Venting of molds

Details Venting of molds Venting of molds

1995-10-11

2001-03-27

Mackey, James P. (Department: 1722)

Static molds

Container-type molding device

Including auxiliary port for transmission of fluid, i.e.,...

C425S028100, C425S812000

Reexamination Certificate

active

06206336

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to molds, particularly for the molding of tires, and in particular to venting techniques used in these molds in order to permit the air imprisoned between the mold and a raw tire blank to escape to the outside.
Molds for tires are frequently equipped with a number of vents arranged at suitable places. These vents are, for instance, generally holes of small diameter connecting the inner cavity of the mold with the outside of the mold. As it is not possible to produce holes of extremely small diameter due to inherent limitations in the machining techniques, it results that the diameter of these holes is generally sufficient to permit rubber to flow, at least slightly, into said holes when the tire is molded. This is the reason why small burrs of rubber protruding slightly from the outer surface can be noted on the surface of numerous tires.
It has been attempted to reduce the dimensions of the vents to a size which is sufficiently small to prevent the raw rubber from flowing into the vent spaces while permitting the air to escape to the outside of the mold. In order to reduce the diameter of the vents beyond what it is possible to do by techniques of direct drilling on the mold, it has been proposed to pierce a hole of relatively large diameter in the mold and to insert into said hole a vent which, in its turn, has a hole or slit or space of extremely reduced size. By way of example, reference may be had to U.S. Pat. No. 4,447,197 which describes a vent of this type. Another example is described in U.S. Pat. No. 3,854,852, in which there can be noted a vent of “valve” type which closes at the time that the raw rubber comes into contact with the head of the valve, after having allowed the air imprisoned between the raw rubber and the mold to escape.
The vents known from the prior art have numerous drawbacks. While some of them permit the air to escape while effectively preventing the appearance of molding burrs on the vulcanized tire, their satisfactory operating life remains, in general, very short. This makes it necessary to repair the mold far too frequently. It is therefore necessary to remove it regularly from the vulcanization press and shut the press down for the time necessary for the repair operations. Furthermore, it is not always possible to restore the venting capacity of these vents, and they must therefore be replaced rather regularly. This leads to an increase in the industrial cost price of these venting techniques. Furthermore, in the case of vents of the valve type, the initial cost price of such vents is relatively high.
With regard to the tread of tires, recourse is had to different means of venting, namely the implanting of vents such as described above, the production of simple drillings, or else one also makes use of the slits which are present between the different parts of the mold (planes of joints between parts) in order to conduct the air from the inside of the molding cavity to the outside of the mold. If necessary, the places where the planes of joints between the different parts of the mold are judiciously selected with respect to the tread pattern of the tire so that, whenever possible, a slit is present at the place where it is necessary to vent the mold.
By way of illustration of this last-mentioned technique, reference may be had to U.S. Pat. No. 5,234,326. In that case, for certain tread patterns, it is possible to develop a complete venting of the tire mold which does not require the drilling of vents. In the case of other tire patterns, however, it is not possible to achieve this purpose. Therefore, even in the tread portion of a tire mold, it is not always possible to avoid having recourse to drillings or vents.
A molding of better and better quality is always desired. The molding burrs which appear on the surface of the tire, whether in the form of small strips or of small bits, are harmful even in the event that the amount of rubber which has flowed out remains very small, since the appearance of the tire is such that the quality perceived by the buyer is considered poor.
Therefore, the problem is always present of succeeding in organizing a complete venting of a tire mold and of succeeding in doing so by dependable means, that is to say means which require only very little maintenance in industrial operation. The problem also arises of designing a less expensive venting, the maintenance of which is as simple as possible, in order that it can be adopted very easily by tire manufacturing workshops without the need of sending the mold back to a mold-construction workshop for it to undergo renovation there.
SUMMARY OF PRESENT INVENTION
The present invention proposes a mold which has a molding cavity which is limited by molding walls made of one or more materials, each molding wall having an air-tight molding surface defining a portion of the form of the outer surface of the object to be molded, characterized by the fact that at least one wall comprises at least one hole extending from said surface through said wall in order to place the molding cavity in communication with the outside of said mold, said hole being closed by a plug having an entirely tight end face flush with said surface in order to constitute a portion of the molding surface, said plug taking up exactly the entire surface of said hole in said molding surface.
The solution proposed by this invention makes it possible to produce only a venting with a relatively slight rate of flow of air. For this reason, it is in general used together with other venting techniques, making use of the slits present between the different mold parts or else, with respect in particular to the sidewalls of the mold, a technique such as described in U.S. Pat. No. 5,382,402. The technique proposed by the invention, despite its slight rate of flow, proves, however, entirely of interest for the perfecting of the venting, in particular in the case of tires having special tread patterns, or in all cases for the tires, in certain regions of the sidewall or the bead, where great difficulties in venting were encountered in the past.
Contrary to the numerous attempts which can be noted, in particular, from the patent references, the present invention does not propose producing a vent of very sophisticated form, nor does it propose producing a vent having movable parts nor using porous materials. Although the clearance between the plug and the hole which receives it is theoretically zero, it has been found experimentally that the air can leak around the plug at the temperature and pressure levels reached during molding. The dimensioning of the plug as venting means proposed is extremely simple, and the venting remains easy to achieve, even in a less specialized workshop, such as a tire production unit, in order to obtain venting spaces where desired.
Very unexpectedly, it is not necessary to try to provide an air-escape hole of very small dimensions at the level of the molding surface of the mold. While all vents known in the prior art allow holes or slits to appear physically on the molding surface, the present invention proposed using a sort of vent which could almost be termed “blind”, if it were not for its experimentally noted ability to allow air to escape. It has been possible to note that, even when effecting a force-fitting between the plug and the hole which receives it, the mounting of the plug in the hole does not take place in an infinitely air-tight manner. There results from this a leakage flow which can contribute to a very great extent to the venting of the mold, since the role played by these vents falls primarily within the so-called final molding regions, which at times are extremely difficult to vent.
Of course, the hole which appears on the surface of the mold is of any shape whatsoever. The simplest is to produce a circular hole with a drill. A drilling diameter which is suitable for production is between 2 and 3 mm. Thus, the trace of the plug appearing on the molding surface of the wall of the mold is less than a value of abou

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