Medical lighting devices

Abstract

A medical lighting device (100) is a dental treatment lighting device (100) particularly for illuminating the oral cavity of a surgical site, wherein the medical lighting device comprises a light source (50) having at least one LED light source (52) arranged on a circuit board (51) and a port-shaped housing (10) for accommodating the light source (50) and forming a light outlet opening (18) for light emission. The housing comprises a bottom area (11) for flatly mounting the circuit board (51) and a wall area (13) extending around the perimeter from the bottom area (11) to the light outlet opening. The housing (10) is formed of an outer shell (20) made of plastic and an inner shell (30) made of metal that is in close contact with the inside of the outer shell (20), and the circuit board (51) is in complete thermal contact with the inner shell (30) extending across the bottom area (11) into the wall area (13).

Description

Technology Field

[0001] The present invention relates to a medical lighting device, in particular to a dental lighting device for illuminating the oral cavity of a surgical site. Background Technology

[0002] To ensure optimal medical treatment, it is essential to properly illuminate the treatment site during surgery, dental procedures, or examinations. Various framework conditions must be considered to protect the patient and optimize lighting. These conditions are reflected in various standards, which, for example, define the shape of the light area, specific minimum values ​​for the Color Rendering Index (CRI), and minimum values ​​for illuminance.

[0003] Lighting that meets these requirements is disclosed, for example, in the applicant’s EP 2 469 158 A2. This document discloses a dental lighting device using five individual lighting units to achieve the desired lighting, each lighting device equipped with an LED light source and an associated optical device. These five lighting units are spatially distributed and integrated into a lighting housing, and the LED light sources are placed on a common support and aligned to project superimposed onto a single target plane. This enables shadowless lighting of the surgical area, which is as uniform as possible and remains uniform even if, for example, the light from an individual lighting unit is blocked by a doctor. As previously mentioned, multiple lighting units are placed on a common support to ensure proper alignment between the lighting units and are thermally connected to the lighting housing. This enables a relatively uniform heat distribution within the lighting housing, which is advantageous since it is essential to maintain a uniform surface temperature that is not too high to prevent irritation when using the lighting or medical device.

[0004] In the case of lighting devices known in the prior art, it is essential to accurately align multiple lighting units with one another to achieve optimal lighting results. Since this requires a relatively large amount of effort, it is advantageous in principle to concentrate light on a smaller area. In this case, light can be controlled or adjusted more easily and efficiently, allowing for equivalent or higher quality lighting to be obtained with less effort.

[0005] However, concentrating light on a smaller area creates a problem in that it generates higher localized heat compared to the dispersed arrangement of light sources in conventional technology, increasing the risk of so-called hot spots forming in the lighting housing. In addition to increasing the risk of damaging electronic components, these hot spots can also pose a risk to patients or doctors. The problem to be solved

[0006] Accordingly, the present invention aims to provide a medical, particularly dental, therapeutic lighting device designed to dissipate heat generated during operation as uniformly as possible through the housing. means of solving the problem

[0007] The present invention solves this problem by a lighting device having the features of claim 1. Advantageous further developments of the present invention are the subject of dependent claims.

[0008] The solution according to the present invention is based on a special design of a lighting housing that accommodates a light source, which is designed to highly efficiently disperse the heat generated within the housing and release it into the external environment over a wide area. Accordingly, the concept according to the present invention is particularly suitable for lighting that uses only a small number of light sources concentrated in a small area to generate light. In principle, it goes without saying that the solution according to the present invention can also be used when the light sources are placed over a wider area.

[0009] According to the present invention, a port-shaped lighting housing having at least one LED substrate disposed on the bottom of the housing has a multilayer structure. In particular, according to the present invention, the housing is composed of an outer shell made of plastic and an inner shell made of metal that is in close contact with the inside of the outer shell, the LED substrate is in complete thermal contact with the inner shell, and the inner shell extends beyond the bottom area of ​​the lighting housing to the wall area.

[0010] According to the present invention, a medical lighting device for illuminating the oral cavity of a surgical site, particularly a lighting device for dental treatment, is proposed, and said lighting device

[0011] - A light source having at least one LED light source disposed on a circuit board, and

[0012] - It includes a port-shaped housing that accommodates a light source and forms a light exit opening for light emission,

[0013] The housing has a bottom area for mounting a circuit board flatly and a wall area extending along the perimeter from the bottom area to a light exit opening, and

[0014] Additionally, according to the present invention, the housing is formed of an outer shell made of plastic and an inner shell made of metal that contacts the inside of the outer shell, and the circuit board is in complete thermal contact with the inner shell, and the inner shell extends from the bottom region to the wall region.

[0015] The lighting housing with a two-layer or double-layer structure according to the present invention has been found to provide a highly efficient method for evenly dispersing heat generated from a light source across almost the entire outer surface of the housing and uniformly releasing it to the external environment. In particular, thanks to the heat dissipation efficiency achieved in this way, even when the lighting is operated at maximum output for a long time, no surface temperature is generated that would cause discomfort to patients, doctors, or general users. Nevertheless, stable heat dissipation essential for the operation of the lighting is guaranteed, which is a surprising result considering that the outer surface of the lighting housing according to the present invention is made of a plastic material with poor thermal conductivity. However, this inherent disadvantage of the outer surface is resolved by using an inner shell according to the present invention. Thus, the entire outer surface of the lighting housing can be made of plastic, which is an electrically insulating material, providing advantages in terms of handling and safety of the lighting. Unlike simple heat sinks, such as metal blocks that absorb heat generated from an LED light source, the concept according to the present invention differs in that the inner shell is an actual functional component of the lighting housing and can also perform additional housing functions, as described in more detail below.

[0016] The outer shell may extend from the wall area of ​​the housing through the inner shell to the light exit opening. In particular, the outer shell may include a translucent cover covering the light exit opening and / or fixing means for fixing an optical element that affects light emitted from a light source in the wall area facing the light exit opening.

[0017] The outer shell, that is, the plastic part of the lighting housing, may also form one or more grip elements that allow the lighting fixture to be moved or aligned to a specific position. In this case as well, it is advantageous for these components of the housing to be made of plastic, which is an electrical insulating material.

[0018] It is desirable for the inner shell to include structural elements in the bottom area for the prescribed positioning of the circuit board, and these structural elements may take the form of pins or bridges that pass through centering holes formed in the circuit board. This is advantageous in that it ensures the precise placement of the light source and enables proper positioning of the light source in relation to other optical elements. Through this, the light can be appropriately controlled to illuminate the processing area uniformly and evenly as desired. It is particularly desirable for the circuit board to have two centering holes, one of which has a cross-section that fits the shape of a pin or bridge, and the other is formed as an elongated hole that allows relative movement between the circuit board and the inner shell. This prevents the generation of stress due to expansion caused by temperature changes. The circuit board can be attached to the inner shell using frame-shaped clamping elements, which elastically compress the circuit board against the inner shell. In particular, the clamping elements are attached to the inner shell via screw holders, and preferably, structural elements form screw channels for attaching the clamping elements.

[0019] It is desirable to place the LED light source in the center of the circuit board and occupy only about 1% of the surface area of ​​the circuit board. In particular, it is desirable for the LED light source to consist of two series circuits, each composed of multiple LEDs, particularly three LEDs.

[0020] By installing a so-called thermal pad on the back of the circuit board opposite the LED light source, heat transfer from the LED light source to the inner shell of the lamp housing can be further improved. This thermal pad extends outward from the center of the circuit board in a radial or fan shape and covers more than 80% of the circuit board area. This thermal pad forms an electrically insulated area and is preferably connected to the terminals of the LED light source through vias.

[0021] According to a particularly preferred embodiment, the inner shell may have a bearing housing for mounting a light on a support arm, and this bearing housing is specifically designed to rotatably accommodate the support arm. This aspect further emphasizes that the inner shell is an actual part of the light housing. This is because it ensures that the light can be permanently and stably mounted on the support arm or, in general, the corresponding support element.

[0022] The thermal conductivity of the inner shell material is about 100 times higher than that of the outer shell material. In particular, the inner shell can be made of aluminum or magnesium, and aluminum or magnesium die-cast parts are particularly preferred.

[0023] For efficient interaction between the inner and outer shells in terms of heat transfer, the two components must be in flat contact with each other, and in particular, there should be no air layer acting as a thermal insulator. Therefore, it has been proven particularly advantageous to form the outer shell in direct contact with the inner shell by spraying plastic material onto it. In this case, although a rigid bond is not formed between the two components due to the different materials, sufficient heat transfer from the inner shell to the outer shell can still occur. Additionally, it is advantageous for the outer shell to at least partially cover the edge region of the inner shell so that the two shells can be firmly connected to each other even without material bonding.

[0024] Ultimately, through these various measures, a lightweight housing suitable for medical lighting devices can be manufactured, which enables uniform heat release to the surrounding environment over a wide area even if localized heat generation concentration occurs. Brief explanation of the drawing

[0025] The present invention will be described in more detail below with reference to the attached drawings. FIG. 1 illustrates an exemplary embodiment of a lighting device according to the present invention installed in a dental treatment table support system. Figure 2 is an enlarged view of the lighting device of Figure 1. Figure 3 shows the interior of a lighting housing in which an LED substrate is placed inside. Figure 4 is a cross-sectional view of Figure 3. Figure 5 shows the front view of an LED substrate with an LED light source. Figure 6 shows the back side of the LED substrate. Figure 7 shows another side view of a light fixture in which a clamping element for fixing an LED substrate is placed inside. Figure 8 is a cross-sectional view of Figure 7. FIG. 9 is a schematic diagram illustrating heat dissipation achievable with a solution according to the present invention. Specific details for implementing the invention

[0026] FIG. 1 shows a schematic diagram of a dental lighting device according to the present invention, indicated by reference numeral 100. The lighting device is mounted on a support structure (110), which is, for example, part of a dental treatment station not shown. Both the support structure (110) and the holder of the lighting device (100) mounted on the support structure (110) (described in more detail below) may include multiple joints, thereby allowing the position of the lighting device (100) to be easily adjusted and properly aligned as is known in dental lighting devices.

[0027] The lighting housing (10), which can be seen in more detail in FIG. 2, is basically pot-like and consists of a bottom area (11) forming the back of the lighting fixture (100) and a surrounding wall area (13) defining the light-outlet opening (18) (e.g., see FIG. 4). In this exemplary embodiment, the lighting housing (10) is designed in a roughly truncated pyramid shape so that the light-outlet opening (18) has a substantially square shape. However, in principle, the shape of the housing (10) may be a truncated cone shape or the shape of the light-outlet opening (18) may be elliptical. Importantly, as described in more detail below, a roughly flat bottom area (11) is formed for housing the light source, and the wall area (13) extends in a closed circumferential direction from the bottom area (11) to the light-outlet opening (18).

[0028] In addition, as illustrated in FIG. 2, in this embodiment, two gripping members (15) facing each other are provided on the housing (10) so that the lighting device (100) can be grasped and positioned or aligned. The joint of the bearing system is designed to maintain the lighting device (100) in a set position and alignment state.

[0029] A component of the lighting housing (10) shown in FIG. 2, namely a grip member (15) preferably integrally disposed on the outer surface of the lighting housing, is formed as a first component of the housing (10) (hereinafter also referred to as an outer shell (20). This outer shell (20) is preferably a piece made of plastic, which is an electrical insulating material, allowing the lighting device (100) to be touched without the risk of electric shock. In addition, using plastic generally provides a more pleasant tactile sensation compared to metal.

[0030] The illustrated shape, including the grip member (15) of the housing (10), can be easily manufactured from plastic material through an injection molding process, and since the structure can be implemented with high precision, the outer surface of the lighting housing (10) can be produced with high quality. However, at the same time, there is a problem in that the plastic itself has low thermal conductivity, so the heat generated during the operation of the lighting device (100) is not properly released to the surrounding environment through the plastic housing. In particular, the problem is that the plastic material cannot efficiently and evenly distribute the heat generated intensively in a specific area.

[0031] To solve these problems, according to the present invention, the housing (10) is designed as a two-layer structure including an inner shell (30) (preferably a fragment) in addition to an outer shell (20). The inner shell (30) has thermal properties that compensate for the disadvantages of the plastic material of the outer shell (20). This concept is explained in more detail below with reference to FIGS. 3 and 4.

[0032] FIG. 3 shows the interior of a lighting housing (10), which is covered with a translucent cover that is not shown in detail in the drawing. FIG. 4 is a cross-sectional view along axis II of FIG. 3.

[0033] According to the present invention, as previously mentioned, an inner shell (30) made of plastic is placed inside a port-shaped outer shell (20). As described below, this inner shell (30), made of metal, forms a bottom area (31) corresponding to the bottom area (11) of the lighting housing (10), and a circumferential wall area (33) extends from this bottom area (31) toward the light outlet opening (18). The inner shell (30) and the outer shell (20) are in flat contact with each other but are not bonded due to different materials.

[0034] The actually flat surface of the bottom area (31) of the inner shell (30) is used to support a light source (50), which is composed of a square circuit board (51) in which an LED light source (52) is placed in the center in the illustrated exemplary embodiment. The LED light source (52) may be one or more LEDs or a combination of several LEDs. As can be seen in the drawing, the LED light source (52) is preferably localized to the central area of ​​the circuit board (51). This means that its size is small. While it is conceivable to mount the LED over a wide area of ​​the circuit board (51), the variation illustrated in the drawing has the advantage that the light from the quasi-point-like light source used here can be better and more efficiently influenced through appropriate optical means. Ultimately, this can improve the quality of lighting, and with the use of appropriate optical means, the surgical or examination area can be uniformly illuminated without shadowing.

[0035] A preferred exemplary embodiment for implementing an LED light source (52) is illustrated in FIG. 5. In this case, the LED light source (52) is composed of two parallel series circuits each consisting of three LEDs, 53a and 53b, so that various lighting modes can be selected according to each medical indication by selectively activating the two LED groups (53a, 53b).

[0036] Two LED groups occupy up to about 1% of the total area of ​​the LED substrate (51), which shows that the board is very small and acts as a nearly point-like light source, allowing the light to be controlled to efficiently illuminate the medical work area. Each of the three LEDs connected in series has four electrical contact tabs (negative and positive) that form a thermal conductor. In order to reduce the heat generated in the form of a hot spot inside the sealed housing as quickly as possible and to induce additional heat dissipation by evenly distributing the heat throughout the housing (10), the electrical connection tabs are perforated through the metal back of the circuit board (51) to come into contact with the heat pad (55) provided therein.

[0037] For efficient interaction between the LED light source (52) and the optical device (not shown), the LED light source (52) must be positioned correctly inside the lighting housing (10). To achieve this, structural elements in the form of pins or bridges (32) are formed on the top of the bottom area (31) of the inner shell (30), and these elements extend through the corresponding openings (54a, 54b) of the circuit board (51). This allows the circuit board (51) to be correctly mounted in the housing (10), and ultimately, the actual light source (50) to be correctly mounted.

[0038] As can be seen in FIG. 6, according to a preferred embodiment, one of the two openings of the circuit board (51) may be designed as a closed hole (54a) with a cross-section aligned with the outer contour of the web (32), and the opposite opening may be designed as an elongated hole (54b). This design allows the circuit board (51) to be fixed in a defined position in the area of ​​the hole (54a), while on the opposite side it can move relative to the inner shell (30). This prevents stress on the circuit board (51) that could ultimately lead to damage, even if the circuit board (51) and the inner shell (30) expand differently due to temperature changes.

[0039] By placing the aforementioned multiple thermal pads (55) on the back of the circuit board (51) shown in FIG. 6, a thermally optimized coupling between the light source (50) and the inner shell (30) can be achieved. These thermal pads extend outward in a radial or fan shape from the center of the circuit board (51) and lie flat on the top of the bottom area (31) of the inner shell (30). These thermal pads (55) form eight electrically insulated segments and preferably cover 80% to 90% or more of the circuit board area. These thermal pads (55) first distribute the heat generated from the centrally placed LED light source (52) across the entire back surface of the circuit board (51) and then transfer it to the bottom area (31) of the inner shell (30). To improve heat transfer from the circuit board (51) to the inner shell (30), it is desirable to use a suitable thermally conductive and electrically insulating interface material, such as a thermal paste or a thermally conductive film.

[0040] The aforementioned bridge or pin (32) initially serves only to accurately align the LED circuit board (51) to the top of the inner shell (30). The actual fixation of the circuit board (51) is achieved through the clamping element (40) shown in FIGS. 7 and 8, which forms a frame that elastically presses against the top of the circuit board (51). In the illustrated exemplary embodiment, the clamping element (40) is attached to the inner shell (30) via a screw connection, and the aforementioned web (32) not only serves to align the circuit board (51) but also forms a screw groove into which two screws are fastened to engage with the clamping element (40). Additionally, as can be seen in FIG. 7, the clamping element (40) can be suspended from the housing (10) via an additional arm or strut.

[0041] In principle, other methods of attaching the circuit board (51) to the housing (10) are possible, but the method illustrated in this drawing has the advantage of achieving particularly excellent thermal bonding and preventing stress generation during temperature changes by pressing the circuit board (51) over a wide area on the top of the bottom region (31) of the inner shell (30).

[0042] Starting from the bottom area (31) of the inner shell, heat is distributed primarily through this area, but also applies to a portion of the wall area of ​​the housing (10). As can be seen in the cross-sectional views of FIGS. 4 and FIGS. 8, the inner shell (30) extends to the side wall area (33) of the housing (10) but does not reach the actual light exit opening (18). Thus, the outer shell (20), made of plastic, protrudes beyond the inner shell (30) in the wall area to form a rigid bond between the two shells (20 and 30). In particular, the outer shell (20) may include an additional structural element (28) in the wall area facing the light exit opening (18), and this structural element can be used to secure an optical element that affects the light emitted from the translucent cover and / or light source (50).

[0043] The lighting housing (10) has a bearing housing (38) on one side of the wall area (13), through which the lighting device (100) is rotatably attached to the aforementioned support arm (110). This bearing housing (38) forms a roughly cylindrical opening in the wall area (13) of the housing (10), into which the corresponding end (115) of the support arm (110) engages. The bearing of this bearing housing (38) allows the lighting housing (10) to rotate about the longitudinal axis of the bearing housing (38). The bearing housing (38) is preferably part of the inner shell (30), which has the advantage that the bearing housing (30), like the entire inner shell (30), is made of metal and can absorb greater force. Even if the weight of the lighting device (100) increases (e.g., additional attachment not shown in detail), the lighting device (100) can be permanently, stably, and safely rotated and mounted on the support arm (110) without risk of wear.

[0044] Therefore, the inner shell (30), which is part of the lighting housing (10), is advantageous for both serving as a heat sink for the light source (10) and for rotatingly mounting the lighting device (100) on the support arm (110).

[0045] The aforementioned dual-structure design of the lighting housing (10) having an inner shell (30) and an outer shell (20) allows heat generated from the LED light source (52) to be efficiently and safely released to the outside. Since the inner shell (30) is mainly bonded to the LED substrate (51) and has a thermal conductivity several times (preferably about 100 times or more) greater than that of the plastic material of the outer shell (20), the heat initially generated locally can be distributed over a wide area, namely both the bottom area (31) and the wall area (33) of the inner shell (30). Because the outer shell (20) and the inner shell (30) are in contact with each other on a flat surface, heat is transferred further from the inner shell (30) to the outer shell (20) and is finally released to the environment through the outer shell (20). Here, the reduced thermal conductivity of the outer shell (20) is no longer a negative factor due to its significantly large surface area.

[0046] In fact, using a plastic material for the outer shell (20) can prevent hot spots from forming on the illuminated surface of the LED light source (52) area. Instead, as shown in FIG. 9, the outer surface of the lighting housing (10) is heated almost uniformly and homogeneously in an area substantially corresponding to the dimensions of the inner shell (30). Here, the two areas (150 and 151) indicated by dashed lines represent the housing surface areas where the temperature rises while the lighting device (100) is operating, but the temperature in both areas remains almost constant. The edges of these areas also indicate the transition to the area where the wall area of ​​the housing (10) is formed solely by the outer shell (20), and it can be seen that there is a continuous temperature drop from this boundary. However, heat is homogeneously released into the surrounding environment at a relatively low temperature over the entire area of ​​the inner shell (30).

[0047] The favorable interaction between the outer shell (20) and the inner shell (30) of the housing (10) is facilitated by the fact that the two shells (20, 30) are in flat contact with each other without air being mixed between them. This can be achieved, in particular, by injection molding a plastic material to form the outer shell (20) on top of the inner shell (30) after it has been fabricated first. This process can be performed using an injection molding process in which the inner shell (30), which has been fabricated first as a die-cast part, is inserted into an injection mold. As previously mentioned, this method does not create a material adhesive connection between the outer shell (20) and the inner shell (30). However, full surface contact between the two shells (20, 30) is ensured, and a solid bond between the two components is ensured because the outer shell (20) covers the inner shell (30) at least partially, as can be seen in the cross-sectional view.

[0048] The procedure described above also has the advantage that the area forming the outer contour of the lighting housing (10) can be produced with high precision during the injection molding process. The inner shell, made of materials such as aluminum or magnesium, can be produced with much lower precision and does not require complex post-processing.

[0049] Ultimately, the described solution provides a lighting housing that can be manufactured relatively simply for medical, particularly dental, applications and effectively dissipates even the heat generated from very small light sources.

Claims

Claim 1 A dental treatment lighting device (100) for illuminating the oral cavity of a surgical site, wherein the dental treatment lighting device comprises: a light source (50) having at least one LED light source (52) arranged on a circuit board (51); and a port-shaped housing (10) for accommodating the light source (50) and forming a light outlet opening (18) for light emission, wherein the housing comprises a bottom area (11) for flatly mounting the circuit board (51) and a wall area (13) extending around the perimeter from the bottom area (11) to the light outlet opening, wherein the housing (10) is formed of an outer shell (20) made of plastic and an inner shell (30) made of metal that is in close contact with the inside of the outer shell (20), and the circuit board (51) is in complete thermal contact with the inner shell (30) that extends across the bottom area (11) into the wall area (13). Claim 2 A dental treatment lighting device according to claim 1, characterized in that the outer shell (20) protrudes from the wall region (13) of the housing (10) through the inner shell (30) and extends to the light outlet opening (18). Claim 3 A dental treatment lighting device according to claim 2, characterized in that the outer shell (20) includes a fixing means (28) formed in a wall area (13) facing the light outlet opening (18) to fix optical elements and / or a translucent cover that closes the light outlet opening (18) that affects light emitted by the light source (50). Claim 4 A dental treatment lighting device according to claim 1, characterized in that the outer shell (20) forms one or more grip members (15). Claim 5 A dental treatment lighting device according to claim 1, wherein the inner shell (30) is provided with a structural element (32) for positioning a defined circuit board (51) in a bottom area (11), and the structural element (32) forms a pin or bridge passing through a centering hole (54) formed in the circuit board (51). Claim 6 A dental treatment lighting device according to claim 5, wherein the circuit board (51) has two centering holes (54), one of the centering holes (54a) has a cross-section that fits the outer contour of a pin or bridge, and the other centering hole (54b) is formed as an elongated hole that allows relative movement between the circuit board (51) and the inner shell (30). Claim 7 A dental treatment lighting device according to claim 5, characterized in that the medical lighting device further includes a frame-shaped clamping element (40) that elastically presses a circuit board (51) against an inner shell (30). Claim 8 A dental treatment lighting device according to claim 7, wherein the clamping element (40) is attached to the inner shell (30) through a screw holder (45), and the structural element (32) forms a screw channel for fixing the clamping element (40). Claim 9 A dental treatment lighting device according to claim 1, wherein the LED light source (52) is positioned in the center of the circuit board and occupies only 1% of the surface area of ​​the circuit board (51), and the LED light source (52) is formed by two series circuits each consisting of a plurality of LEDs. Claim 10 A dental treatment lighting device according to claim 1, wherein the circuit board (51) is provided with a heat pad (55) covering at least 80% of the area of ​​the circuit board (51) on the back side opposite the LED light source (52), and the heat pad (55) extends outward in a radial or fan shape from the center of the circuit board (51). Claim 11 A dental treatment lighting device according to claim 10, wherein the heat pad (55) forms an electrically insulated region, and each of these regions is connected to a terminal of an LED light source (52) through a via. Claim 12 A dental treatment lighting device according to claim 1, wherein the inner shell (30) is provided with a bearing housing (38) for fixing the lighting device (100) to a support arm (115), and the bearing housing (38) is designed to rotatably mount the support arm (115). Claim 13 A dental treatment lighting device according to claim 1, characterized in that the thermal conductivity of the inner shell (30) material is 100 times greater than the thermal conductivity of the outer shell (20) material. Claim 14 A dental treatment lighting device according to claim 1, characterized in that the inner shell (30) is made of aluminum or magnesium. Claim 15 A dental treatment lighting device according to claim 1, wherein the outer shell (20) is injection molded from a plastic material over the inner shell (30) so that the outer shell (20) is seated directly on the inner shell (30), and the outer shell (20) overlaps at least partially with the edge region (33) of the inner shell (30).

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