FIELD OF THE INVENTION The invention relates to improved air filters for hair dryers, and to methods of manufacture of such filters.

BACKGROUND TO THE INVENTION A typical hand-held hair dryer comprises a hand-held housing with an air inlet, an air outlet, and a motor in between to draw air in from the air inlet and drive air out from the air outlet. A heating element is located in the air flow between the air inlet and the air outlet, typically after the motor in the air flow. In some preferred implementations of a hair dryer the motor is coupled to a mix flow impeller, which draws air in axially and generates a high air pressure by thrusting the air outwards, where it is confined by the housing and forced through the hair dryer air outlet. The high pressure achieved by such a technique is useful in forcing apart the strands of hair.

Typically a hair dryer is provided with a filter on the air inlet which, among other things, stops hair from being accidentally drawn into the motor. Known filters are typically either formed by plastic injection moulding or use a metal mesh. However such filters can cause problems because over time dust and other debris builds up in the filter, reducing the air pressure and with more severe clogging causing overheating of the element and cyclical cutting-out the hair dryer resulting in a hot/cold air flow.

It is desirable to improve upon the prior art filtering techniques. SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a hair dryer having a handheld housing comprising an air inlet, an air outlet, a motor between said air inlet and said air outlet to draw air in from said air inlet and drive air out from said air outlet, and a heating element located in said air flow between said air inlet and said air outlet, wherein said hair dryer further comprises a filter on said air inlet, wherein said filter comprises a metal filter plate bearing a plurality of holes, wherein at least some of said holes have a maximum lateral dimension of less than 1 .5mm and wherein walls of said holes through a thickness of said filter plate are substantially straight or at least partially concave.

In embodiments a said wall of a hole comprises at least one concave section. In other embodiments a said wall of a hole comprises a pair of sections, a first concave section and a second substantially straight section such that the hole wall is in part concavely curved and in part straight.

The structure of a metal filter plate according to embodiments of the invention provides an improved air flow through the hair dryer and contributes to addressing the problems outlined in the introduction. In some preferred embodiments at least a subset of the holes of the filter plate comprises hexagonally close-packed holes, which provide the greatest aperture to metal ratio. Preferably at least some of the holes have a maximum lateral dimension of less than 1 mm; preferably the filter plate has a thickness of less than 0.5mm, for example approximately 0.3mm (using a thinner plate facilitates smaller holes). Preferably a majority of the holes have a maximum lateral dimension of less than 1 .5mm or less than 1 mm; in embodiments a majority of the holes are substantially circular. In preferred embodiments the filter plate is substantially flat.

In some preferred embodiments the filter plate includes all or a subset of holes which have a maximum lateral dimension which reduces with increasing radial distance from a lateral centre of the filter. In particular when the motor is coupled to an impeller a velocity profile of the air flow defines a velocity which reduces as the circumferential edge of the air inlet is approached, and it is advantageous to match a proportion of the holes to this velocity profile. Nonetheless in some preferred embodiments a laterally central region of the filter has a reduced density of holes. Thus in embodiments there may be two subsets of holes, a first subset comprising holes of generally uniform size, and a second subset comprising holes of different sizes, more particularly sizes which reduce towards the edge of the air inlet. In some embodiments the lateral dimension of all or a subset of the holes reduces by a factor of at least 2.5. In embodiments of the hair dryer the filter plate includes holes which are aligned along a plurality of spiral arms extending from a laterally central region of the filter; these may be the holes which reduce in lateral dimension towards the edge of the filter plate. This is because the blades of the impeller create less noise if they laterally 'cut across' the filter holes in a stepwise fashion or 'slicing' rather than cutting across holes which are aligned directly along radii. This can be more easily understood if, conceptually, the filter plate is moved so that it is adjacent the blades of the impeller, as then the blades would cut across each hole: although in practice the filter is displaced away from the impeller blades (often there is a finger guard in between) a similar effect arises. Thus, broadly speaking, in embodiments the holes, or at least some of the holes, define a pattern which has a handedness or angular sense of rotation which matches a sense of rotation of the impeller.

In embodiments of the hair dryer the filter is mounted on a user-detachable filter mount, and the filter itself is user-detachable from the filter mount, to facilitate interchange of filters. This allows the user to remove the filter, easing cleaning of the filter and allow the user to customise the hair dryer with different filters.

In some preferred embodiments the holes in the filter plate are formed by acid spray etching which helps to achieve the desired hole wall profile.

Thus a related aspect the invention provides a method of filtering air for a hair dryer, the hair dryer having a hand-held housing comprising an air inlet, an air outlet, and a motor between said air inlet and said air outlet; the method comprising: forming a metal filter plate by acid etching a plurality of holes in a metal plate such that walls of said holes are substantially straight or at least partially concave, and wherein at least some of said holes have a maximum lateral dimension of less than 1 .5mm; and using said metal filter plate to filter air drawn into said hair dryer. In some preferred embodiments a said wall of a hole formed by said method comprises at least one concave section. In other embodiments a said wall of a hole formed by said method comprises a pair of sections, a first concave section and a second substantially straight section such that the hole wall is in part concavely curved and in part straight. In some preferred embodiments the metal filter plate is fabricated from stainless steel.

In a further related aspect the invention provides a filter for a hair dryer wherein said filter comprises a metal filter plate bearing a plurality of holes, and wherein said holes comprise at least a subset of holes for which a lateral dimension the holes reduces with increasing radial distance from a lateral centre of said filter.

In a still further aspect the invention provides a filter for a hair dryer, wherein said filter comprises a metal filter plate bearing a plurality of holes, and wherein said holes comprise at least a subset of holes, aligned along a plurality of spiral arms extending from a laterally central region of said filter.

Preferably a filter as described above is incorporated into a hair dryer comprising an impeller located between the air inlet and an air outlet of the hair dryer.

In any of the above aspects of the invention, the metal filter plate may be flat. When the filter plate is lying flat on a horizontal surface, hole walls in the metal filter plate may be vertical walls. In this specification 'hair dryer' is to be interpreted as covering a similar device used for hair styling.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows a side view a hair dryer according to an embodiment of the invention; Figure 2 shows a cross sectional view of the hair dryer of Figure 1 ;

Figure 3a shows an embodiment of a metal filter plate attached to the hair dryer of Figure 1 ; Figure 3b shows a cross sectional side view the metal filter plate of Figure 3a; Figure 3c shows a zoomed in view of a portion of the metal filter plate of Figure 3a;

Figure 3d shows a zoomed in cross sectional side view of the portion of the metal filter plate of Figure 3c;

Figures 4a-4d show variations in the walls of holes of the filter plate of Figure 2a;

Figure 5 shows a 3D view of the rear of the hair dryer of Figure 1 showing the filter of Figure 3a and a filter mount; and

Figure 6 shows the filter plate of Figure 3a detaches from a filter mount.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a side view of a hair dryer 10 and Figure 2 shows a cross sectional view of hair dryer 10 according to an embodiment of the invention. The hair dyer has an air inlet 12, an air outlet 14, a motor 16 located between the inlet 12 and outlet 14. The motor 16 powers an impeller 18 which draws in air axially from the inlet 12 and expels air radially from the impeller towards outlet 14. A heating element 17 is located between the motor and outlet to heat the air before it is expelled through the outlet. Attached to the handle 1 1 are controls 15 for turning the hair dryer on and off, controlling the rotational speed of the motor (and thereby controlling the flow rate of expelled air) and temperature of the expelled air.

Coupled to the rear of the hair dryer, about the air inlet 12, a metal filter plate 20 is mounted in a filter mount 30. The metal filter plate 20 attaches to the filter mount 30, the filter mount then attaches to the rear of the hair dryer so that the air inlet 12 is covered by the filter plate 20. A finger guard 13 located at the air inlet ensures that a user is prevented from inserting a finger into the body of the hair dryer with the filter mount and plate removed.

Figure 3a shows the metal filter plate 20 that can be mounted on the air inlet of hair dryer 10. The filter plate 20 has smooth flat surfaces with an arrangement of holes through the filter to permit air to be drawn into the hair dryer whilst minimising dust, debris and hair being pulled into the hair dryer. The smooth surface also means that the filter plate is easy to clean. A portion 26 of the metal filter plate 20 is shown in more detail in Figure 3c.

Figure 3b shows a cross sectional side view of the metal filter plate of Figure 3a. In the embodiment of Figure 3a-3d the filter plate is made from 0.3mm stainless steel. It will be appreciated however that other variants are possible and the composition and thickness of the metal plate may vary.

Locating lugs 22, 24 control the positioning of the filter plate when engaged to the filter mount 30 on the hair dryer. Corresponding receptors on the filter mount 30 ensure that the filter plate is arranged to sit in a prescribed position within the mount. This prevents the filter plate rotating unintentionally thereby ensuring the arrangement of holes on the filter plate is optimised. The filter plate 20 is removable from the mount for cleaning and optionally changing with another filter plate.

The lateral dimension of some of the holes in the filter plate decreases with increasing radial distance from the centre of the filter plate as shown by holes 21 a, 21 b and 21 c respectively. This provides a higher proportion of holes approaching the centre of the filter where the velocity profile of the drawn in air can be higher. In Figure 3a for example, the holes are arranged into a spiral formation, with holes decreasing in size towards the outer edge of the filter plate. In the embodiment show, the laterally central region of the filter (which in the embodiment show aligns with the axis of the motor 16 and impeller 18 in Figure 2) has minimal holes.

Referring now to Figure 3c, this shows a five times zoomed in view of a portion 26 of filter plate 20. Figure 3c shows how some of the holes are formed as hexagonal holes 25 in the filter plate to permit a high density arrangement of holes. In the embodiment shown, the lateral dimension of these hexagonal holes 25 are approximately 1 mm (some less) and the holes are separated by a distance of approximately 0.25mm. Figure 3d shows a cross sectional side view of the metal filter plate portion of Figure 3c. Figures 4a - 4d shows cross sectional view of the sections of the metal filter plate 20 of Figure 3a. Figures 4a-4d show variations on the walls of the holes 23a, 23b, 23c, 23d formed by spray acid etching a stainless steel sheet to form the filter plate. Holes 23a - 23d for example show hole walls with concave sections. The spray acid etching may also leave a portion of walls of some holes substantially flat; others may have a section that is flat and a section concave. Spray acid etching from both sides of the sheet, for example, may result in a 'double concave' surface topography such as shown by hole 23b. Very small holes are achievable with such a process with the embodiment herein shown having holes with lateral dimensions down to approximately 1 mm to ensure an adequate air flow (although smaller holes may be present if so desired, preferably in the vicinity or larger holes to maintain an adequate air flow).

Figure 5 shows a 3D view of the rear of the hair dryer 10 with the filter mount 30 and filter plate 20 attached. Also shown in more detail is the finger guard 13. The filter mount 30 couples to the rear of the hair dryer via a thread (or any other conventional locking system).

Figure 6 shows an exploded view of the filter plate 20 and filter mount 30. The filter plate fits into the filter mount 30, guided by lugs 22, 24 being received into corresponding locating holes on the filter plate. These prevent the filter plate rotating within the mount.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.