Dec 6, 2018
The transportation of the molten plastic from the injection unit through the mold to the cavity can be done by a hot runner system.
This consists of at least one sprue bushing, the manifold and one or more hot runner gates.
The single components are provided with heaters, which are controlled by suitable PID-Controllers, to achieve a uniform temperature distribution. The system, on the other hand, is thermally decoupled from the mold so that there is a homogeneous heat balance inside the hot runner system.
Advantages of a hot runner system are for example the saving of material and the reduction of the cycle time.
There are different versions of hot runner systems.
The simpliest is the hot runner system with thermal lock.
At the end of the injection and packing phase the melt is cooling down at the gate due to the heat transfer in the colder mold.
The viscosity of the plastc decreases and a “thermal lock” develops, which prevents the melt from entering into the cavity.
Hot runner systems with valve gates are commonly used.
A needle is added to the gates. It can be moved by cylinders (hydraulic or pneumatic) allowing or preventing the melt to enter the cavity.
Advantages of the usage of valve gates are the controllable filling of the parts and that the break-off point can be designed without a residual pin.
Oct 11, 2019
Hot runner drops can be used with a needle inside. These hot runner drops are called valve gates.
However, the opening and closing of the needle can be done in different ways.
One way is to use air in order to move the needle and is implemented in the internal valve gate nozzle.
This pneumatic valve gated hot runner operates entirely without elastomeric seals, lubricants, or cylinder cooling.
It can operate at extreme temperatures of up to 400°C (750°F).
Image: Open and closing of the
pneumatic hot runner nozzle.
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This glossary of plastic industry is provided by PLEXPERT Canada Inc.
Nov 27, 2018
The hot runner system can be separated in different components.
The area, where the melt is divided into the different hot runner gates, is called manifold.
For molds with multiple cavities, a homogeneous melt temperature and pressure distribution are necessary.
Therefore, the manifold should be preferably symmetrical.
Nov 5, 2018
If there is a hot runner system implemented in the mold a PID-Controller is used to bring the heater band up to a given temperature.
By controlling the heater band, the temperature at the sensor oscillates around the requested temperature.
Dependent on the choice of P-,I- and D-values reaching the given temperature happens faster or slower.
Nov 27, 2018
Nozzles in the hot runner system are divided into open and closed nozzles.
Closed nozzles can be opened and closed with the help of a steel needle, therefore the name valve gate.
The needle pushes the gate flat and only a single round mark is visible after closure of the gate.
The filling and packing phase can be influenced directly by the control of the needle.
