Cold runner

In plastics injection moulding, a “cold runner” has a different meaning: here, cold areas in the mould are avoided as far as possible, as plastic melts must remain in a liquid state. In elastomer processing, by contrast, the cold runner is actively cooled to prevent premature vulcanisation.

The cold runner system consists of cooled channels through which the material is guided before it enters the mould cavities. This cooling prevents the material from vulcanising in the runner, while the mould cavity itself is heated to vulcanise the material in a controlled way. The process comprises the following steps:

• Material injection: the raw material (e.g. rubber or liquid silicone) is injected through a nozzle into the cold runner system.

• Distribution: the material is passed through the cooled channels and guided to the mould cavities.

• Filling the cavities: the material enters the heated mould cavities, where it vulcanises completely.

• Removal: after vulcanisation, the finished parts are removed. The material in the cold runner system remains unprocessed and can be used again in the next cycle.

Cold runner systems offer specific advantages in elastomer processing:

• Material efficiency: the material in the cold runner does not vulcanise and can be reused, which reduces waste.

• Improved quality: even filling of the cavities ensures consistently high component quality.

• Cost-effectiveness: especially when processing expensive elastomer materials, the cold runner pays for itself through lower material losses.

Despite their advantages, there are also challenges in using cold runners in the rubber industry:

• High energy consumption: cold runners must be actively cooled while the mould is heated at the same time. This means high energy consumption, as these two systems work against each other.

• Design complexity: designing a cold runner requires precise planning to ensure even material distribution and avoid material hardening.

• Tooling costs: the investment costs for a cold runner system are higher than for conventional moulds without a cold runner.

Cold runner systems are used above all when complex geometries and high material efficiency are required:

Automotive industry

• Seals: manufacture of door and window seals that require precise dimensions and high weather resistance.

• Vibration dampers: production of heat-resistant rubber parts that remain stable under high loads.

Medical technology

• Silicone parts: manufacture of precise components such as seals and tubes from LSR (liquid silicone rubber).

• Hygiene products: production of prostheses and protective covers with high requirements for material purity.

Electronics industry

• Insulators: production of rubber insulation for cables and electrical connections.

• Protective housings: manufacture of protective covers from elastomers that are resistant to heat and moisture.

Construction industry

• Window and door seals: production of durable, weather-resistant sealing profiles.

• Profile parts: manufacture of sealing profiles and elastomer components for building elements.

The main differences between cold runner systems (for elastomers) and hot runner systems (for plastics) lie in temperature control and material reuse:

Criterion	Cold runner	Hot runner
Temperature	Actively cooled, material remains formable	Heated, material remains molten
Material consumption	Less waste, as material is reused	Higher consumption, as material remains in the runner
Material type	Elastomers (e.g. rubber, LSR)	Thermoplastics
Energy consumption	High, as active cooling and heating are required	High, but from heating only
Costs	High investment costs, long-term savings through material reuse	Medium investment costs, but higher ongoing operating costs

Cold runners are particularly worthwhile for large quantities and complex components, where material costs can be offset by savings. They also make sense when materials are used that are expensive or difficult to obtain, as waste is minimised. By contrast, for small series or simpler geometries, conventional moulds are often sufficient and more cost-effective.

The cold runner system is a key technology in rubber and silicone processing that enables precise manufacturing and material efficiency. It is particularly indispensable in the series production of high-quality seals, dampers and other elastomer components. Despite the higher investment costs, the cold runner offers long-term advantages through material savings and high product quality.