GB2050931A

GB2050931A - Tyre vulcanisation mould

Info

Publication number: GB2050931A
Application number: GB8016666A
Authority: GB
Original assignee: Semperit AG Holding
Current assignee: Semperit AG Holding
Priority date: 1979-05-29
Filing date: 1980-05-20
Publication date: 1981-01-14
Also published as: JPS55161634A; AT363328B; HU180356B; BE883539A; IT1130677B; GB2050931B; FR2457765B1; FR2457765A1; DE3017261A1; YU135180A; ATA389479A; IT8022380A0

Abstract

The mould cavity includes ventilation holes (11) leading therefrom, each covered by a perforated metal plate (7) flush with the interior mould surface (9); the plate is thin (e.g. 1mm) and has small holes (10) or passageways extending therethrough (e.g. 0.1 mm diameter). The plate may be supported as a result of being a press-fit in one end of the ventilation hole or as a result of being supported at said one end by a bush inserted in the ventilation hole or by being of a larger diameter than said ventilation hole. In the last-mentioned and preferred case, the mould body is appropriately removed in the annular vicinity of the hole to provide a seat for the plate. <IMAGE>

Description

SPECIFICATION Tyre vulcanisation mould This invention concerns a tyre vulcanisation mould.

The removal of air from a mould cavity during the moulding of a tyre blank presents a considerable problem, which, if no satisfactory solution is found, always leads to a reduction in the quality of the manufactured product and can even render it useless. Conventional ventilation holes have a diameter of from 0.6 mm to 1.0 mm and extend through a part of the mould in a bolt which has an outside diameter of 3 to 4 mm. In this way the hole into which the ventilation bolt is inserted can have a greater diameter and can thus be more easily made and, if the ventilation hole becomes irreparably blocked, the ventilation bolt can be removed and replaced.

The two most important requirements of ventilation holes are, in fact, contradictory, since on the one hand the air trapped in the cavity must be removed quickly and on the other hand the plastic mass being forced against the inner wall of the mould under high pressure must not be allowed to escape. The fact that both these requirements cannot be fulfilled with equal success is evidenced by the spine-like flashes which are to be found on all new tyres.

Although these in no way affect the performance of the tyres, they must be removed by hand because it has been proved that they are regarded as a sign of inferior quality by the majority of purchasers. In addition, these flashes frequently tear off when the tyres are removed from the moulds and thus block the ventilation holes.

The aim of this invention is to eliminate the above problems, while enabling the moulds provided with holes for the insertion of conventional ventilation bolts to be re-used.

Accordingly, the present invention consists in a tyre vulcanisation mould having a cavity for shaping the external contours of the tyre and having ventilation holes leading away from the cavity, said ventilation holes being covered by a perforated metal plate fitted so as to be flush with an atthe level of the surface which defines the mould cavity, the diameter of the plate being at least twice its thickness and the plate having at least three holes of from approximately 0.1 mm to approximately 0.5 mm in diameter. Preferably, the diameter of the perforated plate is greater than that of the ventilation hole.

However, the perforated plate may be of the same diameter as that of the ventilation hole and the perforated plate may be supported by a bush inserted into the ventilation hole. Alternatively, the perforated plate may be of the same diameter as that of the ventilation hole and the perforated plate may have a U-shaped cross-section.

The present invention will now be more particularly described with reference to the accompanying diagrammatic drawing, in which: Figure 1 shows a cross-section of that part of a tyre vulcanisation mould which is provided with a ventilation bolt in accordance with the state of the prior art to the date of the present invention; Figure 2 shows a similar view of a ventilation hole according to the present invention; and Figures 3 and 4 show suitable perforated plates.

Referring to Figure 1, a vulcanisation mould for shaping and moulding tread and sidewall patterns in a green tyre blank consists basically of an inner part 1 comprising pattern segments for shaping the external contours of the tyre and an outer part 5 forming the body of the mould, which is fitted radially over the inner part 1. A ventilation hole 3 is inside a so-called ventilation bolt 2, which is pressfitted or glued into a corresponding hole or passage Bin the inner part of the mould 1. If, before the vulcanisation process is commenced, the tyre blank is pressed against the mould cavity surface 9 and the ribs 4, the air trapped inside the cavity escapes via the ventilation hole 3. Ventilation bolts of this kind are evenly distributed over the inner part of the mould.A ring-shaped collector duct 6 between the inner part 1 and the outer part 5 conducts the air ejected from the mould cavity through the outer part 5 via central holes.

Turning now to Figure 2 and to the present invention, a ventilation hole 11 (which, when the mould has been designed in accordance with the known state of the art, can be formed by the holes 8 for the known ventilation bolts 2) is covered by a perforated plate 7 fitted at the level of (flush with) the surface 9 of the mould cavity; this plate (shown in top plan view in Figure 3) has nine holes 10 of 0.1 mm in diameter. In order to accommodate the perforated plate 7, the ventilation hole 11 must be enlarged by milling out to the difference in diameter between the hole 11 and the perforated plate 7. The perforated plate is 4 mm in diameter, 1 mm thick and is made of stainless steel.

The U-shaped cross-section of the perforated plate 7a shown in Figure 4 has the advantage that, if the perforated plate is such as to ensure a good press-fit, it can be fitted without the necessity of milling out the ventilation holes 11 or using an additional means of fixing e.g. a bush.

It will be apparent, from what has been said above, that the present invention makes it possible to increase the clear section of passage of the ventilation holes as compared with the clear section of passage of conventional ventilation holes (namely, compare hole 3 in Figure 1 with hole 11 in Figure 2) while reducing the possibility of flash from the material in the mould due to the reduction in the diameter of the individual holes. Thus, on the basis of this invention, time-consuming removal of the flash subsequently to the moulding process is no longer necessary.

The method of using cylindrical, hollow bolts of sintered metal for ventilation of rubber moulds is already known from United States Patent Specification 3,804,566. The diameters of the bores through these sintered metal bolts are between 0.01 mm and 0.1 mm, which means that the danger of the holes being blocked by elastomer entering them or by components exuded from the elastomer mixture during the course of the vulcanisation process is very great. Thus, the operational reliability of such sintered metal bolts is not ensured when used in tyre production. The use of bolts made of synthetic resin material with several ventilation holes with diameters ranging from 0.05 to 0,5 mm is also known from United States Patent Specification 3,822,857.

These bolts are columnar in shape, i.e. their length is greater than their external diameter. This columnar form is necessaryfortechnical reasons because care must be taken to ensure that any flash penetrating into the ventilation holes becomes fully vulcanised in the holes, so that no unvulcanised flash emerges and in order to avoid unquantifiable flash and prevent contamination and possible clogging of the area behind the ventilation bolt. Due to the plastics material used in the manufacture of the bolts, the thermal conductivity of the bolts is so low that their length is necessarily great to ensure complete vulcanisation of the flash.This problem was thought to be likely also to present a hindrance to the realisation of the present invention because it was feared that it would prove impossible to vulcanise any flash fully in the short distance constituted by the thickness of the perforated plate 7 or 7a.

Surprisingly, however, it was found that, if the plate were made of metal, then (e.g. in the case of tread mixtures for lorries) a plate of approximately 1 mm in thickness would be sufficientforvulcanisation of the flash.

A further technical problem, and thus a negative factor with regard to the realisation of this invention, was the difficulty in producing such small holes in a metal plate, where it is no longer possible to use mechanical drills. This difficulty can, however, be overcome by the use of electron-beam drills.

Afurther benefit derived from the present invention lies in the fact that, with the majority of vulcanisation moulds, it used to be impossible to have the advantageous central ventilation system i.e. a system of ducts underneath the pattern segments in the body of the mould linking the ventilation holes running through these segments and leading out the the exterior surface of the mould at at least one point, since if the ventilation holes became blocked, the whole mould had to be taken apart and the ventilation bolts had to be driven out from the back in the direction of the mould cavity surface, thus requiring that each ventilation hole be extended through the body of the mould in order to make it possible for the ventilation bolts to be driven out.

With the ventilation system based on the present invention, it is possible to remove the perforated plate from the front without involving a great deal of time and effort to take th mould apart; for this purpose, a punch can be driven into the perforated plate, thus destroying the plate, which is then removed and replaced. With a central ventilation system, it is no longer necessary to extend the ventilation holes through the outer part 5 of the mould.

As already stated, if the diameter of the perforated plate is greater than that of the hole in the mould part which is provided for a conventional ventilation bolt, it should be milled out in gradations according to the difference in diameters and the thickness of the perforated plate and the plate will then be fixed by means of it being a press-fit or with the aid of a suitable adhesive.

The inner part 1 of the mould which accommodates the ventilation holes requires no machining if the perforated plate is supported by a bush inserted in the hole, of if the perforated plate 7a has a U-shaped cross-section which makes it unnecessary for a bush to be used.

Claims

1. Atyrevulcanisatiion mould having a mould cavity for shaping the external contours of the tyre and having ventilation holes leading away from the cavity, said ventilation holes being covered by a perforated metal plate fitted so as to be flush with and at the level of the surface which defines the mould cavity, the diameter of the plate being at least twice its thickness and the plate having at least three holes from approximately 0.1 mm to approximately 0.5 mm in diameter.

2. A tyre vulcanisation mould in accordance with Claim 1, wherein the diameter of the perforated plate is greater than that of the ventilation hole.

3. Atyrevulcanisation mould in accordance with Claim 1, wherein the perforated plate is of the same diameter as that of the ventilation hole and wherein the perforated plate is supported by a bush inserted into the ventilation hole.

4. A tyre vulcanisation mould in accordance with Claim 1, wherein the perforated plate is of the same diameter as that of the ventilation hole and wherein the perforated plate has a U-shaped cross-section.

5. Atyrevulcanisation mould constructed and arranged substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 3 or Figure 4 of the accompanying diagrammatic drawing.

6. Any features of novelty, taken singly or in combination, of the embodiments of the invention hereinbefore described with reference to Figures 2, 3 and 4 of the accompanying diagrammatic drawing.