A “cold plug“ in the nozzle leads directly to quality problems which are generally apparent from flaws on the surface of the moulded part. If the flow of molten material is completely blocked by a “frozen“ nozzle, production is interrupted. Since the problem can only be rectified manually, this leads to lengthy downtime. Consequently, it was imperative to include the nozzle geometry in the design of the injection mould and to adapt it to the shot weight and sprue diameter. Particular attention had to be paid to the land length of the bore, through which the melt passes to the outlet geometry. The rule of thumb here is that the land length of the bore is 0.5 and 0.8 x Ø, and should be counter-bored at an angle of 20°. The opposing hourglass-shaped cones created by the nozzle bore and the cross-section of the outlet form a predetermined breaking point, enabling the sprue to be demoulded without causing freezing.
If the injection moulding process requires long nozzle contact times, the surface contact area should be optimised, i.e. minimised. For example, heat transfer to the mould can be reduced by means of a recess in the nozzle contact surface because the air in the recess acts as an insulator.
Extended nozzles, which are more than 20 mm longer than the standard machine nozzle, should always be equipped with a heater band in order to avoid problems with freezing nozzles and cold plugs from the outset.