Hot runner technique

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.

hot runner system

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)

Offene Düse ohne Vorkammer

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).

Offene Düse mit Vorkammer

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).

Offene Düse mit Torpedo

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).

Offene Düse mit Angusskegel

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 ).

Nozzle with needle valve

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.

Length of flow path

Fig. 13: Length of flow path as a function of temperature

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