Introduction
An injection mold is a component designed according to a desired product, typically using processes like CNC machining, where molten plastic is injected and cooled. There are many types of injection molds. However, based on the runner and the temperature maintained by the mold, there are two primary mold systems: hot runner and cold runner.
Each mold system uses a different mechanism that affects plastic fabrication in unique ways. Consequently, a proper understanding of the hot runner mold vs. cold runner mold comparison is necessary before making a selection. This article will compare hot runner vs. cold runner injection molding by introducing both systems, how they work, and their differences. It also shows how to select the right mold for your project.
What is a Hot Runner Mold?
A hot runner mold is an injection mold with a manifold system (i.e., one that has a heating component, manifold, and nozzles) that ensures molten plastic is injected into the mold while maintaining its temperature.
Hot runner is suitable for injection molding plastics that require a stable flow and injection pressure for part fabrication. With this method, defects like fluctuations in injection pressure and sink marks caused by low mold temperatures are reduced.
Types of Hot Runner Molds
Based on the method of heat retention, there are two types of hot runner molds used in plastic injection molding:
* Insulated Hot Runner: Insulated hot runner molds have extremely thick runners in the mold plate, which causes a part of the molten plastic to cool and insulate the system. The insulation reduces heat loss, allowing the mold system to maintain an open plastic flow. These molds are less expensive than heated molds, have flexible gates, fewer defects, and favor easy color changes. However, some disadvantages include freeze-ups at the gate, non-uniform mold filling, long start-up periods, and a need for a rapid cycle to maintain the molten state of the thermoplastic.
* Heated Hot Runner: The heated hot runner is the dominant type of hot runner system, where heating is achieved using a heating component in the mold half. They come in two designs: externally heated and internally heated.
* Externally Heated Hot Runner: The heating component is outside the mold. This leads to no obstruction in molten thermoplastic flow and no drop in injection pressure. Molten plastic also doesn't freeze, making it ideal for color changes and suitable for thermosensitive materials.
* Internally Heated Hot Runner: Here, the heating component is inside the mold, which causes a drop in injection pressure, incompatibility with color changes, and material hanging and degrading in the mold. However, there is better flow due to the heated runner and gate tip control.
What is a Cold Runner Mold?
A cold runner mold is the original injection mold with runners at the same temperature as the mold cavity. The system involves injecting molten plastic into a locating ring. Afterward, the molten plastic passes through a sprue and runner before entering the mold cavity, which are all cold. As a result, when the plastic cools, it takes the shape of the mold cavity with solidified plastic in the runner.
The cold-runner mold has a large runner (larger than the part), which overcomes the tendency of the mold to shrink. It is suitable for working with any plastic polymer regardless of heat sensitivity, is less expensive than hot runners, and is easily maintained. However, they generate a lot of waste.
Types of Cold Runner Systems
Based on the number of mold plates, cold runner mold systems are of two types:
* Two-Plate Cold Runner System: This system has two plates with a stationary mold containing the sprue, runner, gate, and cavity. It is the fastest, simplest, and least expensive cold runner system. The runner, however, is attached to the final product and must be trimmed.
* Three-Plate Cold Runner System: This system is similar to the two-plate system. However, the additional runner plate makes the three-plate injection mold flexible. Also, there is no need to trim the runner from the injection-molded part, and the mold does not require an ejection system for part removal. Three-plate injection molds are more flexible and suitable for products with complex designs. They are also a low-cost alternative to hot runner systems. The disadvantages include a long cycle time, complex tool design, and high material waste.
Hot Runner Mold vs. Cold Runner Mold - Advantages
Each runner mold has its advantages that make it suitable for different applications and industries.
Advantages of a Hot Runner System
The hot runner system is widely applied in plastic product fabrication due to the following advantages:
* Shorter Cycle Time: One key difference between hot runner and cold runner injection molding is cycle time. Hot runner injection molds have a faster cycle time due to keeping the plastic at an optimal temperature, increasing flow efficiency.
* Reduced Material Waste: It is more cost-effective because the mold system generates little or no material waste.
* Optimal Part Quality: The final quality of parts made using hot runner molds is better due to the excellent filling profiles and injection pressure.
* Molding Automation: Automation is possible as there is no need for post-processing procedures such as trimming, waste recycling, and runner removal.
* Design Flexibility: There is flexibility in the injection mold design using hot tip gates, valve gates, or edge gates in the placement of the gate. This leads to increased part aesthetics and optimal filling of the mold cavities during injection.
Advantages of a Cold Runner System
The cold runner system has the following advantages:
* Lower Tooling Cost: Cold runner mold systems have a lower investment cost in injection mold tooling. As a result of the lower tooling cost, the molding is cost-effective.
* Lower Inspection and Maintenance Costs: Cold runner molds are not complex. As a result, there is a low need for inspection and maintenance, which reduces maintenance costs and downtime.
* Thermosensitive Plastics: Another difference between hot runner and cold runner molds is their application in molding thermosensitive plastics. The cold runner system is most suitable for thermosensitive polymers and any other plastic.
Hot Runner Mold vs. Cold Runner Mold - Disadvantages
Each runner mold also has its downsides.
Disadvantages of a Hot Runner System
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 like low cycle time, part quality, and flexibility.
* High Requirement for Accuracy in Tooling: The system requires high-end tooling due to the need for accuracy. Otherwise, defects can occur due to machine malfunction. An example is the failure of a plastic seal that can cause molten plastic to spill and damage the hot runner assembly.
Disadvantages of a Cold Runner System
Cold runner injection molding has the following disadvantages:
* Material Waste: Material waste is another difference in the hot runner vs. cold runner molding comparison. Molten plastic solidifies in the cold runner system. Consequently, these lead to scraps that require trimming.
* Injection Pressure Drop: During the plastic's flow, heat transfers from the plastic to the mold plates, leading to a drop in pressure and the creation of defects such as sink marks and underfilled parts.
* High Cycle Time: The cycle time is slow in cold injection molding due to the low flow efficiency of the plastic. Also, the removal of the runner and sprue (especially in the two mold system) also increases the cycle time.
Hot Runner Mold vs. Cold Runner Mold - Selection Considerations
The choice of either should only occur after a careful understanding of how they work. Below are some factors to consider when choosing from any of the molding systems.
Considerations When Selecting Hot Runner Molds
The selection of 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 some you should pay attention to:
* Injection Pressure: Due to the constant temperature of the molten plastic as it passes through the manifold system, there is a lower drop in injection pressure. Consequently, it is crucial 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. Internally heated systems, on the other hand, are suitable for other plastic polymers.
* Gate Type: Each material and product design requires a unique gate design. Therefore, consider factors such as the gate mark, location, and injection of the material type when making the gate.
* Standard or Custom-Made System: Choosing from a standard or custom-made system depends on your needs. Standard systems have standard components and dimensions. Consequently, they are the best option due to cost, delivery time, and maintenance. On the other hand, custom-made systems are ideal for obtaining products with unique designs.
* Type of Plastic Processing: The plastic polymer you are working with also plays a vital role in the selection. For example, glass-reinforced plastic requires molds with gate inserts with good wear resistance or external heating system runners.
* Runner Size: The size of the runner plays a vital role in the system's performance. Getting it wrong can lead to degradation or incomplete filling of the final components.
* Multi-Zone Temperature Control: When working with a large and complex system or thermosensitive plastics, ensure you use a multi-zone temperature control system.
Considerations When Selecting Cold Runner Molds
The important factors you should consider when choosing a cold runner mold are below.
* Nozzle: Choose a nozzle that is not welded to prevent the ingress of particles into the water jacket.
* Nozzle Tips: The type of nozzle tips is dependent on the manufacturers. For example, some manufacturers securely fasten the nozzle tips to the end of the nozzle, while others use spring-loaded nozzle tips as it allows for the thermal expansion of the mold during heating.
* Runner Manifolds: The runner manifold should have a split plate design to make disassembling and cleaning easier. Other designs, such as gun-drilled runners, can cause "dead spots," making it harder to clean the mold.
* Piston Assembly: The use of gun-drilled air passageways can eliminate the need for air hoses and barb fittings to the nozzle to prevent airline damage and eliminate connection errors.
* Pneumatic Connections: A better cold runner mold should either have an external airline or an air terminal box with a quick disconnect coupler.
Conclusion
It is only possible to select between hot or cold runners after understanding the systems of the runners. Consequently, this article introduces the cold runner vs. hot runner injection molding comparison, their differences, and how to select the right one. Do you need high-quality injection-molded parts using a hot or cold runner system? Let us help you get started.
Frequently Asked Questions
* Which mold runner system is suitable for thermosensitive plastics?
The cold runner system is most suitable for working with thermosensitive plastics as no heating system is present. However, externally heated runner molds are also applicable for plastics such as PP and PE.
* Which is better, hot runner or cold runner?
The hot runner system is better due to better flow, support for large production volumes, and low generation of waste. However, for low-volume production and thermosensitive materials, the cold runner would be the better method.
* What is the function of the manifold system?
It is located in hot runner molds and is responsible for heating the mold. It is located in the stationary plate and controls the temperature of the molten plastic.
