GB2466459A

GB2466459A - Folded filter medium

Info

Publication number: GB2466459A
Application number: GB0823232A
Authority: GB
Inventors: Christopher Luke Butcher; Michael Paul Barton; Keith David Rick; Martin Stuart Denning
Original assignee: Dyson Technology Ltd
Current assignee: Dyson Technology Ltd
Priority date: 2008-12-19
Filing date: 2008-12-19
Publication date: 2010-06-23
Also published as: GB0823232D0; WO2010070258A1

Abstract

A cleaning appliance filter assembly (22, Fig 10) comprises a sheet 1 of filter medium folded according to a predetermined pattern so as to form a doubly corrugated structure comprising a plurality of successive peaks and troughs, with each peak and trough having a plurality of undulations along it. Preferably the folded sheet (20, Fig 11e) is located in a housing (23, Fig 11e). The pattern of folds produces a filter preferably having a curved shape, such as a hemisphere. Advantageously the provision of a doubly-corrugated structure for the filter medium permits a filter of greater effective surface area to be achieved. Such a filter has an increased capacity for dirt and dust. It is beneficial to provide filters having a large capacity for dirt and dust, so that they need cleaning or replacing infrequently.

Description

Filter Assembly The present invention relates to a filter assembly for a cleaning appliance such as a vacuum cleaner.

Vacuum cleaners are designed to separate dirt and dust from an airflow. In a typical vacuum cleaner an airflow generator (for example, a motor and fan unit) generates an airflow which draws dirt-and dust-laden air into the vacuum cleaner through a dirty air inlet. The airflow then passes through a form of separating apparatus to remove dirt and dust from the airflow. Some vacuum cleaners make use of a porous bag through which the dirty air is sucked so that the dirt and dust is retained in the bag whilst cleaned air is exhausted to the atmosphere. In other vacuum cleaners, cyclonic separators are used to separate dirt and dust from the airflow.

*:*::* Irrespective of the type of separating apparatus used, there is a risk of a small amount of S...

dirt and dust passing through the separating apparatus and being carried to the airflow generator. It is undesirable for dirt and dust particles to pass through the fan of an airflow generator because the fan may become damaged or may operate less efficiently.

* 20 In order to reduce this problem, some vacuum cleaners include a fine filter in an airflow * path between the separating apparatus and the airflow generator. This filter is : commonly known as a pre-motor filter and is used to extract fine dirt and dust particles remaining in the airflow after it has passed through the separating apparatus.

During normal operation of a vacuum cleaner, fine dirt and dust may be deposited on the pre-motor filter and, after a period of time, it could become blocked. Blockages reduce the efficiency at which a vacuum cleaner operates. Therefore, the pre-motor filter will occasionally need to be replaced or cleaned in order to maintain the performance of the vacuum cleaner. In order to allow cleaning or replacement of the pre-motor filter, it is common for such filters to be removable from a vacuum cleaner. p

It is also known to provide a filter downstream of the airflow generator. This is known as a post-motor filter, and is typically employed to filter carbon dust which may be generated by brushes on the motor, as well as allergens and any microscopic particles of dirt and dust remaining in the airflow. After a period of use, the post-motor filter may need to be cleaned or replaced. Typically, the post-motor filter will need such attention much less frequently than does the pre-motor filter.

It is beneficial to provide filters having a large capacity for dirt and dust, so that they need cleaning or replacing infrequently.

The invention provides a cleaning appliance filter assembly comprising a sheet of filter medium, the sheet being folded in a predetermined pattern so as to form a doubly corrugated structure comprising a plurality of successive peaks and troughs, each peak and trough having a plurality of undulations along it.

* *. The provision of a doubly-corrugated structure for the filter medium permits a filter of * S. i.. greater effective surface area to be achieved than was possible hitherto. Such a filter elS.

* has an increased capacity for dirt and dust.

*: 20 Preferably, the predetermined pattern comprises a plurality of rows of zig-zag lines having alternating vertices, and straight lines running transverse or orthogonal to the :.: ,; rows of zig-zag lines, with the straight lines passing through the vertices. This is a * pattern that is straightforward to produce and allows for a variety of filter shapes to be made.

The zig-zag lines may be parallel to each other, with the pattern comprising a matrix of tessellated parallelograms. Alternatively, the zig-zag lines may be of two different types, having vertices of different angles, with the lines of the different types alternating across the sheet of filter medium. A matrix of tessellated trapezoids may be formed in this way. Such a pattern allows for a variety of curved surfaces, such as a hemisphere, and other three-dimensional surfaces to be made.

Advantageously, the pattern may be impressed on the filter sheet in a variety of orientations, so that an even greater range of filter shapes may be made.

One or more apertures may be provided so that the filter medium is not too densely packed when the filter sheet has been folded and compressed.

Spacers may be provided to maintain a predetermined distance between the peaks and troughs and successive undulations.

Preferably, a housing is provided so as to maintain the folded sheet in its required shape and to assist a user in removing and replacing the filter assembly in a cleaning appliance, such as a vacuum cleaner.

The invention also provides a method of manufacturing a cleaning appliance filter *... assembly comprising the steps of impressing a predetermined pattern onto a sheet of a *::::* filter medium, and folding the sheet according to the pattern so as to form a doubly corrugated structure comprising a plurality of successive peaks and troughs, each peak * * * and trough having a plurality of undulations along it.

Preferably, the method further comprises the step of compressing the folded sheet to * form a predetermined shape.

Advantageously, the method also comprises the step of trimming the folded sheet.

Finally, the method may also include the step of locating the folded sheet in a housing.

The filter assembly may comprise a plurality of folded sheets located in the housing.

The sheet or sheets may be held in the housing by adhesive or may be clamped into the housing so as to be releasable from the housing for cleaning or replacement.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of a sheet of filter medium incorporating a first fold pattern; Figure 2 shows the sheet of Figure 1 in a part-folded state; Figure 3 is a perspective view of a filter assembly constructed according to the invention including the sheet of Figures 1 and 2; Figure 4 is a schematic plan view of a sheet of filter medium incorporating an alternative fold pattern; Figure 5 is a perspective view of a filter assembly constructed according to the invention including the sheet of Figure 4; * -* d * * Figure 6 is a perspective view of a sheet of filter medium folded according to the pattern ofFigure4; Figure 7 is a perspective view of the sheet of Figure 6 in a folded and compressed state; Figure 8 is a perspective view of a filter assembly constructed according to the invention including the sheet of Figures 6 and 7; Figure 9 is a front perspective view of a vacuum cleaner incorporating the filter assembly of Figure 8; Figure 10 is a sectional rear view of part of the vacuum cleaner of Figure 9; Figure 11 a is a side schematic view of part of machinery arranged to manufacture the filter assembly of the invention; Figure 1 lb is a side schematic view of another part of machinery arranged to manufacture the filter assembly of the invention; Figure 1 ic is a plan schematic view of another part of machinery arranged to manufacture the filter assembly of the invention; Figure 11 d is a side schematic view showing another stage of the manufacturing process of the filter assembly; Figure 11 e is a side schematic view of the final stage of the manufacturing process of the filter assembly; and * Figure 12 is a perspective view of an alternative sheet of filter medium. * ** * ** * * * I.

Like reference numerals refer to like parts throughout the specification. * * * * * * **

* : 0 With reference to Figure 1, a sheet of filter medium is shown and indicated generally by * the reference numeral 1. A pattern 2 of folds has been impressed on the sheet 1 by a method which is described further below. The pattern 2 comprises a plurality of substantially identical parallelograms 3, tessellated to cover most of the filter medium 1 with no gaps. The pattern 2 of parallelograms 3 is made up of a plurality of substantially parallel rows of zig-zag lines 4. Each of the zig-zag lines 4 comprises an undulating line having a plurality of alternating vertices. The vertices are substantially equal. The pattern 2 of parallelograms 3 also comprises a plurality of substantially parallel straight lines 5. The straight lines 5 are substantially orthogonal to the rows of zig-zag lines 4 and are disposed to pass through the vertices of the zig-zag lines.

Figure 2 shows the sheet I in the process of being folded. The folds themselves comprise alternating mountain folds and valley folds. In this regard, a mountain fold indicates a fold angle of greater than 1800 about the fold line as measured on the upper surface of the sheet 1, while a valley fold indicates a fold angle of less than 1800 about the fold line as measured on the upper surface of the sheet. For each straight line 5 of the pattern 2, the folding direction changes at every point where the line crosses or meets another. Each zig-zag line 4 comprises one mountain fold or valley fold, with its neighbouring zig-zag lines being of the opposite orientation.

As the sheet 1 of filter medium is folded and compressed, it forms a plurality of successive v-shaped reliefs, as is shown in Figure 3. The sheet undulates in two orthogonal planes: between the zig-zag lines 4 so as to form successive peaks and troughs; and along each peak and trough. Therefore, the surface of the sheet can be said to be doubly corrugated. The sheet 1 of filter medium is shown here in a filter assembly 6, which comprises the folded sheet held in a housing 7. The housing 7 comprises a frame which holds the sheet 1 in its folded configuration. The filter assembly 6 may be located in a cleaning appliance in order to filter particles from a fluid flow. In the example of the filter assembly 6 being employed in a vacuum cleaner, the filter * assembly may be located either upstream of a motor that generates suction, or **t downstream of it. S... * * S...

* Figure 4 shows schematically the sheet 1 of filter medium having an alternative fold pattern 8 impressed into it. In this embodiment, the pattern 8 comprises rows of *..SS.

* zig-zag lines of a first type 9, alternating with rows of zig-zag lines of a second type 10.

Each of the first type 9 of zig-zag lines comprises an undulating line having a plurality of alternating substantially similar vertices 11. Each of the vertices 11 defines a first angle 12. Each of the second type 10 of zig-zag lines comprises an undulating line having a plurality of alternating substantially similar vertices 13. Each of the vertices 13 defines a second angle 14. The first angle 12 and the second angle 14 are not equal, and so the alternating rows of zig-zag lines of the first type 9 and the second type 10 are not parallel. The difference between the first type 9 and second type 10 of zig-zag lines has been exaggerated in this drawing for illustrative purposes.

The pattern 8 also comprises a plurality of substantially parallel straight lines 15. The straight lines 15 are substantially orthogonal to the rows of zig-zag lines of the first type 9 and second type 10 and are disposed to pass through the vertices 11, 13 of the zig-zag lines. Thus, the fold pattern 8 comprises a plurality of trapezoids 16, tessellated to cover most of the filter medium I with no gaps.

For each straight line 15 of the pattern 8, the folding direction changes at every point where the line crosses or meets another. Each zig-zag line 9 comprises one mountain fold or valley fold, with its neighbouring zig-zag lines 10 comprising a fold of the opposite orientation. When the sheet 1 incorporating the fold pattern 8 is folded and compressed, it forms a doubly corrugated structure as before, with successive peaks and troughs, each peak and trough having undulations running along it. However, the nature of the fold pattern 8 means that the sheet also forms a curve. The plane of curvature is parallel to the direction of the parallel straight lines 15. Figure 5 illustrates a filter assembly 17 incorporating a plurality of square sheets 1' of a filter medium folded * according to the fold pattern 8. Each sheet 1' has been folded and compressed to form a filter segment 18. One end portion of each filter segment 18 has been compressed more than the opposite end portion, so that the filter segment also curves in a plane transverse * . : to the plane of curvature of the folded sheet. Thus, each filter segment 18 has a *: curvature defining part of a sphere. The filter segments 18 are mounted in a housing 19, which comprises a plurality of frames 19'. Each frame 19' houses a filter segment 18.

* 5 The resultant filter assembly 17 is hemispherical in shape. One way in which such a *..: filter assembly 17 may be employed will be discussed later in this specification.

Figure 6 shows a large sheet 20 of a filter medium that has been loosely folded according to a predetermined pattern. The sheet 20 is square in shape and has a square aperture 21 at its centre. If one imagines a line running from each corner of the aperture 21 to its corresponding corner of the sheet 20, then the sheet comprises four sections 20a, 20b, 20c, 20d, each of which is a trapezoid oriented at 900 to each of its neighbours. Each of the sections 20a, 20b, 20c, 20d has been impressed with the aforedescribed fold pattern 8 comprising straight lines 15 substantially orthogonal to alternating rows of zig-zag lines of a first type 9 and second type 10. For each section 20a, 20b, 20c, 20d, the fold pattern 8 has been oriented such that the straight parallel lines are substantially perpendicular to the parallel sides of the trapezoid -that is to say, the straight lines 15 are substantially perpendicular to the long edges of the sheet 20.

Thus, the fold pattern 8 of each section 20a, 20b, 20c 20d is oriented at substantially 90° to the folding pattern of its neighbouring sections. It is as though the section 20a, with its fold pattern 8, has been rotationally translated by a right-angle three times in succession. The nature of the fold pattern 8 produces a curvature of each of the sections 20a, 20b, 20c, 20d, which is not particularly noticeable in this drawing because the folded sheet has not been compressed.

Figure 7 shows the folded sheet 20 in a compressed state. The sheet 20 automatically assumes a part-spherical shape, by virtue of the fold pattern 8 and the compression of the sheet. By compressing the sheet 20, the aperture 21 shrinks such that the sections 20a, 20b, 20c, 20d come together at the centre of the sheet. A sheet 20 without an * aperture 21 could be employed, but it has been found that the folds close to the central * 0 region of the sheet are so tightly compressed together as to have a detrimental effect on S...

** the filtration performance of the completed filter assembly. * * * S*

:: A filter assembly 22 incorporating the folded and compressed sheet 20 is shown in Figure 8. It is similar in appearance to the filter assembly of Figure 5, but it comprises the single sheet 20 held in a housing 23. The housing 23 holds the sheet 20 of filter *..: medium in the required predetermined shape -in this case, a hemisphere -as well as enabling a user to remove and replace the filter assembly in a cleaning appliance, such as the vacuum cleaner of Figure 9.

Figure 9 shows a vacuum cleaner 24 comprising a main body 25, a user-operable handle 26 and a large roller 27 for rolling the cleaner 24 along a floor surface. A cleaner head 28 is pivotably mounted on the lower end of the main body 25 and a dirty air inlet 29 is provided in the underside of the cleaner head facing the floor surface. The handle 26 can be manipulated by a user to manoeuvre the vacuum cleaner 24 across a floor surface.

Separating apparatus 31 is releasably held on the main body 25. The separating apparatus 31 comprises a separator 32 and a collecting chamber 33. The interior of the separating apparatus 31 is in communication with the dirty air inlet 29 through ducting on the main body 25. The separating apparatus 31 can be removed from the main body 25 for emptying and for maintenance.

In use, a motor and fan unit 34 located inside the roller 27 draws dirty air into the vacuum cleaner 24 via the dirty air inlet 29. The dirty air is carried to the separating apparatus 31 via the ducting 30. The separator 32 comprises an upstream cyclone 35 in the collecting chamber 33. An air inlet is arranged tangentially to the upstream cyclone so that the incoming air is encouraged to follow a helical path around the interior of the upstream cyclone. The upstream cyclone 35 separates particles of dirt and dust from the airflow. The separated dirt and dust falls into the collecting chamber 33. * S.

A shroud 36 is located in the upstream cyclone 35 and comprises a cylindrical wall having a plurality of through-holes. The shroud 36 provides a communication path between the upstream cyclone 35 and a downstream cyclone assembly, which comprises * : .: a plurality of downstream cyclones 37 arranged in parallel. Each of the downstream cyclones 37 has a diameter smaller than that of the upstream cyclone 35. Therefore, the !5 downstream cyclones 37 are able to separate smaller particles of dirt and dust from the partially-cleaned airflow than the upstream cyclone 35. Separated dirt and dust exits the downstream cyclones 37 and passes into the collecting chamber 33.

Cleaned air then exits the separating apparatus 31 and enters a duct 38. The cleaned air then passes from the duct 38 to a pre-motor filter 39 inside the roller, as is shown in Figure 10. The direction of airflow is indicated by the arrows. The pre-motor filter 39 serves to trap any fine dust or microscopic particles which have not been separated by the two cyclonic separation stages 35, 37. The downstream side of the pre-motor filter 39 communicates with the fan and motor unit 34. This unit 34 accommodates a fan impeller which is driven by a motor to generate the suction airflow.

The outlet of the fan and motor unit 34 communicates with the filter assembly 22. The filter assembly 22 serves as a post-motor filter, to trap any remaining particles in the airflow, as well as carbon particles from the motor. Air then exits the filter assembly 22 and is exhausted from the vacuum cleaner 24 through outlet ports 40 in the roller. Of course, the filter assembly 17 could be used instead of the filter assembly 22.

The aforedescribed filter assemblies 6, 17, 22, with their sheets 1, 20 of filter material comprising a multitude of pleats, have large effective surface areas -much more so than if a simple unfolded sheet of filter medium were employed, or even if the sheet were pleated simply, like a concertina. Such filter assemblies 6, 17, 22 have a large capacity for dirt and dust, and so will have a substantially increased lifetime when compared with conventional filter assemblies. It is thought that the undulating nature of the folded sheets 1, 20 may also have a beneficial effect in damping sound generated by the motor * ** in use. *. 2 S...

The hemispherical filter assemblies 17, 22 also have the benefit of corresponding to the * a., interior contours of the roller 27. Previously, it had been difficult to find room for * : filters inside the roller; the constraints of the roller meant that only relatively small filters could be placed here. The invention permits filters with a large capacity for dirt and dust to be accommodated inside the roller.

A method of forming a filter assembly of the invention will now be described, with particular reference to the filter assembly 22. Figures 1 la to lie illustrate the proposed method.

In Figure 11 a, the sheet 20 of filter medium is drawn between a pair of cylindrical rollers 41a, 41b. The rollers 41a, 41b have the fold pattern 8 formed on their surfaces in relief. One roller 41a may have part of the pattern 8 formed on it, such as the zig-zag lines 9, 10. The remainder of the pattern 8, for example the straight lines 15 may be formed on the other roller 41b. As the sheet 20 passes between the rollers 41a, 41b, the fold pattern 8 is impressed on it, creating lines of weakness to assist in the folding of the sheet.

The sheet 20 then passes between a pair of pleating drums 42a, 42b such as are shown schematically in Figure JIb. The pleating drums 42a, 42b are driven in synchronism with the rollers 41a, 41b and comprise a plurality of blades (not shown) which are urged against the lines of the fold pattern 8. The upper drum 42a presses down against the sheet 20 and so forms valley folds in its upper surface. The lower drum pushes up against the sheet 20 and forms mountain folds in its upper surface.

The resulting folded sheet 20 then needs to be compressed appropriately; the next stage of the process is shown schematically in the plan view of Figure 11 c. The folded sheet of filter medium is placed in a collapsing jig 43. The collapsing jig 43 applies * pressure to the four corners of the sheet 20, as is shown by the arrows in this drawing.

ut0 This causes the folds to come closer together, forming a more compressed filter **** * structure. The jig 43 may also be arranged to apply pressure from above or below in * : S: order to assist the sheet 20 in forming the required curvature. *

S.....

* The folded and compressed sheet 20 is then trimmed by blades 44, such as are shown in Figure 1 Id. The blades 44 trim off excess material from the sheet 20. Finally, the sheet *..: 20 is potted into the housing 23, as shown in Figure Ile. The edges of the sheet 20 are held in the housing 23 by means of adhesive 45. Alternatively, the sheet 20 may be held in the housing by means of a user-operable clamp, so that the user may remove the filter medium from the housing for replacement or cleaning.

The invention is not limited to the detailed description given above. Variations will be apparent to the person skilled in the art. One such alternative is shown in Figure 12.

This drawing shows the filter sheet 20 of Figure 6, but with spacers 46 on each trapezoid. Only some spacers 46 are shown in this drawing for clarity. The spacers 46 ensure that, when the sheet 20 is folded and compressed, each trapezoid is separated from its neighbour by a predetermined distance. The final pleated surface of the filter sheet 20 will undulate uniformly. Thus, airflow through the final filter assembly will be uniform across the surface of the filter, with no tightly compressed portions that may inhibit airflow or clog up with dirt.

The spacers 46 may be formed by glass beads attached to a surface of the filter sheet 20.

Typically, the beads will be very small so as not to affect detrimentally the effective surface area of the sheet 20. Alternatively, the spacers 46 may be formed by small blobs of adhesive or molten plastic applied to the surface of the sheet 20 and left to cool.

As a further alternative, the spacers 46 may be formed by embossing the surface of the sheet 20 itself This has the advantage of not reducing the effective surface area of the sheet 20.

* The filter medium may comprise paper, preferably loaded with glass fibres, * 20 Polytetrafluoroethylene (PTFE) or other fluoropolymer, fiberglass, foam or stiffened **** * * cotton. S. S * S S

:: The invention has been described in relation to a filter assembly having a flat or part-spherical shape, but the filter may be made in any one of a variety of shapes or curves including barrel shapes, cubes or other polyhedrons, cones or cylinders. The contours .: of the folded filter need not be uniform. Different contours may be achieved by varying the distances between folds, the angles of the vertices and the extent of compression of the filter. The sheet may have different fold patterns on different portions of it.

The filter assembly 22 was described as being located downstream of the fan and motor unit 34. Any one of the filter assemblies 6, 17, 22 could be located downstream of the fan and motor unit 34, upstream of it, or both upstream and downstream.

The cleaning appliance need not be an upright vacuum cleaner. The invention is applicable to other types of vacuum cleaner, for example, cylinder machines, stick-vacuums or hand-held cleaners. Further, the present invention is applicable to other types of cleaning appliances, for example, a wet and dry machine or a carpet shampooer.

In the described method, the fold pattern may be scored onto the sheet, or by punching the fold pattern onto it. Alternatively, laser etching, or etching involving a photo-resist stage may be employed. * S. * . . * ** ** * S S... ** . * SI * S.

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* . *... * . S. *5 * . * * S S. S

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Claims

CLAIMS1. A cleaning appliance filter assembly comprising a sheet of filter medium, the sheet being folded in a predetermined pattern so as to form a doubly corrugated structure comprising a plurality of successive peaks and troughs, each peak and trough having a plurality of undulations along it.
2. A filter assembly as claimed in claim 1, in which the predetermined pattern comprises a plurality of rows of zig-zag lines having alternating vertices and straight lines running transverse to the rows of zig-zag lines, the straight lines being arranged to pass through the vertices.
3. A filter assembly as claimed in claim 2, in which the zig-zag lines and straight lines are arranged to form a plurality of tessellated parallelograms.
4. A filter assembly as claimed in claim 2 or claim 3, in which the zig-zag lines are parallel.

*
5. A filter assembly as claimed in cJaim 2, in which the zig-zag lines and straight 10 lines are arranged to form a plurality of tessellated trapezoids. * *

.:
6. A filter assembly as claimed in claim 2 or claim 5, in which the zig-zag lines * : comprise alternating lines of a first type and a second type; the first type of zig-zag line having vertices of a first angle and the second type of zig-zag line having vertices of a *. .* * 5 second angle. ** * * * * I.
7. A filter assembly as claimed in any preceding claim, in which the folded sheet has a curved surface.
8. A filter assembly as claimed in any preceding claim, in which the folded sheet has a part-spherical surface.
9. A filter assembly as claimed in any preceding claim, in which the sheet incorporates the predetermined pattern in a plurality of orientations.
10. A filter assembly as claimed in claim 9, in which the sheet comprises a plurality of sections folded according to the predetermined pattern, each section being oriented transversely to its neighbouring section.
11. A filter assembly as claimed in claim 10, in which each section is oriented at substantially 90° to its neighbouring section.
12. A filter assembly as claimed in any preceding claim, further comprising spacers arranged to maintain a predetermined distance between successive peaks and troughs.
13. A filter assembly as claimed in any preceding claim, further comprising spacers arranged to maintain a predetermined distance between successive undulations.
14. A filter assembly as claimed in any preceding claim, further comprising a housing for the folded sheet. * S.
15. A filter assembly as claimed in any preceding claim, comprising a plurality of *. * sheets of filter medium, each sheet being folded in a predetermined pattern so as to form a doubly corrugated structure and the folded sheets being assembled to form a predetermined shape. 55s55 *
16. A filter assembly as claimed in claim 15, in which the folded sheets are located : in a filter housing so as to form a filter assembly of the predetermined shape.
17. A cleaning appliance incorporating a filter assembly as claimed in any preceding claim.
18. A cleaning appliance as claimed in claim 17, in the form of a vacuum cleaner.
19. A cleaning appliance as claimed in claim 17 or 18, further comprising a motor arranged to generate a fluid flow, in which the filter assembly is located downstream of the motor.
20. A cleaning appliance as claimed in claim 17 or 18, further comprising a motor arranged to generate a fluid flow, in which the filter assembly is located upstream of the motor.
21. A method of manufacturing a cleaning appliance filter assembly comprising the steps of impressing a predetermined pattern onto a sheet of a filter medium, and folding the sheet according to the pattern so as to form a doubly corrugated structure comprising a plurality of successive peaks and troughs, each peak and trough having a plurality of undulations along it.
22. A method as claimed in claim 21, further comprising the step of compressing the folded sheet to form a predetermined shape.
23. A method as claimed in claim 21 or 22, further comprising the step of trimming the folded sheet. S... * I *I.
24. A method as claimed in any one of claims 21 to 23, further comprising the step of locating the folded sheet in a housing. . **: .5 24. A cleaning appliance filter assembly, a cleaning appliance, or a method of manufacturing a cleaning appliance filter assembly, substantially as hereinbefore described, with reference to, or as illustrated in, the accompanying drawings.