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Two types of Pressure Casting are typically employed in wheel construction, Low-Pressure Casting and Counter-Pressure Casting. In terms of quality and strength, the results are similar between both processes, and are a significant improvement over Gravity Casting. Most OEM wheels are manufactured using Pressure Casting.
Low-Pressure Casting uses air pressure to force the molten alloy into the wheel mold. This causes the molten alloy to pack itself more densely.
Counter-Pressure Casting uses a similar, but opposing process. It involves creating a mild vacuum by which the molten alloy is literally sucked into the wheel mold.
Gravity Casting is the simplest (and cheapest) form of wheel manufacturing. The process involves pouring molten alloy directly into the wheel mold, using only the force of gravity to push the alloy into the mold. This creates the least dense metal. Gravity Cast alloy must therefore be thicker and heavier than metal used in other manufacturing methods in order to have enough strength to be used safely for wheels.
and if you're on a budget -- to show that OEM are better than Replicas, or that you can refinish a vintage set on your own.