Open gate (thermal gate)
- Nozzle tip stays open during injection; gate freeze happens at the tip after screw retract
- Lower cost on many layouts
- Gate vestige and stringing risk need process control
Glossary
Hot Runner Injection Molding: Components, Valve Gate & System Selection
A hot runner keeps runner channels molten between shots so only the part solidifies. Heated manifolds and drops cut cold runner scrap on many high volume programs.
Definition
What Is a Hot Runner?
A hot runner is a heated manifold system in an injection mold that keeps the runner channel molten between shots.
Only the part solidifies; the runner stays liquid for the next cycle.
Hot runner systems eliminate cold runner scrap and can shorten cycle on many high volume programs.
For the unheated alternative, see the cold runner page. For feed path balance and a full system comparison table, see the runner glossary entry.
Tooling
Hot runner keeps melt in the manifold between shots
Manifold, drops, valve gates, and temperature control define hot runner scope.
- Residence time matters on heat sensitive resins.
- Valve gates support cosmetics and sequential fill.
- Maintenance plans cover heaters, seals, and valve pins.
Tradeoff
Hot runner in one line
Tooling capital, maintenance, and resin sensitivity belong in the same RFQ thread as volume.
Hot runner eliminates cold runner scrap and can reduce cycle at higher tool cost and service need. Use the runner glossary entry for the full cold versus hot comparison table.
Components
Hot runner system components
Supplier scope varies by manifold brand and cavity count.
Manifold: distributes melt from the machine nozzle to multiple drops; maintains temperature across branches
Manifold layouts include H type, X type, Y type, and stack mold manifolds that serve two parting planes
Nozzle (drop): delivers melt to each gate; open thermal gate, valve gate, and other variants exist
Heater bands or heater rods maintain melt temperature in manifold and nozzle channels
Thermocouple monitors temperature at control points for closed loop regulation
Controller (temperature control module): PID per zone with fault detection for heater or TC failure
Valve pin actuator on valve gate drops: pneumatic or hydraulic cylinder opens and closes the gate
Gates
Open gate vs valve gate on hot drops
Gate style is a tooling decision tied to cosmetics, fill sequence, and budget.
Valve gate
- Mechanical pin opens and closes on command
- Cleaner gate break and less stringing on many programs
- Supports sequential fill; higher cost and maintenance on moving parts
Process
Residence time and color change
Heat sensitive resins need honest shot size versus manifold volume planning.
Melt sits in the manifold between cycles; long residence can degrade heat sensitive resins such as PVC, POM, and some flame retardant grades
Shot weight should be sized to the hot runner system; oversized channels increase degradation risk
Color or material change requires purging manifold and nozzles; purge volume scales with manifold capacity and drop count
Dark to light transitions take longer; frequent weekly color changes can offset hot runner material savings
Maintenance
Hot runner failure modes
Preventive checks belong in the mold maintenance plan, not only after a line stop.
Heater burnout: localized cold spot causes freeze off or degraded melt
Thermocouple failure: loss of control leads to runaway heat or freeze
Leakage: manifold seal failure spills plastic into the mold base
Valve pin wear or sticking: gate fails to close, causing flash or drool
Scheduled heater resistance checks, TC calibration, and seal inspection reduce unplanned downtime
Selection
When hot runner wins or loses
Patterns for scoping RFQs; validate on CAD, volume, resin cost, and quality plan.
Hot runner often wins
- High cavitation multi cavity production
- High annual volume where scrap and cycle dominate economics
- Expensive resin where cold runner slug cost dominates
- Cosmetic programs that need minimal gate witness on the chosen face
- Automated high cycle lines with stable resin and color
Hot runner often loses
- Lower annual volume programs
- Frequent material or color changes
- Heat sensitive resins that need short residence time
- Prototype or bridge tooling with tight capital
- Budget limited programs that accept cold runner scrap and trim
Clamp
Locking force on hot runner tools
Projected area and cavity pressure still drive clamp sizing.
Hot runner melt pressure acts on cavity and manifold plates; high cavitation layouts can need more clamp margin than a simple cold runner impression of the same parts
Verify balance and cavity pressure with mold flow before you lock tonnage on the quote
Go deeper
Feed path links
Each entry covers a different lever on the same tool.
Runner feed path
Balance and cold versus hot comparison table.
Runner feed pathValve gate
Timed shutoff on hot drops.
Valve gateSequential valve gating
Fill sequence on large or multi cavity tools.
Sequential valve gatingCold runner alternative
Unheated feed path and trim scrap.
Cold runner alternativeGate location DFM
Entry after the drop.
Gate location DFMMold base
Manifold stack integrates with plates.
Mold baseRunner design guide
Layout rules on the DFM hub.
Runner design guideVisual reference
Photos from molding and tooling programs
Images are selected from the NetProto Forge media library for this topic.
Hot runner manifold plate assembly on injection mold tooling
Hot runner manifold plate assembly on injection mold tooling
Valve gate hot drop nozzle close up on mold steel
Valve gate hot drop nozzle close up on mold steel
Hot runner temperature control module on injection press
Hot runner temperature control module on injection press
FAQ
Frequently Asked Questions About Hot Runners
Hot runner vs cold runner: which is better for my part?
Neither is universal. Hot runner fits many high volume and low scrap programs. Cold runner fits many low volume and color change programs. Start with volume, cavity count, and resin cost, then see the runner page comparison.
What are the main components of a hot runner system?
Manifold, heated drops, temperature control, and on valve gate tools, pins and actuators. Heaters and thermocouples maintain melt temperature between shots.
What is the difference between open gate and valve gate?
Open gate nozzles stay open during fill and freeze at the tip. Valve gates use a pin to open and close for cleaner break and sequential fill.
How long does a hot runner last before maintenance?
Life depends on resin, cycle, and maintenance discipline. Heater and seal inspection schedules prevent most unplanned stops rather than a fixed hour rating.
Can hot runner handle color changes?
Yes with purging time and scrap during changeover. Frequent changes favor planning purge volume and may favor cold runner on some programs.
What resins are not suitable for hot runners?
Heat sensitive grades with strict residence limits need careful manifold sizing or cold runner instead. Confirm on material data and process window.
How does hot runner affect cycle time?
Eliminating runner cooling and trim often shortens cycle on high volume tools. Controller tuning and gate style still set the practical floor.
What is the typical cost premium for a hot runner system?
Manifold, drops, and controls add tooling capital versus cold runner. Payback depends on scrap avoided, labor saved, and annual volume.
Next step
Confirm hot runner economics
Upload CAD with annual volume, material spec, and cavity count to confirm whether hot runner economics work for your program.