The nozzles of the hot runner system are responsible for receiving the molten polymer from the manifold and delivering it to the mold cavities with the least pressure drop, temperature drop and minimum gate effect on the product. The most diverse parts of the hot runner system are the nozzles, so that with the smallest change in the initial conditions of the process, such as the type of polymer produced, product weight, the minimum and maximum thickness of the product, the variety of colors in the production of the product, the estimation of the number of products produced, the cycle time, the type of gate effect On the product and… we will witness fundamental changes on the type, size, height and overall design of the nozzle. For this reason, choosing the type of nozzle is known as the most challenging part of the design of the hot runner system among the experts in this field of engineering, and it will not be possible to do this step correctly without knowing the relevant sciences and having sufficient skills and experience.
In order to optimize the performance of the channel system, the following points are considered as basic principles in the construction of production nozzles in the Pilan Sazeh complex:
The entire nozzle should have a thermal balance, which means that after the temperature stabilizes on the controller display, the point-to-point temperature of the nozzle body and its other components should have the smallest numerical difference with the displayed value.
The nozzle can easily change the color of the product (Color Chang).
heat loss to its minimum value.
In order to maintain the critical parameters of the injection process, the pressure drop at the nozzle outlet should reach its minimum value.
After the end of the injection phase, the material should not overflow from the tip of the nozzle.
Chemical corrosivity of the material reaches its minimum value when it comes into contact with the nozzle components.
The movement of polymer inside the nozzle channels should be laminar with minimal flow resistance.
The placement of the heater should be in such a way that there is the least possibility of damage and destruction in it.
The placement of the thermocouple inside the nozzle should be such that it displays the most accurate value compared to the temperature of the injected material.
The nozzles of the hot runner system can be divided into three groups based on the injection method.
Open Gate nozzles:
The passage channels of materials in this type of nozzles are straight and without deviation from the beginning of the nozzle (where the molten polymer is received from the manifold) to its end (where it is delivered to the mold gate). For this reason, in order to produce polymer products with hard injection materials (in the term of engineering materials) such as PA6, PMMA, PC, etc., using this method is a very suitable option. Also, these nozzles are very useful in the production of products in which second-hand and ground materials are used. The disadvantage of these types of nozzles is their relatively high gate effect on the product. It is also seen in many materials in the fine material or stringing.
Pin Point Gate nozzles:
In this type of nozzles, the molten polymer is deflected at the end of its path by the tip of the nozzle and enters the mold gate in the form of a needle with minimal gate effect. These types of nozzles are special for the production of parts with easy-to-inject (non-engineering) polymers, such as PP, PE, ABS, etc., in which the MFR value is acceptable. This type of nozzle is not suitable for producing products with second-hand materials or mixed with milled materials due to the deviation of the flow path.
Valve Gate nozzles:
Velogate nozzles are equipped with a pin (Needle), which can be moved forward or backward using spring, pneumatic, hydraulic or servo motor power by the cylinder and piston installed on top of the nozzle. So that when the injection operation starts, this pin moves backwards and opens the exit flow path of the nozzle, and after the injection is finished, the pin moves forward and blocks the exit path, and this action is repeated until the end of the production process. The gate effect of these nozzles is in the form of a circle (like a spring pin) on the product
At first, only these nozzles were used when using gaseous materials such as PA6, where there was a high probability of the material overflowing as soon as the injection or threading was finished on the final product. But with the passing of time and the advancement of the design techniques of the warm corridor system, the use cases of Velogate systems have become more diverse, some of which will be mentioned below:
One of the most important and widely used cases of using Velogate systems is when, due to the use of hard injection polymers, the flow path must be straight, and on the other hand, due to the aesthetic sensitivities required on the final product, the gate effect must be at its lowest value. have In other words, we intend to use the advantages of Open Gate and PinPoint Gate nozzles without receiving their disadvantages at the same time.
Due to the thin wall of the product and as a result the high injection pressure required, the gate inlet must be large.
In cases where the cycle time is so high that there is a risk of the material falling out of the nozzle tip or the nozzle tip freezing.
When gas pressure is used during injection (Gas Injection).
When the sequential method is used to prevent the formation of a welding line on the product.
When Stack Mold is used.
According to single cavity or multi cavity mold, nozzles can be divided into two groups:
(Single) S series nozzles:
These types of nozzles are used in single cavity molds. The difference between these nozzles is in the arc on their heel, which during the injection operation, this arc is placed on the curve on the nozzle of the injection device and causes the nozzle to be sealed with the device’s nozzle. These types of nozzles are restrained by a ring that is installed on the mold shoe.
M series nozzles:
These nozzles are used in multi-cavity molds. Unlike the S series nozzles, the bottom of these nozzles is flat and corroded, and by the corroded bottom of the manifold, the pressure caused by the thermal expansion of the nozzle and the pressure of the clamp of the sealing device is done.
Multi-nozzles can be used in multi-cavity molds with small products where the cavitations are arranged in a circular shape and close to each other. The design of multi-nozzles requires many parameters of the conditions of the injection process, including the weight of the product, the polymer used in the product, the smallest wall thickness of the product, the diameter of the nozzles, the number of nozzles to be produced with a multi-nozzle, etc. For this reason, it is recommended to consult with the relevant consultants and experts before using these systems.