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ISO 28238:2010 — Components for Gating Systems in Compression and Injection Moulds
Compression and injection moulds — Components for gating systems
Introduction to ISO 28238:2010
ISO 28238:2010 specifies the characteristics and terminology of components for gating systems used in compression and injection moulds for thermoplastics, thermosetting plastics, and elastomers. Developed by ISO/TC 29 (Small tools), SC 8 (Tools for pressing and moulding), this standard establishes coherent terminology for conventional gating, externally heated hot runner systems, internally heated systems, and cold runner gating systems. It references extensive component standards including ISO 10072 (sprue bushes), ISO 6751 (ejector pins), and ISO 12165 (general mould component terms).
For mould designers, ISO 28238 is the definitive reference for gating system component selection. A correctly specified gating system can reduce cycle time by 15-30% compared to conventional sprue designs, while also improving part quality and reducing material waste.
Conventional and Hot Runner Gating Systems
The standard categorizes gating systems into three main types: conventional gating (solidifying sprue), externally heated hot runner, and internally heated hot runner. Conventional gating uses sprue bushes (per ISO 10072) with ejector pins and sprue pullers, where the sprue solidifies and is ejected each cycle. Hot runner systems maintain the plastic in a molten state within the manifold, eliminating sprue waste. Externally heated systems use heated distributor bushings (Types A-H in unheated and heated variants) and manifold blocks (I, H, X, S, T, XU, SU forms). Internally heated systems use torpedo heaters and spreader tubes where the plastic flows externally past the heating elements.
| Gating System Type | Key Components | Advantages | Typical Applications |
|---|---|---|---|
| Conventional (solidifying sprue) | Sprue bush, sprue puller, ejector pin | Lowest tool cost, simple design | Prototyping, low-volume production |
| Externally heated hot runner | Distributor bushing (heated), manifold block, nozzle with thermal/mechanical seal | No sprue waste, balanced filling, faster cycles | High-volume automotive, consumer goods |
| Internally heated hot runner | Cartridge heater, torpedo, spreader tube, seal cap | Better temperature control at nozzle tip | Engineering plastics, temperature-sensitive materials |
| Cold runner | Distributor bushing, manifold block, insulating ring, sealing cylinder/needle | Suitable for thermosets and elastomers, no thermal degradation | Rubber moulding, thermoset compression |
Nozzle Types and Critical Design Parameters
Nozzles for externally heated hot runner systems are classified by their sealing mechanism: thermal seal (open nozzle, pointed open nozzle, nozzle for lateral injection with/without cold slug, pointed open multiple nozzle) and mechanical seal (needle valve nozzle). Thermal seal nozzles rely on precise temperature control at the gate to prevent drool and stringing. Mechanical seal nozzles use a reciprocating needle to positively shut off the gate, providing cleaner cut-off and better control for high-speed injection. The nozzle tip design must balance gate vestige requirements with flow characteristics and pressure drop.
Thermal gate balance is the most common challenge in multi-cavity hot runner moulds. Even minor temperature differences between cavities can cause significant filling imbalance, leading to variation in part weight, dimensions, and mechanical properties. Use individual thermocouple control for each nozzle zone.
Cold Runner Systems for Thermosets and Elastomers
Cold runner gating systems maintain the uncured material at a controlled low temperature to prevent premature crosslinking while the mould cavities are heated for curing. The standard defines components including the distributor bushing, manifold block, insulating ring, nozzle, quick-action clamping system, sealing cylinder, and sealing needle. This design allows the runner material to remain reusable in some applications, significantly reducing waste compared to conventional thermoset moulding where the sprue cures and is discarded.
When designing moulds for liquid silicone rubber (LSR) or other injection-mouldable elastomers, the cold runner system per ISO 28238 is essential. Proper insulating ring design and temperature control at the gate-cavity interface are critical to prevent premature curing (scorch) while maintaining material flow.
Engineering Design Insights
Key selection criteria for gating system components include: material melt temperature and viscosity (determining heating requirements), shot weight and cavity volume (manifold sizing), gate location and type (component clearance requirements), and maintenance accessibility. The standard’s detailed figures and tables provide comprehensive dimensional references for all component types. When designing a hot runner system, pay particular attention to the manifold block form selection — the flow path geometry directly affects pressure drop, shear heating, and material degradation risk. Cross-shaped (X) manifolds offer the best flow balance for four-cavity layouts, while H-shaped manifolds are preferred for larger cavity counts with centre injection.
Q1: What is the difference between Types A-D and Types E-H distributor bushings?
A: Types A-D are unheated distributor bushings (Type A = standard, B = immersion nozzle, C = with filter cartridge, D = immersion nozzle with cartridge). Types E-H are the heated equivalents with integrated heating elements.
A: Types A-D are unheated distributor bushings (Type A = standard, B = immersion nozzle, C = with filter cartridge, D = immersion nozzle with cartridge). Types E-H are the heated equivalents with integrated heating elements.
Q2: How do I choose between thermal seal and mechanical seal nozzles?
A: Thermal seal nozzles are simpler and more economical but require precise temperature control. Mechanical (needle valve) nozzles provide positive shut-off, better gate aesthetics, and are preferred for high-speed injection or when gate quality is critical.
A: Thermal seal nozzles are simpler and more economical but require precise temperature control. Mechanical (needle valve) nozzles provide positive shut-off, better gate aesthetics, and are preferred for high-speed injection or when gate quality is critical.
Q3: What is the purpose of the cold runner system for elastomers?
A: It keeps the runner material at a controlled low temperature to prevent premature crosslinking, allowing the runner to remain separate from the cured part and potentially be reused.
A: It keeps the runner material at a controlled low temperature to prevent premature crosslinking, allowing the runner to remain separate from the cured part and potentially be reused.
Q4: Which ISO standards are most critical for component compatibility?
A: ISO 10072 (sprue bushes), ISO 6751 (ejector pins), ISO 6753-2 (machined plates), ISO 10907-1 (locating rings), ISO 15600 (thermal insulating sheets), ISO 16915 (sprue pullers), and ISO 8017 (guide pillars).
A: ISO 10072 (sprue bushes), ISO 6751 (ejector pins), ISO 6753-2 (machined plates), ISO 10907-1 (locating rings), ISO 15600 (thermal insulating sheets), ISO 16915 (sprue pullers), and ISO 8017 (guide pillars).
