In injection molding, runner and gate design directly affect production efficiency and cost control. Traditional cold runner systems, due to generation of solidified material in runner, not only prolong molding cycle time (CT) but also cause raw material waste. Hot runner technology, through precise temperature control and runner optimization, successfully solves these problems, becoming a key to upgrade of modern injection molding processes.

Injection cycle includes stages such as mold closing, injection, holding pressure, cooling, and mold opening.

 

Hot runner technology achieves CT reduction in following ways:
1. Eliminate Runner Cooling Time: In cold runner systems, solidified material in runner needs to cool synchronously with product, consuming a significant amount of time. For example, cooling time for thick-walled runners can account for more than 30% of the entire cycle.
Hot runners, through a heating system, keep plastic in runner constantly in a molten state, eliminating need to cool solidified material and directly reducing cooling stage time.
2. Simplify Mold Opening and Closing Actions: Three-platen cold runner molds require an additional parting action to remove solidified material, while hot runner molds are mostly two-platen structures, resulting in a shorter mold opening stroke and simpler actions.
3. Improve Injection Efficiency: As an extension of injection molding machine nozzle, hot runners reduce melt pressure and heat loss, allowing for faster injection speeds. Especially for thin-walled parts, high-pressure, high-speed filling becomes possible, injection time can be optimized and shortened by more than 20%.

 

In cost structure of general plastic parts, material costs generally account for 30% to 40% of the total injection molding cost, with solidified material in cold runners being main source of waste. Solidified material accounts for a large proportion of material consumption in gating system of ordinary injection molded parts. Reuse of these recycled materials, if involving crushing, extrusion, pelletizing, then mixing with fresh material to remolde plastic parts, presents a problem: foreign matter can easily be introduced, causing contamination and reducing performance of remolded parts. Statistics show that scrap rate of plastic parts mixed with recycled material increases by approximately 5 times. Furthermore, material in runner of thermosetting plastics solidifies completely and becomes waste, unusable for reuse.
Advantages of hot runner systems are:
1. Zero-waste production: In conventional cold runner systems, recycled material accounts for 20% to 60% of the total injection weight, especially significant in multi-cavity molds. Hot runner systems ensure full utilization of melt within runner, achieving "zero-waste" production at dispensing gate and needle valve gate.
2. Precise control of gate size: Hot runner gates (such as needle valve gates) allow for precise adjustment of opening size and closing timing, avoiding excessive holding pressure or material overflow. For example, optimizing holding pressure parameters through DOE can reduce product defect rates, indirectly reducing scrap generation.
3. Adaptability to High-Value Materials
For expensive engineering plastics such as PA66+GF30, hot runners avoid performance degradation caused by repeated processing, ensuring maximum material utilization. Combination of a dedicated screw and hot runner reduces shear heat and minimizes risk of degradation.
Generally, hot runner molds increase mold costs by about 10% to 20%, but this increase is quickly offset by improved economic benefits mentioned above.

Based on different methods of heat acquisition in runner, hot runner injection molds are generally divided into insulated runner injection molds and heated runner injection molds.

 

Furthermore, regardless of whether it is an insulated runner injection mold or a heated runner injection mold, it can be further subdivided into following types based on number of gate entry points and gate structure:
① Insulated runner injection mold with single-point gate entry. Single-point gate entry insulated runner is shown in figure. This structure is only suitable for single-cavity molds.

 

② Insulated runner injection mold with gate sleeve end face participating in molding process. Insulated runner system, where sprue bushing end face participates in molding process, is shown in figure. This structure is also suitable for single-cavity molds, but sprue bushing marks will appear on product surface.

 

 

To achieve cost-effectiveness, hot runner system needs to be scientifically designed:
1. Runner Diameter and Layout Design
Runner diameter needs to be calculated based on material's rheological properties (such as shear rate and viscosity).
Multi-cavity molds should adopt a natural balance layout (such as H-shaped or X-shaped runners) to ensure synchronous filling of each gate, reducing setup time and defect rate.

 

Three types of hot runner gating systems

 

Commonly used multi-cavity molds with natural balance.
2. Precision of Temperature Control System
Nozzles and runner plates need independent temperature control to prevent localized overheating or cold material. Each runner plate and nozzle should have independent zoned temperature control. Thermocouple placement should accurately reflect melt temperature (e.g., near the gate) to ensure accurate and stable temperature control.
3. Selection and Control of Gate Type
Gate is terminal of hot runner system, and its design is crucial.
Open Gate: Simple in structure and low in cost, relying on cooling and freezing of plastic at gate. Suitable for crystalline materials such as PP and PE, but sensitive to temperature control and prone to drooling or stringing.
Valve Gate: Forces gate to close via a mechanical valve needle, effectively preventing drooling, minimizing gate marks, and allowing for time-sequence control to optimize weld line position. Suitable for materials with high appearance requirements, no weld line requirements, or shear-sensitive materials (such as PC and glass fiber-containing materials).
4. Thermal Insulation of Hot Runner Systems
Hot runner system is "heat island" in mold; reducing heat loss to cooling mold is key to energy saving and stable molding. Insulation must be achieved using air gaps (typically ≥3mm), thermal pads (made of low thermal conductivity materials such as titanium alloys), and asbestos boards.

 

Thermal insulation pads should be made of materials with low heat transfer efficiency, such as stainless steel or high-chromium steel. Specific structures of thermal insulation pads supplied by different vendors may vary, but the basic assembly relationships should be same, as shown in Figure 5-43.

 

Hot runner technology is far more than a simple "heated runner"; it is a complex system integrating polymer rheology, precision mechanical design, automatic control technology, and thermodynamics. By fundamentally eliminating cooling time and material waste associated with cold runner solidification, it has revolutionized injection molding production in both cycle time and cost.
In the future, combined with DOE process optimization and intelligent temperature control systems, hot runners will further propel injection molding industry towards efficient and green manufacturing. For modern injection molding companies pursuing high quality, high efficiency, and low cost, a deep understanding and mastery of hot runner technology, continuous innovation based on this foundation, has become an essential path to standing out from competition.