STRUCTURE OF HOT RUNNER SYSTEM

WHAT IS THE HOT RUNNER SYSTEM?

“Hot runner” is a system of electrically heated channels used in plastic injection molds. It is used to keep the plastic in the channel always in a molten state to inject directly into the plastic injection mold. Molds arranged with a hot runner channel system are called hot runner molds. The hot runner is mounted directly on the plastic injection mold and is an integral part of the mold. The temperature in the mold is controlled by controllers, which help create a consistent temperature during the injection molding of plastic products.

HOT RUNNER SYSTEM STRUCTURE

A Hot runner system consists of the following components:

• Sprue bushing
• Manifold
• Nozzle
• Gate

Sprue bushing

Sprue bushing is the part that helps to spray the plastic from the nozzle of the machine to the manifold. In order to ensure uniform heat in the runner system, even at this nozzle, the resin must be heated and adjusted to the same temperature as other parts of the hot channel system. If the temperature of this area is too low for high temperature-sensitive materials, the surface of the product will not be of good quality.

Functions:

• Connect to the injection nozzle of the press.
• Seal the flow system connecting the machine to the nozzle.
• Reduce pressure as needed.
• Filter hot plastic if required.

Manifold

Manifold is used when spraying indirectly through multiple nozzles. Manifold acts like the plastic distribution plate from the hot arm of the machine to the cavity.

Functions:

• Ship and distribute hot plastic flow.
• Maintain resin temperature as a constant while spraying.
• Apply pressure to the plastic stream.

Nozzle

The nozzle is the final position of a hot runner system, and it includes a heat conductor to the nozzle. The nozzle has the role of supplying hot plastic from the channel to the mold cavity.

Functions:

• Supply hot plastic to the gate.
• Maintain resin temperature as a constant while spraying.

Gate

Functions:

• Supply hot plastic to the mold cavity.
• Maintain the plastic path into the mold cavity
• Shut off the plastic flow.

Open Gate

The open gate can be used for crystalline and amorphous plastic materials. We need to pay attention to parameters such as nozzle diameter, nozzle cooling area, and temperature control at the top of the nozzle to optimize the quality of the product. The open nozzle leaves a small mark on the surface of the product. The size of this spot is directly related to the shape of the nozzle and the material properties.

Structure of a hot runner system using the open gate:

Valve Gate

The valve gate is operated by a steam system. In this mechanism, the piston and cylinder are located on the upper plate. Before pressing, the valve is in the retracted position, which causes the orifice hole to widen to allow the plastic to flow into the mold cavity. Before the resin at the injector has completely solidified, the pin moves to the closed valve position.

NOTES IN HOT RUNNER MOLD

Pump pressure through the nozzle

The Runner system is subject to pumping pressure, which is up to 30,000 psi (≈2000Bar). High pressure can cause plastic leakage through crevices during pumping. In addition, the material and design of the hot runner must also be hard enough to withstand the pressure to avoid explosion and nozzle breakage when operating.

Anti-leakage in the system

• The runner must control the deflection to withstand the clamping force and pump pressure of the machine. This tolerance should be controlled to 0.005 mm (0.0002 in).
• The heating system and the cooling system need to work independently
• The temperature from the hot runner system needs to be separate from the temperature of the mold plate to avoid unwanted thermal effects on the product, such as glue tail pulling, drooling, stringing, stress during injection, or clogging of the nozzle.

Level of expansion of Nozzle and Manifold

We need to pay attention to the thermal expansion factor of the hot runner material. Mold and nozzle temperatures can differ by up to 200°C, which will cause significant expansion between the hot runner and the mold during heating and cooling.

Uniform temperature across the flow

The hot runner must have a controlled and balanced temperature flow channel to each nozzle in the system. The flow channel should be smooth, free-flowing, and fill all areas in the mold cavity.

ADVANTAGES OF HOT RUNNER MOLD

Compared to cold runner mold

The advantage of hot runner mold over cold runner mold is that it eliminates the plastic channel system in the production process, thereby creating advantages in cycle time, pressure, pump volume, and post-processing of plastic injection.

Reduce pressing cycle time

The hot runner mold does not have to undergo the cooling or filling process. At the same time, it also reduces the machine stroke during the pressing process.

Reduced mold opening stroke compared to three-plate mold

The hot runner mold reduces the clamping stroke compared to the three-plate mold because just opening the mold is enough to push the product out. Meanwhile, the three-plate mold needs open two times between plates of the mold to get the product and separate the plastic channel into two separate parts.

No need to use plastic channels

There is no need to use a plastic channel in hot runner molding. The manifold is heated continuously. Therefore, during the pressing process, the plastic does not solidify.

• Help save plastic raw materials and reduce energy for reheat operations.
• Remove glue tails and save energy for recycling operations.
• In the medical field, wasted plastics cannot be used for manufacturing other medical devices. Hot runner helps optimize the use of this plastic part.
• Reduce labor costs to process semi-finished products after the pressing process.
• Reduce storage space and transportation costs for plastic scrap.

Optimize product quality

As the resin flows through the lower temperature channel due to the previous cooling operation can cause a pressure drop resulting in dents and underfilled mold cavity. On the other hand, the hot runner system maintains a constant temperature equilibrium flow from the nozzle to the pump port. Therefore, the hot runner mold helps the manufacturer fill all the mold cavities that require high precision, narrow and long cross-section (Test tubes, workpieces). If a mold cavity is damaged or out of specification, it can likely reduce heat to the nozzle and stop production for the defective mold cavity.

Be applicable to stack molds

Although the stack molds can use a cold runner system, most stack molds use a hot runner system to be more optimally.

Mold automation

Due to the need for labor to remove the glue channel from the product, manufacturers can easily integrate the plastic injection process with other automated activities such as robots picking up products, conveyor belts, packaging, and filling.

Design flexibility

The hot runner system enables locating ports at various points on the product. It is also possible to place the gate in the most favorable position to fill the cavities and optimize the product’s aesthetic by using hot tip gating, valve gating, or edge gating in the hot runner system. This flexibility makes it easier for the mold designer to arrange the cooling system and other parts.

Balanced flow

The advantage of flow that a hot runner mold has is that the fusion channels are in manifold tubes, which are heated outside, and insulated from the surrounding mold plates. Unlike the cold runner plate, the manifold can be designed for multi-level flow to help uniform temperature and pressure from the nozzle to the gate.

Can control the pull pump sequence

Eliminate welds by using valve gates with the ability to adjust the injection sequence to help reduce errors on the product.

DISADVANTAGES OF HOT RUNNER MOLD

Technical requirements are higher than cold runner mold

The manufacturing and operation process needs to be careful with problems, such as tail drag, drooling, stringing, and stress during pumping, nozzle clogging, heat control, and foreign matter in the system.

Higher cost of mold making

The initial investment cost is high due to the more complex system (which requires the electric controller and hydraulic or pneumatic control system).

Operation and repair costs

• Due to the more complex structure, the risk of errors in the manufacturing process will also be higher. Production can be interrupted due to clogged gates, damaged electrical components, or leaks.
• Downtime can be prolonged due to a leak, causing severe damage to electrical components and the valve gate.
• There is a possibility of serious damage due to voltage failure of the heater element and controller mismatch.

Less flexible in changing plastic materials or plastic colors

• Materials have different operating temperatures. Using materials not in accordance with the recommended temperature may cause product failure or damage to the mold during use. Different kinds of material also require various pump ports and levels of pressure.
• Some heat-sensitive materials are difficult or impossible to operate in a hot runner system.
• Changing the color of the glue takes longer than the cold runner mold because the glue is still left in the channel pump gate system.

More complex product cooling operations

The injection molding machine and the chambers must be insulated from the hot runner’s heat. At the pump port, the temperature of the pump port junction may differ from other parts during cooling.

Hard to increase the diameter of the manifold and nozzle

The incorrect pump pressure in the hot runner design leads to the need to change the manifold and nozzle size, which can be very costly to repair or completely replace the nozzle.

CONCLUSION

The hot runner system has outstanding advantages in terms of cycle time, automation ability, and reduced number of workers involved in the production process. So, it is preferred by many plastic manufacturers. However, the hot runner system is not suitable for some types of plastic on the market. In addition, the initial investment cost of this technology is pretty high, so we should thoroughly consider it before investing in production.

Duy Tan Precision Mold company has more than 20 years of experience in the research and development of hot runner systems. We manufacture the most reliable, high-performance hot runner systems. The advantage of our hot runner system lies in the design that distributes the resin to the cavity evenly, increases productivity and quality, reduces injection cycle times and maintenance costs, and helps molds operate smoothly and durably. In addition, Duy Tan Precision Mold company applies hot runner systems from world brands to the molds we manufacture with optimal cost.

                                  Contact :

Duy Tan Precision Mold Co., Ltd

Hotline : +84 28 3754 5418 (ext. 728) | Fax: +84 28 37 54 54 16

Info@duytanmold.com | Facbook: https://www.facebook.com/KhuonChinhXacDuyTan