Structure of the Hot runner system in plastic injection molding

1. What is a Hot runner system?

The hot runner system is an essential technology in the plastic injection industry, used to keep plastic in a liquid state throughout the production process. Molds equipped with the hot runner system are referred to as hot runner molds, which are directly attached to the injection mold and cannot be separated. Unlike cold runner systems, where plastic cools and solidifies in the conduits, hot runner systems maintain the plastic at high temperatures, thereby reducing production cycle times and improving product quality. This technology is particularly beneficial for applications requiring high precision and large output, such as automotive components, electronic devices, and household items.

2. Components of the Hot runner system

The hot runner system consists of several key components, each playing a vital role in the plastic injection process:

2.1. Sprue bushing: The sprue bushing is an intermediary part that helps transfer plastic from the machine’s nozzle to the manifold. Similar to other parts of the system, this area also needs to be heated and temperature-controlled to ensure uniform temperature throughout the plastic delivery system. If the temperature at the sprue bushing is too low, especially with heat-sensitive plastics, the product’s surface quality may not meet standards.

• Functions:

Connects to the nozzle of the injection machine.

Ensures a sealed flow system from the machine to the nozzle.

Reduces pressure when necessary.

Filters hot plastic if required.

2.2. Manifold: The manifold is used in indirect injection systems through multiple nozzles, acting as a distribution plate for plastic from the machine’s hot branch to the mold cavities.

• Functions:

Transports and distributes hot plastic.

Maintains the temperature of the plastic throughout the injection process.

Transmits pressure for the plastic flow.

2.3. Nozzle: The nozzle is the endpoint of the hot runner system, consisting of a heat-conducting tube leading to the gate. Its primary role is to deliver hot plastic from the runner into the mold cavity.

• Functions:

Supplies hot plastic to the gate.

Maintains a stable temperature of the plastic during injection.

2.4. Gate:

• Functions:

Supplies hot plastic into the mold cavity.

Maintains plastic flow into the mold.

Stops the plastic flow when needed.

• The open gate can be used for crystalline and amorphous plastics. Key parameters to consider include the diameter of the gate, cooling area, and temperature control at the gate’s tip to optimize part quality. The open gate leaves a small mark on the product’s surface, and the size of this mark is directly related to the gate’s shape and the material’s characteristics.
• The valve gate nozzle uses a pneumatic system to operate the valve opening mechanism. The piston and cylinder are installed at the upper plate in this design. Before the injection begins, the valve pin is retracted, allowing the nozzle opening to widen so that plastic can flow into the mold cavity. As soon as the plastic in the nozzle has not completely solidified, the pin moves to the valve closing position.

3. Advantages of Hot Runner

The hot runner system offers numerous benefits for the plastic manufacturing process:

• Reduced production cycle time: By keeping the plastic at high temperatures, the hot runner system helps decrease the time required to produce each product. This leads to increased productivity and lower production costs.
• Reduced mold opening stroke compared to three-plate molds: Hot runner molds reduce the clamping stroke compared to three-plate molds since they only need to open enough to remove the product and separate the two parts of the runner.
• Improved product quality: This system minimizes the risk of defects and enhances the uniformity of plastic products. Items produced from a hot runner system typically have smooth surfaces and show no signs of solidification.
• Stack mold application: Stack molds can utilize the hot runner system, although they are rarely used; most stack molds use a hot runner system.
• Material savings: Since there is no cooled plastic in the runner, the hot runner system minimizes excess plastic. This not only saves material costs but also contributes to environmental protection.
• Increased flexibility: The hot runner system allows for quick changes between different products without needing to change the mold, optimizing the production process.
• Balanced flow: A key advantage of the hot runner mold is the melt channels within the manifold, which are heated externally and insulated from the surrounding mold plates. Unlike cold runner molds, the manifold can be designed for multi-level flow, ensuring even temperature and pressure from the machine’s nozzle to the gate.

4. Disadvantages of Hot Runner

Despite its many advantages, the hot runner system also has some drawbacks:

• High initial investment costs: Hot runner systems typically require a larger initial investment compared to cold runner systems. This may pose a barrier for small businesses.
• Higher technical requirements than cold runner: The manufacturing and operational processes require greater care due to issues like stringing, drooling, and pressure buildup that must be managed along with temperature control and foreign contaminants.
• Complex maintenance: Maintenance and repair of the hot runner system can be more complicated, necessitating highly skilled technicians. Improper maintenance can lead to production issues.
• Temperature risks: If temperatures are not properly controlled, the plastic may degrade, affecting product quality. Maintaining stable temperatures is crucial for an effective production process.
• Complex cooling operations: The injection molding machine and the cavities must be insulated from the heat of the hot runner. At the glue injection gate, the temperature at the contact point can differ from other components during cooling.

5. Considerations for Hot Runner molds

To ensure the hot runner system operates efficiently, several points should be noted:

• Proper mold design: The mold should be designed to optimize plastic flow and ensure even heat distribution. Careful calculations are necessary to avoid hot or cold spots within the mold.
• Select appropriate plastic: Not all plastics are suitable for the hot runner system. It’s essential to choose materials with good heat resistance that align with the production process to maintain product quality.
• Pump pressure through the nozzle: The pump pressure in the runner system can reach up to 30,000 psi (approximately 2000 bar), creating pressure at the contact point between the hot runner system and the mold. High pressure can lead to plastic leakage through gaps during pumping. Additionally, the materials and design of the hot runner must be robust enough to withstand pressure to prevent blowouts or nozzle breaks during operation.
• Leak prevention in the system: To prevent leaks, the runner system must control the offset to withstand clamping force and pump pressure from the machine. Tolerances should be maintained at 0.005 mm (0.0002 in). The heating and cooling systems must operate independently, with the temperature from the hot runner separated from the mold temperature. This helps avoid unwanted phenomena like stringing, drooling, and stress during pumping or nozzle blockage.
• Expansion of nozzle and manifold: When designing the hot runner, consider the thermal expansion factors. The temperature of the mold and nozzle can differ by up to 200 °C, causing significant expansion between the hot runner and the mold during heating and cooling.
• Uniform temperature supply: The hot runner must have a flow channel that allows controlled and balanced flow to each nozzle. The flow channel needs to be unobstructed and fill all areas in the mold cavity.

6. Conclusion

The hot runner system is a crucial technology in the plastic injection industry, providing numerous advantages in performance and product quality. However, it is also important to consider its disadvantages and maintenance requirements to ensure effective operation. Understanding the structure and functioning of the hot runner system will help manufacturers optimize their production processes and enhance the quality of plastic products. Investing in hot runner technology can yield long-term benefits, boosting productivity and competitiveness in the plastic manufacturing market.