I was just curious how much of a difference having the fins makes in a heatsink vs just a large block of metal.
The heat radiates from the surface of a heat sink, so the greater the surface area, the better the heatsink. Roughly speaking, if you cut a load of slots in your big block of aluminium, you could end up with, say, 5 times the surface area, and the heatsink can then get rid of 5 times the heat.
Have a read of the Wikipedia article - the big block is, in general, rather poor compared to the fins.
The difference between a special heatsinks and an aluminum (metal) block is very big, I can easily tell the following three differences:
The heatsinks will have a significantly larger surface area than the aluminum block through a different structure, so that it has a more efficient heat dissipation function.
The weight of the aluminum block of the same volume is much heavier, which is not suitable for many applications.
Due to its weight, the price of aluminum blocks far exceeds that of special radiators. For example, the most widely used extruded heatsinks are made easily by professional manufacturers and they are basically priced according to the weight of the parts. This heatsink manufacturer has some detailed information about the extruded aluminum heatsinks.
In summary，the above comparison makes the aluminum block's cost-effective heat dissipation completely no advantage.