Hot Runners in Injection Mold


An overview

The process of infusing molten material into the molds for making different parts of objects is called injection molding. As the name shows, the molten material infused into the patterns takes the shape of the mold. A lot of materials are used in injection molding process like metals, glass and plastic.

The molds are made up of metal like steel or aluminum. Designers of industry or engineers carefully design the mold shapes to form the desired parts of different machines, cars and other vehicles, home appliances, screws, plastic products used everywhere in our offices, homes, hospitals, bits and pieces of mechanical parts, bottles caps, hair combs, buttons and almost everything we use in our daily life.

Injection molding not only provides small molds but it is also used in making huge cavities like car panels for entire body.

The injection molding machine has two main parts, the injection unit and the mold clamping unit.

The injection unit has a barrel with one side being fed from the hopper containing the plastic capsules. Barrel has a screw inside it to mixes the plastic and pushes forward the molten material into the mold.

The clamping unit holds the parts of the mold, keeps them aligned against the force of the molting material and opens and closes the parts at right time.

The clamping unit can hold several molds. In the mold design, there are some vital parts which together make the injection process possible. Among those parts one is the runner, the channel through which the molten plastic runs to the specialized mold plates to make the desired shape, the reason this part is called runner. The gate restricts the flow of plastic into the mold tool cavity (the molded shapes to form the desired part).

There are two types of runners; hot runners and cold runners.

In cold runners the channel through which the molten plastic enters the cavity is unheated. The hot runners are opposite to the cold ones. In hot runners, after the plastic leaves the gate, it enters the mold cavity though the parts which are physically heated.

Generally, the cold runner is simply a channel between the two halves of the mold which carries the plastic to the cavity while the hot runner is the system of heated components which carries the molten plastic to the cavity.

Famous brands of hot runners

There are many manufacturers in the arena of hot runner systems manufacturing and supplying all over the world. Some are given below.


The Mold-Masters brand supplies hot runner technology and systems. It is a member of the Milacron family of brands and was established in 1963.

It designs, manufactures and supports a full range of hot runner products like hot runner systems, temperature controllers, hot halves, and gating technologies to produce good quality plastic parts.


Husky supplies injection molding equipments for complex appliances like medical, engineering and heat sensitive products. It provides hot runner systems for the products which require nominal gate mark like pipettes and syringe drums and many other products of plastic industry like drinks bottles and caps, food containers and electronic parts.


Another name in the plastic injection molding industry is Synventive. It manufactures hot runner systems like sequential valve gating, co-injection and stack molds for their customers.


INCOE designs and assembles hot runner systems for injection molding of plastic materiels. It is the principal brand which designed the first existing commercial hot runner nozzle which paved the way for development of molding process all over the world. INCOE has a wide network of services in more than 45 countries.


YUDO is a research and development industry which manufactures and supplies and provides sales services of hot runner system in injection molding. It produces heater, thermocouple and valve nozzles for engineering plastics. It was established in 2002 and has expanded its project in over 60 countries.

Components of hot runner system

The components of hot runner system are generally heated eclectically. The channel size of the hot runner is set by keeping in view the type of the plastic, injection speed and the mold shape.

The hot runner system consists of many parts as given below.

Structure of hot runners-hot runners in injection mold

Locating Ring

It guarantees the alignment of the mold with the injecting machine.


The molten plastic enters the mold from the machine nozzle through the inlet. In order to optimize molding process, inlet can be heated depending upon the design of hot runner and the type of the plastic.


When more than one mold is to be injected from multiple gates, manifold is used. It enables the molten plastic to flow into several injecting points or gates. Manifolds are available in different shapes, designs and materials to serve the desired purpose. They are manufactured by any one of the two main techniques; the gun drilled technique favored for simpler economical systems and 2 piece brazed technique preferred when the performance criteria is tight like in multi material or multi color parts.


The components trough which the molten plastic is supplied to the mold is the nozzle.  Depending upon characteristics of the material and the number of molds to be injected, i.e. with or without manifold, the nozzles are installed on the mold plate.

Heater Technology

The key component of the hot runner system is the heater technology that furnishes the system with the heat to perform its very task. Heater technology affects the quality of the product. Careful selection of the heater technology among several available technologies depends upon the cost, performance and consistency of the part.

Hot runner systems are available in two outlines: internally warmed and externally warmed. The first is portrayed by internally heated, annulus stream sections, with the warmth being outfitted by a probe and torpedo situated in the channels. This framework exploits the shielding impact of the plastic soften to reduce heat loss to the mold. The second comprises of a cartridge-warmed manifold with inside stream sections. The manifold is planned with different insulating characteristics to isolate it from the mold, hence reducing heat loss.

The advantages of hot runner system

Over 50 years ago, the hot runner system has revolutionized the molding process in many ways. Today, hot runners system is proficient enough to produce complex parts with desired size used in every industry.

Disposal of the runner

For more modern and expensive materials, the cold runners cost more.  In many applications like medical and food, cold runners cannot be reused. So putting away them is inflated and dusty. In rapid, high-cavitation molding, the piece and regrind would be colossal issues. When utilizing hot runners, these regrind issues and their related expense is convenient.

Enhanced molding system efficiency

Hot runners system is for the most part simpler to begin in light of the fact that there is no need to for them to be evacuated after each under-filled shot. When operating temperature is reached, the hot runner is ready to use.

Ideal part quality

Hot runners give enhanced pressing conditions. It keeps up an adjusted melt stream at a consistent temperature from the machine nozzle to completely fill and pack the mold cavities.

Automation in molding process

With the increasing demand the industries are automizing their molding process. The use of hot runner systems allow flexible mechanizing with no fear of getting scrambled in automatic machinery like robots, conveyor belts because of their dimensional consistency.

Flexibility in design

A major advantage of hot runner system is the capacity to find the gate on the part. This system allows the gate to be set at the ideal area for perfect filling of the cavity. This in turn allows optimization of cavity direction and cooling.

Adjusted melt flow

In hot runner systems the feed to direct molten plastic in the channels is separate from the mold plates. This system ensures the flow of melt to each cavity with same temperature and pressure profiles and diameter whether there are 2 or 90. This outlined in temperature and pressure control is especially positive for the melts that have narrow processing edges.

Better temperature control

Whether the hot runner is internally heated or externally heated, it provides better temperature control with improved distribution of heat.

The disadvantages of hot runner system

Like every entity in the world, hot runners also have some disadvantages as well.

It is comparatively difficult to change the colors in hot runner system.

Hot runner system is more complex because the melt is to required to keep at certain temperature while the injection mold cavity is requires cooling at the same time. Being complex in turn means higher maintenance cost.

Hot runner system may not be suitable for the materials which are sensitive to heat. Moreover thermal expansion of various mold components should be considered.

Of course for best result in quality production, the professionals have to analyze the type of runner for best injection molding system.

Key technology of application for hot runner system

The product being formed is affected by the two main technologies of hot runners. Temperature and flow of the hot runner system are closely related. Any change in temperature varies the flow of the melt and flow change can resulting temperature disturbance hence making the molding process difficult.

The control of temperature

The temperature controller not only heats the hot runner system and the plastic in the barrel but also maintains the temperature of the components to the desired value. Temperature has substantial effect on flow than pressure because the increased temperature can force the gate tip further to the gate orifice caused by the thermal expansion. Hence the flow is disturbed because of the change in effective radius of the gate.

The control of plastic flow

Flow channels are closely related to the pressure of the molten plastic flow. Variations in pressure through the regions of a manifold cause dead spots in the product. These spots do not wash out well when purged and result in color change problems.

How hot runner system saves the injection cost

Hot runner system saves injection cost in many ways. As the material is always in molten form, the pressure reduces in the flow. This allows simplicity in vast production because realization of the low pressure is easy for injection in multi gate and multi cavity molds.

In the hot runner system, as the gate is hot, it results in good pressure transmission which keeps the product away from imperfect falling, cavity shrinking and product distortion due to lack of feeding.

As there is no recycling in hot runner systems, it reduces the cost of selecting and picking up, wrecking and dyeing. The quality of the product is also improved.

Injection cost is reduced also because of the faster cycle time of the hot runners.

Hot runner mold vs. three plate mold

When the product size gets bigger and needs various injections, hot runner molds are not appropriate as much as necessary.

Three plate mold serves the purpose here. It has more central points to hit upon the gate. The gate is located at the base rather than the side of the part allowing more smooth supply of the melt in the part.

As the three plate mold system has three plates with two notches. It separates runner form the part and the melt drops into the molds giving automatic working.

But alongwith the advantages of three plate systems, there are some shortcomings of this system. The complex apparatus of three plate systems makes it less stable.  It has higher scrap rate, longer run distance and delayed cycle time than hot runner systems.

Gate ways for hot nozzle

Hot runner nozzles are the basic interface between the cavity and the molten conveyance system. Gate quality, working window and how many times the color is changed are straightforwardly affected by the capacity of the nozzle.

Numerous plastics require exceptionally accurate processing temperature. Asymmetric temperature variations in a nozzle badly affect the plastic melt. If the temperature is too warm, the material might be harmed. And if the temperature falls too forcefully, the plastic solidifies and never again streams ideally into the mold cavity. For that reason, heat transfer in the hot nozzle is fundamental.

Traditional hot nozzles need a number of heater bands along their length to maintain uniform temperatures. To keep the molten plastic at the correct temperature, the nozzle is warmed from outside with the help of heating coils. In this way, there is no further warming hardware required at the nozzle tip. The warmed nozzle itself must exchange the warmth to the plastic. Thus, the thermal conductivity of the material is vital. In ordinary metals, for example, steel, this isn’t adequate. But the copper has high thermal conductivity; it can’t withstand the rough plastic.

Following are some gate ways for hot nozzles.

Valve gate hot nozzle

The valve gate Hot Runner System is used for plastic parts that require perfect finishing on the injection point and when there’s a need to sequence the injection, fundamental for technical parts.

Valve gate hot nozzles are used where the plastic parts require idealize finishing on the injection point because it results in a clean and accurate injection. In automobile and electronic industry, where it is essential for to technical parts to arrange during injection phase, valve gate hot nozzle serves the best. Valve gates also have minimal vestige.

Valve gate-hot runners in injection mold


It has even temperature profile all along nozzle length.

The base has high mechanical strength.

The geometry is such it keeps away from standing still and performs very well in color exchange.

It has better flow of injection and less shearing of the material.

Multi-tip gate nozzle

Small and micro molded plastic parts require manifold gates like medical parts with special shapes. Multi-tip nozzles are a compact solution for such parts which also require injection in constrained areas. There are clusters of 2-6 gates tips per nozzle body with minimum distance of 2-3 mm.


It has compact design and is cost effective.

Directly injects in cylindrical cavity with uniform wall thickness.

Nozzle tip is insulated from the matrix.

Open gate hot nozzle

Open gate hot nozzle system stays open constantly. With the valve gate systems, the systems can be cut off with a needle after every injection procedure.

Open systems are the standard way out for different applications. Hot runner nozzles with horizontal injection and smaller than normal hot runner frameworks are just offered as open gate systems.


As the use for the extra control of the needle drive is removed with open gate systems, they are for the most part more cost efficient than the closed one.

This nozzle just has slight nozzle vestige and extremely reasonable for no stream mark required parts like for transparent parts.

There is no bushing mark on the surface of the parts.

Open gate hot nozzle is particularly suitable for engineering plastics and fiber glass material molding.

Hot tip gate nozzle

Hot tip gates also called thermal gates, have a more tightly processing window and are more sensitive to work. It depends upon thermal cycling of the molten plastic in the gate area for good quality operation.

After cavity fills, the melt in the gate area cools and hardens. Gate cooling plays a key role in how quickly solidification takes place. During the subsequent cycle, injection pressure forces the solidified gate material into the cavity, opening the gate. The small slug from the gate melts from shear heating as it enters the cavity.

After cavity fills, the molten plastic in the gate zone cools and hardens. Gate cooling takes a key part in how rapidly hardening happens. After that the injection pressure forces the hardened material into the cavity, opening the gate. The little slug from the gate softens from shear heating as it enters the hole.

It requires an operator to balance temperature of then nozzle with the help of temperature control devices, keeping in view the mold environment.


Hot tip gate molding is thought to be more economic investment compared to valve gating.

Right choice for internal and external injection of the molding parts.

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