The fun of molding PP is that it can take a significant amount of abuse during molding and end use, but only if it has been melted properly.
In the past week, two different customers consulted us for assistance in troubleshooting failures with their molded parts. They were both running PP and were both running back pressures around 1500psi plastic pressure. Not to get into specifics, the parts were exhibiting brittleness during end-use testing. This was not necessarily the only issue, but significantly high back pressures are common issue in this industry.
There tends to be a misunderstanding of the proper use and setting of back pressure. This value is typically set too high and is adjusted far too often during troubleshooting. In general, you want to use enough back pressure for mixing and melt consistency, but not much more. This pressure should not have to be changed during production unless a significant change such as the composition of the material, new additives, new screw, or the screw or barrel have been modified in some manner (lot to lot changes in material do not qualify for a back pressure change).
Essentially, i you need more heat for the colorant to properly disperse, then add barrel temperature… likewise, if you need more melt temperature for the material to melt, mix or inject properly, then add barrel temperature as well. This is why it is critical to know the melt temperature which is used to mold a good part as it is critical to troubleshooting melt-related problems.
You should never use back pressure to add extra heat to the material during recovery as this breaks polymer chains & fibers as well as degrades many of your additives, colorants, & processing aids. All of this will reduce the strength of your part and make it more brittle.