Abstract:
The method for air classification of toner used to develop electrostatic images, whereby a toner product consisting of a powder with a wide particle size distribution is converted into a higher-quality toner product with a narrow particle size distribution in that the residence time of the toner product is controlled by means of components, 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, and which in the bottom section of the classifying chamber permit rapid discharge of the toner product from the classifying chamber, all of which makes it possible to produce a toner end product with an extremely narrow particle size distribution.

Description:
This application is a continuation in part of application Ser. No. 08/953,721, filed Oct. 17, 1997 now U.S. Pat. No. 6,109,448. 
    
    
     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 μm, 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 (μ tot ) is calculated using the following formula: 
     
       
         μ tot =μ 1   ×n   a   
       
     
     with 
     
       
         1&lt;a&lt;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 (μ 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectional elevation of the vertical-axis air classifier described here. 
     FIG. 2 shows a schematic representation of a design variant of the vertical-axis air classifier. 
     FIG. 2A shows an alternative embodiment of a vertical-axis air classifier with a helix that has a pitch that varies along its length. 
     FIG. 3 shows a classifying process for dedusting. 
     FIG. 4 shows a schematic representation of a classifying process for combined coarse and fine classification. 
     FIG. 5 shows graphic results of using the two-stage classifier of FIG.  4 . 
     FIG. 6 shows graphic effects of controlling the residence time of the product. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With the air classifier  6  as shown in FIG. 1, the drive shaft  2  is interrupted at the point where it penetrates the fines discharge chamber  14  and is replaced there by the support  10 ; this permits the fines to discharge from the inside of the classifying wheel  8  to the fines discharge chamber  14 . 
     The support  10  comprises the bottom disc  18 , the annular disc  17 , and the streamlined spacer ribs  10   a , which together form a connecting element via bolt  12  between the drive shaft  2  and the classifying wheel  8  and the apertures for discharge of the fines from the inside of the classifying wheel  8 . 
     The classifying wheel  8  comprises the classifying wheel vane ring  9 , the centrifugal top cover plate  16  with bolt  11  and the bottom cover plate  15 , and is connected firmly to the support  10 . This connection can be in a severable design in the area of the plates  15  and  17  and can, for example, be effected by screws  19  inserted uniformly around the periphery of the classifying wheel. 
     In the area of the plates  15 , 17  and the housing  1  fluid-rinsable seal  20  shown in axial arrangement which reliably separates the classifying chamber  21  from the fines discharge chamber  14 . 
     In the axial transition area between the classifying wheel  8  and the support  10 , the bottom cover plate  15  projects over the inside periphery of the annular plate  17  and thus over the support  10  into the inner zone, thus forming an orifice plate with a throttle effect in the transition area. 
     The product is fed to the top cover plate  16  of the classifying wheel  8 , which forms a centrifugal plate. The annular channel which runs between the outside of the classifying wheel  8  and the inside of the guide vane ring  3  forms the classifying chamber  21  over the entire height of the classifying wheel  8 . 
     The feed material enters through an opening in the housing top  7  and flows through the classifying chamber  21  vertically. To permit control of both the classifying material concentration in the classifying chamber  21  and the residence time, a helix  29  extends over almost the entire radial width of the classifying chamber  21  and over the entire height of the classifying wheel  8 . In the design shown in the figure, a single helix with constant pitch is employed. 
     The flow direction of the classifying air is perpendicular to the stream of feed material. From the classifying air inlet  22 , the classifying air flows horizontally through the classifying air inlet duct  4  and the stationary guide vane ring  3  into the classifying chamber  21  and flows through the chamber at right angles to the flow of feed material. 
     The classified fines are discharged axially through the fines discharge duct  5  and the fines discharge  23  along with the classifying air. The classified coarse material is discharged through the coarse material discharge chamber  13  under the classifying chamber  21  and exits through coarse material discharge  24 . 
     The coarse material discharge ring  25  is fixed securely to the classifying wheel  8  and rotates within the coarse material discharge chamber  13 . The stationary retaining ring  26  is located above the coarse material discharge chamber  13  and is fixed securely to the housing  1 . 
     The aperture  27  for supplying the rinsing air  28  is located between the floor of the coarse material discharge chamber  13  and the coarse material discharge ring  25 . 
     In FIG. 2, the drive shaft  219  is installed above the classifying wheel  221  and projects into the product feed area. The material is charged centrally from above through the annular feed opening  220  onto the classifying wheel  221 . The material is catapulted outwards against the impact ring  222  and is thus distributed uniformly around the periphery. It then falls into the classifying gap between the vane ring  223  and the classifying wheel  221 , where it is rinsed by the classifying air. The classifying air enters the classifying chamber through the spiral housing  224 , flows through the classifying wheel  221  and exits the classifier together with the fines as a result of the gravitational force through the fines discharge  225  at the bottom while the coarse material enters the coarse material discharge chamber  227 . The vane ring  223  is equipped with one or more helixes  226  to permit control of the residence time of the material. Dependent on the application, e.g. coarse or fine classification, different vane rings can be used. 
     FIG. 2A shows an embodiment of the helix  226  that varies along its length. 
     FIG. 3 shows an invention-design classifying process for dedusting which features the above-described type of classifier. The feed material is charged from above by means of a suitable metering element  301 . The ratio of the classifying air Volume flow rate to the feed mass flow rate should range between 0.05 kg/m 3  and 0.3 kg/m 3 , and should preferably be 0.1 kg/m 3 . The classifying air enters the classifier  306  via the classifying air inlet  304  and is suction-transferred through the classifier  306  into an optional cyclone  307  and a filter  308  by means of a fan  309 . The fan  309  is adjusted so that the air flow rate at the outer edge of the classifying wheel ranges between 3 and 7 m/s. 
     In the case of fine classification, the peripheral speeds of the classifying wheel range between 40 and 65 m/s, whereas with coarse classifications, the preferred range is 20 to 45 m/s. The process described here can be operated with either a downstream cyclone  307  and a filter  308 , or just with a filter  308 . The classified fines are first of all routed to the cyclone  307  via the fines ducting  318 , where the main portion is separated from the classifying air and discharged via the fines discharge  303 . Ultrafine particles still entrained in the classifying air deposit in the filter  308  and can be removed via the dust discharge  305  once the filtrate has been detached from the walls of the filter element. The end product is yielded at the coarse material discharge  302 . 
     The process described in FIG. 3, therefore, is a simple method of fine classification for dedusting purposes, whereby as an option, a combined coarse classification and fine classification permitting simultaneous dedusting and top-size limitation can be carried out as detailed in FIG.  4 . 
     FIG. 4 shows such a system configuration for a combined coarse and fine classification and a two-stage classification process. 
     With the combined coarse and fine classifying process, the material is first of all subjected to a fine classification as per FIG.  3 . Thus, the first classifying stage includes metering element  401 , first classifying air inlet  404 , first classifier  406 , first coarse material discharge duct  402 , first fines duct  418 , first cyclone  407 , first fines discharge  403 , first filter  408 , first dust discharge  405  and first fan  409 . Similarly, the second classifying stage includes second classifying air inlet  414 , second classifier  410 , second coarse material discharge duct  415 , second fines duct  419 , second cyclone  411 , second fines discharge  416 , second filter  412 , second dust discharge  417  and second fan  413 . The feed product for the second classifying stage is the coarse material from the first classifying process and is charged to the feed section of the second classifier  410  via the coarse material discharge ducting  402  of the first classifier. The product is then subjected to a coarse classifying process in order to limit the top size, i.e. the fines exiting the second classifying process through the cyclone  411  and the filter  412  are yielded at the fines discharge  416  and the dust discharge  417 , respectively, and represent the actual end product. The extremely coarse portions, whose particle sizes are above the desired top-size limit, are discharged via the coarse material discharge  415  and can be rejected or returned to the grinding process. 
     The combination of fine and coarse classification in one system configuration means that there is no need for intermediate discharge of the product, because it can be charged direct from the coarse material discharge  402  to the second classifier  410 . It is also possible to start off with the coarse classifying process and after discharging the material, to charge it to the second classifier for fine classification. 
     The system configuration shown in the FIG. 4 can be used in the same way for a two-stage fine classification. In this case, however, it is not the medium fraction from the fines discharge  416  and the dust discharge  417  which is yielded as the end product, but rather the coarse material from the coarse material discharge  415 . Provided that the classifier settings are selected appropriately, an extreme dedusting of the product can be achieved. By adding more stages in an analogous manner, this configuration can be extended to form a multi-stage fine classification system. 
     FIGS. 5 and 6 represent particle size distributions measured with a Coulter Counter Multisizer from Coulter Electronics, Inc. (USA). Dedusting, where as much of the fine portion under 5 μm as possible was removed, was the objective here. 
     The feed material displays an extremely high portion of fines and because of this, represents inferior-quality toner. FIG. 5 shows how the product is improved by applying an invention-design, two-stage fine classification as per FIG.  4 . Conspicuous is that the bulk of the ultrafine dust below 5 μm is separated as early as the first classification stage. The second stage succeeds in reducing the portion of ultrafine dust by about another 50%. The end product displays an extremely high gradient just above the cut point of 5 μm. Proof positive that the invention-design process accomplishes a separation with an extremely sharp precision of cut. 
     FIG. 6 shows two particle size distribution curves, one achieved with an invention-design classification process with controlled residence time of the product, the other achieved in a classification process without such a controlled residence time, whereby the classifier types and settings were identical in each case. At otherwise identical classifying conditions, controlling the residence time of the product in the classifying chamber makes it possible to separate considerably more ultrafine dust from the product.