Injection molding process
Hot Runner Systems for Injection Molding
How hot runner systems work, the benefits they can provide, and the technical decisions required to configure a system for the part, material and production targets.
The context
From polymer pellets to the molded component
Injection molding transforms a thermoplastic material into a finished part through a cyclic sequence. The hot runner operates in the section connecting the plasticizing unit to the mold cavities.
Plasticizing
The screw heats, mixes and meters the polymer until the required amount of melt has been prepared.
Filling
The screw moves forward and pushes the material through the machine nozzle, distribution system and gate.
Packing and holding
Pressure compensates for shrinkage while the gate remains capable of transmitting pressure.
Cooling
The part solidifies inside the cavity; the required time mainly depends on the material, wall thickness and temperature control.
Ejection
The mold opens, the component is ejected and the cycle starts again.
Hot runner system
The polymer remains molten up to the injection point
Heaters and thermocouples keep the sprue bushing, manifold and nozzles within a controlled thermal window. The material is distributed to the cavities without the main runner having to cool and solidify.
The benefit is not limited to eliminating runner waste. Reducing the mass that must be cooled and ejected can shorten the cycle, simplify automation and make better use of the machine’s plasticizing capacity. Actual results, however, depend on the part, resin, number of cavities and system configuration.
- Reduction or elimination of the cold runner
- Less handling of runner waste and regrind
- Potential reduction in cycle time
- Greater freedom in gate positioning
- Engineered and balanced multi-cavity filling
- Simpler automation in many applications
Economic assessment
Potential benefits and costs must be considered together
Potential benefits
Lower material consumption per good part, fewer runner-separation and recycling operations, shorter cycles when the runner was the dominant mass to be cooled, easier automation and the possibility of feeding difficult areas directly.
Design trade-offs
Higher initial investment, more heated and controlled components, wiring and spare-parts requirements, a more structured start-up procedure, specialist maintenance and greater attention to leakage, thermal expansion and material degradation.
Melt path
How a hot runner system works
Interface with the molding machine
The machine nozzle mates with the inlet bushing. Geometry, contact radius and alignment must ensure sealing and a smooth melt passage.
Inlet bushing or sprue bushing
It transfers the melt from the molding machine to the manifold and is normally heated and monitored by its own thermocouple.
Manifold
It divides the melt flow among the individual drops. The channels must minimize pressure loss, stagnation, imbalance and excessive residence time.
Heated nozzles
They convey the material from the manifold to the gate while maintaining the thermal profile and withstanding pressure, abrasion and thermal cycling.
Injection point
It is the interface between the hot runner system and the cavity. Its size, cooling and shut-off method affect packing, gate appearance and separation.
Control
The controller reads the thermocouples and modulates heater power. In valve gate systems, it can also manage valve-pin opening.
System architecture
Components that must operate as one integrated system
Manifold
It distributes the melt to the cavities. It may have linear, H-shaped, X-shaped, radial or fully customized geometries.
Nozzles
They define the flow-bore diameter, length, thermal profile, tip design and interface with the gate.
Heaters and thermocouples
They create and measure the thermal zones. Position, power and contact affect uniformity and response.
Temperature controller
It regulates each zone, applies soft start, reports faults and stores recipes or system parameters.
Valve pins and actuators
In valve gate systems, they move the valve pins through pneumatic, hydraulic or electric actuation.
Insulation and supports
Spacers, plates and seals manage heat loss, support loads and thermal expansion without introducing improper forces.
Electrical connections
Cables, connectors and junction boxes must be protected, identified, accessible and compatible with the controller.
Spare parts and documentation
Zone maps, critical spare parts, tip dimensions and assembly procedures reduce downtime.
Architecture options
Available system configurations
Single nozzle or manifold system
From a single injection point to a multi-cavity system.
- Single nozzle: a compact solution for one cavity or for feeding a sub-runner.
- Manifold: distributes the melt to multiple drops and requires both flow and thermal balancing.
Direct or indirect gating
The gate can feed the part directly or feed a small cold sub-runner.
- Direct gating: eliminates the sub-runner but makes the gate area more critical.
- Indirect gating: retains a small cold runner, which can thermally separate the gate from the part or reach difficult locations.
Thermal gate or valve gate
Shut-off through controlled solidification or by means of a mechanical valve pin.
- Thermal gate: simpler construction with fewer moving components.
- Valve gate: better control of separation, sequencing and gate vestige, with greater system complexity.
Individual components or Hot Half
The system can be integrated by the moldmaker or supplied as a fully assembled hot half.
- Individual components: greater freedom for in-house mold design.
- Hot Half: preassembled and tested plates, manifold, nozzles and wiring.
Valve actuation
The choice depends on force, speed, precision and the utilities available.
- Pneumatic: simple and widely used.
- Hydraulic: high force, but requires fluid management.
- Electric: precise, programmable control, with higher cost and integration requirements.
Filling simultaneo o sequenziale
The gates can open at the same time or according to a defined sequence.
- Simultaneous: suitable for naturally balanced systems.
- Sequential: controls the flow front, weld lines and filling of large or asymmetrical parts.
The gate
The injection point affects much more than the entry of the material
Position, diameter, geometry, temperature and shut-off method affect filling, packing, fiber orientation, weld lines, stresses, gate appearance and separation behavior.
| Solution | Strengths | Points to consider | Typical applications |
|---|---|---|---|
| Hot tip / pin-point | Small gate vestige, compact solution and no valve pin. | Careful balance required between freeze-off, stringing and tip temperature. | Small to medium-sized parts and surfaces where the gate vestige must remain minimal. |
| Open gate / flat tip | Simple flow path, low restriction and good flow capacity. | Possible stringing or drooling; the gate mark is generally more visible. | Technical parts, filled resins, high flow rates and non-cosmetic gate locations. |
| Valve gate | Positive shut-off, good control of the gate vestige and sequencing capability. | Actuator, valve pin, guide, alignment, gate cooling and maintenance requirements. | Cosmetic parts, large multi-gated components, thin walls and multi-cavity molds. |
| Hot-to-cold runner | Reduces the main runner and simplifies certain gate geometries. | A sub-runner still has to be cooled and ejected. | Materials or parts that benefit from a more robust cold gate. |
| Side gate | Side feeding of small parts or parts that cannot easily be reached from the front. | Very limited space, local cooling and tip-maintenance requirements. | Compact components, closely spaced cavities and geometries requiring side access. |
Melt distribution
Manifold design: balance, pressure and residence time
Flow balancing
Each cavity should receive a comparable pressure, temperature and time history. Geometrically equal flow paths are often the starting point, but rheology may require further correction.
Pressure loss
Flow bores that are too small, sharp bends and restrictive passages increase pressure and shear. Excessively large diameters increase system volume and residence time.
Residence time
The system volume must be appropriate for the shot size and the thermal stability of the resin. Dead spots and stagnation promote degradation and black specks.
Thermal balance
Heater power, heat loss to the plates, thermocouple position and insulation must maintain a uniform melt-temperature profile.
Thermal expansion
The manifold expands as it reaches operating temperature. Supports, seals and mounting dimensions must compensate for the expected movement.
Simulation
Filling, thermal and structural analyses help define gates, channels, pressure drop, heat distribution and mechanical interfaces.
Temperature control
Every zone must be measured and regulated
The controller does more than display a temperature: it must manage the dynamic behavior of heaters, metal masses and heat losses during warm-up, start-up and production.
- PID control for each zone
- Soft start to remove moisture and protect the heaters
- High- and low-temperature alarms
- Open or reversed thermocouple diagnostics
- Current monitoring
- Recipes and mold identification
- Manual mode or percentage power output
- Integration with a valve gate sequencer
Why the set temperature alone is not enough
The thermocouple measures the point where it is installed, not the entire melt volume directly. Sensor position, contact, wiring, nozzle mass and gate cooling all affect the actual material temperature.
Project data
A sound configuration starts with complete information
The system should not be selected on shot weight alone. The project must consider the part, material, mold, injection molding machine, production requirements and maintenance at the same time.
Material compatibility
The resin determines channel sizes, temperatures, tips and the purging strategy
Amorphous and semi-crystalline polymers do not freeze in the same way; filled materials increase wear and shear; temperature-sensitive resins require short residence times and the elimination of stagnation. POM, PVC, flame-retardant grades, transparent materials, thermoplastic elastomers and high-temperature polymers all require specific evaluation.
Viscosity and shear
They determine channel diameters, allowable velocity and pressure drop. An overly restrictive passage can overheat the material or produce excessive molecular or fiber orientation.
Thermal stability
It determines the allowable residence time, start-up procedure, zone temperatures and shutdown method.
Abrasion and corrosion
Fibers, minerals, pigments and degradation products affect the selection of steels, surface treatments and replaceable tip materials.
Application areas
Applications with different design priorities
Automotive
Cosmetic or structural parts, large surfaces and filled materials. Weld-line control, sequencing, wear resistance and dimensional stability are key priorities.
Medical
Repeatability, cleanliness, high cavity counts and strict process control. The design must promote uniformity and documented maintenance.
Packaging
Thin walls, fast cycles, high flow rates and frequent color changes. Balance, gate cooling and minimum stagnant volume are essential.
Electrical and electronics
Compact geometries, engineering materials and frequently additive-filled grades. The processing window, precision and resistance to wear or corrosion are important.
Consumer products
Cosmetic quality, production flexibility and a wide range of weights, colors and geometries, from single parts to multi-cavity molds.
Design for maintenance as well
Critical issues to prevent
Freeze-off
Premature solidification in the gate area or tip, caused by heat loss, excessive cooling or an unsuitable thermal profile.
Stringing and drooling
The gate remains too hot or does not separate cleanly; material, geometry, decompression and tip temperature may all contribute.
Cavity imbalance
Differences in flow path, temperature or rheology produce non-uniform filling and packing pressures.
Degradation and black specks
Excessive residence time, dead spots, overly high temperatures or incorrect shutdown procedures degrade the polymer.
Melt leakage
Incorrect sealing, preload or thermal-expansion compensation can allow melt to leak between the manifold, nozzles and plates.
Gate and tip wear
Filled materials and misaligned valve pins wear tips, guides and seats, altering gate appearance and behavior.
Electrical faults
Heaters, thermocouples, connectors and cables undergo thermal cycling and must remain accessible and diagnosable.
Slow color change
Large volumes, branching channels and stagnation increase the amount of material to purge and the risk of carrying over the previous color.
Unplanned maintenance
A lack of diagrams, spare parts and procedures can turn a simple fault into a long and costly shutdown.
Technical support in Italy
GDS and HATSYS: from design to start-up
GDS combines its experience in the moldmaking industry with a technology partnership with HAT TEKNIK for HATSYS hot runner systems.
Support can begin with the collection of application data and a technical review of the project, continue with the configuration of nozzles, manifolds, valve gates, controllers or Hot Halves, and extend to documentation, start-up and spare-parts management.
This technical guide deliberately remains independent of any specific commercial range: the final selection is made only after reviewing the material, geometry, number of cavities, molding machine and production targets.
GDS / HATSYS catalogue
Standard and special nozzles, manifolds, control systems, accessories, Hot Halves and applications.
Further reading on the process
Continue reading
Frequently asked questions
Hot runner systems: frequently asked questions
What is the difference between a “hot runner” and a “hot runner system”?
In industrial terminology, the expressions are often used interchangeably. “Hot runner system” more precisely describes the heated assembly that transfers the polymer from the molding machine to the injection points: inlet bushing, manifold, nozzles, heaters, thermocouples, wiring and temperature controller.
Does a hot runner system always eliminate all runner waste?
A fully hot system with direct gating can eliminate the solidified main runner. Hybrid configurations also exist, in which the hot runner feeds a small cold sub-runner: they reduce runner waste without eliminating it completely.
When should a valve gate be selected?
When gate quality, positive shut-off, sequential filling, the management of large or multi-gated parts and the reduction of stringing or drooling are important. The decision still depends on the material, geometry and cycle.
Is an open gate always the least expensive solution?
It generally has a simpler construction, but the total cost depends on the application. If it causes visible marks, stringing, freeze-off or process instability, a more controlled solution may be more economical over the system’s life cycle.
Can all polymers be processed with the same hot runner system?
No. Viscosity, processing window, sensitivity to residence time, abrasive fillers, additives and degradation tendency require different geometries, materials, channel diameters and thermal strategies.
Why is temperature control divided into multiple zones?
The manifold, nozzles and inlet bushing have different masses and heat losses. Independent zones keep the melt within its processing window and make heater or thermocouple faults easier to identify.
What is a Hot Half?
It is the hot side of the mold supplied as a preassembled and tested unit, normally including plates, manifold, nozzles, wiring and connections. In valve gate systems, it may also include actuators and control circuits.
What information is required to request a system configuration?
A 2D or 3D drawing of the part and mold, the complete material specification including fillers and additives, part and total shot weight, number of cavities, wall thicknesses, expected cycle time, molding machine, preferred gate locations, cosmetic requirements and the pneumatic, hydraulic and electrical utilities available.
Can an existing cold runner mold be converted to a hot runner?
Sometimes, but it is not a simple component replacement. Space in the plates, cavity layout, gate cooling, thermal expansion, wiring, opening stroke, ejection and compatibility with the molding machine must all be checked.
Which factors determine maintenance requirements?
Accessibility of heaters, thermocouples, tips and seals; material filtration quality; start-up and shutdown procedures; ease of purging and color change; availability of spare parts and wiring documentation.
Let us discuss your application
Request a hot runner system assessment
Send us the part or mold drawing, the complete material specification including fillers and additives, the number of cavities, shot weight, intended injection molding machine and cycle-time and quality targets. GDS will assess the most suitable configuration together with its technical partner.