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
Hello Andy.
Is it good way to optimize recovery in this way? Picking 3-4 different recovery speeds and charting shot weight variation. Same thing with back pressure. Lowest variation is best.
Milan
Good question Milan,
That works best with back pressure, but it also works well with screw motors which are not very consistent. For example, some of the older, closed-loop molding machines could not maintain a consistent screw recovery because the measurement devices were inferior.
The key is to first optimize the feed temperature, and then work on the recovery speed and pressure.
-Andy
Hello Andy.
I used during feed zone optimization back pressure 70 bar plastic according John Bozzeli recommendation. Is it good way to do this?
Milan
Milan,
The important factor is to use a back pressure that provides consistent recovery for that machine and process. For most cases, a general number such as the one mentioned, works very well.
-Andy
thanks Andy
Milan
Hello Andy.
When I'm increasing rear zone temperature from lowest, screw recovery time should increase or decrease? What is your experience?
Milan
Milan,
If you start low enough, there is usually a decrease and then an increase.
I have some processes where the optimal feed temperature is lower than the material suppliers' recommendations. Many processors will not go below the recommended temperature setting. In such a case, you may just see a rise in recovery time as you increase tempertaures.
-Andy
Andy,
During this test you have nozzle, front, middle temperature in flat profile right?
Milan
Milan,
Usually, this is the case... though the same process can be used to optimize the feed of any profile.
-Andy
Andy,
Are you measuring melt temperature during feed zone optimization to be sure, that you are in supplier range?
Milan
Milan,
You will find that the rear temperature has very little affect on the melt temperature.
-Andy
Andy,
Is the influence of feed zone temperature on screw recovery time lower when I use machine with shot size below 20% of maximum?
Best Regards
Milan
Milan,
If the screw and barrel are appropriate to the material, then the influence of feed zone temperature will be minimized.
-Andy