Pass Through Channels

 

 

1 hot sprue bush (electrically heated)
 2  manifold (electrically heated)
3 hot runner nozzle (electrically heated)

 

Function:

  • provide molten material to mold
  • Hotrunners are basically an extension of the machine nozzle into the mold (Pass Through Channels)
  • important that the hot runner system is dimensioned in relation to the shot volume
  •  no hold-up spots are formed (causing degradation of material)

 

 

 

Risk:

  • Sealing is difficult as well because the melt doesn’t have any self- sealing at a gap formed between hot walls.

  • In case of mass leakage into insulating air gaps troubles will arise because of a 10 time higher heat transfer

  • difficulties with color change restrict usage of internally heated hot-runners
  • Thermal expansion of various components needs to be taken into account 

 

Design:

 

 

 

externally heated hotrunners:

 

  • circular flow cross-section in the center of a heated tube or block.
  • The heating elements should care for maintaining the temperature of the melt which means that the insulation to the outside should get particular attention.
  • The main advantage of externally heated hotrunners is a better defined and better controllable mass flow
  • manifold is insulated using an all-around air gap (8-10mm)

     

a Manifold
 b support disk
c tubular heating elements
d centering pin 
e  heating coil of the nozzle

 

 

 

a  heating rod with cartridge heater
 b  melt channel
c

 torpedo with cartridge heter

  • heating elements are positioned in the center of the runner channel:  heating rods or so called “torpedos” with cartridge heaters inside
  • self-insulation of solidified molding compound
  • inexpensive but difficult to guarantee a safe mass exchange

 

Insulated Runners :

 

  • Melt cools to form an insulating layer of solid plastic along the walls of the runner.
  • The insulating layer reduces the diameter of the runner and helps maintain the temperature of the molten portion of the melt as it awaits the next shot
  •  runner volume does not exceed the cavity volume, all of the molten polymer in the runners is injected into the mold during each shot.
  • This full consumption is necessary to prevent excess build-up of the insulating skin and to minimize any drop in melt temperature.

 Advantages:

  • Less sensitivity to the requirements for balanced runners.
  • Reduction in material shear.
  • More consistent volume of polymer per part.
  • Faster molding cycles.
  • Elimination of runner scrap — less regrind.
  • Improved part finish.
  • Decreased tool wear. 

 

 

Needle shut-off nozzle :

popular due to advantages:

  • good appearance of gate
  • can be individually operated
  • equipped with multiple gates and even no weld lines are appearing between the gates because of sequential opening and closing of the nozzles (“cascade filling”)

 

 

  Actuator for needle
a Pneumatic drive
 b Hydraulic drive
c Electromagnet
d servomotor

 

“cluster nozzles”:

  • For very low cavity distances multi-nozzles are appropriate

 

  • Multi- needle shut- off nozzles with grouped needle drive but individual nozzle