Pleatable nonwoven material and method and apparatus for production thereof

A pleated air filter comprising a pleatable filter material consisting of a nonwoven material, consisting of:

• thicker fibers each comprising a substantially continuous strand for providing a stabilizing support frame for the nonwoven material;
• and
• thinner fibers of the same material as the thicker fibers and relatively thinner than the thicker fibers, the thinner fibers each comprising a substantially continuous strand having a diameter of less than 1000 nm for providing a filter effect,
• wherein the strands of thicker and thinner fibers are tangled and intertwined with each other, and with the thicker and thinner fibers melted and bonded together at least at some of the locations where the thicker and thinner fibers are in contact, and
• wherein the thinner fibers are incorporated largely homogeneously in the thicker fibers in a direction along the surface of the nonwoven material, and
• wherein a continuous distribution-density gradient of the thinner fibers in a direction perpendicular to the surface of the nonwoven material is such that the highest concentration of thinner fibers is in the region at either the center of the filter or one of the two outer surfaces of the filter.