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What is a Hot Runner System?

A hot runner system is a unit within an injection mould which injects molten plastic into the cavities of the mould. It is, essentially, an assembly of heated components.

It can be one of the most sophisticated systems in the injection moulding process.

The process involves the transference of molten plastic from the injection moulding machine into the cavities of the mould. The cavities take the same form and shape as the parts to be produced.

A hot runner system usually contains a heated manifold and several machine nozzles. The molten plastic is injected into the mould from the nozzles.

A controller regulates the temperature of the hot runner, ensuring that the parts produced are consistent in form and shape.

After the molten plastic has been injected into the mould, the mould opens, and the newly created plastic components are ejected. Manual labour is not required to remove excess plastic parts.

For a hot runner system to be effective, it must operate at a constant temperature and a uniform pressure.

Hot Runner System – Main Components

Manifold – the manifold distributes the plastic material into the nozzles

Nozzle – the nozzle injects the molten plastic into the cavities of the mould

Inlet – the plastic is injected from the nozzle and into the mould at this entry point

Thermostat – controls the heat of the manifold

Plate – the source of heat and the location for where the runners are situated

Hot tip hot runner system: This is the most common type of hot runner. It can be used for most crystalline and non-crystalline plastic materials, such as PP, PE, PS, LCP and PA. The hot tip gate is used to produce small to medium sized parts. When hot tips are used, they will leave gate marks on the parts, which may affect how the product looks or compromise its assembly with other parts.

Sprue gate hot runner system: This is more suitable for parts that are of a medium size and density. The advantages of using this type of system include reduced deformity and greater mechanical strength. However, because the sprue gate is larger, the gate marks will also be larger, and may adversely affect the aesthetics of the final component. Sprue gating is often used for internal parts, where visual presentation is less important. It can also be used in combination with a cold runner.

Valve gate hot runner system: Valve gate hot runner systems regulate the flow of plastic into the mould cavity by using mechanical pins that open and close at the tip of the hot runner nozzle. Often used in industries that favour automated production, this system has the following advantages:

– Improved part quality, particularly for complex and or cosmetic components
– Ideal for plastic parts with strict surface requirements
– Shorter moulding cycle timescales
– Less product deformity, reduced warpage and enhanced physical properties
– Eliminates the stringing and drooling of melt at the gate

Advantages of Using Hot Runner Systems

– Can produce very complex parts
– Reduced cycle time
– Lower raw material costs
– Improved quality of final components
– Allows for greater accuracy and consistency
– Reduced waste material due to the absence of a cold runner
– Useful for production automation, where secondary actions are not required
– Ideal for high volume production of low-cost parts

Hot Runner Systems vs Cold Runner Systems

One of the key differences between hot runner and cold runner systems is that hot runners inject plastic directly into the part cavity, so the mould can be cycled again quickly.

On the downside, hot runner systems are more expensive due to heightened set up costs.

Cold runners are more suitable for low volume production cycles. They also allow for a greater choice of polymers to be used and for colour changes to be made quickly. However, despite their flexible design options, cold runner systems also have their drawbacks:

– Runner systems need to be separated from their parts, leading to a more complicated automation process, or the use of manual labour
– Increased possibility of product damage during the separation process
– Greater amount of plastic waste from runners, if it is not reground or recycled
– Increased cycle times