Since the mid-19th century, manufacturers in the U.S. and Europe have used the injection mold to fabricate a variety of plastic parts and products. Today, the injection molding process has blossomed into one of the most versatile and dependable manufacturing processes available. Advances in the types of resins and polymers, innovative processes, and new injection molding machines, with increasingly tighter tolerances and repeatability, have widened the appeal of injection molding. The injection molding process has become one of the best manufacturing processes because of this.

Virtually every industry imaginable including construction, electronics, aerospace, automotive, medical, food and beverage, and more realizes the potential for gaining a competitive advantage. Not only is injection molding a highly effective process for manufacturing accurate plastic parts and components, but it also reduces production time, has a high degree of accuracy, and reduces material waste. At the end of the production process, users can have plastic parts as strong as steel or very soft and flexible.

An injection mold is a component designed according to the intended product using processes such as CNC machining and is where the molten plastic is injected and cooled. There are several injection mold types. However, based on the temperature held by the runner and the mold, there are two mold systems: hot runner and cold runner.

Each mold system uses a different mechanism which affects plastic fabrication differently. As a result, there is a need for a proper understanding of the hot runner mold vs cold runner mold comparison before selection. This article does the hot runner vs cold runner injection molding comparison by introducing both systems, how they work, and their differences. Furthermore, it shows how to choose the right mold for your project.   

HOT RUNNER MOLDS

Often referred to as runnerless molding or hot-manifold systems, hot runner molds describe a set of temperature control heating devices installed in the mold. The purpose of a hot runner injection system is to maintain a molten flow of plastics or rubber from the injection molding machine nozzle to the gate in a plastic injection mold — keeping the runner above the melting point of the plastic.

Hot runner molding systems consist of two plates heated by a manifold system inside one-half of the mold that sends the melted material to nozzles, which feed the part cavities. The system consists of two parts: the hot manifold and the drops. The manifold moves the rubber on a single plane and parallel to the parting line to a location above the cavity. Positioned perpendicular to the manifold, the drops move the rubber from the manifold to the component.

There are two types of hot runner systems. They are insulated runners and heated runners:

Insulated runners. 

Unheated, this type of runner requires extremely thick runner channels to stay molten during continuous cycling. These molds have extra-large passages formed in the mold plate. During the fabrication process, the size of the passages in conjunction with the heat applied with each shot results in an open molten flow path. This inexpensive system eliminates the added cost of the manifold and drops, but provides flexible gates of a heated hot runner system. It allows for easy color changes.

Heated runners. 

This method has two designs: externally heated and internally heated.

Externally heated: 

This system design employs a cartridge-heated manifold with interior flow passages. To separate it from the rest of the mold, the manifold has several insulating characteristics that reduce heat loss. Since it does not require a heater that can block the flow, and all of the plastic is molten, the externally heated hot runner channels have the lowest pressure drop of any runner system. This method works better for color changes because none of the colors in the runner system freeze. In addition, materials do not have surfaces where they stick to and degrade — an attribute that makes externally heated systems an excellent choice for thermally sensitive materials.

Internally heated: Internally heated runner systems have annulus flow passages that are heated by a probe and torpedo located in the passages. Taking advantage of the insulating effect of the rubber melt, it reduces heat loss to the rest of the mold. However, this system requires higher molding pressures, and color changes can be quite challenging. In addition, materials have many places where they stick to the surface and degrade. You should not use thermally sensitive materials in the fabrication process.

Heating the runner can be done through a variety of materials, including coils, cartridge heaters, heating rods, heating pipes, and band heaters. A complex control system ensures a consistent flow and distribution of the melt.

Some advantages of a hot runner system include:

• Extremely fast cycle time
• Improvement of component consistency and quality
• Less waste generated without the cold runner
• Ideal system for fabrication of large parts
• Reduced injection pressure
• The operator has more process control and the ability to fine-tune

Hot runner molding systems cost more for part fabrication and system maintenance. They are not suitable for thermal-sensitive materials like resins and polymers, and you must allow for downtime.

What is a Cold Runner Mold?

A cold runner mold is the basic injection mold with runners at the same temperature as the mold cavity. The system involves injecting the molten plastic into the locating ring. Afterward, the molten plastic passes through a sprue and runner, which are cold, before entering the mold cavity. As a result, when the plastic cools, it takes the shape of the mold cavity with the solidified plastic in the runner attached.

The cold-runner mold features a large runner (larger than the part), removing the tendency of underfilling the mold. Aside from that, it is suitable for working with any plastic polymer irrespective of heat sensitivity, is less expensive than hot runners, and is easily maintained. However, they generate waste.

One of the most important considerations for a cold runner system is to make sure the runner dimension is thicker than the part, to ensure that the melt packs into the part without any limitation as it cools. For most applications, the “round runner” may be the best design option for a cold runner system — each half of the mold contains one-half of the runner diameter. When selecting gates, most users prefer tunnel gates because the action of the injection mold opening or the part ejecting releases the gate from the component.

Factors To Consider For Cold Runner Systems

Nozzles. The nozzles can vary from vendor to vendor. The user must dispose of welded assemblies if any type of particulate penetrates the water jacket. Some nozzles have removable water jackets that allow for cleaning.

Nozzle Tips. Some manufacturers mount the nozzle tips securely to the end of the nozzle. Other vendors offer spring-loaded nozzle tips, which allow for thermal expansion of the mold when heated, which keeps the nozzle in constant contact and prevents leakages.

Runner Manifolds. Be aware that cold runner systems that have a gun-drilled runner manifold design tend to have “dead spots” at the end of the channels, where the material has cured and becomes hard to clean. Conversely, manifolds with a split-plate runner design allow the user to disassemble the plates for cleaning. It also allows for quick material and color changes.

Piston Assembly. Some systems rely on external air lines and barb fittings, but gun-drilled air passageways eliminate the need for air hoses. And barb fittings for each nozzle, which prevents damage to the air lines and connection errors.

Pneumatic Connections. Cold runner systems have either external air lines or an air terminal box with a quick disconnect coupler. The mating umbilical cord abuts with the molding machine to make the required connection from the valve gate controller to the cold runner system.

Technologies for cold runner molding systems continue to improve, which increases the available options for designers and users.

Cold Runner System Advantages

The cold runner system has the following advantages:

Lower tooling cost: The cold runner mold system has a lower investment cost in injection mold tooling. As a result of the lower tooling cost, molding is cost-effective.

Less inspection and maintenance cost: Cold runner molds are not complex. Consequently, there is less need for inspection and maintenance, leading to decreased maintenance costs and downtime.

Thermosensitive plastics: Another difference between hot runner and cold runner molds is their application in molding thermosensitive plastics. Cold runner systems are the most suitable for thermally sensitive polymers. Furthermore, they are also suitable for any plastics.

Hot Runner Mold vs Cold Runner Mold – Drawbacks of Each System

Each runner mold also has its drawbacks. Here, we will talk about their disadvantages and how they compare to each other.

Hot Runner System Disadvantages

The hot runner system has the following disadvantages:

High investment cost: Hot runner systems have a high initial investment and maintenance cost due to the sophistication of the molds. And advantages such as reduced cycle time, part quality, and flexibility.

High requirement for precision in the equipment: The system requires high-end equipment due to the need for precision. Otherwise, there might be defects because of failure in the machine. An example is the plastic seal failure which can lead to the molten plastic spilling and damaging the hot runner assembly.

Cold Runner System Disadvantages

Cold runner injection molding has the following disadvantages:

Material wastage: Materialwaste is another difference in the hot runner vs cold runner molding comparison. The molten plastic solidifies in the cold runner system. Consequently, these lead to scraps that need trimming.

Injection pressure drop: Heat transfers from the plastic to the mold plates during plastic flow, which causes shrinkage leading to pressure drop and further forming defects such as sink marks and underfilling parts.

High cycle time: Cold injection molding has a slower cycle time due to reduced plastic flowability. Aside from that, removing the runner and sprue (especially in the two mold systems) also increases the cycle time.

Hot Runner Mold vs Cold Runner Mold – Considerations When Choosing Each System

Choosing the two must only occur after carefully understanding how they work. Below are a few factors to consider when choosing from any molding system.  

Choosing the right hot runner mold should be based on price, delivery time, and quantity. However, more important factors rest on the technical aspect of the runner system. Below are a few you should take note

Injection Pressure

Due to the consistent temperature of the molten plastic, while passing through the manifold system, the injection pressure drop is low. As a result, it’s important to ensure proper mold flow simulation during injection mold design when working with all materials for injection molding, especially those with poor melt-flow performance.

 Heating

The heating component can be internal or external. External heating systems are suitable for thermosensitive materials. On the other hand, internally heated systems are suitable for other plastic polymers.

Gate Type

Every material and product design requires a unique gate design. Therefore, consider factors such as gate mark, location, and injection of material types when making the gate.

Standard or Custom-Made System

Choosing from a standard or custom-made system depends on your requirement. On the one hand, the standard system has standard components and dimensions. Consequently, they are the best choice due to cost, delivery time, and maintenance. On the other hand, custom-made systems are ideal for achieving products with unique designs.

Types of Plastic Processing

The plastic polymer you are working with also plays an important role in the choice. For example, glass-reinforced plastic needs molds having gate inserts with good wear resistance or an external heat system runner.

Runner Size

The runner size plays a critical role in the system’s performance. Getting it wrong can lead to the degradation of the final components or incomplete filling. Some factors such as the pressure drop, residence time, operation temperature, and shear rate and frequency can determine the size of the runner.

Multi-Zone Temperature Control

When working with a large and complex system or a thermosensitive plastic, ensure you use a multi-zone temperature control system to account for heater quality and heat loss.

Choose StepRapid for Custom Injection Mold Services

Getting a quality part using injection molding should only occur after understanding the hot runner mold vs cold runner mold comparison and using the right service provider. At Step Rapid, we offer services for making injection molds and help you create prototypes. And production parts of the highest quality in a short lead time or provide economical solutions based on requirements.

We are an ISO 9001:2015 company with advanced facilities and teams that can handle your project. Moreover, we have an instant quoting platform that gives accurate quotations and DFM analysis. Upload your design file today and get a real-time quotation, and DFM report for mold design and flow analysis within 12 hours.

Conclusion

Choosing between the hot or cold runners is only possible after understanding the runners’ systems. As a result, this article introduces the cold runner vs hot runner injection molding comparison, their differences, and how to choose the right one. Do you need high-quality injection mold using the hot or cold runner system? Let us help you get started.