Many material manufacturers and suppliers provide processing guides for the materials that they sell. Often, these data sheets are so general as to almost be useless. However, I see a mistake on a great number of processing guides and I think should be corrected. It relates to the nozzle temperature.
Here are the barrel temperatures listed on the processing guide for a major manufacturer of acetal.
|Rear Temperature||338 to 356||°F|
|Middle Temperature||356 to 374||°F|
|Front Temperature||356 to 374||°F|
|Nozzle Temperature||374 to 392||°F|
Notice a problem?
Why should the nozzle temperature be 18 degrees hotter than the front zone?
The only thing that can be accomplished by having the nozzle set that high is too increase nozzle heater band sales. You will literally be burning through heater bands by doing this.
The purpose of the nozzle heater band is to keep the material melted as it travels from the front of the barrel into the sprue bushing (or hot runner system). The surface area of the inside of the nozzle is not large enough to add a bunch of heat to the material. That needs to be done in the barrel.
The material that is in the front of the barrel should be at processing temperature already and the nozzle heater band should be just hot enough to prevent the nozzle from freezing off.
My guess is that when people are entering data into the processing guide they, as a matter of habit, start cooler in the back and then the middle zone is a bit hotter and then the front is a bit hotter yet and then they just enter a slightly higher temperature for the nozzle as well. Many molding companies follow these processing guides and thus I see machines set up like this all the time.
My starting position is that I always set the nozzle temperature the same as the front zone. I have never found a reason to crank up the nozzle temperature, even on nylon which nozzle freeze off seems to be the most common.