Injection Molds: Hot Runner vs. Cold Runner Molds

Since the mid 19th century, manufacturers in the U.S. and Europe have used the injection molding process 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 realize the potential for gaining a competitive advantage. Not only is injection molding a highly effective process to manufacture 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.

When using the plastic injection molding process, the fabrication of any part or product starts with the mold. Molding systems are divided into two categories: hot runner molds and cold runner molds. Hot runners use a screw nozzle that is fed by a barrel using a pump, while cold runners use a closed, thermoset mold. The primary task of any injection runner system is to direct the material flow from the sprue to the mold cavities. The system requires additional pressure to push the material through the runner. Frictional heat, generated within the runner mold by the material, flows through the runner and raises the temperature, which facilitates the flow. One of the best pay-offs for any project relates to the proper sizing of the runner for a given component and mold design.

For example, large runners facilitate the flow of material at relatively low pressure requirements. However, they require a longer cooling time, consume more material and scrap, and need additional clamping force. On the other hand, using the smallest runner system as required for the project will maximize efficient use of raw material as well as energy consumption during the injection molding process. Ultimately, reducing the runner size depends on the molding machine’s injection pressure capability.

A well-designed runner system delivers a number of benefits, such as:

• Achieving the optimal number of cavities
• Delivering melt to the cavities
• Balancing filling of multiple cavities
• Balancing filling of multi-gate cavities
• Minimizing waste
• Allowing for easy injection
• Providing efficient energy consumption
• Controlling the filling/packing/cycle time

This primer explains the basic of cold runner designs and injection molding hot runner designs, and their pros and cons. By obtaining a better understanding of the two systems, you can decide on the system that best suits the design and specifications of the plastic parts.

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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
• Operator has more process control and 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.

CHOOSING A HOT RUNNER SYSTEM

With numerous brands of hot runner systems in the marketplace, you have to take into consideration a number of factors to choose the right one for your needs. Considerations include price, quantity, delivery time and services. In addition, you will need to look at the technical aspects of any system, which are:

Injection pressure. The plastic inside of the hot runner stays hot. This means the hot runner injection pressure drop is significantly smaller than for cold runner systems. You should perform mold flow simulation to get the data necessary to design the proper injection pressure, especially for materials that have poor melt-flow performance and large parts, which require long melt flow and a complete fill.

Heating. The primary difference between the various types of hot runner systems has to do with the method used to heat the melt. External heating systems keep the material. Externally, the raw material flows through the runner without any carriers. This method provides more reasonable melt shear force curves. Internally heated systems mount right on the melt channel and heat the material inside.

Gate type. Hot runner systems have a wide variety of gate types. Aspects you should consider include the gate marks, gate location, and injection of material types. Non-crystalline and crystalline thermoplastic elastomers require different types of gates.

 

Standard or custom-made system. You need to assess whether to go with a standard hot runner system or a custom-made solution. In most cases, the standard solution makes the best choice. The standard length, nozzles, runner boards, gate inserts, and other components means the parts are available. It also reduces delivery time, costs less, and is easier to maintain.

Types of plastics processing. When choosing hot runner systems for injection molds, the user must consider the plastic resin. For example, glass-reinforced plastic requires the gate inserts to provide reasonable wear resistance. If the plastic resins easily discompose, you should employ an external heat system runner to avoid dead corners.

Runner size. The runner size has a significant effect on the overall performance of the hot runner. Get this wrong and it can cause degradation of plastic injection molded components or even an incomplete filling. To determine the optimal size of the runner, you need to consider the pressure drop, residence time, temperature, shear rate and frequency, as well as other factors.

Multi-zone temperature control. When it comes to large, complex hot runner systems and temperature-sensitive plastic resin with tight processing parameters, you should employ a multi-zone temperature controller system. The system can account for heater quality and heat loss.

Make sure you have a professional who understands hot runner molds, the materials and processes to help ensure you consider all the essentials required for a successful production run.

COLD RUNNER MOLDS

Cold runner injection molding systems consist of two or three plates contained within the mold. The system injects the plastic into the mold through the sprue and feeds the runners, which lead to the parts in the mold cavity. The simplest type of cold runner system consists of two plate molds, which has a parting line that separates the two plates. This system has the runner and part attached. You can only gate the part of its perimeter. An ejection system separates both the runner and part from the mold.

Three-plate cold runner systems consist of three plates. There are two parting planes that allow the mold to separate into three sections upon ejection of the part. The runner connects to a separate plate and only the part requires ejection. Three plate molds allow for greater design flexibility and allow flexibility for gate installation based on the requirements of the application. Both types of plates require the runner to be recycled and reground (or disposed of) to reduce waste, which adds to the cycle time.

Here are a few of the benefits of cold runner injection molding systems, including:

• Inexpensive to produce and maintain
• Suitable for a wide variety of polymers
• Allows for quick and easy color changes
• Faster cycle time for a system that includes robotics

The main disadvantages of cold runner systems has to do with slower cycle times compared to hot runner systems and the waste from runners, especially runners that cannot be recycled and reground.

FACTORS TO CONSIDER FOR COLD RUNNER SYSTEMS

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.

You should also consider other factors:

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 allows 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 passage ways 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.