Magazine September 2010

All the Latest Information from the Textile World

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Perfecting the Art of Filter Felt Production

The task of filter felts is to separate solids of the most varied sizes from gaseous and liquid media. The design of the felts varies according to the size of particle that requires filtering and the degree of efficiency needed. PES, PAC, and also Aramid, fluoro-, glass-, ceramic, carbon and metal fibers are used for special applications. Needling here can range from light to very dense. Depending on requirements, base materials in knitted or woven form as well as so-called ‘woven bands' are also needled into the filter. Filter felts can thus be roughly subdivided according to gas or liquid filtration or according to whether they have a base material or not:

The following examples show the decisive factors that have to be taken into consideration when needling filter felts:

 Gas filtrationLiquid filtration
Filter with base materialSample application:
Filter candles for industrial hot gas filtration
Sample application:
Filter bags for industrial oil filtration
Filter without base materialSample application:
Air filters for electric appliances and automobiles
Sample application:
Filtration of foodstuffs such as milk, beverages, edible oils

Example 1: Filter candles with base material for industrial hot gas filtration

  • Fibers: 100% Aramid
  • Fiber fineness: 4.4 dtex
  • Base material: 100% Aramid
  • Weight per unit area: 200-400 g/m²

Hot gas filtration involves so-called cake or surface filtration. The filters used here are exposed to powerful mechanical and thermal stress. As a result they are manufactured from heat-resistant fibers (Aramid), and usually contain a base material. The requirements for the needle to be used can be deduced from the relevant filter properties:

Example 1: Requirements


For the heat-resistant but highly abrasive Aramid fibers used in this example, especially wear-resistant needles are the obvious choice. With very aggressive fibers, chrome or GEBEDUR-coated needles are distinctive for their long service life.

Surface quality
In surface filtration, an optimal filtration effect can only be achieved with the right surface quality. This is why filters are often strengthened in several stages - with pre-needling, intermediate needling and finish needling. Adapting the needle gauge to the right pore size of the filter plays an important role here. Since ultra-fine particles are primarily filtered out in hot gas filtration, fine-gauge needles from Groz-Beckert are ideal. These especially fine needles leave only the tiniest of holes in the product. In the abovementioned example the recommendation is to use 38 gg and 40 gg needles in the pre- and intermediate needling stage and 42 gg needles in the finish phase - for an optimal surface result.

For compact compression of the filter, a combination of needles with large and small barb distance is used. So-called compact barb needles with very short candle distances can also be used here. Lying close together and precisely adapted, the barbs enable needling with only minimal damage to fiber and base material.

Penetration density and penetration depth are important parameters too, however. The correct adjustment of these two values contributes to the optical and physical result of the filter later on. The hot gas filter with base material is powerfully compressed, for instance, requiring a penetration density of 400-600 E/cm2.

Base material

If, as in this example, nonwovens are needled to base materials, the base has to be treated as gently as possible.

Often, needles are used that only have one or two edges with barbs. The size of the kick-up also has to be adjusted to suit the base fabric. Needles with the teardrop-shaped working part contribute to ultra-gentle treatment of the base material because in comparison with the standard working part they only have one edge, while the two remaining edges are completely rounded. To further avoid damage to the base material from overly sharp needle tips, needles with rounded tips are also used.

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Barbs with HL-shape are also frequently used. In comparison to conventional barbs, HL-barbs offer not only the benefit of a rounded input area but also a rounded undercut area. This is gentle on both base materials and fibers simultaneously. The rounded undercut also means improved retention of the fibers in the barb, leading to reduced friction and less wear. The HL shape is thus capable of retaining the original barb shape for far longer than conventional barbs. Here too, of course, the fiber fineness and barb size have to be precisely coordinated.

Example 2: Filtration of foodstuffs

  • Fibers: Viscose/PES
  • Fiber fineness: 4.4 dtex/6,7 dtex
  • Weight per unit area: 60-150 g/m²
In contrast to hot gas filtration, depth rather than surface filtration is taking place with these fluid filters. With depth filtration, a consistently even and homogenous structure across the entire product is especially important. Products are often only lightly needed, and then partially strengthened either chemically or thermally. Here, too, requirements for the needle to be used can be deduced:

Example 2: Requirements


The fibers used place no specific requirements on the needles, enabling the use here of standard needles with triangular working part and normal tips.


With depth filtration, in contrast to surface filtration, the structure of the nonwoven plays a primary role. An optimal filtration effect can only be achieved by means of a highly even and consistent structure across the entire filter cross-section. Naturally the needle gauge has to be adapted to the pore size here as well. The pores on fluid filters are usually somewhat coarser than with air filters, so that needles with gauges from 36-40 gg are generally used.

To retain the volume of the filter as far as possible but also achieve good needling at the same time, the tendency is to use needles with smaller barbs and a large or medium barb distance. The penetration density is also adjusted accordingly. The voluminous filter without base material in this example is being needled with a penetration density of roughly 50-150 E/cm2.

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The barb shape also plays an important role in filter needling. RF-barbs are often used here. They are equipped not only with a rounded undercut but also a more efficient undercut angle, enabling them to needle more efficiently and also more gently than conventional barbs. The probability of fibers leaving the barbs again during the penetration phase of the needles is thus sharply reduced. This enables a defined needling performance per penetration. The wear occurs primarily in the undercut area and not at the kick-up. This means that with RF-barbs, the original shape is preserved for longer. In a best-case scenario, the barb becomes smaller towards the point, and is thus effectively „re-sharpened". This is why RF-barbs also continue to transport fibers when other barb shapes are already worn out.

As can be seen from these two examples alone, filters and their properties and areas of application make complex demands on the production process, machines and tools. As a partner to its customers, Groz-Beckert is available at any time for consulting and discussions - for optimal design of your filter production.

By the way, you can find out about filtration in more detail in the article

"Filter technology"