The PVT curves show that plastic materials are in fact very compressible at the temperatures and pressures encountered in the injection molding process. Process variables such as packing pressure or holding pressure (or more precisely, cavity pressure) should then have a significant influence on part dimensions or mold shrinkage.
If the holding pressure is increased without changes to any of the other process variables, the course of state curve moves further to the right and downwards (to the higher isobaric line), leads to a smaller specific volume because of the greater compression of the melt. This results in a lower value of the volumetric shrinkage and higher molding weight.
Use the holding pressure to press additional melt into the mold to compensate for volume contraction of the part as it cools can be interpreted by the course of state on the PVT diagram of the plastic material.
The PVT-related variables such as part weight, part shrinkage and warpage depend on the course of state on the PVT diagram of the plastic material, they can be controlled by modifying the process variables or settings. For example, modifying magnitude and duration of holding pressure or switch-over point from filling to holding stages, melt and mold temperatures, etc., can result in different courses of state on PVT curves and therefore different shrinkage and warpage behavior.