I received this interesting question earlier this week…
We are changing some of our tools to cylinder nozzles that decompress hot runner tools using carriage retract – closer to the manifold. Is this good?
note: these cylindrical nozzles are designed to insert into the sprue bushing while being allowed to slide in and out as a seal is being maintained between the inner diameter of the sprue and the outer diameter of the nozzle. These systems are used to decompress the melt within the hot runner system as the nozzle retracts during sprue break.
Although the intent of these systems is to decompress the hot runner system to eliminate gate drool, it will introduce variability. Since the carriage movement is not precisely controlled, the amount and rate of displacement both forward and back is not consistent from shot to shot.
Basically, the most effective and consistent method of decompression is to use screw suckback, or retract, after recovery. As a result, the volume of material entering the mold during 1st stage fill is likely to be more inconsistent when this is used. Additionally, you are relying on your maintenance staff to keep the barrel and platens perfectly aligned. When this is not the case, premature wear, leakage, and/or breakage will occur.
After suckback, a shut-off nozzle can be used to maintain a negative pressure in the hot runner system as well as keep the material in the barrel between shots. As shut-off nozzle technology is getting better, the applications which can benefit from a shut-off nozzle.
If you machines already have these nozzle systems, minimize the use of the sprue break and always ensure accurate alignment of the nozzle and and linearity of the carriage movement.