I was recently asked the question…
We are about to build a 2 plate family mold but are not sure which molding machine we will use it in. We may also process two grades of material in the mold. How will these factors effect the size we should cut our runners?
Often, the feed system design is uncertain. You have mentioned two common issues which do effect the runner size.
The capability of your molding machine to fill the mold with a material whose flow characteristics are unknown should always be taken into account. If the runners are cut to large your machine may not be able to fill the mold.
There may be variation between the melt flow rates and pressures to properly balance a family mold, especially when two grades of material are being used.
In this situation I suggest you use a runner dimension that is “steel safe”. “Steel safe” means your design should call for the removal of less mold steel than may be ultimately required. You may want to round down your runner dimensions by one or two standard sizes than what you calculated as nominal. This will result in a lower volume feed system with a greater pressure drop. There is a chance the smaller feed system will work and you will save on scrap material. However, if the undersized feed system is not optimal, this approach will allow you to easily machine the runners and improve the performance of your mold.