GB2210576A - Moulds - Google Patents

Abstract

A metal mould (14), particularly for tyre vulcanisation, has an adherent layer coating as its outer mould surface. The layered coating comprises a first layer (17) adjacent the metal and which is a tungsten carbide layer, and a second outer layer (18) of a P.T.F.E. material <IMAGE>

Description

METHOD OF CURING A RUBBER ARTICLE AND MOLD THEREFOR Field The present invention relates to a method of curing a rubber article and in particular to a method of curing pneumatic tires and mold therefor.

Background In a rubber molding process, rubber compositions are molded under conditions of heat (elevated temperature) and pressure to create a molded, cured rubber article.

In a rubber tire manufacturing process, a tire casing, composed, for example, of a circumferential tread and supporting carcass with spaced beads and sidewalls, is first constructed from green (uncured) rubber components, some of which contain reinforcing elements, and the casing subsequently cured under conditions of heat and pressure in a vulcanization (curing) mold where the casing is given its final shape and where the tread pattern is impressed into it.

The curing molds are traditionally metallic and usually made out of either aluminum or steel alloys.

As a result of the typical porosity of the surfaces of such curing molds in contact with the tire casing, residues from the rubber tire casings often adhere to the curing mold. After numerous curing cycles, (one cure cycle per manufactured article), the mold surfaces often become contaminated to an extent that they need to be cleaned before they can be used for further curing cycles.

Disclosure and Practice of the Invention The inventors have discovered that, if the surfaces of the metal curing molds are coated with a particular and selective two component layered coating, the contamination of the mold surfaces can be substantially reduced and, consequently, the mold surfaces have to be cleaned less frequently.

The inventors have further discovered that the invention can provide rubber articles which have an improved outer, molded surface. The two component layer coatings on the curing mold surface can reduce the irregularities often found on the surface of the metal tire cure mold and thereby is capable of producing rubber articles with a smooth finish surface, depending somewhat upon the quality of the application and finish of the inventors' second layer, or coating, of their invention. This is turn can result in cured rubber products which have an improved appearance or which, depending on their use, have an improved performance.

More specifically, in accordance with the invention hereof, a rubber curing mold composed of a metal selected from steel alloy or aluminum alloy having a layered adherent coating on its curing surface comprised of a first, inner layer adjacent said metal mold surface comprised of a carbide selected from carbides such as a tungsten carbide and a second, outer layer adherent to said inner layer comprised of a tetrafluoroethylene fluorocarbon polymer. If desired, a silicone polymer layer may be applied to the said outer layer.

In the practice of this invention, the carbide coating (inner layer) can be applied to the metal mold surface (the rubber curing mold) by conventional techniques familiar to those having skill in such art.

The resultant surface of the carbide coat is typically somewhat porous and/or irregular (rough) in texture.

If desired, the surface of the carbide coat can be polished to reduce its irregularity or roughness if it displays such characteristic. To the carbide coating is then applied a second coating (outer layer) of tetrafluoroethylene fluorocarbon polymer. It can be applied by conventional techniques familiar to those having skill in such art. A suitable polymer for such purpose is TEFLON, a trademark of the Du Pont de Nemeours, E.I., Co.

As a result, a metal rubber curing mold is obtained which has a rather durable and rubber releasable composite layered coating thereon. The carbide layer is designed to provide a durable, adherent layer "bonded" to the metal mold surface and the TEFLON is designed to provide a durable release surface (to the curing and cured rubber product or article) which is adherent to the carbide.

In some instances, it may be desired that a silicone polymer coating is applied to the said outer coating to enhance or modify its release properties.

While the entire theory of the invention may not be fully understood, it is believed that the carbide layer is "bonded" or adhered to the mold's surface to provide a durable coating thereon in a manner in which the TEFLON is not readily capable of accomplishing.

Further, it is believed that the TEFLON is "bonded" or adhered to the carbide layer to provide a durable coating thereon with a release property (to the curing cured rubber article) not readily obtained with the carbide coating even if it is polished and/or coated with an alternate release coating thereon such as a silicone-based material. It is further believed that the adherence of the TEFLON to the carbide is enhanced by the carbide's porous and/or irregular surface.

-It is considered that, while the thickness of the carbide and TEFLON layers or coatings is not generally critical, it is naturally desired that they have a sufficient thickness to effectively accomplish the purposes of this invention, namely, a durable release composite layered coating on the metal mold surface to provide release surface for the curing and cured rubber product.

In further accordance with this invention, a method for curing rubber articles, particular rubber tires, is -provided which comprises curing the rubber article in such a coated mold under conditions of heat and pressure.

The method of the present invention is characterized in that the surface of the mold ~hat is in contact with the rubber article is the outer anti-adhesion layer which is adhered to the inner abrasive resistant layer which is, in turn, adhered to the metal surface of the mold.

A further advantage that can be gained from the present invention is that the coated rubber curing molds used in the method of the invention can be much more resistant to the generally abrasive cleaning methods than molds without such coating. Hence the useful service life of the curing molds can be expected to be prolonged in most cases.

While it is known to use abrasive resistant and anti-adhesion layers for various molding applications in industry, including carbide coatings and TEFLON coatings, such coated molds have been used in industrial processes for reducing the friction between two surfaces, in particular, such layers have been used for coating guiding surfaces, e.g. in the paper industry. Thus, insofar as it is known, the coated tire mold of the inventors' invention, with the prescribed layers, has not been used in a method of curing tires.

The invention will now be further described with reference to the attached drawing in which FIG. 1 depicts a tire cure mold and FIG. 2 depicts a sectional cross-section of such mold.

FIG. 1 illustrates a typical curing mold 10 for pneumatic tires. The mold 10 is composed of two portions or halves, namely, an upper half 12 and lower half 14 which, when assembled, form a cavity for curing the tire casing 13 that is pressed into it. The mold portions are generally made out of a metal and preferably consist of an aluminum alloy or steel alloy.

While the mold illustrated is a so-called two piece mold, the invention is equally applicable to other forms of molds, in particular to the so-called segmented molds that are extensively used in the tire industry. The surface of each mold portion is provided with a tread pattern (15) and indicia that are to be molded or impressed into the green tire casing to form a desired surface characteristic (16) thereon.

FIG. 2 shows a cross section of the lower mold portion (14) of the tire mold of FIG. 1 taken at position 2-2. It more clearly illustrates the two coating layers that are provided on the mold surface that is in contact with the tire to be cured.

The first (inner) layer (17) that is applied to the metal mold surface is the abrasive resistant carbide layer.

By abrasive resistant layer it is meant a layer of material having sufficient physical hardness and strength that it is placed on the upper end of a hardness scale and that it resists the abrasive action of materials that are placed lower on the scale of relative abrasiveness. Such a scale is known as the MOHS hardness scale. It is preferred that the abrasive resistant layer (17) of the present invention has a MOHS index which is at least nine.

Carbides, for example, particularly tungsten carbide, are known to have a MOHS index which is higher than 9. Indeed, according to a preferred embodiment of the invention, the abrasive resistant layer consists of tungsten carbide.

The second (outer) layer (18) that is applied to the surface of the first layer (17) is an anti-adhesion, or release layer, such as, for example, TEFLON, i.e. a layer which, while it has a very strong adhesion to its substrate, - the abrasive resistant layer -, it has a low adhesion to the rubber article which is cured in the metal mold.

It is well known that Polytetrafluoroethylene and various silicones have a very low adhesion to rubber, and it is, therefore, preferred to use such layers for coating the mold that is used in method of the present invention.

Claims (5)

WHAT IS CLAIMED IS:

1. A metal mold having an adherent layered coating thereon as an outer molding surface, characterized in that said layered coating is comprised of a first inner coating adjacent said metal comprised of tungsten carbide and a second outer layer adherent to said inner layer comprised of a tetrafluoroethylene polymer.

2. The mold of claim 1 characterized in that said metal is selected from at least one of a steel alloy or aluminum alloy.

3. The mold of claim 3 characterized in that a coating of a silicone polymer is applied to said second outer layer.

4. A method for curing rubber articles characterized by curing such articles in the mold of claim 1 under conditions of heat and pressure against the surface of said outer coating.

5. The method of claim 5 where said outer surface has a coating of silicone polymer therein and the rubber article is cured against said silicone coating.