and mold comprising such vent element

The present invention relates to a vent element configured to be arranged within a vent hole of a mold extending from a forming surface of an interior cavity of the mold, and to a mold comprising such vent element.

In the molding of articles, particularly rubber articles such as tires, venting is re quired to allow pockets of air which may become trapped between the tire article and the hot mold surface to escape, so that every part of the curing tire surface contacts the mold and the tire is thus vulcanised with a good impression of the mold pattern. To this end, such mold vents commonly take the form of small di ameter holes through the mold wall normal to the interior surface. Most common venting utilizes so-called insert vents or inserts which are small plugs inserted in the mold vent hole. Once any trapped air has vented through the hole, rubber be gins to flow through the hole. However, the plug insert is plugging the vent hole and sealing the mold so that rubber cannot flow through. After completion of the tire curing process these plugs are pulled out of the vent holes when the tire is demolded.

A problem may occur when a cured rubber part breaks off when the tire is being demolded and thus blocks the vent hole. Moreover, this may leave undesired spikes or runners on the outer surface of the tire. Such a blocked vent may not be immediately apparent and can cause subsequent poor quality moldings because trapped air cannot be vented. Another problem may be that the plug inserts get contaminated with rubber and, after the molding process, cannot be sufficiently cleaned before the molding of a new article. This will result in either blocked vents, or the plug inserts will have to be replaced with new plug inserts, which may be quite cost intensive.

US 4 740 145 A discloses a tire mold having air and gas venting holes provided with synthetic resin plugs mounted in enlarged bore portions of the vent holes ad jacent the inner forming surface of the mold. The plugs are axially compressible and project a slight distance beyond the inner surface of the mold. The molding pressure of the material within the mold compresses the plug whereby an outer surface of the plug becomes generally flush with the inner surface of the mold cav ity to substantially eliminate the formation of runners on the molded tire. The plugs preferably are formed of a PTFE (polytetrafluoroethylene) filter leaf membrane material. Particularly, a mold vent plug is formed of a resin material which is slightly compressible axially when exposed to the pressure and heat of vulcaniza tion in order to achieve the reduction or elimination of runners on the final molded tire product. It is important that the vent plug be axially compressed a proper dis tance so that the plug surface is generally flush with the cavity surface to eliminate the formation of runners. Such plugs require a substantial amount of material and, thus, are quite costly to produce.

DE 28 08 474 A1 discloses an air-permeable mould component made of micro- porous material. It is entirely made, at least within its operative zones, from micro- porous PTFE. The article is used in the de-gassing of moulding and/or vulcanising tools, i.e. to allow gases, vapours, etc. that arise during moulding and/or vulcani sation, to escape. This arrangement requires an aeration nozzle, in which the PTFE is arranged, and a large amount of PTFE material, so that this arrangement is also quite costly to produce.

It would thus be beneficial to provide a vent element configured to be arranged within a vent hole of a mold extending from a forming surface of an interior cavity of the mold which addresses the above mentioned drawbacks and allows a cost efficient production of articles within the mold, particularly rubber articles such as tires.

The invention relates to a vent element configured to be arranged within a vent hole of a mold which extends from a forming surface of an interior cavity of the mold, and to a mold comprising such vent element according to the appended claims.

According to an aspect, there is provided a vent element configured to be ar ranged within a vent hole of a mold extending from a forming surface of an interior cavity of the mold, the vent element comprising at least one porous support mate rial which is configured for venting a gaseous fluid received at a first side of the support material to an outer environment of the mold at a second side of the sup port material opposite the first side, and at least one film layer arranged above the at least one porous support material at the first side and configured to be exposed to the interior cavity of the mold and to permit passage of the gaseous fluid there through, wherein the at least one film layer is formed from a polymeric material with matrix tensile strength greater than 10,000 psi in machine and transverse di rections.

According to a further aspect, there is provided a vent element configured to be arranged within a vent hole of a mold extending from a forming surface of an inte rior cavity of the mold, the vent element comprising at least one porous support material which is configured for venting a gaseous fluid received at a first side of the support material to an outer environment of the mold at a second side of the support material opposite the first side, and at least one film layer arranged above the at least one porous support material at the first side and configured to be ex posed to the interior cavity of the mold and to permit passage of the gaseous fluid therethrough, wherein the at least one film layer is formed from a densified ex panded porous membrane material.

According to another aspect, there is disclosed a mold comprising a mold cavity with a forming surface for forming at least one article, at least one vent hole ex tending from the forming surface to an outer environment of the mold, and at least one vent element according to the aspects and embodiments of the invention as described herein mounted in one or more vent holes for venting a gaseous fluid from the mold cavity to the outer environment.

Advantageously, such vent element according to aspects of the present invention is capable of venting air from the mold to the outer environment while preventing rubber penetration in the vent hole of the mold. Further, rubber material may be released from the vent element without damage to the final molded article, such as a tire. In addition, such vent element, because of its use of at least one film layer formed from a polymeric material with matrix tensile strength greater than 10,000 psi in machine and transverse directions, preferably formed from a densified ex panded porous membrane material, provides a high durability. In consequence, it can be reused multiple times which reduces manufacturing costs of the molded article. The at least one film layer provides an advantageous combination of high air permeability and robustness. It also provides high pressure, thermal and chemical resistance. It avoids also clogging the vent holes in the mold, and avoids that the molded rubber material sticks to the vent element, so that easy release is possible. As a result, the vent element can be reused multiple times in an efficient way, which lowers manufacturing costs. At the same time, with the combination of support material and film layer, a quite cost effective structure with low thicknesses and reduced overall amount of material can be created.

In an advantageous implementation, the vent element is configured to be arranged within a vent hole of a mold for vulcanization of rubber. As such, the mold is con figured for forming the at least one article by means of vulcanization of rubber in the mold cavity. According to an embodiment, the mold is configured for forming at least one tire.

According to an embodiment, the at least one film layer is formed from a densified expanded porous membrane material.

According to an embodiment, the at least one film layer is formed from a fluoro- polymer material, preferably polytetrafluorethylene material, more preferably den sified expanded polytetrafluorethylene material.

Particularly, the at least one film layer may be formed from a densified expanded porous membrane material, which may be microporous.

The at least one film layer may also encompass film layers, the porosity of which is established by one or more holes, such as formed by a laser beam, or even greater holes. Such holes may provide the desired porosity and airflow through it alone or in combination with any micropores, such as formed in a known ePTFE layer. According to an embodiment, the at least one film layer is formed from a densified expanded porous membrane material which is only microporous (i.e. without additional holes). The air permeability of the densified expanded porous membrane material can range from an open structure with greater micropores, thus resulting in greater air permeability, to almost closed structure with almost no air permeability, as set out in more detail below.

The at least one film layer formed from a polymeric material with matrix tensile strength greater than 10,000 psi in machine and transverse directions, such as formed from a densified expanded porous membrane material, may be manufac tured in that the polymeric material is expanded creating micropores and, after ex pansion, is compressed or densified, thus reducing the size of the micropores and densifying the structure. Such process is, in principle, applicable for any mem- brane material which is or comprises at least one of polytetrafluorethylene (PTFE), polypropylene (PP) and polyethylene (PE). For example, in case of polytetrafluore thylene, the at least one film layer may be formed by expanding the PTFE mate rial, thus gaining a membrane layer which is or comprises microporous ePTFE as commonly known. Thereafter, the ePTFE membrane layer is densified by com monly known processes to reduce the pore size and increase stability, durability and/or resistance of the membrane layer (cf. , for example, the process as de scribed in US 7,521 ,010 which is incorporated herein by reference in its entirety). The structure of such densified expanded porous membrane material is different from, e.g., other porous or continuous membrane materials which have not been expanded to be porous and densified thereafter. With respect to this and other polymeric materials, for example fluoropolymer materials, similar or other proc esses which are known to the skilled person to achieve a matrix tensile strength greater than 10,000 psi in machine and transverse directions may also be em ployed.

The at least one film layer can be manufactured, in principle, from any known polymeric materials, for example fluoropolymer materials, preferably PTFE, or from any PP or PE materials, such as employed in common filter or textile applica tions, which are suitable for forming such a film layer.

The skilled person will appreciate that by varying commonly known parameters for forming such film layers or membranes, advantageous properties for air perme ability, thickness, density and/or pressure and temperature resistance, as dis closed herein, may be achieved. Preferably, the at least one film layer has one or more of the following properties which may be achieved by the skilled person by common routine methods:

According to an embodiment, the at least one film layer is configured to withstand a pressure of 24 bar at a temperature of 150°C for a minimum of 40 min.

According to another embodiment, the at least one film layer is configured to with stand a pressure of 16 bar at a temperature of 170°C for a minimum of 18 min.

According to an embodiment, the at least one film layer has a maximum pore size of smaller than approx. 6 pm. According to an embodiment, the at least one film layer is configured to provide an airflow through it of greater than approx. 0.2 l/h, preferably between 0.2 and 3 l/h, at a pressure of 70 mbar.

According to an embodiment, the at least one film layer has an average thickness in a range of approx. 20-60 pm, and/or an average density in a range of approx. 0,6 - 1.5 g/cm ³ .

According to an embodiment, the at least one film layer comprises at least one of polytetrafluorethylene, polypropylene and polyethylene. The at least one film layer may be formed by one of these materials, or any suitable combination thereof, e.g. in respective multiple layers.

According to an embodiment, the at least one film layer is formed by a laminate comprising at least one thermoplastic layer. For example, such thermoplastic layer can be used for bonding the at least one film layer to the support material and/or to any other element, such as a housing element used for housing the film layer and/or support material. For example, the thermoplastic layer is perforated at least in a region thereof configured for venting the gaseous fluid therethrough.

According to an embodiment, the at least one porous support material comprises a sintered metal material or a perforated plastic support.

In a further embodiment, the vent element further comprises a housing element enclosing the at least one porous support material, wherein the housing element is made of at least one of a metal material and solid plastic material. The use of a housing element may be advantageous in cases where the support material can not or can hardly be placed within the vent hole directly between two mold wall portions, or where handling of the vent element is more advantageous if the film layer and/or support material is housed within a housing element. According to an embodiment, the at least one porous support material is formed by at least one support element integrally formed with the housing element.

According to an embodiment, the at least one film layer is bonded to at least a por tion of the at least one support material, e.g. by thermal welding. According to an embodiment, the at least one film layer is wrapped around at least a portion of the at least one support material. Particularly, the at least one film layer may be wrapped around at least a portion of the support material such that the at least one film layer is positioned on top of the support material and bonded thereto to be exposed to the interior cavity of the mold and in at least a peripheral region of the support material between a mold wall and the support material. As such, a secure and all around arrangement of the film layer can be achieved, so that any rubber material can be easily removed without clogging any intermediate spaces between support material and film layer and/or support material and mold walls.

According to an advantageous embodiment, the vent element further comprises at least one compressible intermediate material disposed between the at least one porous support material and the at least one film layer. The at least one film layer is positioned on top of the intermediate material to be exposed to the interior cavity of the mold.

For example, the at least one compressible intermediate material is formed of sili cone material. This embodiment provides the advantage that one or more addi tional vent holes may be provided in the vent element, which however does not significantly decrease the capability of the vent element to be easily releasable from any rubber material after the forming of the article because of the underlying intermediate material which may be designed to close such additional vent holes after the air has escaped. This may be used if air permeability of the film layer shall be increased, and/or if the film layer shall be more densified to increase sta bility and/or durability.

According to an embodiment, the at least one film layer is bonded to the at least one compressible intermediate material. This provides the advantage that rubber material cannot enter into any intermediate spacing between film layer and inter mediate material, if any.

According to an embodiment, the at least one film layer is formed by a laminate comprising at least one first membrane of a polymeric material with matrix tensile strength greater than 10,000 psi in machine and transverse directions, particularly densified expanded porous membrane material, and at least one second mem brane of porous material, wherein the at least one second membrane is bonded to the at least one compressible intermediate material. For the at least one first membrane, the same materials and structures can be used as described herein above with respect to the at least one film layer. Such configuration is advanta geous, since the second membrane can be designed such that there is a good bonding between the film layer and the intermediate material. This can be achieved particularly with sufficient porosity having pores great enough for the ad hesive to enter into the pores of the second membrane. As a result, the membrane can be firmly attached to the intermediate material, so that no rubber material can enter therebetween. In turn, the first membrane can be designed such that it has increased stability and/or durability, which can be achieved with an even more densified expanded porous membrane material. Advantageously, the densified expanded porous membrane material is as dense as possible. As such, the prop erties of the film layer for good bonding to the intermediate material, on the one hand, and increased stability and/or durability, on the other hand, can be advanta geously decoupled from each other.

According to an embodiment, the at least one second membrane has a pore size of greater than approx. 2 pm. This provides the advantage that any adhesive or other material, such as the intermediate material (e.g. made of silicone), can suffi ciently bond to the second membrane by means of engaging with the pores.

According to an embodiment, the at least one second membrane is formed from an expanded polytetrafluorethylene membrane material. Such ePTFE membrane typically has sufficient porosity or pore size for the adhesive to enter into the pores of the second membrane.

According to an embodiment, the at least one first membrane has no pores or a pore size, if the membrane material has any pores, of less than approx. 1 pm. Such densified membrane typically can provide increased stability and/or durability for the exposure to the rough environment within the mold cavity. A determination whether the membrane material has any pores can be made, according to an em bodiment, with a scanning electron microscope (SEM), e.g. with a magnification of 3000. If a SEM with magnification of 3000 does not show any pores, this shall be understood herein as a membrane material densified to an extent that it does not have pores any more. According to an embodiment, the at least one first membrane is configured to pro vide an airflow through it of greater than approx. 4 l/h, preferably between 10 and 200 l/h, at a pressure of 70 mbar.

According to an embodiment, the at least one first membrane has an average thickness in a range of 30-80 pm and/or an average density in a range of approx. 1 - 3 g/cm ³ .

The term“approximately” used herein shall mean that a number slightly exceeding or falling below the respective cited numbers (such as 0.1 pm or 0.9 pm maximum pore size in the above embodiment) shall still be encompassed, since no“hard” structural or physical border for any number of pore size, airflow, layer thickness, density, and/or weight etc. exists. In this regard, for example, a range of +/- 10% shall be encompassed with the term“approximately”.

According to an embodiment, the at least one compressible intermediate material comprises at least one through hole within an area in which the gaseous fluid is vented, wherein the at least one through hole is configured to vent at least por tions of the gaseous fluid through it. As such, the air permeability of the vent ele ment can be increased while not affecting the surface properties of the vent ele ment on the mold cavity side.

According to an embodiment, the compressible intermediate material is configured to close the at least one through hole, at least within portions of the intermediate material along the at least one through hole, by material deformation upon pres sure on the at least one film layer exerted from the interior cavity of the mold. With closing the at least one through hole, the air may escape from the mold before the through hole is closed, while after closing the through hole the rubber material cannot enter the through hole, which reduces the formation of spikes on the formed article in length and/or width. For example, any remaining spikes become shorter in length so that the vent element is easier to release from any rubber ma terial after the forming of the article.

According to an embodiment, the at least one film layer comprises at least one hole which is aligned with the at least one through hole in the compressible inter mediate material and configured to vent at least portions of the gaseous fluid into the at least one through hole. Advantageously, the airflow through the vent ele- ment can be increased. At the same time, the air may escape from the mold be fore the through hole in the intermediate material is closed which reduces the for mation of spikes on the formed article despite the hole provided in the film layer. According to an embodiment, the at least one film layer is wrapped around at least a portion of the support material and the intermediate material such that the at least one film layer is positioned on top of the intermediate material and bonded thereto to be exposed to the interior cavity of the mold and in at least a peripheral region of the support and intermediate materials between a mold wall and the support and intermediate materials, respectively. As such, a secure and all around arrangement of the film layer can be achieved, so that rubber material can be eas ily removed without clogging any intermediate spaces between support material and film layer, intermediate material and mold walls, and/or support material and mold walls.

Further aspects and embodiments of the invention will now be described with re spect to the accompanying drawings, in which

Fig. 1 shows a schematic perspective cross-sectional view of a vent element positioned in a mold according to an embodiment of the invention,

Fig. 2 shows a cross-sectional view of a vent element positioned in a mold according to a further embodiment of the invention, Fig. 3 shows in two cross-sectional views a vent element positioned in a mold according to a further embodiment of the invention in different situa tions during a molding process,

Fig. 4 shows a schematic view of a film layer according to an embodiment of the invention,

Fig. 5 shows a cross-sectional view of a vent element positioned in a mold according to a further embodiment of the invention, Fig. 6 shows a schematic view of a film layer according to a further embodi ment of the invention, Fig. 7 shows a cross-sectional view of a vent element according to a further embodiment of the invention,

Fig. 8 shows a cross-sectional view of an exemplary embodiment of a densi- fied expanded porous membrane material,

Fig. 9 shows a diagram of venting performance of exemplary film layer proto types after a respective vulcanization cycle. In Fig. 1 , there is shown a schematic perspective cross-sectional view of a vent element 10 according to a first embodiment of the invention. The vent element 10 is positioned in a mold 50, a part of which is schematically shown by mold wall 54. Generally, any type of mold can be used in connection with the present invention, or in other words, the vent element 10 according to the invention can be used, in principle, in any type of mold 50. Basically, the mold 50 comprises a mold cavity 53, schematically shown in Figs. 2 and 3, with at least one forming surface 51 for forming at least one article. Generally, a mold for forming any type of article may be used. In a preferred embodiment, the mold 50 is configured for forming an arti cle by means of vulcanization of rubber in the mold cavity 53. In a preferred im- plementation, the mold 50 is configured for forming a tire in the mold cavity 53.

In the molding of articles, particularly rubber articles such as tires, typically venting is required to allow pockets of air, which may become trapped between the article and the hot mold surface, to escape so that every part of the curing article surface contacts the mold 50 and its forming surface 51 . To this end, the mold 50 com prises at least one vent hole 52 (in practice, a mold 50 used in the field of tire manufacturing typically comprises a plurality of such vent holes) extending from the forming surface 51 to an outer environment of the mold 50, so that the trapped air may escape to the outer environment of the mold 50. Such mold vent holes 52 commonly take the form of small diameter holes bored through the mold wall 54 normal to the interior forming surface 51.

In order to prevent the rubber material 5 to flow out of the interior mold cavity 53 to the outer environment of the mold 50, at least one vent element 10 is mounted in one or more of the vent holes 52. Preferably, if the mold 50 comprises a plurality of vent holes 52, a corresponding number of vent elements 10 is placed in each respective one of the vent holes 52, typically one vent element 10 in each vent hole 52. In a typical tire mold, there is provided a high number of vent holes 52, so that the equipment costs for the vent elements 10 to be used scale up with the number of vent holes 52 and can achieve a significant amount when hundreds of vent elements 10 are to be used for the manufacturing of a typical tire.

A basic function of a vent element 10 is to vent a gaseous fluid 1 1 , typically trapped air, from the mold cavity 53 to the outer environment of the mold, prefera bly with high air permeability, while preventing the rubber material 5 from flowing through the vent hole 52 to the outside, and preventing rubber penetration in the vent hole 52 of the mold which may break off when the tire is removed leaving un desired spikes on the tire surface. Further, rubber material 5 should be releasable from the vent element 10 without damage to the final molded article, such as a tire. In addition, such vent element 10 should provide a high durability so that it can be reused multiple times, thus decreasing manufacturing costs. In conse quence, it can be reused multiple times which reduces manufacturing costs of the molded article.

As further shown in Figs. 1 and 2, the vent element 10 comprises at least one po rous support material 3 which is configured for venting the gaseous fluid 11 , par ticularly air, received at a first side 41 of the support material 3 to the outer envi ronment of the mold 50 at a second side 42 of the support material 3 opposite the first side 41. For example, the porous support material 3 comprises a sintered metal material or a perforated plastic support. According to embodiments, the po rous support material 3 may be microporous or may have a porosity with greater pores, e.g. may be macroporous, or may have various kinds of such porous struc tures combined, or the porosity of the support material 3 may be established by through holes (e.g., having small or large diameter, or a mixture thereof), such as in a perforated plate, or any other air permeable structure (such as a canal struc ture schematically shown in Fig. 3), or any combination thereof. The support mate rial has the function of supporting the at least one film layer 1 arranged above it and to withstand the pressures and temperatures exerted from the mold cavity 53. In addition, it shall support the at least one film layer 1 within the vent hole 52 of the mold 50 such that it is safely positioned within the vent hole 52 without slip ping. The dimensions and material properties of the support material 3 may be such that it fits into the vent hole 52 by press fit. For example, it may be slightly compressible in cross-section to achieve a press fit without interspaces or slipping within the vent hole. The at least one film layer 1 is arranged above the at least one porous support material 3 at the first side 41 and configured to be exposed to the interior cavity 53 of the mold. It has a structure which is capable of permitting passage of the gase ous fluid 1 1 , particularly air, from the cavity 53 therethrough. According to the in vention, the at least one film layer 1 is formed from a polymeric material with ma trix tensile strength greater than 10,000 psi in machine and transverse directions. According to an embodiment, it is formed from a densified expanded porous membrane material. According to various embodiments, the at least one film layer 1 may be formed from a fluoropolymer material, such as polytetrafluorethylene material, or densified expanded polytetrafluorethylene material.

According to an embodiment, the at least one film layer 1 is formed from a densi fied expanded porous membrane material which is microporous. The at least one film layer 1 formed from a densified expanded porous membrane material is typi cally manufactured in that the membrane material is expanded creating micro pores and, after expansion, is compressed or densified, thus reducing the size of the micropores. Such process is, in principle, applicable for any membrane mate rial which is or comprises at least one of polytetrafluorethylene (PTFE), polypropylene (PP) and polyethylene (PE). Such densified expanded porous membrane is distinguishable from any structure which has micropores, but has not been densi fied, since the densification or compression alters the structure of the micropores and/or their arrangement within the layer which is significant for the densification or compression and different from an expanded porous membrane material which has not been densified or compressed.

The at least one film layer 1 may also encompass film layers, which are densified quite largely (e.g., leaving only very few and/or small micropores), but the porosity of which is established by one or more holes, such as formed by a laser beam, or even greater holes, such as formed by perforation. Such holes may provide the desired porosity alone or in combination with any micropores, such as formed in a known ePTFE layer. According to an embodiment, the at least one film layer is formed from a densified expanded microporous membrane material (i.e. with or without additional holes).

A film layer 1 with reduced pore size, i.e. which is densified quite largely, is, for example, used in the embodiments of Figs. 2 and 3. In the embodiment of Fig. 2, the film layer 1 comprises at least one hole 24 (e.g. created by means of perfora tion) establishing the desired porosity and air flow (or increasing the porosity and air flow to a desired level) and which may be aligned with a through hole 31 in the support material 3 to create a duct 6 for the gaseous fluid 1 1 to pass to the outer environment.

In the embodiments of Figs. 1 and 2, the film layer 1 is positioned on top of the support material 3 on the first side 41 thereof to be exposed to the interior cavity 53 of the mold. Preferably, the film layer 1 is bonded to at least a portion of the support material 3 to avoid any interspaces between film layer 1 and support ma terial 3 and ensure a proper functioning of the air passage through the compo nents. For example, the film layer 1 is bonded to at least a portion of the support material 3 by thermal welding. In an embodiment not explicitly shown, the film layer 1 may be wrapped around at least a portion of the support material 3 such that the film layer 1 is positioned on top of the support material 3 and bonded thereto to be exposed to the interior cavity 53 of the mold and in at least a periph eral region 43 of the support material 3 between the mold wall 54 and the support material 3 (cf. a comparable embodiment shown in Fig. 5 with film layer 1 wrapped around support material 3 and intermediate material 2, described in more detail below). Such embodiment may avoid any interspaces between the mold wall 54 and the film layer 1 and support material 3, and also enables a safe and durable fixation of the film layer 1 on the support material 3 without interspaces.

Fig. 4 shows a schematic view of a film layer according to an embodiment of the invention which may advantageously be used in connection with bonding the film layer 1 to the support material 3, or to any other underlying material the film layer 1 is positioned on top thereof. As schematically shown, the film layer 1 is formed by a laminate comprising a membrane layer 21 formed with membrane material as described above with respect to the film layer 1 (having preferably the properties as set out above) and at least one thermoplastic layer 22. The film layer 1 may be bonded to the support material 3 with the thermoplastic layer 22 having appropri ate bonding properties, so that the properties as set out above with respect to the molding process involving high temperature and pressure resistance and durability and robustness may be decoupled from the bonding properties to a material layer arranged on the opposite side. According to an embodiment, the thermoplastic layer 22 is perforated at least in a region thereof configured for venting the gase- ous fluid 1 1 therethrough, in order to not negatively influence the air permeability of the film layer 1 as a whole.

According to an embodiment, the membrane material in the film layer 1 has a maximum pore size of smaller than approx. 6 pm. This provides a good compro mise between air permeability on the one hand and durability and robustness on the other hand. For example, the film layer 1 is configured to provide an airflow through it of greater than approx. 0.2 l/h, preferably between 0.2 and 3 l/h, at a pressure of 70 mbar.

According to various embodiments, the film layer 1 may have the following proper ties alone or in combination: an average thickness in a range of 20-60 pm and/or an average density in a range of approx. 0,6 - 1.5 g/cm ³ . According to an em bodiment, the combination of support material 3 and film layer 1 may have a total thickness of approx. 1.5 mm. Such thickness of approx. 1.5 mm may also be em ployed for a combination of support material 3, intermediate material 2 (as de scribed in more detail below) and film layer 1.

Fig. 3 shows in two cross-sectional views a vent element 10 positioned in a mold vent hole 52 according to a further embodiment of the invention. In particular, Fig. 3A shows a situation during a molding process before pressure is exerted onto the film layer 1 by the rubber material 5. Fig. 3B shows a situation during the molding process when pressure is exerted onto the film layer 1 by the rubber material 5.

Fig. 3A depicts a vent element 10 which, as in the previous embodiments, com prises at least one porous support material 3 configured for venting a gaseous fluid 1 1 , such as air, through it from a first side 41 to an outer environment of the mold 50 at a second side 42 opposite the first side 41 into a duct 6 of the vent hole 52. The structure, function and properties of the support material 3 are preferably the same or similar to those as described above with respect to the previous em bodiments. At least one film layer 1 is arranged above the support material 3 at the first side 41 (i.e. above the first side 41 ) and is formed, in this embodiment, from a densified expanded porous membrane material. The structure, function and properties of the film layer 1 are preferably the same or similar to those as de scribed above with respect to the previous embodiments. Different from the above described embodiments, vent element 10 according to Fig. 3A further comprises at least one compressible intermediate material 2 which is disposed between the support material 3 and the film layer 1. More particularly, the film layer 1 is positioned on top of the intermediate material 2 and as such ex posed to the interior cavity 53 of the mold. The intermediate material 2 is posi tioned on the first side 41 of the support material 3. According to an embodiment, the at least one compressible intermediate material 2 is formed of silicone mate rial. Other elastomeric materials like PU (polyurethane), NBR (Nitrile butadiene rubber) are possible or other thermoplastic elastomers (TPE) are possible, too. Preferably, the film layer 1 is bonded to the compressible intermediate material 2, for example by lamination, adhesive or thermal welding. In case of adhesive, the bonding may be established either in regions without airflow through it, or by using a discontinuous pattern or air permeable adhesive. According to an embodiment, bonding can be achieved by positioning the porous film layer 1 on the intermediate material 2 (e.g. silicone material) not yet fully cured, and with curing the intermedi ate material 2 thereafter the film layer 1 gets bonded thereto. For this embodiment the intermediate material 2 needs to have low Shore A values (in the range of 50) such that the material can penetrate into the pores of the film layer 1. According to another embodiment, if the intermediate material is of more fluid-like character when being applied to the film layer, this can be established, e.g. by placing a ring form on the film layer and filling it with the fluid-like material, then curing and re moving the ring form. The bonding of the film layer 1 to the intermediate material 2 is advantageous in order to prevent migration of rubber material into any inter spaces between the film layer 1 and the intermediate material 2 that would proba bly occur if these materials were not bonded to each other.

In order to achieve good bonding properties of the film layer 1 to the intermediate material 2, in a preferred embodiment, the film layer 1 is formed by a laminate, an embodiment of which is shown in Fig. 6. The laminate comprises at least one first membrane 21 , in this embodiment of densified expanded porous membrane mate rial, and at least one second membrane 25 of porous material. The structure, func tion and properties of the membrane 21 are preferably the same or similar to those as described above with respect to the film layer 1 of the previous embodiments. The second membrane 25 is bonded to the compressible intermediate material 2.

This is advantageous, since the second membrane 25 can be designed such that there is a good bonding between the film layer 1 and the intermediate material 2. This can be achieved particularly with sufficient porosity having pores great enough for the adhesive to enter into the pores of the second membrane 25. As a result, the membrane 25 can be firmly attached to the intermediate material 2, so that no rubber material can enter between the film layer 1 and the intermediate material 2. In turn, the first membrane 21 can be designed such that it has in creased stability and/or durability, which can be achieved in one embodiment with a quite densified expanded porous membrane material. Advantageously, the ma terial of the first membrane 21 is as dense as possible to achieve high stability and/or durability. As such, the properties of the film layer 1 for good bonding to the intermediate material 2, on the one hand, and increased stability and/or durability, on the other hand, can be advantageously decoupled from each other.

According to an embodiment, the second membrane 25 has a pore size of greater than approx. 2 pm. This allows any bonding substance between the membrane 25 and the intermediate material 2 to enter into the pores and, thus, achieve a good bonding.

In one preferred embodiment, the second membrane 25 is formed from an ex panded polytetraflourethylene (ePTFE) membrane material. In further preferred embodiment, in combination therewith or independently therefrom, the first mem brane 21 has no pores or a pore size, if any, of less than approx. 1 pm. Such membrane material is still identifiable as an expanded membrane material which had a certain amount of porosity, but which has been densified to an extent that the pore size is reduced to less than approx. 1 pm or even to an extent that there are no pores any more (resulting in closed or almost closed pores), which means that air can hardly or cannot pass through any more through the closed or almost closed pores. A densified expanded porous membrane material having no pores or a pore size as defined above provides a structure with high stability and durabil ity properties because of the dense structure of the expanded membrane material.

In this regard, Fig. 8 shows a cross-sectional view of an exemplary embodiment of a densified expanded porous membrane material in a film layer 1 , which is still open, i.e. has micropores greater than approx. 0 pm.

Again with reference to Fig. 3A, in order for the vent element 10 to provide the de sired airflow through it, according to an embodiment, the compressible intermedi ate material 2 comprises at least one through hole 30 within an area in which the gaseous fluid 1 1 is vented. 24. In addition, according to an embodiment, the film layer 1 comprises at least one hole 23 which is aligned with the at least one through hole 30 and is configured to vent at least portions of the gaseous fluid 1 1 into the at least one through hole 30 of the intermediate material 2. This is pre ferred in particular if the film layer 1 is formed with a quite dense expanded mem brane material in which the pore size has been reduced to an amount in which al- most no airflow can be established. In such embodiment, the gaseous fluid 1 1 can at least be vented through the at least one hole 23 and the through hole 30. De pending on the circumstances and the required airflow, the film layer 1 and the intermediate material 2 can be provided with only one or with multiple holes 23 and through holes 30, respectively.

Fig. 3B shows the embodiment according to Fig. 3A in a situation during the mold ing process in which pressure is exerted by the rubber material 5 onto the film layer 1. As a result of the pressure onto the film layer 1 , the compressible interme diate material 2 is compressed or“squeezed” which results in a material deforma tion of the intermediate material 2 upon pressure on the film layer 1. The com pressible intermediate material 2, upon the pressure and the resulting material de formation, is configured to close the at least one through hole 30 at least within portions of the intermediate material 2 along the at least one through hole 30, as shown in Fig. 3B. With closing the at least one through hole 30, the gaseous fluid (air) 1 1 may escape from the mold before the respective through hole 30 is closed, while after closing the through hole 30 the rubber material 5 cannot enter the through hole 30 any more. This reduces the formation of undesired spikes on the formed article in length and/or width. For example, any remaining spikes become shorter in length and/or thinner in diameter so that the vent element 10 can be re leased easier from any rubber material 5 after it has cured. Moreover, in some embodiments, if the spikes are shorter in length, they are not required to be re moved from the surface of the formed article, such as a tire, but are rather toler able because of their small dimensions. The vent element can, thus, be quickly reused.

Fig. 5 shows a cross-sectional view of a vent element 10 positioned in a mold ac cording to a further embodiment of the invention. The vent element 10 is similar in structure and properties to the vent element as described with reference to Figs. 3A, 3B. Therefore, for any material structures and properties, reference is made to the above description with respect to the previous embodiments. Different from the above described embodiments, the film layer 1 is wrapped around at least a por tion of the support material 3 and the intermediate material 2, such that the film layer 1 is positioned on top of the intermediate material 2 and bonded thereto and in at least a peripheral region 43 of the support material 3 and intermediate mate rial 2 between the mold wall 54 and the support and intermediate materials 2, 3, respectively. As such, a secure and all around arrangement of the film layer 1 can be achieved, so that any rubber material 5 can easily be removed without clogging any intermediate spaces between support material and film layer, intermediate material and film layer, and/or between support material/intermediate material and mold walls. In addition, such embodiment also enables a safe and durable fixation of the film layer 1 on the support material 3 and intermediate material 2 without interspaces.

Fig. 7 shows a cross-sectional view of a vent element 10 according to a further embodiment of the invention. In this embodiment, the vent element 10 further comprises a housing element 4 which encloses the at least one porous support material 3 and, in some embodiments, also the film layer 1 , as in the shown ex ample. Preferably, the housing element 4 is made of at least one of a metal mate rial and solid plastic material. Advantageously, the at least one porous support ma terial 3 may be formed by at least one support element 33 which is integrally formed with the housing element 4, like a perforated plastic support as schemati cally shown.

For example, the film layer 1 can be bonded to such housing element 4 in the pe ripheral parts thereof, e.g. by means of a thermoplastic layer 22 as described with respect to Fig 4. Other configurations may also be useful, for example a configura tion in which the film layer 1 is positioned upon the support element 33 and the top surface of the housing 4, or the peripheral ends of the film layer 1 are enclosed by respective pockets formed in the peripheral housing wall to prevent unintended release of the film layer 1 at its edges from the housing 4. In addition, the use of the housing 4 may be advantageous with respect to the positioning of the vent element 10 in the vent hole 52, for example providing an appropriate press fit through appropriate design of the housing element.

Exemplary parameters of membrane layers which can be used for the at least one film layer according to exemplary embodiments: 1. A densified membrane material with pores (so-called “Open Densified Film (ODF)”), particularly for embodiments without intermediate material and through hole(s) (of. embodiments according to Figs. 1 , 7):

Thickness: 30 pm

Density: 0,89 g/cm ³

Pore size: max. 5 pm

2. A more densified membrane material with no pores or very small pores (so- called“Densified Film (DF)”), particularly for embodiments with intermediate mate rial and through hole(s) (cf. embodiments according to Figs. 3A, 3B, 5). For exam ple, a multilayer structure of a densified film laminated with a FEP (fluorinated eth ylene propylene) layer to a porous ePTFE film.

Laminate parameters:

Thickness: 78 p

Density: 1 ,9 g/cm ³

Pore size of the outer densified membrane layer (cf. first membrane '2T in Fig. 5) too small to be measured with a SEM at magnification of 3000. Densified outer layer has thickness of around 50pm.

Inner porous ePTFE membrane layer (cf. second membrane‘25’ in Fig. 5) to real ize bond to intermediate material has pore size of 2 pm.

Examples:

1. Production of Densified Film (DF) structure:

A densified ePTFE was prepared according to the general teachings of US 7 521 010 to Kennedy et al. , with the exception that the final stretch above the crystalline melt temperature of PTFE was omitted (i.e., an unsintered product). The prepara tion procedure was performed to result in a densified sheet of ePTFE having a thickness of 16,76 pm, a mass/area of 36.3 g/m2, a calculated standard specific gravity (SSG) of 2.15 g/cc, an average matrix tensile strength (MTS) in the ma chine direction of 19,300 psi (approximately 133.1 MPa) and an average MTS in the transverse direction of 14,700 psi (approximately 101.4 MPa). Finally, a lamination step was performed as follows:

For the lamination step a metal mandrel with an outside diameter of about 90 mm and a length of about 300 mm was used with film layers having a width in the or der of 400 mm. 3 densified layers (as described above) were wrapped first, fol lowed by a FEP layer and then a porous ePTFE film having a pore size of 3 pm. Clamps were used to fix the material at the ends of the mandrel to prevent the film from shrinking back. The sintering conditions were 370 °C for 30 minutes.

2. Production of Open Densified Film (ODF) structure:

The open densified ePTFE was also prepared according to general knowledge of densifying PTFE as described above. Here the preparation procedure was per formed to result in a less densified sheet of ePTFE having a thickness of 30 pm, a calculated standard specific gravity (SSG) of 0.89 g/cc and a pore size of 5 pm as determined via bubble point measurement.

3. Prototype of vent housing element with DF (cf., for example, embodiment ac cording to Fig. 7)

The vent housing element is produced with PEEK (Victrex PEEK 150G903 BLACK). The design is adapted to the press fit column in the mold cavity (see ex ample 5) and contains space for a permeable material (sinter metal disc, 1.4404 SIKA-R20, GKN Sinter Metals Filters GmbH). The vent housing element has a cut out to receive the venting construction in a way that prevents disc moving when the molding pressure is applied.

The DF (see example 1 ) with a diameter of 1 1 mm is shaped to a cup form by a drawing process at 250°C. The cup diameter is 4.2 mm with a height of 3.5mm. This cup is filled with a silicon elastomer (Wacker RT601 ) which penetrates into the bottom layer of the DF film. The resulting silicon thickness is 1.50 mm.

After curing the silicone has a hardness of“Shore 50A”. Then, the silicon filled cup is provided with a through hole of 0.30mm in diameter by laser cutting. Finally, the sinter metal disc is positioned on the silicone and the overlapping DF film is wrapped around sinter metal disc.

This resulting unit is pressed into the vent housing element.

This construction offers a venting performance of 13 l/h at 70 mbar air pressure. 4. Prototype of vent housing element with ODF:

The vent housing element is produced with PEEK (Victrex PEEK 150G903 BLACK). The design is adapted to the press fit column in the mold cavity (see ex ample 5) and contains space for a permeable material (sinter metal disc, 1.4404 SIKA-R20, GKN Sinter Metals Filters GmbH). The vent housing element has a cut out to receive the venting construction in a way that prevents disc moving when the molding pressure is applied.

The ODF (see example 2) with typical diameter 1 1 mm is wrapped around the sinter metal disc. This resulting unit is pressed in the vent housing, so the ODF film is facing to the NBR during the process.

This construction offers a venting performance of 1 l/h at 70 mbar air pressure.

5. Molding of plastic parts with vent prototypes

To test the prototypes, a stainless steel mold with a plurality of cavities was cre ated, whereas each cavity was equipped with a vent prototype at the bottom of the cavity. For the molding process the cavities were filed with elastomer (NBR-70 Shore, Kani, Art. Nr.: PP7AHZ) at a pressure of 24 bar at +170°C and cured for 20 minutes. During the molding process, the remaining air was able to exit the cavity via the venting construction.

After the molding process the cured elastomer was released by hand and the sys tem was filled with new uncured rubber for the next molding process. Even after 35 cycles no significant decrease of venting performance was measured (see Fig ure 9).

Fig. 9 shows a diagram of venting performance of exemplary prototype vent ele ments after a respective vulcanization cycle. In each vulcanization cycle, air is vented out through the respective vent element from the molding cavity after the respective vulcanization cycle.

For each of the vent elements with membrane materials ODF and DF, three ex emplary samples were tested. Although manufactured from the same or similar basic material having in principle the same structure, the respective samples are not identical in venting performance, but slightly deviate in venting performance from each other within a certain range as depicted in Fig. 9. This may result from, e.g., a slightly varying pore size among the samples. Prototypes“DF-1” and“DF-2” use film layers from the same basic material (e.g. from the same roll of basic material) with hole diameter of 0.40 mm of the central hole (cf. hole‘23’ in Fig. 3). Prototype“DF-3” uses an alternative film layer with hole diameter of 0.30 mm.

All ODF samples use film layers from the same basic material (e.g. from the same roll of basic material).