You could build a series of models to develop some skills with ANSYS and grow your understanding of this problem, which can become very complex.
Though the real problem is transient, start working with a steady state solution to see what that shows.
Start with a 2D model and ignore the temperature change of the cooling fluid as it travels through the depth of the tool. That means drawing a section of the tool on the XY plane as a surface.
For either 2D or 3D, you have to set boundary conditions on all surfaces of the tool, which in 2D are edges. A simple model could have the temperature on the side of the tool that the molten plastic is flowing past is set as a constant value, while the edges where the coolant is flowing could have the temperature of the coolant. That leaves the other 3 edges, which could have convection with the outside air temperature. Or you could make just one outside edge have convection to outside air while the two sides of the tool have symmetry. This represents a small section of a much longer tool.
Once you have success at a 2D steady state model, you can try more complicated models. The most complex model could have molten plastic advancing into the cavity and freezing onto the walls as it goes.