For reliable and reproducible results in the context of quality assurance, the use of test sieves which correspond to the standards ISO 3310 or ASTM E11 is essential. The technical requirements and monitoring for test sieves are laid down in these standards. If a laboratory carries out quality control in accordance with ISO 9000 ff, the sieve shakers, test sieves and all other involved instruments (e.g. scales) have to be subjected to test agent monitoring. The standard ISO 3310 stipulates which tolerances are allowed for the wire diameter (d) of the woven sieve fabric and for the nominal mesh width (w) of the apertures.
For each mesh width w the value y is defined which indicates how much the mean real mesh width may differ from the nominal mesh width. This can be demonstrated with a 63 μm sieve (fig. 21). The tolerance y for a sieve with the nominal mesh width 63 μm is ±3.7 μm. This means that the mean value of the mesh sizes must lie between 59.3 μm and 66.7 μm. A look at the graph that helps to understand why the knowledge of the real mesh width is so important for sieve analysis: if the mesh size is 63 μm, 40% of the sample are smaller than 63 μm. If, however, the mean real mesh width is 66.7 μm, 44% of the particles are smaller than 66.7 μm. If a user does not know the real mesh width, he would assume that 44% of the sample is smaller than 63 μm.
In addition to the requirements laid down by the standard, RETSCH test sieves offer significant benefits:
The unique manufacturing process of RETSCH test sieves with a one-piece stainless steel sieve frame guarantees unrivaled stability and consistency for your sieving application. Paying close attention to mesh size and other specific requirements, the sieve fabric is precisely joined into the frame. Using a unique technology, which is only found in RETSCH test sieves, the fabric is then permanently and reliably tautened. The complete sieve is made from high alloy corrosion-resistant steel and is suitable for all areas of laboratory analytics, including pharmaceuticals and food. Each individual sieve passes a close optical inspection process ensuring conformity to standards. The sieve data (nominal mesh width, serial number, manufacturer, standard, dimensions) are laser engraved on the sieve frame and cannot be manipulated nor removed.
The 3-D sieving motion of the vibratory sieve shakers is generated by a spring-mass system which is activated by anelectromagnetic drive. All sieve shakers mentioned above (except for the AS 450 basic) can be used for dry and wet sieving. When selecting an instrument, the suitable sieve diameters and the feed capacity (load) have to be taken into account.
All parameters such as vibration height, sieving time and interval operation can be selected and save digitally. All instruments feature a serial interface and can be controlled with RETSCH’s evaluation software EasySieve®. These sieve shakers are activated in their natural frequency, i.e. the sieving frequency is independent of the power frequency. The microprocessor-controlled measuring and control unit ensures a constant vibration height and thus highly reproducible sieving results . To ensure reproducibility of the results even in short-time sieving processes, the default setting of the amplitude A can be switched to sieve acceleration g. This is called sieving with equal acceleration. This means in practice: as RETSCH sieve shakers are activated in their natural frequency, they are independent of the power frequency. The natural frequency of a sieve shaker is influenced by factors such as load (weight of the sieve stack). With increasing load, the natural frequency decreases. When the amplitude has been set, the K value changes. To keep the K value constant and ensure the optimum throwing motion, the sieve acceleration g’ can be preset. If the sieve load increases, it compensates the lower natural frequency with a higher amplitude. The result is a consistent sieve acceleration, i.e. the energy input remains constant.
Air jet sieving is a method where the sieve itself is not moved during the process. The material on the sieve is moved by a rotating jet of air: A vacuum cleaner which is connected to the sieving machine generates a vacuum inside the sieving chamber and sucks in fresh air through a rotating slit nozzle. When passing the narrow slit of the nozzle the air stream is accelerated and blown against the sieve mesh, dispersing the particles. Above the mesh, the air jet is distributed over the complete sieve surface and is sucked in with low speed through the sieve mesh. Thus the finer particles are transported through the mesh openings into the vacuum cleaner or, optionally, into a cyclone.
The tap sieve shaker combines horizontal circular motions via an eccentric with vertical taps generated by a tapping arm. As the number of rotations and taps is fixed, only thsieving time can be digitally adjusted.
The base plate of the AS 400 control performs horizontal circular motions via an eccentric. Speed, sieving and interval time (alternating rotation direction) can be set, displayed and monitored digitally. The AS 400 control is suitable for sieve diameters of up to 400 mm which makes it ideal for coarse bulk materials such as construction materials.