Classifying – separating – and assorting solids – Sifting – Plane sifters
Reexamination Certificate
2000-05-08
2001-09-11
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Sifting
Plane sifters
C209S309000, C209S326000
Reexamination Certificate
active
06286687
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of screening bulk materials and liquids, in particular also coating powder particles, and to an apparatus for effecting the method.
Screening of bulk material is generally used to remove impurities (anti-fouling screening) as well as to separate bulk material into fractions of a specific particle size or of a specific particle size range (grading). Bulk materials may be, for example, granulates, powder and lacquers.
When manufacturing lacquers in powder form with the aid of extrusion techniques, the resulting coarse-grained material is ground and then undergoes a screening process, e.g. to remove unwanted oversize material and impurities.
Various screening techniques are known, wherein the screen may in addition also be excited at ultrasonic frequency.
Thus, for example, eddy current screens, wobble screens, vibrating wobble screens and turboscreens are known.
In the case of eddy current screens, the material to be screened is situated in a screening tube and pressed by a whirling motion against the screening cloth. Here, especially in the case of fine-grained material, clogging and hence impeding of the screening process may occur.
In the case of wobble and vibration screening, the screens which are used are set in motion e.g. by electromagnetic drives. The wobble screens operate with firmly clamped screens, which have to be clamped with a high expenditure of force over the screen frame.
Ultrasound may be applied to generate an additional vibration-like motion of the screening cloth. This leads to heating of the screening cloth with the result that, especially in the case of fine-grained material with a tendency to clog, the screening cloth may choke up completely.
With the described conventional screens, high costs are generally incurred by replacement covering and cleaning work owing to the amount of labour and time involved. The screening capacity does not reach the desired values.
DD-A 227 891 and DD-A 270 476 describe spraying cabins for powder coatings of workpieces, wherein the cabin floor is designed as a vibrating screen deck. The screen pack of said deck may, for example, have a diameter of 200 mm and a mesh width of 200 &mgr;m. An external vibration motor is used as a drive. The maximum amplitude of the screening process has a value of 0.2 mm, wherein the vibration is infinitely variable. A similar principle is described, for example, also in EP-A-0 611 605. The described screening apparatuses are installed as a compact structural unit directly in the spraying cabin and are used for the recovery and recirculation of powder overspray.
The screen decks require a complex construction to enable the necessary angle of inclination. They are not suitable for mobile use.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to discover a method and an apparatus for the screening of coating powders, in particular, which enable an easy-to-operate and hence cost-saving screening process which is independent of special constructions, has a high screening capacity and entails a low and easily effected cleaning effort and expenditure of energy.
Said object is achieved by means of a technically simplified anti-fouling screening and grading method, which is characterized in that the screening cloth used for screening is held in a loose screen covering and during the screening process may be supported on a co-vibrating brace.
The invention therefore provides a method of screening by vibration screening, which is characterized in that a screening cloth is used, which is laid loosely onto a screen frame and fixed to a clamping frame, wherein the screening cloth may additionally be supported on a brace which co-vibrates with the screening cloth.
The screening cloth may be set in vibration-like motion by a conventional drive, e.g. a pneumatic, electric and/or hydraulic drive. Simultaneously with the screening cloth, the brace also co-vibrates. By virtue of the generated vibrating motion of screening cloth and brace, impurities or oversize material may be removed in a desired manner from a bulk material, e.g. coating powder particles, or a liquid material, e.g. A aqueous dispersion or suspension, deposited onto the screening cloth.
According to the invention, the anti-fouling screening or grading is effected by means of a loose screen covering. A loose screen covering means that the screening cloth lies loosely above a screen frame and is loosely fixed by a clamping frame on the screen frame. In said case, the clamping frame is simply laid on top of the screening cloth resting on the screen frame and is fixed between screen frame and clamping frame. Such a loose covering is intended merely to prevent slipping of the screening cloth. An additional tight clamping of the screening cloth by, for example, the additional action of lateral force upon the screening cloth is not necessary.
The co-vibrating brace according to the invention, on which the screening cloth may be supported during the screening process, is preferably integrated in the screen frame. The brace may be provided e.g. in the form of bars or grids in any desired manner, e.g. parallel or diagonally, on the screen frame and be made of any desired material.
It may also be situated as an independent component between screening cloth and screen frame, wherein the screening cloth is preferably situated above the brace.
The vibrating brace is designed and disposed in such a way that no impeding of the screening process and hence no impairment of the screening capacity arises, rather an intensification of the screening process and an increase of the screening capacity is achieved.
The brace is preferably installed in the screen frame situated below the screening cloth in the form of a bar disposed parallel to the screen frame or in the form of a plurality of bars.
During the screening process the screening cloth is preferably supported on the brace.
The usable drive is connected by suitable devices to the screening apparatus. Use is preferably made of an electrically controllable drive, for example, an unbalance motor.
In the case of an electric drive, the frequency of the motion may be controlled by a potentiometer and lie e.g. in a range of greater than 0 up to 480 Hz.
Control of the electric drive is preferably effected continuously.
The invention further provides a vibration screening apparatus, which is suitable for effecting the method according to the invention and is characterized in that it comprises a screening cloth, which is laid loosely on a screen frame and fixed to a frame (hereinafter referred to as clamping frame) and which may be supported on a brace so designed that it may co-vibrate with the screening cloth. The clamping frame in said case serves merely to hold the screening cloth on the screen frame, without clamping of the screening cloth being required. The screening cloth is therefore provided with a low (loose) screen tension.
The apparatus according to the invention may in particular comprise a holding device, a lid with devices for the product feed, for the discharge and for the drive, as well as a screen frame with a screening cloth and a clamping frame.
REFERENCES:
patent: 4224146 (1980-09-01), Kent et al.
patent: 5037536 (1991-08-01), Koch et al.
patent: 270 476 A1 (1989-08-01), None
patent: 195 34 629 A1 (1997-03-01), None
patent: 0 611 605 A1 (1994-08-01), None
Derwent WPI, “Power Coating Spray Cabin . . . Plastics Covered Light Frames”, Abstract DD 270476 A.
Derwent WPI, “Swing Sieve for Classifying Particulates . . . and Working Cycle is Increased”, Abstract DE 195 34 629 A.
Derwent WPI, “Powder Coating Appliancefor Workpieces . . . and Transportation System”, Abstract DD 227 891 A.
Beauchaine Mark J.
DuPont Performance Caotings GmbH Co, KG
Pillsbury & Winthrop LLP
Walsh Donald P.
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