The invention is particularly, but not exclusively, applicable to ski boots. Ski boots conventionally have rigid plastic shells. It is known to customise a ski boot to a user's foot by use of a thermally formable liner. Most commonly, the liner is placed in the boot and heated by a hot air gun. Once heated, the user inserts his foot and tightens the boot, whereupon the liner is moulded to the shape of the foot and sets in that shape as it cools.

This procedure requires heat to be applied until the liner reaches the appropriate temperature. This is difficult to judge, and it is easy to apply excessive heat, leading to discomfort or worse for the user. Moreover, the liner will typically be

thicker in some places (for example, over the ankle bone) than in others (e. g. over the instep). This can lead to excessive heat being applied to the foot at the thinner parts of the liner.

It is known to use electrically heated elements in moulding such inserts in ski boots. See for example US Patent 6003248. However, the electrical heating arrangements proposed hitherto have a number of disadvantages. They use relatively complex constructions with wires or ribbon conductors laminated to carrier films, and do not give a good distribution of heat, the resistance heating elements being relatively localised.

According to one aspect of the present invention, there is provided an electrically heated insert for an article of clothing or footwear, the insert comprising an electrical resistance heating means juxtaposed with a layer of a thermally conductive material.

The heating means preferably comprises one or more pieces of electrically resistive polymer sheet, and is sandwiched between two sheets of flexible thermally conductive material.

The thermally conductive material may be a textile base coated with silicone rubber containing particles of a high heat conductivity material.

Alternatively,

said thermally conductive material may be a woven fabric coated with polyurethane.

Another aspect of the invention provides an electrically heated insert for an article of clothing or footwear, the insert comprising a carrier, a plurality of electric resistance heating elements secured to or embedded in the carrier, and conductors for connecting said elements to a common electrical power source, said elements being adapted to attain different temperatures when so connected.

The carrier is preferably a sheet of thermally conductive material.

Preferably, the insert includes a second sheet of thermally conductive material, the heating elements being sandwiched between said sheet and second sheet.

From a further aspect, the invention provides an electrically heated insert for use in conjunction with a thermally formable liner for customising an article of clothing or footwear, the insert including electric resistance heating means which, when connected to a first, relatively high electrical power level reaches a temperature sufficient to soften said thermally formable liner, and which, when connected to a second, relatively low electrical power level reaches a temperature not sufficient to soften said thermally formable liner but sufficient to provide drying out of moisture.

In one embodiment, the insert forms an integral part of said thermally formable liner.

Embodiments of the present invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 is a front view of a combined liner and insert for use in a ski boot; Fig. 2 is a cross-section on the line 2-2 of Fig. 1 ; Fig. 3 is a view of part of Fig. 1 with layers removed; Fig. 4 illustrates the electrical connections used in conjunction with the liner/insert; Fig. 5 is an exploded cross-section of a modified version of the embodiment of Figs. 1 to 4; and Fig. 6 is a diagrammatic partial perspective view of another embodiment.

Referring to Figs. 1 and 2, a combined liner and insert 10 comprises two layers 12 and 14 of a thermally formable plastic foam of a type known per se. The liner/insert 10 is of a wing shape having a narrow portion 16 which wraps around the back of the ankle in use, and a pair of wings 18 and 20 which overlie the sides of the ankle and the sides of the instep. As seen in Fig. 2, the foam layer 12 is of variable thickness with a thin portion 12a behind the ankle and a thick portion 12b over the ankle bone.

Located within the layers 12 and 14 are electrical heating elements 22 which are disposed within heat conductive layers 24 and 26 which provide an even spread of heat to the thermally formable layers 12, 14. The heat conductive layers 24,26 are of a flexible material of high heat conductivity. A number of specialist heat transfer materials may be used, such as those available from Warth International under the designations K177 and K228.

These materials consist of a glass fibre (or similar) backing material coated with silicone which has been doped with additives exhibiting good thermal transfer properties, for example zinc oxide and aluminium oxide.

Referring to Fig. 3, each of the wings 18 and 20 has a pair of heating elements 22a and 22b. These are formed of a flexible resistance heating material, the preferred material being FabRoc by DC Heat Limited which is synthetic rubber loaded with carbon particles. In this embodiment, the heating elements 22 have metal braid indicated at 28 stitched along their sides, connected by wires 30 to an external connector (not shown) through which electrical power can be applied when desired.

The inner heating elements 22a are selected such that, when a given electrical power is applied to the liner, they will reach a predefined temperature sufficient to thermoform the adjacent foam layers 12 and 14 (typically about 120°C). The outer

electrodes 22b will reach a lower predetermined temperature (typically about 80°C at the same applied power level, in order to thermoform the adjacent foam layers which are of a lesser thickness than the areas 12 and 14. The differing heating element predefined temperatures are achieved by selection of the following factors: -The formulation of the heating element material -The dimensions of the heating elements -The physical dimensions of the conductive paths and their placement on the heating elements -The electrical power applied to the elements -The duration of the applied power.

As shown in Fig. 4, the heating elements 22 are electrically connected in series parallel and can be selectively connected to a low power supply 26 or to a high power supply 28.

To customise a ski boot to the user's foot, the insert/liner 10 is inserted into the boot. The heating elements 22 are connected to the higher power supply 28 and power is applied. At completion of a predetermined time, the heating elements 22 will have generated sufficient heat for thermoforming to occur. The power is disconnected automatically from the heating elements 22 by a timer circuit in the power supply.

The wearer's foot is then inserted into the boot and liner. The boot is tightened and the foam layers 12 and 14 deform to the shape of the foot and the shape of the boot. As the foam cools, it returns to its non-deforming state and retains the customised shape of the foot and the boot.

The thermally conductive layers 24 and 26 ensure that softening of the foam is more rapid and more uniform than would occur with heating elements alone.

Because the heating elements 22 are in the middle of thermally formable foam layers, the liner can be moulded both to the foot and the boot. This allows one style and type of liner to be used across a range of boot sizes, differing designs, and also for use in other manufacturers'boots. This is in contrast to the prior art, where a separate type of liner is required for each boot size, each style, and each manufacturer.

The heating elements are encapsulated permanently in the liner. This allows a second mode of use, where the presence of the heating element allows the liner to be dried after use either in the boot or outside the boot. In this case, the elements are connected to the lower power supply 26 which produces element temperatures of approximately 60°C and 50°C.

The power supplies 26,28 could supply the desired power levels simply by supplying two different voltages, for example 12V and 24V. However, this

may not provide accurately reproducible temperature effects because of non-uniformity of voltage drop owing to variations in conductors and connectors.

It is therefore possible that the power supplies are constant current devices, thus giving accurate 2R heating in the heating elements.

Fig. 5 shows an exploded cross-section through part of a modified embodiment. A heating element 50 of Fabroc material is secured to a heat spreading member 52 by stitching indicated at 54 which is also used to secure flattened braid conductors 56 to the Fabroc material 50. A further heat spreading member 58 is secured over the conductors 56 by adhesive. A thermoformable plastics layer 60 is secured over the further heat spreading member 58 by adhesive, stitching, HF welding, or other fixing method. The assembly is completed by an outer combined fabric and foam layer 62 and an inner fabric layer 64 which may also be secured by adhesive. It will be appreciated that the overall shape of the liner insert is as shown in Figs. 1 and 3.

In this embodiment, the heat spreading members 52 and 58 are flexible, textile-based members made from a material of low thermal inertia. One suitable example is a base fabric weave coated on one side with polyurethane and on the other side with a waterproof coating, such as is used in foul weather clothing. One suitable material is available from Lothian Coated Fabrics under the designation CF0706.

Such materials are considerably cheaper than the specialist thermal transfer materials such as K177.

We have also found that the specialist thermal transfer materials are extremely effective in transmitting heat through the thickness of the liner, they are not entirely satisfactory in transferring heat across the area of the liner and thus reducing hot spots. The arrangement shown and described with reference to Fig. 5 is more effective in achieving this.

A further embodiment is illustrated in Fig. 6. In this embodiment a generally boot-shaped liner 70, only part of which is shown diagrammatically in Fig.

6, is preformed from a thermoformable polymer material which is not electrically conductive but which is capable of spreading heat. One suitable material is non-electrically-conducting polymer.

The liner 70 is moulded around four heating elements, two of which are shown at 72. Electrical connection to the heating elements 72 is provided by means of conductive tracks 74 which are laid down by screen printing with a conductive polymer ink.

This embodiment operates in the same way as the previous embodiments, but can be more easily mass produced.

The use of electrically resistive polymer heating elements is preferred for reasons of simplicity and economy, but other forms of heating element may be used, such as serpentine wires or flexible printed circuits.

The use of metallic braid to connect to the resistive polymer is a relatively simple system and has advantages of flexibility. Varying widths of braid, variable spacing between braid conductors, differing methods of stitching, and differing stitching tensions can be used to provide differing power carrying capacity and power transfer capability for individual system requirements. A suitable form of braid is tinned copper braid of the type used in equipment earthing straps. The braid may be stitched to the polymer with metallic thread to improve the electrical connection.

The use of stitched-on metallic braid does, however, present problems of accuracy and repeatability in terms of electrical parameters, and is relatively labour intensive in manufacture. To avoid or reduce these drawbacks, other conductor schemes are possible.

As indicated above with reference to Fig. 6, one alternative is the use of conductive polymer inks which can be applied by silk screening or other printing techniques. It is also possible to use conductive paints such as those based on silver or nickel, which can be applied by silk screening or other printing processes, or by spraying.

(Conductive paint can also be applied to tinned copper braid to achieve better contact.) Another option is the use of embedded conductors, either by sandwiching conductors between two sheets

of conductive polymer, or by moulding the polymer around conductors as a unitary moulding. The embedded conductors can be wires, flat strips, or flexible circuits.

Other options include: (a) Conductors, which may be flexible copper circuits or nickel flat strips, secured to the surface of the polymer by electrically-conductive adhesive.

(b) Polymer doping, in which the base polymer is doped in some parts to provide a resistance heating element, while other parts are doped to produce low resistance and act as conductors.

(c) Electroless plating of conductor tracks.

Although described with particular reference to ski boots, the invention may be applied to other articles such as skating boots or safety helmets.