Polyetherimide (PEI), commercially known as Ultem, is a high-performance plastic revered for its versatility, but molding it presents unique challenges that can test even experienced manufacturers. Its exceptional high-temperature resistance demands specialized equipment, and minor process deviations can lead to costly defects like warpage or voids. Achieving consistent mechanical strength across batches is another hurdle, especially for critical applications such as aerospace components. Many also struggle with PEI’s high viscosity, which can cause uneven filling or short shots. This guide addresses these pain points, offering expert insights into PEI’s properties, molding processes, and more to help you produce reliable, high-quality parts.
Material Properties of Polyetherimide (PEI/Ultem): Strength and Versatility
Polyetherimide (PEI), or Ultem, is a high-performance thermoplastic celebrated for its balanced properties. Its high-temperature resistance is impressive—it can withstand continuous use at 170°C and short-term exposure to 200°C, making it suitable for applications near heat sources like engine compartments or industrial ovens.
Mechanical strength is another key attribute, with a tensile strength of 85–95 MPa and a flexural modulus of 3.0–3.5 GPa, providing rigidity for structural parts while retaining enough toughness to resist impact. Unlike some plastics, PEI maintains much of its strength at elevated temperatures, critical for load-bearing components in harsh environments.
Thermal stability is paired with excellent dimensional stability (coefficient of thermal expansion: 50–60 ppm/°C), ensuring parts retain their shape across temperature fluctuations—vital for precision assemblies like electronics enclosures.
Chemical resistance is strong against fuels, oils, and many solvents, though it’s not resistant to strong acids or alkalis. Its electrical properties are standout, with high dielectric strength and low flammability (UL94 V-0 rating without additives), making it ideal for electrical components.
For specialized applications, PEI offers UV resistance for outdoor use and biocompatibility (USP Class VI certified grades) for medical devices. It also has useful optical properties, with translucent grades allowing light transmission while maintaining heat resistance.
Injection Molding Process for PEI: Taming the Challenges
PEI injection molding requires precise control and specialized equipment due to its high processing demands. Drying requirements are strict: PEI absorbs moisture readily, so it must be dried at 150–160°C for 4–6 hours in a dehumidifying dryer, ensuring moisture content below 0.02% to prevent hydrolysis and voids.
Melt temperature is critical, ranging from 340–380°C. Temperatures below 340°C lead to poor flow and short shots, while exceeding 390°C causes degradation, reducing strength and causing discoloration.
Injection pressure must be high (120–180 MPa) to overcome PEI’s high viscosity, especially for thin-walled parts (≤2 mm). Injection speed should be moderate (20–50 mm/s) to avoid shear heating, which can degrade the material, while ensuring complete mold filling.
Cooling time is lengthy (20–40 seconds) due to PEI’s low thermal conductivity, contributing to a cycle time of 40–80 seconds—longer than standard plastics but shorter than PEEK. High-temperature processing demands machinery with heated barrels and nozzles capable of maintaining 380°C, while humidity control in the molding area (relative humidity <50%) prevents post-molding moisture absorption.
Runner and gate design should minimize pressure drop—large, short runners and direct gates work best. Process optimization often involves incremental adjustments to temperature and pressure, as PEI is less forgiving than commodity plastics.
Mold Design for PEI Molding: Built for Performance
Mold design principles for PEI focus on withstanding high temperatures and managing viscosity. PEI mold materials must resist heat and wear—H13 tool steel is standard, as it retains hardness at 380°C, while pre-hardened steel works for low-volume runs.
Mold flow analysis is essential to simulate filling, as PEI’s high viscosity can lead to uneven flow patterns. Cooling channel layout should be dense and uniform, with channels 8–12 mm from the cavity to ensure even cooling and reduce warpage. Water temperatures of 70–90°C balance cooling rate and mold longevity.
Venting requirements are critical—PEI traps air easily, so vents 0.02–0.03 mm deep at flow ends and parting lines are necessary to prevent voids. Draft angles of 1–2° facilitate ejection, with slightly larger angles (2–3°) for textured surfaces to avoid damage.
Ejector pin design must distribute force evenly to prevent deformation—use multiple pins, as PEI’s rigidity can resist ejection. Hot runner systems with heated manifolds (350–370°C) reduce waste and improve consistency, though they require precise temperature control. Surface finish of the mold should be smooth (Ra < 0.8 μm) to prevent surface defects in the final part, especially for translucent grades.
Quality Control and Defects in PEI Molding: Ensuring Excellence
PEI’s high performance demands rigorous quality control methods. Common defects and solutions include:
| Defect | Cause | Solution |
| Warpage | Uneven cooling or residual stress | Optimize cooling channel symmetry; reduce packing pressure. |
| Voids | Moisture or poor venting | Extend drying time; add more vents. |
| Short shots | Insufficient pressure or low melt temp | Increase injection pressure; raise temperature to 360–370°C. |
| Flash | Excessive pressure or worn mold seals | Reduce pressure; replace worn components. |
| Surface defects | Mold contamination or degradation | Clean mold; lower melt temperature. |
Statistical process control (SPC) monitors variables like melt temperature and pressure, with allowable deviations of ±5°C and ±10 MPa to ensure consistency. Inspection techniques include CMMs for dimensional accuracy (tolerances as tight as ±0.03 mm for precision parts) and visual checks for surface defects under controlled lighting.
Root cause analysis helps address recurring issues—for example, if warpage persists, checking cooling channel flow rates or mold temperature uniformity can identify the source. Regular material testing (tensile strength, impact resistance) ensures batches meet specifications.
Applications of PEI in Injection Molding: Across Industries
PEI’s unique properties make it indispensable in diverse applications:
- Aerospace components: Cabin interiors and wire harnesses leverage high-temperature resistance and flame retardancy.
- Medical devices: Surgical instrument handles and diagnostic equipment housings use biocompatible PEI grades, which withstand repeated sterilization.
- Electronics enclosures: Circuit board carriers and connectors benefit from PEI’s electrical properties and heat resistance.
- Automotive parts: Underhood components like sensor housings resist oils and elevated temperatures.
- Industrial equipment: Pump impellers and valve bodies use PEI’s chemical resistance and dimensional stability.
Design for manufacturing tips: Use wall thicknesses of 1–4 mm (thinner sections need higher pressure), and avoid sharp corners to reduce stress. For biomedical applications, ensure smooth surfaces to prevent bacterial buildup.
Post-Processing and Finishing of PEI Parts: Enhancing Performance
Post-molding operations for PEI require care due to its heat resistance. Machining and trimming are straightforward with carbide tools, though slow speeds (500–1000 RPM) prevent heat buildup that could affect properties.
Adhesive bonding works with epoxy or cyanoacrylate adhesives, though surface treatment (plasma etching) improves bond strength. Ultrasonic welding is possible but requires higher energy than with softer plastics due to PEI’s rigidity.
Surface treatments include painting (with primers designed for high-performance plastics) or plating, though adhesion requires proper surface preparation. Heat treatment (annealing at 150°C for 1–2 hours) relieves residual stress, improving dimensional stability for precision parts.
Assembly tolerances account for minimal thermal expansion, allowing tight fits in aerospace and medical assemblies. Finishing standards vary by industry: medical parts require USP Class VI certification, while aerospace components must meet AS9100 standards.
Yigu Technology’s Perspective
As a leading custom manufacturing supplier in China, Yigu Technology specializes in PEI injection molding. We understand PEI’s nuances, from precise high-temperature processing to ensuring biocompatibility for medical devices. Our expertise in mold flow analysis and process optimization minimizes defects, while strict quality control methods guarantee consistency. Whether you need aerospace components or electronics enclosures, we deliver PEI parts that meet the most demanding standards, leveraging advanced equipment and years of experience.
FAQs
- How does PEI/Ultem compare to PEEK in terms of performance?
PEI offers good high-temperature resistance (170°C continuous) at a lower cost than PEEK (260°C continuous). PEEK has superior chemical resistance, while PEI excels in electrical properties and ease of processing.
- Is PEI recyclable?
Yes, but recycled PEI has slightly reduced mechanical properties. It’s suitable for non-critical parts, while virgin PEI is recommended for structural or medical applications.
- What makes PEI suitable for medical devices?
Certain PEI grades are biocompatible (USP Class VI certified) and withstand repeated sterilization (autoclaving, ethylene oxide), making them ideal for surgical tools and diagnostic equipment.
