Title:
Paste screener
Kind Code:
A1


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.



Inventors:
Hisakawa, Ikuo (Yokohama-Shi, JP)
Iida, Shoichi (Yokohama-Shi, JP)
Tsai, Ymir (Taoyuan, TW)
Application Number:
11/431952
Publication Date:
11/15/2007
Filing Date:
05/11/2006
Primary Class:
International Classes:
E02B5/08
View Patent Images:
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Primary Examiner:
NGUYEN, ANTHONY H
Attorney, Agent or Firm:
Du Pont I, De Nemours And Company Legal Patent Records Center E. (BARLEY MILL PLAZA 25/1128, 4417 LANCASTER PIKE, WILMINGTON, DE, 19805, US)
Claims:
What is claimed is:

1. A screener for removing foreign substances in a paste comprising: a screen mesh which removes the 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, said tank 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.

2. A screener as set forth in claim 1, equipped with a vacuum unit for reducing the pressure in the tank.

3. A screener as set forth in claim 1, wherein the squeegee is off-contact with the screen mesh.

4. A screener as set forth in claim 1, wherein the area of the underside of said squeegee is 20 or more when the rotation area of the squeegee is 100 arbitrary units.

5. A screener as set forth in claim 1, wherein the area of the underside of said squeegee is 50 or more when the rotation area of the squeegee is 100 arbitrary units.

6. A screener as set forth in claim 1, wherein the squeegee is bar-shaped.

7. A screener as set forth in claim 1, wherein the squeegee is Crisscross-shaped.

8. A screener as set forth in claim 1, wherein the squeegee is disc shaped.

9. A screener as set forth in claim 8, wherein the paste is supplied through an opening in the underside of the squeegee.

10. A screener as set forth in claim 1, wherein the underside of the squeegee has a tapered edge in the direction of rotation.

11. A screener as set forth in claim 1, wherein the squeegee is made of polyacetal resin.

12. A method of removing foreign substances in a paste, comprising the steps of: (a) providing a screener comprising a screen mesh which removes the foreign substances in a paste, (b) stirring the paste of (a) with a squeegee, said squeegee having a top side and an underside, and (c) 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 and wherein 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.

13. The method of claim 12, wherein the screener is equipped with a vacuum unit for reducing the pressure in the tank.

14. The method of claim 12, wherein the squeegee is off-contact with the screen mesh.

15. The method of claim 12, wherein the area of the underside of said squeegee is 20 or more when the rotation area of the squeegee is 100 arbitrary units.

16. The method of claim 12, wherein the area of the underside of said squeegee is 50 or more when the rotation area of the squeegee is 100 arbitrary units.

17. The method of claim 12, wherein the squeegee is bar-shaped.

18. The method of claim 12, wherein the squeegee is Crisscross-shaped.

19. The method of claim 12, wherein the squeegee is disc shaped.

20. The method of claim 12, wherein the paste is supplied through an opening in the underside of the squeegee.

21. The method of claim 12, wherein the underside of the squeegee has a tapered edge in the direction of rotation.

22. The method of claim 12, wherein the squeegee is made of polyacetal resin.

23. A paste screened by the method of claim 12.

24. A paste passed through the screener of claim 1.

Description:

FIELD OF THE INVENTION

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

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.

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.

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).

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

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.

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

FIG. 1. Sectional view of a screener

FIG. 2. Diagram of the structure around the squeegee shown in FIG. 1.

FIG. 3. Diagram of a squeegee.

FIG. 4. Plain view of the squeegee shown in FIG. 3.

FIG. 5. Rotation area of the squeegee.

FIG. 6 shows the cross section of a part indicated as VI.

FIG. 7 shows the underside parallel to the screen mesh in the squeegee indicated by S2.

FIG. 8 shows a bar shaped squeegee.

FIG. 9 shows the underside of the squeegee in FIG. 8.

FIG. 10 and FIG. 11 show increasing the width of the squeegee. FIG. 12 shows a disc-like squeegee.

DETAILED DESCRIPTION OF THE INVENTION

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (S1) is the area shown in FIG. 5. FIG. 6 shows the cross-section of the part illustrated as “VI”.

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 S2 in FIG. 7.

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 (S2/S1)×100.

It is preferable that (S2/S1)×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.

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 (S2/S1)×100 can be increased by increasing the width of the squeegee as shown in FIG. 10 and FIG. 11.

In a particular embodiment, the value of (S2/S1)×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 S2 can be increased by using such a disc-like shaped squeegee.

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.

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.

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.

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.

In a further embodiment, polyurethane squeegees are used.

EXAMPLES

The relation between squeegee shape and screening time is evaluated in the following experiments.

Paste Preparation

A silver paste was obtained by mixing the following components. The viscosity of the paste was 40-50 Pa/s.

Polymer resin25wt. %
Silver powder60wt. %
Glass powder5wt. %
Solvent5wt. %
Additives5wt. %

Comparative Example 1

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 cm2. Also, the area of the section parallel to the screen mesh under the squeegee was about 88 cm2. That is, taking the rotation area of the squeegee to be 100 arbitrary units, the underside area parallel to the screen mesh is 7.

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.

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

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 cm2 and the area of the section parallel to the screen mesh under the squeegee was about 240 cm2. 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

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 cm2 and the area of the section parallel to the screen mesh under the squeegee was about 633 cm2. 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
ComparativeWorkingWorking
Example 1Example 1Example 2
Squeegeecm2119411941194
rotation
area (S1)
Area of thecm288240633
squeegee
parallel
with the
screen mesh (S2)
Area ratio72053
((S2/S1) × 100)
GapMm202020
Rotation rateRpm/min757575
Processing ratekg/hr13.725.249.5
Operation timehr/Batch21.911.96.1
ProcessingModified/Initial11.843.61
capacity

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, (S2/S1)×100 in the working example 1 is 20 and the processing capacity will improve to 1.84. Additionally, for working example 2 with (S2/S1)×100 is 53, the processing capacity will improve to 3.61.





 
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