Element for molding a pattern in a tread

Details Element for molding a pattern in a tread Element for molding a pattern in a tread

1998-04-22

2001-02-27

Mackey, James P. (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Reshaping, resizing or vulcanizing means for tire, tire...

C425S046000, C425S438000, C425SDIG005, C152SDIG003

Reexamination Certificate

active

06193492

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to molding elements intended to equip molds for molding articles or objects made of rubber and in particular treads for tires. It relates more specifically to molding elements for molding patterns of non-demoldable shape.
A non-demoldable pattern is intended to mean a pattern which has at least one undercut in its shape which means that if the molding element used for molding said pattern is trapped in rigid material, then there is no possible sliding between the element and the surrounding material because the part with the undercut opposes such sliding. The term rigid is intended to mean a material which does not allow enough elastic deformation for it to be possible for the element to be demolded, unlike the elastomeric material used, for example, to make a tire tread. In the latter case, and although the material based on a rubber compound is flexible enough to deform elastically and thus allow the demolding of the non-demoldable molding element, there may, however, be substantial deformation which, during the molding, may cause breaks in the molded material. These deformations of the molding material are all the greater if each molded pattern has at least one part with an undercut.
It has been observed that to meet an increase in performance with tires over time there is an increasing demand for tire treads which have patterns in which at least part of the tread pattern has an undercut.
It is known, for example from U.S. Pat. No. 4,794,965, that to improve the wear life of a tread with relief elements, these can be provided with a plurality of incisions which, on the running area of the tread, present straight lines, and in cross-section present broken or undulating lines over the depth.
Furthermore, and to avoid the appearance of the fatigue cracks at the bottom of the incision, it is known, see for example the patent U.S. Pat. No. 2,121,955, for the bottom of the incisions of a tire tread to be provided with a widened part forming a channel of an appropriate cross-section (which channel can also play a part in removing the water present on the area on which the tire is running). To make such an incision use is generally made of a molding element in the form of a somewhat thin metal blade which at one of its ends has a bulge intended to mold the widening of the pattern at the bottom of the incision.
With the purpose of overcoming the drop in grip during the life of a tire, U.S. Pat. No. 4,994,126, describes a tread which has incisions which, in cross-section, present lines in the shape of a fork with at least two branches, so that when said tire has been partially worn, a greater number of incisions appear. To produce such incisions, the use of rigid and non-deformable blades, generally metal, is known, the cross-sectional geometry of which is equivalent to the desired geometry of the incision. It will be readily understood that for some geometries of such blades, it is particularly difficult to extract them from the tread after molding, the branches of the block blade having to force their way out by parting the walls of that part of the incision which opens to the area of the tread.
Furthermore, when a great many non-demoldable molding elements are provided at the area of a mold, and such that they are fairly close together, it has been seen that the demolding force becomes very high; this results in an increased expenditure of energy and in an increase in the risk of material being torn out of the molded strip as a result of the demolding operation.
SUMMARY OF THE INVENTION
The object of the present invention is to propose a molding element for molding a non-demoldable pattern in a strip made of elastomeric material, said molding element not having the drawbacks of the molding elements usually employed.
In particular, there is proposed a non-demoldable molding element which, through its architecture, allows easy demolding while at the same time appreciably reducing the force needed to achieve said demolding and appreciably reducing the risk of the molded material being torn out or broken.
Among other things, the molding element according to the invention offers the possibility of producing a pattern of which the part that opens to the area of the new tire is extended under said area by a part which occupies a volume which is very much larger than the volume of the emerging part.
The structure of the non-demoldable molding element proposed in the context of the invention also makes it possible to produce patterns whose geometries are such that it could not reasonably be envisaged to produce them using conventional methods employing conventional molding elements.
The molding element in accordance with the invention is intended to equip a mold comprising a molding area for molding a tread intended in particular for a new or re-treaded tire. Another application of such a strip is also for fitting to a caterpillar track.
This molding element, once in place on the mold, projects from the molding area of said mold to mold a non-demoldable pattern, that is to say a pattern, at least part of which has an undercut and can therefore not be demolded in the demolding direction associated with said molding element. The molding element comprises at least one part for molding a part of a pattern with an undercut.
The molding element is one wherein, during at least part of the demolding operation, the projected area of each part molding a pattern with an undercut onto a plane perpendicular to the demolding direction is smaller than the area of the same part projected onto the same plane in the molding configuration.
This feature makes demolding easier with a lower force by comparison with what is needed to demold an element that would mold the same pattern but does not have said characteristic, that is to say a rigid and geometrically non-deformable element. Indeed, in the case of a rigid and non-deformable molding element, only the molded material can and should deform to allow said molding element to be extracted after molding.
During demolding, the molding element in accordance with the invention is moved in the demolding direction and is subjected to a demolding force which is opposed by the molded material surrounding said molding element bearing in mind the undercut part or parts; on each part of the molding element that forms a part of the pattern with an undercut, the forces exerted by the molded material result in a reduction in the size of the molding element through a substantial decrease in the area projected, onto a plane perpendicular to the demolding direction, of the part of the element molding a part of the pattern with an undercut. At the same time, the molded material which is elastomeric in nature and has elasticity properties, deforms enough to allow those parts of the pattern that open to the area of the tread to open and thus allow the molding element to be extracted from said material.


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