Liquid purification or separation – Filter – Movable medium
Reexamination Certificate
1999-07-28
2001-04-17
Walker, W. L. (Department: 1723)
Liquid purification or separation
Filter
Movable medium
C210S390000, C210S429000, C210S432000, C210S433100, C210S435000, C210S498000
Reexamination Certificate
active
06216880
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a filter device for separating unwanted particulate material from molten or flowable plastic material, such as thermoplastic polymer, wherein a stream of plastic material having particles, such as dirt, lumps and/or carbonised polymer entrained therein, is fed to the inlet side of the filter device, for processing.
More precisely the invention relates to an automatic screen changer particularly suitable for production of pellets in recycling of plastics, in which use is made of a filter element comprising a filter band or wire net which is step advanced or moved throughout a filter chamber, across a flow path for the plastic material to be processed and cleaned from the unwanted particles.
RELATED PRIOR ART
It is known that in recycling plants for recovery of plastics, in particular polymer regeneration plants, filtration assemblies capable of ensuring the homogeneity and the purity of the processed material are increasingly required. It is therefore usual practice to dispose a filter element across the stream of molten plastic material, at the outlet side of an extruder to remove the particulate material mixed therein.
As the filter retain more and more of the filtered particles, whatever filtering element is adopted inevitably the same becomes soiled and clogged, thus requiring replacement of the clogged filtering element with a new clean one.
Many types of filtering devices in the past have been proposed for plastic materials, in which the replacement of the filtering element is performed by different procedures, according to whether the filter device is of the slide plate type or of counter-flow type; desirably, changing of the filter should not interfere or negatively affect the working of a plant, and should be performed in a very short time.
Furthermore, in plants for the recovery of waste plastics, in which the economical value of the recovered material is very poor, the automation or the use of automatic screen changers requires filter devices having a simple design and low cost, involving a limited number of operators for the maintenance and use.
Filter devices of the slide type are described for example in U.S. Pat. No. 4,167,384, U.S. Pat. No. 4,358,262 and U.S. Pat. No. 4,701,118; according to these documents a slider or each slider carries one or more filter packs that can be reciprocated between a first position in which the filter or filters register with a flow path for the molten plastic material, and one additional position in which the filter or filters are made accessible for inspection and/or substitution thereof.
Although in this type of device the change of the filter can be made in a very short time, nevertheless the change of a filter involves many problems in the processing of the plastic materials, particularly due to a large quantity of air entrained into the highly viscous flow of plastic material, by the perforations of a back-plate during the displacement across the flow path. Entrainment of air causes bubble formation i affecting the flow of the plastic material downstream the filter device; pre-filling of perforations of the back-plate as suggested by U.S. Pat. No. 4,025,434, partially solve the air problem only. In fact, due to the necessity to remove each clogged filter from the slide plate at a very high temperature by an operator, the change of the filter can not be automatically performed and involves some risks for the same operator; in conclusion this type of filter devices did not proved to be suitable for a practical use in the recovery of plastic materials, owing to the problems arising at the changing of the filter and for the involved high costs.
In an attempt to partially automate the change operation of the filter, preventing the breaking of the flow, the use of filter devices has been suggested provided with two or more flow-paths arranged in parallel between inlet and outlet sides; when an appropriate pressure sensing device indicates that a filter is clogged, the flow-path is-closed by a valving member to shut down the pressure, in order to allow for the replacement of the clogged filter element with a new one, while the flow of plastic material is continued in the remaining path or paths; the use of pressure sensing means does not provide any useful remedy to the disadvantages of this known type of filter devices.
Finally, U.S. Pat. No. 3,940,335 and U.S. Pat. No. 4,159,953 suggest the use of a filter device or filter apparatus in which a filter band is step moved on a reciprocable back-plate, across the molten stream of the plastic material; the filter band is passed through inlet and outlet apertures which are sealed, for example, by thermal solidification therein of the filtered material, to prevent air entrance and leakage of the same material after the filter band being advanced to locate a fresh part thereof into the flow-path.
Apart the use of a reciprocable back-plate, which again will entrain air as in the previously known filter devices, the sealing at the inlet and outlet apertures for the filter band by the solidified material involve new problems for the high share forces acting on the filter-band during any advancement. Again these filter devices do not solve the problem of a complete automation of the filter change operation, and require the use of wire nets which must be sufficiently strong to resist to the share forces. This type of filter proved to be not suitable or advisable for the use in plastic recovery systems, for example in the production of plastic pellets, in which use must be made of a very simple and low-cost filter device, allowing a complete automation.
In the attempt to provide a solution to the above mentioned problems, U.S. Pat. No. 4,842,750 which constitutes the closest prior art, suggests the use of a filter apparatus for cleaning viscous materials, and more a particular molten down scraps of synthetic materials, comprising a screen in the form of a strip which can be moved across the flow of plastic material to replace a clogged area of the filter strip with a fresh one; braking and sealing means are provided on both sides of the strip to prevent leakage. Furthermore, in order to prevent synthetic material form escaping when the filter strip is advanced, the volume of a chamber up-stream the screen is enlarged, for example by axially moving tubular members in the flow-path or by any suitable means, to reduce or eliminate the pressure upon the screen; therefore the filter strip can be easily advanced avoiding any leakage of molten plastic material.
Apart breakage of the flow of plastic material during replacement and advancement of the clogged strip, the proposed solution requires a very complex and costly filter device; furthermore the use of breaking and sealing devices on both sides of the filter strip substantially hinder the total removal of the thick layer of the particulate material on the upstream side of the filter strip, involving loss of material.
OBJECT OF THE INVENTION
Therefore the need for a filter device or automatic screen changer of simple design and low cost steel exist.
Accordingly, an object of the present invention is to provide a filter device or automatic screen changer of the continuous filter band type, which is suitable for solving the problems mentioned above and for use in the processing of plastic materials, for example for the production of pellets in plastics recovery plants.
Another object of the present invention is to provide a filter device of the type mentioned above which ensures, in a very simple manner, a perfect peripheral seal around a filtering area of the filter band, allowing at the same time an easy and rapid step advancement of the band for the replacement of a clogged filtering area with fresh one, without manual intervention by an operator, and without breaking the flow of material during a change.
Another object of the invention is to provide a filter device of the type mentioned above, in which the replacement of a clogged area of the filter may be performed without any risk of
Ocampo Marianne
Previero N. S.r.l.
Walker W. L.
Young & Thompson
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