A variety of factors such as solids content, type of particles or operating conditions (pressure, temperature, etc.) influence the separation effect of a filter medium. In most cases, different types of filtration overlap, such as depth filtration through a filter cake that has formed during surface filtration with a screen mesh.

During the production of paints and varnishes, various impurities often end up in the product, which can be removed with the help of our sieve and filter bags:

• Pigment and filler agglomerates resulting from insufficient dispersion
• other undissolved particles (e.g. clumped pearlescent pigments)
• Dried paint residue due to inadequate cleaning
• Gel particles
• other impurities such as fibers, dust lint, hair, etc.

These impurities can not only have a negative impact on the quality of the coating, but also on the production and application process:

• Metal particles that accidentally get into the product can damage the pump,
• The nozzles on spraying units can become clogged and blocked
• Impurities in the coating can have a negative effect on the surface finish, depending on the thickness and size of the particle.

The screen fabrics used by Schwegmann consist of individual, smooth (monofilament) threads which, after heat setting (heating), retain their typical wavy shape even under pressure. The fabric thus becomes “stitch-proof”, i.e. the individual threads can no longer be displaced and therefore guarantee a definable stitch width. The mesh size initially defines the separation limit of a fabric. All particles whose diameter is larger than the mesh size used cannot pass through the fabric and are retained on the surface of the fabric. The separation limit can shift due to various influencing factors: Individual particles, which should normally pass through the mesh due to their size, form bridges that are retained by the mesh. The filter cake that forms on the fabric surface can significantly increase the separation effect. Depending on the type, quantity and distribution of the particles in the suspension, a filter cake forms more or less quickly. It is possible for gel-like particles to deform and be pushed through the smaller mesh opening. The “open screen area” also has an influence on the separation effect of the mesh. It indicates what percentage of the available area of the screen mesh is taken up by the openings (and not by the mesh thread). It should be noted here that the thread thickness plays an important role. Although the stability of the mesh increases with increasing thread thickness, the open screen area decreases and with it the separation effect. The open screen area of different fabrics with different gauges varies greatly depending on the thread thickness: while a fabric with a mesh size of 1000 µm can have an open screen area of 59%, with a mesh size of 25 µm it is only 19%. This is due to the fact that the thread thickness does not decrease proportionally to the mesh size. The thread would be too thin, the mesh not stable enough. Despite these influences on the separation effect of screen meshes, they still have a very high separation efficiency and a high reproducibility of the filtration result. The mesh size of the mesh and the size of the particles are specified in µm
(1000 µm = 1 mm). The main materials used are polyamide, polyester and polypropylene. These materials differ in their resistance to chemicals and temperatures.

We offer needle felts and various filter cartridges for depth filtration. With these filter media, there are no defined mesh sizes as with the screen cloths used for surface filtration. The fibers of these materials are arranged randomly and form pores with different pore sizes. In depth filtration, the particles are also mechanically retained on the surface, but mainly mechanically and adsorptively inside the filter medium. The example of needle felt makes it clear: it consists of individual fibers that are brought into a tangled position during production with the help of needle passages and thus consolidated. The outer side of the felt is usually flamed or calendered, i.e. thermally treated to prevent fibers from being released into the filtrate. The number, thickness and stitch depth of the needles determine the compaction of the fibers and thus the nominal separation limit of the needle felt. The particles of a suspension to be separated are now initially retained in the three-dimensional structure of the felt. It should be noted that some of these particles can pass through the felt, i.e. the actual separation efficiency of a felt is approx. 60 - 70 %. To counteract this property, a felt with a nominal fineness of 25 µm instead of 50 µm can be selected, for example. However, the three-dimensional structure of the depth filter elements also has advantages: The large number of pores offers a high solids collection capacity combined with good throughput rates. This means that although the filter element is already loaded with a high proportion of solids, the suspension still finds open pores and channels through which it can flow through the filter element. A filter cake therefore only builds up very slowly on the surface of the filter element. This in turn favors a long service life of the filter element. Depth filters often have an asymmetrical pore structure, i.e. the pores become smaller and smaller as the depth of the filter medium increases from large pores on the inflow side. This has advantages for a suspension with a large particle size distribution, as the larger particles can penetrate the filter medium and do not immediately block the surface. The random position of the fibers of a depth filter is well suited for the separation of gel-like particles, provided that a low filter speed is used.

Control sieving to protect pumps, sieving of unwanted residues from preparation tanks or pipelines, control sieving before filling, e.g. for..:

• Lubricants
• Process water treatment (e.g. removal of metal chips)
• Binder production
• Aerosols