Patent Publication Number: US-2015080989-A1

Title: Moulding Method

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of moulding an article, and to an article produced by that method. The method can be used to manufacture a pad for applying stimulation to a human or animal body, but the method can also be used to manufacture a wide variety of other articles. 
     BACKGROUND OF THE INVENTION 
     For a variety of therapeutic applications, several treatment modalities are currently known in the art including electrical stimulation, heat therapy and thermostimulation. Electrical stimulation involves the application of an electrical current to a single muscle or a group of muscles through one or more stimulation pads that are temporarily attached to the skin. A conductive gel is often used to improve the electrical conductivity between the stimulation pads and the skin. The muscle contraction that results from the applied electrical current can produce a variety of effects from strengthening injured muscles and reducing oedema to relieving pain and promoting healing. Heat therapy involves the application of heat to the body. Heat therapy is very useful as it has a number of effects such as relaxation of muscle spasm and increased blood flow that promotes healing. However, combination therapy, i.e. the synergistic use of other modalities such as massage, ultrasound and/or electrical stimulation has been found to be more effective than heat therapy alone. Thermostimulation is one such combination therapy that involves the use of heat therapy and electrical stimulation simultaneously. With thermostimulation, the healing benefits of heat are provided along with the strengthening, toning, pain relieving and healing benefits of electrical stimulation. Moreover, the application of heat has been found effective in that it allows the patient to tolerate higher currents. This yields higher electric field strengths, greater depths of penetration and, therefore, more positive results than could be achieved with electrical stimulation without heat. Thermostimulation can be performed using pads that are temporarily attached to the skin. 
     The inventors have identified a number of deficiencies with currently-available stimulation pads. There is a need for improved stimulation pads for electrical stimulation, heat therapy and thermostimulation. The inventors consider that an improved stimulation pad should: be able to deliver a sufficiently high electrical current to cause effective stimulation to the patient&#39;s muscles; be able to deliver sufficient heat to keep the patient&#39;s skin temperature constant at temperatures up to 43° C.; have a high quality appearance; be possible to clean; be sufficiently flexible to allow good contact with the patient&#39;s skin; not cause irritation to the patient&#39;s skin; be watertight; and be durable. 
     Existing moulding methods are unable to produce stimulation pads that meet these exacting requirements. In particular, the inventors have found that, when existing moulding methods are used to mould one portion of an article that comprises a soft polymer against another portion of the article that also comprises a soft polymer, there is a tendency for flash to form at the interface of the two portions (which is detrimental to the appearance of the article), for the portions to deform (which is detrimental to the functionality and appearance of the article) or for the two portions not to bond to each other properly (which is detrimental to watertightness and durability). Furthermore, when electronic components are to be embedded within the article, there is a risk that the components could be damaged by the heat and pressure that are experienced during the moulding process. 
     WO 2011/150092 relates to a method of insert moulding of liquid silicone rubber. U.S. Pat. No. 7,739,791 relates to a method of producing an overmoulded electronic assembly. U.S. Pat. No. 6,319,448 relates to a process for the production of a measuring apparatus by total or partial overmoulding of its functional elements, particularly of its components and electric and electronic circuits. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides a method of moulding an article, comprising: forming a first portion of the article by injecting a first material into a first mould, the first mould being shaped so as to define a flange on the first portion of the article; placing the first portion of the article into a second mould, the second mould having a first plate and a second plate; compressing the flange between the first and second plates of the second mould; and forming a second portion of the article by injecting a second material into the second mould whilst the flange is compressed between the first and second plates of the second mould. 
     The hardness of the first portion of the article increases when the flange is compressed. This increase in hardness makes the first portion of the article more able to withstand the compressive force that is experienced when the second portion of the article is formed, and thereby prevents deformation of the first portion. 
     Preferably, compressing the flange causes the flange to deform, thereby creating a seal between the flange and the second mould. The seal reduces the risk of flash on the parting line between the first and second plates of the second mould. This improves the visual appearance of the article and can avoid the need for a finishing process to remove flash. 
     Preferably, the first and second materials each comprise a soft plastic material. Preferably, the hardness of the first material and the hardness of the second material are each measurable on the Shore A durometer scale. At least one of the first material and the second material preferably has a Shore A hardness less than or equal to 85. When the first material is soft, the flange is able to deform when compressed between the first and second plates of the second mould. This creates a good seal between the flange and the second mould, and thereby reduces the risk of flash. 
     Preferably, both the first material and the second material have a Shore A hardness less than or equal to 85. When both the first and second materials are soft, the temporary increase in hardness that is achieved by compressing the flange allows one soft material to bond to another soft material without deformation of the first portion. 
     More preferably, at least one of the first material and the second material has a Shore A hardness of 45 to 70. More preferably, both the first material and the second material have a Shore A hardness of 45 to 70. In particular examples of the invention, both the first and second material have a Shore A hardness of 70. 
     Preferably, the first and second materials each comprise a polymer. More preferably, the first and second materials each comprise a thermoplastic material. More preferably, the first and second materials each comprise thermoplastic polyurethane. Thermoplastic polyurethane (TPU) is well suited for injection moulding, can be made relatively soft and exhibits good mechanical and chemical properties. 
     The method preferably further comprises placing an electrical circuit onto the first portion of the article prior to forming the second portion of the article. The electrical circuit is thereby encapsulated between the first and second portions of the article. The good bonding between the first and second portions of the article that results from the moulding method described herein prevents this ingress of water and other undesired substances into the circuit. 
     The method preferably further comprises covering at least a portion of the electrical circuit with a protective housing prior to forming the second portion of the article. The protective housing protects the circuit from the heat and pressure that are experienced when the second material is injected. 
     The second material is preferably injected into the second mould from an injection point that is oriented to direct the second material towards the protective housing. This results in the second material making contact with the protective housing before it makes contact with the circuit, and thereby reduces the risk of the circuit being damaged by the heat and pressure of the second material. 
     The circuit is preferably connected to a cable, wherein one end of the cable is covered by the protective housing and wherein the other end of the cable extends outside the protective housing. The protective housing protects the delicate connection between the circuit and the cable from the heat and pressure that are experienced when the second material is injected. 
     The cable preferably comprises a sheath, wherein the protective housing is adapted to grip the sheath. The protective housing thereby protects the connection between the circuit and the cable from being damaged by tensile forces that may be applied to the cable. The protective housing is preferably adapted to form a seal with the sheath. The seal between the protective housing and the sheath prevents the second material from entering the protective housing, and thereby prevents damage to the connection between the circuit and the cable. 
     Preferably, the circuit comprises a substrate and the protective housing is adapted to form a seal with the substrate. The seal between the protective housing and the substrate prevents the second material from entering the protective housing, and thereby prevents damage to the portion of the circuit that is contained within the protective housing. 
     Preferably the circuit is connected to a cable comprising a sheath and the second portion of the article is bonded to the sheath. The bonding of the second portion to the sheath prevents this ingress of water and other undesired substances into the circuit. 
     The article is preferably a pad for applying stimulation to a human or animal body. The method described herein is particularly advantageous when used to manufacture a stimulation pad. For example, the use of soft materials results in a stimulation pad that is flexible and able to conform to the contours of the user&#39;s body, thereby allowing good contact to be made with the skin during stimulation. The stimulation pads have a high quality appearance due to the reduction of flash on the parting line between the first and second portions of the mould. Furthermore, the moulding method described herein results in a good bond between the first and second portions, which makes the stimulation pad durable, waterproof and easy to clean. 
     Preferably, the step of forming a first portion of the article comprises: placing an electrically conducting member into the first mould; and injecting the first material into the first mould, such that the first material bonds to the electrically conducting member. This results in a good bond between the electrically conducting member and the first material that forms the remainder of the first portion of the article. 
     The electrically conducting member preferably has an elliptical shape. An elliptically-shaped conducting member is less prone to being distorted by the compressive force that it experiences when the first material is injected. Reducing distortion of the conducting member is desirable in its own right and, additionally, helps to avoid flash forming at the interface of the conducting member and the first portion of the article. 
     The electrically conducting member preferably comprises a surface to which the first material is bonded, and a groove disposed substantially parallel to the surface. The electrically conducting member is usually harder than the first material. The groove allows the electrically conducting member to flex near to its interface with the first material, which reduces the risk of the first material detaching from the electrically conducting member when an applied force causes the stimulation pad to bend. 
     The first mould preferably comprises a ridge, and the groove is preferably adapted to engage with the ridge. The ridge thereby reinforces the conducting member when the first material is injected. This reduces distortion of the conducting member and thereby helps to avoid flash forming at the interface of the conducting member and the first portion of the article. 
     The electrical circuit preferably comprises a substrate having a slot extending therethrough, and the groove is preferably aligned with said slot when the electrical circuit is placed onto the first portion of the article. When the second material is injected, it can pass through the slot and bond with the interior surface of the groove. This improves the durability of the stimulation pad, by allowing the second portion to bond to the conducting member and by holding the circuit in position. 
     The first mould preferably comprises an insert, the insert being separated from the first mould by a gap, wherein the gap allows air to exit the first mould when the first material is injected into the first mould. By allowing air to exit the first mould in this manner, the first material is able to fill the whole of the first mould. 
     A further aspect of the invention provides an article produced by the method described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred features of the invention will now be described, purely by way of example, with reference to the accompanying drawings, wherein like elements are indicated using like reference signs, and in which: 
         FIG. 1  is a top isometric view of a stimulation pad; 
         FIG. 2  is a bottom isometric view of the stimulation pad shown in  FIG. 1 ; 
         FIG. 3  is an exploded view of the stimulation pad shown in  FIGS. 1 and 2 ; 
         FIG. 4  is the top isometric view of the stimulation pad shown in  FIGS. 1 to 3 , in which a flange is highlighted; 
         FIG. 5  is a flow diagram of a method of moulding the stimulation pad shown in  FIGS. 1 to 4 ; 
         FIG. 6  is a cross-sectional view of a first mould during the first step of the method shown in  FIG. 5 ; 
         FIG. 7  is an isometric view of a first plate of the first mould shown in  FIG. 6 ; 
         FIG. 8  is an isometric view of a second plate of the first mould shown in  FIG. 6 ; 
         FIG. 9  is a cross-sectional view of the first mould when it has been closed; 
         FIG. 10  is a cross-sectional view of the first mould during the second step of the method shown in  FIG. 5 ; 
         FIG. 11  is a magnified view of  FIG. 10 ; 
         FIG. 12  is a plan view of the stimulation pad during the second step of the method shown in  FIG. 5 ; 
         FIG. 13  is a top isometric view of a first portion of the stimulation pad that results from the second step of the method shown in  FIG. 5 ; 
         FIG. 14  is a top isometric view of the first portion of the stimulation pad shown in 
         FIG. 13  following the addition of electronic components; 
         FIG. 15  is a cross-sectional view of a second mould during the third step of the method shown in  FIG. 5 ; 
         FIG. 16  is an isometric view of a first plate of the second mould shown in  FIG. 15 ; 
         FIG. 17  is an isometric view of a second plate of the second mould shown in  FIG. 15 ; 
         FIG. 18  is a cross-sectional view of the second mould during the fourth step of the method shown in  FIG. 5 ; 
         FIG. 19  is a magnified view of  FIG. 18 ; 
         FIG. 20  is a cross-sectional view of the second mould during the fifth step of the method shown in  FIG. 5 ; 
         FIG. 21  is a magnified view of  FIG. 20 ; 
         FIG. 22  is an isometric view of a conducting member of the stimulation pad shown in  FIGS. 1 to 3 ; 
         FIG. 23  is a partial cross-sectional perspective view of a conducting member of the stimulation pad shown in  FIGS. 1 to 3 ; 
         FIG. 24  is a cross-sectional view of the stimulation pad shown in  FIGS. 1 to 4 ; 
         FIG. 25  is a schematic diagram of a stimulation system comprising the stimulation pad shown in  FIGS. 1 to 4 ; 
         FIG. 26  is a top plan view of the circuit shown in  FIG. 3 ; 
         FIG. 27  is a bottom plan view of the circuit shown in  FIG. 3 ; and 
         FIG. 28  is a schematic diagram of a connector for the circuit shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a method of moulding an article, and to an article produced by that method. The method will be illustrated by referring to an example in which the moulded article is a pad for applying stimulation to a human or animal body. However, the method can be used to mould other types of article and, therefore, the invention is not limited solely to the manufacture of stimulation pads. 
     A stimulation pad  1  will now be described with reference to  FIGS. 1 to 4 .  FIGS. 1 and 2  are top and bottom isometric views respectively of the stimulation pad  1 .  FIG. 3  is an exploded view of the stimulation pad  1 . The stimulation pad  1  comprises a first portion  2  and a second portion  4 . The first portion  2  is bonded to the second portion  4  by the moulding method that is described below. The stimulation pad  1  has a first surface  3 , shown facing upwards in  FIG. 1 , which is defined by both the first portion  2  and the second portion  4 . The stimulation pad  1  also has a second surface  5 , shown facing upwards in  FIG. 2 , which is primarily defined by the first portion  2 . The first surface  3  is oriented in substantially the opposite direction to the second surface  5 . 
     The first portion  2  of the stimulation pad  1  comprises a flange  8 . The flange  8  is shown by the shaded area in  FIG. 3 . The flange  8  forms part of the first surface  3  of the stimulation pad  1 . The flange  8  surrounds at least part of the perimeter of the second portion  4  of the stimulation pad  1 . 
     Various components  12 ,  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26  are located inside the stimulation pad  1 , and are encapsulated between the first portion  2  and the second portion  4 . These components include an electrical circuit  16  that is operable to generate stimulation that can be provided to a human or animal body. For example, the circuit  16  can generate electrical stimulation and/or heat. The circuit  16  may comprise a first connector  24  for connection to a corresponding second connector  22  of a cable  12 . The cable  12  extends outside the stimulation pad  1 , such that only part of its length is contained within the stimulation pad  1 . The cable  12  comprises a core surrounded by a sheath, the core comprising one or more electrically conducting wires. The cable  12  is operable to provide electrical power and, optionally, one or more control signals, from a console (indicated by reference numeral  210  in  FIG. 25 ) to the circuit  16 . The cable  12  may also be operable to provide one or more feedback signals from the circuit  16  to the console. 
     A strain relief  10  preferably surrounds the cable  12  at its point of entry into the stimulation pad  1 . The strain relief  10  restricts the bending of the cable  12 , and thereby reduces the risk of bending causing damage to the cable  12  itself or to the first and second connectors  22 ,  24 . The strain relief  10  is preferably integrally formed with the second portion  4  of the stimulation pad  1 . The strain relief  10  is preferably bonded to the cable  12  by the moulding method that is described below. 
     The circuit  16  may also comprise a visual indicator  14 . The visual indicator may comprise a light emitting diode (LED), for example. The visual indicator  14  is operable to provide a visible indication of the status of the circuit  16 . For example, the visual indicator  14  may indicate that electrical power is being supplied to the circuit  16  and/or may indicate that the circuit  16  is ready to apply stimulation to a human or animal body. As the visual indicator  14  is located inside the stimulation pad  16 , the second protective housing  20  may comprise a transparent region  28  that penetrates the second portion  4 , thereby allowing a visual indication provided by the visual indicator  14  to be seen from outside the stimulation pad  16 . The whole of the second protective housing  20  is preferably formed from a transparent material, such that the transparent region  28  is defined by a projection formed on the surface of the second protective housing  20 . 
     The first connector  24 , second connector  22  and visual indicator  14  are preferably disposed between a first protective housing  18  and a second protective housing  20 . The first and second protective housings  18 ,  20  protect the connectors  22 ,  24  and visual indicator  14  from the heat and pressure that are experienced during the formation of the second portion  4  by the moulding method that is described below. Additionally, the first and second protective housings  18 ,  20  protect the connectors  22 ,  24  against damage from tensile forces that may be applied via the cable  12  (as discussed in more detail below, with reference to Detail B of  FIG. 24 ). One or more screws  26  can be provided to fasten the first protective housing  18  to the second protective housing  20 . 
     The stimulation pad  1  comprises two electrically conducting members  6   a ,  6   b . The conducting members  6   a ,  6   b  are provided on the second surface  5 . The first portion  2  of the stimulation pad  1  surrounds each conducting member  6   a ,  6   b . The first portion  2  is bonded to the conducting members  6   a ,  6   b  by the moulding method that is described below. The purpose of the conducting members  6   a ,  6   b  is to effect electrical stimulation by conducting electricity from the circuit  16  to a human or animal body placed in contact with the conducting members  6   a ,  6   b . The stimulation pad  1  could comprise just one conducting member, more than two conducting members or, in the case of a stimulation pad that does not provide electrical stimulation, no conducting members at all. Each conducting member  6   a ,  6   b  comprises a groove  7 . Each conducting member  6   a ,  6   b  preferably further comprises one or more pins  27 . 
     The circuit  16  is formed on a substrate that comprises one or more holes  29  and/or one or more slots  30  that extend through the substrate. The holes  29  align with the pins  27  on the conducting members  6   a ,  6   b  when the stimulation pad  1  is assembled. The slots  30  align with the grooves  7  of the conducting members  6   a ,  6   b  when the stimulation pad  1  is assembled. Whilst the slots  30  are preferably elongated in the plane of the substrate, as shown in  FIG. 3 , the slots  30  could alternatively have a circular cross-section (i.e. the slots  30  could be round holes). 
     A method  100  of manufacturing the stimulation pad  1  will now be described with reference to  FIGS. 5 to 21 . 
     The method  100  starts at step  102 , in which components  6 ,  18  are inserted into a first mould  61 . Step  102  is illustrated by  FIG. 6 . The first mould  61  comprises a first plate  60  and a second plate  62 . Isometric views of the first plate  60  and the second plate  62  are shown in  FIGS. 7 and 8  respectively. The surface of the second plate  62  corresponds to the shape of the components that are to be inserted into the first mould  61 . Thus, the conducting members  6   a ,  6   b  and the first protective housing  18  are inserted into the first mould  61  by positioning them on the surface of the second plate  62 , as illustrated by the arrows in  FIG. 6 . Step  102  is optional, and need not be performed if the article to be moulded does not require any inserts. The conducting members  6   a ,  6   b  and the first protective housing  18  are formed prior to step  102 , preferably by a moulding process. 
     The first mould  61  is then closed, as illustrated by  FIG. 9 . When the first mould  61  is closed, a cavity  66  is defined by the first plate  60 , the second plate  62 , the conducting members  6   a ,  6   b  and the first protective housing  18 . A parting line  68  is defined by the abutting surfaces of the first plate  60  and the second plate  62 . The first mould  61  is shaped so as to define the flange  8  on the first portion  2 . More specifically, the second plate  62  comprises a recess  63  that defines the flange  8  when material is injected into the cavity  66 . 
     In step  104 , a first material  50  is injected into the cavity  66  of the first mould  61 , thereby forming the first portion  2 . Step  104  is illustrated by  FIGS. 10 to 12 . The hatched area in  FIG. 10  shows the first portion  2  of the stimulation pad  1 .  FIG. 10  illustrates that a flange  8 , having a shape corresponding to the shape of the recess  63 , is formed when the first material  50  is injected into the cavity  66  of the first mould  61 . 
       FIG. 11  is a magnified view of  FIG. 10 , to illustrate how air leaves the first mould  61  when the first material  50  is injected into the cavity  66  in step  104 . The second plate  62  comprises one or more inserts  64   a ,  64   b . A gap  65  exists between the inserts  64  and the surrounding regions of the second plate  62 . Air can exit the cavity  66  through the gap  65  when the first material  50  is injected into the cavity  66 . The escape of air from the cavity  66  is illustrated by the arrows  70 . However, the gap  65  is too narrow to allow the first material  50  to exit the cavity  66 . For example, the gap  65  can have a thickness of 0.005 millimetres, although other suitable thicknesses are possible. Allowing air to escape in this manner avoids air being trapped in the cavity  66 , which allows the first material  50  to fill the whole of the cavity  66  and improves bonding between the first material  50  and the conducting members  6   a ,  6   b . Allowing air to escape also avoids the Diesel effect, whereby compressed air that is trapped in the cavity  66  could cause the first material  50  to burn. 
     Each insert  64   a ,  64   b  comprises a ridge  72 . The shape of each ridge  72  corresponds to the shape of the groove  7  in a respective conducting member  6   a ,  6   b , such that each ridge  72  engages with a respective groove  7  when the conducting members  6   a ,  6   b  are inserted into the first mould  61  in step  102 . As can be seen in  FIG. 8 , each ridge  72  has an elliptical shape, thereby allowing engagement with the entirety of a groove  7 , which also has an elliptical shape. The ridges  72  reinforce the conducting members  6   a ,  6   b , thereby preventing the conducting members  6   a ,  6   b  from being deformed by the force that is exerted upon them when the first material  50  is injected into the cavity  66  in step  104 . The direction of the force exerted upon the conducting members  6   a ,  6   b  by the first material during step  104  is illustrated by the arrow  71  in  FIG. 11 . Alternatively or in addition, the conducting members  6   a ,  6   b  can be compressed between the first plate  60  and the second plate  62  of the first mould  61 , so as to increase the hardness of the conducting members  6   a ,  6   b  and thereby prevent them from being deformed by the force experienced when the first material  50  is injected into the cavity  66  in step  104 . These measures to prevent deformation of the conducting members  6   a ,  6   b  also help to prevent flash (i.e. unwanted excess material) forming around the conducting members  6   a ,  6   b  when the first material  50  is injected. 
       FIG. 12  shows the flow path  82  of the first material  50  during step  104 . The first material  50  is in the molten state when it is injected into the first mould  61 . The first material  50  is injected into the first mould  61  at an injection point  80 . A vertical injection machine is preferably used to inject the first material  50 . The injection point  80  is preferably located near to the centre of the cavity  66 , so as to reduce the distance over which the first material  50  has to flow, thus lowering the pressure needed to fill the first mould  61  completely. The flow path  82  in the region denoted by reference numeral  84  shows that the elliptical shape of the conducting members  6   a ,  6   b  helps to reduce the approach angle of the first material  50 , thus resulting in a more laminar flow of the first material  50  around the conducting members  6   a ,  6   b . This helps to minimise deformation of the conducting members  6   a ,  6   b  when the first material  50  is injected. 
     When injected into the first mould  61 , the first material  50  bonds to the conducting members  6   a ,  6   b  and the first protective housing  18 . This results in a watertight seal between the conducting members  6   a ,  6   b  and the surrounding regions of the first portion  2 . This advantageously prevents sweat and conductive gel from entering the stimulation pad  1  during use, which may cause the circuit  16  to malfunction. Each side of the first protective housing  18  preferably comprises a plurality of holes  19  (shown in  FIG. 3 ). The holes  19  increase the bonding strength of the first protective housing  18  to the first material  50 , and thereby reduce the risk of the first protective housing  18  being displaced by the force experienced during injection of a second material during step  110 . 
     The optimum pressure, flow rate and temperature at which to inject the first material  50  during step  104  will depend on numerous factors, such as the properties of the first material  50 , the geometry of the cavity  66  and the ability of air to exit the cavity  66 . In general, higher pressure, flow rate and temperature improves the bonding between the first material  50  and the conducting members  6   a ,  6   b , but increases the risk of flash forming around the conducting members  6   a ,  6   b . Thus, the optimum pressure, flow rate and temperature are found through trial and error, so as to find the maximum values for these parameters that do not result in flash. 
     When the first material  50  has cooled, the first portion  2  is removed from the first mould  61 .  FIG. 13  shows the first portion  2  immediately after step  104 . The first material  50  is bonded to the conducting members  6   a ,  6   b  and the first protective housing  18  by the moulding process. A circuit assembly (comprising circuit  16 , first connector  24 , second connector  22  and cable  12 ) is then positioned on the first portion  2 . The second protective housing  24  is then secured to the first protective housing  18  by screws  26 , which results in the assembly shown in  FIG. 14 . The method then proceeds to step  106 . 
     In step  106 , the first portion  2  is placed into a second mould  91 . Step  106  is illustrated by  FIG. 15 . The second mould  91  comprises a first plate  90  and a second plate  92 . Isometric views of the first plate  90  and the second plate  92  are shown in  FIGS. 16 and 17  respectively. The surface of the first plate  90  corresponds to the shape of the second surface  5  of the stimulation pad  1 . Thus, the first portion  2  (which constitutes the majority of the second surface  5 ) is placed into the second mould  91  by positioning it on the surface of the first plate  90 , as illustrated by the arrows in  FIG. 15 . 
     In step  108 , the flange  8  is compressed between the first plate  90  and the second plate  92  of the second mould  91 . Step  108  is illustrated by  FIGS. 18 and 19 . The flange  8  is compressed by closing the second mould  91 . The distance between the first plate  90  and the second plate  92 , measured at the point  93  at which the second plate  92  abuts the flange  8  when the second mould  91  is closed, is less than the thickness of the first portion  2  at the flange  8 . Thus, closing the second mould  91  causes the flange to be compressed between the first plate  90  and the second plate  92 . This can be seen clearly in  FIG. 19 , which is a magnified view of  FIG. 18 . The dashed line  95  illustrates the profile of the flange  8  in the absence of a compressive force. The compressive force  97  that is applied to the flange  8  when the second mould  91  is closed causes the flange  8  to deform. Compression of the flange  8  creates a seal between the first portion  2  and the second plate  92 . Compression of the flange  8  also causes the hardness of the first portion  2  to increase, as described in more detail below. The surface of the flange  8  is substantially parallel to the surface of the second plate  92  at all points  93  at which the which the second plate  92  abuts the flange  8  when the second mould  91  is closed. The compressive force  97  is substantially perpendicular to the surface of the flange  8 . 
     When the second mould  91  is closed, a cavity  96  is defined by the first plate  90 , the second plate  92  and the first portion  2 . A parting line  98  is defined by the abutting surfaces of the first plate  90  and the second plate  92 . 
     In step  110 , a second material  55  is injected into the cavity  96  of the second mould  91  whilst the flange  8  is compressed between the first plate  90  and the second plate  92 , thereby forming the second portion  4 . Step  110  is illustrated by  FIGS. 20 and 21 . The hatched area in  FIGS. 20 and 21  denotes the second portion  4 .  FIG. 21  is a magnified view of  FIG. 20 , to illustrate how air leaves the second mould  91  when the second material  55  is injected into the cavity  96  in step  110 . As previously mentioned, compression of the flange  8  creates a seal between the first portion  2  and the second plate  92 . This seal prevents the second material  55  from exiting the cavity  96 , and thereby reduces the risk of flash forming on the parting line  98 . However, the seal allows air to exit the cavity  96 , by flowing between the flange  8  and the second plate  92 , as illustrated by arrows  99 . This avoids air being trapped in the cavity  96 , which allows the second material  55  to fill the whole of the cavity  96  and improves bonding between the first material  50  and the second material  55 . Allowing air to escape also avoids the Diesel effect, whereby compressed air that is trapped in the cavity  96  could cause the second material  55  to burn. 
     The second material  55  is injected into the second mould  91  at an injection point  86 . The second material  55  is in the molten state when it is injected into the cavity  96 . A vertical injection machine is preferably used to inject the second material  55 . The injection point  86  is located as far away from the circuit  16  as possible, so as to reduce the risk of the electrical components of the circuit  16  being damaged by the heat and pressure of the second material  55 , which are greatest near to the injection point. Thus, the injection point  86  is preferably located above the second protective housing  20 . Furthermore, the injection point  86  is preferably oriented so as to direct the second material  55  towards the second protective housing  20  when it first enters the cavity  96 . The first and second protective housings  18 ,  20  protect the connectors  22 ,  24  and visual indicator  24  against damage from the heat and pressure that are experienced when the second material  55  is injected. Furthermore, by locating the injection point  86  above the second protective housing  20 , the pressure applied by the incoming second material  55  when it makes contact with the circuit  16  forces the circuit  16  towards the first portion  2 . This reduces the risk of the second material  55  flowing underneath the circuit  16 , which would have the undesirable effect of preventing the circuit  16  making good electrical contact with the conducting members  6   a ,  6   b . The injection point  86  is most preferably located above the region of the second protective housing  20  that surrounds the cable  12 . The presence of the cable  12  stiffens this region of the second protective housing  20 , and thereby improves the ability of the second protective housing  20  to withstand the pressure applied by the incoming second material  55 . Furthermore, locating the injection point  86  closer to the cable  12  helps to improve the bonding between the second material  55  and the sheath of the cable  12 , by reducing the flow distance and thus ensuring that the second material  55  is relatively warm when it makes contact with the sheath. 
     When injected into the second mould  91 , the second material  55  bonds to the first material  50 . This results in a watertight seal between the first portion  2  and the second portion  4 . This advantageously prevents sweat and conductive gel from entering the stimulation pad  1  during use, which may cause the circuit  16  to malfunction. The second material  55  also bonds to the second protective housing  20 . Since one component (i.e. the second portion  4 ) is moulded upon another component (i.e. the first portion  2 ), step  110  may be referred to as an overmoulding process. Similarly to step  104 , the optimum pressure, flow rate and temperature at which to inject the second material  55  during step  110  are found through trial and error, so as to find the maximum values for these parameters that do not result in flash. 
     As mentioned previously, the grooves  7  of the conducting members  6   a ,  6   b  are aligned with the slots  30  in the substrate of the circuit  16 . When the second material  55  is injected in step  110 , the second material  55  passes through the slots  30  and bonds with the interior surface of the grooves  7 . This improves the durability of the stimulation pad  1 , by allowing the second portion  4  to bond to the conducting members  6   a ,  6   b  and by holding the circuit  16  in position. The grooves  7  increase the available surface area of the conducting members  6   a ,  6   b  to which the second material  55  can bond. 
     The strain relief  10  is defined by the shape of the second mould  91 . Thus, the strain relief  10  is formed by injection of the second material  55  into the second mould  91  in step  110 . The second material  55  bonds to the sheath of the cable  12  in the region of the strain relief  10 . This forms a watertight seal between the cable  12  and the strain relief  10 . 
     When the second material  55  has cooled, the stimulation pad  1  is removed from the second mould  91 . This results in the stimulation pad  1  shown in  FIGS. 1 to 3 . 
       FIG. 24  illustrates preferred features of the stimulation pad  1 , any or all of which can be provided. As shown in Detail B, the inner surfaces of the first protective housing  18  and the second protective housing  20  each comprise one or more ribs  120  to engage with the sheath of the cable  12 . The ribs  120  grip the sheath and hold the cable  12  in position. Thus, tensile forces applied to the cable  12  are borne by the sheath and the ribs  120 , rather than by the connectors  22 ,  24 . This protects the connectors  22 ,  24  from being damaged by tensile forces applied to the cable  12 , and reduces the risk of the cable  12  being detached from the stimulation pad  1 . The ribs  120  also form a seal with the cable  12 , which prevents the second material  55  from entering the first protective housing  18  and the second protective housing  20  during step  110 , and thereby prevents damage to the connectors  22 ,  24 . The first protective housing  18  and the second protective housing  20  are shaped so as to anchor them to the second material  55 . For example, as shown in Detail B, the first protective housing  18  and the second protective housing  20  may comprise a hook-shaped portion  122 . As another example, shown in Detail C, the outer surface of the first protective housing  18  comprises one or more ridges  124  for engagement with the first material  50  and the second material  55 . Similar ridges could be provided on the second protective housing  20 . 
     As shown in Detail D of  FIG. 24 , the second protective housing  20  comprises a lip  126  for engagement with the second connector  22 . The lip  126  prevents the second connector  22  being detached from the first connector  24  when a force is applied to the cable  12 . 
     As shown in Detail E of  FIG. 24 , the first protective housing  18  and the second protective housing  20  comprise one or more elongated corrugations  134 . The corrugations  134  form a seal on each side of the substrate of the circuit  16 , so as to prevent the second material  55  from entering the first protective housing  18  and the second protective housing  20  during step  110 . The circuit  16  preferably has a flexible substrate, which is caused to deform by the corrugations  134 , thereby improving the quality of the seal. Detail E also shows that the transparent region  28  is defined by a projection formed on the surface of the second protective housing  20 . In addition to allowing the visual indicator  14  to be seen from outside the stimulation pad  1 , this projection preferably has the additional purpose of holding the second protective housing  20  in position when the second material  55  is injected into the second mould  91  in step  110 . To achieve this additional purpose, the projection is shaped so as to abut the internal surface of the second plate  92  when the second mould  91  is closed. 
     As shown in Detail F of  FIG. 24 , the second surface  5  of the stimulation pad  1  comprises one or more grooves  136  to increase the flexibility of the pad  1 , thereby improving the ability of the pad to conform to the contours of the body. The grooves  136  also reduce the tendency for sweat and conductive gel to form a short circuit between the conducting members  6   a ,  6   b , which would reduce the efficacy of electrical stimulation. 
     As shown in Detail G of  FIG. 24 , the first portion  2  and the conducting members  6   a ,  6   b  comprise one or more pins  27 . The pins  27  engage with corresponding holes  29  (shown in  FIG. 3 ) in the circuit  16 . The engagement of the pins  27  with the holes  29  helps to keep the circuit  16  in position during step  110 , when the circuit  16  experiences large forces as the second material  55  is injected. Furthermore, the pins  29  bond with the second material  55 . The bonding of the pins  29  to the second material  55  is particularly advantageous because the substrate of the circuit  16  (described below, with reference to  FIGS. 26 and 27 ) is usually formed from a material that will not bond to either the first material  50  or the second material  55 . Thus, the pins  29  provide additional bonding points between the second portion  4  and the first portion  2  or the conducting members  6   a ,  6   b , thereby improving the durability of the stimulation pad  1 . Detail G also shows the groove  7  in the conductive member  6   b.    
     As shown in Detail H of  FIG. 24 , the surface of the second portion  4  comprises a depression  142  adjacent its interface with the flange  8 . The depression  142  reduces the risk of flash forming during step  110 . However, if any flash is formed during step  110 , the depression  142  reduces the visibility of the flash. The second mould  91  is shaped so as to define the depression  142  on the surface of the second portion  4 . 
     The first material  50  and the second material  55  are both electrically insulating materials. This ensures that the conducting members  6   a ,  6   b  are the only regions on the external surface of the stimulation pad  1  that are able to conduct electricity. The first material  50  may be the same as the second material  55 . Alternatively, the first material  50  may be different from the second material  55 . 
     The first material  50  and the second material  55  each comprise a soft polymer. In this context, the term “soft” is preferably understood to mean that the hardness of the polymer is measurable on the Shore A durometer scale. The term “soft” is more preferably understood to mean that the hardness of the polymer is below 85 Shore A. Due to the softness of the first material  50 , there is a risk that the first material  50  could be deformed by the compressive force that it experiences when the second material  55  is injected. Deformation of the first material  50  would be detrimental to the appearance and functionality of the article produced by the moulding method, and could also increase the risk of flash forming when the second material  55  is injected in step  100 . The risk of the first material  50  deforming is mitigated by compressing the flange  8  during steps  108  and  110 . Compression of the flange  8  causes a reversible, temporary increase in the hardness of the first portion  2 , particularly its hardness in the vicinity of the flange  8 . The first portion  2  returns to its original hardness (i.e. becomes softer) when the compression is released. This increase in hardness allows the first material  50  to withstand the compressive force that is experienced when the second material  55  is injected, which results in a high quality article being produced by the moulding method. 
     The compression of the flange  8  during steps  108  and  110  is particularly advantageous when the first material  50  is different from the second material  55 . The first material  50  and the second material  55  may comprise the same soft polymer, but each material may comprise different additives. For example, the first and second materials  50 ,  55  may comprise different fillers or plasticisers, so as to give the first portion  2  different properties from the second portion  4 . As another example, the first and second materials  50 ,  55  may comprise different colorants, so as to give the first portion  2  a different colour from the second portion  4 . Alternatively, the first material  50  and the second material may comprise different soft polymers. Compression of the flange  8  during steps  108  and  110  prevents the second material  55  from overshooting the first material  50  when the second material  55  is injected into the second mould  91 . This allows the first and second portions  2 ,  4  to retain the different properties that result from their different constituent materials  50 ,  55 . 
     The first material  50  and/or the second material  55  preferably comprise thermoplastic polyurethane (TPU). TPU is advantageous because it has good chemical resistance to oils, fats and alcohols. This is particularly important for a stimulation pad, which will be exposed to fats and oils when placed into contact with the skin, and which will be cleaned with disinfectants (typically isopropanol) after use. Thus, the use of TPU results in a durable stimulation pad. A further advantage of TPU is that it is relatively soft, thereby allowing the stimulation pad  1  to be sufficiently flexible to conform to the contours of the body when in use. TPU is relatively elastic, which allows the hardness of the first portion  2  to increase when it is compressed during steps  108  and  110 , and which allows the first portion  2  to revert to its original softness when the compression is released. Furthermore, TPU is a good electrical insulator. The particular composition of the TPU is not of critical importance, and the teachings disclosed herein are applicable to a wide variety of commercially-available TPU compositions. 
     Alternatively, the first material  50  and/or the second material  55  may comprise Styrene Ethylene Butadiene Styrene (SEBS). An advantage of SEBS is that it is available in very soft grades (down to 10 Shore A) and is easy to process. However, SEBS has limited chemical resistance to oils, fats and alcohols. 
     The sheath of the cable  12  preferably comprises the second material  55 . This allows the sheath of the cable  12  to bond with the second material  55  when it is injected in step  110 , which improves the watertightness of the stimulation pad  1  and also reduces the risk of the cable being detached from the stimulation pad  1 . Preferably, the sheath of the cable  12  and the second material  55  both comprise TPU. 
     The conducting members  6   a ,  6   b  will now be described with reference to  FIGS. 22 and 23 .  FIG. 22  is an isometric view of a conducting member  6 , whilst  FIG. 23  is a partial cross-sectional view of a conducting member. 
     Each conducting member  6  has an elliptical shape. The reason for the elliptical shape is to reduce distortion of the conducting members  6  during step  104 . By way of explanation, there is a tendency for the shape of the conducting members  6  to be distorted by the compressive force that is experienced when the first material  50  is injected into the first mould  61 . The inventors have discovered that a conducting member  6  is less prone to distorting when it has an elliptical shape. Furthermore, the elliptical shape also helps to promote laminar flow when the first material  50  is injected, as discussed above with reference to region  84  of  FIG. 12 . In contrast, the inventors have found that a circular or square conducting member distorts significantly and does not bond reliably to the first material  50 . Thus, the optimal shape for the conducting members  6   a ,  6   b  is an ellipse. 
     The conducting members  6   a ,  6   b  comprise an electrically conducting polymer. In a preferred example, the conducting members  6  comprise TPU and an electrically conducting additive. The advantage of forming the conducting members  6  from TPU is that, when the first material  50  also comprises TPU, good bonding between the conducting members  6  and the first material  50  can be achieved. The electrically conducting additive preferably comprises carbon black. The advantages of carbon black are that it is cheap, easily dispersed in the polymer matrix and does not cause skin irritation when the conducting members are placed on a human or animal body to deliver stimulation. It is possible to achieve a volume resistivity of less than 5 Ωm by adding carbon black to TPU. 
     As less-preferred alternatives, the electrically conducting additive could comprise carbon nanotubes or stainless steel fibres. Carbon nanotubes have a higher conductivity than carbon black and, therefore, a conducting member  6  with a particular conductivity can be produced using a smaller amount of carbon nanotubes than if carbon black were to be used, which results in a softer conducting member. However, it is difficult to disperse carbon nanotubes uniformly throughout the polymer. Stainless steel fibres have an even higher conductivity than carbon nanotubes, thereby allowing relatively soft and highly conductive conducting members to be produced. However, the steel fibres have a tendency to break when injection moulded to form the conducting member and it is difficult to disperse the steel fibres uniformly throughout the polymer. 
     The presence of an electrically conductive additive usually causes the hardness of a polymer to increase, which results in the conducting members  6   a ,  6   b  being harder than the first material  50 . For example, when the electrically conducting additive comprises carbon black, the conducting members  6   a ,  6   b  have a hardness of 85 Shore A, whereas the hardness of the first material  50  may be 70 Shore A or even lower. Thus, the conducting members  6   a ,  6   b  are less flexible than the first portion  2 , which results in a risk of the conducting members  6   a ,  6   b  detaching from the first material  50 . This risk is mitigated by the groove  7  in the conducting members  6   a ,  6   b . The groove  7  increases the flexibility of the conducting members  6   a ,  6   b  in the vicinity of their interface with the first material  50 . This improves the durability of the stimulation pad  1 . The groove  7  preferably has a U-shaped cross-section, as shown in  FIG. 23  and Detail G of  FIG. 24 . 
     The conducting members  6   a ,  6   b  comprise a surface  9 , to which the first material  50  is bonded. The groove  7  is formed adjacent the surface  9 . The groove  7  is preferably substantially parallel to the surface  9  for substantially the whole of the surface  9 . This ensures that the conducting members  6   a ,  6   b  can flex at every point at which they are bonded to the first material  50 . The groove  7  is formed on an internal surface of the conducting member  6 . In other words, the groove  7  is formed on the side of the conducting member  6  that faces the inside of the stimulation pad  1 , i.e. the side opposite to the surface  5  of the stimulation pad  1  that makes contact with the skin when the stimulation pad  1  is in use. This allows the surfaces of the conducting members  6   a ,  6   b  that make contact with the skin to be smooth, thereby maximising the surface area that is available to conduct electricity to the human or animal body and facilitating cleaning of the stimulation pad  1 . 
     The first protective housing  18  and second protective housing  20  each comprise a hard polymer. For example, the first and second protective housings  18 ,  20  preferably comprise TPU with a hardness measurable on the Shore D durometer scale. More preferably, the first and second protective housings  18 ,  20  comprise TPU with a hardness of approximately 75 Shore D. Advantageously, the first and second protective housings  18 ,  20  are able to bond to the first portion  2  and second portion  4  when all are formed from TPU. Optionally, the first protective housing  18  and/or second protective housing  20  can be potted (e.g. by filling with epoxy) before the circuit assembly is positioned on the first portion  2  of the stimulation pad  1 , so as to increase the strength of the housings  18 ,  20 . Optionally, the first and second protective housings  18 ,  20  can be reinforced with glass fibres if additional mechanical strength is required to withstand the pressures experienced during injection of the second material  55  in step  110 . Alternatively, the first and second protective housings  18 ,  20  could comprise polycarbonate if yet more mechanical strength is needed. 
     The surfaces of the conducting members  6   a ,  6   b  can be treated prior to step  102 , so as to improve their bonding to the first material  50  in step  104 . Similarly, the surfaces of the first portion  2  and/or the cable  12  can be treated prior to step  106 , so as to improve their bonding to the second material  55  in step  110 . Plasma or corona treatment may be used to treat the surfaces. Alternatively, the surfaces can be cleaned with methyl ethyl ketone (MEK) or isopropanol. 
     The first mould  61  and the second mould  91  preferably have a rough surface finish. This reduces the risk of the first portion  2  or the second portion  4  sticking to the moulds  61 ,  91  during ejection. The resulting rough texture of the first surface  3  and the second surface  5  also advantageously helps to prevent the stimulation pad  1  from slipping whilst in use. A suitable surface finish for the first and second moulds  61 ,  91  is VDI-33 (MT-11530 MoldTech/Standex). 
       FIG. 25  shows a stimulation system  200 . The stimulation system  200  comprises a console  210  and a stimulation pad  1 . The stimulation pad  1  is electrically connected (and, preferably, detachably connected) to the console  210  by a cable  12 . In use, the stimulation pad is placed upon a human or animal body. The console  210  is operable to apply electrical stimulation to the body via the conducting members  6   a ,  6   b . In addition to applying electrical stimulation to the body, the stimulation system  200  may also be able to apply heat to the body. That is, the stimulation system  200  can be a thermostimulation system. Such a thermostimulation system is preferably operable to apply heat and electrical stimulation to the body simultaneously or independently of each other. 
     An example of a circuit  16  will now be described with reference to  FIGS. 26 to 28 .  FIG. 26  is a top plan view of the circuit  16  shown in  FIG. 3 .  FIG. 27  is a bottom plan view of the circuit  16 .  FIG. 28  is a schematic diagram of the first connector  24  shown in  FIG. 3 . The circuit  16  comprises a substrate  500 . The substrate  500  has a first surface  53  (shown in  FIG. 27 ) and a second surface  52  (shown in  FIG. 26 ), wherein the first surface  53  has an opposite orientation to the second surface  52 . The circuit  16  further comprises a heating element  502  and one or more electrodes  514 . The circuit  16  can further comprise electronic components including a temperature sensor  510 , a visual indicator  14  and a first connector  24 . 
     Electrical conductors  511 ,  512  are patterned on each surface  52 ,  53  of the substrate  500  to form electrical connections between the components of the circuit  16 . As used herein, the term “patterned” is preferably understood to describe the result of a process whereby an electrically conducting region having a predefined shape is formed upon a surface of the substrate  500 . The conductors are illustrated by the grey shaded areas in  FIGS. 26 and 27 . The conductors on the first surface  53  are denoted by reference numeral  511  in  FIG. 27 , whilst the conductors on the second surface  52  are denoted by reference numeral  512  in  FIG. 26 . One or more electrodes  514  are also patterned on the first surface  53  of the substrate  500 . The electrodes  514  are also illustrated by grey shaded areas in  FIG. 27 , since the electrodes  514  are preferably formed from the same electrically conducting material as the conductors  511 . Insulating regions that do not comprise a conductor are illustrated in  FIGS. 26 and 27  by the unshaded areas denoted by reference numeral  513 . 
     The electronic components  502 ,  505 ,  507 ,  510 , conductors  511 ,  512  and electrodes  514  are provided on both surfaces  52 ,  53  of the substrate  500 . The electrodes  514  are formed on the first surface  53 , whilst the heating element  502  is formed on the second surface  52 . In use, the heating element  502  faces away from the skin of the user and the electrodes  514  face towards the skin. The temperature sensor  510 , visual indicator  505  and first connector  24  are also preferably provided on the second surface  52 . Since electronic components  502 ,  505 ,  507 ,  510 , conductors  511 ,  512  and electrodes  514  are provided on both surfaces of the substrate  500 , the substrate  500  should have electrically insulating properties in order to prevent unwanted electrical conduction between components and conductors on different surfaces. 
     The circuit  16  comprises a first connector  24  to allow the circuit to be electrically connected to the cable  12  and thereby connected to the console  210  (shown in  FIG. 25 ). The first connector  24  is preferably provided on the second surface  52 . The first connector  24  is preferably a surface-mount connector. The first connector  24  comprises connection pins, which can be connected to a corresponding connector on the cable  12 . In an example, the first connector  24  comprises six connection pins, as illustrated in  FIG. 28 . The pins labelled ‘Heat+’ and ‘Heat−’ are connected to the heating element  502 . The pins labelled ‘Temp+’ and ‘Temp−’ are connected to the temperature sensor  510 . The pins labelled ‘EM 1 ’ and ‘EM 2 ’ are connected to the electrodes  514 . 
     The heating element  502  preferably comprises a plurality of resistors  503  and one or more conductors  512 . The resistors  503  are distributed across the second surface  52  of the substrate  500 . For the sake of clarity, only three resistors  503  are labelled in  FIG. 26 ; however, it can be seen that the circuit comprises many more resistors, each of which is illustrated as a small black rectangle in  FIG. 26 . The resistors  503  are electrically connected to each other by the conductors  512 . In the example illustrated in  FIG. 26 , conductor  512   a  is connected to the ‘Heat−’ pin of the first connector  24  such that, in use, the conductor  512   a  operates as a negative voltage supply rail. Similarly, conductor  512   b  is connected to the ‘Heat+’ pin of the first connector  24  such that, in use, the conductor  512   b  operates as a positive voltage supply rail. 
     When a voltage is applied across the resistors  503 , power is dissipated as heat. The positive and negative supply voltages are supplied to the resistors  503  by the pins labelled ‘Heat+’ and ‘Heat−’ respectively in the first connector  24 . The resistors  503  are soldered to the conductors  512 , and are thereby electrically connected to the first connector  24 . The power dissipated by each resistor  503  is defined as: 
         P=I   2   R   (1)
 
     where P is the power dissipated (measured in watts), I is the current through the resistor (measured in amperes), and R is the resistance of the resistor (measured in ohms). 
     In an example, thirty resistors  503  are distributed over the area of the second surface  52 . The resistance values of the resistors  503  preferably range from 3.3 kilohms to 6.8 kilohms in order to avoid localised areas generating more heat than surrounding regions. The resistors  503  are preferably connected in parallel, but it will be appreciated that they could also be connected in series or in a combination of series and parallel connections. In an example, a direct current input voltage of twenty-four volts is applied across the resistors  503 . The invention is not limited to any particular input voltage or resistance values. 
     The temperature sensor  510  is mounted on the second surface  52  of the substrate  500 , using surface-mount technology. The temperature sensor  510  is preferably mounted at the point equidistant between the electrodes  514   a ,  514   b . This is to give an indication of the temperature near the region where electrical stimulation is applied, although the temperature sensor  510  could be placed at any other suitable point on the second surface  52 . The positive and negative supply voltages for the temperature sensor  510  are supplied by the pins labelled ‘Temp+’ and ‘Temp−’ respectively in the first connector  24 . The temperature sensor  510  is coupled to the first connector  24  by the conductors  511  patterned on the first surface  53  of the substrate  500 . Vias through the substrate  500  connect the conductors  511  on the first surface  53  to the temperature sensor  510  and first connector  24  that are mounted on the second surface  52 . The temperature sensor  510  can be a resistance thermometer or a thermocouple. The temperature sensor is preferably a platinum resistance thermometer (PRT), and is more preferably a Pt1000 element. A Pt1000 element is preferable due to its high accuracy. 
     Optionally, the resistors  503  and the temperature sensor  510  can be coated with a thin layer of TPU, prior to the circuit assembly (comprising circuit  16 , first connector  24 , second connector  22  and cable  12 ) being positioned on the first portion  2  of the stimulation pad following step  104 . The thin layer of TPU may be sprayed onto the resistors  503  and  510 . This provides a simple way to protect these components from being damaged by the heat and pressure experienced when the second material  55  is injected in step  110 . 
     An electrical stimulation current is delivered from the console  20  to the electrodes  514   a ,  514   b  by the pins of the first connector  24  labelled ‘EM 1 ’ and ‘EM 2 ’ respectively. The electrodes  514  are coupled to the first connector  24  by the conductors  511  patterned on the first surface  53 . Vias through the substrate  500  connect the conductors  511  on the first surface  53  to the first connector  24  that is mounted on the second surface  52 . 
     Other electronic components could be mounted on the substrate  500  and, preferably, mounted on the second surface  52  of the substrate. For example, logic components such as a programmable logic device, microprocessor or microcontroller could be mounted on the substrate  500 . Such logic components could be used to control the heat and/or electrical stimulation that is applied to a user. As another example, one or more sensors could be mounted on the substrate  500 , in addition to the temperature sensor  510 . A visual indicator  14  can be mounted on the second surface  52  of the substrate  500 . The visual indicator  14  is preferably a light emitting diode. 
     In use, the heating element  502  faces away from the skin of the user and the electrodes  514  face towards the skin. Thus, heat generated in the heating element  502  on the second surface  52  is conducted through the substrate  500  to the first surface  53 , and is subsequently conducted to the body of a user through the casing body  100  of the stimulation pad  1 . 
     Preferably the substrate  500  is flexible and so conforms to the contours of the body when the stimulation pad  1  is placed on the body. Preferably the substrate  500  comprises plastics material and preferably the plastics material is chosen to allow the substrate  500  to be flexible. Examples of suitable materials include polyimide and polyether ether ketone (PEEK). Those skilled in the art will appreciate that the substrate  500  could comprise any other suitable material. Thus, the substrate  500 , electronic components  502 ,  505 ,  507 ,  510 , the conductors  511 ,  512  and the electrodes  514  collectively form a flexible circuit. Flexible circuit technology is defined by industry standards, such as IPC standards IPC-T-50, IPC-2223A and IPC-4202. Despite its flexibility, the substrate  500  is preferably substantially planar in the absence of an applied force. 
     As mentioned above, the moulding method described herein can be used to mould other types of article and, therefore, the invention is not limited solely to the manufacture of stimulation pads. It is envisaged that the moulding method described herein will be particularly useful in other situations where it is desired to mould one soft material against another soft material, due to the method&#39;s ability to reduce distortion of the resulting article, reduce flash and ensure good bonding between the materials. By allowing one soft material to be moulded against another soft material, the end product is advantageously soft and flexible. It is also envisaged that the moulding method will be useful in other situations where it is necessary to encapsulate an electrical circuit within a water resistant cover. For example, the moulding method may be useful for manufacturing: sensors; soft and/or flexible consumer products, such as headsets; shock resistant and/or water resistant consumer products, such as cameras, watches and telephones; and medical devices, such as hearing aids, defibrillators, heart rate monitors, ultrasound devices and electroencephalogram (EEG) sensors. 
     It will be understood that the invention has been described above purely by way of example, and that modifications of detail can be made within the scope of the invention.