The inherent cause for the shrinkage, in injection molded parts, is the thermodynamic behavior of the material.
This behavior is shown in the pvT-diagram, in which the specific volume (v) is presented as a function of pressure (p) and temperature (T).
It characterizes the compressibility and thermal expansion of polymers.
There is an essential different pvT behavior between amorphous and semi-crystalline materials.
In the picture there are three different pressure curve shown p0, p1, p2 for each material type.
As a melt, both show a linear dependency of the specific volume on the temperature.
For the solid phase, amorphous materials (green) also have a linear dependency.
While semi-crystalline materials (red), on the other side, show an exponential decrease of the specific volume at decreasing temperatures.
These different behaviors, in the pvT-Diagram, explain the reason for the higher shrinkage of semi-crystalline thermoplastics compared to amorphous materials.