Method for the pneumatic classification of toner

The present invention relates to the production process of toners of a particular particle size distribution for the development of electrostatically generated images, and more particularly to a visualization process for adjusting the required particle size distribution as a means of achieving excellent toner product quality.

The preparation of prior art toner involves blending appropriate components, extruding, cooling, and subsequently comminuting. The crushed material is then sent to a sighting to remove unwanted particulate fractions and to obtain a product with the desired particle size distribution. The particle size distribution (PGV) is usually measured on a Coulter Counter Multisizer from Coulter Electronics, Inc. (USA) and is a number distribution. The aim of the sighting is usually a separation of very fine particles in the range below 5 microns, but sometimes also a Oberkornbegrenzung or both together.

Conventional processes use for this purpose classifiers, such as from DE 39 15 641 A1 are known. In such a classifier, in which there is no directional product guide, the already viewed product can be mixed again with the feed material. It comes thereby to a contamination of the product with unwanted fines, which affects the success of the sighting. These classifiers also do not have sufficient dispersion of the feed material directly in front of the viewing zone, so that it can come to agglomerates, which can actually get too fine particles in the coarse material. This contamination of the product with fine particles can lead to a loss of quality in the printed image when used as a toner.

The object of the invention is to provide a process for the production of toner which solves the problems described above and makes it possible to produce a toner with a required, narrow particle size distribution as effectively as possible. Another object is to provide a process that allows to reduce the fines of the product. Furthermore, it involves a process in which both the fines fraction can be reduced and the oversized grains can be limited, thus achieving a narrow particle size distribution by directional material guidance, with the starting material being obtained by blending, extruding and crushing a master batch.

The object of the invention is achieved by that a uniform distribution of the toner product over the circumference of the classifier wheel is achieved by a diffusing screen, the toner product is guided in a directional material guide in the direction of gravity through the viewing zone and are provided for controlling the residence time of the toner product means in the viewing zone, which, in the upper region of the viewing zone, quickly introduce the toner product in the homogenous state into the viewing zone, which allow the toner product to linger longer in the central region of the viewing zone and allow a rapid removal of the toner product from the viewing zone in the subsequent lower region of the viewing zone.

The feed material, coming from the comminution process, is subjected to one or more sightings, depending on whether only dedusting or a combined dedusting / upper grain boundary is required. With dedusting alone, you get a coarse fraction that represents the product and a fine fraction that can be recycled. In a combined dedusting / Oberkornbegrenzung one receives a fine fraction, a coarse fraction and a middle fraction, which represents the product. The two other fractions are fed back either to extrusion or comminution. During the sighting, attention must be paid to an even distribution of the feed material, a directed material guidance during the sighting, a control of the dwell time and a rapid removal of the coarse material in order to ensure a clear sighting and to prevent contamination of the material to be seen.

To increase the amount of product to be observed, a multiple sighting is performed. Here, the coarse material of the previous sighting is again given to a classifier and re-sighted. The fines can be collected from each vision level in a common filter. The advantage of this procedure is that the loading far exceeds the usual range of 0.05 to 0.3 kg / m³ can lie.

An estimate of the total load is made according to the formula:<maths id="math0001" num=""><math display="block"><mrow><msub><mrow><mtext>μ</mtext></mrow><mrow><mtext>ges</mtext></mrow></msub><msub><mrow><mtext> = μ</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><msup><mrow><mtext> xn</mtext></mrow><mrow><mtext>a</mtext></mrow></msup><mtext> with 1 &lt;a &lt;1.6</mtext></mrow></math><img file="EP0933142A2_D0001.tif" /></maths>

Preferably, the value of 1.3 is chosen for a. This results in a possible loading of 0.2 to 0.83 with a triple sighting. Of course, a multiple sighting in the range of a &lt;1 is possible. This optimizes the product quality with maximum coarse material accumulation.

According to the invention, a vertical-axis air classifier is used, the one central property task with tangential, arranged at the level of the sifter sight air intake, a fixed vane ring arranged radially at the circumference of the sifter rotor, an annular, by a blade rotor-classifying rotor mounted on one side and a guide space arranged coaxially at a radial distance from the outer circumference of the classifier rotor, Having a drive shaft for the cantilevered classifier rotor and a housing with fine material and Grobgutaustritt.

The material to be sighted is placed centrally and distributed by a spreading plate surface and bell-shaped over the outer circumference of the sifter rotor as evenly distributed Gutschleier past the Sichterrad-blades over. The separator wheel is flowed through from the outside to the inside by the view air and the fines are directed into the interior of the separator rotor. The rejected coarse material continues to follow gravity and is absorbed by an annular Grobgutaustragsraum.

The viewing zone is flowed through by the classifying air radially from outside to inside. The coarse material is rejected radially outward by the rotating paddle wheel and the fine material is transported together with the classifying air into the interior of the classifying rotor. The sighted fines are then deflected in an axially downward direction and then discharged from the sifter rotor through the perforated drive shaft.

In this vertical axis air classifier, both the apertured drive shaft, the annular coaxial to the drive shaft arranged fines discharge space, as well as the annular to the drive shaft coaxially arranged Grobgutaustragsraum, and the Sichterradlagerung are arranged on the same side and below the sifter rotor.<ul id="ul0001" list-style="none" compact="compact"><li>Fig.1 shows the used vertical axis air classifier.</li><li>2 shows a construction variant of the vertical axis wind sifter.</li><li>3 shows a visual process for dedusting.</li><li>4 shows a viewing process for a combined coarse and fine sighting.</li></ul>

In a separator according to FIG. 1, the section of the drive shaft penetrating the fine material discharge space is shown <b>2</b> as a broken shaft part, the support device <b>10</b>, trained and thereby allows the passage of the fine material from the interior of the Sichterrades <b>8th</b> into the fines discharge room <b>14</b>,

This support device <b>10</b> includes the bottom plate <b>18</b>, the ring disk <b>17</b> , as well as the streamlined spacer bars <b>10a</b> and form a link between the drive shaft <b>2</b> and classifier wheel <b>8th</b> and the passages for the discharge of the fine material from the inside of the classifier wheel <b>8th</b>,

The Sichterra<b>d 8</b> consists of the classifier wheel rim <b>9</b>, the spreading disc <b>16</b> and the cover disk <b>15</b> and is with the support device <b>10</b> rotatably connected. This connection in the area of ​​the discs<b>15</b> and <b>17</b> can be designed detachable and z. B. by evenly over the circumference of the sifter rotor arranged screws<b>19</b> respectively.

In the area of ​​the discs <b>15.17</b> and the housing <b>1</b> is a flushable with a fluid seal <b>20</b> shown in an axial arrangement and reliably separates the viewing space <b>21</b> from the fines exit space <b>14</b>,

In the axial transition region between Sichterrad <b>8th</b> and support device <b>10</b> protrudes the lower cover disk <b>15</b> over the inner diameter of the annular disc <b>17</b>, and thus on the support device <b>10</b> into the interior and thus forms an aperture with throttle effect in the transition region.

The material task takes place on the cover disk <b>16</b> the Sichterrades <b>8th</b>which forms a lens. The annular channel between the outer diameter of the classifier wheel<b>8th</b> and the inner diameter of the vane ring <b>13</b> forms over the height of the classifier wheel <b>8th</b> the visual space <b>21</b>,

The visual space <b>21</b> is flowed through by the Sichtgut in the vertical direction. For controlling both the concentration of visual matter in the viewing area<b>21</b>, as well as the residence time extends a helix <b>29</b> almost over the entire radial width of the viewing space <b>21</b> and runs over the entire height of the Sichterrades <b>8th</b> time. In the illustrated embodiment, a single spiral helix with constant pitch is used.

The view air flow runs perpendicular to the product flow. The classifying air passes from the visible air inlet<b>22</b> horizontally through the fixed vane ring <b>3</b> into the classifier room <b>21</b> and flows through it perpendicular to the Sichtgutstrom.

About the fines exit <b>23</b> the fined material is discharged axially with the classifying air. The classified coarse material is passed through the coarse material discharge space<b>13</b> below the visual space <b>21</b> about the coarse material exit <b>24</b> discharged.

The coarse material discharge ring <b>25</b> is fixed with the classifier wheel <b>8th</b> connected and rotated within the Grobgutaustragsraumes <b>13</b>, Above the coarse material discharge area<b>13</b> is the fixed retaining ring <b>26</b> arranged and with the housing <b>1</b> firmly connected.

Between the bottom of the coarse material discharge space <b>13</b> and the coarse discharge ring <b>25</b> is the gap <b>27</b> for the introduction of the scavenging air <b>28</b>,

According to FIG. 2, the drive shaft is arranged above the classifying wheel and penetrates the product feed area. The material feed takes place centrally from above through the annular feed opening<b>220</b> on the classifying wheel <b>221</b>, The material becomes outward against the baffle ring<b>222</b> hurled and distributed so evenly on the circumference. It then falls into the viewing gap between the blade ring<b>223</b> and classifying wheel <b>221</b>where it is flushed by the classifying air. The classifying air passes through the volute casing<b>224</b> into the viewing area, flows through the classifying wheel <b>221</b> and leaves the classifier with the fines down in the direction of gravity through the fines exit <b>225</b>, The scoop wreath<b>223</b> is with one or more spiral flights <b>226</b> equipped to control the residence time of the material. For various visual tasks, eg. B. coarse or fine sighting and different blade rings can be used.

An inventive dedusting process incorporating the classifier described above is shown in FIG. The task is carried out from above by a suitable dosing<b>301</b>, The ratio of the view air volume flow to the feed mass flow should be in the range of 0.05 kg / m³ up to 0.3 kg / m³, preferably at 0.1 kg / m³ lie. The classifying air enters through the visible air inlet<b>304</b> in the sifter <b>306</b> and is by means of a blower <b>309</b> through the sifter <b>306</b> in an optional cyclone <b>307</b> and a filter <b>308</b> sucked. The fan<b>309</b> is adjusted so that there are air velocities in the range of 3 m / s to 7 m / s at the outer edge of the classifying wheel.

The circumferential speeds of the classifying wheel in a fine classification are between 40 m / s and 65 m / s, with a rough view, they are preferably in the range of 20 m / s to 45 m / s. The process mentioned here can be either with a downstream cyclone<b>307</b> and a filter <b>308</b> or just with a filter <b>308</b> operate. About the fines line<b>318</b> The selected fines are first the cyclone <b>307</b> supplied and the essential part separated from the classifying air and can through the fines <b>303</b> be discharged. Ultrafine particles, which were still entrained with the classifying air, settle on the filter<b>308</b> off and can after blowing off the filter material on the dust discharge <b>305</b> be dissipated. The finished product is about the coarse material discharge<b>302</b> receive.

It thus presents itself in accordance with FIG. 3, the possibility of a simple Feinsichtung for dust separation, but optionally, a combined Grobsichtung and fine sighting for simultaneous dedusting and Oberkornbegrenzung shown in FIG. 4 are performed.

Fig. 4 shows such a system scheme for a combined coarse and fine sighting and a double fine sighting.

In the case of the combined coarse and fine classification, the material is first subjected to a fine classification analogous to FIG. The product to be processed is the coarse material from the first sighting and is sent via the line of the first coarse material discharge<b>402</b> the product task of the second classifier <b>410</b> fed. In the second sighting, the feedstock is subjected to a coarse sighting to limit the upper grain, ie the fines from the cyclone obtained in the second sighting<b>411</b> and the filter <b>412</b> is in each case via the fines discharge <b>416</b> and the dust discharge <b>417</b> The very coarse fractions, whose grain diameters are above the desired upper grain limit, are transferred via the coarse material discharge <b>415</b> discharged and can be discarded or, for example, fed to a new grinding.

By the application of fine-sighting and coarse-sighting in one arrangement, an intermediate separation of the product, since it is made from the coarse-product discharge, is unnecessary <b>402</b> directly on the second classifier <b>410</b> can be abandoned. It is also possible to carry out the first coarse sighting and, after deposition of the material and task on the second classifier, to connect the fine sighting.

The system structure according to FIG. 4 can be used in the same way for a two-level fine-sighting. However, it is not the middle fraction from the fines discharge<b>416</b> and the dust discharge <b>417</b> obtained as a product, but the coarse material from the coarse material discharge <b>415</b> represents the product. By the appropriate choice of the classifier settings an extreme dedusting of the product can be achieved. This structure can be extended to a multiple Feinsichtung by adding more levels in an analogous manner.

Diagrams 1 and 2 show the particle size distribution curves measured with a Coulter Counter Multisizer from Coulter Electronics, Inc. (USA). The goal was a dedusting by as complete as possible separation of fines below 5 microns.

The starting material has a very high content of fines and thus represents a toner of inferior quality. Diagram 1 shows the product improvement as represented by a two-stage fine-vision according to the invention according to FIG. 4 is reached. It can be clearly seen that even the first sighting can separate the vast majority of fine dust particles below 5 μm. Through the following second fine classification, the proportion of fine dust is again increased by approx. 50% reduced. The finished product has a very high slope just above the cut-off of 5μm. The inventive method thus a very sharp separation is achieved.

In Diagram 2, the particle size distribution curves of a sighting inspection of the present invention with controlled dwell time are compared to a sighting without such control using the same class of sifter and same classifier settings. By controlling the residence time of the visible material in the viewing zone, it is possible, with otherwise identical viewing conditions, to separate considerably more fine dust from the material to be visualized.