There are many different types of plastic injection molds, each with its advantages and disadvantages. This blog post will discuss the differences between hot runner molds and cold runner molds. Both types of molds have their pros and cons, so it’s important to know which one is right for your project. Let’s get started!

The cold runner is the basic form of injection mold, the so-called cold runner is the conventional injection mold, after the end of the plastic injection molding manufacturing process the runner has runner material that needs to be removed.

With the reduction of resources, competition in the market, the price of raw materials has increased significantly, which is very necessary to save material on this issue, which is also an important reason for the emergence of hot runner, can be said to be the inevitable product of social industrialization.

The emergence of the hot runner to solve the problem of waste of resources, cost savings, while in the manufacturing injection molding process has also achieved relatively large results. The hot runner has gradually developed into the general direction of the development of injection molding, and will certainly be popular!

Due to the cost accounting problem, many small and medium-sized molds have not yet been applied to hot runners, and there are a few parts of the plastic material that must be applied to cold runners injection mold.

So now in the injection molding industry, there are cold runners and hot runners.

What is a cold runner?

Cold runner: It is the part between the mold inlet and the product gate. The plastic is kept flowing in the runner by the injection pressure and its heat. The runner is used as part of the molded material but is not part of the product. (The cold runner part is the injection residue.)

Cold Runner Injection Molds In a cold runner system, the runners and the molds are maintained at the same temperature. Within this system, two or three plates are contained within the mold.

As opposed to hot runner injection molding where the sprue system is cooled in the mold and removed with the product, removing the cold material from the sprue and trimming the sprue day can result in a final product with no visible traces of the incoming sprue.

Advantages of cold runner molds

(1) Cold runner systems have the advantage of being easy to use and cold runner systems also meet certain aesthetic needs very well.

(2) Cold runner systems can reduce the part of acrylic or polycarbonate used for light transmission in the injection channel, avoiding the visible band effect in some parts of the injection.

(3) Simple cold runner molds design, widely used.

The disadvantages of cold runner mold

More waste of raw materials, the resulting cold material needs to be crushed again for recycling. It makes additional steps in the whole production process. Not suitable for the multi-cavity system.

What is a hot runner?

As a common part of the injection molding service system, it is heated to keep the plastic in the runner and gate in a molten state. When choosing hot runner systems, consideration should also be made to the thermal sensitivity of the polymer.

Since there are heating rods and heating rings near or in the center of the runner, the entire runner from the nozzle outlet to the gate of the injection molding machine is in a high-temperature state, which keeps the plastic in the runner molten.

Therefore, the hot runner process is sometimes referred to as a hot collector system, or as runners molding.

Many conditions are taken into consideration when selecting a hot runner, such as different plastic properties, shape, size, thickness, the weight of the product, mold cavity arrangement, and gate location.

There are several different shapes and sizes of hot nozzles and runner plates available in the market today to suit various products.

The advantages of hot runner mold

(1) Save raw materials and reduce cost.

(2) Shorten molding cycle and improve machine efficiency

(3) Improve the surface quality and mechanical properties of products.

(4) It is not necessary to use a three-plate type mold that can use a pointed gate.

(5) Economically mold single products with side gates.

(6) Improve the degree of automation.

(7) The gate sealing can be controlled by a needle valve type gate.

(8) The quality of injection molded parts of multi-cavity molds is consistent.

(9) Improve the surface aesthetics of injection molded products.

(10) Can use smaller injection pressure, which can effectively reduce the post-deformation of thin-walled products.

Disadvantages of hot runner molds

However, every technology has its disadvantages, and hot runner technology is no exception:

The mold structure is complex, the cost is high, and the maintenance cost is high. It takes a period for the process to stabilize, resulting in more scrap at the beginning.

In case of melt leakage and heating element failure, it will have more impact on product quality and production schedule. The third disadvantage above can be reduced by purchasing top-quality heating elements, hot runner plates, and nozzles and by careful maintenance when using them.

Hot runner vs. cold runner material cost savings

The hot runner has no sprue and gate waste loss, no grinding and recycling after molding, which can save sprue waste and grind and recycling cost, especially when the weight of the sprue system is larger than the molded product.

Molding without a sprue system saves molding energy.

The scrap generated by the cold runner and sprue is very large, especially the larger the size of the part, the more scrap there is due to the long run. Some of these scraps can be reused, some cannot.

Even if they can be reused, the overall performance of the material is greatly reduced, and it is necessary to use material crushing equipment.

Hot runner vs. cold runner improves production efficiency

The molding cycle of an injection molded part = injection time + holding time + cooling time + ejection time + trim time.

The longest of these is the cooling time. In plastic molds manufacture, the thicker the wall of the product the longer the cooling time. Since the cold runner needs to feed multiple mold cavities or more pouring points at the same time, the cooling time is usually longer.

Therefore, the wall thickness of the product in the cold runner is usually greater than the thickness of the injection molded product itself.

Because there is a cooling time difference between the melt in the cold runner and the injection molded product, the hot runner eliminates the need for a cold runner.

The hot runner eliminates the cold runner and the cooling time will be shortened. The difference in injection time is also an aspect of using a hot runner instead of a cold runner.

The primary difference in injection time is attributed to the additional time required to fill the cold runner. This is because the cold runner mold increases the opening/closing stroke of the injection molding machine.

This increase in stroke is to ensure the safe ejection of the cold runner. Hot runner injection molding is more suitable for automatic product removal.

Without the interference of the cold runner on the product ejection, the secondary manual operation of the injection molding process.

For example, the time for product separation from the runner, product trimming, and packaging can be greatly reduced or eliminated. Thus, the capacity per unit time is improved, i.e., the productivity is increased.

Hot runner vs. cold runner improves part quality

The use of hot runners can effectively improve the surface quality and mechanical properties of products, greatly improve the warpage of thin-walled parts and ensure the consistent quality of injection molded parts with multiple cavities.

It can ensure the consistent quality of injection molded parts in multi-cavity molds.

In particular, the needle valve nozzle is more obvious to improve the appearance quality of injection molded products.

The main advantages are no residual marks left on the product from the gate. The larger diameter gate can be used to speed up the cavity filling and further reduce the injection pressure and deformation of the product.

It can prevent the phenomenon of drawing and salivation when the mold is opened, and prevent the back-absorption of material from the mold cavity when the injection molding machine screw is backed up.

Summary

Through this article, I believe you have a certain understanding of hot runner injection molds and cold runner injection molds, in the actual injection molds, before making inejction molds must consider its cost-effectiveness. The cost of the mold and the cost of the product should be considered according to many aspects.

If the number of products is large, hot runner molds are more suitable, although the initial mold input costs may be higher, the price of the product will be lower, but the hot runner is not suitable for the frequent need to change the color of the product.