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Should my mould have a hot runner or a cold runner?
For prototypes, low volumes and frequent colour changes, a cold runner is usually the cheaper and simpler choice. For high, steady volumes a hot runner often pays for itself by removing runner scrap and shortening the cycle; one mould maker's published planning ranges put the tooling premium at roughly 20 to 50 percent and the cycle saving at 20 to 40 percent.
The runner is the channel system that carries molten plastic from the machine nozzle to each cavity. In a cold runner mould that channel is simply cut into the mould plates, so the plastic in it cools along with the part and is ejected every shot. In a hot runner mould a heated manifold and nozzles keep the plastic in the channels molten between shots, so only the part itself is ejected.
How the two compare
| Factor | Cold runner | Hot runner |
|---|---|---|
| Tooling cost | Lower | Higher: manifold, heated nozzles, temperature controller |
| Runner scrap | A runner every shot, reground or discarded | None from the hot section |
| Cycle time | Often set by how long the thick runner takes to cool | Usually shorter |
| Colour and material changes | Quick | Slower; the manifold has to be purged |
| Heat-sensitive resins | Forgiving | Needs care, because material sits hot between shots |
| Gate mark | Trimmed gate stub | Small, and near flush with a valve gate |
| Maintenance | Simple | Heaters, thermocouples and seals to look after |
The table summarises the comparison published by Carmo, a Danish moulder that runs both kinds of tool, and matches the outline in Wikipedia's hot runner article, which notes that hot runners make the mould dearer to build and run but save plastic and cycle time.
Published numbers
Hard figures vary by part, so treat these as one supplier's planning ranges rather than rules. Axiom Molds' comparison guide gives:
- Cold-runner scrap of about 15 to 45 percent of total shot weight.
- Hot-runner tooling at roughly 20 to 35 percent above a cold-runner tool for open thermal tips, and 35 to 50 percent above for valve-gated systems.
- Cycle times about 20 to 30 percent shorter with thermal tips, and 25 to 40 percent shorter with valve gates.
- Payback often within 80,000 shots for programmes above about 100,000 parts a year; below about 50,000 parts a year, a cold runner usually wins.
Dienamics, an Australian toolmaker, adds a practical limit on reusing cold-runner scrap: if more than about 20 percent of the material fed to the machine is regrind, the risk of defective parts rises, so a heavy runner cannot all be recycled. Carmo also points out that regulated and medical parts often cannot use regrind at all, which turns every runner into true scrap.
How to decide
- Estimate annual volume honestly. Volume is the factor that most often decides it.
- Weigh the runner. Ask the toolmaker for the runner weight per shot and multiply by your resin price and annual shots.
- Check whether regrind is allowed for your part. If not, the runner is pure cost.
- Consider colours. Many short runs in different colours favour a cold runner.
- Check the resin. Some heat-sensitive grades need a specific hot-runner design; Eastman, for example, recommends nozzles with heaters outside the melt stream for its copolyesters.
- Look at the gate position. If the gate must sit on a visible face with almost no mark, a valve gate may be the practical way to get it.
These figures come from published supplier guides and are planning ranges, not quotes. Ask the toolmaker for a runner weight and cycle estimate for your part before choosing.
Questions buyers ask
Is there anything between the two?
Yes. A hot sprue, sometimes called a semi-hot runner, heats only the first section from the machine nozzle and feeds a short cold runner. It reduces scrap without the full cost of a manifold.
Do prototype moulds use hot runners?
Rarely. Prototype and bridge tools are usually cold runner because the extra build cost and lead time are hard to justify for small quantities.
Does a hot runner improve part quality?
It can, because the plastic reaches the cavity hotter and often with less pressure loss, and Wikipedia's article lists fewer sink marks and short shots among the benefits. It also adds temperature zones that need to be controlled, so a poorly balanced hot runner can cause its own problems.
Can a cold runner mould be converted later?
Sometimes, but it usually means significant rework of the mould plates. If volumes are expected to grow, discuss the option at the design stage.
Sources
- Wikipedia, Hot runner
- Carmo, Hot runner vs cold runner
- Axiom Molds, Hot runner vs cold runner molds: selection guide
- Dienamics, Injection moulding tool configurations
- Eastman, Keys to successful molding (PDF)