Hot Runner vs Cold Runner: Which Runner System to Specify

What a runner system actually does

Melt enters the tool through the sprue, travels through a runner system, and arrives at the gate of each cavity. The runner system's job is to deliver melt to every cavity at the same pressure, at the same temperature and at the same moment. Everything else about runner design — hot or cold, balanced or not, valve gated or thermal — is a way of achieving that, or a compromise about what it costs.

The fundamental choice is whether the runner solidifies with the part or stays molten between shots.

• A cold runner is a channel machined into the tool. Melt fills it, packs the cavity through it, then freezes in it. The frozen runner is ejected with the part and becomes scrap, regrind or waste.
• A hot runner is a heated channel — a manifold and nozzles held at melt temperature — so the plastic in it never freezes. Each shot delivers only the part, with no runner attached.

That single difference drives material cost, cycle time, part quality, colour change time, tool cost and maintenance. It is worth understanding properly before accepting either as a default.

Cold runners: two-plate and three-plate

Two-plate tool

The standard, cheapest, most reliable runner system. The tool opens on one parting line, the part and the runner are ejected together, and the runner is degated afterwards — by hand, by a snipping fixture, or in the tool itself if the gate is designed to break cleanly on ejection.

Advantages: simple, cheap, robust, easy to maintain, works with every resin including heat-sensitive ones, and colour changes in minutes. Disadvantages: runner material is wasted, degating is a secondary operation, and the gate is usually on the parting line, which constrains gate position and therefore flow and weld line placement.

Three-plate tool

A three-plate tool has two parting lines, which allows the gate to be positioned anywhere on the top face of the part rather than only at the edge. The runner is pulled apart at the second parting line and drops separately from the part, so degating is automatic.

Advantages: gate anywhere on the part's upper face, automatic degating, better gate position for cosmetic parts and for multi-point gating of large panels. Disadvantages: more expensive than two-plate, longer cycle (two openings instead of one), more wear items, and a larger tool stack height which needs a bigger machine.

Multipliers relative to a two-plate edge-gated tool for the same part. The three-plate cycle penalty comes from the second opening stroke and the longer ejection sequence.

Hot runners: thermal gate, hot tip and valve gate

A hot runner system is a heated extension of the machine's nozzle, built into the tool. It has three parts: a manifold that distributes melt from the sprue to each drop, heated nozzles that carry melt down to each gate, and a controller that holds each zone at temperature.

Thermal gate (hot tip)

The simplest hot runner. The nozzle tip stays hot enough that the gate freezes only at the very surface, and the frozen plug is pushed out by the next shot's pressure. There is a small gate vestige and a short string that breaks off.

Cheapest hot runner option, no moving parts in the tool, works well for commodity resins and for parts where a small vestige is acceptable. Less good for heat-sensitive resins (the gate area is a dead spot), for very small parts (the vestige is proportionally large), and for materials that string badly.

Valve gate

A mechanical pin opens and closes each gate positively, driven by pneumatic or hydraulic actuation. This gives the cleanest gate — a small circular witness mark — and, more importantly, independent control of when each gate opens and closes.

Valve gating enables sequential filling: gates open in a programmed order so the melt front advances as a controlled line across the part. On large panels this eliminates weld lines, controls the last-filled point, and reduces the clamp force needed. It is the standard solution for automotive panels, large appliances and any part where a weld line in a cosmetic area is unacceptable.

Cost: significantly more than thermal gate, both in the tool and in the controller, plus a maintenance item for every pin and actuator.

Hot runner vs hot runner: the practical differences

A 16-drop thermal gate system is often cheaper than a 4-drop valve gate system, because the cost is dominated by the actuators and controller rather than the nozzle count.

The maths: when a hot runner pays back

A hot runner is a capital decision with three savings and one cost. The savings are material, cycle time and degating labour. The cost is the hot runner system plus its maintenance.

That number is the whole argument. At 300,000 parts a year, a hot runner pays back in about a year. At 30,000 parts a year, it pays back in ten years — by which time the controller has been replaced twice.

Payback in parts assumes the savings per part are constant. It ignores the maintenance cost, which shifts every row to the right — see section 9.

The numbers move a lot with runner weight. A small part with a proportionally large runner — a 5 g part with a 6 g runner — has more than 50 % of its material in the runner, so the hot runner saving is enormous and payback can be under 100,000 parts. A 500 g part with a 20 g runner barely benefits, and a hot runner is usually the wrong choice for it.

The rule of thumb: if the runner is more than 15 % of the shot weight, a hot runner is worth calculating. If it is more than 30 %, it is usually right at any volume above 100,000 parts a year.

Material savings, and the resins that forbid hot runners

Cold runner scrap is not always wasted. Most suppliers regrind it and blend it back, typically at 10 – 25 %. So the real saving from a hot runner is not the full runner weight — it is the difference between regrind value and virgin resin value, plus the regrind handling cost, plus the fraction that cannot be reused.

Where regrind cannot be used at all — medical, food contact, optical, some automotive interior — the saving is the full runner weight. That is one reason hot runners are near-universal in medical moulding despite the maintenance burden.

The general principle: the narrower the material's processing window, the more a hot runner punishes you. A hot runner holds material at temperature for the whole production run, and every dead spot in the manifold is a place where material degrades.

Cycle time and the real production difference

A cold runner adds two things to the cycle: the runner has to cool (it is often the thickest section in the tool), and the part cannot be ejected until the runner is rigid enough to survive ejection. On a thick sprue this can be several seconds.

A hot runner removes both. Typical savings are 2 – 8 s per cycle, and the higher end applies to parts with a heavy sprue or a long runner.

Cycle savings come from removing sprue cooling, removing the degating stroke on three-plate tools, and in valve-gated tools from eliminating the packing stage that a frozen gate would otherwise truncate.

There is a second, larger effect that does not show in the cycle time: uptime. A hot runner tool runs continuously at temperature; a cold runner tool has to re-stabilise after every interruption. On a high-volume programme running three shifts, the reduced start-up and the shorter restart after a stoppage is worth as much as the cycle saving.

Colour change and material change

This is where hot runners hurt, and it is the reason many suppliers keep a cold runner tool for short runs and colour variants.

A cold runner changes colour in the time it takes to purge the barrel — a few shots. A hot runner has a manifold full of the old colour, and that volume has to be displaced. On a large manifold with long drops, a colour change can take 50 – 200 shots of purge, or the manifold has to be pulled apart and cleaned.

Shot counts for a 4-drop manifold with a 45 g part. A 16-drop manifold with long drops is proportionally worse.

If a part runs in six colours at 20,000 pieces each, a cold runner is almost certainly the right tool, even at a volume where a hot runner would otherwise pay back. Colour change cost is a per-change penalty, and short runs multiply it.

What each system does to part quality

The runner system is not quality-neutral. It changes where the gate is, when the gate freezes, and therefore how the part packs and where weld lines land.

Gate freeze and packing. A cold gate freezes when the runner freezes, which may be before the part has finished packing. This is the single biggest quality argument for hot runners on thick parts: the gate stays open, so packing pressure reaches the cavity for as long as the machine applies it, and sink and voids become controllable.

Weld lines. A cold runner restricts gates to the parting line, which often means multiple gates and therefore weld lines. A hot runner — especially a valve gate — can gate into the centre of a part or fill sequentially, which can eliminate weld lines entirely.

Gate vestige. A cold runner leaves a vestige that usually has to be trimmed, and trimming marks a cosmetic surface. A valve gate leaves a clean circular mark that is often acceptable without any secondary operation.

Internal stress. A hot runner gate that stays open longer allows a lower packing pressure over a longer time, which generally means lower moulded-in stress and less warpage. A cold gate that freezes early forces higher pressure in a shorter window.

On thin-wall parts the quality difference is small and the cost argument dominates. On thick parts and large cosmetic panels it is the reverse.

Maintenance: the cost nobody quotes

A hot runner has heaters, thermocouples, seals, connectors, a controller and — on valve gates — pins, bushings and actuators. Each of those fails, and each failure stops the machine.

Lives assume clean resin, correct temperature settings and a maintained tool. Running abrasive or corrosive resins shortens everything.

Two practical points. First, a hot runner fault is usually a total stoppage, because the tool cannot run without the manifold at temperature. A cold runner tool with a chipped gate still runs. Second, hot runner repairs are specialist work — not every tool room keeps spares, and a nozzle that has to be ordered from Europe can stop a line for a week.

Budget 3 – 8 % of the hot runner's cost per year in maintenance and spares. On an $18,000 system that is $540 – 1,440 a year, plus downtime. It is not a reason to avoid hot runners; it is a reason to include it in the payback calculation.

A decision table you can actually use

The last row is commercial rather than technical and it is often decisive: a hot runner tool is harder to move, because the new supplier needs a compatible controller and the manifold has to be re-commissioned.

Hot runner problems and their causes

The first three account for most hot runner downtime, and all three are temperature-control issues rather than mechanical failures.

Frequently asked questions