Hot runner techniques and hot runner nozzles
Hot runner feed systems are becoming increasing popular in the processing of thermoplastics. With Desmopan® and Texin™, too, this technique is being used more and more often.
Fig. 7 lists some of the main criteria for this method.
Fig. 7: Demands on the hot runner system
Type 1: Open nozzle for direct gating
- Open nozzle without plastic insulating cap
Not very common for processing TPU, because of poor thermal separation, resulting in "shiners", haloing, sticking and material drooling. (Fig. 8)
Fig. 8: Open nozzle without plastic insulating cap
-Open nozzle with plastic insulating cap
The insulating cap and cooling system lead to better thermal separation (Fig. 9).
Fig. 9: Open nozzle with plastic insulating cap
-Open nozzle with torpedo
The narrow annular slit leads to excessive shear. Normally unsuitable for TPU (Fig. 10).
Fig. 10: Open nozzle with torpedo
Type 2: Open nozzle with sprue
This system is used primarily for large molds (single/multiple cavity).
The particular advantages of this construction are its low pressure losses and good thermal separation (Fig. 11).
Fig. 11: Open nozzle with sprue
Type 3: Nozzle with needle valve
A clean gating point can be placed directly on the component.
This gating system is characterized by effective heat insulation and low pressure losses (Fig. 12 ).
Fig. 12: Nozzle with needle valve
Flow characteristics of the mold
The flow behavior of Desmopan® and Texin™ is basically the same as that of other thermoplastics. The length of the flow path is dependent on the melt temperature, the wall thickness of the part to be filled, the injection speed and the rheological properties of the material.
The injection speed can be varied according to the machine and the technical circumstances.
The following figure shows the flow path of Desmopan® and Texin™ as a function of melt temperature.
Fig. 13: Length of flow path as a function of temperature