The fundamental idea of extrusion is quite simple; a square extruded into a third dimension becomes a cube; a circle becomes a cylinder. Given spherical layer surfaces, the circle can also become a sphere.
[Note: It is important to consider carefully the characteristics of any given problem, to determine the orientation most amenable to extrusion.]
What happens if we extrude our simple 2D heat flow problem into a third dimension? Setting the extrusion distance to half the plate spacing, we get a cylinder imbedded in a brick, as we see in the following figure:
A cross-section at any value of Z returns the original 2D figure.
A cross-section cut at Y=0 shows the extruded structure:
