, strength, and flame retardancy, while the ether linkages improve processability compared with fully aromatic polyimides.

2.2 Physical Properties

💡 PEI is amorphous, so it generally has less machining warp and more predictable dimensional stability than many semi-crystalline high-temperature plastics. This is one reason it can hold tight tolerances better than PEEK in some geometries.

3. Mechanical & Chemical Properties

3.1 Mechanical Properties

⚠️ PEI is strong and rigid, but it is notch-sensitive. Avoid sharp internal corners, knife edges, and abrupt section changes, especially in parts exposed to load, heat, or chemicals.

3.2 Chemical Resistance

3.3 Notable Characteristics

• High heat resistance: continuous service around 170~180 ℃, with Tg around 217 ℃
• Inherently flame-retardant: commonly UL94 V-0, with very low smoke and low toxicity
• Excellent FST performance: a major advantage for aerospace interior applications
• High strength and stiffness: suitable for structural plastic parts and precision fixtures
• Dimensionally stable: amorphous structure, low moisture absorption, low warp tendency
• Good dielectric properties: widely used for electrical/electronic insulators and connectors
• Steam sterilization capable: suitable for selected medical instruments, trays, and reusable components

4. CNC Machining Process ⭐⭐ Core

4.1 Machinability Rating

★★★★☆ Excellent machinability for a high-performance plastic — PEI machines predictably and can hold tight tolerances:

• Amorphous structure gives stable cutting behavior and less post-machining movement than many semi-crystalline plastics
• Produces clean edges and good surface finish when tools are sharp
• Capable of tight tolerances for precision insulating, aerospace, and semiconductor parts
• More notch-sensitive than POM or nylon, so geometry and toolpath planning matter

4.2 Recommended Tooling

4.3 Recommended Cutting Parameters

📌 Parameters are for reference only; adjust based on machine rigidity, tool diameter, part geometry, grade (unfilled vs glass-filled), and tolerance requirements.

4.4 Machining Challenges & Solutions

4.5 Annealing Recommendation ⭐

To reduce machining distortion and lower the risk of stress cracking, annealing is recommended before/after machining precision PEI parts:

Reference Annealing Process:
• Temperature: 160~180 ℃
• Time: approx. 30~60 min per 25mm of wall thickness
• Cooling: slow furnace cooling to below 80 ℃ before removal

💡 For high-precision PEI parts, the rough machining → annealing → finish machining workflow improves dimensional stability and reduces cracking risk, especially for thick sections or tight-tolerance aerospace/semiconductor components.

4.6 Cooling Methods

• Air cooling / air blast: preferred for most PEI machining; removes chips without introducing chemical stress risk
• Compatible water-soluble coolant: acceptable only after compatibility testing; useful for deep drilling and heat control
• ❌ Avoid aggressive cutting fluids, chlorinated solvents, strong alkaline cleaners, and solvent wiping on stressed parts

5. Surface Treatment

PEI has good inherent appearance in natural transparent amber, but surface treatment options are more limited than ABS or PC. For functional parts, PEI is usually used as-machined, polished, bead-blasted, or laser-marked.

💡 If the part needs a decorative metal finish or reliable painting, PEI is usually not the first choice. Use PEI for heat, flame/FST, electrical, and dimensional-stability performance, not for rich cosmetic finishing.

6. Applications & Material Selection

6.1 Typical Application Industries

6.2 Pros & Cons Summary

6.3 Material Selection Guide

✔ Recommended for PEI:

• Aerospace interior or structural plastic parts requiring excellent FST performance
• Precision CNC parts needing high strength, heat resistance, and dimensional stability
• Electrical/electronic insulators, connectors, and dielectric components
• Semiconductor test sockets, wafer handling fixtures, nests, and high-temperature tooling
• Medical parts requiring repeated steam sterilization or hot-water resistance
• Applications where PEEK performance is desired but lower cost than PEEK is important

✘ Not recommended for:

• Maximum continuous temperature or chemical resistance → choose PEEK or PI
• Sharp-cornered, high-impact, highly notched parts → redesign with radii or choose tougher materials
• Sliding/wear parts requiring self-lubrication → choose POM, PTFE, or filled PEEK
• Exposure to chlorinated solvents, ketones, aromatic solvents, or aggressive cleaners → choose PTFE, PVDF, or verify compatibility
• Cosmetic parts needing reliable painting/electroplating → choose ABS or PC

📌 Compared with PEEK, PEI is usually cheaper, amorphous, easier to hold dimensionally stable, and excellent for aerospace FST, but it has lower continuous temperature capability, lower chemical resistance, and greater solvent stress-cracking sensitivity.

⚠️ Safety & Handling Notes

⚠️ Do not assume every coolant or cleaning solvent is safe for PEI. The most common shop-floor failure mode is a precision part that machines correctly, then cracks later due to residual stress plus incompatible solvent exposure. Anneal precision parts and verify fluid compatibility.

🔗 Related Pages

• Plastic Materials Handbook (overview)
• PEEK — higher-temperature, higher-chemical-resistance upgrade
• PI (Polyimide) — ultra-high-temperature engineering plastic