Do you ever hear a faint hissing sound approaching a plastic injection molding machine? If so, the machine is likely fitted with a hot runner system.
Often, we take for granted the incredible complexity and advanced engineering that goes into making the objects we use in our everyday lives. Hot runner technology is one example of a vital but often unseen component in plastic injection moulding.
We also hear some myths that hot runner technology is expensive and only used in high-end applications. The reality is that hot runner systems can be found in all sorts of plastic injection moulding applications, from the smallest medical devices to the largest automotive parts.
Let’s explore applications of hot runner technology in plastic injection moulds and dodge the myths about their use.
What is a hot runner technology in plastic injection moulds?
A hot runner system is simply a means of keeping the plastic melt within the injection nozzle heated. Doing this eliminates the need to heat and cool the mould cavity continually- an energy-intensive and time-consuming process.
This results in increased production efficiency as well as improved product quality. In short, a hot runner system ensures that the plastic is kept at a constant temperature from when it leaves the barrel until it solidifies in the mold cavity.
How does it work?
A hot runner system consists of two main parts: a manifold and a series of gate nozzles ( also called tips) connected to the manifold. The manifold is a temperature-controlled block of metal that evenly distributes the molten plastic to each gate nozzle. The gate nozzles are also temperature controlled and have tiny orifices through which the plastic is injected into the mold cavity.
The hot runner system is powered by either electricity or oil and uses heaters to maintain the plastic melt at a consistent temperature. Thermocouples are used to monitor and control the temperature of both the manifold and the gate nozzles.
If you still have questions about how hot runner systems work, Read our blog, Hot Runner System In Injection Molding.
Applications of hot runner technology in plastic injection moulds
Many people think that hot runner systems are only used in high-end applications. But certainly, it is not valid. If you ever use any of the following objects, chances are they were made with the help of hot runner technology:
- Dental floss containers
- Medical syringes
- Eyeglass frames
- Plastic hangers
- PET preform
- Bottle cap
- Remote controls
These are not high-end applications. But these are examples of the products we use daily, and they were made with the help of hot runner technology only. With the help of hot runner technology, we can make good-quality products in less time. And not just these. There are lots of other applications too in which hot runner technology is used, such as:
1. Heat conduction and cooling
Every product has its own cooling time. And it is essential to maintain the cooling time because if the cooling time is less than, the product will be of bad quality, and if the cooling time is more than required, then it will increase the production time and cost too. But with the help of hot runner technology, we can easily control the cooling time.
The design concept of the hot runner’s system is to have a heated manifold with individually controlled nozzles that inject molten plastic directly into the cavities of the mold.
This process differs from conventional injection molding, which features a cold runner system where material flows through a channel etched or milled into the mold.
Hot runner systems can be designed with a freedom or point gate arrangement. In a freedoms system, each nozzle is connected to the manifold by a short runner that terminates in an opening or gate at the mold surface. This arrangement offers considerable design flexibility but can make it difficult to distribute the material to all cavities evenly.
The thermal conductivity of the runner is an essential factor that affects the heat transfer from the manifold to the gate. The thermal conductivity of the runner material should be as high as possible to minimize heat loss and improve temperature control.
Aluminum is the most commonly used material for hot runner runners because it has high thermal conductivity and can be easily machined. However, aluminum is unsuitable for all materials because it can cause galling and welding.
Stainless steel is another common material used for hot runners. It has a lower thermal conductivity than aluminum but is more resistant to wear and corrosion.
The molten plastic entering the cavities must be cooled quickly to prevent defects and ensure that the parts have the required properties. The hot runner system must provide adequate cooling to all mold parts.
The cooling system consists of channels that carry the coolant from a water source to the mold cavities. The coolant is typically water or a water-based solution but can also be oil or air.
The size and number of cooling channels depend on the size of the mold and the material being molded. The channels must be large enough to carry the required amount of coolant and evenly distribute throughout the mold to ensure uniform cooling.
2. Heat distribution and intensity control
When the mold adopts the hot runner to realize glue feeding, the heat required for melting and plasticizing the glue must be transferred from the nozzle to the mold cavity so that the molten state of the heated melt can be maintained in the gate area and the runner system.
This requires a good heat conduction path to be set up between the heater and various parts of the mold. The heating method of the mold can be divided into three types:
- Internal heating,
- External heating,
- Direct heating
The most common method is indirect heating, which uses a heat exchanger to transfer heat from the heater to the mold. The heat exchanger can be a metal plate, a spiral tube, or a tubular heat exchanger. The spiral is often used for smooth flow channels and is easy to clean.
A tubular heat exchanger is mainly used in a hot runner system. Its structure is simple and compact, and the heat exchange efficiency is high. The advantage of this type of heat exchanger is that it can be installed in the mold to save space.
The direct heating method uses a heater to heat the mold directly. This method is primarily used in small molds or when space is limited.
The heater can be a resistance heater, an inductance heater, or a dielectric heater. The most commonly used heater is a resistance heater, which is simple in structure and easy to control.
Induction heating is suitable for high-power applications. The dielectric heater is ideal for applications where space is limited.
For intensity control, a temperature controller is required to feedback on the actual temperature of the heater and compare it with the set value. You don’t need to worry about the temperature of the nozzle because the set value indirectly controls the temperature of the nozzle.
The typical control method is ON/OFF control, phase-shifting control, proportional integral derivative (PID) control, and PID with fuzzy logic control.
3. Reducing the cycle time
The most extensive application of the hot runner technology is in reducing the cycle time. The conventional cold runner system comprises a mold with runners, gates, and sprues.
In this kind of system, after every shot, the plastic that remains solidified in the runners and sprues needs to be removed manually before the next shot can be initiated. This step lengthens the cycle time, and causes wear and tear to the mold.
On the other hand, the hot runner technology does not have runners or sprues. The molten plastic is directly injected into the cavities through the gates. Since there are no solidified plastics in the runner system, the need for manual removal is eliminated, thereby reducing the cycle time.
To further reduce the cycle time, some hot runner systems have an inbuilt heater to keep the molten plastic at a constant temperature. This ensures no time is lost waiting for the plastic to melt.
4. Improved quality of parts
Since the molten plastic is directly injected into the cavities, the possibility of contamination is eliminated. Contamination often occurs in cold runner systems when the solidified plastic from the runners and sprues gets mixed with the fresh molten plastic. This contaminated plastic is then injected into the cavities, which results in defective parts.
The hot runner technology also eliminates the sink marks often seen on the surface of the parts produced by the cold runner system. This is because the molten plastic is injected under high pressure into the cavities, which fills them.
With that said, it is worth noting that, in some cases, the surface finish of the parts produced by the hot runner system can be inferior to that of the parts produced by the cold runner system.
5. Accurate adjustment of melt flow
Do you ever notice how some molded parts have uneven walls? This is often the result of an inaccurate adjustment of the melt flow. When the melt flow is not adjusted correctly, the plastic does not fill the cavities evenly, which results in parts with uneven walls.
The hot runner system makes it possible to adjust the melt flow, thereby eliminating this problem accurately. By accurately adjusting the melt flow, you can achieve a more consistent weight for the parts. Here a question arises: how is it possible? The answer is the hot runner system allows for more precise control of the molten plastic.
This is because the molten plastic is injected directly into the cavities through the gates without passing through the runners and sprues. As a result, there is less flow resistance, making it possible to adjust the melt flow accurately.
6. Reduced cost per part
Forget the old saying, “you have to spend money to make money.” With the hot runner system, you can save money while improving the quality of your parts.
How? The hot runner system reduces the cycle time so that you can produce more parts in less time. In addition, the hot runner system also eliminates the need for manual removal of the runners and sprues. This reduces the labor cost and the wear and tear of the mold. The increase in productivity and the reduction in labor costs decrease the cost per part.
Hot runner vs. Cold runner
Injection moulding is a manufacturing process for producing parts by injecting molten material into a mould. Injection moulding can be performed with a host of materials, including metals, glasses, elastomers, confections, and, most commonly, thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed, and forced into a mould cavity, where it cools and hardens to the configuration of the cavity.
The main difference between a hot runner and cold runner injection molding is that hot runners have heating elements in the runner system while cold runners do not. Hot runners are more expensive than cold runners but offer many benefits.
Whereas cold runners require the entire runner system to be cooled so that the plastic can solidify before being ejected from the mould, hot runners keep the plastic in a molten state until it is injected into the cavity.
Many industrialists prefer hot runner technology over cold runner because it is more efficient and can save money in the long run. Hot runners also offer superior shot-to-shot consistency, which is why they are often used for mass production. In addition, hot runners generally have a longer lifespan than cold runners.
Every product that is made of plastic is first made in a mould. Making the mould and the plastic product both require hot runner technology.
Hot runner technology aid the manufacturing process by providing uniform and consistent heating to the mould so that the plastic can be easily injected into it and cooled down quickly to harden in the desired shape.
We hope the above guide will help you learn about hot runner technology and its applications in plastic injection moulds.
Go4 mould is the only leading manufacturer of hot runner systems that provides world-class products and services. We have the most innovative and advanced technology to offer you the best possible solutions for your needs. Contact us today for all your hot runner requirements!
You can also read our blog Plastic China Injection Molding Services for more information about our services.
Frequently asked question
How many types of hot runner technology are there?
There are two types of hot runners, internally heated and externally heated. An internally heated system is recommended if your application requires high precision. Externally heated systems are recommended for lower temperatures and when speed is essential.
Which one is a better hot runner or cold runner?
Both options have their pros and cons. It entirely depends on your application as to which one is better. A cold runner system is cheaper, but a hot runner system provides more precise control over the flow of molten plastic.