I received a question which relates to an issue that many injection molders face…
MJ
Is there a technique for optimizing recovery speed?
My Response
Basically, there is a way to optimize recovery speed… but it is a two step process:
1) First, determine the optimal feed zone temperature for your process by performing a feed zone temperature study. The purpose of this study is to determine the optimum feed temperature by graphing feed zone temperature versus screw recovery time. Starting with a low feed zone temperature, incrementally increase the temperature and document the screw recovery time at different increments. When graphed, the screw recovery time will drop and then rise as the temperature is increased. The optimal feed zone temperature is the temperature at which the screw recovery time is the lowest. This is the temperature where the polymer sticks best to the barrel, causing it to convey most efficiently.
2) Once the optimal feed zone temperature is determined, you should adjust the rotational speed of the screw so that screw recovery consumes 80 percent of the overall cooling time. Note that the back pressure used during screw recovery should be high enough to provide a consistent recovery time and consistent mixing. Your recovery times should not vary more than 5% from shot to shot, and 10% from material lot to material lot.
Additional Thoughts
Many older molding machines cannot maintain consistent screw speeds at low RPM. In such a case, you may want to consider a longer delay before recovery to ensure the machine can maintain the desired consistency.
-Andy
Hi Andy
I work for a medical company therefore most of the processes are monitored with close tolerances. Recently we were processing Acetal material with black masterbatch. During the whole run the screw recovery time kept going up excessively before coming back to normal values after a few minutes. This was repeated throughout the run and it happens on this particular job all the time. we are not allowed to change either screw speed or back pressure excessively. Purging the barrel through with purging material helps for a while but it goes back doing the same thing.could it be the masterbatch causing screw slip? your thoughts on this please.
Hello Saeed,
I will address this in a separate posting.
-Andy