Mold Design for High-Temperature Polymers: Gate, Runner, and Cooling Considerations

High-Temperature Molding Is Different

Designing an injection mold for high-temperature polymers (PEEK at 360-400°C, PPS at 300-340°C, LCP at 280-350°C) requires fundamentally different thinking than designing for commodity polymers. The high melt temperatures, narrow processing windows, and crystalline behavior of these materials impose constraints that are not present in standard mold design. Running a PEEK mold designed for ABS will produce poor parts — if it produces any parts at all.

Gate Design

For semicrystalline high-temperature polymers, gate location determines the flow direction, which determines the orientation of crystalline domains and fiber fillers. This is critical because the mechanical properties of these materials are anisotropic — the strength in the flow direction is 1.2-1.5× higher than the transverse direction for PEEK, and 3-5× higher for LCP. Gate location must be chosen to align the flow direction with the primary load-bearing axis of the part. Edge gates are preferred for most high-temperature polymer applications — they provide unidirectional flow that maximizes the orientation effect. Pin-point gates are acceptable for smaller parts but create a high-shear zone at the gate that can cause localized degradation in shear-sensitive materials like PEEK and LCP. Fan gates are recommended for parts requiring low orientation or balanced filling. The gate diameter should be 80-100% of the wall thickness — not smaller, as is common for commodity polymers. Undersized gates cause excessive shear heating that can degrade the polymer and cause splay, burn marks, or molecular weight loss. For PEEK, the gate freeze-off time must be carefully controlled — the gate must remain open during the holding phase to allow melt packing during crystallization shrinkage, which is approximately 1.2% for unfilled PEEK.

Runner System Design

Full-round runners are essential for high-temperature polymers — trapezoidal or half-round runners increase pressure drop and shear heating. The runner diameter should be 6-10mm for most high-temperature polymer applications, with the larger diameter for longer flow lengths and higher-viscosity materials. Hot runner systems are strongly recommended for high-temperature polymers, particularly for PEEK and PPS. A hot runner eliminates the regrind from cold runner systems (regrind is acceptable at 20-30% for PEEK but not recommended for LCP and high-temperature PPS grades). The hot runner manifold must be designed for the specific melt temperature of the material — PEEK requires a manifold rated for 400°C, with ceramic heater bands and high-temperature seals. The nozzle tip temperature should be independently controllable, typically set 5-10°C below the manifold temperature to prevent drool. For PEEK, the hot runner must be purged with a thermally stable polymer (PP or PE) before shutdown to prevent the PEEK from solidifying in the manifold.

Cooling System Design

Mold temperature control is the most critical aspect of high-temperature polymer mold design. For PEEK, the mold temperature must be maintained at 160-200°C throughout the cycle — this requires oil-heated mold temperature controllers (not water-heated, which are limited to 140-150°C). The cooling channels must be positioned to provide uniform temperature distribution across the mold surface, with a target temperature variation of less than ±5°C across the cavity. Conformal cooling (3D-printed mold inserts with cooling channels that follow the part surface) is the most effective approach for high-temperature polymer molds. Conformal cooling reduces cooling time by 30-50% compared to conventional drilled channels, and more importantly, provides uniform temperature distribution that eliminates warpage from differential cooling. The cooling channel material must be selected for compatibility with the high mold temperature — copper alloys (AMPCO 940, thermal conductivity 208 W/mK) are preferred for rapid heat extraction, but must be protected from corrosion when using oil-based heat transfer fluids. The cooling circuit should be designed with turbulent flow (Reynolds number >4,000) to maximize heat transfer efficiency. For PEEK molds, the cooling circuit must be capable of both heating (during start-up) and cooling (during the cycle) — this requires a dual-function temperature controller that can switch between heating and cooling modes.

References & Industry Standards

• ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
• ISO. ISO 1043 — Plastics — Symbols and Abbreviated Terms. iso.org
• National Institute of Standards and Technology (NIST). Polymer Properties Database. nist.gov
• UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
• MatWeb — Material Property Data. matweb.com