Classifying – separating – and assorting solids – Fluid suspension – Gaseous
Reexamination Certificate
1999-01-20
2001-07-17
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Fluid suspension
Gaseous
C209S132000, C209S133000, C209S134000, C209S138000, C209S142000
Reexamination Certificate
active
06260708
ABSTRACT:
FIELD OF THE INVENTION
The invention described here concerns a manufacturing process for toner of a defined particle size distribution to permit electrostatically generated images to be developed, and especially concerns a classifying process to permit adjustment of the required particle size distribution as a means of achieving an ultrahigh-quality toner.
BACKGROUND OF THE INVENTION
The state-of-the-art method of manufacturing toner includes a number of processes such as the mixing of suitable components, extrusion, cooling, and downstream comminution. The comminuted material is then routed to a downstream classifying process with the aim of removing the undesirable particle fractions and of producing an end product of the desired particle size distribution. The particle size distribution (PSD) is usually measured with a Coulter Counter Multisizer made by Coulter Electronics, Inc., USA. The objective of the classifying process is generally the separation of extremely fine particles in the range under 5 &mgr;m, but is sometimes also achievement of an upper particle limit or of both objectives together.
For this purpose, conventional processes use classifiers as are known from German patent DE 39 15 641 A1. With such classifiers, in which there is no controlled product feed, the classified product can backmix with the feed product. This leads to the product becoming contaminated by undesirable fines, which adversely affects the success of the classifying process. Furthermore, the dispersion of the feed material directly upstream of the classifying chamber in these classifiers is inadequate, meaning that agglomerates can form and thus transport particles which are actually too fine into the coarse material. This type of contamination, where fine particles contaminate the end product, can lead to a loss of quality when using the toner for printed images.
SUMMARY OF THE INVENTION
The core objective of this invention is to devise a toner manufacturing process which solves the above-described problems and makes it possible to produce a toner powder with the required narrow particle size distribution in the most effective way. Another objective is to devise a process which makes it possible to reduce the fines portion in the end product. Over and above this, the objective concerns a process which is not only capable of reducing the fines portion, but also of limiting the top size, by which means a narrow particle size distribution through classifying with a controlled material feed is achieved, whereby the feed material is obtained by means of mixing, extruding and comminuting a base mixture.
The objectives of the invention are solved in that a centrifugal plate achieves a uniform distribution of the toner product across the extent of the classifying wheel, the toner product is routed by means of a controlled material feed section in gravitational direction through the classifying chamber, and that components are installed in the classifying chamber to permit controlling the residence time of the toner product, components which in the upper section of the classifying chamber quickly introduce the toner product in a homogeneous state into the classifying chamber, which in the central section of the classifying chamber permit a longer residence time of the toner product in comparison to the upper section of the classifying chamber, and which in the bottom section of the classifying chamber permit rapid discharge of the toner product from the classifying chamber.
The feed material exiting the comminution process is subjected to one or more classifying stages, dependent upon whether pure dedusting or a combination of dedusting and top-size limitation is required. If the demand is for pure dedusting, a coarse fraction which represents the end product is yielded as well as a fine fraction which can be reused. With combined dedusting and top-size limitation, a fine fraction, a coarse fraction, and a medium fraction—which represents the end product—are yielded. The other two fractions are either returned to the extrusion process or the comminution process. All-important to ensure a high precision of cut and to prevent product contamination is uniform distribution of the feed material, a controlled material feed during the classifying process, regulation of the residence time, and rapid discharge of the coarse material.
To increase the amount of product being classified, a multiple-stage classification is carried out. The coarse material from the previous classifying stage is charged to another classifier for final classification. The fines from each classifying stage can be collected in a common filter. The advantage of this method is that the loading factor can be far in excess of the otherwise standard range of between 0.05 and 0.3 kg/m
3
. An estimate of the total loading factor (&mgr;
tot
) is calculated using the following formula:
&mgr;
tot
=&mgr;
1
×n
a
with
1<a<1.6
The preferred value for “a” is 1.3. In the case of a three-stage classification (i.e., n=3), this then results in a possible loading factor (&mgr;
tot
) of between 0.2 and 0.83 kg/m
3
. Multi-stage classification is naturally also possible with the value of “a” lower than 1. This serves to optimise the product quality at maximum coarse yield.
The invention design comprises a vertical-axis air classifier equipped with a central feed section with a tangential classifying air inlet located on a level with the classifying wheel, a stationary guide vane ring surrounding the classifying wheel at a radial distance, an annular classifying chamber bounded by a deflector wheel classifying wheel supported on one side and a guide vane ring located coaxially at a radial distance to the outside periphery of the classifying wheel, a drive shaft for the classifying wheel supported on one side and a housing with fine material and coarse material discharge.
The material to be classified is charged centrally to the classifier, is then distributed over a large surface area by a centrifugal plate and routed as a uniformly distributed, bell-shaped cloud of product over the periphery of the classifying wheel past the classifying wheel vanes. The classifying air flows through the classifying wheel in a centripetal direction; the fines are routed to the inside of the classifying wheel. Gravity causes the rejected coarse material to move downwards, where it deposits in an annular-shaped coarse material discharge chamber.
The air flow pattern through the classifying chamber is centripetal. The rotating deflector wheel deflects the coarse material radially to the periphery and conveys the fines together with the classifying air to the inside of the classifying wheel. The classified fines are then deflected axially downwards and are discharged from the classifying wheel through the interrupted drive shaft to the outside.
This vertical-axis air classifier has the following components all located on the same side beneath the classifying wheel: the interrupted drive shaft, the annular fines discharge chamber located coaxially to the drive shaft, the annular coarse material discharge chamber also located coaxially to the drive shaft, and the classifier bearing.
REFERENCES:
patent: 4304360 (1981-12-01), Luhr et al.
patent: 5115989 (1992-05-01), Poeschl
patent: 39 15 641 (1990-11-01), None
patent: 40 14 342 (1991-11-01), None
Adam Marcus
Furc Hner Bodo
Zampini Stefano
Hosokawa Alpine Aktiengesellschaft
Martin Mark J.
Walsh Donald P.
Winston & Strawn
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