Abstract:
The present invention relates to a screener for removing foreign substances contained in a paste, and a method for removing foreign substances contained in paste by using a screener.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a screener for removing foreign substances contained in a paste, and a method for removing foreign substances contained in paste by using a screener.  
       TECHNICAL BACKGROUND OF THE INVENTION  
       [0002]     When preparing a paste, a certain degree of foreign substances are contained in the paste. Since the foreign substance might cause defective products, it is commonly required that it be removed from the paste. For instance, a panel coated with a paste containing a foreign substance might turn out to be defective, which results in lower yield of plasma display panel (PDP) production. In recent years, screen panels are getting larger and larger, and the content of foreign substances in the paste used for the panels is strictly regulated, in order to reduce the number of defective products.  
         [0003]     For more efficient screening, of the paste, it is preferable to use a fine screen mesh. When a fine screen mesh is used to remove the foreign substance, although the screening quality is improved, much more time is required. Therefore, there is a need for fast screening methods. In particular, the screening time tends to be prolonged if the paste is highly viscous, and quick, effective screening methods are greatly in demand.  
         [0004]     JP2002-239311A disclosed a screener provided with a feeding hole to supply paste to be filtered, a squeegee which stirs the supplied paste and presses it onto the screen mesh, a screen mesh which removes the impurities in the paste, and a receiving tank which stores the paste passed through the screen mesh. In JP2002-239311A, a flat squeegee is used (refer to  FIG. 5 ).  
         [0005]     However, the use of such a screener does not reduce the screening time. The present invention addresses the need for improvements in order to achieve reduced screening time.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention relates to a screener comprising, in combination, a screen mesh which removes foreign substances in a paste, a squeegee which stirs the supplied paste and squeezes it onto the screen mesh, and a tank which stores the paste passing through the screen mesh, characterized in that the area of the underside of said squeegee parallel with the screen mesh is 10 or above when the rotation area of the squeegee is 100 arbitrary units.  
         [0007]     The present invention further relates to a method of removing foreign substances in a paste, comprising the steps of: providing a screener comprising a screen mesh which removes the foreign substances in a paste; stirring the supplied paste with a squeegee, having a top side and an underside, and squeezing, with the squeegee, the stirred paste onto the screen mesh, wherein the paste that passes through the screen mesh is stored in a tank. The area of the underside of said squeegee parallel with the screen mesh may be 10 or above when the rotation area of the squeegee is 100 arbitrary units. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1 . Sectional view of a screener  
         [0009]      FIG. 2 . Diagram of the structure around the squeegee shown in  FIG. 1 .  
         [0010]      FIG. 3 . Diagram of a squeegee.  
         [0011]      FIG. 4 . Plain view of the squeegee shown in  FIG. 3 .  
         [0012]      FIG. 5 . Rotation area of the squeegee.  
         [0013]      FIG. 6  shows the cross section of a part indicated as VI.  
         [0014]      FIG. 7  shows the underside parallel to the screen mesh in the squeegee indicated by S 2 .  
         [0015]      FIG. 8  shows a bar shaped squeegee.  
         [0016]      FIG. 9  shows the underside of the squeegee in  FIG. 8 .  
         [0017]      FIG. 10  and  FIG. 11  show increasing the width of the squeegee.  FIG. 12  shows a disc-like squeegee.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     The present invention provides a means for shortening the time required for screening out foreign substances in a paste. The screener in the present invention comprises a screen mesh to remove impurities in paste, a squeegee to stir the supplied paste and squeeze it onto the screen mesh, and a tank to store the paste passing through the screen mesh; the area of the underside of the squeegee parallel to the screen mesh is 10 or more when the rotation area of the squeegee is 100 units. The typical screen mesh size is in the range of 325 to 1,400. In one embodiment, the mesh size is 800 to 1,400 mesh.  
         [0019]     First, a sectional view of an embodiment of the screener of the present invention is shown in  FIG. 1 . The screener  10  has a screen mesh  100 , a squeegee  102 , and a tank  104 . When the paste passes through the screen mesh  100 , foreign substances such as aggregates, large particles, and foreign bodies are removed. The squeegee  102  stirs the paste supplied from a feeder. Moreover, the paste is pressed against the screen mesh  100  by the squeegee  102 . Screening of the paste is completed by forcing the paste towards the screen mesh with the squeegee  102 . The paste which passes through the screen mesh  100  is stored in a tank  104 . These are the basic components of the screener of the present invention.  
         [0020]     The screen mesh  100  serves to remove foreign substances. There is no special restriction on the shape and diameter of the screen mesh. A honeycomb-like mesh with many hexagonal openings is preferred from the viewpoint of screen mesh strength. The mesh diameter can be determined according to the size of the foreign substance to be removed. For instance, to remove impurities ≧50 μm in diameter, a screen mesh ≧50 μm can be used.  
         [0021]     The squeegee  102  stirs the paste and squeezes it against the screen mesh. In a particular embodiment, the shape of the squeegee  102  is adjustable. The shape of the squeegee  102  is described in detail below. In a further embodiment, the squeegee  102  may be movable in the up and down vertical directions. If the squeegee is movable, it can be positioned at an appropriate height according to the type of the paste and the shape of the squeegee.  
         [0022]     In an embodiment, the squeegee  102  is not in contact with the screen mesh  100 . When the screen mesh  100  is in contact with the squeegee, that is, when they are in the on-contact state, the foreign substance might be squeezed into the screen mesh  100  by direct force from the squeegee. As a result, the foreign substances that are supposed to be removed by the screen mesh  100  pass through the mesh  100  and might not be removed sufficiently. On the other hand, when the screen mesh  100  is off-contact with the squeegee, that is, they are in the non-contact state, the aforementioned problems can be avoided. However, in the non-contact mode the time required for screening is longer, due to the insufficient pressure applied to the screen. The present invention can be used to screen impurities in a comparatively short period of time even in the off-contact mode, since the time required for screening is shortened. Namely, efficient screening is made possible by using the off-contact mode. Another benefit of the off-contact mode is the prevention of wear and tear on the screen mesh  100  and squeegee  102 . If the screen mesh  100  and squeegee  102  are in contact with each other, there is a risk that powder from the squeegee generated by the wear and tear of rotation might get into the paste. Problems like this can be prevented with the off-contact mode.  
         [0023]     There is no special limitation on the gap between the screen mesh  100  and the squeegee  102  in the case of non-contact operation. If the gap is too wide, the squeezing force applied by the squeegee  102  to the screen mesh may be insufficient and the time required for screening might be long. Also, if the gap is too narrow it will be difficult to control so as to maintain an off-contact state. With these considerations in mind, it is preferred that the gap between the screen mesh  100  and the squeegee  102  be between 15 μm to 35 μm. However, the scope of the present invention is not limited to this range.  
         [0024]     The tank  104  stores the paste after passage through the screen mesh  100  to remove the impurities. There is no special restriction on the size and configuration of the tank  104 . The use of a tank  104  with a large capacity allows processing a large amount of paste at one time. It is also acceptable to install a tank drain to remove paste from the tank continuously, according to circumstances. Continuous long-time screening is possible if such a tank drain is installed.  
         [0025]     It is also acceptable to use a screener  10  with a paste feeder section  106 , paste container  108 , motor  110 , rotary shaft  112 , support plate  114  (refer to  FIG. 2 ), and vacuum unit  116 , if necessary.  
         [0026]     The feeder section  106  supplies paste to the paste container  108 . It is desirable that the feeder section  106  be isolated from the external atmosphere to prevent contamination by foreign substances from outside. There is no special restriction on the concrete configuration of the feeder section  106 . For instance, the configuration disclosed in JP2002-239311A can be adopted. JP2002-239311A is hereby incorporated herein by reference in its entirety.  
         [0027]     The paste container  108  temporarily holds the paste to be supplied to the screener before passing through the screen mesh. There is no special restriction on the size of the paste container  108 . It is sufficient to choose its size in proportion to the amount of paste to be screened.  
         [0028]     The motor  110  is the power source for rotating the squeegee. Although the squeegee can be operated manually, if necessary. It is desirable to use a motor to rotate the squeegee, because the consistency of the rotation and the screening uniformity are improved, and less manual labor is required.  
         [0029]     The rotary shaft  112  performs the task of transmitting power from the motor to the squeegee. In the mode shown in  FIG. 1 , the paste feeder section  106  surrounds the rotary shaft.  
         [0030]     The tank  104  should be connected with the vacuum unit  116  to reduce the pressure in the tank. The speed with which the paste passes through the screen mesh  100  can be improved by decreasing the pressure in the tank by using the vacuum unit  116 . Moreover, air in high-viscosity paste can be removed.  
         [0031]      FIG. 2  is a diagram illustrating the structure around the squeegee. As shown in the diagram, the screen mesh  100  is located under the paste container  108 . It is acceptable to place a support plate  114  under the screen mesh  100  to prevent shifting and distortion while the screen mesh is operating. Steady screening becomes possible with such a support plate  114 . Also, a retainer such as a clip  118  can be mounted to hold the screen mesh  100  and support plate  114  in place.  
         [0032]     A squeegee with the above properties is installed in the screener in the present invention. The type of squeegee used is such that if the rotation area of the squeegee is 100, the area of the underside of said squeegee parallel to the screen mesh is 10 or more. By using a squeegee with a large underside area parallel to the screen mesh, the time required for screening can be greatly shortened.  
         [0033]     The rotation area of the squeegee refers to the area where the squeegee rotates and moves. For instance, the case of a squeegee with the shape shown in  FIG. 3  is illustrated.  FIG. 3  is a model diagram of a squeegee in an embodiment.  FIG. 4  is a plain view of the squeegee shown in  FIG. 3 . The dotted line shows the tapered part of the underside of the squeegee. When the squeegee shown in  FIG. 4  rotates in the direction of the arrow, the rotation area (S 1 ) is the area shown in  FIG. 5 .  FIG. 6  shows the cross-section of the part illustrated as “VI”.  
         [0034]     On the other hand, the lower side parallel to the screen mesh means the side facing the screen mesh. When the screen mesh lies beneath the squeegee, it means the area under the squeegee. To illustrate with  FIG. 6 , the section indicated by A is equivalent to the underside parallel to the screen mesh. In this case, the underside area parallel to the screen mesh in the squeegee is the area indicated by the section S 2  in  FIG. 7 .  
         [0035]     The time required for screening can be greatly shortened by increasing the underside area parallel to the screen mesh when the rotation area of the squeegee is 100, calculated by the formula (S 2 /S 1 )×100.  
         [0036]     It is preferable that (S 2 /S 1 )×100 be 10 or more, more preferably ≧20, ≧30, ≧40, and ≧50, wherein the properties improve as the numbers increase, and wherein ≧50 is the most preferred.  
         [0037]     The squeegee may have various shapes known to one of skill in the art. Examples include the bar-shaped squeegee shown in  FIG. 8  and the Crisscross squeegee shown in  FIG. 3 . For purposes of reference, the underside area in the bar-shaped squeegee shown in  FIG. 8 , which is parallel to the screen mesh, is the area of the shaded section shown in  FIG. 9 . The ratio (S 2 /S 1 )×100 can be increased by increasing the width of the squeegee as shown in  FIG. 10  and  FIG. 11 .  
         [0038]     In a particular embodiment, the value of (S 2 /S 1 )×100 is increased by using the disc-like squeegee shown in  FIG. 12 .  FIG. 12  is a plain view of a disc-like shaped squeegee. The value of S 2  can be increased by using such a disc-like shaped squeegee.  
         [0039]     When using this type of disc-like shaped squeegee, there may be difficulties in feeding the paste under the squeegee, because of the large surface area of the squeegee. To solve this problem, an opening  120  may be formed in the squeegee to supply the paste from underneath. The paste is supplied through the opening  120 , which accelerates efficient screening of the paste. Alternatively, a slit  112  can be made in the periphery of the disk and paste supplied through it.  
         [0040]     The underside of the squeegee should be tapered at the edge in the direction of rotation of the squeegee, regardless of the shape of the squeegee. That is, it is preferable that the forward, rotating edges of the blades be slit downward as shown in  FIG. 3 ,  FIG. 6 ,  FIG. 9 , and  FIG. 10 . With the formation of this type of taper, the paste stirred by the squeegee is squeezed toward the underside of the squeegee, that is, onto of the screen mesh. As a result, more efficient screening is achieved.  
         [0041]     There are no particular limitations on the size of the squeegee. Nevertheless, for effective stirring and squeezing of the paste toward the screen mesh, in a particular embodiment, the squeegee size may not be much smaller than the width of the paste container. Therefore, although there is no special restriction, when the width of the paste container is 100, the width of the squeegee should be ≧80, with ≧85 better and most preferably ≧90.  
         [0042]     The size of the squeegee can be determined based on the rotation area of the squeegee to the area of the screen mesh or the width of the paste container. In this case, taking the width of the paste container or the area of the screen mesh to be 100, although there is no special restriction the rotation area of the squeegee should be ≧70 with ≧75 better and ≧80 best. There is no special restriction on the composition of the squeegee. Rubber materials such as the polyurethane or resins such as polyacetal resin are suitable. In a particular embodiment, polyacetal resin squeegees are used. In an aspect of this embodiment, polyacetal resin squeegees are used for screening in the on-contact mode. The use of polyacetal resin squeegees for screening in the on-contact mode may prevent the formation of powders generated by the wear and tear of other squeegees.  
         [0043]     In a further embodiment, polyurethane squeegees are used.  
       EXAMPLES  
       [0044]     The relation between squeegee shape and screening time is evaluated in the following experiments.  
         [0000]     Paste Preparation  
         [0045]     A silver paste was obtained by mixing the following components. The viscosity of the paste was 40-50 Pa/s.  
                                                           Polymer resin   25   wt. %           Silver powder   60   wt. %           Glass powder   5   wt. %           Solvent   5   wt. %           Additives   5   wt. %                      
 
       Comparative Example 1  
       [0046]     A 300 kg screener comprising a Crisscross-shaped squeegee, a motor, a rotary shaft, a tank, a paste container, a screen mesh, and a support plate as shown in  FIG. 3  was prepared. The screen mesh and the support plate were attached on the lower side of the paste container. The rotation area of the squeegee was 1194 cm 2 . Also, the area of the section parallel to the screen mesh under the squeegee was about 88 cm 2 . That is, taking the rotation area of the squeegee to be 100 arbitrary units, the underside area parallel to the screen mesh is 7.  
         [0047]     The squeegee was lowered until it came in contact with the screen mesh and then raised up 20 μm, so that the squeegee was in non-contact mode.  
         [0048]     Paste was placed in the paste container and the screening was started. The rotation speed of the squeegee was 75 rpm. The paste to be screened was placed into the container gradually according to the screening speed. The time required for screening about 300 kg silver paste was measured, and the processing rate (kg/hr) and the processing time (hr/Batch) were found. The processing time is the time required to process 300 kg of silver paste. The above procedure was repeated five times, and the average value of the processing rate and the processing time was calculated. The results are shown in Table 1.  
       Working Example 1  
       [0049]     A squeegee wider than the one used in the Comparative Example 1 was used (refer to  FIG. 9 ). The rotation area of the squeegee was 1194 cm 2  and the area of the section parallel to the screen mesh under the squeegee was about 240 cm 2 . That is, taking the rotation area of the squeegee to be 100, the underside area parallel to the screen mesh is 20. Except for the different squeegee used, the screening conditions were identical with those in Comparative Example 1. The results are shown in Table 1.  
       Working Example 2  
       [0050]     A squeegee wider than the one used in the Comparative Example 1 was used (refer to  FIG. 9 ). The rotation area of the squeegee was 1194 cm 2  and the area of the section parallel to the screen mesh under the squeegee was about 633 cm 2 . That is, taking the rotation area of the squeegee to be 100, the underside area parallel to the screen mesh corresponds to 53. Except that the squeegee was replaced, screening was conducted under the same conditions as in Comparative Example 1. The results are shown in Table 1.  
                                                     TABLE 1                                   Comparative   Working   Working           Example 1   Example 1   Example 2                                    Squeegee   cm 2     1194   1194   1194       rotation       area (S1)       Area of the   cm 2     88   240   633       squeegee       parallel       with the       screen mesh (S2)       Area ratio       7   20   53       ((S2/S1) × 100)       Gap   Mm   20   20   20       Rotation rate   Rpm/min   75   75   75       Processing rate   kg/hr   13.7   25.2   49.5       Operation time   hr/Batch   21.9   11.9   6.1       Processing   Modified/Initial   1   1.84   3.61       capacity                  
 
         [0051]     To facilitate comparison, the processing capacity is indicated in Table 1 relative to the processing capacity of the Comparative example 1, taken as 1. As shown in Table 1, the time required for screening can be greatly shortened by increasing the area of the underside of the squeegee. Specifically, taking the processing capacity in Comparative example 1 to be 1, (S 2 /S 1 )×100 in the working example 1 is 20 and the processing capacity will improve to 1.84. Additionally, for working example 2 with (S 2 /S 1 )×100 is 53, the processing capacity will improve to 3.61.