Various Cardboard Frame Prefilters
Have you seen such filters?
Yes, they are all coarse filters with cardboard frame. Usually, people use them in various conditions, such as residential, commercial and industrial ventilating systems. It`s very widely used. And there are also a lot of sizes available (see following chart). Each type is mold made.
As for the material, the only difference between white type and blue type is color. But when it comes to the green one. It is blended fiber of cotton and synthetic fiber. The fiber is finer thus the efficiency is ensured and dust holding capacity is higher. (while with about 10Pa pressure drop above)