As the name implies, multifilament fabrics are produced from yarns that have many individual continuous filaments. Multifilament yarns are referred to in their size as denier. Denier is the weight in kilograms of 9,000 meters of a multifilament yarn. Therefore, the larger the denier number, the larger the yarn, and it is a directly proportionate number. An 840 denier yarn would be twice the size as a 420 denier yarn. One thing to consider when talking of multifilament yarns, in addition to the denier, is the number of filaments in the yarn bundle. For example, 840 denier yarns may have 70 filaments per yarn bundle, they may have 128 filaments per yarn bundle, 144 filaments per yarn bundle. In terms of the filtration efficiency of the fabrics used by these various yarns, we would assume that the higher the number of yarns per yarn bundle, the more efficient the fabric produced. The lower number of filaments per yarn bundle would give us better release, better throughput and less tendency to blind, and may effect adversely our filter efficiency.
Compared to the monofilament yarns and yarns to be discussed below, multifilament yarns have poorer release qualities than does the monofilament, yet better retention (filter efficiency); throughput will be less than the monofilament (flow rate); efficiency will be better than the monofilament. They may be more likely to blind than the monofilament.
Multifilament fabrics are used extensively again on all types of fabric filtration, vacuum and pressure for both plate and frame presses, rotary vacuum filter belts, horizontal filter belts, vertical filter discs, and horizontal pan filters, etc.
The weaves used to produce these fabrics again will be stain weaves, twill weaves, oxford weaves and some plain weaves. The weave pattern pretty well is dictated by the application.
Fabric finishing is very critical in the multifilament fabrics. When multifilament fabrics are woven, there is a significant amount of yarn tension remaining in the fabric once it is removed from the loom. If this fabric is fabricated into a filter media, then exposed to temperature in a wet environment, the tensions in the yarns will be relaxed and this fabric will shrink, stretch, or otherwise deform, because it has not been stabilized by finishing. finishing takes the following forms: scouring and heat setting. Scouring basically incorporates wetting the fabric and then running it across an oven to reestablish a memory in the yarns. Calendering is as described above, running the fabric between two rollers to squeeze the fabric or reducing the permeability again imparting some stability into the weave pattern. Fabric which has come from the loom and not finished is called “greige”. The multifilament yarns used in these fabrics may be either single yarns or plied yarns. A plied yarn consists of two single yarns twisted together to create a two-ply or three-ply, etc., yarn.
Multifilament yarns are held together by twists and this is measured in terms of TPI (twists per inch). For example, we have 840 denier polypropylene yarns that have, what we refer to as, producer twist. Producers twist is one turn or less per inch. This is just a sufficient twist to hold the yarn bundle together. If you cut this fabric, you will notice that the filaments within the yarns tend to fray very readily. On the other end of the spectrum, we have some polyester yarns used in our style 15-285, for example, that have 30 turns per inch. The higher the number the turns per inch, the more monofilament-like the multifilament yarn becomes. In many cases, for example, we may use a high twist multifilament in the warp direction and use a producer twist yarn in the fill direction. The monofilament yarn gives us a very hard finish and significantly contribute to the way the fabric will release the filter cake, whereas, the fill yarns being less twisted will greatly effect the efficiency of the fabric. Generally, twist per inch will range from 7 to 12 turns per inch’ 15 turn and 30 turn twist per inch is considered high twist yarn.