After weaving a “greige” fabric composed of the optimum weave and yarn combinations for an intended application, proper finishing is critical for this fabric to perform in the filter.
In woven fabrics, both the warp and fill yarns are placed under extreme tension and will remain that way until they are finished. The finishing process must reestablish the yarn member so that when wet out the yarns remain in the finished condition and the cloth characteristics do not change. Improper finishing will allow the fabric to shrink or contract.
Hydrophilic fibers shrink due to the fiber (filament) swelling when immersed in a hot water bath. Scouring and heat setting the fabric consists of wetting out the fabric and drying it on a tenter frame. The heat setting on the tenter frame reestablishes the yarn memory and stabilizes the fabric for filter service.
Thermoplastic yarns, while they may have some moisture absorption, also may elongate during weaving. The fabric during the finishing process is allowed to relax, the yarns contract. The contraction can be stabilized by a combination of heat as well as rolling the fabric up in a relaxed condition. Failure to do this will cause the fabric to contract during the fabrication process where it has been unrolled and relieved of its constrained condition.
Both contraction and shrinkage will contribute to variations in fabric permeability. This can cause a fabricated bag to fit poorly onto a filter frame. This will cause installation problems and can cause premature bag failure because of improper sizing.
There are two types of fabric finishing used for filtration fabrics. These are described as wet finishing and dry finishing. Wet finishing consists of scouring and heat setting as described above to stabilize the fabric by wetting it out prior to heating. This type of heat setting will affect fabric stability and also affect permeability.
The second type of finishing is described as calendering. This is a totally dry process, consisting of passing the fabric through a series of two or three hot rolls and squeezing the fabric between them. This is generally done on synthetic thermal plastic type yarns. The combination of the heat which is used to soften the yarns in combination with the pressure between the rolls will adjust the permeability as well as impart stability to the fabric. The calendering process may also give the fabric surface a shiny or smoother appearance which will enhance cake release.