Key facts:
- Also called: hot runner mold / cold runner mold (US), hot-runner system, cold-runner system
- What’s heated: cold runner = nothing (channels cool each shot) · hot runner = a manifold + heated nozzles keep the feed molten
- Runner scrap: cold runner = a trimmed runner every shot · hot runner = none
- Tooling cost: cold runner = lower · hot runner = higher (manifold, heaters, controls)
- Cycle time: hot runner is usually faster (no runner to cool)
- Best for: cold runner = lower volume, frequent colour/material changes, sensitive materials · hot runner = high volume, automation, clean gates
- Middle ground: a hybrid (hot-to-cold) runner, a heated manifold feeding short cold sub-runners
What is a cold runner?
A cold runner is an unheated set of channels, the sprue and runners, formed into the mould itself, that carry molten plastic from the machine nozzle to the cavities. Because the channels aren’t heated, the plastic inside them cools and solidifies along with the part each cycle, and the whole runner is ejected with the part and trimmed off. That makes cold-runner tooling simpler and cheaper to build and run, and forgiving of colour changes and heat-sensitive materials: at the cost of a trimmed runner every shot and a small gate mark on the part.
What is a hot runner?
A hot runner keeps the feed channels molten between shots, using a heated manifold and temperature-controlled nozzles. Only the part cools and ejects; there’s no runner to remove or trim, so there’s no runner scrap and the cycle is usually faster. Hot runners also enable clean, controllable gates (including valve gates that open and close on command). The trade-off is costlier, more complex tooling with more to maintain, slower colour changes, and care with heat-sensitive materials, which degrade if held hot too long.
Hot runner vs cold runner: the key differences
| Cold runner | Hot runner | |
|---|---|---|
| Feed channels | Unheated; cool each shot | Heated; stay molten |
| Runner scrap | A trimmed runner every shot | None |
| Cycle time | Slower (runner must cool) | Usually faster |
| Tooling cost | Lower | Higher (manifold, heaters, controls) |
| Maintenance | Simpler | More to maintain |
| Colour / material change | Easy | Slower, more involved |
| Heat-sensitive materials | More forgiving | Needs care (residence time hot) |
| Gate | Trimmed gate mark | Clean / valve-gated |
| Best volume | Lower to medium, prototype | High, automated |
Mould it with Carmo
We don’t just compare the two: we run both, and help you pick the right one for your volume. See our injection moulding, or two-component (2K/3K) moulding, where hot-runner valve gates often do the work.
Which should you choose for your part?
The runner choice usually tracks one thing above all: volume, but four factors decide it together:
- Volume. The higher the annual volume, the more a hot runner pays back: no runner scrap, faster cycles. At lower or prototype volumes, a cold runner’s cheaper tooling wins, and the material saving doesn’t justify the manifold.
- Material. Heat-sensitive grades, and any part that changes colour often, favour a cold runner, which doesn’t hold plastic hot between shots. Stable, high-throughput materials suit a hot runner.
- Gate & cosmetics. Need a flush, mark-free gate, gating on demand, or sequential filling of a large part? That’s hot-runner valve-gate territory. A trimmed cold-runner gate mark is fine on many technical parts.
- Regrind reality. Cold-runner scrap can sometimes be reground and reused, but regulated and medical parts often can’t reuse regrind, so the runner becomes true scrap. That tilts high-volume medical work toward a hot runner.
This is also where the runner choice maps to the development journey: a cold runner is often right for early and low-volume builds; a hot runner earns its place as you scale to regulated volume. Designing for that step early avoids re-tooling later.
If a hybrid (hot-to-cold) runner is the smarter middle ground for your part, we’ll tell you that.
Practical considerations
- Gate vestige. A cold runner leaves a trimmed gate mark; a hot runner, especially a valve gate, leaves a minimal, often flush vestige. It matters where the gate lands on a visible or sealing surface.
- Regrind economics. Whether cold-runner scrap can be reused depends on the material and the spec; for regulated parts, assume it can’t, and count the runner as scrap when you model cost.
- Colour-change downtime. Frequent colour changes are quick on a cold runner and slow on a hot runner, a real cost on short, varied runs.
- Thermal degradation. Heat-sensitive materials sitting in a hot manifold can degrade; nozzle and residence-time design matter, or a cold runner sidesteps the risk.
- Runner balancing. In multi-cavity tools, the runner layout must fill every cavity evenly, a design discipline in both systems.
Worked examples
- Medical (illustrative). A high-cavitation medical device component at volume → a hot runner with valve gates: no runner scrap (important when regrind can’t be reused in a regulated part), clean gates, and a faster cycle. The tool and process are tested to the part’s requirements.
- Technical / non-medical (illustrative). A lower-volume or prototype technical part → a cold runner: cheaper tooling, easy colour and material changes, and a trimmed gate that the application can live with.
Generic illustrations of where each system fits: not specific Carmo projects.
How Carmo approaches it
We run both hot- and cold-runner tooling under one ISO 13485 quality system, so the runner choice is part of the design conversation, not a constraint handed to you. A cold runner often suits early and low-volume builds; a hot runner earns its place at regulated volume. The same engineering team carries the part across that step, with no supplier handover at scale-up.
Tell us the part, the material and the volume you’re heading for, and our engineers will tell you which runner system fits, and when it’s worth switching.
