The most popular production method for heatsinks is extrusion. With the aid of high pressure and temperature, a flow of aluminum is forced through a shaped opening. This results in a long stick having the same form as the opening. Later, the aluminum is stretched, which straightens it and improves its mechanical properties (better strength through re-alignment on a molecular level). Finally, the long aluminum sticks are cut into heatsink-sized pieces, and possibly milled to improve flatness of the contact area. Even though the concept is simple, the machines involved are huge.

The classic extruded heatsink has a base plate with fins on one side. If a fan is used, the direction of the air flow is orthogonal to the direction of the extrusion.

Extruded heatsink with classic design

With this classic extruded heatsink design, the air from the fan will at some point "hit" the base plate; it can escape only at two sides. A high pressure may occur within the heatsink, which is bad for air flow.

Modern CPUs have only a small contact area between die and heatsink. Therefore, it is possible to design extruded heatsinks where the direction of the air flow is identical to the direction of extrusion; they feature a thick core which leads the heat upwards. Air can more easily flow through the heatsink, and escape at all sides. The core is located below the fan motor, where little air flow occurs anyways.

Direction of air flow same as direction of extrusion