The answer combines both experience and science, and it may be one of those molding details that gets overlooked but could be stealing a chunk of your profits.
It may seem like a dull topic, but it will overcome the emotional experience that follows when you put a new mold into a machine and you find out there is not enough barrel capacity to make a full shot.
Deliberately making a short shot permits you to do a scientific molding viscosity curve. It will also prevent you from damaging the mold by overpacking if you set shot size incorrectly.
Mold-filling analysis is the right direction to go, but if you want to arrive at your destination—good parts and an efficient process—you’ll need to pay attention to all the details and select an experienced professional to do the analysis.
We all recognize that the non-return valve is a problem. The question is where is the research to figure out how to correct this so we have a better shot at making more consistent parts