GB2584316A

GB2584316A - End cap and filter unit

Info

Publication number: GB2584316A
Application number: GB1907676.9A
Authority: GB
Inventors: Walker Brian
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-05-30
Filing date: 2019-05-30
Publication date: 2020-12-02
Also published as: GB201907676D0

Abstract

An end cap 56, for a fluid filter unit having a filter tube 54, has recesses or protrusions 58 in a wall surface 57. A curable sealant 55 can be inserted into the end cap 56 to seal the filter element 54 to the end cap 56. The recesses or protrusions 58 can provide a key for the seal, to help prevent detachment of the end cap56 from the filter tube 54 (e.g. due to expansion or vibration). The grooves 58 may be tapered or square screw threads 58 with liquid adhesive 55 bonding the filter medium 54 to the end cap 56. The curable liquid sealant 55 may be epoxy resin, polyurethane resin, or silicon rubber. The filter medium 54 may be flanked by cylindrical foraminous support cylinders 52, 53 embedded in the potted seal.

Description

END CAP AND FILTER UNIT

The present invention relates to a filter unit comprising an end cap and to a method of fabricating a filter unit with an end cap. Preferably, but not necessarily, the filter unit is an air filter unit to separate solids and/ or liquids, such as droplets and/ or particulates, from an air stream. The filter unit may for example be configured for use as a compressed air filter, a vacuum pump filter, an oil/air separator, a gas filter, an adsorption filter or a liquid filter. The present invention further relates to an end cap for a filter unit and to a method of fabricating an end cap for a filter unit.

Introduction

A known cylindrical air filter unit or separator unit comprises a cylindrical filter element sealingly bonded at either or both ends to a gas-impervious end cap.

The filter element primarily comprises a tube of filter medium through which air to be filtered passes from an upstream side to a downstream side. The tube is arranged such that one of the upstream side or the downstream side is the internal volume of the tube and the other is external of the tube. For example, air to be filtered is passed into the internal volume of the tube and through the filter medium.

The filter element may additionally comprise a foraminous cylindrical support sleeve located coaxially with and externally of the filter medium tube, and/ or a foraminous cylindrical support sleeve located coaxially with and internally of the filter medium tube. The cylindrical support sleeve may be a metal or rigid plastics material, and is for example rolled and spot welded perforated or expanded stainless steel but spiral wound perforated cylinders can also be used. The filter element may additionally comprise other structural or functional components.

The material for endcaps varies depending on application but pressed metal, pressure diecast aluminium, injection moulded plastic or machined metal or plastic are the more common materials. Injection moulded plastics, and for example thermoplastics and thermoplastic composites, such as glass-filled nylon, are widely used.

Where end caps are provided at both ends at least one of the end caps has to have a conduit allowing the air to flow in or out of the internal volume. This may be a single central conduit (hole) but some designs use multiple holes and/ or offset holes.

For low pressure applications the filter medium may not need any rigid foraminous support in both radial or longitudinal directions. Support in the radial direction is generally used to counteract pressure loss across the filter medium due to fluid flow and the build of particulate matter. Support in the longitudinal direction may be to counteract compression forces due to mechanical clamping methods or tensile forces due pressure loss from inside to outside. A single support cylinder is often used for low pressure applications.

However, a pair of cylindrical support sleeves (one on the outside and one on the inside of the filter medium) is more common on higher pressure applications. For example, high efficiency separators that are used in oil lubricated compressors and vacuum pumps can operate at high temperatures and pressures. Conventionally, such separators have metal or plastic foraminous cylindrical support sleeves on each side of a filter medium tube to prevent its collapse or rupture, with end caps bonded to the filter element, and preferably also to the cylindrical support sleeves, at either end. The present invention relates in particular to a filter unit comprising a cylindrical filter element having such foraminous cylindrical support sleeves on either or both sides of a filter medium as above described.

In the manufacture of cylindrical filter elements, it is common practice to use a curable medium such as an adhesive, for example a rigid single or two-part resin such as Epoxy or Polyurethane, to bond and seal a pair of endcaps at least to the filter medium of the filter element. The end caps are normally required to affect a seal between the filter medium and a pressure containing housing by means of suitable gaskets. It is also common practice to resin bond either one or two foraminous cylindrical support sleeves, where present, to the endcaps.

The integrity of the seal, and hence the bond, between the filter medium and the end caps is critical for the functioning of the filter unit. It is imperative that the resin bond to the filter medium and to the end caps is maintained at all times. It is known in the art that end caps becoming detached can have catastrophic consequences which must be avoided at all costs. These consequences can cause severe damage to plant and equipment, destroy products and in some cases can cause harm to human life.

A typical end cap has a pair of coaxial annular walls, projecting from or recessed within the end cap structure, an inner surface of the outer wall and an outer surface of the inner wall together defining with the base portion between them an annular channel into which the cylindrical filter element is received and into which bonding adhesive may additionally be received to effect a gas-impervious sealing bond between the cylindrical filter element and the end cap. For some applications a simpler end cap with a single wall comprising just the outer wall into which the cylindrical filter element is received in use may be sufficient. In this case the inner surface of the outer wall and the base surface of the end cap define a volume into which the cylindrical filter element is received and into which bonding adhesive may additionally be received to effect the sealing bond.

In a typical bonding operation, a liquid bonding resin is poured into the annular channel or into the entire area surrounded by the single wall as the case may be, and the cylindrical filter element, comprising the filter medium tube with or without support cylinders, is located in the channel and the resin allowed to set. Setting times will vary depending on the type of resin used and by example a two-part polyurethane may take 6 minutes whereas silicone rubber may take up to 24 hours.

The inner surface of the outer wall and the base, and where applicable the outer surface of the inner wall defining the annular channel, thus comprise bonding surfaces with which the resin effects a bond.

In a case where the filter element additionally comprises a foraminous cylindrical support sleeve, a bond is effected between the foraminous cylindrical support sleeve and the immediately facing bonding surface of the wall(s). When a foraminous cylindrical support sleeve is located into the newly poured resin, the resin penetrates into the perforations of the sleeve and when the resin cures it makes a rigid keyed joint in the perforations of the sleeve.

It is known in the art that end caps can become detached due to various circumstances, e.g. lack of bond strength, incompatible materials, end cap expansion due to heat, vigorous mechanical vibration and increasing pressure drop across the filter medium as it removes particulate matter. However, the channel wall(s) of conventional plastic end caps are generally fabricated with smooth sides which are further polished over time due to opening and closing of the moulding tool. Consequently, the corresponding bonding surfaces provide for no keying taking place but just a simple surface to surface bond. End cap detachment is more prevalent if one end is unsupported, for example if one end has a threaded portion which connects to the housing and the other end cap is unsupported.

Various modifications have been proposed to resist the tendency for endcaps to become detached. For example, further mechanical support may be provided by means of a clamping force between each end and there are various ways of achieving this, a tie rod being the most common. Additionally or alternatively, secondary process steps may be employed to modify the surfaces of the annular channel in the end cap with view to enhancing the integrity of the bond/ seal produced between the adhesive and the surface. These may include bead blasting to roughen the surface, machining of radial grooves, and plasma cleaning.

Such secondary structures or process steps are generally undesirable, adding to the complexity and/ or the cost of the operation of fabricating a filter unit with end cap(s).

It is generally desirable to develop a modified end cap and a method of attaching an end cap that offers an improved mechanical strength and/ or integrity of seal with a tubular filter element in use in a filter unit, and/ or that reduced the likelihood of the end cap becoming detached during normal use.

Summary of Invention

In accordance with the invention in a first aspect there is provided a fluid filter unit comprising: a tubular filter element comprising: a tube of filter medium; and optionally additionally comprising a foraminous support sleeve located on either or both sides of the tube of filter medium; the tubular filter element having an open end; an end cap configured to constitute a fluid-impervious end seal when positioned in use on an end of the tubular filter element comprising: an optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface; a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface; wherein the open end of the tubular filter element is positioned in close association with the end cap; and wherein a curable medium is provided in the end cap to effect a fluid-impervious seal between the tubular filter element and the end cap; and wherein at least one wall surface is fabricated with recessed structures.

As the skilled person will understand, the purpose of an end cap in a fluid filter unit in accordance with the invention is to effect a fluid-impervious end seal on the end of the tubular filter element. An endcap may be provided to seal each end of the tubular filter element. Nevertheless, where end caps are provided at both ends at least one of the end caps then requires a conduit allowing the air to flow in or out of the internal volume. Accordingly, the skilled person will understand that the end cap is both in the foregoing sense configured to constitute a fluid-impervious end seal while at the same time optionally requiring, as part of the filter apparatus in which it is to be incorporated, an aperture within its closure formation to allow fluid communication into or out of the internal volume defined by the tubular filter element. References to a closure formation will be read herein accordingly to encompass a closure formation with one or more such apertures.

The first aspect of the invention applies in particular to two principal preferred embodiments of end cap. In a first principal embodiment inner and outer wall structures define a receiving channel into which the open end of the tubular filter element may be received and which also serves as a reservoir for curable medium. In a second principal embodiment a single outer wall structure surroundingly defines a receiving region into which the open end of the tubular filter element may be received and which also serves as a reservoir for curable medium.

Thus, in the first principal preferred embodiment the end cap comprises a mounting formation having two wall formations comprising first and second channel formations projecting from or recessed within the closure formation, each channel formation defining a channel surface, with the second channel formation being disposed to surround the first channel formation such that the channel surface on the first channel formation and the channel surface on the second channel formation together define a receiving channel for receiving a tubular filter element; wherein at least one of the channel surfaces is fabricated with recessed structures; wherein the open end of the tubular filter element is received within the receiving channel; and wherein a curable medium is provided in the receiving channel to effect a fluid-impervious seal between the tubular filter element and the end cap.

Thus, in the second principal preferred embodiment the end cap comprises a mounting formation comprising a single wall formation projecting from or recessed within the closure formation, the single wall formation having a wall surface defining a perimeter of a receiving region for receiving a tubular filter element; wherein the wall surface is fabricated with recessed structures; wherein the open end of the tubular filter element is received within the receiving region; and wherein a curable medium is provided in the receiving region to effect a fluid-impervious seal between the tubular filter element and the end cap.

An end cap in accordance with the first aspect of the invention carries a mounting formation comprising at least one wall formation projecting from or alternatively recessed into the cap body, being at least a surrounding wall defining a region for receiving a tubular filter element and with a tubular filter element received therein for additionally receiving a curable sealing medium. Optionally an end cap in accordance with the first aspect of the invention carries a mounting formation comprising two wall formations constituting first and second channel formations, which constitute inner and outer walls of a channel for receiving a tubular filter element and with a tubular filter element received therein for additionally receiving a curable sealing medium. The curable sealing medium is then cured to effect a fluid-impervious seal between the filter medium of the tubular filter element and the end cap, the end cap thereby acting to close that end of the tubular filter element in familiar manner.

The invention is distinctly characterised in that the end cap in its as-fabricated condition is provided with recessed structures in at least one of wall surface that sits in use adjacent to the tubular filter element. In the case where there is only one wall surface, for example being the inner surface of a single surrounding wall, this surface is recessed. In the case where there are two wall surfaces in use adjacent to the tubular filter element comprising the channel surfaces defined by the mutually facing outer surface on the first channel formation and inner surface on the second channel formation at least one of these is recessed.

The recessed surface(s) provide a keying surface to key with the curable medium and improve the mechanical strength and/ or integrity of seal with the tubular filter element and/ or that reduced the likelihood of the end cap becoming detached during normal use. The curable medium, which is for example an adhesive, is as will be familiar applied in a fluid state in which it is able to penetrate the recesses and if applicable the apertures in the foraminous support sleeves and is then cured to form a solid medium and to effect an improved fluid-impervious seal between the filter medium of the tubular filter element and the end cap.

As discussed above, a typical tubular filter element may be structured to comprise a tube of filter medium through which fluid to be filtered passes from an upstream side to a downstream side and additionally comprise a foraminous support sleeve located on either or both sides of the tube of filter medium. Advantageously in such a case uncured curable medium in flowable form fills the recessed structures and the foraminous apertures of the support sleeve(s) and upon curing produces a rigid mechanical key between all the parts. In this way detachment of the end cap due to lack of bond strength alone in the absence of mechanical failure may be resisted.

The tubular filter element is typically a cylindrical tube.

As will be familiar, end caps may be fabricated from pressed metal, pressure diecasting, injection moulded plastic or machined metal or plastic. Injection moulded plastics materials, and for example thermoplastics and thermoplastic composites, may be particularly suited to the invention.

Preferably, the end cap comprises a plastics material. Preferably the end cap comprises a thermoplastic material which term encompasses the use of plural thermoplastic materials and thermoplastic matrix composite materials. Preferably, the or each thermoplastic has a melting point of at least 180°C, allowing the end cap and the filter unit including the end cap to be used in high temperature operating environments such as can be found in compressor and vacuum pump applications. Suitable materials include PBT (polybutylene terephthalate), nylon, acetal or other thermoplastics which have melting points of at least 180 C, and thus are suitable for use in high temperature operating environments.

Preferably, the end cap is injection moulded. Preferably, the end cap is injection moulded plastics material and for example comprises an injection moulded thermoplastic which term encompasses an injection moulded thermoplastic matrix composite. In some embodiments the end cap comprises an injection moulded glass-filled nylon material.

The end cap in its as-fabricated condition and for example as-moulded condition is provided with recessed structures in one or both of the channel surfaces of the receiving channel. In some preferred embodiments the end cap is injection moulded to include recessed structures in one or both of the channel surfaces of the receiving channel in an as-moulded condition.

The end cap includes in the usual way an optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface. Preferably, the sealing surface of the closure formation comprises a planar surface, except for the wall formation(s). In use the sealing surface combines with the cured curable medium to effect a fluid-impervious closure of the end of the tubular filter element when positioned in use on the end of such a filter element. The filter element primarily comprises a tube of filter medium through which air to be filtered passes from an upstream face through the filter medium to a downstream face and optionally additionally comprising a foraminous support sleeve located on either or both sides of the tube of filter medium. The tube has open ends. The closure formation acts to close an open end, excepting, where applicable, that it allows flow through one or more apertures therein. Accordingly, the closure formation is suitably configured to effect such a closure. For example, the closure formation is correspondingly shaped to an open end of the tubular filter element with which it is to be used, defining surface shaped to extend in use fully across the said open end.

In some embodiments, the closure formation comprises a planar formation.

In some embodiments, the closure formation comprises a planar formation having a geometrically similar shape to that of the end of the tubular filter element with which it is to be used, and sized to be at least co-extensive with and extend in use fully across the said open end.

In some embodiments, the closure formation comprises a planar disc formation configured to be used with a cylindrical tubular filter element, and sized to be at least co-extensive with and extend in use fully across a cylindrical open end of the said tubular filter element.

The mounting formation comprises at least one wall formation, and in some embodiments comprises first and second channel formations, projecting from or recessed into the closure formation. The at least one wall formation is preferably arranged perpendicularly to an otherwise planar sealing surface of the closure formation. For example, the at least one wall formation projects perpendicularly from a planar closure formation or is recessed perpendicularly into a planar closure formation.

In the first principal embodiment comprising a pair of channel formations each channel formation defines a channel surface, with the second channel formation being disposed to surround the first channel formation such that the mutually facing outer channel surface on the first channel formation and inner channel surface on the second channel formation together with the portion of the sealing surface of the closure formation define a receiving channel for receiving a tubular filter element and a curable sealing medium.

The channel formations preferably together define a continuous closed channel. The continuous closed channel is complementarily shaped to the end of the tubular filter element with which it is to be used such that the end of the tubular filter element may be received therein in use.

In a particular embodiment, the end cap is adapted for use with a cylindrical tubular filter element, and the channel formations together define an annular channel.

Each channel formation for example comprises a wall, an outer surface of the wall comprising the first channel formation and an inner surface of the wall comprising the second channel formation defining facing walls of the receiving channel. The facing walls of the receiving channel are complementarily shaped and evenly spaced.

In a particular embodiment, the end cap is adapted for use with a cylindrical tubular filter element, and the channel formations comprise a pair of coaxial annular walls, an inner surface of the outer wall and an outer surface of the inner wall together defining an annular channel.

In a preferred embodiment, particularly advantageous in the case where end caps are injection moulded or otherwise fabricated on a structural core former, the recessed structures may comprise a screw threaded form. This has the advantage that during the fabrication process the fabricated end cap may readily be released from the structural core former by rotation. The threaded form on the channel surface of the as-fabricated end cap then constitutes an effective keying surface to key with the curable medium when the filter unit is assembled and sealed. It is also known in the plastic moulding industry that more expensive collapsible cores can be used to provide a number of recesses in a channel wall which will effect a keying surface.

It is advantageous if the thread form(s) are tapered which will allow the fabricated end cap to release and rotate off the core former more easily. For example, the channel surface on which the recessed structures comprising such a screw threaded formation are carried may be tapered. Additionally or alternatively the threaded forms themselves may be tapered.

The threaded form is used on at least one wall surface that sits adjacent to and bonds with the filter element in use. In the case where there is a single wall surrounding the filter element a threaded form is used on at least the inner surface of the wall facing the filter unit. In the case where there are two walls for example an inner channel and an outer channel wall defining a channel in which the filter unit is seated, a threaded form is used on at least the inner channel wall or the outer channel wall. In some embodiments a threaded form is used on both inner and outer channel walls, and recessed structures comprising a screw threaded formation are provided on each of the channel surface on the first channel formation and the channel surface on the second channel formation. If a threaded form is used on both inner and outer channel walls, then the tooth pitch of each thread is preferably the same. It may also be advantageous if the threads are synchronized.

In accordance with the invention a fluid filter unit is provided comprising a tubular filter element sealingly bonded at either or both ends to a fluid-impervious end cap by means of a cured curable medium.

Typically, a second end cap may be provided at a second end of the tubular filter element remote from the first. The second end cap may be the same as or different from the first.

Where end caps are provided at both ends at least one of the end caps has to have an aperture allowing the air to flow in or out of the internal volume. This may be a single central aperture, but may comprise multiple apertures and/ or offset aperture.

That is, the tubular filter element has two remotely spaced open ends, with an end cap at each end, at least one of which is apertured.

More completely, the tubular filter element has a second open end remote from the first end; and a second end cap is provided comprising an optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface and a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface, wherein at least one wall surface is fabricated with recessed structures; wherein the second open end of the tubular filter element is positioned in close association with the second end cap; and wherein a curable medium is provided in the end cap to effect a fluid-impervious seal between the tubular filter element and the end cap.

For example in the first principal embodiment, the tubular filter element has two remotely spaced open ends and the fluid filter unit comprises two end caps each configured to constitute a fluid-impervious end seal when positioned in use on an end of such a tubular filter element and each comprising a closure formation comprising a fluid-impervious body defining a sealing surface, wherein at least one of the closure formations is apertured; a mounting formation having two wall formations comprising first and second channel formations projecting from or recessed within the closure formation, each channel formation defining a channel surface, with the second channel formation being disposed to surround the first channel formation such that the channel surface on the first channel formation and the channel surface on the second channel formation together define a receiving channel; wherein at least one of the channel surfaces comprising the channel surface on the first channel formation or the channel surface on the second channel formation is fabricated with recessed structures; wherein each open end of the tubular filter element is received within a respective receiving channel of one of the said two end caps; and wherein a cured curable medium is provided in the respective receiving channel to effect a fluid-impervious seal between the tubular filter element and each respective end cap.

For example in the second principal embodiment, the tubular filter element has two remotely spaced open ends and the fluid filter unit comprises two end caps configured to constitute a fluid-impervious end seal when positioned in use on an end of such a tubular filter element each comprising a closure formation comprising a fluid-impervious body defining a sealing surface wherein at least one of the closure formations is apertured; a mounting formation comprising a single wall formation projecting from or recessed within the closure formation, the single wall formation having a wall surface defining a perimeter of a receiving region for receiving a tubular filter element; wherein the wall surface is fabricated with recessed structures; wherein the open end of the tubular filter element is received within the receiving region; and wherein a curable medium is provided in the receiving region to effect a fluid-impervious seal between the tubular filter element and the end cap.

The channel formations preferably together define a continuous closed channel. The continuous closed channel is complementarily shaped to the end of the tubular filter element received therein.

In some embodiments, the tubular filter element comprises a cylindrical tubular filter element. In such embodiments the closure formation conveniently comprises a planar disc formation at least co-extensive with and extending fully across a cylindrical open end of the said tubular filter element.

In such embodiments the channel formations where present together define an annular channel. For example, the channel formations comprise a pair of coaxial annular walls, an inner surface of the outer wall and an outer surface of the inner wall together defining an annular channel.

In such embodiments the tubular filter element comprises a cylindrical tube of filter medium and optionally additionally comprises a foraminous cylindrical support sleeve located coaxially with and externally of the tube of filter medium, and/ or a foraminous cylindrical support sleeve located coaxially with and internally of the tube of filter medium.

The tubular filter element comprises a tube of filter medium. Suitable filter elements are known. For example, the medium can be a vacuum moulded fibre tube, sintered plastic, carbon or metal tube, a woven mesh tube, a wrapped filter paper tube, wrapped activated carbon media tube or a pleated filter tube. Suitable materials for the filter medium are well known and established and may include fibres and for example microfibres and for example glass microfibres. In some embodiments the tube of filter medium may comprise borosilicate glass microfibres.

Optionally, the tube of filter medium may be electrically conductive. For example, the filter medium may comprise a mixture of non-electrically conductive fibres and electrically conductive fibres. Alternatively, the filter medium may comprise a body of non-electrically conductive fibres coated with an electrically conductive film.

The tubular filter element optionally comprises a foraminous support sleeve located on either or both sides of the tube of filter medium. In some embodiments the foraminous support sleeve may comprise metal or rigid plastics material. In some embodiments the foraminous support sleeve may comprise stainless steel or coated carbon steel.

In some embodiments the curable medium is a resin. A potting resin, such as a single part heat cured epoxy resin, a two-part room temperature cured epoxy resin, a two part polyurethane resin or a silicone rubber, may be used.

In accordance with the invention in a second aspect there is provided an end cap suitable for use with a tubular filter element to constitute a fluid filter unit in accordance with the first aspect of the invention.

Thus, there is provided: an end cap configured for use in a fluid filter unit that comprises a tubular filter element sealingly bonded at either or both ends to such an end cap and configured to constitute when so bonded a fluid-impervious end seal on an end of the tubular filter element, comprising: an optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface; a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface adapted to be positioned in use in close association with an end of a tubular filter element; wherein at least one wall surface is fabricated with recessed structures.

In a first principal embodiment the end cap comprises a mounting formation comprising first and second channel formations projecting from or recessed within the closure formation, each channel formation defining a channel surface, with the second channel formation being disposed to surround the first channel formation such that the channel surface on the first channel formation and the channel surface on the second channel formation together define a receiving channel for receiving a tubular filter element; wherein at least one of the channel surfaces is fabricated with recessed structures.

In a second principal embodiment the end cap comprises a mounting formation comprising a single wall formation projecting from or recessed within the closure formation, the single wall formation having a wall surface defining a perimeter of a receiving region for receiving a tubular filter element; wherein the wall surface is fabricated with recessed structures.

Other preferred feature of the end cap of the second aspect of the invention will be understood from the discussion of the same in the context of the first aspect of the invention.

In accordance with the invention in a third aspect a method of fabrication of an end cap configured for use in a fluid filter unit that comprises a tubular filter element sealingly bonded at either or both ends to such an end cap and configured to constitute a fluid-impervious end seal when positioned in use on an end of such a filter element is provided, the method comprising: forming an end cap comprising an optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface and a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface to be positioned in use in close association with an end of a tubular filter element; forming recessed structures on at least one wall surface.

In a first principal embodiment the method comprises forming an end cap comprising a mounting formation comprising first and second channel formations projecting from or recessed within the closure formation, each channel formation defining a channel surface, with the second channel formation being disposed to surround the first channel formation such that the channel surface on the first channel formation and the channel surface on the second channel formation together define a receiving channel for receiving a tubular filter element; forming recessed structures on at least one of the channel surfaces.

In a second principal embodiment the method comprises forming an end cap comprising a mounting formation comprising a single wall formation projecting from or recessed within the closure formation, the single wall formation having a wall surface defining a perimeter of a receiving region for receiving a tubular filter element; forming recessed structures on the wall surface.

Thus, the end cap in its as-fabricated condition following performance of the method of the third aspect of the invention is provided with recessed structures in at least one of the surfaces that will in use face a surface of and end region of the tubular filter element to provide a keying surface to key with the curable medium and improve the mechanical strength and/ or integrity of seal with the tubular filter element and/ or that reduced the likelihood of the end cap becoming detached during normal use.

Preferably the step of forming an end cap and the step of forming recessed structures are performed simultaneously, which is to say that the recessed structures are formed with and at the same time as the end cap. Advantageously, this obviates the need for a secondary processing step performed after fabrication of the end cap to provide a keying surface.

Preferably, the method of forming the end cap comprises injection moulding. In some preferred embodiments the method of forming the end cap comprises injection moulding to include the said recessed structures in one or both of the channel surfaces of the receiving channel in an as-moulded condition.

An end cap fabricated in accordance with the method of the third aspect of the invention is for example an end cap in accordance with second aspect of the invention for use in a filter unit in accordance with the first aspect of the invention and preferred features and advantages of those aspects will be applicable by analogy to the foregoing method.

In accordance with the invention in a fourth aspect a method of fabrication of a fluid filter unit is provided comprising: providing an end cap in accordance with the second aspect of the invention or fabricating an end cap in accordance with the method of the third aspect of the invention; mounting the end cap on an end of a tubular filter element and sealingly bonding the same thereto.

More completely, in accordance with the invention in a fourth aspect a method of fabrication of a fluid filter unit is provided comprising: forming an end cap comprising an optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface; and a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface; forming recessed structures on at least one wall surface; providing a tubular filter element comprising a tube of filter medium; and optionally additionally comprising a foraminous support sleeve located on either or both sides of the tube of filter medium; the tubular filter element having an open end; causing curable medium to be provided to the end cap; bringing the tubular filter element into contact with the curable medium; curing the curable medium to effect a fluid-impervious seal between the tubular filter element and the end cap.

In a first principal embodiment the method comprises forming an end cap comprising a mounting formation comprising first and second channel formations projecting from or recessed within the closure formation, each channel formation defining a channel surface, with the second channel formation being disposed to surround the first channel formation such that the channel surface on the first channel formation and the channel surface on the second channel formation together define a receiving channel for receiving a tubular filter element; forming recessed structures on at least one of the channel surfaces; causing curable medium to be provided in the receiving channel; receiving the open end of the tubular filter element within the receiving channel; curing the curable medium to effect a fluid-impervious seal between the tubular filter element and the end cap.

In a second principal embodiment the method comprises forming an end cap comprising a mounting formation comprising a single wall formation projecting from or recessed within the closure formation, the single wall formation having a wall surface defining a perimeter of a receiving region for receiving a tubular filter element; forming recessed structures on the wall surface; causing curable medium to be provided in the receiving region; receiving the open end of the tubular filter element within the receiving region; curing the curable medium to effect a fluid-impervious seal between the tubular filter element and the end cap.

Preferably the step of forming an end cap and the step of forming recessed structures are performed simultaneously. Thus, the end cap in its as-fabricated condition is provided with recessed structures in at least one of the surfaces that will in use face a surface of and end region of the tubular filter element to provide a keying surface to key with the curable medium and improve the mechanical strength and/ or integrity of seal with the tubular filter element and/ or that reduced the likelihood of the end cap becoming detached during normal use.

Conveniently, the method of forming the end cap comprises injection moulding. In some preferred embodiments the method of forming the end cap comprises injection moulding to include the said recessed structures in one or both of the channel surfaces of the receiving channel in an as-moulded condition.

Typically, a second end cap in accordance may be provided at a second end of the tubular filter element remote from the first.

In such cases, the method for example comprises: forming a first end cap and a second end cap in accordance with the foregoing method, and in particular therefore: forming recessed structures on at least one of the wall surfaces of each end cap; providing a tubular filter element in accordance with the foregoing with two remotely spaced open ends; causing curable medium to be provided; receiving each open end of the tubular filter element within the curable medium; curing the curable medium to effect a fluid-impervious seal between the filter medium of the tubular filter element and each end cap.

The curable medium is caused to flow into the channel as a flowable material and sets to a solid as it is cured. The curable medium is for example poured into the channel as a liquid.

The tubular filter element is inserted into the channel and the curable medium is cured to effect a rigid joint between the end cap and the tubular filter element. In the preferred case where the tubular filter element comprises a foraminous support cylinder, the curable medium is first caused to flow into the channel as a flowable material to a suitable depth such that when the tubular filter element is subsequently inserted into the channel apertures of the foraminous support cylinder and the recessed structures on the channel surface both fill with curable medium which is then cured to effect a rigid joint between the end cap and the tubular filter element.

A fluid filter unit fabricated in accordance with the method of the fourth aspect of the invention is for example a unit in accordance with the first aspect of the invention and preferred features and advantages of the first aspect will be applicable by analogy to the foregoing method.

In particular, in some embodiments the end cap is formed of a plastics material. Preferably the end cap is formed of a thermoplastic material which term encompasses the use of plural thermoplastic materials and thermoplastic matrix composite materials.

Preferably, the end cap comprises injection moulded plastics material and for example comprises an injection moulded thermoplastic which term encompasses an injection moulded thermoplastic matrix composite. In some embodiments the end cap comprises an injection moulded glass-filled nylon material.

Preferably, the closure formation is correspondingly shaped to the open end of the tubular filter element, defining a surface extending fully across and thereby closing the said open end.

In some embodiments, the closure formation comprises a planar formation, for example having a geometrically similar shape to that of the end of the tubular filter element received within the receiving channel, and sized to be at least co-extensive with and extend in use fully across the said end.

Other preferred features of the method will be understood by analogy.

The present invention relates in particular to a fluid filter unit and to a method of fabricating a fluid filter unit. The fluid to be filtered may be a primarily gaseous fluid, the filter unit for example being a gas filter unit to separate solids and/ or liquids, such as droplets and/ or particulates, from a gas stream. References to fluid hereinabove being then interpreted as references to gas, but in other embodiments the fluid to be filtered may include liquids and mixtures. The gas may be air. Preferably, but not necessarily, the filter unit is an air filter unit to separate solids and/ or liquids, such as droplets and/ or particulates, from an air stream. The filter unit may for example be configured for use as a compressed air filter, a vacuum pump filter or as an oil/air separator.

In further aspects the invention may encompass a filter unit produced according to the foregoing method, the use of the filter unit according to the foregoing as a compressed air filter or air/oil separator, and the use of the filter unit according to the foregoing as a vacuum pump filter or air/oil separator.

Brief Description of Drawings

The invention will now be described by way of example only with reference to figures 1 to 4 of the accompanying drawings, in which: Figure 1 shows a plastic injection moulded filter cartridge endcap which includes the features of the invention with a filter element received within and closed by the end cap; Figure 2 is a detailed view of a portion of the receiving channel showing the characteristic threaded structures of the embodiment of figure 1; Figure 3 is an end view (direction A in figure 2) of the end cap of figure 1; Figure 4 shows an alternative embodiment of plastic injection moulded filter cartridge endcap which includes the features of the invention with a filter element received within and closed by the end cap.

Detailed Description

Figure 1 shows a plastic injection moulded filter cartridge endcap 1 with a tubular filter element received within and closed by the end cap.

The filter element comprises a pair of cylindrical foraminous support cylinders 2, 3, either side of a tube of filter medium 4 which can be of any known type.

The filter element is for example a filter element for an air/ oil separator through which air to be filtered passes from an upstream side to a downstream side. Suitable materials for the filter medium might include high efficiency, glass microfibres, such as borosilicate microfibres, which may be held together by binder. The foraminous support cylinder sleeves (typically providing an open area of greater than 40%) allow the passage of air, and can be made, for example of metal or from plastic. The tube is arranged such that one of the upstream side or the downstream side is the internal volume of the tube and the other is external of the tube. For example, air to be filtered is passed into the internal volume of the tube and through the filter medium and out through the outer surface to separate out the oil carried by the incoming air.

The filter element is received in the annular channel defined by side walls of channel formations defined by the end cap. Figure 1 represents schematically in cross-section a channel portion modified in accordance with the principles of the invention on the left-hand side and a channel portion of conventional construction with unmodified walls on the right-hand side. This is for purposes of illustration. A practical embodiment would have a channel portion modified in accordance with the principles of the invention for its full annular circumference.

The modified channel portion is shown in greater detail in figure 2. The channel formations define a pair of coaxial annular walls 6, 7, an inner surface of the outer wall and an outer surface of the inner wall together defining an annular receiving channel and resin reservoir. A curable sealing medium comprising in the example a bonding resin 5 is added, first in liquid form but which after curing becomes rigid, bonding to the end of the filter element and the side walls of the resin reservoir. A suitable bonding resin is a two-part epoxy. Preferably, the end caps are formed from a thermoplastic or thermoplastic composite which is resistant and compatible with synthetic and mineral oils at high temperatures (up to 130 C). An example material is glass-filled nylon.

Figure 2 is a detailed view of a portion of the receiving channel showing the characteristic threaded structures of the illustration of figure 1. The figure shows a v-form screw thread 8 on the inside of the outer wall 6, a square-form screw thread 9 on the outside of the inner wall 7 and by illustration shows the resin 5 keyed into the apertures of the cylinders 2 and 3 and also into the screw threads 8 and 9.

Figure 4 shows an alternative embodiment of plastic injection moulded filter cartridge endcap which includes the features of the invention with a filter element received within and closed by the end cap.

The filter element again comprises a pair of cylindrical foraminous support cylinders 52, 53, either side of a tube of filter medium 54 which can be of any known type. A simpler design of end cap 56 is proposed which merely has an outer wall 57, an inner surface of which has recessed structures 58. The filter element is received in similar manner within the outer wall and adjacent to the inner surface. The whole inner area of the end cap defines a resin reservoir. A curable sealing medium comprising in the example a bonding resin 55 is added, first in liquid form but which after curing becomes rigid, bonding to the end of the filter element and the outer wall. The resin keys into the apertures of the cylinders 52 and 53 and also into the threaded recessed structures 58 on the inside of the outer wall in a generally similar manner to that illustrated in figure 2.

In a preferred case the end cap is fabricated, and given an as-fabricated screw thread, by forming about a threaded structural former from which is removed by unscrewing, and for example by injection moulding about such a threaded structural former.

Moulding a screw thread into a plastic injection moulding has been done for many years. In the past it required a costly addition to the moulding tool which comprised a complex mechanical rotational element which unscrewed the solidified moulded part when the tool opened. Alternatively, it required an operator to manually unscrew it from the tool. Nowadays robots are used extensively in the plastic injection moulding industry and have the means to unscrew mouldings cost effectively.

Figure 3 is an end view (direction A in figure 2) of an end cap such as is shown in figures 1 and 2 that has been specifically adapted for such an injection moulding operation. The figure shows two ribs 10 illustrating a gripping means for unscrewing the moulded end cap 1 from the moulding tool. However, there are many different shapes and forms which can be used to mechanically rotate the moulding from the moulding tool.

In a possible embodiment when the two parts of the moulding tool open to reveal the newly formed plastic end cap they must open sufficiently to allow an arm of a robot, complete with a motorized unscrewing mechanism, to enter the cavity between the open faces. The motorized mechanism will then traverse towards the end cap, engage with the end cap and rotate the end cap off the stationary core of the tool whilst traversing away from the core. The arm will then rotate out of the space and deposit the end cap away from the moulding machine.

In other words, the moulding sequence is:- * Moulding tool closes * Plastic is injected into the tool cavity * Time allowed for the plastic to cool * The two parts of the tool opens * The plastic endcap is revealed and is held onto the threaded structural former by the screw thread * A robot arm swings into the space between the open faces of the tool * The robot arm traverses towards the end cap * The motorized element at the end of the arm engages unscrewing facets on the end cap * The motorized element unscrews the end cap off the core * The end cap can either drop downwards out of the moulding machine or be gripped and taken away * The robot arm swings out of the space with or without the end cap * The tool closes and the process is repeated.

The design of the screw thread will depend on the size of the end cap but more especially the wall thicknesses of the moulding. For instance, an end cap with a very thin wall of say 1 mm may only have a screw thread 0.5mm deep whereas with a wall thickness of 2 mm then the thread depth could be 1 mm. This depth of thread and the thread form will determine the pitch of the thread. The depth of the channel design will determine the maximum amount of full threads that can be achieved in the moulding.

Material selection may be based on established principles. A preferred end cap material for injection moulding as above may be a thermoplastic, which term includes thermoplastic matrix composites. Material strength may be selected by application.

For high pressure application, plastic moulded end caps made from glass filled nylon can have tensile strengths in the region of 85 MPa. The tensile strength of bonding resins also varies e.g. 2-part polyurethane may be 15 to 22 MPa whereas epoxy resin may be up to 85 MPa. Temperature plays a big part in the strength of plastics and resins therefore it is important to choose the correct materials to suit the application.

Similarly, the foraminous cylinders may be made from metal i.e. perforated or expanded steel strip which are inherently stronger than plastic, but such cylinders can also be moulded in plastic.

The invention provides a very simple, cost effective, system for ensuring plastic injection moulded endcaps will not become detached during normal service. The system includes moulding a screw thread form into the one or both side walls of the resin channel such that the liquid resin fills the thread form and fills the foraminous apertures of the cylinder and upon curing produces a rigid key between all the parts.

In this way the end cap detachment due to lack of bond strength is prevented.

While the invention has been described in conjunction with the exemplary embodiment described above, modifications and variations will be apparent to those skilled in the art when given this disclosure. For example, in discussion above the downstream side of filter element is suggested as the outer side, with air passing from inside the tube to outside. However, embodiments which draw incoming air through the external surface of the filter element, and discharge filtered air from the central hole of the filter element, are possible. Accordingly, the exemplary embodiment of the invention set forth above is considered to be illustrative and not limiting to the spirit and scope of the invention as defined by the appended claims.

Claims

CLAIMS1. A fluid filter unit comprising: a tubular filter element comprising: a tube of filter medium; and optionally additionally comprising a foraminous support sleeve located on either or both sides of the tube of filter medium; the tubular filter element having an open end; an end cap comprising: an optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface; a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface; wherein at least one wall surface is fabricated with recessed structures; wherein the open end of the tubular filter element is positioned in close association with the end cap; and wherein a curable medium is provided in the end cap to effect a fluid-impervious seal between the tubular filter element and the end cap.
2. A fluid filter unit in accordance with claim 1 wherein the end cap comprises a mounting formation comprising first and second channel formations projecting from or recessed within the closure formation, each channel formation defining a channel surface, with the second channel formation being disposed to surround the first channel formation such that the channel surface on the first channel formation and the channel surface on the second channel formation together define a receiving channel for receiving a tubular filter element; wherein at least one of the channel surfaces is fabricated with recessed structures; wherein the open end of the tubular filter element is received within the receiving channel; and wherein a curable medium is provided in the receiving channel to effect a fluid-impervious seal between the tubular filter element and the end cap.
3. A fluid filter unit in accordance with claim 1 wherein the end cap comprises a mounting formation comprising a single wall formation projecting from or recessed within the closure formation, the single wall formation having a wall surface defining a perimeter of a receiving region for receiving a tubular filter element; wherein the wall surface is fabricated with recessed structures; wherein the open end of the tubular filter element is received within the receiving region; and wherein a curable medium is provided in the receiving region to effect a fluid-impervious seal between the tubular filter element and the end cap.
A fluid filter unit in accordance with any preceding claim wherein the end cap is fabricated from a plastics material.
A fluid filter unit in accordance with any preceding claim wherein the end cap comprises an injection moulded material.
A fluid filter unit in accordance with any preceding claim wherein the sealing surface of the closure formation comprises a planar surface, except for the first and second channel formations, and wherein the first and second channel formations are arranged perpendicularly to the planar sealing surface of the closure formation.
A fluid filter unit in accordance with claim 6 wherein the closure formation comprises a planar disc formation configured to be used with a cylindrical tubular filter element, and sized to be at least co-extensive with and extend in use fully across a cylindrical open end of the said tubular filter element.
A fluid filter unit in accordance with any preceding claim wherein the end cap comprises first and second channel formations projecting from or recessed within the closure formation which together define a continuous closed channel. 4. 5. 6. 7. 8.
9. A fluid filter unit in accordance with claim 8 wherein the filter element comprises a cylindrical tubular filter element and the first and second channel formations together define an annular channel.
10. A fluid filter unit in accordance with any preceding claim wherein the recessed structures comprise a screw threaded formation.
11. A fluid filter unit in accordance with claim 8 wherein the screw threaded formation is tapered. 10
12. A fluid filter unit in accordance with claim 8 or 9 wherein the end cap comprises first and second channel formations projecting from or recessed within the closure formation and recessed structures comprising a screw threaded formation are provided on each of the channel surface on the first channel formation and the channel surface on the second channel formation.
13. A fluid filter unit in accordance with any preceding claim wherein: the tubular filter element has a second open end remote from the first end; and a second end cap is provided comprising an optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface and a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface, wherein at least one wall surface is fabricated with recessed structures; wherein the second open end of the tubular filter element is positioned in close association with the second end cap; and wherein a curable medium is provided in the end cap to effect a fluid-impervious seal between the tubular filter element and the end cap.
14. A fluid filter unit in accordance with claim 13 wherein the tubular filter element has two remotely spaced open ends, and the fluid filter unit comprises two end caps each comprising a closure formation comprising a fluid-impervious body defining a sealing surface, wherein at least one of the closure formations is apertured; a mounting formation comprising first and second channel formations projecting from or recessed within the closure formation, each channel formation defining a channel surface, with the second channel formation being disposed to surround the first channel formation such that the channel surface on the first channel formation and the channel surface on the second channel formation together define a receiving channel; wherein at least one of the channel surfaces is fabricated with recessed structures; wherein each open end of the tubular filter element is received within a respective receiving channel of one of the said two end caps; and wherein a cured curable medium is provided in the respective receiving channel to effect a fluid-impervious seal between the tubular filter element and each respective end cap.
15. A fluid filter unit in accordance with any preceding claim wherein the tubular filter element comprises a cylindrical tube of filter medium and additionally comprises a foraminous cylindrical support sleeve located coaxially with and extemally of the tube of filter medium, and/ or a foraminous cylindrical support sleeve located coaxially with and internally of the tube of filter medium.
16. A fluid filter unit accordance with any preceding claim wherein the curable medium is an epoxy resin or a two part polyurethane resin.
17. An end cap configured for use in a fluid filter unit that comprises a tubular filter element sealingly bonded at either or both ends to such an end cap, and configured to constitute when so bonded a fluid-impervious end seal on an end of the tubular filter element, comprising: An optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface; a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface adapted to be positioned in use in close association with an end of a tubular filter element; wherein at least one wall surface is fabricated with recessed structures.
18. A method of fabrication of an end cap configured for use in a fluid filter unit that comprises a tubular filter element sealingly bonded at either or both ends to such an end cap and configured to constitute when so bonded a fluid-impervious end seal on an end of the tubular filter element, comprising: forming an end cap comprising an optionally apertured closure formation and a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface; forming recessed structures on at least one wall surface.
19. A method in accordance with claim 18 comprising: forming an end cap comprising a mounting formation comprising first and second channel formations projecting from or recessed within the closure formation, each channel formation defining a channel surface, with the second channel formation being disposed to surround the first channel formation such that the channel surface on the first channel formation and the channel surface on the second channel formation together define a receiving channel for receiving a tubular filter element; forming recessed structures on at least one of the channel surfaces.
20. A method in accordance with claim 18 comprising: forming an end cap comprising a mounting formation comprising a single wall formation projecting from or recessed within the closure formation, the single wall formation having a wall surface defining a perimeter of a receiving region for receiving a tubular filter element; forming recessed structures on the wall surface.
21. A method in accordance with one of claims 18 to 20 wherein the step of forming an end cap and the step of forming recessed structures are performed simultaneously.
22. A method in accordance with one of claims 18 to 21 wherein the method of forming the end cap comprises injection moulding.
23. A method in accordance with one of claims 18 to 22 wherein the method of forming the end cap and recessed structures comprises forming about a structural former from which is then removed.
24. A method in accordance with claim 23 wherein the recessed structures comprise a screw threaded formation and the method comprises forming about a screw threaded structural former from which is then removed by unscrewing.
25. A method of fabrication of a fluid filter unit comprising: forming an end cap comprising an optionally apertured closure formation comprising a fluid-impervious body defining a sealing surface; and a mounting formation comprising at least one wall formation projecting from or recessed within the closure formation, each wall formation defining a wall surface; forming recessed structures on at least one wall surface; providing a tubular filter element comprising a tube of filter medium; and optionally additionally comprising a foraminous support sleeve located on either or both sides of the tube of filter medium; the tubular filter element having an open end; causing curable medium to be provided to the end cap; bringing the tubular filter element into contact with the curable medium; curing the curable medium to effect a fluid-impervious seal between the tubular filter element and the end cap.