WO2018219557A1 - Method for manufacturing spectacle frame components and frame components produced using said method - Google Patents

Abstract

A method is described for manufacturing spectacle frame components, such as a front frame (1) of the frame or a side arm (2) of a frame or part of the front frame or the side arm, comprising the following successive phases: preparing a cellulose acetate slab (11) having a preselected, defined thickness between opposite faces (11a, 11b) of the slab, placing the slab (11) in an over-injection mould, over-injecting elastomer over at least one of the faces of the slab (11) over at least part of the surface extent of that face to produce a slab-shaped, multilayer semifinished product (13), removing the semifinished product (13) from the mould, shaping the semifinished product (13) by cutting and/or milling to produce the preselected final shape of the component, the slab-shaped structure of the component thus made being formed of at least two layers, respectively a layer of cellulose acetate and a layer of elastomer.

Description

Method for manufacturing spectacle frame components and frame components produced using said method

Technical field

The present invention relates to a method for manufacturing spectacle frame components.

The invention also relates to spectacle frame components produced by the aforesaid method.

Technological background

In the technical domain of spectacles, both for eyesight and for protection from the sun, it is known that frames can be made from plastics material supplemented with inserts or components made of elastomeric material, also described in technical jargon by the term "rubber". Frames made of plastics material are known, for example, which have nose pieces or nose pads made of elastomer in the zones resting on the nose, to increase the comfort of the spectacles for the user. Elastomer is in fact characterised by greater softness, yielding guality and resilience than the plastics materials generally used for producing frames, and therefore makes it more comfortable for the front frame to rest on the nose of the user. This is due both to the feeling of softness offered by the elastomer, and to the actual ability of the elastomer to adapt to some extent to the shape of the nose.

Moreover, there are known frames made of plastics material that have elastomer inserts within the surface of the side arms, also with the aim of increasing the wearing comfort of the spectacles. For example, it is known that an elastomeric material can coat or be applied to all or part of the surface of the inner side of the side arm, intended to rest in direct contact with the user's head, so that the portion of side arm that rests behind the ear is soft and adaptable. The application of inserts on the side arms is not aimed exclusively at improving the wearing comfort of the spectacles. It is, in fact, known that elastomer inserts can also be applied on the outer side of the side arms (as an alternative or in addition to application on the inner side), in order to produce decorative elements enhancing, for example, the possible colour combinations that can be obtained, respectively, in the material of the side arm and the elastomer of the added component.

Any decorative or aesthetic effect added by the elastomer is not necessarily determined by any colouring it may have. The materials belonging to the elastomer families are in fact very often characterised by a different surface finish from that achievable in ordinary plastics materials. In particular, elastomers tend to have an opaque surface, in this case not so much related to any lack of transparency of the material as to the substantial lack of surface gloss. This produces an "opaque" effect, often known in the sector by the English term "matt" or the similar term "matte" (this effect may be enhanced by providing a suitable surface finish to the walls of the injection mould impression), in contrast with the gloss or generally shiny surface that characterises injected plastics materials. The contrast between the two types of surface can therefore further enrich the aesthetic effect of the frame, as an alternative or in addition to the result that can be obtained by deliberate combinations of the relevant colours.

In the prior art there are also other known applications of inserts made of elastomer or plastics material located on the front frame, also in zones other than the nose support region. It is in fact known that details made of elastomer can be provided on outer portions of the front side of the front frame, purely for decorative purposes, for the same reasons as stated for applications on the side arm.

Typically, these frames are nowadays produced using the "injection moulding" technique, applied both to plastics material and elastomer. This process is preferred and proves decidedly more effective than the method based on gluing elastomer components and plastics components together, both because of its cost-effectiveness (since it is easier to automate the operations) and because the adhesion between the different materials is stronger and holds better.

Injection moulding is also preferred to mechanical or slotted fixing systems, mainly because the process is economical.

In greater detail, provision is made in the known production process to first inject the front frame and the side arms with harder and more rigid plastics material, and then over-inject the elastomer over the intended surfaces or zones of the side arms or the front frame.

This involves using a first mould for injection, and then a second mould for over- injection.

In the relevant sector of the application considered here (elastomer inserts on a plastics mounting) the materials that are or can be used are in practice limited to the use of polyamide as an injected material for producing the frame ("rigid frame")- This is because, for this class of plastics materials, there is a huge range of elastomeric materials, in terms of chemical nature and/or composition, that are compatible from the chemical point of view, and can therefore ensure a sufficiently high elastomer/plastics adhesion force.

In contrast, the systematic use of polyamide-based plastics materials has major limitations from the aesthetic and functional point of view, in relation to the characteristics of the frames that can be made using them.

In particular, with polyamide-based plastics materials it is not technically possible to achieve the aesthetic colouring effect known in the sector as the "avana (tortoiseshell)" finish or effect.

The "avana" effect consists of the typical variegated surface colouring and also thickness of the component, produced by a series of transparent or semi-transparent spots of two different tones (for example, yellow and brown), which can typically be found in what is known as the "cellulose acetate" plastics material, and which has now become one of the most in demand, commonest and most popular aesthetic variants. For reasons related to the process of producing the raw material (plastics material) to be used for producing the frames, it is known in the reference sector that only cellulose acetate is available in versions characterised by the "avana" effect, made in many possible shades, variants and colour combinations. It is also known that these effects can be obtained in cellulose acetate produced strictly in slab form.

Typically, cellulose acetate in slab form is produced by two alternative methods: lamination or extrusion or "calendering", on the one hand, and "block" production, on the other. Said methods allow the producer to modify and vary its composition, with extremely high degrees of freedom, even in terms of pigments or additives that have specific colouring properties.

The producer is therefore able to control the appearance and structure that the final slab material will have, depending on the particular choices or specific requirements of whoever will then be processing the cellulose acetate to make spectacle frames with it.

The industrial process that typically leads to the production of the injectable raw material does not, however, allow the aesthetic properties of said material to be controlled. Polyamide is therefore not available in versions that have "avana (tortoiseshell)" colouring.

Actually, it is known in the sector that replication of this effect can be attempted on injected polyamide frames, by using surface finishing or decoration techniques capable of recreating the characteristic "tortoiseshell" patches. These techniques include varnishing processes, processes based on sublimation, processes of depositing water- soluble decorative films applied by immersing the part (what are known as "dip- printing" processes), decorations obtained by digital printing, and others.

The results that can be obtained cannot, however, be considered equivalent to the finish of the original " avana (tortoiseshell)" effect.

The aforesaid surface decoration processes do by their nature make it possible to produce colourings or aesthetic effects restricted to the surface of the part treated. The typical, original " avana (tortoiseshell)" effect of cellulose acetate in slab form is, however, distinguished by the highly "three-dimensional" effect or depth of the spotted structure, since the spots are designed as three-dimensional figures, distributed through the depth of the material, i.e. within the thickness of the side arm or the front frame made of acetate, and are clearly visible if at least one of the two base colours of the patches is at least partly transparent.

Cellulose acetate in slab form can also offer other decorative effects, which prove not to be reproducible in conventional injectable plastics materials, such as polyamide and many other thermoplastic materials.

For example, it is possible to produce cellulose acetate slabs consisting of layers of different coloured materials (even three or more layers), which can easily be obtained by the method of producing acetate by means of calendering.

A cellulose acetate slab that has two, three or more layers in different colours is also a form of acetate widely used for manufacturing front frames or side arms for spectacles, since the aesthetic characteristics deriving therefrom are very popular in the market, and are not available in other, alternative plastics materials, or only by using fairly complex and costly production processes.

Other effects still can be produced in acetate and only using slab-form acetate, and these effects make this material unique and highly noticeable against all other, conventional plastics materials, which in practice can be divided into injected materials (e.g. polyamide, cellulose propionate, polycarbonate) and glued materials.

Among these effects, we mention only the further example of slabs containing inserts or decorative elements of various shapes, colours, sizes and constituent materials, visible on the surface or by transparency (what are known as "incorporated material effects").

A further advantage offered by cellulose acetate can also mentioned, that is to say the possibility of adapting the shape of the side arm (generally in the end part) made from that material by heating it to a temperature of around 70°C to 80°C, and shaping it by exploiting the plastic deformability of the material. This characteristic is particularly highly prized when a "registrable" frame is to be produced, i.e. one that can be adapted to the face and head of the user.

This property is particular to cellulose acetate, characterised by a softening temperature that will, precisely, act at about the aforesaid value. This temperature value can also be easily achieved by using standard optician's, optical laboratory or point of sale equipment, which consists in essence of a hot air fan.

This same characteristic of acetate is also reflected in the possibility of "hot" fitting the lenses, allowing the front frame in the two eyepieces to be temporarily softened by heating to a temperature of around 70°C to 80°C, so as to insert the two lenses more easily, without having to overcome the resistance presented by an otherwise rigid, hard material, similar in this respect to other known plastics materials.

The other plastics materials used in the production of frames, on the other hand, would require much higher temperatures to soften (in the case of polyamide, over 120°C), which generally cannot be achieved by using normal optician's or lens mounting laboratory equipment.

Frames made of a plastics material other than cellulose acetate in slab form therefore actually prove unsuitable for manufacturing "registrable" frames, i.e. ones that have adaptable size or shape.

The sole exception consists of the plastics material called cellulose propionate, which has hot malleability characteristics, i.e. a low softening temperature, that are substantially the same as those of cellulose acetate. Cellulose propionate is not, however, available in mechanically processable slab form for producing spectacle frames, given that it is produced only in a granular material form that can be used for injection moulding. It therefore suffers from the same aesthetic or decorative limitations described above for injected plastics materials in general.

According to the teaching of the prior art, adopting a process for over-injecting or co- injecting elastomer over a cellulose acetate component in slab form, whether a front frame or a side arm, although it may seem obvious, does have major limitations. Therefore, providing a front frame or a side arm made of ready-shaped acetate, i.e. cut out with the final intended shape for the component (side arm or front frame) in a first phase and, in a second phase, placing said component in an over-injection, co- injection or "insert moulding" mould, and subsequently injecting the rubber or elastomer, does not actually prove adequate or effective because of other intrinsic characteristics of the cellulose acetate.

In particular, it is known that slab-form cellulose acetate has a marked tendency to "spontaneous deformation", which can occur first and foremost on slabs leaving the production process for calendering or in "block" form (and which therefore constitute the base material for manufacturers of spectacle frames). This can also occur, with even more marked and problematic effects, on products made from the slab, following processes such as cutting or milling (for example, side arms or front frames at the semifinished stage, or even finished parts).

This tendency is due to the many internal stresses that occur within the material during the industrial production processes.

The internal stresses remain "frozen" fairly diffusely (and randomly distributed) in semifinished products coming from production plants, i.e. in slabs or panels delivered by the acetate producer to the spectacles producer.

Through the effect of the substantially mechanical treatments carried out during the process of manufacturing the frames, which therefore chiefly comprises cutting and milling using various tools and in several successive phases, the internal stresses are released, with consequences that can be seen in the parts emerging from said treatments.

After the treatments, these parts actually suffer almost instantaneously from deviations from the shape initially given.

Among other things, every individual part can have a different deformation from that found in the other parts belonging to the same production batch, the various parts exhibiting deviations with a different extent or direction from one another.

One such undesirable effect can easily occur even on a semifinished product cut to a more advanced shape (e.g. a side arm) and then further processed by means of the curving operation provided afterwards, to give the front frame or the side arm the ergonomic curves or bends of the final frame.

This tendency to post-production deformation of slab-form acetate cannot be controlled in any way, and therefore constitutes a problem for possibly using it to produce pre-shaped (pre-cut) components for placing in an over-injection (or co- injection, or "insert moulding") mould.

The over-injection technique does require products pre-inserted into the mould to all have substantially the same shape and size as each other, within certain acceptable variations described as "process tolerances".

In the over-injection process the tolerances are particularly low, which means that there needs to be high repeatability and precision in the preceding phase of manufacturing the inserts or components to be placed in the mould.

This requirement results from the need to place in the mould a part that always perfectly or almost perfectly corresponds to the impression of said mould.

If the shape corresponds well, the molten elastomer flows over the insert at the time of injection and is restricted to filling the cavity in the intended mould, remaining contained therein.

If there is not much correspondence, the high fluidity (or low viscosity in the molten state) of the injected elastomer tends to make it flow into the empty spaces that have been created at various points in the mould impression, because of the imperfect match between the walls of the insert and those of the impression. If the free spaces are filled with elastomer during injection, this will also adhere to the insert in positions or portions in which it was not intended (for example, along the side edges of the insert), with the result that the part will have to be rejected, or possibly recovered by means of additional, exacting processes for removing the elastomer.

Obviously, therefore, the process of manufacturing frame components (e.g. front frames, side arms) to be pre-inserted into the over-injection mould, where these components have already been given their final or next-to-final shape by cutting and/or milling treatments on the acetate slabs, has clear limitations because of the poor precision and repeatability demonstrated by the result of these treatments.

Description of the invention

The main purpose of the invention is to provide a method for manufacturing spectacle frame components, designed to deal with the requirements and overcome the limitations set out above with reference to the prior art mentioned.

This aim is achieved by the present invention by means of a method and by means of spectacle frame components produced in accordance with the following claims.

Brief description of the drawings

The features and advantages of the invention will be more clearly apparent from the following detailed description of a preferred exemplary embodiment thereof, illustrated non-restrictively and for information, with reference to the attached drawings, in which :

- Fig. 1 is a flow chart showing the main operational phases of the manufacturing method according to the invention,

- Fig. 2, 3 and 4 are perspective views of a slab-shaped semifinished product in respective and different phases of the method of the invention,

- Fig. 4A is a view corresponding to that in Fig. 4, referring to other frame components produced from the semifinished product in Fig. 3,

- Fig. 5 is a perspective view of a front frame produced using the method of the invention,

- Fig. 6 is a perspective view of a frame comprising a front frame supplemented by side arms hinged thereto, produced according to the present invention.

Preferred exemplary embodiments of the invention

With reference to the figures mentioned, a spectacle frame component, in the form of a lens-holder front frame 1 or a side arm 2 of the frame, is produced according to the manufacturing method of the present invention. It should be noted that the method lends itself also to producing parts of the aforesaid components or other, different frame components not explicitly indicated.

The term "frame component" will therefore, unless explicitly stated otherwise, hereinafter refer irrespectively to the front frame or the side arms of the frame or to parts thereof.

With particular reference to Fig. 1, which shows a flow chart of the succession of major phases of the method, a first phase, marked as 10, provides a cellulose acetate slab 11 of a preselected thickness S, which is defined between opposite faces 11a, l ib of the slab.

In a preferred embodiment, the slab 10 is provided in the form of a strip or tablet that has rectangular plan and face profiles 11a, l ib, in a plane shape that has a substantially constant thickness S of the slab.

This shape can be obtained by cutting out from a slab-shaped material, for example one supplied industrially. Provision can also be made to reduce the thickness of the slab-shaped material supplied, for example by milling, to produce the preselected thickness S of the slab 11.

In a subsequent phase, marked as 20, the slab 11 is placed in a suitable over-injection mould, conventional per se, designed to allow a layer 12 of elastomer, which is specifically identified among injection mouldable thermoplastic polymeric materials, and is also known in technical jargon by the term "rubber", to be over-injected over one of the faces of the slab.

Using over-injection, the layer 12 of elastomer, of thickness SI, is then deposited on the slab 11, producing a multilayer slab-shaped semifinished product, indicated as a whole as 13, the thickness of which is the sum of the thicknesses S and SI. Adhesion between the layers is ensured by the compatibility of the preselected materials subjected to the over-injection process.

The final colouring intended for the elastomer component can be obtained in the phase of over-injecting the elastomer itself, by mixing into the elastomer that is in raw form (granulate materials) and has a "natural" colour, suitable pigments (generally in powder form), or suitable polymer-based colourant materials, also in granule form, known in the sector as "masterbatch" materials.

Alternatively, it is possible to obtain the elastomer in raw form (granulate material) already provided with the final colouring, i.e. supplied from its producer with the required colouring.

In the embodiment described, the layer 12 of elastomer is over-injected homogeneously over one of the faces of the slab 11, over the entire surface extent of the face and so as to have a constant thickness SI. Alternative embodiments, mentioned in detail below, can provide different shapes of the layer 12 of over- injected elastomer, for example typified by differentiated thicknesses in the layer. Once extracted from the mould, the semifinished product 13 is subjected, in a subsequent phase 30 of the method, to a shaping process by means of which, by cutting and/or milling, the final shape of the frame component is produced.

Fig. 4 shows diagrammatically the profiles of the outer contours of the front frame 1 and the side arms 2, along which the operation of cutting the semifinished product is carried out, which cutting therefore involves the whole thickness (S+Sl) of the semifinished product. This operation produces frame components, whose slab-shaped structure is therefore formed by two superimposed layers, respectively the cellulose layer and the elastomer layer.

It should be noted that the thicknesses S and SI are chosen appropriately both on the basis of technical reasons, for example to ensure adequate strength of the component, and for aesthetic reasons, for example to obtain the desired decorative effects.

The advantage of using an acetate slab, particularly in strip or tablet form, lies in the fact that the processes for producing such a slab-shaped product made of cellulose acetate are extremely limited, in terms of number and complexity, compared with the processes needed to produce finished or almost finished frame components from said slab, i.e. components in a substantially final form (side arm or front frame). Consequently, in the case of a strip from a slab, any release or relaxation of the stresses originally present within the material takes place to a much lesser extent, or practically not at all, compared with the magnitude of the release that can generally be found in shaped parts that have more complex forms, close to the final shapes intended for the frame components.

A slight release of internal stresses therefore corresponds to a slight post-treatment deformation that can be found in the strips to be used subsequently in the over- injection operation. Consequently, there is also a slight variability in the extent and direction of said deformations and, in the final analysis, high repeatability and precision in the shape of the strips, which proves compatible with the low tolerances permitted by an over-injection process.

In phase 30, by means of processes that are conventional per se (milling and cutting) done on both the acetate portion and the elastomer portion of the semifinished product 13, the frame component is then produced so as to have the profile and thicknesses provided for the final frame.

It is known that, for the embodiment of the method described here, it is sufficient to use a single mould for injection (mould for producing the bi-material tablet), in contrast with the conventional method used for producing polyamide frames comprising details in over-injected elastomer, which requires the use of two different moulds (respectively, a mould for injecting the polyamide and a mould for over- injecting the elastomer).

Compared with the conventional method, therefore, the method described here offers the advantage of being more economical when preparing the equipment for production, since it is a matter of common knowledge that the cutting and milling processes do not require the use of such complex and costly moulds as injection moulding moulds are typically.

The side of the semifinished product 13 that has the elastomer layer can be intended to constitute the outer side of the frame component (front frame or side arm) or the inner side thereof, based on the functional or aesthetic aims intended for the elastomer.

If required, a subsequent phase, indicated as 40, is designed for inserting into or applying onto the component any metal details intended for completing the frame. For example, it is possible to insert into the front frame 1 the metal details suitable for forming the hinges for articulating the side arms (Fig. 6). Accordingly, it is possible to insert (or fix) similar metal details into the front portions of the side arms. If the objective is the application of metal components for hinges onto the front frame (i.e. to the rear surface of the front frame, or to the end of two respective bent "lugs") it is, for example, possible to insert the metal details into an elastomer layer (Fig. 6). Alternatively, it is possible to insert the metal details into the layer made of acetate. Insertion can be carried out by means of various known techniques common in the sector. For example, it is possible to make a seating in the acetate layer, and then insert the metal component for the hinge by means of an ultrasonic technique.

Alternatively, it is for example possible to pre-insert the metal component into the mould for over-injecting the elastomer over the acetate, and to ensure that the metal component is partly incorporated into the elastomer during injection into the mould. It is also anticipated that a metal core or insert (not illustrated) could be inserted into the acetate portion of the side arm, using operating techniques known in the sector, with the aim of strengthening the side arm, or even giving the side arm an internal structure that is pliable but strong and permits the operation of "registering" (adapting) the shape of the side arm, possibly assisted by heating to between around 70°C and 80°C, which results in a curve that is stable over time.

In such an eventuality, it is preferable to insert the core into the frame component intended to form the side arm, in a phase preceding the phase of cutting and milling for producing the final form of the part. In other words, it is preferable to insert the metal core directly into the bi-metal tablet or optionally into a semifinished product in intermediate form (strip or bar or bi-material cut out of the tablet), and to only do the final cutting or milling later on.

In this case too, the acetate portion can be the one placed on the outer side or the one placed on the inner side of the side arm.

In a subsequent phase, indicated as 50, final processes are optionally provided for surface finishing the component, suitable for obtaining for example a shiny or bright surface, particularly as regards the surfaces of components made from acetate. For the final surface finish, for example, provision is made to use the mechanical finish by means of surface abrasion, known by the technical term "tumbling".

This surface finish requires the selection of a composition of the abrasive material contained in the tumbling barrel (form and composition of the abrasive particles, or liquid or paste or other) that will be sufficiently effective on the acetate, and will at the same time prevent the elastomer frame component from being damaged. Since elastomer is typically softer and is characterised by a much lower surface hardness than is characteristic of acetate, there is in fact a risk of the abrasive material of the tumbling barrel acting excessively on the elastomer portions, causing them superficial damage, and therefore detracting from the aesthetic or functional purpose.

Another, subsequent phase 60 can be provided to give the final curvature to the components (front frame, side arms). In this phase, known techniques that are common in the sector can be used (bending or curving the part with the aid of heat or pressure).

In a variant embodiment of the method, it is possible to actuate the surface finishing phase after the bending phase.

In a variant embodiment, in the elastomer over-injection phase 20, provision can be made for the over-injection to take place in preselected, localised zones of the corresponding face of the cellulose acetate slab 11. In this case the frame component will then be provided with the elastomer coating only in preselected zones.

In that event, it is possible (by producing the half-mould for injection as appropriate) to provide for the elastomer deposited on limited and predetermined zones to form projections protruding from the surface profile of the face of the slab, and characterised by local variations in thickness and by suitably selected dimensions and contour profiles, whether intended or not for subsequent shaping processes together with the remaining portions of the slab.

In another variant embodiment, shallow impressions or cavities can be produced in the localised zones of the acetate slab involved in the over-injection in advance (for example by milling), the elastomer being injected just enough to fill said cavities or so as to form projections or, at any rate, structures protruding from the surface of the cellulose acetate layer. Fig. 5 shows an example front frame 1 produced using the method of the invention, in which the elastomer is extended so as to partly cover the inner side of the lens-holder rims.

Fig. 6 shows a front frame 1 in which the elastomer is extended so as to cover completely the inner side of the lens-holder rims. The frame is supplemented by a pair of side arms 2, also produced using the method of the invention and in which the elastomer extends, on the inner side, over the whole longitudinal extent of the side arm.

The method of the invention therefore achieves the proposed aims by obtaining the advantages claimed with respect to the proposed solutions with reference to the prior art mentioned.

Claims

1. Method for manufacturing spectacle frame components, such as a front frame (1) of the frame or a side arm (2) of a frame or part of said front frame or side arm, comprising the following successive phases:

- preparing a cellulose acetate slab (11) having a preselected, defined thickness (S) between opposite faces (11a, l ib) of said slab,

- placing said slab (11) in an over-injection mould,

- over-injecting elastomer over at least one of the faces (11a, l ib) of the slab (11) over at least part of the surface extent of said face to make a slab-shaped, multilayer semifinished product (13),

- removing the semifinished product (13) from said mould,

- shaping the semifinished product (13) by cutting and/or milling to produce the preselected final shape of the component, the slab-form structure of the component thus made being formed of at least two layers, respectively a layer of cellulose acetate and a layer of elastomer.

2. Method according to claim 1, wherein a final phase of surface-finishing the component by means of surface abrasion is provided.

3. Method according to claim 2, wherein a phase of bending the component to produce a preselected curve, by applying heat and/or pressure on said component, is provided.

4. Method according to any one of the preceding claims, wherein said slab (11) is prepared in the form of a uniformly thin strip.

5. Method according to claim 4, wherein said slab (11) has planar faces (11a, l ib) and a rectangular plan profile.

6. Method according to any one of the preceding claims, wherein the layer of elastomer produced by over-injection has a uniform thickness (S).

7. Method according to any one of the preceding claims, wherein provision is made to insert a metal insert or core into the cellulose acetate layer of the component.

8. Method according to any one of claims 1 to 5, wherein the elastomer is over- injected in preselected, localised zones on at least one of the faces (11a, l ib) of said cellulose acetate slab (11).

9. Method according to claim 8, wherein said localised zones are shallow cavities in the face of the slab (11) and the elastomer is over-injected just so as to fill said cavities.

10. Method according to claim 8, wherein over-injection is carried out in said zones while generating respective projections protruding from the surface profile of the face of the slab (11).

11. Frame component, such as a front frame (1) or a side arm (2) or part of said frame or arm, produced using the method of one or more of the previous claims.

12. Spectacles comprising a lens-holder front frame (1) articulated to side arms (2), wherein the front frame and/or the side arms are produced using the method of one or more of claims 1 to 10.