Hot runners are electrical heaters that heat the flow galleries within a tool, to allow for faster processing and to reduce materials use per shot, as the tool is run.
This is an option in tooling development that allows for increased efficiency in production, compared with more traditional cold runner tools. The choice also carries some commercial and technical burdens that must be taken into account.
This article gives you a primer into the function, operation and options in specifying hot and cold runner tooling and introduces you to the costs and benefits.
I. What is Hot Runner Injection Mold?
Hot runner injection molding is a manufacturing process used to produce plastic parts with greater precision and efficiency. In this process, a plastic injection molds galleries or runners that are maintained above the melting point of the plastic being injected.
The plastic is fed into the heated mold under pressure, filling the cavities, while the galleries remain full of liquid plastic, ready for the next cycle.
A key advantage of hot runner injection molding is that it allows for faster cycle times, as the tools temperature requires less cycling between shots. This results in higher production rates and lower manufacturing costs.
Additionally, hot runner injection molding can produce parts with a higher level of consistency and quality, as there is less variability in the temperature of the molten plastic as it is injected into the mold.
One of the most significant impacts is in reducing plastic usage per part on large multi cavity tools, as galleries are not ejected with the parts, as galleries that remain liquid result in ONLY parts using virgin plastic
Hot runner injection molds can be designed as single, dual, or multiple gates, depending on the specific application and part geometry. They are commonly used in a wide range of industries, including automotive, medical, and consumer goods.
II. What is Cold Runner Injection Mold?
As the name suggests, this type of tooling uses runners or galleries that are not internally heated.
In such a cold runner tool, the cavities are fed plastic in the injection process through galleries that sit at the part line of the tool. As the galleries and then the cavities fill, both gallery and component parts cool and then the tool opens and all is ejected.
- parts be trimmed from runners, often by hand
- solidified runner plastic parts are reground and used again, or discarded depending on the component quality requirements
- Lower production rates and higher cost per part are consequences of choosing a cold runner tool design
Extra plastic use can be a small percentage of the part material use, if the parts are large. The economic impact of this wastage is more significant in smaller components, in extensive multi cavity tools (requiring long galleries) and in processes that require proportionally higher cost materials.
III. Hot Runner vs Cold Runner Injection Mold: Which Method is Best for Your Application
The decision to use a hot runner system is generally a simple commercial one. Where part price is very sensitive and volumes are large, hot runners are the norm. Where tooling cost is a barrier and production rates/volumes are lower, cold runners are the logical choice.
Hot Runner Injection Mold: Benefits and Applications
Hot runner injection molding offers several benefits over cold runner techniques:
- Material use: In cold runner injection molding, the plastic material that is used to create the runner system is typically discarded after each cycle. This can result in significant waste, particularly for larger or more complex parts. With hot runner injection molding, there is no runner material to discard, which can result in significant cost savings.
- Processing time: Because the plastic material in a hot runner system is kept at a consistent temperature, there is no need to wait for the material to heat up or cool down between cycles. This can result in faster cycle times and higher production rates.
- Part quality: Hot runner injection molding can produce parts with a higher level of consistency and quality, as there is less variability in the temperature of the molten plastic as it is injected into the mold.
- Design flexibility: Hot runner injection molds can be designed in a variety of configurations, including single, dual, or multiple gates, which allows for greater design flexibility and can result in parts with more complex geometries and tighter control over weld lines.
Hot runner injection molding is used wherever high volume parts are required:
- Automotive: Hot runner injection molding is commonly used in the automotive industry to produce parts such as dashboards, fill caps, driver controls and interior trim components.
- Medical: Hot runner injection molding is used in the medical industry to produce high volume consumable parts such as syringes, IV components etc.
- Consumer goods: Hot runner injection molding is used in the production of consumer goods such as toys, electronics, and appliances.
- Packaging: Hot runner injection molding is used to produce parisons (pre blow molding blanks), packaging materials such as bottles, caps, and closures etc.
Cold Runner Injection Mold: Benefits and Applications
Cold runner injection molding is the more traditional manufacturing process used to produce plastic parts. The plastic material is injected into the cooled mold, filling the cavities with molten plastic to form the desired shape.
The material in the runner system is typically discarded after each cycle, resulting in more waste than hot runner injection molding.
Despite its disadvantages compared to hot runner injection molding, cold runner injection molding still offers several benefits:
- Lower tooling cost: Cold runner injection mold tools are simpler, lower cost and faster to manufacture.
- Simplicity: Cold runner injection molds are simpler to make and easier to maintain than hot runner injection molds.
- Versatility: Cold runner injection molding can be used with a wide range of plastic materials, including thermoplastics, thermosetting plastics, and elastomers. Not all materials are suited to hot runners and setup is considerably more involved.
- Small production runs: Cold runner tools are easier to run, and stabilize sooner, so smaller production runs are feasible than with hot runners.
Cold runner injection molding is used across all sectors of the molding industry:
- Consumer goods: Cold runner injection molding is used to produce most consumer goods, other than at the highest volumes
- Packaging: Cold runner injection molding is used to produce packaging materials such as containers, caps, and closures where volumes are low or intermediate.
- Automotive: Cold runner injection molding is widely used in the automotive sector, when volumes are moderate.
- Medical: Cold runner injection molding is used to produce lower volume medical parts such as syringes, pipettes, and medical device housings – and it is often used in higher volume sectors because of the greater flexibility and shorter lead times for tooling.
IV. Hot Runner Mold vs Cold Runner Mold – Drawbacks of Each System
Hot Runner System Disadvantages
Hot runner tooling is not suited to every molding job:
- Tooling is considerably more costly
- Tools suffer longer lead times
- Production runs require more setup and settling time to achieve good moldings
- Not all mold machines are adapted to hot runner tooling
Cold Runner System Disadvantages
Cold runner tooling also has disadvantages to be considered in choosing which type to use:
- Material usage is higher, especially in small components
- Gate and gallery trimming can be a relatively high cost, as it is often manually performed
- Cold runners are less well adapted to high volume molding
- Part quality can be more affected by machine settings as there is less buffer in the injection process
V. Hot Runner Mold vs Cold Runner Mold – Considerations When Choosing Each System
Careful consideration must be given to adhering to best practise in tool specification, design and operation. A variety of factors differentiate the development and operation of these two classes of tool and using services with extensive experience in these matters is key to good outcomes.
Considerations When Choosing the Hot Runner Molds
Choosing the right hot runner mold can have a significant impact on the quality and efficiency of your plastic injection molding process.
Here are some key considerations to keep in mind when choosing hot runner molds:
- Injection pressure: The injection pressure required for the plastic material being used is an important consideration when choosing a molder to operate a hot runner, multi cavity tool. The mold should be designed to handle the required injection pressure without damaging the mold or causing issues with the plastic flow. Hot runner, multi cavity tools require high injection pressures and high clamping forces.
- Heating: Hot runner molds use an electrical heating system to keep the runners and nozzles at the correct temperature. When specifying a hot runner mold, it is important to consider the heating system power and quality, to ensure that it can maintain appropriate operating temperatures.
- Gate type: The type of gate used in the hot runner system can significantly affect part quality. The gate type should be chosen to suit the part being produced, to achieve smooth fill and good quality welds.
- Standard or custom-made system: Hot runner molds can be purchased as standard systems or custom-made to meet specific requirements. When choosing a hot runner mold, it is important to consider whether a standard system will meet the needs of your process, or if a custom-made system is necessary. Custom made systems carry a price differential and can pose future maintenance issues.
- Types of polymer: High-temperature plastics may require a different type of hot runner system than low-temperature plastics.
- Runner size: The size of the runners in the hot runner system can impact the flow of the plastic material and the overall efficiency of the process. The runner size should be specified with knowledge of the intended molding machine and the flow viscosity of the intended fil polymer.
- Multi-zone temperature control: Hot runner molds can have multiple zones of temperature control to ensure that the plastic material is heated evenly throughout the mold. This offers advantages in larger, more complex molds – but at a price in tooling and operational costs.
Considerations When Choosing the Cold Runner Molds
Good specification/design of cold runner molds is crucial to ensure the quality and efficiency of your plastic injection molding process.
- Nozzles: Nozzles distribute the plastic material evenly into the mold cavities, and must be designed to withstand the high injection pressures and temperatures required for plastic injection molding. Ensure that the nozzles are made from high-quality materials and are compatible with the plastic material being used (some polymers are corrosive in the melt, for example, and require stainless steel components).
- Nozzle tips: The nozzle tips in cold runner molds are responsible for controlling the flow of the plastic material into the mold cavities. The tips must be designed to provide a consistent flow rate and minimize any turbulence in the plastic material.
- Runner manifolds: Runner manifolds are the channels that connect the nozzles to the mold cavities in cold runner molds. The design of the runner manifolds can have a significant impact on the efficiency and quality of the injection molding process.
- Piston assembly: In cold runner tools, the piston assembly is responsible for controlling the flow of the plastic material into the mold cavities. The assembly must be designed to provide a consistent flow rate and minimize any turbulence in the plastic material.
- Pneumatic connections: The pneumatic connections in cold runner molds are used to supply air pressure. They are quick release couplings that must be of high quality.
VI. Hot Runner Mold vs Cold Runner Mold: An Overview of Each Welding Process
In principle, the welding processes in hot and cold runner tools are identical – propagating fronts/lobes of molten plastic fill the tool and rejoin at the last fill point. Where this plastic is properly liquid, the weld will be strong and generally not visible.
Weld management is an art within tool design and molding, and hot runner tools are often less prone to the problems that arise in poor weld. In particular, dual and multiple gate cavities allow aggressive management of welding, in hot runner molding.
Hot Runner Mold: Principle of Process
In relation to welding, the hot runner injection molding process can offer some advantages. Because the hot runner system delivers more highly temperature controlled plastic directly to the mold cavity, the plastic material often flows more evenly, and the better fluidity results in stronger welds within the parts.
Additionally, because the hot runner system allows for greater design flexibility in multi-gating the cavities, parts can be designed with specific weld points in mind, which can further enhance the strength of the weld.
Overall, hot runner injection molding can produce strong welds, offering some advantages when it comes to achieving consistent, high-quality welds in complex parts with specific design requirements.
Cold Runner Mold: Principle of Process
In cold runner injection molding, the weld strength can be heavily influenced by factors such as the material being used, the cooling rate, and the pressure at which the plastic is injected into the mold.
These factors can vary with production rate and pauses for settings adjustment. While cold runner injection molding can produce strong welds, the process requires more fine-tuning to achieve consistent results.
VII. Should you Choose Hot Runner Mold or Cold Runner Mold
The choice to build a hot runner tool is not a simple one:
- Where the art is very high volume, hot runners make commercial sense.
- Where the part must be of the most consistent quality, hot runner tools are advantageous.
- Where material wastage (due to cost, environmental concerns etc) then hot runner tools are a good choice, where volume justifies this.
- Where tooling cost is a factor in the decision making, hot runners can be harder to justify.
- If volumes OR product run sizes are more modest, then hot runners are disadvantaged.
- Where material flexibility is required (i.e. same tool, alternate materials), hot runners can complicate this.
It is common to begin production with a modest number of cavities, in a cold runner tool, until volumes climb to justify the extra expense and potential part cost savings that come with hot runners.
VIII. Choose Kemal for Your Injection Molding Needs
We are happy to introduce Kemal. We are a company that has remarkable range and depth of skills, qualification and experience. We have a deep understanding of our customers’ needs and we are thorough and professional in our operations. We deliver excellence in quality and customer service and competitive pricing.
- Molding Manufacturing
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Trust Kemal with your project, we will help you along the road to production in ways that will surprise and delight.
To know more about our services in all aspects of injection molding and so much more, contact us to discuss your project, meet our team and let us explore together how we can make your product, make it better, and make your life easier.