This air filter / grease filter is made of multiple layers of expanded aluminum and enclosed in a lipped aluminum channel frame and secured on one corner with an aluminum rivet. This air filter / grease filter is not as efficient as the bonded aluminum mesh filter, but it is nearly indestructable. It is an extra heavy duty air filter / grease filter that can be used for various filtering applications including primary filters and pre-filters. The frame is hand closed so it grips the filter media more securely. All corners are hammered to remove any sharp edges and hammering also helps close the frame very firmly on the filter media, giving both the frame and the filter media more strength in the corner areas. The standard duty lipped aluminum frame has rounded edges so it won't split like most of the sharp edged filters. The frame is closed on one corner by an aluminum rivet and that corner is flattened so the rivet seats tighter for more holding power. This is a very strong, rigid durable filter. It will perform well for extended periods under heavy use. It is recommended for medium and heavy air volumes in industrial type applications.