Preliminary Study on The Application of Hot Runner Injection Molding

The Application of Hot Runner Injection Molding

Introduction: When discussing whether to use cold runner or hot runner injection molding, two factors are mainly considered, one is the factor of economic cost, and the other is the factor of technical requirements. The following briefly discusses the economic cost considerations, then discusses the technical considerations, and finally introduces the typical application fields of hot runner technology.

1. Economic consideration when choosing the type of cold runner or hot runner injection molding

The economic superiority of hot runner molds is mainly achieved by reducing and eliminating production waste and realizing the automation of injection molding production. Generally speaking, the production design and manufacturing cycle of hot runner molds is longer than that of cold runner molds and involves more links. Hence, in terms of mold cost, hot runner molds are much more expensive.

Suppose the output requirements of plastic products are very large (such as output requirements on millions of pieces), and the productivity requirements are high. In that case, the application of hot runners is very advantageous. Generally speaking, the most important factor affecting the injection molding cycle (CYCLE TIME) is the cooling and solidification time (COOLING TIME) of plastic products.

On the cold runner mold, because the cross-sectional size of the runner system is often larger than the wall thickness of the plastic product, the cooling time is longer. This often results in longer overall injection molding cycle times. On the contrary, because no cold runner requires a long cooling time on the hot runner mold, the injection molding cycle can be significantly reduced. In addition, forming long lengths with cold runner molds requires secondary processing operations such as trimming gates, recycling runner system waste, etc.

The application of hot runner molds can avoid secondary processing operations and other problems and realize the automation of injection molding production. For projects where plastic raw materials are expensive, product output requirements are large, and recycled materials are not allowed to be processed, hot runner molds should be the preferred mold category.

When the output of plastic products is small, the proportion of mold cost in the price of plastic products is relatively high. At this time, it is more cost-effective to choose cold runner molds, the delivery time of molds is short, and the use and maintenance are relatively simple. Because hot runner components are expensive, and the production design and manufacturing cycle of hot runner molds is much longer than that of cold runner molds, it is not suitable to use hot runner molds for short, flat, and fast injection molding projects in terms of economic costs. The channel mold is processed and shaped, and the cold runner mold is considered.

Many mold companies also combine cold and hot runner molds. For example, before making an expensive hot runner mold, first make a more economical cold runner mold to produce small batches of plastic products, research and demonstrate production plans, and test the use characteristics of plastic products. After gaining experience, purchase a hot runner system to convert the original cold runner mold into a hot runner mold.

The initial investment and cost are relatively large for companies that are just starting to learn to use hot runner molds. In addition to purchasing the hot runner system itself, it is also necessary to purchase a temperature controller.

Because the hot runner mold consumes a lot of electricity, the electricity bill will increase significantly. This is an important economic cost factor for regions with tight power resources. The user’s mastery of the hot runner technology is very important, and adequate personnel training is also an expense.

Compared with cold runner molds, hot runner molds are more prone to various production failures. The use and maintenance of hot runner molds are more complicated. Hot runner components work under high temperatures and high dynamic load conditions, and many factors lead to their failure. Many hot runner components are also wearable parts and need to be replaced regularly.

Therefore, hot runner users often need to purchase spare components in addition to purchasing normal hot runner systems. This will increase the cost of using additional hot runner molds. The cold runner mold has much less chance of failure. For many faults of hot runner molds, the hot runner supplier often needs to send technical service personnel to help them solve them smoothly. And these technical services are often charged services, which will increase the cost of using hot runner molds.

At the same time, the economic benefits of plastic injection molding processing are mainly guaranteed by non-stop mass production, so once there is a production stop failure, the economic loss will be great. Therefore, when determining whether to use a cold runner or a hot runner mold during the cost calculation of injection molding, it is necessary to consider whether the shutdown failure of the hot runner mold can solve this factor smoothly.

To comprehensively consider. From the above discussion, there are many factors in the economic cost of deciding whether to use hot runner molds. Generally speaking, hot runner molds should be considered for projects with large batch production requirements, expensive plastic raw materials, and companies with rich technical experience. On the other hand, for projects with small production batches, insufficient user technical experience, and general product quality requirements, it is more economical to use traditional cold runner molds.

2. Technical considerations when deciding whether to choose a hot runner injection molding

Injection molding is like any other economic activity; the economic benefit is the most important goal. But at the same time, it should be noted that the technical requirements are also very important. Because for many, especially various new injection molding processes that have emerged in recent years, it is technically impossible to use trad cold runner molds. Although a hot runner mold costs relatively high, it is technically the only choice. At the same time, hot runner technology has also raised the traditional injection molding process to a new level. After applying hot runner technology, the mold design is more flexible and diverse. Design solutions that could not be achieved with the original cold runner can now be realized. Some typical applications of hot runner technology will be introduced in more detail below.

3. Typical applications where hot runner injection molding technology must be used

The application range of hot runner technology is very rich and extensive. Hot runner technology is especially required in the applications discussed below. Traditional cold runner molds cannot realize these injection molding processes.

3.1 Plastic injection molding products with strict quality requirements, such as medical plastic parts

injection molding gate-design

For many medical plastic injection molding products, one of the most basic requirements is that no gate marks (GATE VESTAGE) are allowed. Because gate marks may scratch the gloves and skin of medical staff, causing disease transmission and so on. The solution is to use the valve gate (VALVE GATE) molding method in hot runner technology to remove gate marks completely. In recent years, the plastic product processing industry has not been booming, but the processing of medical plastic injection molding products is unique. And most of them are molded by hot runner valve gate mold. Companies preparing high-quality medical plastic products should consider developing hot runner molding technology.

3.2 Multiple plastics, multiple colors co-injection process (CO-INJECTION)

Traditional injection molding involves molding one plastic or color at a time on a set of molds. At this time, cold runner or hot runner molds can be used. However, hot runner technology must be used to achieve the co-injection of multiple plastics or colours. Currently, many parts are processed by co-injection of various plastics and colors for hot runners. For example, advanced long-term fresh-keeping beverage and beer bottles are made by co-injection processing multiple plastics through hot runners with two kinds of plastics in a sandwich structure. A small amount of oxygen-barrier fresh-keeping plastic is sandwiched in a cheaper outer plastic.

In automobile manufacturing, integral front and rear headlights composed of multiple colors (such as red, orange, and transparent) are currently very popular, requiring multiple hot runner systems to be installed on the same mold. Use one hot runner system to inject red, another hot runner to inject orange, and the next hot runner to inject transparent color, etc. It cannot be realized with a traditional cold runner mold.

3.3 Gate direction and mold opening direction form a certain or perpendicular angle

To avoid opening the gate on the front of the part to affect the appearance and use, it is often necessary to place the gate on the side of the part. At this time, if the gate direction is consistent with the mold opening direction, the strength of the mold at the gate will be weak. It is better to gate at an angle perpendicular to the side of the part (see Figure 1). For example, some large parts with complex shapes on automobiles often need to make the gate direction and the mold opening direction form a certain angle. This can only be done with a hot runner. Because of the mold opening action, pulling out the long cold runner and gate from the angled direction is difficult.

3.4 The gate is opened inside the deep part

Some industrial products have extremely strict requirements on appearance, and no sprue traces are allowed to be exposed during use, such as small parts such as cosmetic lipstick pen caps and large automotive interior parts. At this time, the gate needs to be opened inside the part. When the depth of the part is too large, it is impossible to use the cold runner gate, but only use the hot runner nozzle to protrude into the inside for injection molding.

3.5 Multi-layer mold (STACKMOLD)

Multi-layer mold (STACKMOLD)

A way to increase production without increasing the tonnage of injection molding machines is to use multi-layer molds. That is, there are multiple parting surfaces on the mold. The hot runner system completely realizes this kind of mold.

3.6 Production of preformed preforms (such as PET preforms)

The beverage bottles widely used by people daily are produced by injection molding method to produce PET performed preforms and then made into industrial beverage bottles by blow molding method. The production of PET preforms is, without exception, completely realized by hot runner technology. The world’s largest PET preform mold can produce 144 preforms at a time.

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