The manifold is the most basic part of the hot runner system and its task is to receive molten polymer from the nozzle of the plastic injection machine and divide it to the beginning of the nozzle. According to the number of injection points, the distance of the injection points from each other, the type of polymer materials used, manifolds are designed in different dimensions and shapes, but two basic principles must apply to all of them: heat balance and flow balance.
Definition of heat balance:
Thermal balance means that the dimensional design of the manifold as well as the selection of the number and parameters and the geometry of the heaters should be such that after turning on the heaters and the temperature stability on the controller display, the actual temperature throughout the manifold has the most uniformity and the point to point temperature of the surface The manifold has the smallest difference with the value displayed by the thermocouple. Lack of thermal balance can lead to freezing of materials (Hesitation) in colder parts and burning of materials (Degradation) in hotter parts of the manifold.
Output current balance:
In addition to the thermal balance, the output flow balance will play a very important role in the performance of the hot runner system, so that the dimensional and geometrical design of the manifold channels should be such that the pressure of the molten polymer material during injection in all manifold outlets (injection points) , have the lowest numerical difference with the rest of the output points. Also, these values should have the lowest pressure drop compared to the input pressure of the material.
Excessive pressure in the high pressure points of the manifold:
Excessive pressure in the high-pressure points of the manifold will lead to an increase in the static pressure of the molten polymer in Quetta, an increase in the residual stress, and as a result, warpage of the final product. Also, the difference in polymer pressure at the low pressure points of the manifold can also lead to short shots not being completely filled, shrinkage out of control of the product, reduction in surface quality, reduction in weight and mechanical strength of the product, etc.
Achieving a flow balance, especially in molds with a high number of cavities and molds that have cavities of different sizes (Family Mold), is one of the most challenging problems in the design of the hot runner system, which is solved by using computational and experimental sciences and software. Emulators are available without any restrictions.
The manifolds of the hot duct system can be divided into two different groups based on the sealing method:
Manifolds with adaptive sealing
In this type of system, the manifold and the nozzle are installed separately in the mold and the ground surface of the bottom of the manifold is placed on the top ground surface of the nozzle and the sealing between the manifold and the nozzle is through the pressure resulting from the thermal expansion of the height of the nozzle (as a result of heating nozzle) as well as the clamping force of the plastic injection machine is obtained during the injection operation.
In this type of system, the outlet points of the manifold have threaded holes with a calculated pitch, and specially designed nozzles are screwed and fixed inside these points. Unlike the previous type in this system, the manifold, nozzles and electrical wiring channels are made in an integrated manner, which facilitates its maintenance and installation. The sealing between the manifold and the nozzle is achieved without the need for expansion force, or clamp pressure or any other force through the force resulting from the closing of the nozzle inside the manifold.
Manifolds with adaptive and fixed sealing each have many advantages and disadvantages, and choosing the right type of them requires considering the computational and experimental parameters of the hot runner system design. For this reason, it is recommended to use the advice of experts before choosing.
In order to optimize the maximum efficiency of the manifold of the hot duct system built in the industrial complex of Pilan Sazeh, in addition to observing the thermal and flow balance, other parameters are also considered in the construction and design, some of which will be mentioned below.
To prevent the creation of sharp corners (Dead Corner) resulting from the intersection of vertical and horizontal channels that are the center of accumulation of motionless polymers. As a result of the passage of time, the non-movement of the material at these points causes the material to burn and black streaks appear on the final product. Also, not fixing these points will eliminate the possibility of Color Change (use of several colors in production) of the hot runner system.
The frequent increase and decrease of temperature in the manifold will cause many problems, such as changing the center-to-center distance of the injection points, causing thermal stress and damage to the manifold body, etc., which can be predicted using computer science and simulation software. It is fixed.
The heaters used in the manifolds of this collection are flexible and special for bending. This advantage increases the lifespan of the heater and prevents the destruction of the element texture due to bending. The surface of these heaters, after being placed in the respective channels, is covered by profiles with a special alloy, and this increases the heat transfer coefficient and reduces the temperature loss of the system.
In order to facilitate the movement of materials inside the manifold and prevent pressure drop, the internal surfaces of the channels of the channels are polished with a very high quality. Also, standard parts are used to change the direction of movement of the polymer inside the manifold.
Blocking channels is done with standard insert parts. This makes it possible to easily remove these parts when the channels are closed due to stones, iron shavings, etc. and cleaning operations are carried out.
The use of standard parts in the manufacture of manifolds, such as spacers, sprues, dwell pins, etc., facilitates the replacement of manifold components when needed.
It should be noted that due to the increasing growth of the plastic industry and the production of plastic products by the injection method and the emergence of new polymer materials with new and complex properties and behaviors, the design of channel systems as one of the leading sciences in the polymer engineering industry, mechanics, Electricity and electronics are always changing and progressing. For this reason, there is a great diversity in the design and construction of hot runner systems, the explanations given are only a small part of it, and it will not be possible to introduce all of them in one article. For this reason, it will be tried to investigate and research about other types of it in future articles.