Should You Use Hot Runner Systems in Injection Molding?

What is a hot runner system?

Hot runner systems are systems of heated components used in injection molds to inject molten plastic particles into the cavity of the mold.

Hot runner systems are heated to ensure that the plastic in the runners and gates remains molten. Since there are heating rods and rings near or in the center of the runner, the entire runner from the nozzle outlet to the gate of the injection molding machine is under a high temperature, the plastic in the runner is kept molten. It is generally not necessary to open the runner to take out the condensed material after shutdown.  For this reason, the hot runner process is sometimes referred to as runnerless molding.

Advantages of hot runner systems

• Saving raw materials and reducing material costs
  • No need to take out the remaining feed every time
• Shortening the molding cycle and improving machine efficiency.
  • That means less time spent cleaning out the runner every single cycle.
• Fewer follow-up processes and increased automation.
  • The finished product is formed by the hot runner mold, no need to trim the gate and recycle the cold sprue and further processes, which is conducive to production automation.
• Consistent quality of injection molded parts in multi-cavity molds improves product quality.
  • During the hot runner molding process, the plastic melt temperature is accurately controlled in the runner system.  The plastic flows into each cavity in a more uniform state, resulting in consistent quality parts. Hot runner molded part features high quality of gate, with low residual stress and part deformation after demolding.
• The possibility of a lower injection pressure remarkably reduces the possibility of deformation of thin-walled products.

However, every technology has its own drawbacks, and there is no exception for hot runner technology.

Here's the caveat:

• Together with high production and maintenance costs, mold structure is more complex and harder to pull off.
• It takes a while for the start-up process to stabilize, resulting in more scrap at the beginning of a cycle.
• The possibility of melt leakage and heating element failure has a greater impact on product quality and production schedule.
  
  • Therefore you shouldn't let an inexperienced plastic manufacturer handle hot runners. This type of machinery needs solid maintenance.

Therefore, instead of offering an all-around better alternative for all plastic injection parts, hot runners are suitable for certain types of products, not everything.

When Should You Use Hot Runner Systems Then?

• When the overall size of the product is large or when the product wall is thin
  • For example, large size product such as car liners, balance plates, ... needs casting at multiple spots at the same time, and the hot runner systems are needed for making the plastic flow at stable temperatures within the mold. Otherwise, you may get Weld Lines, Burn Marks or Splays on your molded parts. Click the link below to download our free ebook on 20 Most Common Issues in Injection Molding

• When the requirement for product appearance or precision is demanding.
• When the product yield is high
• When the material used in the product is thermally sensitive
• When the product requires special materials
  • For example: the hot runner system addresses the requirements of high viscosity, low stickiness, high forming temperature materials

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Components of a Hot Runner System

A hot runner system generally consists of a heated nozzle, manifold, temperature control box, and accessories. Generally, there are two types of heated nozzles: open type and needle valve type. The heated nozzle directly determines the selection of the hot runner system and mold used in the process. Therefore, the hot runner systems are often classified into open-type hot runner systems and needle valve-type hot runner systems.

Key Factors

Two factors are crucial to ensure the success of a hot runner mold application project.  These are plastic temperature and flow.

1) Plastic Temperature

The control of plastic temperature is extremely important in hot runner molding applications. Many of the processing and product quality problems that occur in the production process derive directly from poor temperature control during the injection and cooling processes.

1. The examples include the issue of poor gate quality during injection molding using a heated pin gating method.
2. Difficulty in closing the valve pin when molding by valve gating method.
3.  The issue with inconsistent injection time and quality of parts in multi-cavity molds, etc.

When possible, try using a hot runner system capable of independent temperature control for multiple cavities to increase flexibility and adaptability in usage.

2) Plastic Flow

A balanced flow in the hot runner systems should be maintained for the molten plastic.  Gates should be opened at the same time so that the plastic fills each cavity simultaneously.  The sprue size should be designed in a balanced manner for parts with a significant difference in weight.

1. Otherwise, problems like insufficient or excessive injection or holding pressure can lead to short shots or flashing. 
2. The hot runner sprue size should be reasonably designed. Under-sized sprue will result in loss of mold filling pressure.
3. Over-sized sprues might cause the plastic to stay for an extended amount of time in the hot runner system, damaging the material properties and causing the part to fail after molding.

Have you had any experience using hot runner systems? Share them with us in the comments section below!