Hot runners cost more up front but can pay back within months; cold runners are simpler and more flexible. A practical comparison of cost, cycle time, scrap, colour changes and maintenance, with the questions that decide it.
A closure manufacturer was moulding 32 million caps a year in polypropylene on cold runner tools. The runners weighed almost as much as the parts. They were granulated and fed back at 20%, which looked like free material until the quality team traced a recurring black-speck problem to contaminated regrind, and engineering calculated that cooling the thick sprue was setting the cycle time, not the parts. Converting the highest-volume tool to a hot runner cost a significant sum. It paid back in about seven months through material, cycle time and scrap savings.
But hot runners are not always the right answer. The decision depends on volume, material, colour changes and the kind of supplier support available.
How each works
- Cold runner: the channels that carry plastic from the machine nozzle to the cavities are part of the mould. They cool and solidify with the part and are ejected every shot, then separated as scrap or regrind.
- Hot runner: a heated manifold and nozzles keep the plastic molten all the way to the gates. Only the parts are ejected. The runner material stays in the system for the next shot.
Where hot runners win
- Material savings. No runner scrap. On small parts the runner can weigh as much as the parts themselves.
- Cycle time. The runner no longer needs to cool, so the cycle is governed by the part. Savings of 10–30% are common on small, thin parts.
- Quality and process control. Less pressure loss, better balance between cavities and the ability to control each nozzle's temperature.
- Automation. No runner to separate or handle.
- Gate options. Valve gates can sequence filling to control weld lines, and leave a cleaner gate mark.
Where cold runners win
- Lower tool cost. A hot runner system adds significant cost, especially on multi-cavity tools.
- Simplicity and robustness. Fewer heaters, thermocouples and controllers to fail.
- Colour and material changes. Cold runners purge almost instantly. Hot runners can take many shots to clear a strong colour, especially with poorly designed manifolds.
- Heat-sensitive materials. Some materials, such as PVC and certain flame-retardant grades, are prone to degradation if held hot in a manifold for too long.
- Low volumes, where the runner material and cycle savings never repay the extra tool cost.
The cost comparison that matters
Compare total cost over the life of the product, not tool price:
- Extra tool cost for the hot runner.
- Runner material saved per shot × shots per year × material price, minus any value of regrind.
- Cycle time saved × machine-hour rate × annual hours.
- Scrap and quality differences.
- Additional maintenance and the cost of hot runner downtime.
At high volumes the payback is often months. At a few thousand parts a year it may never arrive.
Hybrid options
Hot-to-cold systems use a hot manifold feeding short cold sub-runners, which combine much of the material saving with simpler gating and easier colour changes. They are common in family tools and multi-cavity tools for small parts.
Questions to ask before deciding
- What are the annual volume and expected product life?
- How often will colours or materials change?
- Is the material heat-sensitive or abrasive?
- What is the runner-to-part weight ratio on the cold runner design?
- Does your moulder have hot runner maintenance experience and spares?
- Is the hot runner brand supported locally?
The cheapest tool to buy is often not the cheapest tool to run. Ask your toolmaker to quote both options on the same specification, as we described in how to compare moulding quotes fairly, and let the life-cycle numbers decide.
