Learn About Common Filter Bag Finishes Such as Calendering (Glazing), Singeing, Napping, and Coating

There are a few different finishes and treatments that can help lengthen the life of your filter bag, improve its stability, and make cleaning an easier process. Filter performance is directly related to how well filters can tolerate the work environment it’s in.  So applying a specific finish or treatment is one way to achieve better performance.

Fabrics for filters are usually made of woven felts, non-woven felts, natural fibers, synthetics, or polyphenylene sulfide. Natural fabrics, such as wool and cotton, are usually preshrunk to eliminate bag shrinkage during operations. Certain fabrics can be treated with silicone to give them better cake release proprieties.  When it comes to both synthetic and natural fabrics, processes such as calendering (glazing), napping, singeing, or coating are typically applied.

Calendering (Glazing)

Calendering, also known as Glazing, is the high-pressure pressing of the fabric by rollers to flatten or smooth the material. Calendering pushes the surface fibers down onto the body of the filter medium.  Calendering can be applied to various media such as polyester, polypropylene, and aramid. This is done to increase and uniform surface life, improve stability and reduce shrinkage.

Napping

This process is the scraping of the filter surface across metal points or burrs on a revolving cylinder. Napping raises the surface fibers, creating a “fuzz”, that provides a large number of sites for particle collection by interception and diffusion. Fabrics used for collecting sticky or oily dusts are sometimes napped so they can provide better collection and an easier cleaning process.

Singeing

Singeing is done by passing the filter material over an open flame. This removes any straggly surface fibers and results in a more uniform filter bag surface.  You can typically see this process done on the following fabrics; polyester, polypropylene, acrylic, aramid, ryton, and P84 felts.

Coating

Also known as resin treating, this involves immersing the filter media in natural or synthetic resin. This process helps lubricate the woven fibers, provides high-temperature durability, and various fabrics.