Hot runner vs cold runner mold

The short answer

Build a cold runner mold when volumes are low or uncertain, when the part changes colour or material often, when the resin is heat-sensitive, or when the runner is small next to the part. Build a hot runner mold when volume is high and stable, when the runner would be a large share of every shot, when the resin is expensive and regrind is not allowed, when the gate has to sit on a cosmetic face, or when the thick cold sprue is what sets the cycle time. Between the two sits a hybrid, a hot sprue or hot drop feeding a short cold sub-runner, which is often the right answer for small multi-cavity parts.

What the two systems are

In a cold runner tool the sprue, runner and gates are channels machined into unheated mold plates. They fill with melt, cool with the part, and are ejected with it as one shot. The runner is then cut off, by hand, by a sprue picker, or automatically through a submarine gate or the stripper plate of a three-plate tool, and either reground into the next batch or scrapped.

In a hot runner tool a heated manifold and nozzles (drops) carry the melt from the machine nozzle to each gate. Cartridge or band heaters and a thermocouple on every zone, run from a temperature controller, hold the plastic molten between shots. Only the part is ejected. The gate at the nozzle tip is either a thermal gate, which freezes and reopens with temperature, or a valve gate, where a pin closes it mechanically. Layouts, runner cross sections and gate types are explained in runners in injection molds, and where the runner plane sits in the mold base in two-plate vs three-plate mold.

Side by side

	Cold runner mold	Hot runner mold
Tool cost	Lower. Channels are machined into the plates.	Higher. Manifold, heated drops, wiring and a controller zone per drop are added to the same cavitation.
Tool build time	Shorter	Longer. The manifold has to be specified, sourced and commissioned.
Material per shot	Part plus runner and sprue	Part only
Cycle time	Can be set by the thickest section of the sprue or runner, which must freeze before ejection	Set by the part wall alone
Degating	Trimming, a picker robot, a submarine gate or a three-plate tool	None needed. Parts drop free.
Gate position	Edge of the part, or the face with a three-plate tool	Anywhere on the part face
Gate mark	Trim mark at the gate	Small round vestige with a thermal gate, a near-flush witness with a valve gate
Colour and material change	Fast. Nothing is held in the tool.	Slow. The manifold holds melt that has to be purged out.
Heat-sensitive resins	Tolerant	Residence time in the manifold can degrade them
Maintenance	Simple	Heaters, thermocouples, seals and the controller all need servicing, and a leak at a nozzle stops the tool
Best fit	Prototypes, bridge and short runs, frequent colour changes, PVC and similar resins	High, stable volumes, expensive resins, cosmetic gates, many cavities

Where the cost difference comes from

A hot runner is a set of parts added to a mold base, so its cost rises with the number of drops. Each drop carries a nozzle, a heater, a thermocouple and a controller zone, and the manifold has to be balanced so every drop delivers the same melt temperature and pressure. On a single cavity part with one drop the added cost is modest. On a 16 cavity tool it is sixteen nozzles and a larger manifold. Valve gates add a pin and an actuator to every drop on top of that.

What the hot runner earns back comes from three places: the plastic that no longer becomes runner, the seconds that come off the cycle when a thick sprue no longer has to freeze, and the handling that disappears when there is no runner to cut, sort and regrind. The first two are measurable before a tool is built, so the decision does not need to be made on instinct.

A worked breakeven

Take a 4 cavity tool for a 20 g part. Each shot is 80 g of parts. Suppose the cold runner and sprue weigh 24 g, which is 23 percent of a 104 g shot and not unusual for a small multi-cavity layout. The program runs 200,000 parts a year.

• Plastic in the runner. 200,000 parts is 50,000 shots. At 24 g of runner per shot that is 1,200 kg of resin a year that is melted, cooled and ejected without becoming a part.
• If regrind is allowed. Most of that 1,200 kg goes back into the hopper, and the real loss is the cost of grinding, the drying time, and the property loss the drawing will tolerate. A common cap is 20 to 25 percent regrind blended with virgin material, and less on cosmetic or structural parts.
• If regrind is not allowed. Many medical and automotive drawings forbid it. Then the full 1,200 kg is bought and thrown away every year, and at engineering resin prices that alone can pay for a hot runner inside a year or two.
• Cycle time. If the sprue is thicker than the part wall, it can hold the tool closed for extra seconds. Taking 3 seconds off a 25 second cycle is about 14 percent more parts per press hour, which reaches the piece price directly.

The breakeven is the added hot runner cost divided by the yearly saving from those lines. When that comes out at well under the life of the program, build the hot runner. When it is several years, or the volume forecast is uncertain, build the cold runner and revisit it at the second tool. Your own numbers go into the same arithmetic: part weight, cavity count, annual volume, resin price, and whether regrind is permitted. What drives injection molding part cost shows how tooling and piece price combine into the number on a quote.

When the resin or the colour decides it

• Heat-sensitive resins. PVC, some flame-retardant packages and POM (acetal) degrade when they sit molten for too long. A hot runner holds melt between shots, so a small shot on a large manifold can hold it for many cycles. These resins either run cold, or run hot only with a manifold sized to keep residence time short.
• Frequent colour changes. A cold runner changes colour as fast as the barrel purges. A hot runner has to purge its manifold and drops as well, with streaks until it is clean. A part molded in six colours in short batches is usually a cold runner part.
• Glass and mineral filled grades. They run hot, but they wear nozzle tips and gate inserts, so the tool is specified with hardened or replaceable tips.
• Expensive and regulated resins. PEEK, PEI, medical grades with lot traceability, and any grade where the drawing forbids regrind move the arithmetic firmly toward a hot runner. See medical injection molding for how lot control applies.

The hybrid: hot drop, cold sub-runner

A hot sprue bushing, or one hot drop feeding a short cold runner to several cavities, removes the thickest and heaviest part of the feed system, which is usually the sprue, while keeping the tool simple. The cold runner that remains is small, the cycle is no longer set by the sprue, and there is one heater zone instead of one per cavity. For small parts in 4 to 16 cavities this often recovers most of the saving for a fraction of the hot runner cost.

What the choice does to the schedule

A hot runner adds a manifold to specify, source and commission, which pushes a tool toward the long end of the 4 to 8 week build range. The injection molding lead time guide breaks down where those weeks go. For a first tool on a design that may still change, a cold runner is the faster and cheaper way to get approved samples, and a hot runner can be designed into the production tool once the part and the volume are settled.

How to choose

1. Weigh the runner. If it is a small share of the shot and regrind is allowed, a cold runner is almost always right.
2. Check the resin. PVC, POM and other heat-sensitive grades, or many colour changes, point to cold.
3. Check the drawing. No regrind permitted, or a gate that must sit on a cosmetic face, points to hot.
4. Run the breakeven above on your volume and resin price.
5. If it is close, price the hybrid, a hot sprue or hot drop into a short cold runner.

What Trumould builds

The tool room in Delhi builds two-plate, three-plate, hot runner, multi-cavity, family and stack tools, 1 to 32 cavities as standard, inside a 600 by 900 mm platen envelope; see injection molds and tooling. Send a model with the annual quantity and the resin, and the quote states whether a cold runner, a hot runner or a hybrid is proposed and why, with the tool and the piece price shown separately so the trade-off is visible.