Hot Runner Mold

What is a hot runner mold?

A hot runner mold is a mold that uses a heating device to keep the melt in the runner from solidifying. Because it is shorter than the traditional mold forming cycle, and more economical raw materials, so the hot runner mold in today’s world industrial developed countries and regions are very widely used.

In the world’s more developed industrial countries and regions hot runner mold production is extremely active. The proportion of hot runner mold is increasing. Many small injection mold factories with less than 10 employees are engaged in the production of hot runner molds.

Hot runner mold structure

A hot runner mold consists of two plates that are heated by means of a manifold system. Both internally heated and externally heated hot runners are available. In hot runner systems, the mold is ready to run when the system reaches operating temperature. 

The hot runner system generally consists of a hot nozzle, manifold, temperature control box and accessories, and other parts. There are two types of hot nozzles, open nozzle and needle valve nozzle.

Because the nozzle form directly determines the hot runner system selection and mold manufacturing, and therefore often corresponding hot runner system into open hot runner system and needle valve type hot runner system.

The manifold is used in one mold with multiple cavities or multi-point feed, single-point feed but with offset material level. The material is usually P20 or H13.

The manifold is generally divided into two categories, standard and non-standard. Its structure form is mainly determined by the distribution of cavities on the injection mold, nozzle arrangement, and gate position.

The temperature control box includes the main unit, cables, connectors, and wiring male and female sockets, etc. Hot runner accessories usually include heaters and thermocouples, runner seals, connectors and junction boxes, etc.

Although there are many hot runner manufacturers and many hot runner product lines in the world, a typical hot runner system consists of the following components.



Temperature controller

Auxiliary Parts

Hot runner mold classification

Generally speaking, hot runner systems are classified as single-head hot runner systems, multi-head hot runner systems, and valve gate hot runner systems.

Single-head hot runner system mainly consists of a single nozzle, nozzle head, nozzle connection plate, temperature control system, etc.

Single-head hot runner system plastic injection mold structure is simple. The molten plastic is injected from the injection molding machine into the nozzle connection plate, and after reaching the nozzle head through the nozzle, it is injected into the cavity.

It is necessary to control the dimensions d, D, L and the axial displacement of the nozzle by adjusting the thickness dimension of the nozzle connecting plate so that the fixed mold fixing plate presses the end face of the nozzle connecting plate, or directly use the injection molding machine nozzle to hold the end face of the nozzle connecting plate, which can also achieve the same purpose.

A lead slot is set at a suitable position on the fixed mold fixing plate to allow the power cable to be led from inside the mold to connect with the terminal block installed on the mold.

Multi-head hot runner system plastic injection molds are more complex in structure. The molten plastic is injected from the injection molding machine into the nozzle connection plate, flows through the hot runner plate to the nozzle, and then reaches the nozzle head and is injected into the cavity.

The nozzle of the hot runner system has radial dimension D fit requirement and axial dimension limit requirement with the fixed mold plate. The nozzle head has radially sized with a fixed mold insert to ensure that the molten plastic does not overflow into the non-cavity parts, and the hardness of the fixed mold insert is hardened by about 50HRC.

The distance L between the parting surface and the axial positioning surface of the hot nozzle must be strictly controlled, and the size should be determined according to the actual distance L′ of the nozzle at room temperature plus the actual extension ΔL of the nozzle at the normal working temperature of the mold.

To ensure that the nozzle and the hot runner plate fit reliably and do not deform the hot runner plate, an adjustment pad is provided above the top of the nozzle, which together with the axial positioning surface of the nozzle itself limits the movement of the nozzle in the axial direction and effectively controls the possible deformation of the hot runner plate.

At room temperature, the adjustment pad is controlled with a 0.025mm gap between the hot runner plate and the fixed plate of the injection mold so that the adjustment pad is properly pressed at working temperature after the injection mold is heated.

The hot runner system has a positioning seat and a positioning pin that together control the position of the hot runner plate in the mold. The positioning seat has a radial dimension D2 with the fixed template, and the depth H must be controlled accurately. The axial direction of the positioning seat plays the role of supporting the hot runner plate, and directly bears the injection pressure of the injection molding machine.

The positioning pin and the hot runner plate fixed plate have matching requirements. There must be enough clearance between the hot runner plate and the template to wrap the insulation material.

The hot runner plate and fixing plate must be provided with sufficient wiring slots to allow the power cable to be led from inside the mold to connect with the terminal block mounted on the injection mold.

There is a radial dimension D1 fit requirement between the nozzle connection plate and the fixed plate of the fixed mold so that the injection head of the injection machine fits well with the nozzle connection plate on the mold.

Near the hot runner plate, the fixing plate, hot runner plate fixing plate, and mold fixing plate are connected with screws to enhance the rigidity of the hot runner plate.

Valve gate hot runner system plastic injection mold structure is the most complicated. It has the same structure as the common multi-head hot runner system plastic injection mold, but there is an additional set of valve pin transmission devices to control the opening and closing movement of the valve pin.

This transmission device is equivalent to a hydraulic cylinder, using the hydraulic device of the injection machine connected to the mold, forming a hydraulic circuit to achieve the opening and closing motion of the valve needle, controlling the molten plastic into the cavity.

Hot runner mold advantages

Shortened cycle time

The cycle time of molding is shortened because there is no limitation of the cooling time of the sprue system, and the part can be ejected in time after injection molding and curing. Many thin-walled parts produced with hot runner molds can be molded in less than 5 seconds.

Plastic savings

In a pure hot runner mold, there is no production cost because there is no cold sprue. This is especially significant for applications where plastic is expensive. The major international hot runner manufacturers have grown rapidly in an era when oil and plastic materials were expensive in the world. This is because hot runner technology is an effective way to reduce material costs.

Scrap reduction

Reduce scrap and improve product quality. During the hot runner molding process, the plastic melt temperature is accurately controlled in the runner system. The plastic can flow more uniformly into each cavity, resulting in consistent quality parts.

Hot runner molded parts have good gate quality, low residual stress after release, and low part distortion. So many high-quality products in the market are produced by hot runner molds. For example, many of the plastic parts in familiar HP printers and DELL laptops are made with hot runner molds.

Production automation

Eliminate the subsequent process, facilitate the automation of production. The finished product is formed by the hot runner mold, so there is no need to trim the gate and recycle the cold sprue. It is conducive to production automation. Many foreign product manufacturers are combining hot runners with automation to greatly improve production efficiency. 

Many advanced plastic molding processes are developed based on hot runner technology. Such as PET preform production, multi-color co-injection in the mold, multi-material co-injection process, STACK MOLD, etc.

Hot runner mold disadvantages

Although hot runner molds have many significant advantages over cold runner molds, mold users also need to understand the disadvantages of hot runner molds. To summarize, there are the following points.

Increased costs

Hot runner components are more expensive and the cost of hot runner tooling can increase significantly. If part production is small, the high percentage of injection mold tooling costs is not economically viable. For many injection mold users in developing countries, the high price of hot runner systems is one of the main issues affecting the widespread use of hot runner molds.

High equipment requirements

Production process equipment requirements are high, hot runner mold needs precision processing machinery for guarantee. Hot runner system and mold integration and cooperation requirements are extremely strict, otherwise, the mold in the production process will have many serious problems. 

Such as poor plastic sealing leads to plastic overflow and damage to the hot runner components interrupting the production, and the nozzle inserts are not well-positioned relative to the gate leading to a serious decline in product quality.

Complex operation and maintenance

Compared with cold runner molds, hot runner molds are complicated to operate and maintain. If the use of improper operation is very easy to damage the hot runner parts so that production can not be carried out, resulting in huge economic losses. New users of hot runner molds take a long time to accumulate experience in using them.

Hot runner mold design procedure

First, according to the structure of the plastic molded parts and use requirements, determine the location of the inlet. The inlet of the hot runner system can be placed anywhere on the plastic molded part as long as the plastic part structure allows it and the nozzle and nozzle head do not interfere with the injection molding structure within the fixed mold insert.

The location of the inlet for conventional injection molding is usually chosen empirically. For large and complex shaped plastic parts, the injection molding inlet position can be determined by using computer-aided analysis (CAE) to simulate the flow of molten plastic in the cavity and analyze the cooling effect of each part of the mold to determine the ideal inlet position.

Second, determine the nozzle head form of the hot runner system. The material of the plastic molded part and the characteristics of the product are the key factors to choose the nozzle head form, the production lot of plastic parts, and the manufacturing cost of the mold is also an important factor to choose the nozzle head form.

Third, the number of cavities per mold is determined according to the production batch of plastic molded parts and the tonnage size of the injection equipment.

Fourth, the number of nozzles is determined by the determined location of the inlet and the number of cavities per mold.

If a product is molded, choose an injection mold with one feeding port, then only one nozzle, that is, the choice of the single-head hot runner system.

If the injection molding of a product, choose an injection mold of multiple cavities or an injection mold of a cavity with more than two inlets, then we need more than one nozzle, that is, the choice of multi-head hot runner system, but there is a cross-runner mold structure except.

Fifth, according to the weight of the plastic molded parts and the number of nozzles, to determine the size of the nozzle radial size. The same form of the nozzle has more than one size series, respectively, to meet the molding requirements of the different weight ranges of plastic parts.

Sixth, according to the structure of the plastic part to determine the injection mold structure size, and then according to the thickness of the fixed mold insert and fixed template size to select the nozzle standard-length series size, and finally trim the thickness of the fixed template size and other dimensions related to the hot runner system.

Seventh, determine the shape of the hot runner fixing plate according to the shape of the hot runner plate, arrange the power line lead slot on its plate, and design a sufficient cooling water loop near the hot runner plate, nozzle, and nozzle head.

Eighth, complete the design drawing of the plastic mold of the hot runner system. 

Application range of Hot runner molds

Material types

Hot runner molds have been successfully used to process various plastic materials. Such as PP, PE, PS, ABS, PBT, PA, PSU, PC, POM, LCP, PVC, PET, PMMA, PEI, ABS/PC, etc. Any plastic material that can be processed by cold runner molds can be processed by hot runner molds.

Size and Weight

The smallest parts made with hot runner molds are above 0.1 grams. The largest is under 30 kg. The applications are extremely wide and flexible.

Industrial field

Hot runner molds are widely used in various industrial sectors such as electronics, automotive, medical, daily necessities, toys, packaging, construction, and office equipment.

Compared with ordinary runner molds, hot runner molds have the advantages of high injection efficiency, good quality of molded plastic parts and saving raw materials, etc. With the development of the polymer industry, hot runner technology is constantly developing and improving, and its application is becoming more and more extensive.

A hot runner is a method of heating to ensure that the plastic in the runner and gate remains in a molten state. 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 at a high temperature, so that the plastic in the runner stays molten, and there is no need to open the runner to take out the condensed material after shutdown.


Saving raw materials and reducing the cost of products are the most significant features of hot runner molds. In a normal pouring system, a large amount of stalk is produced, and in the case of small products, the weight of condensate in the pouring system may exceed the weight of the product.

Since the plastic is always molten in the hot runner mold, the product does not need to trim the gate and is basically a scrap-free process, thus saving a lot of raw materials. Since there is no need to recycle, select, crush and dye the scrap, it saves labor, time, and energy.

Since the injection material is no longer mixed with the repeatedly processed gate material, the quality of the product can be significantly improved. At the same time, since the plastic remains molten in the pouring system, the pressure loss during flow is small, so it is easy to realize multi-gates, multi-cavity molds, and low-pressure injection of large products.

Hot gates facilitate pressure transfer, and to a certain extent can overcome the defects such as depression, shrinkage, and deformation of plastic parts due to insufficient material replenishment.

Wide range of applicable resins and easy-to-set molding conditions. Due to the improvement and development of hot runner temperature control system technology, the hot runner can be used not only for polyethylene and polypropylene with wide melting temperature but also for heat-sensitive plastics with narrow temperature range, such as polyvinyl chloride and polyformaldehyde (POM). For polyamide (PA), which is prone to salivation, hot runner molding can also be achieved by selecting a valve-type hot nozzle.

In addition, simplifying operation and reducing injection molding cycle time is also an important feature of hot runner molds. Compared with the ordinary runner, shorten the opening and closing stroke, not only the mold release and molding cycle of the parts shortened, but also conducive to the realization of automated production. According to statistics, compared with ordinary runners, the molding cycle time can be shortened by 30% after changing to hot runners.

Development trend

The hot runner system has some defects, such as complex mold structure, heater components are easy to damage, high manufacturing costs, the need for more sophisticated temperature control devices, molding resin must be clean and free of debris, resin replacement and color change more difficult, more complex maintenance, etc., but these defects are gradually being overcome. At present, the main development trend of domestic and foreign hot runner mold can be summarized as the following aspects.

The miniaturization of components, to achieve small products of a mold multi-cavity and large products multi-gate filling mold. By reducing the nozzle space, more cavities can be configured on the mold to improve the output of products and the utilization rate of injection machines.

The overall reliability of the hot runner mold design is improved. Nowadays, major mold companies at home and abroad pay great attention to the research and development of the design of hot runner plate and the pressure distribution, temperature distribution, and sealing of the connected part of the hot nozzle. The development and utilization of stacked hot runner injection mold is also a hot spot.

Stacked molds can effectively increase the number of cavities, while the requirement for injection machine clamping force only needs to increase by 10%~15%. Stacked hot runner molds have been used for industrialization in some foreign developed countries.

The purpose of improving hot-runner component materials is to improve the wear resistance of nozzles and hot runners and for sensitive material molding. Such as the use of molybdenum and titanium and other tough alloy materials to manufacture nozzles to metal powder injection molding by sintering made of hot runner elements has become possible.

Development of accurate temperature control system. In hot runner mold molding, developing more precise temperature control devices to control the temperature of the molten resin in the hot runner plate and gate is an effective measure to prevent resin overheating degradation and product performance degradation.