Rigid mould for moulding and vulcanizing tyres.

p0001The present invention relates to molds used for molding and vulcanisation of tires.

p0002In the manufacture of new tires, one type of mold has become widespread: it comprises two lateral portions, each for molding a tire sidewall, and, for the molding of the tread, a peripheral ring divided into several segments having side faces perpendicular to the axis of the mold and the radial transverse faces (see e.g. U.S. patent 3,779,677). To close such a mold, move the segments radially toward the axis and axially closer to the sides. In the closed position, these various elements (side portions and segments) are joined and define the outer surface of the tire. The raw blank of the future tire must be firmly applied and held against the mold to obtain the desired geometric dimensions and, if necessary, print the sculpture of the tread. To this end, it usually employs a flexible membrane that is applied against the inner surface of the blank, by inflating the required pressure, which causes or tends to cause expansion of the blank. To ensure a good quality casting, it is necessary that the rubber is subjected to a molding pressure of about 10 bar. This pressure is applied through the membrane. To minimize the molding burrs, it is desirable that the membrane pressure increase occurs only when the mold is in the closed position. To avoid the appearance of these burrs, patent CA 765 745 proposes producing segments having parallel transverse faces. The peripheral ring therefore comprises alternately parallel transverse faces segments and segments with transverse faces are parallel to the transverse faces of the adjacent segments. Thus, at the end of closing the mold, parallel transverse faces segments can slide between the other segments, in the manner of a piston, which prevents pinching of the rubber between two adjacent segments.

p0003Molding and vulcanizing a rubber tire with conventional means, namely a segment mold to define the outer surface of the tire and a flexible rubber diaphragm to define the inner surface of the tire does not allow to control perfectly the shape and the internal dimensions of the tire.

p0004Another disadvantage resulting from the use of molding techniques and curing of the prior art is related to the different thermal behaviors of the various parts in contact with the tire during vulcanization, namely the side portions and crown metal segments the one hand, and the membrane in the other rubber. This complicates the development of vulcanization laws and their implementation.

p0005On the other hand, it is also known to use a removable rigid core defining the inner surface of the tire blank future. As an illustration, for example, consult the US patent 1 877 751. The rigid cores sometimes define, with the outer mold, a dimensionally stable molding space. In practice, we can use that to produce tires by casting or injecting into said space a material in liquid or paste. This is the production process of polyurethane tires, illustrated for example by US Patent 4,279,856.

p0006By cons if the rigid core supports an unvulcanized green tire carried out with materials conventionally used in the tire industry (i.e. unvulcanized rubber, textile and / or metallic reinforcing elements), and which is combines the core of a conventional type segment mold described by US patent^o 3,779,677, the segments will start to enter the raw blank to print the sculpture of the tread before complete closure of the mold. When the mold is not closed completely, there are still gaps between all segments. The raw rubber may well creep between segments. According to the groove ratio of sculpture and raw rubber resulting movements, this flow will cause significant burrs and even the inability to ensure proper closure of the mold due to the excessive amount of gum present in the gaps, preventing relative circumferential approximation of segments which is necessary to enable their radial movement until total closure.

p0007To overcome all these drawbacks, the solution according to the invention consists in using a rigid mold to impose the shape and final dimensions of the tire made from a green case and to take advantage of the differential in expansion between the unvulcanized rubber and the material constituting the rigid mold, due to the temperature rise during vulcanization, to achieve an adequate molding pressure.

p0008segment mold according to the invention used for molding and vulcanising rubber tires, of the type comprising<ul><li>a) a rigid core defining the inner surface of the tire,</li><li>b) two side portions, each serving for exterior molding of a sidewall,</li><li>c) a peripheral ring divided into a plurality of segments ensuring exterior molding of the tread,</li></ul> is characterized in that all parts (rigid core, side parts, peripheral ring of segments) defines the tire molding space, and in that all parts of the mold are configured so that they are related sliding one on the other at least during the final phase of the closing movement of said mold.

p0009The invention will be better understood by reviewing the following figures and the description relating thereto, illustrating a nonlimiting manner a particularly advantageous embodiment of the mold according to the invention. <ul><li>- Figure 1 is a partial and schematic radial section of a mold according to the invention, shown in closed position.</li><li>- Figure 2 is a section along the line II, II shown in Figure 1.</li><li>- Figure 3 is a peripheral ring of the plan view in partial open position segments, according to a first embodiment of the mold.</li><li>- Figure 4 is a radial sectional view of an alternative embodiment of the invention.</li><li>- Figure 5 is a plan view of the peripheral ring of segments, in the closed position, showing another embodiment of the mold.</li><li>- Figure 6 is a plan view of the peripheral ring of segments, in partially open position.</li><li>- Figure 7 is a peripheral ring of segments plan view showing another embodiment.</li></ul>

p0010The mold according to the invention, is constituted by a peripheral ring 1 segments for molding the tread, and two side parts 2, each ensuring the molding of a sidewall of a tire, as well as a 3 removable rigid core defining the inner surface of the tire. In this way, the mold ensures molding the outer and inner surfaces of the tire and therefore completely defines a molding space.

p0011The mold according to the invention to mold and vulcanize the tire with a much greater geometric precision than that which can be achieved by the conventional method of molding with a flexible membrane because all the mold parts which require the final tire geometry are rigid while in the conventional technique, to consider possible deformations and behavior of the inflatable membrane. With "rigid" is understood "substantially non-deformable" compared to an inflatable vulcanization membrane which is by definition and highly deformable structure compared to other parts of a traditional mold that undergo only very small elastic deformations due the forces exerted by the molding pressure. It follows that it is possible to achieve a much greater variety of forms, including the designer of tires can enjoy. The vulcanization is carried out at constant volume instead of the perform at constant pressure as is the case in the conventional technique. Being able to use with the mold according to the invention a different molding pressure between different areas of the tire is still an asset.

p0012Molding the inner surface of the tire being provided by a ring of essential characteristic rigidity, it can choose from wide range of materials to obtain this characteristic. In particular, the core can be metal like other parts of the mold. The calorific flow provided by inside the tire is then propagated to the raw rubber without encountering the heat shield consisting of traditional inflatable membranes, made of rubber. In addition, the heat transfer coefficient between such metallic core and the raw rubber is excellent, much better than when using a flexible membrane. This is still a significant advantage of the invention. It is possible to have a reduces cooking times without increasing the curing temperature and thus increase productivity vulcanizing equipment without any penalty in materials.

p0013The abolition of recourse to a flexible rubber membrane is also an intrinsic advantage of the invention. The use of such a membrane is indeed expensive because its lifetime is very small and because it requires the use and therefore to apply stick products.

p0014In addition, because the inner surface of the tire is defined by a rigid core while ensuring that the mold closing is possible and that the molding will occur at the interfaces of the mold parts, that very small burr, compatible with a very high quality of the molded product can be used as said rigid core supporting the tire blank at the beginning of manufacture. With the mold according to the invention one is freed in a very large extent on the strength properties of the rubber compounds in the unvulcanized state since these mixtures are not mechanically stressed, contrary to what is the case when 'tire blank undergoes, in the raw state, sometimes significant conformations. Thus the mold of the invention opens the way for the use of different mixtures of those used hitherto in the manufacture of tires, homogeneous or heterogeneous, in particular mixtures cheaper and / or conferring better performance to the tire.

p0015Similarly, removing the use of any conformation during the manufacture of the tire renders said much more accurate manufacturing since the products which enter into the constitution of the tire, once placed on the rigid core, will suffer more relative movements up such time as the structure is fixed by vulcanization. For example, the laying angle of the reinforcing son constituting the belt of a radial tire is no longer subject to variations due to these conformations.

p0016In Figure 1, we see that each segment has, in the closed position, in concordance with the side parts 2 through contact surfaces 101. Each segment also has transverse surfaces (not visible in Figure 1) joined, in the closed position, with the transverse faces of the adjacent segments. The radially inner faces 302 of the core 3 come, in the closed position in contact with the corresponding faces 202 arranged in the extension 20 of each side part 2 beyond the area 200 assuring the molding of the radially inner surface of the heels of pneumatic. These faces 202 and 302 are cylindrical, coaxial, and such that they can slide one on the other in the manner of a piston in a bore.

p0017Referring more particularly to Figures 2 and 3, we see that the mold segments according to the first embodiment includes a rigid three core and two types of segments (segments of the first type called Type A and designated by the reference 1a and segments of the second type called type b and designated by reference 1b) alternately arranged along the periphery. The rigid core 3 is divided also in two kinds of members (3a and 3b) arranged alternately. Elements 3b type b have transverse surfaces the extensions of which intersect radially outside and type 3a elements a are of complementary shape to define the inner surface of the tire, as described for example in US Patent 4,279,856 . thus, the tire requiring the introduction and removal of elements of the kernel is done by the only access, namely between the tire beads and radially inside of each heel, it is always possible complete the assembly of the core and start dismantling by elements 3b type b, the latter having no volume against undercut for radial movement towards the axis of the tire.

p0018Returning now to the ring segments 1 and according to a similar principle, one of two types of segments (type b) has transverse faces 1b1 and 1b2 parallel to each other, as taught by patent CA 765 745, which allows to obtain a piston effect when closing segments, closing type segments was before the segments of type b.

p0019We will now detail the implementation process of such a mold explaining the closing movements through the same Figures 1, 2 and 3.

p0020A core 3 consists of elements 3a and 3b of two complementary types is introduced inside of the green tire, as described above or the tire blank is constructed on the mounted core. Next, the side parts 2 are brought together the core bearing the raw blank of the tire until the extensions 20 slide under the faces 302 of the core 3, without, however, up to the closed position.

p0021At this stage, the rubber is not yet placed in compression. Then, the segments are brought into their closed position in accordance with the kinematics for which they are designed, namely the segments 1a before segments 1b. The blank supported by the rigid core 3 is in its final geometry before the mold is closed. When approximation of radial segments 1a, molding the sculpture of the tread will begin before the segments are not in the closed position. Thereby uncured rubber creeps of both sides of said segments 1a. In position of closure of said segments (see Figure 3), all the transverse faces of the segments are contiguous in pairs, so that the segments 1b are still down at least partially. Since all are contiguous transverse faces (1a1 and 1b1, 1b2 and 1a2), the final approximation of radial segments 1b helps push back the raw rubber which has flowed under the effect of the molding segments 1a while ensuring complete molding tread.

p0022Thus, type segments 1b, acting like a piston between the type 1a segments can avoid pinching raw rubber between the transverse faces of the segments.

p0023Finally, the lateral parts 2 are moved to their closed position, thereby again to push the rubber having been able to flow on either side of the crown 1 segments. Indeed, the contact surfaces between side parts 2 and ring segments 1 device are generated by a constant straight line parallel to the last mold closing movement. In the embodiment described, each contact surface 101 and 201 is a cylinder whose axis coincides with that of the mold. To prevent smudging, the axial approach of the side parts 2 is made at least partly after the closure of all segments. Alternatively, each 101' and 201' contact surface can be constantly generated by a line perpendicular to the axis of the mold, as shown in Figure 6. In this case, the axial approach of the side portions until their final position must precede all segments closing movement.

p0024For all the movements of the different elements of the mold that just indicated, the skilled person may use any suitable means, including mechanical jacks, hydraulic or pneumatic.

p0025This shows that the complete closure of the mold is done by sliding the parts over each other, without there ever approximation of an edge of a part to another part. With this, one can design a completely rigid mold, which can however be closed, if necessary even requiring to apply a closing pressure to one or more of these parts.

p0026Figures 5 and 6 illustrate another embodiment of a mold according to the invention. This embodiment provides for a different arrangement of the transverse faces of the segments. Thus, it can even prevent uncured rubber nip between the transverse faces of the segments when the segment mold comprises two types of segments and 1a' 1b' arranged alternately along the periphery, one of two types of segments (type b) having transverse faces 1b'1 1b'2 and whose radial extensions have a radially intersection outside the mold, the other type of segment (type a) having, when the mold is in the closed position, the transverse faces 1a'1 and contiguous 1a'2 previous 1b'1 and 1b'2. approximation radial movements of both segments must be coordinated. The implementation process of this embodiment of the mold according to the invention is characterized in that, at least in the final phase of closing the mold, all the transverse faces of the segments adjoin each other in pairs and, the movement toward type segments is being imposed, the approximation of type b segments is made according to a kinematic imposed by the sliding of the transverse faces of the segments of type b on the transverse faces of the segments of type a.

p00277 illustrates another embodiment of the segments of a mold according to the invention. It is still possible to obtain, according to the invention, a sliding of the transverse faces of the adjacent segments at least during a portion of a mold closing movement comprising any number n of segments when said segments are connected circumferentially between them and grip the article to be molded in the manner of a chain. The segment mold of this embodiment of the invention is characterized in that the transverse faces of each segment (1c, 1d, 1e) are such that their intersection with any plane perpendicular to the axis of the mold is an arc of circle. The closed mold is shown in Figure 7 in solid lines. Were overlaid, in interrrompus features, the representation of a segment 1d in the open position. The segment chain begins with segment 1c, 1d is continued by the segments, and ends with the first segment. Each segment, except the first (1c) is rotatable about an axis 4 located in an adjacent segment, to which it is connected by an arm 5. Each shaft 4 is parallel to the mold axis. To allow relative movement between the segments, the transverse faces and 1da 1DA adjacent segments in question are defined by the rotational movement of a curve contained in a plane passing through and tilting about the axis of rotation. In the simplest embodiment, this curve is a straight and defines a transverse face in the form of a partial cylinder, shown in plan view (Figure 7) by an arc of circle of radius r, r being the distance separating each point of a transverse side of the axis of rotation considered. All the transverse faces are thus defined, except the first face 1c1 (belonging to the first segment 1c) and the last 1e1 face (of the last segment 1e) which are defined in an analogous manner, but depending on the radius R with R = r + d, where d is the length of the segments, measured in a plane passing through the axis of rotation considered. The reason for this difference in configuration will better appear from the description of the closing of a mold thus formed. The segment 1c is moved to its closed position. Gradually, by a rotation around the axis 4 of the previous segment, the segments 1d are brought to their closed position, the transverse faces remaining contiguous. Finally, still by rotation about the axis 4 of the previous segment, the segment slides 1d between the last segment and the segment 1c. Closing this mold is ensured while maintaining, permanently, the transverse faces of adjoining segment pairs. At each step of closing, the segment that closes acts as a piston to push the raw rubber which has been able to flow upon closing of the previous segment. In an alternative embodiment, the peripheral ring of segments may be constituted by two half-channels which close to one in a circumferential direction, the other in the opposite direction.