Designing injection molds for high-temperature materials (such as PEEK, PEI, PPS, high-temperature nylon PPA, LCP, etc.) is a highly specialized field. It differs significantly from molds designed for standard plastics (like ABS, PP) and requires consideration of much more demanding factors.
Key focus areas when designing injection molds for high-temperature materials can be broken down into five main aspects: material selection, thermal management, gating system, ejection system, and corrosion/wear protection.
I. Mold Material and Heat Treatment
This is the most fundamental aspect, as the high-temperature materials themselves and their processing temperatures place extreme demands on the mold steel.
- High-Performance Mold Steels:
- It is essential to select steels with excellent heat resistance, high hardness, and high strength, such as pre-hardened mold steels or higher-grade powder metallurgy steels.
- Recommended Steels: S136H and other pre-hardened steels are suitable for general high-temperature plastics; for highly corrosive and abrasive materials (like PEEK), extremely corrosion-resistant steels such as M333, Elmax, or CPM-10V should be used.
- Core Advantage: These steels maintain high hardness and stability at elevated temperatures, resisting thermal fatigue and heat checking caused by long-term thermal cycling.
- Risk of Excessive Hardness and Insufficient Toughness:
- While high hardness provides wear resistance, excessive hardness can increase the brittleness of the mold, making it prone to cracking, especially in thin-walled sections or sharp corners. A balance must be struck between hardness and toughness.
- Coating Process for Cores/Cavities:
- Surface treatments such as Nitriding, PVD, or DLC coating are highly recommended.
- Benefits: They significantly increase surface hardness, wear resistance, and corrosion resistance, while also reducing the coefficient of friction. This improves release performance and reduces damage to the mold.
II. Thermal Management and Cooling System Design
The processing temperatures for high-temperature materials often exceed 300°C, making effective thermal control crucial.
- Efficient Cooling Channels:
- Design Principle: Cooling must be uniform and efficient. Water channels should be placed as close as possible to the cavity surface and laid out evenly.
- Recommended Method: Prioritize the use of 3D-printed conformal cooling channels. They perfectly follow the contour of the part shape, enabling uniform cooling, significantly reducing cycle times, and minimizing part warpage and internal stress caused by uneven cooling.
- Mold Insulation Measures:
- Add insulation plates between the mold and the injection molding machine platens.
- Purpose: To prevent excessive heat from the mold from transferring to the machine, protecting the machine’s hydraulic and electrical systems, and stabilizing the process.
- Mold Temperature Controller Selection:
- It is mandatory to use high-temperature oil-type mold temperature controllers, as their heating capacity and temperature range far exceed those of standard water-type controllers.
- Control Precision: The temperature controller must provide precise control (±1°C), as minor fluctuations in mold temperature directly affect the crystallinity and dimensional stability of high-temperature materials.
III. Gating System Design
The gating system directly affects melt filling, material properties, and product quality.
- Runner and Gate Size:
- High-temperature materials typically have high melt viscosity and relatively poor flowability.
- Design Point: Runner and gate dimensions should be appropriately larger to reduce flow resistance and ensure complete filling. The sprue should also be designed to be shorter and thicker.
- Cold Slug Well and Venting System:
- Cold Slug Well: Must be sufficiently large to effectively trap the cold material at the flow front, preventing it from blocking the gate or entering the cavity.
- Venting System: High-temperature materials are prone to decomposition gases, making it essential to provide adequate and properly deep venting channels to prevent air traps that can cause burns, short shots, and other defects.
IV. Ejection System Design
High-temperature materials are often stiffer and more brittle, and their shrinkage rates can differ from standard plastics, placing higher demands on the ejection system.
- Draft Angles:
- Maximize the draft angle within allowable limits to prevent part dragging, scoring, or cracking due to high ejection forces or friction.
- Ejection System:
- Ejector pins should be sufficient in number, evenly distributed, and possess adequate strength and rigidity.
- Recommendation: Use large-area ejection methods such as sleeve ejectors or air-assisted ejection to distribute the ejection force and avoid localized stress concentration that could damage the part.
V. Corrosion and Wear Protection
- Corrosive Gases:
- Some high-temperature materials can decompose during processing, releasing corrosive gases that accelerate mold corrosion. Therefore, selecting corrosion-resistant steels and protective coatings is critical.
- High Wear Risk:
- High-temperature materials are often reinforced with glass fibers or minerals, and these additives are highly abrasive to the mold, especially gates and cavity surfaces. High-hardness steels and wear-resistant coatings are essential.